284 



PHYSICAL GEOLOGY 



wind work is hindered through lack of suitable tools. Many 

 lofty mountains are being reduced, also, by glaciers. In many 



. other mountains with few 



or no glaciers at present, 

 the striking results of the 

 work of former glaciers 

 maybe seen (p. 219). In 

 many cases neighboring 

 glaciers drove their val- 

 ley sides and cirques back 

 into the mountain m'ass 

 until they were separated 

 only by sharp crests and 

 narrow ridges (Fig. 300). 

 Mountain scenery is due 

 far more to the agents of 

 land sculpture than to the 

 forces of diastrophism. 



The subdued and gentle 

 slopes of the later life of 

 a mountain are worn less 

 rapidly than the steeper 

 slopes of its earlier career, 

 so that its old age is likely 

 to be longer than its youth 

 and maturity combined. 

 All lofty mountains are 

 comparatively young, geologically speaking; if old, they 

 would have been worn low. While very old mountains are 

 low, obviously not all low mountains are old. 



FIG. 300. Serrate mountain peaks due 

 to erosion by valley glaciers. (Trow- 

 bridge.) 



PLATEAUS 



A plateau is a relatively elevated area of comparatively 

 flat land, which is commonly limited on at least one side by 

 an abrupt descent to lower land. While plateaus are usually 



