THE WORK OF SNOW AND ICE 107 



The center of a valley glacier is a little higher thanits sides in most 

 cases and its surface may be smooth or rough. The causes of rough- 

 ness are several. 1 . In many cases the ice is cracked, and the cracks, 

 or crevasses, frequently gape. One cause of the crevasses is the 



Fig. 110. Aletsch glacier, Switzerland. 



movement of the brittle ice over an uneven bed (Fig. 2, PI. XXX, 

 p. 101). Crevasses formed in this way usually run across the glacier 

 from side to side. Some glaciers have crevasses parallel to their 

 sides or oblique to them, and such crevasses are due to other causes. 

 The breaking of the ice as it moves is one of the many ways in which 

 a glacier differs from a river. 



2. Valley glaciers often extend far below the snow-line, and 

 their lower ends are within the region of active melting during the 



