256 GEOLOGY 



points of rock over which it has passed; (2) smaller pieces of rock 

 of the size of cobbles, pebbles, etc., either picked up by the ice from 

 its bed or broken off from larger masses; (3) the fine products 

 (rock-flour) produced by the grinding of the debris in the ice on the 

 rock-bed over which it passes, 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 materials which the ice carries (called drift) are of all 



Fig. 208. Figure showing contrast between a glaciated rock surface below, 

 and non-glaciated crests above. Kearsarge Pinnacles, Bubbs Creek 

 Canyon, Cal. 



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 predomi- 

 nates 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 

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

 V-shaped valley (Fig. 206) U-shaped (Fig. 207). The change in 

 topography at the upper limit of glaciation is often marked (Fig. 

 208). 



The deepening of a valley by glacial erosion may throw its 



