2IO 



ELIOT BLACKW ELDER 



Local evidence alone is not sufficient to permit a close determina- 

 tion of the age of this elevatory movement. Obviously it followed 

 the development of the Wind River peneplain, which the avail- 

 able evidence indicates occurred in the late Miocene and Pliocene. 

 In later pages it will appear that there have been several 

 changes of level separated by long intervals of quiet. All, however, 

 seem to have occurred before the last stage of glaciation. It is 

 significant that many of the geologists who have studied the 

 western mountain states have recently concluded that the begin- 

 ning of Quaternary time was marked by important changes of 

 level, to which the present relief and ruggedness of our western 



Fig. 23. — Diagram illustrating the progress of the same erosion cycle in rocks 

 of three different degrees of hardness : {A ) residual areas of older surface with canyons 

 in granitic complex; {B) maturely mountainous surface on Paleozoic limestones, etc.; 

 (C) plain on Tertiary clays: (Z?) topography of granitic mountains more advanced 

 5han at A because of proximity to deep basin. 



mountains is in large measure due. These considerations lead to 

 the somewhat indefinite conclusion that the several changes of 

 level occurred between the late Pliocene and the close of the Pleisto- 

 cene epoch. 



General effects of erosion in the Quaternary period. — On every 

 hand there are deep canyons, wide river basins, and other speaking 

 evidences of denudation in geologically recent times. It is reason- 

 able to believe that the district has been subject to erosion under 

 slowly varying conditions and hence with varying results ever since 

 the beginning of the uplift which seems to have accompanied the 

 close of the Tertiary period. Although I am not disposed to belittle 

 the efficiency of the wind in fashioning topographic details, nor in 

 deporting dust from the region and in that way gradually lowering 

 its elevation, it is clear from their forms that nearly all the coarser 

 and many of the finer topographic features of western Wyoming 

 have been carved by running water, and that in general the topog- 

 raphy of the district has been under the control of the three prin- 



