POST-CRETACEOUS HISTORY OF WESTERN WYOMING 203 



of the same region. If this inference is sound, the peneplain must 

 have been made either just before or just after the Eocene-OHgocene 

 epoch of deposition. We may first consider the hypothesis that the 

 peneplain is part of the pre-Wasatch (Lower Eocene) topography. 



The eroded surface which underlies the Eocene strata is clearly 

 exposed at many points among the ranges of western Wyoming. 

 Where it trunkates the soft Mesozoic strata, this surface is rela- 

 tively fiat, but on the harder rocks nearer the axes of the ranges, 

 it is hilly and even mountainous in relief. Thus in the southwest 

 part of the Kirwin quadrangle, between Double Diamond ranch 

 and Mountain Meadows, the Wind River Eocene beds rest upon 

 an eroded surface which has a visible relief of more than 1,200 feet. 

 Immediately upon it there generally lies a coarse conglomerate 

 (Pinyon) of variable thickness. Northeast of the Gros Ventre 

 Range this reaches a thickness of about 1,000 feet, and on the south 

 side it is apparently even thicker. From these facts we seem com- 

 pelled to infer that there were conspicuous hilly or mountainous 

 tracts when the early Wasatch deposits were laid down, and 

 that streams later leveled up the lesser inequalities with their 

 deposits. 



As a second test of the hypothesis of Eocene age, attention 

 should be turned to the relation between the peneplain and the 

 Eocene formations, for if those relations are clear they may decide 

 the age of the plain. The Eocene deposits now generally lie at 

 much lower elevations than the existing remnants of the peneplain, 

 and the basal contact in some places is more than 6,000 feet below 

 it. I can conceive of but three ways (Fig. 19) in which this could 

 be brought about: {a) the Eocene sediments may represent the 

 filling of Eocene valleys excavated in the peneplain; (b) they may 

 be remnants of once horizontal deposits laid down upon the pene- 

 plain and now preserved by down- warping or down-faulting; or 

 (c) they may be bodies of sediment, either deposited in earlier 

 depressions or warped or faulted down, upon the surface of which, 

 together with adjacent formations, the peneplain has subsequently 

 developed. In the first two cases the age of the peneplain would 

 be pre-Wasatch. In the last case it would be post-Oligocene. 

 These three possibilities will now be examined briefly. 



