374 



TRANSPORTATION OF ROCKS BY GLACIERS. 



[Ch. XVI. 



o 



can permit of a differential motion of its parts, without at 



same 



The agency of glaciers in producing permanent geological 

 changes consists partly in their power of transporting gravel, 

 sand, and hnge stones to great distances, and partly in the 



smoothin 



r, polishing, and scoring of their rocky channels, 

 and the boundary walls of the valleys through which they 



pass 



At the foot of every steep cliff or precipice in high 

 Alpine regions, a talus is seen of rocky fragments detached 

 by the alternate action of frost and thaw. If these loose 



masses 



move 



place of a single heap they will form in the course of years a 

 long stream of blocks. If a glacier be 20 miles long, and its 

 annual progression about 500 feet, it will require about two 

 centuries for a block thus lodged upon its surface to travel 

 down from the higher to the lower regions, or to the extre- 

 mity of the icy mass. This terminal point remains usually 

 unchanged from year to year, although every part of the ice 



motion 



m 



may be compared to an endless hie o± soldiers, pouring into 

 a breach, and shot dow T n as fast as they advance. 



The stones carried along on the ice are called in Switzer- 



* of the glacier. There is always one line 



moraines 



of blocks on each side or edge of the icy stream, and often 



middle 



mounds 



The 



reason of their projecting above the general level is the non- 

 liquefaction of the ice in those parts of the surface of the 



from 



the 



action of the wind, by the covering of earth, sand and stones. 

 (See fig. 22, p. 368.) The cause of ' medial moraines' was 

 first explained by Agassiz, who referred them to the conflu- 

 ence of tributary glaciers.* Upon the union of two streams 

 of ice, the right lateral moraine of one of the streams comes 

 in contact with the left lateral moraine of the other, and 





in 



* Etudes sur les Glaciers, 1840. 



