Spal t srt
FOR THE PEOPLE FOR EDVCATION
FOR SCIENCE
LIBRARY
OF
THE AMERICAN MUSEUM OF
NATURAL HISTORY
GEOLOGY OF MONGOLIA
I ALVId
PLATE I.
THE FLAMING CLIFFS OF DJADOKHTA. (Painted by C. Lester Morgan after field sketch in water color by Frederick K. Morris.)
CENTRAL ASIATIC EXPEDITIONS ROY CHAPMAN ANDREWS, Leader
PeEOrOGY OF MONGOLIA
A RECONNAISSANCE REPORT BASED ON THE INVESTIGATIONS OF THE YEARS 1922-1923
BY CHAREES” P.”BERKEY
CHIEF GEOLOGIST
PROFESSOR OF GEOLOGY, COLUMBIA UNIVERSITY
FREDERICK kK. MORRIS GEOLOGIST
AMERICAN MUSEUM OF NATURAL HISTORY
With 44 “Plates and 161 Illustrations in the ext Six eMaps in Pocket at End
NATURAL HISTORY OF CENTRAL ASIA VOE. ik
THE AMERICAN MUSEUM OF NATURAL HISTORY HENRY FAIRFIELD OSBORN, ‘President NEW YORK
1927
Copyright, 1927 by The American Museum of Natural History
G. P. Putnam’s Sons, Publishers New York and London.
Made in the United States of America
PREFACE
THE series of volumes of which this one forms a part is intended to cover a wide range of investigations conducted by expeditions into Central Asia under the scientific supervision of the American Museum of Natural History. The Third Asiatic Expedition of 1922-23 was one of these, operating in Mon- golia under the leadership of Roy Chapman Andrews, and the volume now presented grew out of the observations made at that time. The succeeding volumes will include investigations in all of the branches of science repre- sented, and will cover the results of many years of field exploration and re- lated study by different groups of men. The whole effort is directed to a better scientific understanding of the continent of Asia, especially its central and less readily accessible portions. Field observations and the collecting of material have already occupied several years, and the exploratory side of the work is still in progress. Doubtless it will take a much longer time to describe and interpret the collections and field notes.
This volume is devoted entirely to geology. It is confined to reconnais- sance studies, especially the traverses of the Third Asiatic Expedition in the season, 1922, when both geologists were in the field together. The results were used in 1923 when the junior author had additional opportunity to revisit certain districts and see new ground. Since the manuscript of this volume was first written, the Expedition of 1925 gave further opportunity to both geologists to carry the investigations forward, but the results are not included in this volume. The reconnaissance observations of 1925, together with a series of special studies, are to form the next volume.
The traverses furnishing the material for this volume amount to about 5000 miles of travel in central Mongolia. Observations are distributed from the vicinity of Kalgan on the southeast, to Urga on the north, and, on the west, to Sain Noin in the Khangai Mountains and Baga Bogdo of the Altai Ranges. The area has been crossed and recrossed with little duplication of trails; nevertheless, there are wide spaces untouched and large areas whose geologic details are quite undetermined. The general geologic structure, however, has proved to be readable; the major units have been recognized so many times and at so many different places that it is believed the general
iv;
vi PREFACE
geologic features and history are now reasonably well established. The area to which these studies apply amounts to not less than 200,000 square miles. It is an enormous area to cover in so short a time, and if it were not for a certain simplicity of geologic history the results could not be presented with any confidence. Doubtless an immense amount of detail will be added as the work proceeds, but it is believed that no large item in the geologic state- ment is likely to be greatly changed.
This reconnaissance is put forward as a step in the unraveling of the geology of Central Asia. We recognize its incompleteness and its many unsolved problems, yet we are confident that it is based upon true lines with respect to structure and historic succession, and that it ought to serve a use- ful purpose as a background for additional exploratory investigations.
Many problems have presented themselves as the work has proceeded. It is hoped that some of them may be given further study. Data already in hand bear on problems that have not been discussed in this report but are expected to yield results on laboratory investigation. There are yet other problems which cannot be solved except by additional field work of a detailed and extended nature. In so far as future work makes it possible, we hope to present these in succeeding volumes as opportunity may arise.
The four parts of this volume exhibit very different range and have dif- ferent purposes.
Part I is essentially an introduction to the field and methods of work.
Part II covers the traverses and records observations along the route.
Part III includes a series of locality studies where more careful and detailed work could be done.
Part IV is an attempt to summarize in geologic terms the meaning of the mass of data assembled from these studies.
Readers not primarily concerned about the geological record will find cer- tain portions of more general interest. To them we recommend the introduc- tions under each Part as well as certain chapters in each Part, as illustrative of the nature of the investigations. Chapters I, II], X11, XIV, XXI and XXII present a series of pictures to the general reader, and we commend them as of broad enough interest to be of service to the layman.
We are indebted to many organizations and many more individuals for helpful suggestions and for assistance in assembling and presenting this work. Due recognition is given some of them in the body of the report.
To the generous friends of scientific exploration who have contributed the funds that have made this work possible we owe a special debt of grati-
PREFACE vil
tude. Without their support neither the field investigations nor the sub- sequent research could have been carried out. The sponsors have also borne the expense of publication; thus enabling the Museum to offer the volume at cost. Their interest in pure science and their faith in this venture have helped to materialize visions that otherwise must have vanished as they came, as many others before have vanished.
It is fitting that special mention should be made in this place of the helpful coéperation of the Geological Survey of China, and that we should acknow- ledge the interest of individual members of the staff, particularly Dr. V. K. Ting, former Director, and Dr. Wenhow Wong, the present Director. To _Dr. A. W. Grabau, Chief Stratigrapher and Palzontologist, we have turned repeatedly for helpful suggestions. Dr. Grabau has undertaken the identi- fication of the invertebrate fossils collected by the Expedition and will pub- lish his results in a subsequent volume of this series.
In the preparation of this material for publication, still more vital assist- ance has been rendered by members of the staff of the American Museum of Natural History, whose close connection with the work of the Expedition has given special acquaintance with and insight into its scientific problems.
It is a pleasure to express special obligation to President Henry Fairfield Osborn whose prophetic vision is being fulfilled and whose guiding hand in these Asiatic explorations has been felt from the beginning.
We are deeply indebted to Acting Director George H. Sherwood on whose shoulders has fallen the home administration.
To Dr. W. D. Matthew, whose wide knowledge of paleontology and Cenozoic history make him an authority, we have referred many questions, particularly as to the correlation of a number of the Tertiary formations.
The heavy labor of editorial work has been borne by Dr. Chester A. Reeds, assisted by Miss C. M. Beale and Mrs. Mary V. Forster. It is an espe- cial pleasure to record our gratitude to Dr. Reeds, whose constructive sug- gestions have been exceedingly helpful. Dr. E. W. Gudger has aided in the preparation of the bibliography.
Among the institutions whose services should be acknowledged is Colum- bia University, from whose faculty ranks the chief geologist came, on an as- signment to field investigation. This service was rendered at a sacrifice made by the staff of the Department of Geology, since an absence of this kind thrust additional responsibility on every remaining member. Special recog- nition should be accorded to Professors James F. Kemp, Roy J. Colony, and Douglas W. Johnson, who together shared the chief burden left to the Depart- ment. Professor Johnson has read portions of the manuscript and has given generously the benefit of his keen and scholarly criticism. Contributions to such a piece of work, therefore, are made by many whose actual participation
vill PREFACE
is not apparent, and it would have been much more difficult to carry it to completion without their help.
Professor V. A. Obruchev, of the Academy of Sciences at Moscow, has kindly contributed a list of references to Russian geological publications which proved a most helpful guide in reviewing the literature on Central Asia. Professor Obruchev has also aided in the preparation of the map showing the routes of Russian explorers (Plate II).
To Mrs. Florence Eddowes Morris, Mrs. Flora Cook Parks, Miss Helen R. Fairbanks, and Miss Agnes Molloy, we are indebted for valuable help on the manuscript. The illustrations are the work of many hands, some of which are acknowledged in the text. Messrs. Andrews, Granger, and Shack- elford have furnished some of the photographs. Professor George B. Bar- bour, of Yenching University, Peking, contributed the photographs appearing as Plates VIII A, XLI A and B. Mr. Edward J. Alexander drafted the geo- logic cross-sections of the first ten chapters, chiefly from the field notebooks of the senior geologist; Mr. Erwin J. Raisz drafted the base maps, and Mr. Leon B. Hills made a number of the diagrams.
A place of highest honor for contributory service must be reserved for the members of the field staff of the Expedition with whom we worked day after day amid the privations and perplexities of strange and uncharted re- gions. With these men we endured the hardships that months of isolation always bring. With them we worked in surroundings of seemingly boundless desolation where only the unaccountable enthusiasm of the explorer, or the zeal of the scientific investigator, or the lure of the confirmed wanderer could have made the drudgery of those days appear to be worth what it cost. With their codperation and encouragement the secrets of the desert were slowly searched out. And, now that we can look back on those toilsome days that stretched out almost endlessly month after month, with never a glimmer from the outside world, and with self-appointed tasks that taxed endurance to the last dregs, all seems like an entrancing dream that slowly takes on the glamour of a golden age.
These men we came to know intimately and, as our problems were dis- cussed together, we learned to rely on them for daily help and encouragement. Sympathetic and helpful companions they were through all the confusions and discouragements that in the nature of the case belong to the borders of the unknown. Their service cannot be measured. We can only say that we appreciate it all, and realize full well that except for their fine spirit of codper- ation the results must have been very different indeed.
To the leader of the Expedition, Dr. Roy Chapman Andrews, whose humanness and sanity under disappointments and successes alike helped to stabilize what might have become an ineffective effort, is due a kind of credit
PREFACE ix
unlike that of any other. Except for his genius the Expedition would never have been formed; except for his success in interesting public support and inspiring faith in the undertaking, the work could never have been kept going, and, except for his confident leadership and good fortune in the face of moun- tains of difficulty, calamity might easily have overtaken the venture.
To Walter Granger, Chief Palzeontologist, whose patient quiet manner and scientific competence inspired general confidence, we owe another spe- cial acknowledgment. The work of the geologist and of the paleontologist interlock and overlap. Both use the same data. Some of the records that both search for in the field are to be found in the same strata, and the history that both strive to unravel is closely interwoven. Their stories supplement each other. Their discoveries are often made together or are made one for the other and must be freely contributed to the general good. Therefore, it is vital to the success of both that the fullest coéperation and the frankest relations should be maintained. With such a man as Granger the task is easy.
To J. B. Shackelford, the photographer, whose artistic work speaks for itself, we owe an equal debt for other qualities. His ingenuity and practical knowledge in lines quite beyond his own special field made him the general advisor of the camp, and his genial nature and wonderful gift of humor never failed.
During the field season of 1925, Major L. B. Roberts, assisted by Lieu- tenant F. B. Butler and Lieutenant H. O. Robinson, made a series of -route maps from Kalgan to Orok Nor, about 865 miles. These maps will be pub- lished in a later volume of this series, but the altitudes, which were deter- mined by means of a large Gurley transit, have been used to correct the ane- roid readings of the earlier traverses covered in the present volume. The out- line of Tsagan Nor (Plates XX1X and XXX) has been corrected also to agree with Major Roberts’ special map of that lake.
Each member of the whole staff has his own special claim to recognition and acknowledgment. In a kind of work where the chauffeur of one day may become a discoverer the next, or where a camp helper of one season may develop into an efficient scientific aid the next, and where competence and reliability in any single task make for so much greater efficiency in the whole organization, it is not possible to single out every individual for his full share of credit. Sometimes even our lives were in the hands of our com- panions, and that may have been true more times than we knew. If Bayard Colgate, in charge of transportation, had been less efficient or less accommo- dating; if Merin, the leader of the caravan, had been incompetent or untrue; if the supporting staff had failed in any major service, the whole story of accomplishment must have been very different.
x PREFACE
So we recall with gratitude and appreciation the services of F. A. Lar- sen, the interpreter, and of the native Mongol guides, Sirimpil, Bato, Aioshi, and Tserin; of Merin’s caravaneers, Banjien, Bato, Okher, Otoburun, and Sanjarav; of Colgate’s transportation staff, Wang Hung Ping and Ah Sah; of the taxidermists, Chi Shou Lun and Hsia Wen Chiang; of Granger’s staff of collectors, George Olsen, Peter C. Kaisen, and Albert F. Johnson; and those men whose constancy in their daily service about the camp made our own work possible, Loh and his staff, Chang Kwei Mo, Hwei Hsiu Yen and Kan Ch’uen Pao.
There have been scores of others, of lesser influence only because of fewer opportunities, but whose interest has been keen and whose sympathetic sup- port and encouragement have been a spur to such accomplishment as is rep- resented. We do not forget the importance of their service although it is not practicable to enter here all of the names that might properly claim a place.
To them all we offer this tribute of appreciation. That one of our num- ber deserves most credit who executed his own task the best and thereby helped to make the whole interdependent organization so much the more efficient. Such results as have been attained in any field are only in part the simple product of the individuals directly responsible; in much larger part than is readily indicated they are a joint accomplishment. This is true also of the two authors of this volume. The interpretations here presented have grown up under continuous unreserved codperative contribution of observa- tion, interpretative understanding and experience. Certain parts of the volume took form originally as individual contributions, but all have been subjected to drastic revision by both authors. The work has grown in our hands so gradually and under such intimate discussion, and has been modi- fied so many times that we ourselves are not able to separate clearly the ele- ments that belong to each. The actual labor of preparation has fallen most heavily on the junior author; in the field, the larger experience of the senior author may have counted more heavily in coérdinating and interpreting obser- vations; but the result as presented is a joint product on which we have agreed, both as to form and substance.
CHARLES P. BERKEY, FREDERICK K. Morris, Geologists. New York Clty, October 25, 1926.
CONTENTS
PREFACE . : : : : i : : A C = 2 : List oF PLATES 2 , : a = . : r - A A List oF FIGURES : é : : , ri : A ; : PART I.—GENERAL INTRODUCTION ‘ 5 3
CHAPTER I.—GEOLOGICAL OBJECTIVES AND METHODS OF THE EXPEDITION WITH A REVIEW OF FORMER EXPLORATIONS . : - . : 5
INTRODUCTION
THE APPROACH TO MONGOLIA
THE GOBI REGION
PREHISTORIC ADVENTURE
HISTORIC TRAVEL AND MODERN EXPLORATION
EARLIER STUDIES OF SPECIAL SIGNIFICANCE é * . .
II.—BOUNDARIES OF THE GOBI REGION
INTRODUCTION
THE NORTHERN BOUNDARY Northern boundary—western section Northern boundary—central section Northern boundary—eastern section . Summary of the Transbaikal border
THE EASTERN BOUNDARY Summary of the eastern boundary . 5 : x :
THE SOUTHERN BOUNDARY . : E : 3 : Southern boundary—eastern section . ; ¢ - 5 Southern boundary—central section . : : c Southern boundary—western section . : ¢ 3 ¢
THE WESTERN BOUNDARY 3 ; 3 , : = é 3
xii CONTENTS
CHAPTER PAGE PART II.—ROUTE STUDIES OR ITINERARY . - A 3 ee kei INTRODUCTION . ; ; : : : : , ; : AQ THE TASK OF A GEOLOGICAL RECONNAISSANCE EXPEDITION ; = - 39 RECORDING OBSERVATIONS. ‘ : ; : : : ea IIL.—FROM KALGAN TO IREN DABASU . ; : : 5 c - : 45 KALGAN TO WAN Cu’UAN Pass é : : 5 : : Br ls
The lower pass : : : : ; - - . eat
Wan Ch’uan basin and the upper pass : 6 é : AS
THE PACIFIC DIVIDE TO P’ANG KIANG : : 5 ; 5 = 49
The granite hills of Chakhar . : : 3 : : ay BAS
The P’ang Kiang hollow . : : : F ‘ ‘ ies
P’aANG KIANG TO IREN DABASU , : : 5 5 e 21055
The first fossils : : j é A 5 - 4 - 59
IV.—FROM IREN DABASU TO URGA : 3 5 : ° A . 4 60 IREN DABASU TO CAMP JURASSIC . : ; 5 - - - 60
The structural relations at Camp Jurassic . ; : : Gy,
Camp Jurassic TO Mount TUERIN ; : : < é = go
The granites of Mount Tuerin . ; ? : ‘ : Syed
Mount TuUERIN TO BOLKUK GoL .. . ‘ ; : : = a7 6
The Arctic divide . : , F ‘ : ‘ : 5 Ge OBSERVATIONS IN THE VICINITY OF CAMP BOLKUK GOL. : ar 83
Course of the Bolkuk Gol , : 2 : : : .. 85
Glacial evidence in the Gangin Daba : : : ‘ 5 4245)
Groundice . : 5 : : : : ; : Se roy
Local culture . : ; j d : : ; 5 5 he
Side traverse toward Urga ; : é ; - ; 5 teks)
Mineral resources . F 4 : A s ‘ - 5 ute)
V.—FROM URGA TO TSETSENWAN 4 f “ A 5 > : 5 go TRAVERSE ALONG THE TOLA RIVER ‘ : c ‘ ‘ - 90
THE VICINITY OF FIVE ANTELOPE CAMP . 3 : 2 6 3 £93)
FROM THE TOLA RIVER TO TSETSENWAN ‘ c - , = | 204
THE VICINITY OF TSETSENWAN : é ; : . : 1 97
A side traverse north of Tsetsenwan . ‘ 5 é ‘ - 98
CONTENTS CHAPTER The serpent-form dikes : ; 7 5 é Contact effects of the granite margin . 5 4 Jurassic structural relations . : ; 5 :
A side traverse south of Tsetsenwan . 5 : ‘
VI.—FROM TSETSENWAN TO SAIN NOIN AND THE ARCTIC DIVIDE .
WESTWARD FROM TSETSENWAN
THE VICINITY OF CAMP CANYON BROOK .
From Camp CANYON BROOK TO THE ONGIN GOL Rock-bound hollows Continuation of the traverse . ; : : : The Ongin Gol
FROM THE ONGIN GOL TO SAIN NOIN The vicinity of Sain Noin
From Rainy GULCH TO THE ARcTIC DIVIDE . : 5
A step across the divide . : ¢
VII.—FROM THE HOT SPRINGS OF SAIN NOIN TO MOUNT USKUK ARISHAN TO GORIDA The Gorida basin The old Uliassutai trail . : 3 F 5 OLD ULIASSUTAI TRAIL TO Mount Uskuk The saltpan of Guchu Burt ONDAI SAIR
VIII.—THE RETURN JOURNEY FROM TSAGAN NOR TO ARTSA BOGDO CHOOSING A ROUTE TsSAGAN NoR TO THE VOLCANIC CLIFFS Desert hollows ; : VoLcANIc CLirrs TO ARTSA BOGDO NORTH MARGIN OF ARTSA BoGbDo . : : ;
Picture writings ‘ ‘ : ; : : 4
IX.—FROM ARTSA BOGDO TO SAIR USU - . 6 ArtsA Bocpo To DJADOKHTA i : The flaming cliffs of Djadokhta :
xiii PAGE
98 102 103
105
xiv CONTENTS CHAPTER DJADOKHTA TO ONGIN GOL IN SUMU Ongin Gol in Sumu . ONGIN GOL TO Sarr Usu A 2
X.—FROM SAIR USU TO KALGAN . PALAOZOIC STRATA e 5
CONTINUATION OF TRAVERSE TO ARDYN OBO
Sediments of Ardyn Obo .
ArpyN Oso To SHARA MuruN Paleozoic strata of Jisu Honguer Tertiary sediments of Shara Murun
SHARA MURUN TO KALGAN Chinese settlements .
The return to Kalgan
PART III.—SPECIAL AND LOCALITY STUDIES
INTRODUCTION Method of Mapping
XI.—IREN DABASU AND IRDIN MANHA . INTRODUCTION
GENERAL ASPECT AND LARGER RELATIONS OF THE BASIN
Origin of the hollow at Iren Dabasu .
GENERAL ROCK STRUCTURE The Iren Dabasu formation Variegated Tertiary beds .
The Houldjin formation
The Arshanto and Irdin Manha formations
Shara Murun .
AGE AND CORRELATION .
XII—ARISHAN, THE SACRED MOUNTAIN OF SAIN NOIN
INTRODUCTION FEATURES OF THE LOCALITY GEOLOGICAL FORMATIONS
Graywacke series
CONTENTS
CHAPTER
The granite . j : : 5 5 The Jurassic formation . : - : Special products ‘ ‘ , c - STRUCTURAL FEATURES . : : ; : THE SPRING WATERS AND THEIR ORIGIN . :
XIII.—MOUNT USKUK AND THE TSAGAN NOR BASIN
PRINCIPAL STRUCTURAL FEATURES i ; - THE ROCK FORMATIONS : : : : 5 ROCKS OF THE ANCIENT FLOOR : - 5 The crystalline metamorphics . : 9
The graywacke series § f : 6
Bathylithic granite
Jurassic strata : ‘ é : - Post-Jurassic intrusives . : ; : ROCKS OF THE SEDIMENTARY COVER A é Cretaceous strata. : : : Fossil content . : : s 5 Tertiary strata : 5 : - ‘
Fossil content . j : : a Former extent of later sediments
Evidence of the faulted margins
Evidence of the sedimentary remnants .
Evidence of the basalt remnants
Evidence of planation . EROSIONAL HISTORY OF THE SOUTH BLOCK OF USKUK DEFORMATION OF THE Mount USKUK REGION . THE SALTPAN OF GUCHU BURT
TsAGAN Nor 5 ‘ ‘ 5 5 .
XIV.—THE GURBUN SAIKHAN RANGES
THE JOURNEY F : é 4 : 4 A desert well . é : , : : The return journey . ‘ : é 5
GEOLOGICAL OBSERVATIONS AT ARTSA BOGDO . The south basin P , ; s =
xvi CONTENTS CHAPTER APPROACH TO THE GURBUN SAIKHAN THE FORMATIONS OF THE GURBUN SAIKHAN Ancient rocks . Limestones Serpentine . Graywacke-slate series : : ;
Topographic character of the Gurbun Saikhan
XV.—ARTSA BOGDO AND OSHIH THE ArtsA Bocpo MounTAIN BLOCK The marginal shelf History of the shelf Rock formations of the mountain Ancient meta-crystallines and intrusives Folded sediments and associated eruptives Later igneous rocks Tectonic features Traces of former sedimentary cover Topographic features Stream pattern Glaciation . THE OsHIH HOLLOW Features of the locality
The geologic column
XVI.—PROBLEMS AND AREAS DESERVING SPECIAL STUDY INTRODUCTION
SEDIMENTARY BASINS AND POTENTIAL FOSSIL FIELDS .
