EXTINCT AND EXISTING GLACIERS OF COLORADO 43 



of the shallow lakes freeze to the bottom, and the snow by drifting 

 builds a steep snow-drift extending far out into the lake. Some melting 

 goes on throughout the winter, the resulting water percolating into 

 the snow and freezing, so that in the early summer the bank of snow 

 is thoroughly consoHdated. The lake exposes a flat surface to the 

 summer sun and receives its full midday rays, while the steep ice- 

 bank receives the sun's rays at a low angle during the hottest part of 

 the day. Hence, while the ice-bank remains unmelted the lake ice 

 is melted, the water is raised considerably above the freezing point 

 and undermines the ice which projects out into it, by melting and by 

 wave erosion, until it breaks off. In case of neve the breaking is 

 often perhaps facilitated by former crevasses which have not been 

 well healed, so that in such cases the motion has some influence upon 

 the production of the bergs, but even in such cases the bergs would 

 often form without crevasses. 



Phenomena of Glaciated Regions 



For the recognition of the work of extinct glaciers there are other 

 phenomena. Among the principal ones applicable to our mountain 

 region are the following: 



U-shaped Valleys. — The typical valley caused by stream erosion 

 is V-shaped. Glaciers did not create our glaciated canyons, but 

 greatly modified pre-existing stream-cut canyons.^ In case of the 

 stream, lateral erosion is going on all the while that the stream is 

 cutting deeper. Alternate freezing and thawing break down the 

 cHffs on both sides and furnish the stream with a large quantity of 

 material which it must remove, thus hampering its work of deepening 

 the channel. Hence, the valley is usually widened at the top more 

 rapidly than it is deepened. When a glacier fills the valley, the side 

 walls are protected from alternate freezing and thawing by the body 

 of ice, which preserves a fairly uniform freezing temperature. At the 

 same time the whole mass of the ice is scouring away the rocks. The 

 greatest weight rests along the sides of the canyon at the bottom, so 



' Endlich, F. M., "Report on the Geology of the White River District," Tenth Ann. Rept. U.S. Geol. and 

 Geog. Sum. Terr., for 1876, p. ii6, 1878; Cross, Whitman, and Howe, Ernest, "Geography and General 

 Geology of the Quadrangle," Silverton Folio No. 120, Geologic Atlas 0/ the United States, U.S. Geol. Surv., p. 24, 

 190S. 



