POST-CRETACEOUS HISTORY OF WESTERN WYOMING 205 



Eocene sediments. On the other hand, the remnants of the pene- 

 plain must have been preserved on a large scale from the early 

 Eocene to the present. Without convincing evidence in its favor 

 this is not, therefore, an attractive hypothesis. 



On the supposition that the surface we now find beneath the 

 Eocene sediments is part of the old peneplain, which has merely 

 been warped and partly uncovered in subsequent time, we should 

 anticipate that the softer Paleozoic and especially the Mesozoic 

 strata would be reduced to base-level long before the massive 

 granitoid rocks of the Archean were worn low. Nevertheless, 

 there is abundant evidence that the basal Eocene surface was only 

 maturely hilly upon the outcrops of the Paleozoic and, in some 

 places, of even the Mesozoic strata. That surface was a plain 

 only on the weakest beds. Again, if we are to ascribe the preserva- 

 tion of the Eocene to down-warping after it was deposited upon 

 the peneplain, we should rightly expect to find the Eocene strata 

 parallel to the base on which they rest. On the contrary, in many 

 places they grade horizontally into thick beds of conglomerate 

 which themselves rest against a steeply inclined surface of the 

 older rocks, thus indicating that the beds were actually deposited 

 in a valley or basin and not upon a plain. 



To the suggestion that the peneplain is younger than the Eocene 

 sediments, and has beveled them as well as the older rocks, I can 

 find no objection. It is free from the difficulty occasioned by the 

 Miocene deformation of the entire district. It explains the absence 

 of Eocene remnants upon the peneplain and does not conflict with 

 the existence of the rugged basal Eocene contact overlain by hori- 

 zontal beds. It also corresponds with determinations in other 

 parts of western United States, that the principal peneplains are 

 not older than the Miocene. I therefore accept, for the present, the 

 conclusion that the peneplain is younger than the early Tertiary 

 strata, and later than the mid-Tertiary deformation that expressed 

 itself in warping and faulting. 



In subsequent pages it will be shown that a number of events 

 requiring a relatively long time must be referred to the Quaternary 

 period, and there is so much good evidence all over the western 

 states that the opening of that period was characterized by 



