394 



HISTORICAL GEOLOGY. 



is certain that the whole Sierra was at that time covered 

 with perpetual snow, from which ran great glaciers forty 

 to fifty miles long to the valleys below. It is certain that 

 all the valleys and canons which trench the flanks of the 

 Sierra were filled with glaciers of enormous size. Many 



FIG. 349. Glaciated surface and scattered bowlders near Lake Tenaya, Cal. 

 (From a photograph by J. N. Le Conte.) 



of these have been traced in the clearest manner by their 

 polished pathways, their scattered bowlders, and their 

 lateral and terminal moraines (Fig. 349). 



Lakes. All the lakes of that time, especially in the 

 Basin region, were greatly enlarged. About Lake Mono, 

 terraces rise, one above another, to 700 feet above the 

 present lake-level, and inclosing an immense area. The 

 lake-waters then washed against the foot of the Sierra, 

 and glaciers ran into its waters and produced icebergs. 

 At the same time, the whole lower part of the Utah and 

 Nevada basins was filled each with a great lake. That 

 which filled the Utah basin, called Lake Bonneville, was 