The area of the great pass P’ang Kiang ; The paper-shales of mile 299 The flaming cliffs of Uskuk Oshih (Ashile) basin Djadokhta
Ardyn Obo
Shara Murun . -
CONTENTS CHAPTER
Ulan Nor F 3 : : - 5 The badlands of the Ongin Gol . :
STRUCTURAL AND HISTORICAL PROBLEMS . The Camp Jurassic area : : The crystalline upland east of the Ongin Gol The transition country of Gangin Daba
Sair Usu Losin Sumu . ; 0 ; : . Jisu Honguer . ‘ ‘ : a 5 GENERAL PROBLEMS. A : 5 PART IV.—SUMMARIES AND DISCUSSIONS . 3 INTRODUCTION
XVII.—STRUCTURAL ELEMENTS OF THE OLDROCK FLOOR Two MAJOR DIVISIONS SUBDIVISION OF THE OLDROCK FLOOR Mesozoic intrusives . ; : : 2 Jurassic sediments Paleozoic strata The great Mongolian bathylith . The Khangai graywacke series .
THE ANCIENT CRYSTALLINE COMPLEX j . The Wu T’ai system The T’ai Shan complex . : : 5
XVIII.—SURFACE FEATURES AND THEIR ORIGIN BasINs AND MouNTAINS
THE BASIN OF MONGOLIA Deserts in general The shaping processes
MAJOR SUBDIVISIONS OF THE MONGOLIAN BASIN Talas of the eastern and southern province The western faulted talas .
Mountains : 3 : : Fault-block mountains of the Altai type
XViii CONTENTS
CHAPTER
Ikhe Bogdo : - - “ P ° “ . Baga Bogdo : : - ° : - 5 : Artsa Bogdo ‘ ‘ : c = A 5 Gurbun Saikhan - - ~ - : The mountains of the eastern province . 5 - Volcanic mountains . , : 5 : - THE STRUCTURE LINES OF MONGOLIA . ‘ - - ; XIX.—SURFACE FEATURES AND THEIR ORIGIN (continued) . . PLATEAUS, PLAINS AND FLATLANDS : ; é : . 5 : INTRODUCTION—THE LEVEL LANDS OF THE DESERT AGENCIES WHICH CARVE THE EROSION PLANE . é ; Erosion by wind F , é c : Spats Rainfall . ; ; : : ‘ A 0 : 5 - Drainage : ; ; : ; : s :
Immigrant streams Native streams Erosive work of running water . THE PIEDMONT SLOPE THE GOBI EROSION PLANE Original slopes of the erosion plane Deformation of the Gobi surface Deposits on the Gobi upland Redissection of the Gobi erosion plane Summary of the Gobi erosion plane DESERT HOLLOWS THE OLDER PENEPLANES The pre-Cretaceous peneplane The Mongolian peneplane The Khangai peneplane Relations of the Khangai and Mongolian peneplanes Relations of the Mongolian peneplane and the Gobi erosion plane Relations between the Gobi erosion plane and the lowlands of the P’ang Kiang stage COMPARISON OF PENEPLANES WITH THOSE OF SURROUNDING REGIONS . LAKES
HUMMOCKS AND DUNES .
PAGE 310 3II 313 314 317 318 319
323 323 323 324 325 326 327 327 328 329 330 332 333 333 333 335 336 336 341 341 342 343 343 345
347 347 348 350
CONTENTS CHAPTER XX.—STRATIGRAPHY OF THE LATER SEDIMENTS THE LATE MESOZOIC ERA
CRETACEOUS DEPOSITS
The Ondai Sair formation
The Oshih (Ashile) formation
The Iren Dabasu formation
The Wan Ch’uan formation UPPER (?) CRETACEOUS DEPOSITS
The Djadokhta formation COMMENTS ON CRETACEOUS SEDIMENTS
THE CENOZOIC ERA
PALEOCENE PERIOD
The Gashato formation EOCENE PERIOD
The Arshanto formation
The Tukhum formation
The Irdin Manha formation
The Shara Murun formation OLIGOCENE PERIOD :
The Ardyn Obo formation
The Houldjin formation
The Hsanda Gol formation MIOCENE PERIOD .
The Loh formation . PLIOCENE PERIOD . :
The Hung Kureh formation
SUMMARY OF THE CRETACEOUS AND TERTIARY DEPOSITS
XXI.—SUCCESSIVE CLIMATES OF MONGOLIA INTRODUCTION . PAST CLIMATES OF MONGOLIA LATE PRE-CAMBRIAN OR SINIAN PERIOD PALEOZOIC ERA MESOZOIC ERA
Lower Jurassic period
xx CONTENTS
CHAPTER
Cretaceous period . : : - S Oshih and Ondai Sair time . “ 5
Tren Dabasu time P : - : Djadokhta time . : ~ 3 CENOZOIC ERA. ‘ - - - -
Eocene period . Gashato time Arshanto time Irdin Manha time Shara Murun time Oligocene period Ardyn Obo time . Houldjin time Hsanda Gol time Miocene period Pliocene period Hung Kureh time Ertemte time Pleistocene period RECENT CHANGES OF CLIMATE EVIDENCE BEARING ON CLIMATIC CHANGE Dominant denudation Double dissection ‘ ; : : The choked gorge of Wan Ch’uan Hsien Pass Desert trees The forest margin Tree growth at Tiger Canyon Ancient stream courses Terraced alluvial fans Minor scarps in erosion hollows Lakes : Ancient lake beaches of Tsagan Nor . Glaciation A vanished race An abandoned dam MEANING OF THE EVIDENCE PRESENT CLIMATE
PAGE
376 376 376 377 378
379
379 379 379 380 380 380 380 380 381 381 381 382 382 385 386 386 386 387 387 388 389 390 390 391 391 392 393 393 395 395 395
CONTENTS CHAPTER XXII.—SUMMARY OF GEOLOGIC HISTORY . ANCIENT ERAS CHARACTERIZED BY METAMORPHISM. F : The T’ai Shan complex . : ; : F s : The Wu T’ai system The Khangai series . The granite bathylith : 5 PALAOZOIC MARINE HISTORY
Mip-MESOZOIC SEDIMENTATION INITIATING THE CONTINENTAL HISTORY OF CENTRAL ASIA
DEVELOPMENT OF BASIN STRUCTURE WITH COVERING OF LATER SEDI- MENTS . .
A SEDIMENTATION SUMMARY DEVELOPMENT OF VOLCANISM Deep-seated magmas Serpent-form dikes . The brittle porphyries : : : ; : Basaltic and other lavas and pyroclastics DEFORMATION HISTORY . Epeirogenic movements Orogenic disturbances Warping : : : : : : Faulting : ; : : ; Metamorphic deformation : : 3 Grouping of deformation products Epochs of erosion Disconformities ORIGIN OF PRESENT TOPOGRAPHY . : : . ‘ ‘ RELATION OF GEOLOGIC HISTORY TO THE DEVELOPMENT OF MAN .
BIBLIOGRAPHY . é . : . INDEX . . . . . . . . . : .
Xxi PAGE 397 398 398 398 399 400 400
402
403 495 408 408 409 409 410 410 410 411 411 412 413 413 414 416 417 418
421
449
AT
ab viyye
PLATES
FACING
PLATE PAGE I.—The Flaming Cliffs of Djadokhta (colored) ‘ ; . Frontispiece II.—Map of the routes of Russian geologists in Central Asia (colored) . 6 III.—Map of the routes of non-Russian geologists in Central Asia (colored) . 17 IV.—Location map of the borders of Mongolia . : ; ; : 5 oP V.—(A) The first mountain barrier Bt : : : : ‘ Be ee (B) The Great Wall above Nan K’ou Pass P F : : ees VI.—Conventional symbols used in the geologic sections. : j me 45 VII.—Conventional symbols used in the geologic sections (continued) . 46 VIII.—(A) View from head of the Wan Ch’uan Pass. : f ‘ 45 (B) Cultivated fields of Chinese farmers . : : : ; t. 748 IX.—(A) The upland as seen from the second camp . ‘ : 3 5 lg) (B) Camp in the granite hills of Chakhar . é , : : ; ) 349 X.—(A) Dissected dinosaur beds at Iren Dabasu : 3 5 :) 60 (B) Residuary granite bowlders : F é : : : 5 (eo) XI.—(A) Updragged sandstone beds at Camp Jurassic : : é PeeoL (B) Granite residuals on the slopes of Mount Tuerin . : : = OI XI1.—(A) Peneplaned granite floor along the Urga trail : : j 1K) (B) Residuals of the granite bathylith, west of Tsetsenwan 3 SLO XIII.—(A) The aggraded trench at Canyon Brook : ; : c Bo, a (B) Rock terrace at Rainy Gulch. : : : ' ; = Te XIV.—(A) The Arctic Divide in the Khangai Mountains . : : 830
(B) Sain Noin Khan and the Khangai range . : : : 5 8%)
Xxiii
XxXiv PLATES
PLATE
XV.—Mongol lama and his small daughter at Sain Noin XVI.—Mongol herdsman and his son at Sain Noin
XVII.—(A) Block disintegration in the granite at Arishan (B) The salt pan of Guchu Burt
XVIII.—(A) The sand dune belt of Tsagan Nor (B) The lava-capped mesa at Oshih .
XIX.—Wind-scoured cliffs at Djadokhta
XX.—(A) The great temple and lamasery, Ongin Gol in Sumu (B) A nest of dinosaur eggs at Djadokhta
XXI.—(A) The scarp of Oligocene beds at Ardyn Obo . (B) Structural detail of the scarp at Ardyn Obo
XXII.—(A) The beveled slate floor of Bilgoho (B) Badlands of the Shara Murun formation at Ula Usu
XXIII.—Geologic map of the Iren Dabasu area (colored) . XXIV.—The Sacred Mountain of Arishan (colored)
XXV.—(A) Guardians of the Sacred Mountain (B) Xenolith of graywacke capping a granite hill
XXVI.—Geologic map of the Arishan area (colored) XXVII.—Geologic map of the Mount Uskuk area (colored) XXVIII.—Geologic map of the Hsanda Gol area (colored) . XXIX.—Geologic map of part of the Tsagan Nor basin (colored) XXX.—Geologic map of the Hung Kureh area (colored)
XXXI.—(A) The Hsanda Gol - (B) The southern margin of the south Uskuk block XXXITI.—(A) The Uskuk block from Dzun Hsir ‘ (B) Sedimentary remnant in the south Uskuk block .
XXXIII.—(A) The flaming badlands south of Mount Uskuk (B) Dissected sand deposits in Tsagan Nor basin
FACING PAGE
183 In pocket 213
218 218
In pocket In pocket In pocket In pocket In pocket 230 230 231 231 248 248
XXXIV.—(A) The intermont basin between Artsa Bogdo and the Gurbun Saikhan 249
(B) Typical morning scene at a desert well
249
PLATES
PLATE
XXXV.—(A) A fault zone in the Oshih basin . (B) The badlands at Oshih Nuru
XXXVI.—(A) The tilted Lower Cretaceous strata at Oshih Nuru (B) Cannonball Canyon . XXXVII.—Residuary stack of jaspery silica at Oshih
XXXVIII.—(A) Beveled surface of upturned phyllites at Gorida (B) Shattered graywacke near Jisu Honguer
XXXIX.—(A) Deformed jasperite in the graywacke series (B) The phyllites of Artsa Bogdo
XL.—(A) An outpost forest at the Arctic Divide (B) The terraced alluvial fan at Tiger Canyon .
XLI.—(A) Two stages of dissection at Wan Ch’uan (B) The choked gorge at Wan Ch’uan Pass
XLII.—(A) Shrinkage cracks in the floor of a playa near Tsagan Nor
(B) The abandoned beaches of Tsagan Nor
XLIII.—(A) A pre-Mongol grave . : : : 3 , (B) Gap through the Great Wall at Wan Ch’uan Pass
XLIV.—An ancient monolith
XXV
FACING PAGE
268 268
269 269 272 302 302 303 303 388 388 389 389 396 396 397 397 418
FIGURES
FIGURE
1.—Generalized geologic section from Tientsin to Wan Chu’an Pass
2.—Two cross profiles of the Gobi basin
3.—Profile and section illustrating methods of recording observations
4.—A general location and route map 5.-Geologic section, Miles 0-30, Wan Ch’uan Pass 6.—Geologic section, Miles 30-60, Miao T’an 7.—Geologic section, Miles 60-90, Ertemte 8.—Geologic section, Miles 90-120, Chap Ser 9.—Geologic section, Miles 120-150, the Chakhar Hills 10.—Geologic section, Miles 150-180, P’ang Kiang 11.—The P’ang Kiang basin 12.—Geologic section, Miles 180-210 i 13.—Geologic section, Miles 210-240, Irdin Manha 14.—Geologic section, Miles 240-260, Iren Dabasu 15.—Geologic section, Miles 260-290 16.—Geologic section, Miles 290-320 17.—Geologic section, Miles 320-350, Ude 18.—Geologic section, Miles 350-380, Camp Jurassic 19.—Geologic section, Miles 380-410 20.—Geologic section, Miles 410-440 21.—Geologic section, Miles 440-470 22.—Geologic section, Miles 470-500 23.—Geologic section, Miles 500-530, Mount Tuerin 24.—Geologic section, Miles 530-560 25.—Geologic section, Miles 560-590 26.—Geologic section, Miles 590-620 F : 27.—Geologic section, Miles 620-650, Continental Divide
XXVii
.
XXVili FIGURES
FIGURE
28.—Geologic section, Miles 650-670, Bolkuk Gol 29.—Glacial cirque, Bolkuk Gol . “ 4 < - ; 30.—Bowl-shaped valley heads in the Gangin Daba Mountains 31.—Geologic section, Miles 668-688, traverse to Urga . 32.—Geologic section, Miles 670-700, to the Tola River 33-—Geologic section, Miles 700-730, Tola River Valley 34.—Geologic section, Miles 730-760, Tola River Camp 35.—Geologic section, Miles 760-790, Five Antelope Camp 36A.—The fault-block at Tsetsenwan .
B.—The fault-block at Tsetsenwan (continued) 37.—Geologic section, Miles 790-820, Tsetsenwan 38.—Geologic section, Miles 0-20, north of Tsetsenwan 39.—Margin of a roof-pendant 40.—Geologic section, Miles 0-30, south of Tsetsenwan 41.—Geologic section, Miles 30-45, south of Tsetsenwan 42.—Route map, Miles 820-850 . 43.—Geologic section, Miles 820-850 ieee 44.—Geologic section, Miles o-8.3, north of Canyon Brook 45.—Rock drawings near Canyon Brook 46.—Route map, Miles 850-880 . ; : 47.—Geologic section, Miles 850-880, Canyon Brook 48.—Route map, Miles 880-910 . ‘ 49.—Geologic section, Miles 880-910, oan Gol . 50.—Route map, Miles 910-940 . 51.—Geologic section, Miles 910-940, popes Gulch 52.—Sketch map of Sain Noin 53-—Route map, Miles 940-961 . 54.—Geologic section, Miles 940-961, Forest ee 55.—Route map, Miles 960-990 . : 56.—Geologic section, Miles 961-990, Arishan 57-—Geologic section, Miles 990-1020, Gorida 58.—Route map, Miles 990-1020 59.—The ragged mountains of Gorida 60.—Route map, Miles 1020-1051 61.—Geologic section, Miles 1020-1051, Uskuk
PAGE
81
83
86
89
92
92
95
95
98
99 IOI IOI 102 107 107 112 112 113 115 117 117 121 121 123 123 126 129 129 133 134 134 137 138 140 140
FIGURES FIGURE 62.—Geologic section, Miles 0-30, Tsagan Nor 63.—Geologic section, Miles 30-60, Volcanic Cliffs 64.—Geologic section, Miles 60-90, Artsa Bogdo . 65.—Geologic section, Miles o-30, Gun Usu 66.—Geologic section, Miles 30-60, Dubshih 67.—Geologic section, Miles 60-90, Djadokhta 68.—The Khurul Obo : j 69.—Geologic section, Miles 90-120, Ulan Nor 70.—Geologic section, Miles 120-150, Ongin Gol . 71.—Geologic section, Miles 150-180 72.—Geologic section, Miles 180-210 73.—The margin of a sediment basin at Los in Sumu 74.—Geologic section, Miles 210-240, Sair Usu Trail
75.—Geologic section, Miles 240-270, Porphyry Camp .
76.—Geologic section, Miles 270-300 77.—Geologic section, Miles 300-330, Sair Usu 78.—Geologic section, Miles 330-360, Paleozoic inlier 79.—Geologic section, Miles 360-390 80.—Geologic section, Miles 390-420 81.—Geologic section, Miles 420-450
82.—Geologic section, Miles 450-480 : 83.—Geologic section, Miles 480-510, joie Obo . 84.—Map of Ardyn Obo 85.—Dissection of the scarp at pee Obo 86.—The geologic column of Ardyn Obo 87.—Geologic section, Miles 510-540, Taliu in Sumu 88.—Geologic section, Miles 540-570, ‘‘Choppy Sea”’ 89.—Geologic section, Miles 570-600, Jisu Honguer 90.—Geologic section, Miles 600-630, Shara Murun 91.—Geologic section, Miles 630-660, Boltai Urtu 92.—Geologic section, Miles 660-690, Shara Khata 93.—Geologic section, Miles 690-720
94.—Geologic section, Miles 720-750, Chinese cultivation
95.—Geologic section, Miles 750-780 96.—Geologic section, Miles 780-810
Xxix PAGE 145 145 150 155 155 159 159 160 160 163 163 165 167 167 168 168 171 171 172 172 175 175 176 176 177 179 179 180 180 185 185 186 186 188 188
XXX FIGURES FIGURE 97-—Geologic section, Miles 810-827 . : : 5 - 2 - 98.—Location map of the eastern Altai region : 3 2 99.—Sketch map of the region between Iren Dabasu, Shara ue and P’ang Kiang 100.—Block diagram of the area between Iren Dabasu, Shara Murun, and P’ang Kiang 101.—The scarp at Irdin Manha 102.—Generalized geologic section between Iren Dabasu and Irdin Manha 103.—The stratigraphic section at Iren Dabasu 104.—The scarp at Holostai 105.—Location map of Arishan 106A.—Field sketch of the Baga Bogdo front B.—Field sketch of the Baga Bogdo front (continued) 107.—Generalized geologic section across the Tsagan Nor basin ; 108.—Domed and dissected planation surface of the Mount Uskuk area . 109.—Field sketch of Baron Shiliuta 110.—Field sketch of the eastern margin of the Mount Uskuk block 111.—Gobi plantation surface beveling tilted strata ; 112.— Generalized geologic section from Artsa Bogdo to the Gurbun Saikhan 113.—Generalized geologic section of the Gurbun Saikhan 114.—Generalized structure section of the piedmont shelf at Artsa Bogdo 115.—Generalized structure section of the eastern end of Artsa Bogdo 116.—Analytical profile of the upland of Artsa Bogdo 117A.—The upland meadows of Artsa Bogdo B.—The upland meadows of Artsa Bogdo (continued) 118.—The mesa and stripped inliers of the Oshih basin .
119.—Two fault zones in the Oshih basin. , : 5 - A“ 120.—Location map of proposed studies : é : c - : 121.—Section of the Flaming Badlands of Uskuk .
122.—Dome at Camp Jurassic . ‘ ; ‘ - F ; ‘
123.—Typical margin of a sediment basin <
124.—Geology of the oldrock floor along the route b 5 - : : 125.—Typical structure section of the oldrock floor
126.—Geologic section north of Tsetsenwan .
127.—Geologic section of an area of marine Permian beds
128.—Typical geologic section of the Khangai series
129.—Typical geologic section of the pre-Cambrian complex .
PAGE
189 194 198 199 200 201 203 209 214 224 225 229 239 240 242 245 253 257 261 263 264 266 267 269 270 275 277 281 289 290 291 293 295 297 299
FIGURES
FIGURE 130.—Summary table of the formations of the oldrock floor . 131.—Sketch map of Mongolia ; , 132.—A generalized block diagram of a faulted basin 133.—Four stages of sedimentation at Gurbun Saikhan 134.—View of Oshigo Ola from the south 135.—Tectonic map of Transbaikalia (after Obruchev) . 136.—General structure lines of Asia 137.—The Gobi upland at Ardyn Obo . 138.—The Gobi upland at Irdin Manha 139.—The piedmont slopes of Los Ola 140.—Diagram showing origin of residuary gravels 141.—The P’ang Kiang lowland . 142.—The short gullies at Ardyn Obo . 143.—Re-dissection of a smooth scarp at Shara Murun . 144.—Early stage in the development of a reversed scarp 145.—Late stage in the development of a reversed scarp 146.—Reversed scarps at Shara Murun F 147.—The Mongolian upland in the Chakhar Mountains 148.—The Tola River terraces 149A.—The Arctic Divide . :
B.—The Arctic Divide (continued) . 150A and B.—Alternative correlations of the older erosion surfaces 151A and B.—Alternative explanations of the Mongolian peneplane 152.—The zolian crossbedding at Djadokhta
153.—Successive stages in the development of a gobi basin _
154 and 155.—Two diagrams to illustrate the relation of warping to ee in the
formation of basins 156.—Columnar diagram of the later sediments
157.-—Glacial map of Eurasia
158.—Field sketch of a bowl-shaped ee head in the on Daba Mountains
159.—Drawings cut upon a basalt block at Artsa Bogdo
160.—Sketch map of central Mongolia, showing geologic units along the traverses
161.—Generalized columnar section of the oldrock floor
XXXi PAGE 301 304 310 316 318 320 321 324 324 330 334 337 338 339 339 340 340 342 342 344 345 345 346 356 367
369 370 383 384 394 404 415
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4
VOL. II—I
A GEOLOGICAL RECONNAISSANCE OF THE
GOBI DESERT REGION OF CENTRAL ASIA
PART I GENERAL INTRODUCTION
PART I—GENERAL INTRODUCTION
CHAPTER I—GEOLOGICAL OBJECTIVES AND METHODS OF THE EXPEDITION, WITH A REVIEW OF FORMER EXPLORATIONS
CHAPTER II—BOUNDARIES OF THE GOBI REGION
CHAPTER I
GEOLOGICAL OBJECTIVES AND METHODS OF THE EXPEDITION,
WITH A REVIEW OF FORMER EXPLORATIONS
INTRODUCTION
THIS volume is confined to the geological reconnaissance of the seasons of 1922 and 1923. In it are recorded observations made in the course of an extended itinerary across the desert regions of Mongolia. The major traverses include the following:
I.
