Studies for Students. 



SUPERGLACIAL DRIFT. 



I. ALPINE GLACIERS. 



Lateral moraines. — On the surface of alpine glaciers, there is 

 sometimes an abundance of stony material which takes the form 

 of lateral moraines. The material composing these moraines is 

 derived principally from the slopes above the ice. In its acquisi- 

 tion, the ice is for the most part passive. Alpine glaciers occupy 

 the bottoms of valleys. So far as general topography is con- 

 cerned, the valley in which a glacier lies may be said to have an 

 ice bottom. From the slopes of the ice-bottomed valley, rock 

 masses, large and small, descend. They may be loosened by 

 the expansion and contraction due to rapid changes of tempera- 

 ture, by the wedge-work of ice forming in the crevices of the 

 rock, or by the growth of roots in the same position. Once 

 loosened, the blocks of rock begin their journey of descent. 

 This may be accomplished rapidly or slowly, depending upon 

 the steepness of the slopes, and other local conditions. Descend- 

 ing the slopes, the loose masses of rock may reach the bottom 

 of the valley, that is, the surface of the glacier. Avalanches 

 which sometimes descend the steep slopes of valleys, are likely 

 to carry down considerable quantities of stony or earthy material. 

 Where avalanches reach the surface of glaciers, the stony material 

 they bear is deposited on the glacier near its lateral margin. A 

 similar result may be effected by landslides. Locally the amount 

 of material carried down by avalanches and landslides may be con- 

 siderable but on the whole it is not great. The occasional 

 torrents which come into existence during rain storms, or during 

 the season when the snow of the higher mountains is being 

 rapidly melted, descending from the slopes above to the ice 

 below, carry larger or smaller quantities of rock material. 



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