98 ELIOT BLACKW ELDER 



PART I 

 INTRODUCTION 



Subject and scope.- — -In this essay it is my purpose to discuss the 

 geologic history of a part of western Wyoming, from the close of 

 the Cretaceous period to the present. The field under considera- 

 tion is limited naturally by the district which it has been my 

 privilege to explore. For the interpretation of Cenozoic history 

 the district is especially favorable because that era is represented by 

 deposits of several ages, and the physiographic conditions are so 

 varied that they afford the means of interpreting many of the later 

 events. It is scarcely necessary to state that many of the problems 

 which arose in the course of the study remain unsolved or but partly 

 solved, for such is the nature of most complex questions. 



Data and acknowledgments. — The data for this work have been 

 obtained largely from my own field examination of the district for 

 the United States Geological Survey during the summers of 1910, 



1911, 1912, and 1913. In the summer of 1910 a somewhat rapid 

 reconnaissance trip was made from Montpelier, Idaho, through 

 Jackson Hole and the Wind River basin to Thermopolis, Wyoming. 

 In 191 1 we ascended Green River from the Union Pacific Railroad, 

 made a brief examination of the southern part of the Wyoming 

 Range, carefully studied the Fall River basin and the headwaters of 

 Green River, and devoted the remainder of the season to an exami- 

 nation of the Gros Ventre Range and the highlands north of it. In 



191 2, a detailed study was made of the west slope of the Teton 

 Range near the Idaho line. The summer of 1913 was devoted to 

 the Owl Creek Range and the Wind River basin and Range. In this 

 field work I have had the assistance of Messrs. J. M. Jessup, C. W. 

 Tomlinson, Hyrum Schneider, and D. Dale Condit. 



Earlier seasons in surrounding districts afforded me an oppor- 

 tunity to gather information which has value by way of comparison 

 with western Wyoming. Thus the Bighorn Mountains in north- 

 central Wyoming were studied in 1902, the Laramie Range in the 

 southeastern part of the state in 1907-8, and the Wasatch Range of 

 northern Utah in 1909. 



In addition, some facts and many interpretative suggestions 

 have been obtained from the writings of other students of the 