From Kalgan to Urga, a distance of six hundred and seventy miles, through Wan Ch’uan Hsien Pass over the Pacific divide, across the Gobi basin to the Arctic slopes of the Tola River region.
From Urga southwestward along the Tola River, through the Tsetsenwan district across the Ongin Gol, a distance of four hundred miles, to Sain Noin Khan on the Arctic divide.
From Sain Noin southwestward and southward three hundred miles to Tsagan Nor at the base of Baga Bogdo, one of the Altai mountain ranges.
From Tsagan Nor in the basin at the foot of Baga Bogdo, a hun- dred miles and more eastward to the Artsa Bogdo and Gurbun Saikhan ranges.
> From the Gurbun Saikhan northward across the desert to the Sair Usu Trail, returning on that trail six hundred miles to Kal- gan.
These traverses, together with side trips from certain base camps, gave an itinerary of more than three thousand miles—most of it in desert or semi- desert country, part of it in the mountainous country of the Arctic divide, and a much smaller part in the isolated mountain groups of the easternmost
3
4 GEOLOGY OF MONGOLIA
ranges of the Altai mountain system. Our traverses reached the center of the great continent of Asia, which in all probability has been inhabited by man longer than any other continent. Across its immense areas of desert and steppe country, caravans have journeyed from the time commercial inter- course developed between peoples, and travelers have toiled over these stretches of sand in the Gobi ever since man began to wonder about the earth and its possibilities. Here are the oldest trails of the world. Some of them are primitive transcontinental highways made by the tread of countless bare and padded feet, and dating back, perhaps, to the time of the great racial migrations.
Notwithstanding such a history, surprisingly little had been learned about the content and the meaning of the underground of the Gobi region. But as students of science continued to gather data of other regions of the earth, especially those facts bearing on early man and the animal life that preceded him, the conviction grew that Asia must have seen the evolution of man, and may have seen as much of many other races of living things. The race of man may have originated here; there was, however, no basis for a more authoritative statement, since no one had yet found much evidence in Asia itself, especially in central Asia.
It is surely not accident that has made the continent of Asia so impor- tant a factor in the development of man. Both man and beast have been more directly dependent on the corresponding growth and behavior of the earth than has been generally assumed. Geologic environment,—the whole physical setting of the past while this great continent was in the making,— may have been a more important factor in producing interrelated floras, faunas, races, and cultures of the present day than is fully appreciated.
Geologic history and the history of life run side by side, and have been intertwined and interlocked from the beginning; yet they are not equally dependent one on the other, for life is always subject to the geologic setting, and to some degree is compelled to adjust itself to the endless changes that the surface of the earth presents. Too seldom have geology and evolution been studied together as complementary parts of the same story. Thus these geological explorations in Asia take on a special significance, for the unraveling of whatever story there is to be told should give a better under- standing of the long eras through which life progress has been made.
Some such considerations as these were in the minds of the sponsors and of the scientific staff of the Third Asiatic Expedition, and served to give direction and character to their undertaking. The general itinerary was chosen because the region which it covered appeared to have been touched by geological exploration less than any other of similar extent in central Asia, but it was modified in accordance with the problems that arose as the work
GEOLOGICAL OBJECTIVES AND METHODS 5
progressed. The major traverse was projected across the Gobi basin with the conviction that somewhere in this basin country a series of sediments ought to have been preserved, which could contribute important facts to a better understanding of the geologic evolution of Asia, and of the development of present life forms.
The Expedition hoped to establish a better foundation for the study of all forms of life in late geologic time. It was, therefore, in large part, an expe- dition devoted to the unraveling of geologic and palzontologic history as re- corded in the rocks of the desert regions of central Asia. i
It had been predicted by Professor Henry Fairfield Osborn, many years before, that central Asia would one day yield important paleontologic contri- butions and perhaps connecting links in the chain of ancient forms of life, and that it would prove to have been a great center of origin for forms found in North America and Europe. It was not the province of the Expedition, how- ever, to predict or to promise, but rather to investigate and to interpret.
Travelers crossing the desert seem to have found little support for these more or less speculative predictions, and the general reputation of the Gobi region was far from encouraging to such an undertaking as the Third Asiatic Expedition. It was evident, however, from the observations of such men as von Richthofen (1877) in the region farther south, and Obruchev (1900) in the Gobi proper, that there were sediments of somewhat uncertain age which ought to fulfil the requirements; but it took courage to project so expensive an expedition into a region apparently so desolate and un- promising as the Gobi. The region, to be sure, has been crossed by many other explorers, but with less advantage in operation and apparently with less success in detecting the elements of its complicated structure. In part this is because of the many natural difficulties that face the would-be inves- tigator in such surroundings. Slow, tiresome travel; privation and occasional danger; heat, cold, and ceaseless winds; drifting sands and almost trackless wastes; scanty food and uncertain water-supply,—all these combine to make more complicated the task of an investigation whose uncertainties and puzzles are only one part of the practical problem.
It is quite outside our present purpose, and surely not necessary, to give a descriptive account of this now well-known Expedition. Its scientific staff was prepared to make every exertion to prove the quality of the enormous expanses of desert country in central Asia. The members were resolved to do as much as was humanly possible, on a hurried reconnaissance, toward interpreting the physiographic features and the geologic structure, the fossil content and the prospective paleontology of the region, and to determine, if possible, the significance of the processes represented in the rocks made in former times. They set out to discover and to read the story of the fossil
6 GEOLOGY OF MONGOLIA
remains that might lie buried in the strata. Whether any such finds could be made no one knew, but whatever contribution the country had to offer must be found. Geographer, paleontologist, geologist—together all bent their energy to one purpose. Each turned his best effort to unraveling that part of the story which was written in the language of his own science, from the present back to the beginning.
The major purpose was to see whether the Gobi region had fossil fields of late geologic time, to find how promising they might be, and to determine the structural and stratigraphic history of the plateau and basin country cov- ered by Mongolia. In any case, whether the major object proved to be attain- able or not, the staff of the Expedition was prepared to make a line of scien- tific determination across the northern desert, from China on the one side to Siberia on the other, so that it could be used as a standard for future reference and a guide to additional exploratory work. At least it should serve as a starting point for studies in other parts of the region.
As a step toward this end, a continuous cross-section, representing the geological changes and structure, was carried from the beginning to the end of the traverse. A good deal of ground was covered, of course, before much system could be discovered in the accumulating mass of data. In the course of time, however, a fair working basis was evolved, and certain localities were found to be of sufficient importance to deserve special and more extended study. At those places local, detailed investigations were made on a scale commensurate with their apparent geologic and palzeontologic importance. These have become the key studies of the Expedition, and are supported by maps and large collections, both of rocks and of fossils.
In this manner there was constructed about three thousand miles of cross-section, part of it with accompanying route maps, and five special areas were given intensive study as local problems. Areal and structural reconnaissance maps were made, covering more than seven hundred square miles. At the same time, the physiographic features and processes of the desert were observed and studied, so that those features constitute an addi- tional contribution. The maps of the region were entirely inadequate, and, therefore, attention was given to their correction, and to the more accurate location of important physical features or points of special interest.
The major results, in addition to the details of observation, include the following items:
1. Several great fossil fields of reptiles and mammals have been located.
2. The general geologic structure of the major rock formations of the Gobi basin region has been determined.
3. The major elements of geologic history of the Mongolian region have been unraveled, and a fairly complete geologic column has been worked out.
GEOLOGICAL OBJECTIVES AND METHODS 7
4. Extensive collections, both of fossils and of rocks, have been made, some of which are among the rarest and most valuable ever found in any region.
5. The principal steps in the physiographic development of central Mon- golia have been detected in the features of the region.
6. A chain of scientific determinations has been welded between China on the one side and Siberia on the other.
7. The major problems deserving additional investigation have been indicated, and steps toward their solution have been suggested.
8. A great field for subsequent research has been opened in central Asia, which is certain to attract increasing attention in many other related sciences.
g. A series of special key studies has been made, to serve as starting points for future work.
In preliminary summary, it is, therefore, possible to say that, instead of being barren and hopeless, the Gobi Desert has proved to be one of the most productive geologic and paleontologic regions in the world. This volume records the observations and conclusions covering some of these results.
THE APPROACH TO MONGOLIA
The Expedition outfitted in Peking, and traveled inland by rail as far as Kalgan before taking to its own means of transportation. From that point motors were used for the scientific party, its immediate equipment and a few supplies. A caravan of seventy-five camels had been sent into the interior about two months in advance, with additional supplies sufficient for the whole season. After making connections with this caravan at Mount Tuerin, five hundred miles out, the movements of the two sections were codrdinated in such a way as to keep the supply caravan within reach of the motor section.
This system of transportation and supply proved to be adequate during most of the season, and only an occasional side traverse or piece of local work had to depend on a different method. Rarely were supplies short. The sev- eral junction points were reached by the whole Expedition, and the move- ments of both sections were so well timed that no delay was caused by failure to codrdinate.
Since it is not the purpose of this report to describe the incidents of the trip, or to cover the several other fields of investigation carried on at the same time, but instead to emphasize the geological observations, little further at- tention will be given in this volume to that side of the Expedition’s operations.
The itinerary account will be carried without special reference to the method of travel, the incidents of camp life in the desert, or other happenings of the Expedition. These have been given in considerable detail by the
8 GEOLOGY OF MONGOLIA
leader of the Expedition, Roy Chapman Andrews, in semi-popular articles contributed to Asia Magazine and other publications (1922 to 1925).
Although the scientific work of the Expedition began beyond the outer walls of Kalgan, a better appreciation of the features first encountered there can be gathered from a statement of the geology along the route from Peking to Kalgan.
Peking stands not far from the inner border of the great alluvial plain of China formed by deposits carried by rivers that have come down from the mountainous interior of the continent. These alluvial deposits cover a com- plex erosion floor of older geologic formations (Fig. 1, page 8). The surface of the deposits is a smooth plain composed of gravel, sand, and silt, rising almost imperceptibly from the sea to a very abrupt and rugged inner margin which stands as a mountain wall nearly three thousand feet above the plain (Plate V, A, page 23). The Nan K’ou Pass pierces this barrier, and through it winds the old trail—northern China’s principal gateway to the interior— over which seven centuries ago the Mongol hordes came on a very different mission. On a modern railroad that parallels the ancient trail to Kalgan, the Expedition set out to reach the heart of the country of the Mongols, which has become again a center of world interest, not because of anything the Mongols have done, but because of the remarkable geologic history of their vast desert region.
oe ors
SI
Archean "Proterozoic Granite Cambro- Lower Tertiary Gneiss Limestone Ordovician Jurassic Cretaceous Lava
Ficurp 1.—Generalized geologic section from Tientsin to the Wan Ch’uan Hsien Pass. The section is intended to emphasize the step-like approach to the Gobi region from the great plain of northern China. The major underground geologic structures and the relations of the principal rock formations are indicated. They fall conveniently into three groups: (a) a simple delta, made of alluvial sands and clays built out into and dis- placing the ancient Yellow Sea; (b) a basement of complex rocks emerging from beneath the delta at Nan K’ou; (c) a cover of Mesozoic and Tertiary gravels, sands and clays with associated basaltic lava flows, beginning at Wan Ch’uan Hsien Pass,
In the Nan K’ou Pass, through which one must go to reach the higher lands of the interior, the older sedimentary rock formations come to the sur- face and are plainly exposed in all their complicated structure in the walls of the gorge. On entering the gorge, simple sedimentary strata are encoun- tered, dipping southeastward beneath the plain. Then more complex for- mations appear in the core of the range. They are cut by large intrusions of granite, which continue to the upper reaches of the Pass. The geology of
GEOLOGICAL OBJECTIVES AND METHODS 9
Nan K’ou Pass is comparatively well known, and it is eminently fitting to turn to it as a key to help unlock the secrets of the ground that lies beyond. Many times in the course of the work the features of this pass were recalled, and the interpretations accepted for them were read into many of the rock structures of the interior.
On reaching the top of the Pass, however, one is surprised to see another plain, not very different in appearance from the one at the foot of the Pass, but this second plain stands at a much greater elevation, for it is not far below the crest of the mountain range (Fig. 3, page 42). The higher plain looks very similar to the lower one at Peking, but it lies between mountain ranges, is far smaller, and has a more variable floor and a much thinner cover of sedi- mentary strata. We may travel inland on this second level for many miles, but in due course we come again to mountain barriers that stand across the way, and if the interior is to be reached we must climb through other passes to yet higher levels. .
Kalgan lies in a valley at the foot of these passes. It is located on the alluvial deposits of the Yang Ho, a stream whose variable flow cuts and fills and shifts enough at times to endanger the very foundations of the city. Occa- sional floods cause great damage and loss of life, for the inhabitants push their dwellings and tiny farms to the very edge of the river, on ground which may have been flooded only a year or two earlier. The trail to the famous Wan Ch’uan Hsien Pass climbs a tributary of this valley to the Pacific divide, and by way of it we may reach the edge of the plateau where Mongolia begins.
THE GOBI REGION
The Desert of Gobi occupies a broad, shallow, basin-like depression in the very extensive plateau of central Asia. From every side it is approached over a mountainous rim, and the trail stretches away for one thousand miles and more over desert and semi-desert country (Fig. 2). The monotony is broken by relief features which are chiefly the result of warping and uplift. Some of these features, like the Altai ranges, are of mountainous quality, for each one is a complex uplifted fault-block. Many of the ranges are rugged, and comparatively difficult to traverse. The less prominent hill-masses mark areas of gentler warping and smaller vertical displacement. Some of them are older fault-blocks that have been more completely worn down. Elsewhere the region stretches out in a succession of plains, marking the location of minor enclosed basins where down-warped areas have caught the sediments washed down from the uplifted segments (Fig. 10, page 53).
In spite of these irregularities, the average relief of the Gobi basin 'as a whole is remarkably slight. The basin measures six hundred miles in width, and more than one thousand miles in length, yet it is marked by an average
10 GEOLOGY OF MONGOLIA
depression of less than six feet to the mile—an amount not appreciable by ordi- nary methods of observation. For all essential purposes, then, the region is a great sweeping upland country, so many hundred miles in extent in every direction that one feels hopelessly lost in it.
! KALGAN hart IDE. ee |DABAsU [RIANG PACIFIC DIVIDE y | 50 100 200 300 = 400 500 600 MILES " P PROFILE A AD is H bre He BB ———————— PROFILE B —_—0 oso
FIGURE 2.—Two cross profiles of the Gobi basin. Both profiles are based on hundreds of systemati- cally distributed aneroid readings. The vertical scale is exaggerated ten times.
Profile A crosses the whole of the eastern Gobi and shows a broad, shallow downwarp between the Arctic and Pacific divides.
Profile B crosses only the northwestern extension of the Gobi, lying north of the Altais, and shows much stronger warping and block-faulting than does the eastern section. Index letters are as follows: AD, Arctic Divide; S N, Sain Noin Khan; G, Gorida; U, Mount Uskuk; TN, Tsagan Nor; BB, Baga Bogdo.
In very early geologic time, the region had an entirely different physical habit from the present. Epochs of mountain-folding, of volcanism, and of great elevation and subsidence, with all their complexities of product and structure, have characterized the geologic history. In the beginning the region was mountainous and rugged, with rivers flowing in all directions to the sur- rounding seas (Fig. 153, page 367). After a very long period of erosion, the ancient continent was worn down almost to base-level, and the traces of this old peneplan are what one sees in some of the dissected remnants of the old rock floor. Since that time mountain-folding has not occurred there; but, under the influence of some of the inner forces of the earth, the whole region has been lifted and warped and faulted, so that the streams, which formerly carried sediments to the sea, turned inland and made deposits in the bottoms of the inland basins.
Ever since Lower Cretaceous time, when these changes came about, the region has been continental in habit. Centers of deposition shifted as new warpings took place, but deposits were nearly always being made somewhere, while in places deposits were being stripped from areas previously covered. The complete story of the later sediments of the Gobi region could be com- piled by interpreting the data of all these basins. Bones of animals that roamed these plains from Cretaceous time to the present must be buried here and there in the deposits of the Gobi basins, and the story of the ancient life of the region, with its changes, could be written if their remains were found. Many epochs are unrecorded, so far as we know, and the continuity of the story is broken; but some of the gaps may be filled by future exploration.
GEOLOGICAL OBJECTIVES AND METHODS II
If it were not for the fact that the story is more complex than the part of it already given, the complete history could not be unraveled. If the pro- cesses had stopped when the basins were filled with sediment, the history of the Gobi could not be worked out, because only the surface of the basin could be studied. However, since deposition began, there have been periods of deformation so pronounced in certain places that great series of sediments have been turned on edge and exposed by later erosion (Plate XI, A, page 61). Here one can measure each bed in detail and examine its content as success- fully as though he had gone down through the whole series from top to bottom. In addition to this, there were, in former times, periods of greater rainfall than now, and streams have cut trenches into deposits already made. There are gorges and gulches, escarpments and uplands, where strata are exposed, in some places throughout considerable thickness, even down into the ancient rock floor itself (Fig. 11, page 54). At such places the meaning of the strata is legible, and ultimately a geologic column, covering nearly the whole depo- sitional history, may be constructed by piecing together the data afforded by the various localities. A trial column, based chiefly upon the work of this Expedition, has been assembled, and will appear in Chapter XXII.
These are the obvious features of the Gobi region. The geologist soon learns their significance, so that the character of the country suggests the nature of the underground structure and something of its geologic possibil- ities. There is an alternating succession of broad, nearly level, smooth, grav- elly plains which mark the presence of sediments, separated by more rugged areas of the uplifted and dissected hard-rock floor. Occasionally a character- istic dissection of the sediments themselves actually opens to observation the edges of the strata, and these alone are favorable for fossil collecting (Plate I). They are the great fossil fields of Mongolia.
Caravan trails reach out bewilderingly across this apparently endless country, intersected in a confusion of possible courses by the trails of herds and flocks and wild animals. Across such an open plains country, where tireless winds sweep the shifting sands, which are the only mantle obscuring the rock floor beneath, one must find his way and determine the geological structure, unravel the history of the region, and locate the fossil fields. Each day’s traverse looks so much like the last in general surface features and land- marks that the traveler feels hopelessly lost; yet every trail leads somewhere, for these highways of the desert have served for centuries as the only connect- ing threads between otherwise isolated centers of culture in central Asia.
A Mongol guide is immensely more useful than any existing map. Save for his instinct in finding the right trail out of hundreds of possible courses, or, failing that, except for the use of the compass almost as the mariner uses it, one would inevitably come to grief in the Desert of Gobi.
12 GEOLOGY OF MONGOLIA PREHISTORIC ADVENTURE
One of the amusing foibles of a dominant race is its belief that explora- tion begins when men of its own blood travel into a ‘‘new land’’—disregard- ing the peoples who have made it their home for untold centuries. So Mon- golia, which has been a land of varying climate, favorable enough at times to attract races, and then arid enough to drive them out again, seems to have been forever under exploration. Before men lived in Mongolia, race after race of other creatures surged through the land and died in its broad basins, where we are now deciphering their history.
The tale of early human exploration will make a fascinating chapter when increasing knowledge enables us to piece it together. As we tell it now, the story is incomplete. Yet it seems advisable to fit the fragmentary evidence together as best we can, even though it will be many times corrected and added to as knowledge increases. We believe that future studies will con- firm the inference that ever since there were men in Mongolia, their tale has been one of tribal migrations. As in Europe, clan after clan has entered this region and driven out or absorbed the earlier inhabitants. As their numbers grew, streams of nomad wayfarers went out to other lands. Each invasion brought new blood into Mongolia, and the new conquerors were in turn lured forth by rumors of better lands or were absorbed by the next immigrant horde, just as fate, climatic changes, and the laws of inheritance ruled.
History does not begin with the written page. Records are also made by bones, chipped flints, and ash heaps of vanished settlements, and are as much a part of human history as the Bible and the Rosetta Stone.
The earliest record is a single tooth obtained from a Chinese apothecary by Haberer (1903, page 20), and described by Dr. Max Schlosser, who reported on his suggestive discovery in 1903. It is not wholly certain that the tooth be- longed to a human being; Dr. Schlosser claims only that it belonged to an an- thropoid which was structurally nearer to man than any known ape. Clinging in the roots of this tooth is a bit of red clay. These are the only facts, and speculation upon them should be made with caution. No one knows where the tooth was found, or from what geologic horizon it came, but its primitive character suggests that it belonged to one of the earliest inhabitants of cen- tral Asia. We can only hope that future discoveries will throw more light upon this interesting problem.
The second chapter in the history of human adventure in Asia lies quite outside our region. In 1892, Professor Dubois (1894) discovered in Java the remains of a primitive human being, or, as some scientists believe, a crea- ture rather more primitive than man, to which he gave the name Pithecan- thropus erectus. The age of the beds in which the bones were found is generally
GEOLOGICAL OBJECTIVES AND METHODS 13
considered to be early Pleistocene. No one knows how widely this early Javanese roamed, but it seems unlikely that the same species lived in both Java and central Asia.
