PHYSIOGRAPHY OF BISHOP CONGLOMERATE 613 



of this for along the eastern face of Little Mountain the peneplain 

 truncates rocks of this age. From long-distance observations 

 west of Green River it is thought that the planed surface will be 

 found to truncate rocks of the Bridger formation along the southern 

 rim of the Bridger basin. Gravel-covered, planed surfaces which 

 may have originated at the same time as those of the Rock 

 Springs region are reported by Woodruff' from the Wind River 

 basin of Wyoming. The evidence there indicates that the plana- 

 tion was not completed until after the deposition of the White 

 River Oligocene. The fossils in the gravels do not definitely 

 determine the age, but the beds lie above various formations from 

 Colorado up through and including Mesaverde, Cretaceous, and 

 Wind River Eocene, younger than Fort Union. They were not 

 seen in contact with White River beds, but deformations which 

 moved the Wind River beds as well as the White River Oligocene, 

 overlying, occurred previous to the deposition of these gravels, 

 hence the inference that they are younger than White River. If 

 these planed surfaces in the Wind River basin are the result of the 

 same period of planation as those of the Rock Springs region, then 

 the date of this is established with little question as later than 

 White River Oligocene. 



In the absence of more definite evidence from the area under 

 discussion we can only assert that the planation took place in the 

 late Tertiary, and that it was doubtless in late Tertiary time that 

 the culmination of the base-leveling process was reached. Further 

 observations in other regions or more extended observations in 

 this region may later make it possible to assign a more definite 

 date for the planation. 



For the present discussion we will assume that planation was 

 in progress during the latter part of the Tertiary, and that it 

 ceased with the beginning of the crustal movements which occurred 

 between the Miocene and the Pliocene; the reader meanwhile 

 bearing in mind that this date is merely provisional pending the 

 discovery of more exact data. 



Climatic conditions at the time of planation. — Planation over 



^ E. G. Woodruff, unpublished data. See U.S. Geological Survey bulletin on 

 the "Coal Beds of the Wind River Basin, Wyoming," soon to be published. 



