EROSION BY GLACIERS 151 



bed over which it passe^, and similar products resulting from the 

 rubbing of stones in the ice against one another; and (4) sand, clay, 

 soil, vegetation, etc., derived from the surface overridden. Thus 

 the mutt-rials which the ice carries (called drift] are of all grades 

 of coarseness and fineness, from huge bowlders to fine clay. The 

 coarser materials may be angular or round at the outset, and their 

 forms may be changed and their surfaces striated as they are moved 

 forward. Whether one sort of material or another predominates 

 depends primarily on the nature of the surface overridden. 



The topographic effects of glacial erosion. In passing through 

 its valley, an alpine glacier deepens it, widens its lower part, and 

 smoothes its slopes up to the limit of the ice. It tends to make a 



I-'ii;. 14 j. A UK unit ain valU-y which li;is been strongly glaciated, Wasatch 

 Mountains. (Photo, liv Church.) 



Y-shaped valley (Fig. 148) U-shaped (Fig. 149), and to make its 

 head hi::, blunt, and steep-sided. Such a valley head is a cirque 

 (PI. XIII). The change in topography at the upper limit of 

 glat iation is striking in many places (Fig. 150). 



The deepening of a valley by glacial erosion may throw its 

 tributaries out of topographic adjustment. Thus if a main valley 

 is lowered 100 feet by glacial erosion while its tributary is not 

 deepened, the lower end of the latter will be 100 feet above the 

 former when the ice disappears. Such valleys, called hanging val- 

 leys (Fig. 151), are common in the western mountains of North 

 America which were recently glaciated. 



