POST-CRETACEOUS HISTORY OF WESTERN WYOMING iii 



members of the Hayden Survey, the strata were tentatively cor- 

 related with the Wasatch of Utah. Fossil mammals from the 

 Wind River basin^ have recently been correlated with those of 

 the Wind River and Uinta formations of the Eocene and with those 

 of Lower Oligocene. In the southwest part of the Bighorn basin 

 abundant mammals belonging to the typical Wasatch have been 

 found by Wortmann, Loomis, Granger, and others. The beds in 

 which they occur are so closely similar to those in the Upper Wind 

 River valley and near the head of the Gros Ventre River that 

 the intercorrelation of all three is strongly suggested. Since the 

 volcanic Tertiary beds lie almost entirely at higher elevations, and, 

 where both formations are present, at stratigraphically higher 

 horizons than the Wind River formation, they are probably not 

 older than Upper Eocene. Sinclair and Granger have found Lower 

 Oligocene fossils in beds of tuff and agglomerate east of Lander. 

 Older Tertiary beds in that vicinity do not contain volcanic ma- 

 terials, except for two or three beds of ash in the Middle or Upper 

 Eocene. 



In review, the evidence relating to the Tertiary sediments seems 

 to indicate that, from the Wasatch or Lower Eocene epoch until 

 at least the early Oligocene, west-central Wyoming was subject to 

 a semi-arid climate in which there were, however, considerable 

 fluctuations; that streams made widespread deposits in the low- 

 lands; that these were largely of clay, silt, and sand but locally of 

 gravel; and that the wind was a modifying rather than a con- 

 trolling agency in the process; that lakes and marshes existed locally 

 from time to time; and that volcanic eruptions in the western part 

 of the district, and late in the sedimentary interval, scattered ash 

 and cinders over much of the district. No matter what hypothesis 

 of the origin of the Tertiary beds is adopted, it leads to the con- 

 clusion that by the Oligocene epoch the processes of sedimentation 

 had produced one or more broad flat or gently graded plains beneath 

 which were buried deeply the old Eocene hills and many of the lower 

 mountains. There seems to be no means of knowing whether all 

 the ranges were buried by the Tertiary deposits. They now cover 



^ W. J. Sinclair and W. Granger, "Eocene and Oligocene of the Wind River and 

 Bighorn Basins," Bull. Am. Miis. Nat. Hist., XXX (igii), 83-117. 



