206 DESCRIPTIVE GEOLOGY. 



previously existing topographical features. Some idea may be liad, then, of 

 the magnitude of the erosion which has taken place in this region since 

 Pliocene times, of which the Wyoming Conglomerate is the latest represent- 

 ative, when we consider that the bed of the stream which represented Green 

 River must have been at a level corresponding to a present elevation of 

 between 8,000 and 9,000 feet above sea-level, and that a thickness of from 

 1,000 to 2,000 feet has been gradually removed from a great part of the 

 area of the Green River Basin, and earned away by its waters through the 

 ever-deepening canons. The bulk of this material must have been carried 

 away by the floods which followed the Glacial period, while at the present 

 day, under conditions of comparatively slight precipitation, the amount 

 removed from the basin region by the actual agency of water is relatively 

 slight. From this period also dates the present configuration of the mount- 

 ain summits ; the shallow amphitheatre-shaped valleys between the higher 

 peaks were carved out by n^vd-i^e, while the shapes of Jhe canons of the 

 principal streams, and the moraines found at their mouths, show that they 

 were cut nearly to their present depth by glaciers. As indicated by the 

 moraines, which have remained to the present day, these glaciers were, in 

 many cases, over 25 miles in length, and extended down to a level of but 

 little over 6,000 feet above sea-level. 



