POST-CRETACEOUS HISTORY OF WESTERN WYOMING 195 



southeast end of the Wind River Range. Instead of assuming 

 the origin of the plain, it is fitting that we examine the question 

 before proceeding. 



It is apparent that the plateau surface is a cut plain, with 

 residual hills and mountains still standing above it. There appear 

 to be three methods by which extensive cut plains may be produced 

 in the interior of a continent: {a) by the action of ice-sheets;^ 

 (b) by the long-continued action of the wind; (c) by river systems 

 that have endured to old age. 



Fig. 13. — Mountain ridge (11,500 feet) near Gj^sum Creek at the west end of 

 the Wind River Range, showing a syncline in Paleozoic rocks cut off at the level of 

 the peneplain. 



There is so httle in favor of the hypothesis that the plain was 

 produced by ice-sheet glaciation, that it may be quickly dismissed. 

 It will be shown that the plain is one of the oldest topographic 

 features in the district and therefore probably pre-Quaternary. 

 It now carries locally upon its surface deposits of residual soil. 

 There is no evidence of general ice-sheet glaciation in the surround- 

 ing districts. It would have to be assumed that the present axial 

 range has been developed since the ice sheet disappeared, for ice 

 sheets inevitably tend to destroy rather than to make such features, 



'F. E. Wright, "Effects of Glacial Action in Iceland," Bull. Geol. Soc. Am., 



XXI (19 10), 717-30- 



