POST-CRETAC^OUS HISTORY OF WESTERN WYOMING 339 



are apparently subject to the removal of dust rather than to its 

 accumulation. 



Sand dunes, which might be expected in the driest parts of the 

 district, appear to be rare, although a few were observed along the 

 middle course of Green River. This tends to confirm the view 

 that in this region the dominant phase of eolian activity is erosive. 



SUMMARY OF CONCLUSIONS 



Divested of details and quahfications, the history of the dis- 

 trict may now be reviewed in general terms. About the close of 

 the Cretaceous period, the entire region was probably a monoto- 

 nous plain underlain by unconsolidated sediments. Soon there- 

 after, tangential forces within the earth's crust produced a series 

 of moderate folds, trending northwest and southeast. The process 

 of folding tended to produce a series of mountain ranges and basins 

 corresponding to the anticHnes and synclines; but the uplifts were 

 certainly being eroded even while they were growing, and after 

 the disturbance ceased they continued to be worn down until the 

 surface became a well-dissected mountainous or hilly region inter- 

 spersed with wide, relatively flat basins. 



In the Wasatch or Lower Eocene epoch, terrestrial sediments 

 began to accumulate in the lowlands, probably in response to mild 

 warping movements and perhaps cHmatic changes, which disturbed 

 the activities of the previously graded streams. These sediments 

 were distributed chiefly by rivers, but doubtless in part by the wind, 

 and some seem to have lodged in lakes and playas. Sedimentation 

 thus continued with but little interruption under a rather warm, 

 subarid continental climate until some time after the early Oli- 

 gocene. Early in this sedimentary cycle there were volcanic 

 eruptions in the western part of the district, during which acidic 

 lava flows and occasional beds of ash were deposited; but the much 

 more extensive eruptions of the Absaroka Range took place later. 

 The grassy plains of the time were occupied by ancestral horses, 

 cloven-hoofed ungulates, rodents, gigantic titanotheres, and their 

 appropriate carnivorous enemies. 



Somewhat later, probably near the middle of the Miocene 

 epoch, the district was notably warped and locally faulted, in part 



