POST-CRETACEOUS HISTORY OF WESTERN WYOMING 105 



ontologically they are rather closely related to the underlying Cre- 

 taceous. Following current usage in referring these formations to 

 the base of the Tertiary system, it appears that the principal 

 deformation occurred after the laying-down of the Paleocene series, 

 although in some parts of the Rocky Mountains an important oro- 

 genic movement had preceded the Paleocene. 



The oldest strata which were not affected by this particular 

 episode of deformation within the limits of the district belong to the 

 early Tertiary series, represented east and southeast of Jackson 

 Hole by the Pinyon conglomerate, in Green River valley by the 

 Green River formation and associated strata, along Wind River 

 by the Wind River formation, and in the Bighorn basin by the 

 so-called "Wasatch" beds. Mammalian and plant fossils found 

 at various points in these districts are said to prove that the early 

 Tertiary strata are Eocene and that the sequence generally, al- 

 though not everywhere, begins with the Lower Eocene or Wasatch 

 stage. This is in harmony with conditions generally throughout 

 the Rocky Mountains. 



The old familiar fact is thus redetermined — that the folding 

 took place between the close of the Cretaceous and the deposition 

 of the Lower Eocene. Assuming that the strata in the canyon of 

 Buffalo Fork, mentioned above, have been correctly referred to the 

 Fort Union formation, it is suggested as probable that the most 

 vigorous folding took place between the Paleocene and Eocene 

 epochs. 



Early Eocene erosion. — So nice is the adjustment of stream activ- 

 ities that no sooner did the post-Cretaceous folds commence to be 

 bulged above the grade-level of their time than they were subject 

 to erosion. The fact that they were thus eroded is amply attested 

 by the unconformity at the base of the Eocene (Wasatch, etc.) 

 strata. Since the latter now rest upon the trunkated edges of any 

 and all of the Paleozoic and Mesozoic formations, and in some 

 places even upon the Archean, it is evident that the post-Cretaceous 

 folds were completely trunkated during the early part of the Eocene 

 epoch. 



Whether this resulted at any time in bringing the entire region 

 to the condition of a peneplain is still an open question. The 



