POST-CRETACEOUS HISTORY OF WESTERN WYOMING 21 1 



cipal water drainage systems — the Snake, the Green, and the 

 Bighorn. Glaciation and eolation have merely modified the effects 

 of erosion — the one locally, the other generally. 



Before proceeding to describe the effects of stream denuda- 

 tion in the district, attention is called to the fact that the surface 

 is underlain by a great variety of rocks which differ vastly in the 

 resistance which they offer to erosive processes. Under a subarid 



Fig. 24. — Plane surfaces developed upon the Wind River Eocene beds on the 

 flanks of Black Mountain, in the Wind River basin. 



climate, the Tertiary sandy clays are the weakest rocks; the mas- 

 sive Archean gneisses and granites stand near the other extreme 

 of resistance. Between them all gradations exist. It is evident 

 that in the softest of these materials, erosion proceeds with great 

 rapidity in contrast with its progress upon the most refractory 

 formations. This fact (in addition to the structure) is the key to 

 the origin of the principal topographic features of the region (Fig. 23) . 

 In the notably weak beds of Tertiary and Cretaceous age, 

 broad plains have been developed at elevations several thousand 

 feet below the surrounding highlands (Fig. 24). Good examples 



