THE WORK OF SNOW AND ICE 121 



the glacier are steep, rocks may fall or slide down to the surface of 

 the ice, or be brought down by slides of snow (avalanches) . Some 



Fig. 127. Diagram illustrating one way in which a glacier gets material up 

 above its bottom. 



of the debris on the ice comes up through the ice, as illustrated by 

 Fig. 2, PL XXXIV, p. 113. 



If the debris which reaches a glacier from the cliffs above lodges 

 along its margin, it makes a lateral moraine (p. Ill), and if two 

 glaciers bearing lateral moraines unite, as sometimes happens, the 

 two lateral moraines of the sides which come together may form a 

 single medial moraine (Fig. 113). But some medial and lateral 

 moraines arise in other ways. 



Deposition 



Some of the debris which the ice carries in its bottom lodges 

 on the surface beneath while the ice is in motion. That which is 

 deposited at one time may be taken up and carried on again later. 

 In this respect, deposition by glaciers is somewhat like the deposi- 

 tion of streams.- The material deposited by glaciers is called 

 glacial drift. 



If the ice at the end of a glacier moves forward two feet a day, 

 the end of the glacier would advance two feet if none of the ice 

 melted. If the ice of the glacier moves forward two feet a day, and 

 if, at the same time, two feet of ice at the end are melted each 

 day, the end of the glacier does not advance, even though the ice 

 keeps moving. When the end of a valley glacier or the edge of an 

 ice-cap stays in the same place for a long time, a thick body of drift 

 is lodged beneath it (Fig. 3, PL XXXIV and Fig. 128), for drift is 

 continually brought to this position by the on-coming ice, and left 

 there. 



The thick drift accumulated beneath the end of a valley glacier, 