The next cultural stages in the history of the region are much more defi- nite and understandable. They are represented by a Paleolithic culture dis- covered in the Ordos by the distinguished explorers Pére Emil Licent (1925) and Pére Teilhard de Chardin (1924 0), and one Paleolithic and two Neo- lithic stages found in the Gobi by our own Expedition. Licent and Teil- hard found primitive stone implements, not unlike those used by the Nean- derthal man of Europe, and bones of extinct animals in gravels which were laid down before the vast deposits of ‘‘loess’’ were formed in northern China —probably in the Middle or Lower Pleistocene. Of the three stages found by the Third Asiatic Expedition, the palzoliths are apparently much like those of the Mousterian culture in Europe, but similar implements need not imply the same or closely related races or even the same period. These stud- ies are as yet so incomplete that all we dare to say is that several stone-age civilizations existed in Mongolia long ago, and it is reasonable to think that they represent many successive invasions of the land by tribes which brought much of their culture with them.
Dr. J. G. Andersson (1923 b, c, and 1925) described an ancient Eneo- lithic culture in China that antedates all written and traditional records of the Chinese. Excellent pottery, ornaments and implements were made, but no metals were found in any of the old dwelling-sites. The material includes vases, plates and ornaments, with patterns so closely related to those of later Chinese art that he believes that the ancient culture was indeed Chinese. Dr. Davidson Black has examined the human bones which Andersson col- lected, and concludes that they belong to a people essentially similar to the present-day North Chinese (1925 b, page 98). Therefore the Chinese people probably had occupied China and had developed the elements of their civil- ization at least several thousand years before the beginning of their written history. They may have been invaders, too, but not even in their legends do we find the myth of an original homeland.
In many parts of Mongolia, especially in the north, there are stone mon- uments of many different types, built by races which were in the land before the coming of the Mongols. The Encyclopedia Britannica, quoting Radlov, says that the earliest inhabitants of this northern region were Yeniseians; remnants of these tribes still linger in the mountain-country of Uriankhai. They were succeeded in possession of the land by the Urgo-Samoyeds, immi- grant tribes, who brought a finer art in gold, silver, and bronze than the land had known before. The Huns drove out these less warlike people about the third century B.c. The succeeding centuries were periods of vast tribal move-
14 GEOLOGY OF MONGOLIA
ments, and we hear of the Huns invading India on the one hand and Europe on the other—lured by the hope of better lands than central Asia afforded. Some of the monuments can be traced to the Turkish tribes which appeared in the fifth and sixth centuries A.D. They were masters of much of northern Mongolia and southern Siberia, but they were overcome in turn by the rising might of the Mongol empire which Jenghiz Khan built up in the thirteenth century. Gradually the Turkish tribes shifted away from Mongolia, or merged with the overwhelming Mongol peoples. As the power of the short- lived Mongol empire declined, the Chinese conquered Mongolia, and made it one of the ‘‘outer provinces.’’ At about the same time, the Russian peoples were beginning to turn their eyes eastward to the vast lands beyond the Ural Mountains.
This sketch of man’s ancient migrations in central Asia is dim and frag- mentary, yet we think that it contains the first lines of a splendid picture which future studies may enable someone to paint in full color. We discern two parts in the composition of this picture: the older part, which deals with man’s origin and descent, is but faintly sketched; the other part is a picture of migrat- ing tribes and changing cultures, rather than the history of an evolving race. The first part reaches back into Pliocene and earlier periods; the second begins with the Ice Age, and continues to the present. Ancient adventure and the discovery of central Asia by successive migrating tribes constitute good sub- ject matter for future scientific study.
HISTORIC TRAVEL AND MODERN EXPLORATION
Exploration implies travel for the purpose of collecting data of one kind or another. Travel for adventure and to satisfy one’s urge for a larger world is older than exploration. The list of travelers is legion, but only a few names, chiefly of those who have made extensive traverses in the interior of Asia, may be mentioned here. Exploration proper is of very recent development, and scientific exploration in central Asia dates only from ‘‘day before yester- day,” with a very short list of names. Among the travelers, Marco Polo is the dominant figure; among the explorers, von Richthofen is one of the earliest and most illustrious.
As early as 1253, Saint Louis sent William of Rubrouck to Mongolia to visit the court of Mangu, the grandson of Jenghiz Khan. William reached the court in the year in which Marco Polo was born. Fifteen years later, Marco with his father, Nicolo, and his uncle, Matteo, began the astonishing pilgrimages that were to fill his life and fire his only too active imagination (Yule translation, 1921). In the next century, Father Odoric traveled through Persia, India, and the East Indian islands to China, returning by way of Tibet.
GEOLOGICAL OBJECTIVES AND METHODS 15
These and other travelers brought back to medieval Europe the earliest accounts of the Far East; but Marco Polo has given us the richest and most colorful picture of the land and people of Mongolia and China that was ever prepared before science came to guide exploration.
In the sixteenth century, at about the time when adventurous peoples from western Europe were beginning to colonize the new-found Americas, the Russians began to cross the Ural Mountains into Asia. We may picture two streams of migrating peoples, one pouring toward the west, the other toward the east: the one to cross the salt waste of the Atlantic to America, the other to traverse the tundras and swamps of western Siberia—the one to face the red Indian, the other the bronze Mongol. Each stream went forth to a pioneer’s life, into a vast unconquered land, and each, despite the emphasis that written history lays upon wars, went armed rather with the plough than with the sword. Even the earliest records of Russian occupation of Siberia tell of farmers who were sent-out with the soldiers: one group to build the blockhouse, the other to break the soil around it.
The ghastly story of exile and the prison camps of Siberia is one of those black chapters which no one palliates or defends. Most countries have tried the experiment of exile colonies in one form or another, and all have found, sooner or later, that it does not pay. Yet, curiously enough, one of the sto- ries which concerns us in this book grew out of the prison camps of Siberia. _ After the battle of Poltava, in 1710, a Swedish officer named Philipp Johann von Strahlenberg was taken prisoner by the forces of Peter the Great. He was sent, with many others, to Siberia. During their long exile, Strahlenberg and his comrades studied peoples, languages, resources, and the geography and history of Siberia. With older maps to build upon, Strahlenberg made a new and astonishingly good map of Siberia, Turkestan, Mongolia, northern China and Tibet. An excellent critique of this map has been published by Sven Hedin (1917, I, pages 246-252) in his Southern Tibet. Two other pris- oners of this same war, Schénstrom and Renat, made important geograph- ical and ethnographical observations in Siberia, and Renat constructed a map of central Asia which gives the best picture of Dzungaria and the Tarim basin known up to that time.
Strahlenberg and Schénstrom bought from a Boucharan merchant a man- uscript which they translated into several European languages. It proved to be a brilliant account of the Tatar or Mogul (Mongol) people, which was written by Abul Ghazi, prince of Khiva (translated 1729). The book is un- finished, for the writing was broken by the prince’s death in 1663. He carries the history of the world rather rapidly from the creation of Adam to the rise of Jenghiz Khan, who was Adam’s lineal descendant and Abul Ghazi’s lineal ancestor. For a man who believed in magic and who dealt in traditional his-
16 GEOLOGY OF MONGOLIA
tory, the work of Abul Ghazi is remarkably literal, and bears evidence on every page of an honest and scrupulous mind. Although we cannot include him among the explorers of Mongolia, his Genealogical History of the Tatars has become one of the source books for the history of the Mongol people.
EARLIER STUDIES OF SPECIAL SIGNIFICANCE
Following the work of Strahlenberg, Schénstrom, and Renat, an epoch of scientific exploration of Siberia, Turkestan, and Mongolia opened. French, German, and Russian investigators searched out the resources of the vast region, studying its plants, animals, soils, rocks, and minerals. Naturally the work of the eighteenth century was crude and poor, and had to be done over again by the more fortunate and better trained scientists of the nineteenth century. A new era in the exploration of this region dawned in 1864, when an American geologist, Raphael Pumpelly, made the first geologic section across Mongolia from Kalgan to Urga (Plate III). In his short but excellent account (Pumpelly, 1866) he tells that Mongolia is an inland basin; that much of it is a plateau, or series of plateaus, formed of almost level strata of sandstone, clay, and lava sheets, interrupted by areas of granite and other massive rocks belonging to the oldrock floor; and that folded rocks, chiefly slates and schists, form another important structural element. This sane and simple analysis was the first great step toward the unraveling of the complex structure and history of Mongolia. |
When Pumpelly crossed Mongolia in 1864, Ferdinand Freiherr von Richt- hofen (1877) had been two years at work upon his pioneer reconnaissance in the Far East. He examined the southern part of the great basin, and recog- nized the Khingan range as a continuation of the great line of flexure and faulting along which the mountains of Chihli bend down under the soft silts of the Great Plain. He noted the level strata of sand and clay, and called these beds the Han Hai Series. The name is Chinese and means the “dry sea.” It is a beautifully figurative name for these immense and marvelously level wastes of desert above which the hard-rock hills rise like islands. But von Richthofen carried the analogy further, for he believed that these depos- its had been laid down in an ancient sea.
Von Richthofen finished his researches in China in 1873, and was followed by the new school of Russian explorers. The character of their work changed from that of travelers to that of more scientific investigators. I. V. Mushke- tov (1876, 1906) had visited the Tien Shan while von Richthofen was yet in China, and, like the German pioneer, continued his travels for twelve years (Plate II). He noted the complexly folded rocks, cut by many kinds of once molten intrusives—granites, syenites, diorites, and porphyries. He collected
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GEOLOGICAL OBJECTIVES AND METHODS 7,
fossils which told of Devonian, Carboniferous, and Cretaceous seas, and plant remains which recorded swamp-lands in the Triassic and Jurassic periods. He made a careful study of the coal in the Kuldsha basin, in Turkestan, and noted many other mineral resources. Romanovsky (1878) began exploring Turkestan rather later than did Mushketov; but the two pioneers joined hands to produce in 1884 a map of Turkestan which has been the foundation of all our scientific knowledge of that region.
Pievtsov and Bogdanovich (1892) traveled through eastern Turkestan, Dzungaria, Tibet and western Mongolia in 1889 and 1890. Bogdanovich (1892) made out the general geology of the region traversed, and laid the foundations for all later studies.
While the Russians were pushing their investigations outward from Siberia, the Hungarian count, Béla Széchenyi, undertook an extensive jour- ney through China, southern Mongolia, and Tibet. He was accompanied by Gustav von Loczy (1893, 1899), who constructed a geologic cross-section throughout the entire course of the journey.
The explorations, which began in the west with Turkestan, were con- tinued eastward by later investigators. It is not possible in a reconnaissance volume to do justice to the many colleagues who have contributed to this work through their writings, and the dry acknowledgments convey little of the appreciation due to the many deeds and thoughts of great summarists like Suess, and of master pioneers like Obruchev. The maps (Plates II and III) give some idea of the maze of routes traced across central Asia before us, but even this picture leaves much untold. We can give space to a very few outstanding names, and trust that in a later volume we may tell the whole story.
V. A. Obruchey, greatest of Russian explorers, began his travels in 1892 (1893). His advent marks a new era in the study of these great regions; the coming of a keener insight into geologic problems, with a better training in the science than that of any of his predecessors. He began, as was natural, where von Richthofen, Mushketoy, Roborovsky, Bogdanovich, and others had left the story. He recognized at least two periods of mountain-building: an earlier, following the Permian period, when the strata were folded into ranges like the Appalachians; and a later, in Tertiary time after the earlier mountains were wholly worn away, when the land was broken along rifts, and tilted up as block-mountains (Obruchev, 1900, 1912, and 1923). Obru- chev was the first geologist to discover vertebrate fossils in the Han Hai beds. His utmost care in collecting the fragments could not prevent their breaking still further; nevertheless, a tooth of general rhinocerotid type was pieced to- gether, and was tentatively determined by Edouard Suess (1899) as a Rhi- noceros or Aceratherium, which he believed to be of Miocene age. Obruchev’s
VOL. IIl—2
18 GEOLOGY OF MONGOLIA
account of this discovery was in the hands of the geologists of the Expedition, and led them to pay special attention to this region, although, curiously enough, vertebrate fossils were found at two other horizons before the site of Obruchev’s discovery was examined. In the northwestern corner of the Ordos, Obruchev found the skull of a fossil rhinoceros, probably of Pleistocene age, very near to the place where, more than thirty years afterward, the Jesuit Fathers Licent (1925) and Teilhard de Chardin (1924 b) made their great dis- covery of the implements of fossil man.
From the foregoing list of contributions, it is evident that a large number of exploratory investigations have been made in central Asia. The funda- mental elements of the work accomplished have been taken as a ready-made foundation for the new investigation, and the original sources of a particularly helpful character require some additional comment and acknowledgment. These are: the contributions of Ferdinand von Richthofen (1877), and of V. A. Obruchev (1900); Research in China, by Bailey Willis (1907); and Explorations in Turkestan, by Raphael Pumpelly (1905). Each in his own field has pene- trated an almost trackless wilderness of scientifically new territory, and has brought information and order out of difficult, obscure, and comparatively little-known regions.
The structural and stratigraphic elements of China, as given by Willis, furnished by all means the best starting point for the geologists of the Third Asiatic Expedition, because the work had to be projected from China as a base of operation. Furthermore, his work affords most direct and ready com- parison, because, after the season’s work, one returns again from the interior to the standard sections of China. We now know that a remarkable similar- ity of development is represented in the geology of the interior and that of the Chinese border. The two regions have approximately the same history. In past time they comprised a more uniform structural unit than the present physiographic differences suggest, so that the major structural features are essentially the same.
It is fortunate, therefore, that the Third Asiatic Expedition found it convenient to organize in China, as the staff was given special encouragement and help by the geologists of that country. It was of advantage, also, that one of the members of the geologic staff of the Expedition, Frederick K. Mor- ris, had already accumulated considerable field experience in China and had become familiar with its formational and structural habit. Probably from no other base and under no other conditions could the work have been under- taken with so great advantage or so great likelihood of success.
It is certainly not advisable at this stage of the investigation to carry detailed correlation to such an extreme as to assume the identity of many of the individual formations in these two widely separated regions,—China
GEOLOGICAL OBJECTIVES AND METHODS 19
proper and the interior desert plateau. Nevertheless it is true that the major structural units, as well as the principal deformation epochs and changes in the dominant processes, as marked out by Willis in China, can be recognized definitely, in the same order and with much the same proportions in the north- ern region.
The geological statement as given by Willis is, for another reason, par- ticularly helpful. It not only recognizes formational units, but establishes them on a broad dynamic basis. Moreover, in describing them Willis man- ages to allow for expectable variations in the character and physical condition of the sediments, so that corresponding representatives or equivalents in new territory might be placed in the geologic column without serious revision. This is materially aided by a careful determination of igneous and deforma- tion epochs, so that the whole geologic column is built up with suitable empha- sis on the major breaks and the dominant forces or processes of each part. The Expedition came to appreciate the great service of such a contribution as this of Willis in China. One is the more impressed with its soundness after working, as this Expedition did, over several thousand miles of adjacent ter- ritory, where, again and again, this proved to be the only really helpful geo- logic guide.
In addition to the work of Willis in China, that of Pumpelly (1866, 1905), Davis (1904, 1905), and Huntington (1905 a, 1907 a), on the west side of the continent in Turkestan, ranks especially high in its applicability and help- fulness in a study of problems of the interior. For our region the geologic column representing the older formations, as given in the Explorations in Turkestan, is of little service; but the determinations bearing on physiographic changes, the significance of the processes and effects represented, and the order of events in later time are of far-reaching application on the continent of Asia. It would be strange, of course, if this were not so; for the men who participated are masters in their own fields, and their observations and conclusions could fail of wider application only in case the geological conditions were actually different. We now know that there is enough unity in the continent of Asia, despite its immense size, to enable us to correlate with reasonable success many important steps in its development, from China on the one side to Turk- estan on the other.
The emphasis placed by the work in Turkestan on the changes that have taken place in late geologic time might be applied also in Mongolia, where the deformations, the processes, and the climatic changes are of similar type and of corresponding magnitude. The features are more easily read, are more definite, and are of greater significance in the desert region of the interior than in China proper. One finds, therefore, in the reports of these western explora- tions the clearest statement of the history of late geologic time, of the physio-
20 GEOLOGY OF MONGOLIA
graphy, and of the processes that have made present conditions. The prin- ciples used and the explanations given were found to be a constant help in our studies of the interior, and we are confident that the summary given in this volume will be found to establish a reasonably consistent geologic link of field observation between these two justly famous investigations—that of Willis and Blackwelder in China, and that of Pumpelly, Davis and Hunting- ton in Turkestan.
The most suggestive observations of the latter expedition are those which postulate a fluctuating climate, reaching back to the limits of human occupa- tion and probably beyond; and those which establish the correlation of uplift and depression, with the shift of deposition centers during late Tertiary time.
Displacements of ten thousand feet, recognized by Huntington, are equaled in such movements as that of the Altai uplift of the desert interior; and the repeated warpings which Huntington refers to in similar connection are not only duplicated in many parts of Mongolia, but, because of the detailed determination of horizons in many adjacent basins, are established more firmly than ever before by the work of the Third Asiatic Expedition. By means of this work, epochs of deposition, of erosion, and of deformation have been dated and brought into definite sequence. Davis and Huntington in Turkestan recognized the significance of the evidence indicating these changes, and placed them for the first time, in their proper geologic and physiographic setting. The work of the Third Asiatic Expedition, while it supports the general conclusions of the expedition to Turkestan, attempts to go further in the definiteness of its correlations.
The work done in Siberia is more scattered, deals with more limited problems, and is difficult of access. Nevertheless, important observations have been made, and some of the conclusions derived from them are readily fitted to the Mongolian region.
CHAPTER II
BOUNDARIES OF THE GOBI REGION
INTRODUCTION
THE Gobi Desert is merely the arid part of a great inland basin that lies enclosed between mountainous divides (Fig. 4, page 43). The highland bor- ders of the basin are for the most part well watered grasslands that change by almost imperceptible gradations into the central desert. The boundaries of the desert proper are therefore not only indefinite, but inconstant, because a few rainy years extend the grassland, while a dry period makes the desert broader. The boundaries of the inland basin in which the desert of Gobi lies must be placed along the crests of the mountains that slope into the basin on the one hand, and outward toward the sea on the other. At first sight it seems as though the boundary would coincide with the watersheds which part the inland drainage from the waters that reach the sea, but this is not every- where true, for the Kerulen River, after draining a part of the Gobi region, escapes to the sea through the Argun and Amur Rivers; and in the west, the Gobi is not bounded by marine drainage, but is separated by low, inconspicu- ous divides from other inland desert basins. In attempting to summarize the nature and structure of its boundaries, one has to rely largely upon a crit- ical review of literature concerning the surrounding regions, and the record is very imperfect.
THE NORTHERN BOUNDARY
The northern boundary is everywhere mountainous, and may be divided into three general parts. The western part consists of the fault-block range called Tannu Ola, which connects with the Sailugem Mountains on the west, and, by several minor fault-blocks, with the Khangai uplift on the east. The central section includes the Khangai, Gangin Daba, and Kentai ranges, which may be considered as a series of arched or domed masses of complex rocks. The eastern section is formed of the Transbaikalian block-mountains which
21
a2 GEOLOGY OF MONGOLIA
overlap one another obliquely along the Gobi basin, the southwestern end of each range plunging down beneath the Gobi sediments.
Northern boundary—western section
The northwesternmost part of the Gobi basin is a group of mountain- rimmed depressions called by Pievtsov the ‘‘Valley of the Lakes” (Plate IV). Its western wall is the Sailugem mountain chain, concerning which we have very little information. The northern rim of the Valley of the Lakes is formed by the Tannu Ola range, which is a stretch of the Arctic divide, parting the headwaters of the Yenisei on the north from the wholly enclosed Ubsa Nor basin on the south (PlateIV). The abrupt scarp of the Tannu Ola rising above the broad lake-basin strongly suggests that the range is faulted, forming as it were a shorter northern counterpart to the longer fault-block range of the Altai. Suess has assembled (1902, III, pages 111-117) three traverses of the Tannu Ola, and shows that the western traverse reveals only Devonian rocks —conglomerates, quartzites, and porphyries. The middle traverse brings in gneiss and granite, limestone, schists, marls, and sandstone. The sandstone beds contain plants belonging to the Carboniferous age (Culm). Just east of this section, on the north side of the range, the same traverse records Devo- nian strata, followed by Culm and then by the Jurassic Angara series. The third traverse begins on the north with the Jurassic Angara beds, and passes to the Devonian on the south side of the range. The rocks are folded and faulted.
Hausen, in 1917 and 1918, studied the part of the Tannu Ola lying east and west of the lake Kara Kul (1925). He reports that the rocks are largely igne- ous masses,—granite, grading into diorite and even gabbro,—which invade greenstones, limestones, and schists of uncertain age. Devonian sandstones and limestones with porphyries and lava sheets overlie the older rocks uncon- formably. The range is made up of several faulted blocks, and some of the longitudinal valleys, including that of the Kara Kul, are regarded as grabens. Hausen has thus shown that there are at least two ancient periods of folding, and that the folded structures of the mountain are broken by faulting of more recent date.
Northern boundary—central section
The Tannu Ola passes eastward into the general mass of the Khangai Mountains. The latter name is here used collectively for a mountainous region, rather than a range, lying between the Valley of the Lakes on the west and the Tola River on the east (Plate IV and Fig. 4). Suess says (1902, III, page 118), ‘“Eastward the Khangai merges into the Kentai Shan and the mountainous regions north of Urga which are continuous with those of Trans-
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A. THE FIRST MOUNTAIN BARRIER. THE WESTERN HILLS FRONT NEAR PEKING,
B. THE GREAT WALL ABOVE NAN K’'OU PASS.
This view was taken at Ch'ing Lung Chao, and shows the rugged crest of the first mountain barrier back of Peking. In an- cient times this was the inner defense line for the great plain of China, the outer defense being the Great Wallabove Kalgan, more than a hundred miles farther toward the interior of Asia. (See Plate XLIII B.)
BOUNDARIES OF THE GOBI REGION 23
baikalia. . . . No tectonic boundary separates the Gobi from the Khangai, since denuded remnants of a similar structure may be recognized in adjacent parts of the Gobi itself.’’ Suess recognized horsts and grabens in the Gobi region, but as both the folding and the later faulting follow in general much the same structural trend, Suess did not clearly differentiate an age or ages of dominant folding from another age characterized by dominant faulting.
