BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA PRE-CAMBRIAN GEOLOGY OF THE MEDICINE BOW MOUNTAINS 1 BY ELIOT BLACKWELDEK (Read before the Society December 28, 1917) CONTENTS Page Introduction........................................................................................................................ 615 Outline of the geology of the district...................................................................... 619 Stratigraphy........................................................................................................................ 621 General statem ent.................................................................................................... 621 Tabie of pre-Cambrian formations.................................................................... 622 Gneissic complex...................................................................................................... 624 Deep Lake metaquartzite........................................................ ............................. 625 Headquarters schist................................................................................................ 627 Heart metagraywa eke............................................................................................. 631 Medicine Peak metaquartzite.................................................. ........................... 632 Lookout schist.............................................................. ................ ........................... 634 Sugarloaf metaquartzite........................................................................................ 635 Nash marble series.................................................................................................. 636 Anderson .phyllite......................................................................... ........................... 641 Ranger marble............................................................................. j ........................... 642 Towner greenstone.................................................................................................. 644 French slate............................................................................................................... 645 Acidic intrusives........................................................................... ........................... 647 Basic intrusives............................................................................. ........................... 647 Younger form ations................................................................................................ 648 Geologic structure............................................................................................................ 648 Metamorphism.................................................................................................................... 653 Paleogeography.................................................................................................................. 655 Summary.............................................................................................................................. 657 I ntroduction The Medicine Bow Mountains lie south of the Union Pacific Railroad and about 40 miles west of the city of Laramie, in southeastern'Wyoming. They extend southward into Colorado. A short alpine ridge, known as the Snowy Range, rising to an elevation of 12,000 feet, surmounts a 1 Manuscript received by the Secretary of the Society May 26, 1926. (615) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/4/615/3414690/BUL37_4-0615.pdf by guest on 01 October 2021 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/4/615/3414690/BUL37_4-0615.pdf S c a l e o f M il e s q /MLROAD C i 0 5 ;. BLACK WELDER---------------------------------------------------------------------------------------------------------------------------------------------------------GEOLOGY OF MEDICINE BOW MOUNTAINS WELDER---------------------------------------------------------------------------------------------------------------------------------------------------------GEOLOGY w;. BLACK P r e -C a m b r i a n P a l e o z o ic P o s t -P a l e o z o ic F ig u r e 1.— Geologic Sketch .Map of a Part of Southeastern Wyoming Showing distribution of pre-Cambrian formations. Adapted from maps by U. S. Geological Survey. INTRODUCTION 617 broad plateau which varies from 9,000 to 10,000 feet in elevation and is intrenched by canyons. The plateau, partly margined by fault-line scarps, is nearly surrounded by a broad, terraced plain that has been eroded from outcrops of Cretaceous shales by streams and wind. In spite of its ready accessibility, this interesting range has been strangely neglected by geologists until the last decade. In 1868 Hague2 made a reconnaissance, noted the general character and distribution of the rocks, and offered a tentative interpretation of the internal structure. Yan Hise3 crossed the range on horseback in three days in 1889 and thereby added some further information regarding the pre-Cambrian rocks. Subsequently the range-was visited from time to time by Wilbur C. Knight and his son, S. H. Knight, and perhaps by other geologists; but no detailed work seems to have been attempted and the results of such examinations do not appear to have been published. In 1917 the writer, assisted by H. F. Crooks, then a student of the University of Illinois, devoted six weeks to a survey of the pre-Cambrian rocks in the central part of the range. During a part of the season he also had the company of F. M. Van Tuyl and S. H. Knight. The World War interrupted this work, and Mr. Crooks went to France as a soldier a few months later. The writer presented jan outline of the results at the Saint Louis meeting of the Geological Society, but never submitted the paper for publication because he felt the need of more field-work. A t the time, it was hoped that Mr. Crooks would be able to return and complete the study himself, but that hope was never realized. Again, in the summer of 1925, assisted by A. L. Crawford, the writer devoted a month to reexamination of the same district, but the unprece­ dented rainy season so reduced the available time that certain outlying parts of the district, such as French Creek and Medicine Bow River canyons, were not reached. During the same season J. F. Kemp, in com­ pany with S. H. Knight and students from the University of Wyoming, spent some weeks on the eastern side of the range, and J. J. Runner, assisted by students from the University of Iowa, made a detailed study of the pre-Cambrian rocks at the request of the State Geologist of Wyo­ ming. Aside from the brief abstract4 of the paper read by the present writer at the Saint Louis meeting of the Society in 1917, none of the results of any of these explorations have as yet been published. 2 Arnold Hague: U. S. Geological Exploration of the 40th Parallel, vol. ii, 1877, pp. 94-111. 3 C. It. Van H ise: Unpublished notes, summarized in “Pre-Cambrian Geology of North America.” U. S. Geol. Surv. Bull. 360, 1909, pp. 837-883. 4 E. Blackwelder and H. S. Crooks: Pre-Cambrian rocks in the Medicine Bow Moun­ tains of Wyoming. Bull. Geol. Soc. Am., vol. 29, 1918, pp. 97-98 (abstract). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/4/615/3414690/BUL37_4-0615.pdf by guest on 01 October 2021 628 B. BLACKWELDEK-----------------------------------------------------------------------------------------------------------------------------------------------------------GEOLOGY 01' MEDICINE BOW MOUNTAINS Adapted from U. S. Geological Survey topographic map, with slight corrections. The small squares are one mile wide. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/37/4/615/3414690/BUL37_4-0615.pdf by guest on 01 October 2021 INTRODUCTION 619 The general geographic features of the district are represented on figure 3. For greater detail the reader is referred to the contour map of the Medicine Bow Quadrangle published by the U. S. Geological Survey. Thus far only a few of the more important topographic features of the range have been definitely named. Sheep herders, prospectors, and the rangers of the Forest Service have until recently been the only inhab­ itants of the plateau. Scarcely any of the mountain summits—not even the high eastern summit of the Snowy Eange—have names that are in current use. Likewise, of the 160 or more glacial lakes distributed over the surface of the Medicine Bow plateau, only a few of the largest or most accessible have been named. U ntil recently, access to the district, was chiefly by trails and a few very poor roads over which wagons could be taken with difficulty. In .1925, however, a good automobile road was completed from Centennial to Brooklyn Lake, and thence southwestward across the range to Brush Creek and Saratoga. There is also a passable road for automobiles to Sand Lake by way of Rock Creek. As the district is rather generally forest-covered, affording attraction for fishermen and a refuge from the torrid plains of the Central States, its use each year as a summer playground is increasing rapidly. O u t l i n e o f t h e G e o l o g y o e t h e D i s t r i c t On referring to the general map, the reader will see that the Medicine Bow Mountains consist of a large anticline in Mesozoic and Carbon­ iferous strata, the top of which has been so deeply denuded that the pre- Cambrian formations in the core are now broadly