358 THE GREAT ICE AGE 



is discharged at the lower end, accumulating there in the mass 

 of detritus known as the terminal moraine. 



Glaciers have been termed rivers of ice ; but there is one 

 respect in which they differ remarkably from rivers. They are 

 broad above and narrow below, or rather, their width above 

 corresponds to the drainage area of a river. This is well seen 

 in a map of the Mer de Glace. From its termination in the 

 Glacier du Bois to the top of the Mer de Glace proper, a dis- 

 tance of about three and a half miles, its breadth does not ex- 

 ceed half a mile, but above this point it spreads out into three 

 great glaciers, the Geant, the Du Chaud, and the Talefre, the 

 aggregate width of which is six or seven miles. The snow and 

 ice of a large interior tableland or series of wide valleys are 

 thus emptied into one narrow ravine, and pour their whole 

 accumulations through the Mer de Glace. Leaving, however, 

 the many interesting phenomena connected with the motion of 

 glaciers, and which have been so well interpreted by Saussure, 

 Agassiz, Forbes, Hopkins, Tyndall, and others, we may con- 

 sider their effects on the mountain valleys in which they 

 operate. 



1. They carry quantities of debris from the hill tops and 

 mountain valleys downward into the plains. From every peak, 

 cliff and ridge the frost and thaw are constantly loosening 

 stones and other matters which are swept by avalanches to the 

 surface of the glacier, and constitute lateral moraines. When 

 two or more glaciers unite into one, these become medial 

 moraines, and at length are spread over and through the whole 

 mass of the ice. Eventually all this material, including stones 

 of immense size, as well as fine sand and mud, is deposited in 

 the terminal moraine, or carried off by the streams. 



2. They are mills for grinding and triturating rock. The 

 pieces of rock in the moraine are, in the course of their move- 

 ment, crushed against one another and the sides of the valley, 

 and are cracked and ground as if in a crushing mill. Further 