Tolstikhin (1920) made an extensive study in the region of the Selenga River in 1919 (Plate IV). We have not a copy of his paper, and must rely for the present upon Obruchev’s review of it (1921, page 149). Tolstikhin noted a thick formation of steeply folded crystalline limestones, striking N. 30° W. to N. 50° W. and extending from the Kosso Gol to the river Telgir Merin (Plate IV). Northward from the river Murin, dolomitic limestones striking N.N.E. have been cut and metamorphosed by granites and granite porphyries. Associated with these limestones are quartzites, sandstones, and silicious schists, all of which Tolstikhin considers to be pre-Cambrian. He goes on to say that after a long erosion period, the region came again under water which covered the lower course of the rivers Murin, Buksui, and Eder and extended eastward along the Selenga to the mouth of the river Egin Gol. Gray and black limy clays and sands, with green and red sands and conglom- erates, were deposited, in which obscure plant remains are found. This water sheet, he says, retreated toward northeast and east. While the sedi- ments were being deposited, volcanoes poured out dark basic lavas and their tuffs. All of these rocks were folded at a later date, and strike W. N. W. to E. N. E. with dips less steep than those of the pre-Cambrian. The basic erup- tive rocks were converted into green chloritic schists. Tolstikhin assigns these sediments and associated igneous rocks to the Jurassic Angara series of Suess. He describes a later granite-syenite intrusion, which closed with eruptions of keratophyre.
Obruchev (1921), commenting upon this paper, thinks it more likely that the later sediments may be not Jurassic, as Tolstikhin supposed, but possibly may be of Paleozoic age, citing his own discovery of Palzozoic bryozoans and corals between Urga and Kiakhta in rocks of much the same character as those observed by Tolstikhin. Obruchev considered it probable that “in northern Mongolia, as in the rest of Central Asia, except the region of Kashgar, the lat- est advance of the sea was in Upper Carboniferous or Lower Permian time”’ (1921, page 150). The possibility that these chloritic schists may be of pre- Palzozoic age should not be overlooked.
The route of the Expedition in 1922 led southwestward from Urga through a wide belt of the slates and graywackes. As this section is followed south- ward, igneous rocks appear, cutting the graywacke series, and increasing in number and size until the granite of the Mongolian bathylith is exposed over
24 GEOLOGY OF MONGOLIA
large areas, while the graywackes lie isolated as roof-pendants and xenoliths (Figs. 34 and 35, page 95). In the region of Tsetsenwan (Lat. 46° 15’ N., Long. 104° 10’ E.), the mountains are disposed in broad, relatively low blocks whose tops are beveled by an arched peneplane and whose fronts descend ab- ruptly to basins with almost flat bottoms, some of which are floored by the gran- ite, while others bear a cover of nearly horizontal sediments (Fig. 37, page 101). In the block-mountains we found in one place gneisses and marbles which we assigned to the Archean complex; and in other places schists and crystalline limestones,—striking N. 75°-80° W. and dipping northward,—which we judged to be Proterozoic, comparable to the Wu T’ai series of China. They are all unconformably overlaid by the folded conglomerates, sandstones, and surface volcanic rocks which we tentatively consider to be Lower Jurassic. As we went northwestward again to Sain Noin, the graywackes reappeared. They form the entire region for many miles about Sain Noin Khan, save for infolded synclines of the Jurassic conglomerate. The graywackes are indeed present on a scale so vast that we called these rocks the Khangai series.
Thus the Khangai mountain region, taken as a whole, includes a great complex of folded sediments and igneous rocks, ranging from the Archzan to, possibly, the Lower Jurassic. Their upturned edges are beveled by a pene- plane which forms the upland of the Khangai, standing as high as ten thou- sand feet above sea-level. All of these rock types were observed also in the Gobi desert, where they maintain the same folded structure and essentially the same strike as in the Khangai. The Gobi and the Khangai are not sep- arated by a fault line or by an abrupt scarp, and Suess may have had this in mind when he wrote that there is no tectonic boundary between these two regions. The Khangai hills become lower, and extend in long tongues out into the desert; while the lowland of the desert surface extends up the valleys between the projecting spurs of the mountains. The mountain mass as a whole rises gradually above the desert and has the aspect of a broad dome in which faults and fault lines are not so conspicuously in control of the topog- raphy as they are in the Altai ranges to the southwest or in the Transbaikal ranges to the northeast. The streams heading on the two opposite sides of the Khangai watershed form a long dendritic drainage pattern which clearly suggests that the drainage is consequent upon the Khangai arch. There is no evidence in the river pattern of a former connection of the Gobi basin with the Arctic drainage. Along the Khangai front, however, the earth movements that have made the mountains were essentially a gentle doming or upwarping of ancient, complexly folded rocks which had been peneplaned prior to the uplift.
The Kentai Ola continues the Khangai northeast of Urga, and forms the Arctic divide in this region. J. Morgan Clements (1922) examined part of
BOUNDARIES OF THE GOBI REGION 25
the Kentai hills, and expressed the belief that the graywacke series is of late pre-Cambrian age. Ussov (1915) reports in general an Archean “ Barchin Series” and an Algonkian graywacke series which are cut and metamorphosed by great intrusions of granodiorite. The Expedition in 1922 crossed the Gangin Daba Mountains, which may be considered the southwestern exten- sion of the Kentai, and observed in the southern part a belt of schists, which we regarded as later than the Archean, and probably of much the same age as the Wu T’ai system of Bailey Willis (1907, II, page 4). This correlation would not necessarily cast any doubt on Ussov’s identification of Archzan rocks in the Kentai. Like the Khangai, the Kentai appears to rise gradually out of the great basin of the Gobi, without any marked tectonic boundary beyond a broad, gentle upwarp or doming.
Northern boundary—eastern section
Thus far the boundary of the Gobi has been placed along mountain crests that form the Arctic watershed. All water falling south of this divide runs southward into the desert, to escape only by evaporation. But the Kerulen River, heading in the Kentai hills, runs southward into the great basin of the Gobi, and then flows eastward nearly four hundred miles to the lake Dalai Nor, from which the waters escape to the ocean along the Argun and Amur Rivers (Fig. 4). The course of the Argun River follows the fault-graben along the eastern base of the Argun range to about latitude 50°, where it cuts across the mountain. Between this and the Khingan range the river follows some of the fault lines, and cuts across others. It swings around the northern end of the Khingan and joins with the Shilka River to form the Amur River (Fig. 4). The genetic origin of the Kerulen-Argun is an unsolved problem. The course is clearly a complex one, and shows two types of adjustment: along parts of the course the river is adjusted to the structure of the present ranges, and along other parts it cuts across these structures. If the moun- tains arose by faulting at somewhat diverse times, and at diverse rates of inter- mittent movement, a large river would maintain its course across the minor ridges, and especially across those whose growth was slow and of more recent beginning, while larger, older ranges and those whose increments of fault move- ment were greater, would force the river to take a course adjusted to the trend of the ranges. The Argun range has in part, and the Great Khingan has wholly, forced such adjustment to its own lines.
The mountains north of the Kerulen belong to the Transbaikal system of fault-blocks in which the rivers have a peculiar trellis pattern. Whether the topography is primarily controlled by block faulting, or whether the rivers have subsequent courses and have etched out their valleys along the weak zones of ancient fault lines, is another of the unsolved problems of the region.
26 GEOLOGY OF MONGOLIA
Lvov (1916) says that the dominant topographic feature of the western Amur region is a group of short parallel-oriented massifs, separated from one another by narrow or broad, down-sunken depressions.
Obruchev (1899, pages 192-206, and 1926, page 432) has given a descrip- tion of the broad features of Transbaikalia, in which he emphasizes the great width of the divides or mountain uplands, and also the great width of the val- leys. He considers the mountains to be fault-blocks, and his map, which is redrawn and transliterated in Fig. 135, page 320, shows a long series of faults defining the present ranges. On the other hand, he does not throw direct light upon many important problems. He says that several ranges are “broken through’’ by rivers, but does not explain the origin of the river’s present course. He stresses the fact that the ranges are bow-shaped, lying in arcs which are slightly convex toward the south; but he does not tell whether he believes that this shape is due to pressure or thrust from a given direction. He shows that the strike of the bedded crystalline rocks diverges in places quite markedly from that of the ranges. In the west, near Lake Baikal, the folded structure strikes east-southeast, while the faulted mountains lie east-northeast to northeast (Fig. 135, page 320). Farther east the prevail- ing strike is east-northeast, and here it falls into agreement with that of the mountains. Still farther east the strike of the beds is again east-southeast to southeast, and locally east-northeast, or northeast or even north, so that it lies now across, now diagonal to, and now parallel with the direction of the mountain ranges. Obruchev continues (1899, page 201):
The Mesozoic and Tertiary rocks . . . in most cases strike parallel to the de- pressions, in which exclusively these rocks are found. It seems that they have been broken into larger and smaller tablets or plates, which sank steeply in this or that direction, and, locally, have thereby even formed shallow folds.
The post-Tertiary deposits are not dislocated. Obruchev says (1899, page 202):
The depositional relationships of the metamorphic schists, the Paleozoic, the Mesozoic and Tertiary rocks, show that the fault movements repeated themselves in later periods, and in general remained true to the first established strikes . . . Secondary horizontal movements accompanied the dominant vertical movements, as the folding of the sediments shows . . . The vertical movements of the earth’s crust were accompanied by outbursts of massive rocks, which arose along fault- clefts. Many of these rocks pressed in as laccoliths into the Archean masses, and have been laid bare only by later erosion and denudation. Elsewhere they form many dikes in the older rocks. But the existence and the wide distribution of tuffs, breccias and conglomerates which are associated with various porphyrites, mela-
BOUNDARIES OF THE GOBI REGION 27
phyres, trachytes, rhyolites, and basalts, and which in other places are interbedded and alternate with the ordinary clastic sediments, prove that many outbreaks reached the surface and that long chains of volcanoes were active along the faults at the margins of the ancient (‘‘Archaische’’) horsts. The products of eruption came to rest partly on the dry land, and partly in the lakes, which occupied the valleys during many periods. :
In the east in the district of Nerchinsk, the igneous rocks are as widely distributed as the crystalline schists and sedimentary strata; and in some localities the horsts consist only of the igneous rocks. Therefore the arrangement of the rocks in this district is not so regularly zoned as in the other parts of the region, and the structure is far more complex.
. . . Besides the dominant fault lines striking east-northeast, which mostly, even though with interruptions, have a great length, there are in the region other far shorter fault lines which run north-northwest to north. Such cross-faults, which in places appear to be connected with horizontal displacements, are seen in the west, but more especially in the district of Nerchinsk.
Obruchev says that these cross-faults are rudely parallel to the Khingan range, and therefore infers that they are related to the displacement history of the Khingan. He calls them the “ Khingan faults” in contrast to the “‘ Baikal faults’’ which lie parallel to Lake Baikal and form the prevailing mountain lines of Transbaikalia. He points out that the great Khingan range forms the eastern tectonic boundary of the Baikal region. At the western boun- dary is Lake Baikal, which Obruchev considers to be a great faulted depres- sion, the deepest graben in Transbaikalia. Westward from this graben we meet with broad areas of Cambrian and Silurian marine deposits, which are relatively slightly dislocated. Here, then, very different rocks and very different tectonic relations dominate, so that Lake Baikal closes the western limit of the Transbaikalian fault lines.
“South and southwest of our region lies northern Mongolia, in which we find the same ancient metamorphic and massive rocks and the same tectonic lines as in Transbaikalia.”’
Summary of the Transbaikal border
As in the Khangai-Gobi contact, so also in the contact between the moun- tains of Transbaikalia and the great basin of the Gobi, there is no difference in the rock formations or in the lines of ancient folding. These facts point to the conclusion that the great basin of the Gobi is of more recent origin than any of the folded ranges. All of the folded mountains, including the Altaids of Suess, were wholly planed away before central Asia became an inland, un- drained hollow. The present Gobi basin is relatively young, and was formed coincidentally with the uplift of the Transbaikal ranges. Both the Trans-
28 GEOLOGY OF MONGOLIA
baikal mountain region and the basin of the Gobi were formed by deforma- tion of a single great province of peneplaned ancient mountains.
THE EASTERN BOUNDARY
The eastern boundary of the great basin of Mongolia may be taken as the Ta Hsing An or Great Khingan range, which reaches from the Amur River at the north to the mountains of the Kalgan and Nan K’ou region in the south (Fig. 4). The Khingan range strikes about N. 25° E., and so lies athwart the structure of the Altai, Khangai, and the Transbaikalian mountains. The following brief account of this range is taken from the summary of Suess (1902, III, pages 117-122):
According to Gedroits, the northern part of the range has a width of one hundred eighty to two hundred versts, or about one hundred and twenty to one hundred and thirty miles. It is formed of igneous rocks and ancient schists, overlaid by deposits probably of the Palzozoic age, with intrusions of granite, porphyry, and diabase. Kropotkin (1865) and Manakin (1898) have recorded in Lat. 49° 30’ N., “‘clay-slate and granite and in places porphyry and upturned red sandstone’”’ (Suess, 1902, III, pages 151-152). Potanin, crossing at the headwaters of the Nomin River (Plate IV) noted ‘parallel chains of granite and porphyry with an almost meridional trend and particu- larly steep eastern declivity.”” Potanin, coming from the north, encountered granite, porphyry, and clay-slate down to about 45°N. (Suess, III, page 153). Prjevalski (1877), traveling northward from Peking to Dolon Nor, reports gneiss and granulite for one hundred and eighty versts on the western slope of the Great Khingan (about Lat. 42° 20’ N., Long. 116° 20’ E.).
Northwest and west of Dolon Nor the Mongolian platform rises at once to over 2000 meters; on the summit, the previously rocky character of the landscape dis- appears and we suddenly enter the monotonous Gobi... The town of Dolon Nor stands on ancient rocks traversed by quartz veins; then follows to the northwest quartz-porphyry, and the steps which take us to the summit of the Gobi appear to be formed of the same rock. (Suess, 1902, III, page 153.)
Upon the ancient complex rocks lie floods of lava, chiefly basalt, which probably are genetically related to fault movements. Suess mentions ‘recent voleanic rocks . . . near Nerchinski-Savod, along the upper Argun and on Lake Kulun (1902, III, page 151)."". Nerchinski-Savod is on the Argun River, about Lat. 51° 20’ N., Long. 119° 30’ E. Manakin reports an extinct volcano with a well-preserved crater on the east side of the Khingan, at the bend of the Nomin River (1898, pages I-79). Potanin notes remains of two other craters near the town of Mergen. ‘‘Thus the town of Mergen is surrounded by a
BOUNDARIES OF THE GOBI REGION 29
volcanic region of recent date, which measures two hundred versts from west to east.” . . . and ‘forms an eastward extension of the volcanic rocks of the Khingan and at the same time a part of the border of the eastern Gobi.” (Quoted by Suess, 1902, III, page 152.)
Other extensive lava fields lie west of the Khingan. Mushketov (1881) describes a lava field north-northwest of Dolon Nor, in the hills called Bogdo Ola (Lat. 43° 35’ N., Long. 115° 30’ E.), where he saw a volcanic cone that still retained its crater. Very recently Pére Emil Licent and Pére Teilhard de Chardin visited this general region, and at about Lat. 42° 30’ N., Long. 115° E., discovered large areas of basalt and a chain of volcanic cones. The basalt overlies sediments carrying an Upper Pliocene fauna which would serve to suggest an approximate date of either late Pliocene or Pleistocene for these outpourings. (Teilhard, 1924 c, and personal communication, 1924, to W. D. Matthew.)
Suess believed (1902, III, page 120) that the lavas of the Khingan Moun- tains probably were correlated with the lava fields of southern Mongolia. But later studies have shown that the lavas range in age from probably as old as Oligocene to certainly as young as late Pliocene, so that correlation of the lavas must be left for future careful investigation.
The eastern side of the Khingan descends abruptly to a broad lowland, and is considered to be a fault-scarp by Mushketov. Von Richthofen also took this view, adding: ‘‘. . . it seems that the name Khingan applies to the steep eastward-facing scarp of a gentle up-swelling of the margin of the plateau from the west.’’ (1877, page 34.) On page 147 (1877) von Richthofen also says: ‘‘the gentle up-arching of the Khingan forms the boundary of the north- ern and eastern transition region [from the Gobi outward to the drained and watered regions], but is more of a connecting link between them than a separa- tion of them.”” On page 519 of Volume II, he adds:
If we omit the tongues of the plain that thrust in long valleys into the mountains,
the plain [the great plain of China] is separated from the mountains by a straight line
If we prolong this line north-northeast, it coincides with the eastern descent
of the mountains of Shansi rimming the Bay of Peking. This line, nearly at right
angles to the trend of the Kuen Lun ranges, is of great significance in the configura-
tion of northern China, and it is perhaps not by accident that the Khingan, which
is likewise an up-arched plateau margin and only offers the aspect of mountains when seen from the east, lies in the prolongation of this very line.
He considers that the uplift of these northeastward-trending mountains took place at some time later than Lower Jurassic.
Willis, commenting upon von Richthofen’s line of dislocation, the Khin- gan line, says in part (1907, II, page 106):
30 GEOLOGY OF MONGOLIA
He does not cite any evidence of faulting on the line itself, it being drawn indeed in the plain of alluvium; and according to our observations on three different sections, the passage from the plain to the mountains is a zone of warping, not a line of dis- location. Where, in latitude 49°, the Siberian railroad descends to the Sungari, the eastern slope of the Khingan is a tilted, dissected, but unbroken peneplane. Where the Sha-ho, in latitude 39°, has cut its autogenous valley, . . . the effects of modern warping are obvious. Normal faulting, though present in the Ning-shan basin, occurred at a remote Tertiary date, and erosion has reversed the relief to which it gave rise. Again, in latitude 31°, where the Yangtze emerges from its profound gorges at Ichang, the mountain slope that faces the far-spreading river plain is a tilted surface of erosion, showing a continuous stratum of Carboniferous limestone, which toward the base is overlaid by the K’uichow red beds in appropriate strati- graphic sequence. It is a warped surface, not a fault; and there is no evidence that it is limited by a fault at the base.
Willis then cites two places at which von Richthofen crossed the Khingan line—one in latitude 33°, the other in latitude 38°. Von Richthofen found a simple flexure at the first, but at the more northerly crossing he found that the descent to the plains was made by a series of step-faults with the down- throw on the east. Willis points out that the moderate faulting in this case “is not inconsistent with the warping observed elsewhere,’’ and concludes that the weight of evidence favors the view that the Khingan line is to be regarded ‘‘as a zone of monoclinical flexure, not as a fault’’ (1905, page 7).
Summary of the eastern boundary
From the foregoing facts, we may conclude that the history of the Khin- gan range has been essentially as follows: An ancient mountain range com- posed of folded schists and igneous intrusives was wholly planed away by ero- sion before the uplift of the present Khingan. The new range is due to simple warping or arching of the peneplaned rocks, which are similar to those of the oldrock floor of the Gobi. The lava fields along the Khingan were formed by volcanic outpourings which accompanied the growth of the present range during Tertiary and Pleistocene time.
THE SOUTHERN BOUNDARY
The southern boundary of the Gobi is complex, and locally difficult to define. In the east it is the edge of a great dissected plateau. The central section in the Ordos is indefinite. Here the Yellow River makes a great loop toward the north, east, and south, encircling the Ordos platform: Whether the Gobi basin should include this block, or whether the boundary should be placed along the hills north of the Yellow River is an open question. On the
BOUNDARIES OF THE GOBI REGION 31
whole, however, it seems advisable not to separate the two regions, because in the later history of basin making, they have behaved essentially as a unit. West of the Ordos, the boundary is to be placed along the Nan Shan and Richt- hofen ranges. There are thus three sections of the southern boundary—the eastern section, composed of a plateau edge, largely capped with lava flows; the central section, composed of hills that swing around the southern end of the Ordos; and the western section, composed of great mountain chains.
Southern boundary—eastern section
The southern boundary of the Gobi in the region of Kalgan is formed by the edge of uptilted Tertiary lavas (Figs. 1, page 8, and 5, page 47). The margin of the lavas presents an imposing scarp, descending abruptly into China on the south, and sloping very gently away into Mongolia on the north, so that it forms another “of the great steps by which Central Asia descends to the sea,’’ as Mushketov so aptly said of the Khingan. The scarp at Kalgan does not mark a fault, but is simply an erosion scarp. The only faults we have been able to trace in this region are downthrown toward the north, that is, toward Mongolia (Fig. 5, page 47). The scarp, therefore, indicates a truly impressive retreat, through erosion, of lava sheets that once must have cov- ered a far greater area than at present.
The lavas overlie a thick formation of conglomerate, sand, and sandy clay in which dinosaur bones have been found. There is apparently a slight but significant angular unconformity between the sandstones below and the lavas above. The lavas dip northward at a gentler angle than do the sand- stones. Andersson (1923 a, page 99) and Barbour (1924,) have reported the presence of plant beds, believed to indicate Lower Oligocene age, between the lava sheets.
According to von Richthofen’s map, the lavas continue to the westward for fifty miles to about Lat. 40° N., Long. 114° E. From this point (Plate IV) we must again follow Suess’s summary for the general description of the region (1902, III, pages 255-266). Prjevalski observed limestone hills called Khara Khada, or Shara Khada, which appear from underneath the lava cover in about Lat. 41° 30’ N., Long. 113° E., and form a range that strikes southwestward toward the Yellow River. Two other ranges, the In Shan and Muni Ola, continue the general line of the Shara Khada along the northern side of the Yellow River valley. The ranges are not in one line, but overlap one another (Plate IV). A similar chain of ranges lies farther to the north and includes the Suma Khada, Sheiten Ola and Narin Ola ranges. They are all formed of short overlapping ranges, and all are of ancient rocks—dominantly schists, gneisses, and crystalline limestones, cut by large bodies of granite. Suess remarks (1902, III, page 257) that the strike of these old disturbed rocks
32 GEOLOGY OF MONGOLIA
does not agree with the present strike of the ranges. Probably the ranges follow Tertiary or post-Tertiary fault lines.
