MOKAIXES. 



55 



its tributaries shows clearly the manner in which these moraines are 



formed. Both lateral and medial moraines are generally situated on a 



ridge of ice, sometimes fifty to eighty feet high, evidently formed by 



the protection of the ice, in this 



part, from the melting power of 



the sun. The fragments of rock 



brought down by glaciers are 



often of enormous size. One 



described by Forbes contained 



244,000 cubic feet. 



The ground moraine is the 

 mass of debris carried between 

 the glacier and its bed. It is 

 derived partly from erosion of 

 the bed, and partly from top ma- 

 terial (lateral and medial mo- 

 raines) ingulfed and carried 

 down to the bottom. 



Everything which falls upon 

 the surface of the glacier is slow- 

 ly and silently carried downward 

 by this ice-stream, and finally 

 dropped at its point. Much 

 finely-triturated matter is also 

 pushed along beneath the gla- 

 cier, and finds its way to the 

 same point. In the course of 

 time an immense accumulation 

 is formed, of somewhat cres- 

 centic shape, as seen in Fig. 44. 



This accumulation is called the terminal moraine. It is the delta 

 of this ice-river. The existence of moraines is a constant witness of 

 the motion of the glaciers. 



Glaciers as a Geological Agent. 



Glaciers, like rivers, erode the surface over which they move, carry 

 the materials gathered in their course often to great distances, and 

 finally deposit them. In all these respects, however, the effects of their 

 action are perfectly characteristic. 



Erosion. — When we consider the weight of glaciers and their un- 

 yielding nature as compared with water, it is easy to see that their 

 erosive power must be very great. This is increased immensely by 

 fragments of stone of every conceivable size carried along between the 

 glacier and its bed. These partly fall in at the sides and become 



Fig. 44.— Mer de Glace. 



