THE WORK OF SNOW AND ICE, 



287 



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 



Fig. 257. — A glacial cirque. The lake occupies a rock basin, produced by glacier 

 erosion. Head of Little Timber Creek, Montana. 



from the surface overridden. Thus the materials which the ice carries 

 {drift) are of all grades of coarseness and fineness, from large bowlders 

 to fine clay. The coarser material may be angular or round at the 

 outset, and its form may be changed and its surface striated as it is 

 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 and widens its bottom and smooths 

 its slopes up to the upper limit of the ice. It tends to change a Y" 

 shaped valley (Fig. 258) into a U-shaped one (Fig. 259). The change 

 in topography at the upper limit of glaciation is often marked (Figs. 260 

 and 261). 