Southern boundary—central section
The Khara, Narin, Argilintai, and, farther south, the Ala Shan Mountains bend like a great framework around the corner of the rectangular course of the Yellow River. The eastern side of this frame is made by the fault-block mountains of Shansi. Within the mountain-framed bend of the big river lies the semidesert land of the Ordos. It is a platform of low relief which the Comte de Lesdain says has sunk somewhat and now lies at about 1500-1600 meters, without high mountains or deep valleys (1908). It is bounded rather indefinitely on the south by hills from which flow a number of tributaries to the Yellow River and the Wei Ho.
The Cambrian limestones in the Ordos are still nearly horizontal, indicat- ing that the region is part of a very stable block of the continent,—a plateau land very like the Angara element in central Siberia (Fig. 127, page 295).
Suess points out the strong contrast between this ancient plateau struc- ture and the complexly folded oldrock floor of the Gobi. He adds (1902, III, page 258): ‘‘The difference between the Gobi and the Ordos could not be more complete. In the contrast between the ancient folds of the Altaides and their foreland, the Ordos, we find a parallel to the contrast between the folded Alps and their foreland.’’ However, Suess has compared only the folded and the non-folded units of the oldrock floor; the contrast does not extend to the younger sediments that overlie this floor. Since the time when the latest folded mountains of Mongolia were peneplaned, Ordosia has been broken up into a series of elevated and depressed blocks, and the Ordos proper has shared the warping and inland deposition of the Gobi. In the Gobi, the oldest sedi- ments resting upon the peneplaned floor of complex rocks are of Lower Creta- ceous age. The peneplaned oldrock floor of the Ordos is similarly covered by inland basin sediments, the oldest of which, according to W. H. Wong, are now believed to be Cretaceous (1924, personal communication). Despite the warping that has caused the great detour of the Yellow River toward the north, around the margin of the Ordos platform, it seems fair to reckon the Ordos as essentially a subprovince of the Gobi.
Southern boundary—western section
The southern boundary of the Ala Shan desert, which lies west of the Ordos, is formed by the Nan Shan ranges. In describing the relations of the Nan Shan to the Gobi basin, Suess (1902, III, page 223) draws his data chiefly from Obruchev’s work (1900, page 581). The Nan Shan is a great and com- plex mountain range, striking a little south of east, and is cut by a series of
BOUNDARIES OF THE GOBI REGION 33
longitudinal faults. North of it lies a broad lowland bounded by the Lung Shan, often called Pei Shan, which means North Mountains, in contrast to the South Mountains or Nan Shan. The intermontane depression which forms the lowland of Kansu, is watered chiefly by streams from the Nan Shan and Richthofen ranges. Most of these streams cross the depression at right angles to its trend, and pass through gaps in the Lung Shan into the desert beyond; only one runs parallel to the trend, collecting affluents from the mountains, and then passes through a gap in the Lung Shan to become the Edsin Gol in the Gobi. Their courses lie upon basin sediments in the low- lands, and upon the complex rocks of the ancient floor in the mountains. A critical study of the relations of the rivers, the basin sediments, and the old- rock floor, would yield a rich harvest of discovery concerning the structure and age of this part of the Gobi basin.
Obruchev’s cross-sections of the Lung Shan (1900, I, page 553) show clearly that the rocks have been folded in one revolution, and later have been faulted by quite another series of earth movements. The folding took place after the Carboniferous period, as Carboniferous strata are involved in the folded structures. Three of the four faults are represented in the section as downthrown on the north side, indicating that the depressed region lay north of the range, where the Gobi lies. Along the north flank of the Lung Shan, later sediments of the inland Gobi basin are tilted up, indicating that part, or all, of the uplifting of the Lung Shan is of later date than the deposition of the sediments. Similarly in the Nan Shan and Richthofen ranges, the basin sediments have been displaced, according to Obruchev; therefore, in part at least, the uplift must be younger than the basin sediments.
Briefly summarizing these facts and the inferences they support, the Nan Shan and Lung Shan owe their present height to warping and faulting, not at all to folding. Here, as elsewhere throughout the Gobi region, the ancient folded mountains were eroded away before the basin sediments were deposited. The modern uplift is later than some at least of the basin sediments.
THE WESTERN BOUNDARY
On the west, the boundary is still more difficult to define, because we pass farther into the vast undrained interior of Asia, and must choose a boundary between basin and basin, not between inland basin and continental slopes that drain to the sea. Two such enclosed lowlands open from the Gobi,— the Tarim or Lop Nor on the southwest, and Dzungaria on the west.
Between the Gobi and each of these great basins there is a lowland gate- way which probably is a peneplaned agd warped divide, not unlike the broad low saddles that separate the minor basins within the Gobi. Between the
VOL. II-—-3
34 GEOLOGY OF MONGOLIA
Dzungarian gateway and that of Lop Nor, the Tien Shan mountain system thrusts far eastward, dying out at about the 1ooth meridian. The two gate- ways can hardly be considered as boundaries of the Gobi. They are, indeed, merely the connecting links between subdivisions of one great physiographic unit. It seems logical to include the mountain-circled basins of Dzungaria and Lop Nor and the Gobi in one central Asiatic basin province.
The Tien Shan is one of the largest mountain systems of Asia, with a total length of about thirteen hundred miles, and a maximum breadth of nearly two hundred miles. The range Bogdo Ola, east of Urumchi between latitude 88° and 92°, is gedlogically the best known part of the Tien Shan. Here Merz- bacher (1916), Groeber (1914), Machatschek (1918), Friederichsen (1899, 1904, and 1919), and others have conducted careful studies. We briefly summarize the work of these authorities. The Bogdo Ola rises from the southern border of the Dzungarian lowland to an altitude of nearly twenty thousand feet, above which three splendid peaks rise to 21,330, 21,370, and 20,690 feet. From the lowest peak the western part of the chain drops abruptly in sheer cliffs nearly ten thousand feet, to a depression that sepa- rates the lower western part of the chain from the high central part (Friede- richsen, 1919). On the south the range sinks abruptly to a long desert depression, lying about 4,200 feet above sea level. Beyond this lowland is a minor range called the Dyjargos, the southern front of which drops sheer into the Turfan lowland, whose floor is 720 feet below sea level. On the north, the first and lowest part of the range consists of Jurassic conglo- merates, sandstones, shales, and marls, cut by younger igneous intrusive rocks: this series is eroded to a peneplane, and downthrown along a fault (Friederichsen, 1919). South of the Jurassic belt is a series of more or less strongly metamorphosed slates, quartzites, graywackes, breccias, hornfels, and altered eruptive rocks. Groeber (1909, 1914) considers these to range from Lower Carboniferous to Permian, but fossils are lacking. The high cen- tral range includes older rocks that rise in a monstrous scarp, three thousand meters above the northern parts of the range, harboring splendid hanging gla- ciers. The disturbed Jurassic beds and the folded and metamorphosed ser- ies which Groeber has called Paleozoic are dislocated along fault lines which cut and displace the folded structures. Therefore the folding must have been wholly completed before the growth of the present faulted mountains began. Such a history is very similar to that of the eastern Altai as determined by the Third Asiatic Expedition. It also agrees with the masterly summing up of this range by Obruchev, who concludes (1915, page 321):
But all later observations on the TiengShan system, as well as the Russian Altai, compel us to doubt seriously the conclusions of Suess as to the simultaneity of the
BOUNDARIES OF THE GOBI REGION 35
folding and faulting movements in central Asia . . . Large mountain-making move- ments, creating entire ranges of bow-shaped folds with their convexity turned to- ward the south, and extending from the Russian Altai to the Tien Shan, were com- pleted at the very close of the Palaeozoic or at the beginning of the Mesozoic. Then disjunctive movements began creating numerous faults . . . which appeared with special vigor at the end of the Mesozoic or in the beginning of the Tertiary period. The lines of rupture . . . diverged . . . from the direction of the Paleozoic folds much oftener than they coincided with that direction. By these two diagnostic evidences—diversity of the time of occurrence and lack of agreement in strike—the movements of folding and of disjunctive dislocation prove to be separated one from another; and it seems hardly possible, as Suess proposes, to reckon the dislocations as a supplement to the plication-movements.
This analysis constitutes a definite advance from the view propounded by Suess, in that Obruchev has recognized a period of mountain making charac- terized by fault movements, as distinct from the more ancient revolution which was characterized by folding of the rocks.
The correctness of Obruchev’s general conclusions, as far as he may have intended them to apply to the Gobi region, is thus confirmed by our more recent studies, and the fact that our analysis of the region has brought out many additional details rather enhances than diminishes the value of his pioneer work. Our own results agree very well with this sequence of events. We go farther, however, in recognizing the virtual peneplanation of the folded Jurassic series, and the development of the Han Hai in entirely new depres- sions, formed by the warping of the post-Jurassic peneplane. We also recog- nize (1924, b) that the Han Hai deposits began earlier than Neogene, in Lower Cretaceous time, that they are of diverse origin, and that they are not by any means the simple filling of large lakes. We believe that warping and fault- ing took place intermittently through the time occupied by the deposition of the basin sediments, and that the relief formed by faulting was more than once peneplaned. Although movements went on intermittently throughout the Cretaceous period and the Cenozoic era, the chief mountain-making periods. were probably the Pliocene and the Pleistocene.
The following facts and inferences have been developed in this discussion of the Gobi borders:
1. Warped borders form the lip of the great basin on the Khangai,. Kentai, and Khingan fronts.
2. The border on the Kerulen River front is probably a compound fault-- and-warp.
3. On the Tannu Ola and Nan Shan fronts, the wall of the basin is a. fault-scarp.
4. Along the Ala Shan, Yellow River, and In Shan front the borders are:
36 GEOLOGY OF MONGOLIA
faulted, if we agree with Suess in classing the Ordos as a region quite apart from the Gobi, but if we include the Ordos as part of the Gobi, the boundary is the warped southern margin of the Ordos.
5. The basins of Dzungaria and Lop Nor, lying west and southwest of the Gobi, are virtually continuous with it and are connected with it by low warped gateways.
6. The structure lines of the Gobi are quite in accord with those of the surrounding mountains. The oldrock floor of the Gobi is composed of rocks essentially the same as those of the surrounding lands. In the ancient dis- turbed rocks and in their structure lines, there is, therefore, as Suess has justly remarked, no boundary between the Gobi and the surrounding regions. Nev- ertheless there is traceable a tectonic boundary, formed of later warps and faults, which marks out the Gobi as an inland basin; and the movements that have determined this basin character are not older than post-Jurassic. They seem on the whole to have culminated in the late Tertiary and Pleistocene.
| ROUTE STUDIES OR ITINERARY _
“sy PM Pegi ely ways ORD 1S OE em
tke
PART II—ROUTE STUDIES OR ITINERARY
INTRODUCTION
CHAPTER |] III—FROM KALGAN TO IREN DABASU
CHAPTER IV—FROM IREN DABASU TO URGA
CHAPTER V—FROM URGA TO TSETSENWAN
CHAPTER VI—FROM TSETSENWAN TO SAIN NOIN KHAN AND THE ARCTIC DIVIDE CHAPTER VII—FROM THE HOT SPRINGS OF SAIN NOIN TO BAGA BOGDO OF THE ALTAI CHAPTER VIII—THE RETURN JOURNEY FROM TSAGAN NOR TO ARTSA BOGDO
CHAPTER IX—FROM ARTSA BOGDO TO SAIR USU
CHAPTER X—FROM SAIR USU TO KALGAN
38
INTRODUCTION THE TASK OF A GEOLOGICAL RECONNAISSANCE EXPEDITION
It is of prime importance for a geological expedition to keep in close inter- pretative touch with the strata or other rock formations along the route of travel, for the resulting information forms the basis of such additional explora- tory investigations as the country seems to warrant. This further study may resolve itself into acquiring collections of fossils for palaontologic purposes, or into a search for important fossil fields, with a view to their future exploita- tion. It may strive to determine the ages of the strata and to read something of their geologic history, to locate valuable mineral deposits, to solve the detail of local geologic structure, or to interpret more clearly the meaning of the surface features of the country.
Important finds in any field, of course, may be made by accident, but no scientist expects to reach his larger discoveries other than by careful and systematic work. In any case, he tries to weld his observations into some form of reconnaissance interpretation. A mineral deposit, a fossil field, or a surface feature has a good reason for being where it is, and it acquires added significance because of its surroundings and history. It may not be a geo- logic achievement to discover such a feature, but it is a geologic problem to explain it, and to use such factors from the record as may lead to other dis- coveries. Without such interpretation most finds have little more signifi- cance than the isolated specimens of a museum, whereas, when properly con- nected, they not only constitute a definite chapter in local geologic history, but may lead to still more important finds which would otherwise escape observation.
Fossil bones had previously been found in central Asia, but they meant little or nothing to the people who found them—perhaps they were ‘‘ dragon bones,” good for medicine or magic! An occasional fragment was carried away from the desert and reached hands of greater competence, but without corresponding field observations and opportunity to connect with the geologic story, they have added little to an understanding of this great continental area.
39
40 GEOLOGY OF MONGOLIA
Such considerations explain in a measure why the Third Asiatic Expedi- tion has been successful. To the chances of accidental discovery common to all expeditions was added the result of scientifically directed search based on a step-by-step field interpretation of the geology; to the large collections of fossil material were added the determination of the structural setting and of stratigraphic relations, and the evidence of origin and subsequent changes belonging to the same deposits. Thus the strata of the desert basins have been subdivided into formations having a reasonably definite age and history. Their relations, one to another, have been traced structurally as well as by their fossil content. The detail of the geologic column has been extended beyond anything that had previously been attempted in central Asia, the history has taken on a more definite form, and now the region can be com- pared more satisfactorily with other regions of the earth.
This is not the first time, of course, that the sedimentary formations of the Desert of Gobi have been described. Von Richthofen, in the winter of 1871-1872, entered the southern part of the basin, where, it appears from his description (1882), similar geologic conditions prevail. He called the sedi- ments the ‘‘Han Hai beds” (1877, I, page 25). Obruchev (1900, I, page 69) covered a portion of the same route in 1892 and gave the name ‘‘Gobi Series”’ to all these superficial sedimentary deposits. Neither scientist subdivided the series or determined their age with any considerable accuracy. Neither seems to have suspected their complexity of make-up, and neither unraveled the structural or the palzontologic story. One of the accomplishments of the Third Asiatic Expedition was to subdivide, identify, and correlate the members that make up the desert basin sediments, to discover their structural relations, and to collect their fossil content. We now know that they form a complex of several distinct groups of strata, representing an immensely long period of geologic time, from the Lower Cretaceous to the present. Nearly every period is represented in the Gobi, and no fewer than fifteen separate identifi- able formations of historical and paleontological significance have been added to the geologic column. The identification of these members, showing that definite periods of Mesozoic and Tertiary age are represented and that they contain fossils both characteristic and rare, is the most fundamental strati- graphic contribution of the Expedition.
In this way the Gobi series of Obruchev has been subdivided and has taken on a more definite meaning, expanding its history to include a much greater range of the geologic column than previous investigators had assigned to it. The limits of some of the formations have been drawn, general terms have been replaced by others of more specific time value, and the several mem- bers have been given a very different significance in the geologic history of the region. For example, we find that three sedimentary formations—the Ondai
INTRODUCTION 41
Sair, the Hsanda Gol, and the Hung Kureh—which are found in the district immediately north of the Baga Bogdo range of the Altai Mountains, and which together represent the Gobi series, are of widely different age from one an- other. In the first, there are dinosaurs of Lower Cretaceous type; in the sec- ond, mammals of mid-Tertiary type; and in the last, mammals of a time just preceding the Ice Age. At Iren Dabasu, at Djadokhta, at Oshih, as well as at many other localities, still different groupings occur, dependent on the shifting conditions of deposition and denudation that accompanied the suc- cessive warpings of the Gobi floor.
At most places the ‘‘Gobi Series” is not a series at all, but remnants of several different series separated by significant breaks in the record. Some of these gaps correspond to deposits which are still to be found in adjacent territory. The whole list of distinguishable constituent members together makes a formidable array, covering a long series of events and a long succes- sion of life forms, and adds a big chapter to the story of central Mongolia.
Whether such a reconnaissance deserves to be regarded as sufficiently complete to warrant publication, is a fair question. It is at best a hurried, preliminary study with the object of determining major features as a guide for later, more detailed work of many kinds. Its value, of course, may be measured by its adequacy in meeting the demands for which it was organ- ized. A geological reconnaissance which has solved the formational struc- ture, outlined the major historical events, established their sequence, and assembled the criteria in such a way that they can be used successfully, has perhaps a right to be considered a unit of investigation. If, in addition, these criteria have already been used to advantage in the progress of the field work in a region which bids fair to become a famous source of scientific returns, there may be good reason to put these reconnaissance results into permanent form for unrestricted use.
RECORDING OBSERVATIONS
It is not easy to record in usable form the thousands of observations made in so extended a traverse as that of the Third Asiatic Expedition in its reconnais- sance of 1922 and 1923 in central Mongolia. Unless they are organized they become too unwieldy to be of much value beyond that already served in the progress of the reconnaissance itself. Probably the simplest method is to avoid printing the record, and to give instead only the conclusions and inter- pretations reached. It is evident, however, that this alternative is open to serious objection, particularly because the work has had to be done too rapidly for uniformly reliable results. It is only fair, therefore, that the major facts of observation should be given without involving them in much interpretation
42 GEOLOGY OF MONGOLIA
beyond the simplest inferences, wherein there is little likelihood of error. It is our wish to make these observations serviceable to others, even though they may lead to a different interpretation, and to this end we have chosen to record the great bulk of elementary data in an itinerary account.
a
Ficure 3.—Profile and section illustrating methods of recording observations. The figures below the cross section are the altitudes in feet, as determined by barometer readings. The figures above are miles, representing the distance traveled from Kalgan. The profile of the section is constructed to scale, both horizontally and vertically. The bedding of all stratified rocks is drawn in approximately correct position, recording folding, tilting or horizontal structure. The igneous rocks are indicated by symbols suggesting crystalline structure. The conventional rock symbols are set forth in Plates VI and VII. In the succeeding sections, beginning with Figure 5, only a few of the altitude readings are entered, though all were used in constructing the profile; and the mileages are represented by a graphic scale.
1 460 9 8 7 6 5 454 MILES FROM TSAGAN-NOR <—— TRAVELLING S40*E
it
We believe that this account will be more interesting and more readily used if it is in part narrative and descriptive, and in part graphic, represented by profiles and cross-sections running parallel with the text (Fig. 3, and Figs. 5-97). We are well aware that this is an unusual method—one which, as far as we know, has never been followed by a reconnaissance expedition. We are mindful also that to some degree the record thus takes on certain elements of interpretation, in spite of the wish to avoid it; but only such interpretation has been used as that attached to field structure, which is represented in this volume by geologic cross-sections. If presented, therefore, with the explana- tion that much of the work has been done under rapid movement, requiring hurried observations, and that this graphic method was found in the field to be the most practicable way of recording the mass of data gathered, it will probably serve a useful purpose, and should not lead to any misconception of its value as compared with those special studies where more careful work could be done. It is possible by this means to follow the route mile by mile, and to find the profile, the geologic structure, and the specific rock formation at any point along the trail without difficulty (Fig. 5, et seq.). Thus the route sec- tion becomes a base from which additional investigations can be projected.
Wherever we were fortunate enough to make many observations, we are confident that the suggested determinations and structures are correct. Wher- ever the observations are particularly scattered or scanty, and where there is
INTRODUCTION 43
obviously greater doubt about the structural interpretation, we have at- tempted to indicate that difference of reliability in the accompanying descrip- tion. Yet rather than leave such stretches blank, we have suggested the underground structure which the evidence best supports, believing that even this is a step toward a better understanding of this very little known region. Wherever the field notes are sufficiently complete, we have used them to construct not only the profile and cross-section of the traverse, but a route map on which it is possible to indicate more or less of the immediate side coun- try and guide points.
The scales of the itinerary diagrams are uniform, each covering thirty miles, although special cross-sections are constructed on a larger scale. In
54°
uf UF tb LA PPENING 1500 SSS
RES Vea “A
Ficure 4.—General location and route map. Mountainous areas are shaded in slanting lines. Lowlands are white, and the deeper depressions are stippled. Lakes are colored black. The routes of the Expedition in 1922 and 1923 are drawn in broken lines. Index letters along the routes indicate the following places: K (northwest of Peking) Kalgan; P K, P’ang Kiang; I M, Irdin Manha; I D, Iren Dabasu; C J, Camp Jurassic; T W, Tsetsenwan; O S, Ondai Sair; O, Oshih; G S, Gurbun Saikhan; D, Djadokhta; A O, Ardyn Obo; S M, Shara Murun; B U, Boltai Urtu; U (south of the K of Khangai), Uliassutai; K (north of the A of Altai), Kobdo. The Ubsa, Kirghiz and Kara basins constitute Pievtsov’s “Valley of the Lakes.”
order to avoid possible confusion from this cause, we have taken pains to carry a scale of miles on every sheet, indicating not only the scale of the drawing but also the distance from the starting point of the traverse.
The running descriptive account which accompanies the continuous pro- file and cross-section covers the major points of interest in the corresponding portion of the itinerary (Fig. 4 and Plate IV). Of course, the text and its corresponding illustration do not in all cases, fall on opposite or on following
44 GEOLOGY OF MONGOLIA
pages but the description always follows the same order as the cross-section and forms a continuous story of observations. In large part, these observa- tions constitute the basis of such conclusions as the Expedition has been able to formulate concerning the geology of Mongolia.
But the data are not wholly confined to the route traverses. Where an area was found which combined unusual opportunities for all the scientists of the Expedition, time for longer study was given, and more detailed obser- vations were made. These studies form the basis of contributions included in Part III of this Report. Some of the special and local studies covered par- ticularly critical spots, and some of them have been examined in as great detail as is ordinarily done in countries much more highly favored with geological service. Details of stratigraphy, structure, and process were worked out in certain areas, which led to extensive correction of the geologic conclusions based on the reconnaissance traverses alone.
The itinerary observations, together with the special and local studies, have been reorganized, therefore, in the later discussion of the larger geologic problems. The assembled material, representing the summaries and tenta- tive conclusions of the geological section of the Expedition, constitute Part IV of this Report.
