264 



GEOLOGY. 



gaping crevices on many glaciers show. The crevasses are sometimes 

 longitudinal, sometimes transverse, and sometimes oblique. In the 

 case of Arctic glaciers, longitudinal crevassing is especially conspicuous. 



Fig. 238. — Cracking of glacier due to change in grade of bed. A North Greenland 

 glacier overriding a mound of moraine-stuff. 



Crevasses appear to be developed wherever there is appreciable 

 tension, and the causes of this tension are many. An obvious cause 

 is an abrupt increase of gradient in the bed (Fig. 238). If the change 

 of gradient be considerable, an ice-fall or cascade results, and the ice 

 may be greatly riven (Fig. 228). Below the cascade, the surface may 

 bristle with wedges and pinnacles of ice (seracs, Fig. 239). Transverse 

 crevices at the margin sometimes appear to be the result of the tension 

 developed on a curve. ObHque crevices on the surface near the sides 

 are commonly ascribed to the tension between the faster-moving cen- 

 ter and the slower-moving margins, and in like manner crevasses that 

 rise obhquely from the bottoms are attributed to the tension between 

 the faster-moving portions above and the slower-moving portions below. 

 All these crevasses indicate strains to which a hquid, whose pressures 

 are equal in all directions, does not offer a close analogy. Fongitudinal 



