148 



PHYSICAL GEOLOGY 



rise obliquely from the bottom, since the latter portion of the ice is 

 retarded by friction with the bed. Crevasses when first formed are 



usually separated from 

 one another by rela- 

 tively level surfaces, 

 but since their upper 

 portions are soon wid- 

 ened by melting, the 

 intervening ice often 

 becomes blade-like in 

 its sharpness, so that 

 the surface of the gla- 

 cier presents a maze of 

 sharp ridges. Such a 

 ridge of ice is called a 

 serac (Fig. 131). In 

 crossing a glacier, such 

 as the Mer de Glace, 

 the chief difficulties encountered are these sharp, steep ridges 



Fig. 131. — The Aletsch Glacier, Switzerland. 



of ice. 



The most conspicuous roughness of a glacier's surface develops 

 where there is a sudden change in the slope of the bed (Figs. 132, 133). 

 In a river this would produce a waterfall, and in a glacier it produces 

 an icefall. Such icefalls make travel on a glacier extremely difficult 

 and dangerous. The ice 

 passes over the fall slice 

 by slice, the fall (as in a 

 river) remaining station- 

 ary. Below the fall the 

 blocks heal together, but 

 the resulting surface is 

 extremely rough, although 

 it gradually becomes 

 smoother. 



It is not to be under- 

 stood that a glacier is much fractured in all parts. The absence of 

 cracks on portions of the Aar Glacier is shown by the fact that a 

 pond 20 feet deep and covering 10 acres existed for 24 years and 

 was carried a distance of 600 feet. 



(2) Irregularities Due to Streams and Ice Tables. The surfaces 



Fig. 132. — Longitudinal sections of a glacier 

 showing icefalls formed where the slope of the bed 

 of a glacier increases suddenly. (After Heim.) 