, 4 i i ; ; 1 ; in : f P > i : ; f ; : , J ' ; 7. ¢ , : 1 4 ; i , , 4 ; i 5 y 1 i > y : . . i i 1 F na Wi ' 4 " ; 7 “ 7 2 ) i 6 ‘al = | ae « , } _ . ‘i ‘j : , an o ; 2 , t nf : | ‘ i ' ab } " 7 ; A i. - ; | Lh , . an & pel) Toa tay a
PLATE VI.
CONVENTIONAL SYMBOLS USED FOR THE ROCK FORMATIONS OF THE SERIES OF STRUCTURE SECTIONS
CHAPTERS Il TO X INCLUSIVE
SIMPLE LATER SEDIMENTS
OF THE INLAND BASINS CENOZOIC PLIOCENE ie od Hung Kureh and Ertemte PROBABLY MIOCENE
P’ang Kiang
OLIGOCENE =| Hsanda Gol
aaa Houldjin Vt «
PALEOCENE BEES Glehcto MESOZOIC CRETACEOUS Iren Dabasu
Djadokhta
4 Oshih and Ondai Sair
PERIOD UNDETERMINED
DEFORMED SEDIMENTS OF THE OLDROCK FLOOR
MESOZOIC PROBABLY JURASSIC
Tsetsenwan Series
Sandstones Conglomerates
PALA-OZOIC CARBONIFEROUS AND PERMIAN
Jisu Honguer and Sair Usu
| Sandstone Series
PROTEROZOIC LIMITS OF THE SERIES UNCERTAIN RHR | - INS Limestone
SIRENS ASHES i SASS Cherty Limestone
NN Argillite N i\\ Slate
Quartzite
Khangai
Series
Jasper
Graywacke
PLATE VII.
CONVENTIONAL SYMBOLS USED FOR THE ROCK FORMATIONS OF The SERIES OF STRUCTURE SECTIONS CHAPTERS Tl 10 xX INGLUSIVE (CONTINUED) EARLY PROTEROZOIC IGNEOUS ROCKS THE WU TAI SYSTEM INTRUSIVE PORPHYRIES BELONGING TO THE OLDROCK FLOOR Sy Dolomites
“3 Crystalline Limestones : in Granite Porphyries
\ Quartzites \ Phyllites
Schists
Trachyte Porphyries Andesite Porphyries
Basalt Porphyries
ARCHA-OZOIC T’Al SHAN COMPLEX
, : MASSIVE IGNEOUS ROCKS Crystalline Limestones CHIEFLY BELONGING TO THE MONGOLIAN BATHYLITH
Injected Schists : “| Granites Gneisses sonnei! Syenites
Diorites
IGNEOUS ROCKS
VOLCANIC FLOWS mya Cebbros ASSOCIATED WITH THE LATER SEDIMENTS
Rhyolites Trachytes
+= Andesites BASIC IGNEOUS ROCKS PROFOUNDLY ALTERED
Basalts ‘s,0-| Serpentine
CHAPTER III FROM KALGAN TO IREN DABASU
THE ground on which Kalgan stands is alluvium, or flood-plain deposit, of the river Yang Ho, which periodically overflows its banks and spreads destruc- tion along its course; but the rugged hills above Kalgan, at the entrance to the Wan Ch’uan Hsien Pass, are made up chiefly of igneous rocks (Figs. 1 and 5).
KALGAN TO WAN CH’UAN PASS
For the first few miles the traverse was made over a low spur on the side of the valley, where the roadway is cut down fifteen or twenty feet into vol- canic tuffs (Specimen 6, A.M. 9695) and associated lavas (Specimen 7, A.M. 9696), which together form the foundation rock. A small tributary gulch just above this spur is partially filled with loess, which exhibits the character- istic habit of this material. Nowhere else on the whole summer’s traverse was typical loess again encountered. Apparently it is entirely a border accu- mulation and not at all characteristic of the interior desert region.
The lower pass
The igneous formations belonging to the lower half of the Wan Ch’uan Hsien Pass are in peculiarly fresh condition except on the lower portions of the valley sides, where long exposure to weathering has led to the usual decay, and it is here only that the rocks of this formation show enough weakness to yield noticeably to the wear of travel. In the course of centuries the wear of wheels and the tramp of caravans have cut veritable miniature canyons into the rock. The types of rock include rhyolites, trachytes, trachy-andesites, porphyritic felsites with corresponding porphyries, and a great variety of tuffs. Structurally they present great confusion, and leave one in uncertainty as to their intimate relation. Some of them are clearly lava flows and surface tuff accumulations in which there is a crude structure that gives locally some clue to original conditions, but for the most part the structure is obscure.
45
46 GEOLOGY OF MONGOLIA
It is evident that intrusions break through the simpler surface flows, not only cutting off the original continuations, but otherwise disturbing the earlier members.
The variety of rock observed is much greater than this list of types would suggest. A very great range is represented, the like of which had not been seen elsewhere in our traverse, and was not to be encountered again for sev- eral hundred miles. The whole complex, however, has a definite character, due to the prevalence of felsitic and fine-grained porphyritic types of rock, to the obscurity of their structural relations, and to their habit of breaking into small angular bits, so that it has been possible to identify the same forma- tional conditions at many other places in the course of our work in central Mongolia. The igneous eruptive complex continues halfway through the pass —indeed, through the whole of the narrow portion of about five miles. Be- yond it the valley opens out considerably, and although traveling is difficult, the gorge form disappears.
We found no clue to the age of the igneous rocks exposed in the lower portion of the Pass, except the fact that they are not metamorphosed and not intimately deformed, although much broken. The deformational habit, coupled with the peculiarities of rock type and structure, belongs, as we after- wards learned, to great igneous eruptive complexes associated with strata which we judged to be of Jurassic age. At other places, similar porphyries underlie the Lower Cretaceous sediments, a fact which tends to support the inference of the Jurassic age of the porphyries.
A very rugged topography is carved on this complex and forms a frown- ing, jagged barrier rising to one thousand feet and more in the immediate hills about the city (Fig. 5). It is surmounted by a deeply dissected escarp- ment, rising two thousand feet higher to the top of the Pass and extending to the edge of the Mongolian plateau.
Wan Ch’uan basin and the upper pass
Where the road emerges from the narrow gorge into the open portion con- stituting the upper half of the Pass, the igneous complex is terminated abruptly against a fault. Higher ground lies beyond, but its slopes are smoother, because the underlying rocks are of a simpler and less resistant type. They are sediments that lie nearly flat, dipping only slightly northward toward the interior of the continent. Curiously enough, they are dragged abruptly upward at the fault margin, indicating that the ground upon which we have just entered, despite its greater present surface elevation, belongs in reality to a down-dropped block rather than to an uplifted one. Above the dis- sected edge of the escarpment lies the Mongolian plateau.
Stream erosion has carved the usual complexity of a dissected surface, but
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47
48 GEOLOGY OF MONGOLIA
for two or three miles beyond the fault the topography is comparatively smooth, and the trail rises gently. Beyond this, the climb is more difficult, and the last three miles are as steep as can be traversed successfully with motor equipment such as that of the Third Asiatic Expedition. Thus one mounts, over the edges of a great series of strata, to the top of the Pass.
Two features of this second portion of the Wan Ch’uan Hsien Pass deserve special note:
(a) The double erosion form noted on the dissected escarpment, and
(b) The occurrence of volcanic outbreaks and intrusions.
Midway in the Pass, the sediments bear traces of an ancient, smoothly rounded topography, which was developed by long continued erosion during a preceding epoch (Plate XLI, page 389). In the present epoch these slopes have been sharply trenched by new, steep-sided, precipitous gullies, which give a peculiar double form to the hillsides. Whatever the cause, there has been a rejuvenation, and local erosion is proceeding far more rapidly at the present time than in the preceding cycle. This is probably due to changed climatic conditions—apparently to greater aridity. When rainfall is heavy, a thick mat of vegetation protects the soil so that rainwash tends to produce a gently rolling surface. At the present time the slopes are poorly protected by vegetation, and the attack of running water meets with little resistance. This explanation appears to be more satisfactory than that of change of local base-level, or greater rainfall, or even that commonest of explanations, defor- estation by men. Climatic change as an explanation is consistent with many evidences seen in other regions on this extended traverse. (See Chapter XX1I.)
At many points in the upper half of the sedimentary series in this stretch of ground there are butte-like hills and unexpectedly rugged spots, which, on inspection, prove to be due to igneous outbreaks. Some of them are necks and plugs and irregular, projecting intrusions; others are dikes and sills; still others are local induration effects produced by escaping gases. All together they represent igneous activity on a considerable scale. At no place is there a very extensive modification of the surrounding strata by the invading mate- rial, but locally the structure is disturbed. The most marked feature is an abrupt change of rock type, introducing a great difference in erosion effect. The outbreaks are mainly basalts; some are vesicular, and doubtless volcanic ashes and tuffs were also furnished, which were distributed with the sediments. A thick series of basalt flows forms a cap which constitutes the surface forma- tion inland for many miles.
Here, at the very top, stands the ancient wall that divides China and Mongolia. Here, also, we could see that we had attained, not the top of a mountain range, but the outer edge of a great plateau, a portion of which has been carried away by erosion, while the ground remaining constitutes the
PLATE VIII.
A. VIEW FROM THE HEAD OF THE WAN CHU'AN PASS.
Valley carved in the peneplaned basalts on the inland basin side of the Pacific divide beyond the Wan Ch’uan pass.
B. CULTIVATED FIELDS OF CHINESE FARMERS.
Grain fields of the Chinese settlers developing the agricultural possibilities of the grassland belt of southern Mongolia.
PLATE IX.
A. THE UPLAND AS SEEN FROM THE SECOND CAMP.
Upland of the oldrock hills preserving remnants of the dissected Mongolian peneplane in the Chakhar district of southern Mongolia.
B. CAMP IN THE GRANITE HILLS OF CHAKHAR,
The photograph, Plate IX A, was taken from the top of this hill.
FROM KALGAN TO IREN DABASU 49
rugged topography and steep ascent of the Wan Ch’uan Hsien Pass from the valleys below—a distance of twenty miles. In that distance the Expedition had climbed nearly three thousand feet and had traversed the splendidly exposed representatives of two great geologic formations,—in the first ten miles the igneous porphyry complex, and in the last ten miles a great thickness of sedimentary strata, having the appearance characteristic of formations belong- ing to the later periods of geologic time.
The sediments of the younger formation have not been much disturbed or reorganized since the strata were laid down, and present all the evidences of rapid accumulation by water currents. Sediments of this kind are now usu- ally considered to be of continental origin. They consist of conglomerates, sands, and clayey sands—varicolored and rapidly changing in quality and color both vertically and laterally. Bedding is prominent, but differences of resistance to erosion in successive beds are not great enough to have much effect on the topography.
This was our introduction to the simple sediments of the interior coun- try. Such strata ought to carry fossils. Strata of like appearance, found in the interior, yielded abundant returns. On the outward journey little atten- tion could be given to this stretch of ground and a hurried inspection at two or three points yielded nothing. On the homeward trip, however, fragments of reptile bones were found close beside the trail. In structure and dynamic history the Wan Ch’uan formation resembles some of the Cretaceous and early Tertiary formations seen elsewhere in Mongolia, and the fossil evidence favors this view, although an exact determination has not yet been made.
THE PACIFIC DIVIDE TO P’ANG KIANG
The edge of the plateau reached by the Wan Ch’uan Hsien Pass, twenty miles above Kalgan, is a true continental divide. From that point toward the interior the average elevation gradually decreases, and one enters the great basin occupied by the Desert of Gobi (Plate VIII, A). While the elevation at the divide is more than five thousand feet above the sea, at the center of the basin, three hundred miles away, it is less than three thousand feet. From the top of the Pass the streams turn inland, and for nearly six hundred miles the drainage is toward the interior.
For the most part the country is either smoothly rolling or nearly flat (Plate VIII, B). The surface features are all erosion forms governed by the underground rock structure. We soon learned that the topography is related to the deformational history of the region, its faultings and warpings, its igne- ous outbreaks, and its sedimentary accumulations. The origin of the land forms is genetically connected with the physiographic history that goes back
VOL. II—4
50 GEOLOGY OF MONGOLIA
to the time when this region first became an inland basin. Traces of a com- paratively ancient erosion are preserved on some masses of the hard rock, whereas features which were developed at a much later time appear on sedi- ments resting upon the oldrock floor. The topography therefore ties up not only with the underground structure but with the dynamic history, and as the relations of these forms come to be better understood, they furnish the first, and in some respects the best, aid in unraveling the geological relations in rapid reconnaissance.
For the first twenty miles after crossing the divide, the rolling basalt hills continue, with enough exposures to determine the character of the form- ations below (Fig. 6). Weathering and decay have formed a soil that has obscured most of the rock floor, and these processes, together with the stream wash, have produced an alluvium that covers almost the entire surface. The surface material is thus, in part, residuary from decay, and, in part, a deposit from the wash. In many places, therefore, and occasionally over long stretches, it is impossible to determine the exact quality of the underlying rock floor, but its general structural habit is, as a rule, indicated by the topo- graphic features just described. Relying on this evidence, we judge that the basin of sediments, across whose edge the Expedition climbed at Wan Ch’uan Hsien Pass, extends inland many miles, capped by basalt, and, in places, also by sediments covering the basalts. Some of the sediments are doubtless much younger than those of the Pass, and vary greatly in quality from place to place. Granite and other crystalline rocks appear abruptly on the inner margin of the basin. They are doubtless a part of the still more ancient floor on which all the so-called later sediments rest, and they have been brought to the present surface either by faulting or by warping.
From this point onward there is a succession of complex features belong- ing chiefly to the ancient floor (Figs. 6 and 7). The first large basin in which sedimentation occurred was crossed at or near the 50th mile. A much smaller basin, only a few miles beyond, has one of its limits at mile 75, and then a series of schists, quartzites and slates, cut by diorite, porphyry and granite masses, forms the floor for fifty miles (Figs. 7 and 8). The minor structural relations of these different formational units are not determinable with so little field observation, but the major ones are plain. The schists, quartzites, and slates are comparatively ancient rocks which have been invaded by the granite, and the diorite porphyries have cut both the granite and the schists. It is possible that the oldrock hills are the remnants of an ancient fault-block mountain.
This was our first encounter with a type of granite that we were to see subsequently in a thousand other places, exhibiting a great variety of facies, but always with similar relations to the major formations of the region. Per- haps it is the same also as that seen in Nan K’ou Pass, forming the core of the
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52 GEOLOGY OF MONGOLIA
mountain barrier above the plains of China. It is this granite that we came afterward to recognize structurally as part of a great bathylith underlying the whole Mongolian region, in places penetrating the various oldrock formations of the roof, forming the surface for long stretches, or appearing as intrusions of more limited extent and of more variable quality.
At mile 116 a third small basin was reached, where, because of the cover, ‘the nature of the rock is more obscure; but it is certain, judging from the topography, that the underlying beds should be classed with the basin sedi- ments. Several miles of granite and graywacke separate this tract from a larger basin, which begins at about mile 137, and, except for a low inlier of schists and graywacke, continues for nearly fifty miles, with simple, nearly flat-lying sediments, capped by, or interbedded with, basalts (Fig. 9).
The granite hills of Chakhar
In the granite hills the first glimpse was obtained of one of the great peneplanes of Mongolia. The camp here, Camp Granite Hills, lay one hun- dred and thirty miles from Kalgan and was reached on the second day (Plate IX, B). It was pitched in a country of rolling hills and broad, open valleys, but from the summit of any of the surrounding hills we looked over a veri- table sea of hilltops in every direction (Plate IX, A). From this eminence the irregularities of relief seemed to fail completely in the distance, as they might if the relief had been carved by simple erosion from an original plane surface. In every direction the same features were in evidence. This surface may be a trace of the ancient erosion floor, originally lifted and warped to make the great Gobi basin, or it may be the result of subsequent planation, with this portion still uncovered by sediments. Or perhaps it has been stripped of a former sedimentary covering and somewhat dissected. Whatever the explana- tion, it is one of the finest exhibits of planation on crystalline rocks to be seen anywhere in the region. We named this upland surface the Mongolian Pene- plane. (See Chapter XIX.)
The P’ang Kiang hollow
Our third camp was established near the telegraph station at P’ang Kiang, which stands in the midst of the fourth basin of sedimentary strata, on the border of a hollow which has been cut deeply enough to expose the edges of the beds (Figs. 10 and 11).
Although the locality promises fossils, diligent search in the time avail- able failed to reveal any. During the season of 1923, however, a single fossil was found in this vicinity, proving that the P’ang Kiang beds are of Tertiary age. The topography of the basin introduces certain physiographic questions not hitherto presented. There is a post-mature, almost peneplaned upper
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53
54 GEOLOGY OF MONGOLIA
surface on the sediments. It is clearly not the original surface of the depos- its, but represents a long period of erosion, probably in some late Tertiary epoch. Later when the significance of the smooth upland came to be more fully recognized, we referred to it as the ‘‘Gobi erosion plane.” The broad, undrained hollow, with an almost level floor, carved in this ancient plane, rep- resents a definitely later stage in the local physiographic history. Similar relations were later recognized at many other places, so that the erosion fea- tures of this locality were selected as a type and referred to as the P’ang Kiang stage of dissection. It is essentially the latest important stage of erosion
Crossbedded Sandstone Light red soft sandy Clay 1 White Sandstone | fe Red Sand and Clay
Hard red Sandstone -—— Soft red Sand and Sandstone 15
Ficure 11.—Field sketch drawn southeast of the telegraph station at P’ang Kiang. The station lies ina large, undrained hollow eroded in the sands and clays, and the sketch shows the nearly horizontal sediments exposed in the wall of the hollow.
marking the dissection of the almost level surface presented by the sediment- basins. Even though the rock floor of the hollow is composed of compara- tively soft sands and gravels, there is very little residuary soil. At most places there are only a few inches of residuary gravel or a few feet of shifting sand in the form of mounds.
At P’ang Kiang there is no way of determining how thick the deposits are. They lie almost flat, for they dip not more than four degrees from the horizontal, generally less (Fig. 11). ‘The sediments are cross-bedded, and their structure seems to indicate that they were brought to their present position
FROM KALGAN TO IREN DABASU 55
from the northwest rather than from the south. There are slight discrepan- cies of dip between the coarser upper beds and the lower fine, clayey beds, but this is not a marked feature. The whole structure is slightly deformed, or warped, since the strata of the east side of the valley dip gently to the east, while those of the west side dip even more gently to the west. It is, of course, entirely reasonable to believe that the sediments were formerly more exten- sive, and that they may have covered portions of the oldrock floor which are now bare. It is possible, also, that many of the last fifty miles of ground, where only small basins are indicated by the topography, have been stripped —the warpings that followed original deposition having exposed some of this ground to an erosion sufficiently deep to remove the whole cover.
The P’ang Kiang basin was the first to throw some light on the geologic history of the region, and although many other places furnished much more striking and conclusive evidence, none was more suggestive or helpful to the geologists of the Expedition.
P*ANG KIANG TO IREN DABASU
A few miles beyond P’ang Kiang we came, without a break in the topog- taphy, to the ancient floor (Fig. 12). The relations clearly show that at one time these Tertiary sediments must have formed a complete cover. The P’ang Kiang stage of erosion has stripped them from a small area, because here, either by warping or by faulting, the old floor is lifted higher than in the basin just crossed. It is probable, judging from the apparent dips of the strata, that the floor is brought up by faulting. This effect is repeated within ten miles. In both places small patches or strips of the ancient floor form outcrops at the surface, and are abruptly terminated and succeeded in each case by smooth gravel-covered ground having the characteristic appearance of simple, overlying sediments. After the last outcrop, at mile 197, the basin sediments appear once more and continue unbroken for sixty miles, to the tele- graph station known as Erlien, or Iren Dabasu—the scene of one of the most important finds of the Expedition. (See Chapter XI.)
At miles 187, 197, and 217, three quite different types of rock are repre- sented in outcrops, which are crossed by the trail after leaving P’ang Kiang. The first is a limestone belonging to old, much deformed units, whose age can- not be determined from the material itself but whose appearance is similar to limestones seen in Nan K’ou Pass, and similar also to limestones seen farther along on the traverse and regarded as of late pre-Cambrian age. The out- crops examined in the narrow strip exposed on the trail are not fossiliferous. The rocks dip steeply and are crumpled and sheared. The outcrop adjoins the sediment-basin along a comparatively straight marginal line, suggesting
56 GEOLOGY OF MONGOLIA
that it is brought up by faulting or by very abrupt flexure, although it is fully appreciated that original eminences in the old basement might present simi- lar exposures after stripping. The ten-mile stretch from these limestone outcrops to mile 197 crosses a broad depression about one hundred and twenty- five feet deep, and on the next rise of ground, at mile 197, slates are exposed. Brief examination of these slates indicates that they were ancient sediments which have undergone considerable deformation. There is no possible way of determining either their age or their relation to other floor formations of the district, because the exposures are small, very obscure, and separated from the nearest outcrops of a different kind of oldrock by several miles of covered ground. They are similar, however, to many other slates noted on this trav- erse, most of them much farther along on the trail.
The third exposure of a type of rock different from the usual superficial sediments is located in the traverse between miles 217 and 229 (Fig. 13). In this stretch are numerous outcrops of limestone of dolomitic appearance. The rock has a peculiar porous structure, and there are traces of fossils, the nature of which was not determinable in the field. Structurally this rock is much simpler than that of the two outcrops previously described. It does not show deformation, metamorphic reorganization, or any of the complexi- ties belonging to the most ancient types. It does not at all resemble the Palzozoic limestones which were encountered farther to the northwest. It has the structural appearance of a later time, but the evidence as to its exact age is not clear. Nothing resembling this formation had been seen previously along the journey of more than two hundred miles. Later on, we found patches of similar limestone interbedded in the basin sediments; we believe that they represent deposits formed in inland lakes and ponds.
The last appearance of the oldrock floor in this stretch from Kalgan to Iren Dabasu is a very small outcrop of slate, in the depression of Iren Dabasu at about mile 258 (Fig. 14). At this point a patch, very similar to that noted at mile 197, has been stripped by the erosion that has made the Iren Dabasu lowland. Its only significance comes from the evidence that it bears as to the shallowness of the basin itself, since the floor rocks are actually exposed at several places and form the northern edge of the sediment-basin. Doubt- less the basin is deeper between these windows—i.e., between miles 197 and 258—but there is no way of measuring it.
