458 HISTORICAL GEOLOGY 



the last ice sheet left its deposits there are lakes, marshes, 

 aimless rivers, waterfalls, and scattered bowlders. Elsewhere, 

 lakes and marshes are confined largely to the river bottoms 

 and seashores ; waterfalls are few ; the rivers are grouped in 

 branching, treelike systems ; bowlders from distant regions are 

 not to be found ; and the hill soils are chiefly residual. 



Valley glaciers in the mountains. In the mountains 

 to-day there are small valley glaciers wherever there is suffi- 

 cient cold and snowfall. In the Glacial epoch these were 

 larger than now and vastly more numerous. Only the lower 

 ranges in western United States were free from them. It is 

 easy to identify the places where these alpine glaciers have 

 been at work, long after they have passed away, for they 

 not only scoured and striated the valley floors, but made 

 the original valleys U-shaped, sharpened the mountain 

 peaks into crags and pinnacles, and built loop-shaped mo- 

 rainic ridges farther down the valleys. Along the aban- 

 doned valleys many lakes now testify to the work of the 

 ice. The wild scenery of the high, snowy ranges to-day 

 is due largely to the sculpturing by Quaternary glaciers. 



Great Quaternary lakes of Utah and Nevada. The basin 

 which lies between the Rocky Mountains and the Sierra 

 Nevada is now arid, and most of the rivers flowing into it 

 dwindle away in the desert soils, or feed salt lakes from which 

 no streams flow out. During the Glacial epoch, all of these 

 lakes were much larger than now. Great Salt Lake in Utah 

 is only a remnant of a lake, called Bonneville (Fig. 476), 

 which was two thirds as large as Lake Superior and one 

 thousand feet deep. The former existence of this great lake 

 is shown plainly by the series of cliffs and terraces which 

 parallel the slopes of the adjacent mountains (Fig. 477). 

 These terraces were made by the waves on the lake. At 

 that time, Lake Bonneville overflowed northward into the 

 Snake River. In lakes with outlets the water is continually 

 being changed and so is not allowed to become salty. Bonne- 

 ville was therefore a fresh lake. In western Nevada a series 



