GLACIERS 



221 



their sides. Thus V-shaped valleys are changed to U-- 

 shaped troughs (Figs. 234 and 235, Plate XIII). The en- 

 larged heads of glaciated valleys have broad bottoms, often 

 containing ice-worn rock basins, and high, precipitous walls. 

 Such valley heads are called cirques (Figs. 236 and 237, Plate 

 XIII) . In winter the 

 neve and ice of the 

 upper glacier freezes 

 to the valley walls. 

 In spring, the ice 

 pulls away from them 

 and dislodges and 

 carries with it many 

 rock fragments. 

 During the summer 

 the walls of the val- 

 ley head may be 

 more or less exposed 

 to the agents of 

 weathering, and ma- 

 terial prepared for 

 later removal by the 

 ice. This process 

 helps to drive the 

 sides and head of 

 the valley back into 

 steep cliffs. 



Lakes dot the bot- 

 toms of most glaci- 

 ated valleys (Fig. 

 238, Plates X and XIII). Some of them occupy rock basins 

 gouged out by the glacier (Fig. 239), and others fill depres- 

 sions on the up-valley sides of morainic dams (Fig. 240). 



Tributary valleys normally join their main valleys at even 

 grade. But main valleys are often deepened by glaciers 

 more than their tributaries. Because of this, and because 



FIG. 234. Glacial trough near Green River 

 lakes, Wind River Range, Wyoming. Shows 

 contrast between glaciated topography be- 

 low, and unglaciated topography above. 

 The lake in the foreground is held in by a 

 morainic dam. (Baker.) 