Between mile 197 and Iren Dabasu at mile 260, we passed continuously over simple, nearly flat-lying sediments. The trail is on the upland to mile 238, where it descends abruptly into an undrained erosion lowland, repre- senting the P’ang Kiang stage, and continues for seven miles in the hollow. At mile 245 the upland is reached again. Here a much smoother and much more gravelly surface, representing an erosion plane, extends to the Iren
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58 GEOLOGY OF MONGOLIA
Dabasu lowland, which begins at mile 255. These two different erosion forms, the level upland and the enclosed hollow, are especially well developed at Irdin Manha (Fig. 13).
Little of the topography, except in very minor ways, is constructional in origin. The smooth, almost level, gravel-covered surfaces that are encoun-
NW Camp Iren Dabasu Houldjin = anni
WLLL I) Mf “Probakis Granite Basement _ Slates
240 MILES 245 250
FiGcurE 14.—Geologic section from Irdin Manha to Iren Dabasu, crossing a narrow strip of upland between two undrained hollows. At Iren Dabasu the lowest sediments are Cretaceous, resting directly upon the folded slates and followed by Eocene and Oligocene in the Houldjin escarpment. (See Chapter XI.)
tered here and there, stretching away for miles, are strictly erosional forms, and are typical of the Gobi region. Since the Mongols call these great open areas of nearly level country ‘‘gobis,’’ we chose the name ‘‘Gobi erosion plane”’ for the surface which we later were able to trace across nearly the whole desert region.
All the landforms thus far mentioned are destructional. The most per- sistent constructional form consists of small hummocks or dunes of sand. They are, perhaps, the “‘tamarisk mounds” described by Huntington (1907, page 179). In some places they shift more or less, but in most cases they are small, very closely and irregularly placed, and owe their lodgment to shrubs and other small plant growths, which knit them together and give them some slight fixity. It is the shifting surface sands, however, that offer the greatest difficulty to the traveler and make it quite impossible in certain areas to depart far from the beaten trail. It is always easier to follow the trail where the dis- turbance caused by caravan travel enables the winds to keep the trail clear, so that the loose sands are, to a large degree, swept away. These surface accu- mulations are, in general, more numerous and troublesome in lowland areas represented by the P’ang Kiang dissection than on the Gobi erosion plane. The gravelly residue of the level upland seems to serve as a protection against the furnishing of sand for drifting, whereas the strata exposed in the walls of hollows of the P’ang Kiang stage are readily attacked, and sands from them are spread on the lowland and in places are swept over portions of the adjacent upland.
FROM KALGAN TO IREN DABASU 59
Nowhere in this whole traverse of 260 miles did we see any large drift- ing dunes, and nowhere is the constructional deposit more than a thin, super- ficial covering. No large features are controlled by it, and at no place is the accumulation, including both the soil mantle and the drifting cover, more than a film on the surface. Instead of being compelled to travel over great accumu- lations of loose sand, the Expedition actually traversed rock with only a few inches or a few feet of obscuring cover. To be sure, the floor itself is com- posed, in part, of simple and almost unconsolidated sediments, not very unlike the sands themselves; but they are, nevertheless, definite sedimentary forma- tions and of varying age from place to place, quite different from one another, both in quality and in fossil content. The sandy and little consolidated char- acter is responsible for the common failure of travelers to distinguish between loose sand and sedimentary rock formation.
Fully half the distance, however, is over a floor of much more complex rocks, and they also are exposed with little cover. The obscurity due to over- lying soil is greatest in the first hundred miles, where some of the country has agricultural capacity—some, indeed, is cultivated by Chinese settlers, who are in this way encroaching upon Mongol territory (Plate VIII, B, page 48). The land could be farmed for a considerably greater distance northward, but by the time P’ang Kiang is reached, conditions have become too desert-like for agriculture, and the rock formations are too nearly bare, since here the wind is so successful in blowing away the finer débris that there is little devel- opment of soil mantle. This, therefore, is the edge of the Gobi Desert proper, which, as far as we have seen it, is essentially a rock desert.
The first fossils
At P’ang Kiang and again at Irdin Manha and Iren Dabasu (Plate X, A), the sedimentary formations are especially well exposed in the escarpments, where the succession of strata can be seen and intimately inspected. On the first traverse no fossils were found at the first two places, but upon arriving at the Houldjin escarpment above Iren Dabasu, at mile 255, numerous frag- ments of fossil mammal bones and teeth were found, while dinosaur bones were discovered at Iren Dabasu. Later, an extensive fauna was collected at Irdin Manha, and in the following season a single rodent jaw was brought to light - at P’ang Kiang. The finds at Iren Dabasu were recognized at once as of very great importance, and on this account it was decided to spend additional time on this ground. Here the first local study was made. The results are set forth in special study No. 1 of Part III, ‘‘The Iren Dabasu Basin,” constitut- ing Chapter XI of this volume.
CHAPTER IV FROM IREN DABASU TO URGA
ALTHOUGH sediments are extensively exposed at Iren Dabasu, the slate floor lies near the surface. One small outcrop is uncovered at the south side of the basin (Fig. 14), and a comparatively large area is exposed on the north border of the lake Iren Nor and near the Chinese telegraph station (Plate XXIII in pocket). The hollow is margined on the south by a scarp of sedi- ments, and on the north by an abrupt ascent over crystalline rock to a nearly level erosion plane 200 feet above the lake (Fig. 15 and Plate XXIII in pocket). The trail over this northern edge’ passes across slates cut by numer- ous quartz veinlets like those in the bottom of the basin.
An inspection of the veins failed to show mineralization of any conse- quence. In spite of the abundance of vein quartz—a condition that was noted at many other places in the course of the summer—there is everywhere a sur- prisingly small amount of metallic content. Prospectors have dug pits here and there, and doubtless have made tests of the material for the precious metals. But we saw nothing that gave any encouragement to such a search, except the presence of the veins of quartz.
Farther to the east, the Iren Dabasu basin is bordered by intrusive granites which, with the slates, have been eroded to form the ancient floor (Plate XXIII in pocket). Within a mile of the north margin, on the way toward Urga, the slates are cut by basalts or traps (Specimen 35, A. M. 9724), which continue for about two miles (Fig. 15). The exact structural relations are not determinable, neither is it certain whether these rocks are intrusive or are in part surface flows. Some of the material has the aspect of surface flows, but in any case these rocks are much later than the slates with which they are associated. There is little evidence of deformation in the basalt, whereas the slates are considerably metamorphosed.
IREN DABASU TO CAMP JURASSIC
Six miles north of Iren Dabasu, at mile 267, the trail again descends to a basin-like area, smaller than Iren Dabasu but similar in superficial form 60
PLATE X.
A. IREN DABASU.
Dissected dinosaur beds at Iren Dabasu. The distant skyline is the Houldjin escarpment
B. RESIDUARY GRANITE BOWLDERS.
Bowlders of granite left as residuals on a level erosion surface of almost bare rock, along the Urga trail.
PLATE XI.
A. UPDRAGGED SANDSTONE BEDS AT CAMP JURASSIC,
B. MOUNT TUERIN.
Granite residuals on the lower slopes of Mount Tuerin, Irregular jointing controls weathering of the rock into peculiar shapes.
FROM IREN DABASU TO URGA 61
(Fig. 15). The floor is doubtless the same slate formation, but it is covered for several miles with later sediments which have been characteristically eroded into low, rounded bluffs and undrained hollows. The sediments con- tinue to mile 272, where basalts are again encountered; then, a little way be- yond, granites and syenites break through the slates and form the floor through a somewhat more rugged stretch of country.
At mile 275.8 there is a very striking outcrop of poikilitic syenite which displays a great many variations of composition and coarseness of grain. It probably represents, in part, the results of syntexis, and, in part, the end products of a differentiating magma. The chief local types include horn- blende syenite, hornblende granites, and quartzose pegmatites. The locality bears a monument known as Kwei Ming Kwan, near which we noted a most complex group of variations in the syenitic rock. The dominant type is a hornblende syenite (Specimen 36, A. M. 9725), but there seems to be no real difference in age or origin between these and other varieties. Even the granite (Specimens 36 and 39, A.M. 9725 and 9728), from this same ground probably has essentially the same significance. At mile 278, there is granite porphyry (Specimen 40, A.M. 9729), and at mile 280, a very coarse granite (Specimen 41, A.M. 9730), both of which are probably only varietal expressions of the igneous activity of this peculiar area. In the Kwei Ming Kwan area, also, there are some older metamorphosed sediments, through which the syenites and granites have penetrated. The patches of these metamorphics now re- maining seem to be merely portions of the roof into which the original igneous mass had advanced. They include a group of strongly veined argillites which probably have been tuffs (Specimens 37, 38, A. M. 9726 and 9727). Their peculiar greenish color and their structure suggest intermixture of ash. At mile 283, an ancient conglomerate is associated with the slates and tuffs. It carries bowlders of granite and many kinds of porphyry and may be a basal conglomerate (Specimen 42, A.M. 9731), but a detailed examination could not be made. There is evidence, at least in this locality, for an important structural break between two ancient series, and it is possible that this con- glomerate represents the basal portion of the Nan K’ou series. We believe that the slates, as well as the limestones seen much farther back along the trail (miles 187 to 220), belong to the same series.
Argillite outcrops again at mile 286 and is cut by a very coarse hornblende granite, which is itself cut by dikes, and which continues for two miles or more. Slates or graywackes reappear at mile 289, and in a short distance are overlaid by sediments in a basin which extends from mile 289 to mile 302, a distance of 13 miles (Fig. 16). The slates of the margin are very badly deformed and stand in all attitudes, cut by granite porphyry and by basalt sills and dikes in great abundance (Specimen 46, A.M. 9735). The slates
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FROM IREN DABASU TO URGA 63
themselves are variable in quality and extremely thin-bedded, but are well indurated. They are closely folded, and the strata stand vertically in places; but on the north side of the basin, where the slates form the surface for seven miles, the dips are chiefly to the north. The whole formation is undoubtedly very thick.
The sediments in the basin from mile 289 to mile 302 have a basal con- glomerate of rather coarse quality on the north margin, at mile 302. At mile 296, there are red shales overlying a pebbly fresh-water limestone and sand- stone, together with carbonaceous paper shales which contain scattered gyp- sum crystals. It was appreciated at once that the quality of these strata is unusual and that nothing quite like them had been seen. A rapid search was made for fossils, but none was found. Their possibilities in some respects were not fully understood until three months later, when we studied the Ondai Sair locality, north of the Altai Mountains. At Ondai Sair, exactly similar dark paper shales and red and gray sandstones furnished fossil insects, fishes, and plants, as well as dinosaur bones, by which they were determined to be of Lower Cretaceous age. The paper shales alone are so characteristic that there is really little uncertainty in correlating these deposits with the Lower Cretaceous of Ondai Sair. The sediments in this particular basin, between miles 289 and 302, are quite different in physical aspect from the Cretaceous strata which had already been identified at Iren Dabasu. A small salt pan lies in the bottom of the depression. The conglomerates at the base may indicate that basal beds of the Lower Cretaceous series are represented here. It is entirely possible, of course, that the higher red shales are of Ter- tiary age, but no satisfactory evidence of their age could be found.
This basin deserves additional study. It contains formations of the same type as those which elsewhere furnished a unique fauna, and probably would furnish also a valuable contribution if it could be inspected in detail. Un- doubtedly important stratigraphic data could be gathered in a more extended investigation of the ground, from mile 289 to mile 302, especially by broaden- ing the scope of the search to the east and west, within the basin. It is the only area noted on the Kalgan-Urga trail where the carbonaceous paper shales are developed in typical form. A week, instead of an hour, should have been devoted to this locality.
At mile 310, a very coarse granite, of almost binary composition, cut by pegmatite veins and quartz stringers, breaks through the slates and continues for two miles (Specimen 47, A.M. 9736). Since the ground slopes gently to the north, the granite gradually passes beneath a mantle of later sediments obscured by arkosic sand and many dunes. In a gulch at mile 320, however, we noted green clays, which are probably Tertiary beds (Fig. 17). The whole basin is very shallow and is terminated on the north, at mile 323, by slates
64 GEOLOGY OF MONGOLIA
similar to those seen before. From mile 325 the ground slopes rather rapidly between granite hills, northward toward Ude. An alaskite granite (Specimen 49, A.M. 9738), was gathered beside the trail. Probably a thin sediment, or at least an arkosic alluvium, occupies the bottom of the valley, in the midst of which stands a granite hill (Specimen 50, A.M. 9739), which marks the site of the station of Ude, now entirely deserted. From this point the trail follows the sandy wash of a stream bed with rugged valley sides of crystalline rock, probably granite, bowlders of which appear abundantly in the wash. No motor exists that can plow up this mixture of sand and cobbles without coming toa stop. And at mile 332, just where the heavy sand stops the car, the door-flap of a yurt opens, and a long-skirted Buriat official comes down, inspects the traveler’s permit to enter Outer Mongolia, and invites him to pay atax. In the hills on either hand sits an invisible garrison, ready to fire upon a car that is mad enough to try torun. The exaction may not be pleas- ant, but the strategic choice of a perfect motor trap is admirable.
Above the permit station, we cross a low divide to where, on the farther slopes, sedimentary strata overlie the granite. The road is smooth and gravel- ly for five miles, continuing down a gentle slope to a well at mile 341. Although there was no opportunity to determine the quality of this ground, the sub- stantial nature of the material led us to believe that it may be older than Tertiary.. The structure is basin-like, and is terminated abruptly on the north at mile 343, making a ten-mile stretch of sedimentary strata not very different from the others passed over in the last 75 miles. All of the sedimentary basins encountered in that day’s run are smafl and shallow—none more than Io or 12 miles across.
The country at Camp Snow Storm (mile 341) was covered with snow on the morning of May 8. This entailed some delay, but in the afternoon the clouds began to break, and a start was made. After a twenty-five mile run, however, the weather turned bad again, and a long-threatened storm broke in a furious blizzard. There was no alternative but to keep on as cautiously as possible, in the hope of driving through. By the time mile 373 was reached, the clouds had lifted again, and not only had the Expedition passed through the storm itself but it had come into ground where no snow had fallen. At this point we noticed a decided difference in the topography, which invited geological investigation to determine just what change of structure had taken place (Fig. 18). For some distance the storm, with its blinding snow, had obscured everything, but it was evident, at least, that a small basin had been crossed.. Herea well gave opportunity for camp, and the party stopped at mile 374, in one of the most interesting structural areas to be found on the Urga trail.
Immediately after leaving Camp Snow Storm at mile 341, the trail leads over slates with a strike nearly east and west, and a dip south at an angle of
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66 GEOLOGY OF MONGOLIA
about 60 degrees. There are many interbedded limestone and quartzite layers, but the most striking feature is the occurrence of great numbers of granite sills, which penetrate the slates. Weathering and erosion have left residuary bowlders of granite scattered along the course of the sills, as though they were the ruins of long, low masonry walls built upon the marvelously level surface of the desert. No more splendid exhibit of planation across different types of rock has been seen in the whole of Mongolia than is shown in this stretch of ground. From mile 343 to mile 353, folded slates which stand nearly on edge are interbedded with limestones and quartzites. They are associated with granite sills and all are planed off so perfectly that one can drive almost anywhere by motor. This kind of ground continues for twenty- four miles.
It is clear that the formations are folded, but the structure could not be determined in greater detail, because of the rapidity of travel. Possibly the limestones furnish evidence of the age of these beds, but no fossils were found in them. In the northerly half of this stretch of ground the limestone beds are much more numerous, and there is some variety in the quality of associated slates and sandstone. Northward from mile 355 there is a valley-like de- pression in which the character of the material is less easily made out, but the slates dominate, with some graywacke and a few quartzite beds.
At mile 362 one of the most striking features seen thus far in the traverse is represented by a volcanic blowout, which forms buttes and minor ridges bursting up through the slates. The rock is an olivine basalt (Specimen 66, A.M. 9755). North of this blowout the ground carries more graywacke and less slate, becomes pebbly, and obscures the relation to the slates and lime- stones farther south. At mile 367, conglomerates, the like of which we had not yet seen in our traverse, lie nearly flat beside the trail. Observation of these beds was interrupted by the snowstorm, and the point at which change to later sediments takes place was not accurately determined. Their pres- ence, however, was not long in doubt, because they were found again at Camp Jurassic, mile 374, on the north edge of the small basin which occupies the ground from miles 370 to 374.
The belt of slates, from mile 343 to mile 367, deserves more careful in- spection than could be given. There must be an immense thickness, perhaps 15,000 to 20,000 feet. Their variation in character, the amount of deforma- tion they have undergone, the strikingly abundant intrusives in the form of sills, and the numerous limestone interbeds, are prominent features. Prob- ably more than a hundred granite sills were seen in this stretch of ground, all of them intersecting the strata in a clean-cut, definite fashion, instead of impregnating or absorbing or markedly metamorphosing the country rock. There are many quartz veins and occasional jaspery bands. Doubtless these
FROM IREN DABASU TO URGA 67
latter are related genetically to the intrusives in some way. The slates im- pressed us as having finer grain and a better slaty structure than most of the slates subsequently encountered farther north. Indeed, the relation between these fissile slates and the argillites and graywackes farther north is one of the questions not satisfactorily settled. They may be equivalent, but, if so, there is a marked change in lithologic quality.
The structural relations at Camp Jurassic
Because the physiographic features showed that a change in structure had been reached at mile 374, camp was made there on the evening of May 8, so as to give an opportunity for more detailed inspection of the ground (Fig. 18). The strata which occupy the narrow basin extending from mile 370 to mile 374 have been turned on edge by drag against the older formations. The plane of movement could be readily located (Plate XI, A). In the distance, in line with this fault, several small volcanic cones were seen. The strata thus turned on edge are sandstones belonging to the later sediments; their exact age is undetermined, but, judging from their petrographic character and condition, they may belong to the Tertiary series.
Special interest centered in the formations standing almost vertically in the uplifted block against which these later beds are faulted. The older strata are comparatively hard sandstones in great variety, and are devoid of fossils except for scanty and obscure plant remains. The agencies of weather- ing and erosion have carved them into fantastic shapes. The structure and history of the rocks seem to be consistent with what was expected of the younger strata belonging to the floor formations; there is no metamorphism, and the beds are simply folded. In China, rocks which in all essential re- spects resemble these are regarded as of Jurassic age, and the same correlation was assumed for this locality. Later, at three other places, strata were found, similar in composition, structural relation, and history, and apparently con- sistent with the assumption that they are all of Jurassic or mid-Mesozoic age.
The area is more significant than it at first appears to be. Here one can see that the basins carrying the later sediments, such as had been crossed re- peatedly in the traverse from Kalgan, may have originated, in part, by de- formation of comparatively recent date, for in places the strata themselves are turned up on edge, indicating that some of the deformation is younger than the sediments. The making or deepening of basins, therefore, probably was a long-continued and oft-repeated process.
Within two miles the entire exposure of the steep-standing sandstones of the floor is crossed, and younger overlying beds appear again. Evidently there is an uplifted area adjacent to the down-dropped one. The erosion surface, representing the original floor, has been thrown into broad arches
68 GEOLOGY OF MONGOLIA
and troughs by gentle warping, but there are sharp flexures and faults along certain marginal lines. It is apparent, also, that volcanic activity found vent along some of these lines of weakness. An extensive igneous accumula- tion, both of lava and of tuffaceous material, is associated with the overlying deposits. There are evidences of volcanism also within the supposed Jurassic area, but the relations there are obscure.
The supposed Jurassic strata strike N. 60° E., and dip steeply north. There is a marked angular unconformity between them and the beds coming in above (Figs. 18 and 19). The area exposing the Jurassic strata is the center of a breached, dome-like uplift, with annular valleys, dip slopes, hogbacks, and the other erosion-forms characteristic of a dissected dome of sedimentary strata (Fig. 122, page 281).
On the whole, this locality was the most illuminating in the matter of dynamic history and structural relations of any thus far seen in the Mon- golian region. There is definite evidence of the succession of sets of strata and of their relation to deformation and igneous activity. Here one can see the youngest strata that belong to the ancient floor, and this is the first place where strata suggesting Jurassic age had thus far been seen. It is a clear case of late faulting, and volcanism seems to be structurally related to that weakness. It shows that there is a great unconformity between the Jurassic strata of the floor and the so-called later sediments. It seems also to carry evidence of an important break in the midst of later sediments, particularly in the ground forming the north side of this dome. Apparently there are three important series here, one of Mesozoic age belonging to the old floor; another, the first series of sandstones above the unconformity, lying on that floor; and a still younger series, including the topmost beds.
At the time these features were first seen, it was not fully appreciated that the structural elements of this locality are representative of the most fundamental structure of the Gobi region. Although not fully proved at the time, it appears certain from later developments in adjacent regions that the two sedimentary members at Camp Jurassic, overlying the eroded edges of the so-called Jurassic strata forming the floor, include a Cretaceous series and, after a slight unconformity, a series of Tertiary strata of simpler history. This splendid structural exhibit presents a good opportunity for working out in much greater detail the sedimentary history of the region, and warrants the recommendation that this area be given a more extended investigation. The strata of these different types are well enough exposed to allow a com- paratively complete column to be constructed, as there must be at least 3,000 feet exposed in the area, a large portion standing almost on edge. Here, an area that could be extended so as to include the basin to the north deserves mapping, as well as a structural and paleontologic study.
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70 GEOLOGY OF MONGOLIA CAMP JURASSIC TO MOUNT TUERIN
The ‘‘Camp Jurassic’’ dome proved to be a very limited one. It is dis- sected in perfectly typical form, and from the higher eminences one can follow the hogback border from the south side eastward and northeastward until it curves back to the west again, completely closing that end of the uplift.
For a distance of little less than a mile, the trail crosses steeply upturned sandstone beds supposed to be of Jurassic age (Fig. 17). The thickness rep- resented is at least 3,000 feet, with neither the top nor the bottom exposed. At mile 375, a new series of sandstone beds appears unconformably on the up- turned edges