POST-CRETACEOUS HISTORY OF WESTERN WYOMING 207 



indicate that it was Pliocene. At that time western Wyoming 

 must have been in a state of tectonic rest which permitted the 

 streams or the wind, or both, to reduce nearly all of the district 

 to the condition of a peneplain, interspersed with post-mature 

 mountain masses, situated on the most resistant rocks (Fig. 20). 

 Appropriately we find in the region no sediments of presumptive 



Fig. 20. — Model of a part of the Wind River basin and vicinity as it may have 

 appeared at the completion of the peneplain. 



Miocene or Pliocene age and hence know little of the climate, the 

 life, or the changes of the time. 



Early Quaternary rejuvenation. — Even where now best preserved, 

 the peneplain is trenched by deep valleys, and over a large part of 

 the district it has been destroyed by their growth (Fig. 21). The 

 remnants now stand at elevations (11,000-13,000 feet) so high that 

 no agency of land sculpture seems capable of fashioning them as 

 they stand. Since the graded streams of the district now flow 

 in channels between 5,000 and 8,000 feet above sea-level, the 

 remaining parts of the peneplain are obviously in a vulnerable 



