THE WORK OF GLACIERS 1 59 



Since much of the rock of the valley floor over which a glacier moves 

 as well as the pebbles which it holds in its grasp are ground to powder, 

 it is not surprising to find the water of glacial streams so turbid with 

 sediment as to be spoken of as glacier milk. The light color of such 

 streams differs from the yellow color of ordinary streams because the 

 former carry freshly ground, unoxidized rock, and the latter the prod- 

 ucts of weathering. The Aar Glacier, for example, a comparatively 

 small glacier of the Alps, is estimated to discharge 280 tons of rock 

 flour a day during a summer month. 



Factors Influencing the Rate of Erosion. — As has been said, 

 glaciers accomplish little erosion in passing over smooth surfaces. 

 The amount of morainic material carried by Greenland glaciers, for 

 example, is surprisingly small. This is due to the fact that they have 

 moved over their beds so long as to render them comparatively 

 smooth. If, however, a glacier moves over a bed whose surface is 

 sufficiently rough to permit the ice to tear away fragments by pluck- 

 ing (p, 157), it is likely to deepen its bed rapidly, since under these 

 conditions a large surface is exposed to the wear of the debris held in 

 the base of the ice. Erosion is also favored by the incoherence of the 

 material over which the ice moves, by the weight of the ice, and by a 

 rapid rate of movement ; but is unfavorably affected by an over- 

 loading of the basal portion of the glacier with debris and" By the 

 resistance of the rock. 



Deposits of Mountain Glaciers 



Terminal Moraines. — At the lower end of a glacier, where the 

 melting equals, or nearly equals, the advance, all of the debris (the 

 superglacial, englacial, and subglacial) is deposited as a terminal 

 moraine. Terminal moraines (Fig. 145) are usually crescent or 

 horseshoe-shaped, concave towards the glacier, and often form con- 

 spicuous hills in valleys once occupied by glaciers. The heights of 

 terminal moraines vary greatly, since the quantity of material de- 

 posited in them depends upon a number of factors. (1) The length 

 of time during which the front of a glacier remained stationary is 

 important. If a glacier advances 600 feet a year and for a number of 

 years melts back at the same rate, it is evident that each year all of 

 the debris carried on, in, and under it will be left at the same place, 

 with the exception of that which is carried away by the stream which 

 flows from it. If, on the other hand, the ice melts back 600 feet a year, 



CLELAND GEOL. — II 



