258 



The Realm of Nature 



CHAP. 



Some change in the channel alters the stresses, and as time 

 goes on the old crevasses close up and new ones open. 



FIG. 54. Map of a Glacier showing the formation of medial moraines, by 

 union of tributary glaciers. The arrows show the direction of flow, and the 

 lines radiating from the edges represent crevasses. 



The regions where glaciers occur are coloured dark blue on 

 Plate VII. 



338. Glacial Work. Glaciers work both by transport- 

 ing the moraine material that falls upon them and by 

 powerfully eroding the ground they pass over. Moraine 

 rubbish falling down crevasses gets wedged in the ice, 

 which presses the angular stones firmly against the bed-rock 

 as the glacier slides forward, the action exactly resembling 

 the cutting of glass by a diamond. Immense quantities of 

 sand and clay result from the grinding down of rock and 

 stones, and are carried along the bed of the glacier, forming 

 the ground or bottom moraine or boulder clay. When the 

 climate of a glacial region grows warmer, as the Alpine district 

 has been doing for the last twenty years, the glaciers melt 

 away at the lower end, which shrinks up the valleys, while the 

 boulders which may have been carried far by the ice are 

 deposited on the slopes amongst rocks of an entirely 

 different nature, and sometimes in very precarious positions. 

 Such travelled and perched blocks are called erratics, or 

 simply boulders. The rocks of the valley uncovered by the 

 ice are seen to be deeply grooved or striated by the stones 

 dragged over them, the run of the striae showing the direc- 

 tion in which the glacier was moving. The surface scratched 



