MOVEMENT OF GLACIERS 135 



and other likenesses, some of which are apparent rather than real, 

 gave rise to the view that glacier ice moves like a stiff, viscous 

 liquid. 



Hut while the points of likeness between glaciers and rivers are 

 several, their differences are numerous and significant. The most 

 ol>\ious difference is the fact that the glacier is fractured readily, 

 as the numerous gaping crevasses on many glaciers show. Some 

 of the crevasses are longitudinal, some are transverse, and some 

 are oblique. In the case of arctic glaciers, longitudinal crev- 





Kii;. 137. Crevassed glacier, the cracking due to change in grade of bed. 

 North Greenland. 



assing is especially conspicuous. Crevasses appear to be de- 

 veloped wherever there is appreciable tension, and the causes 

 of tension are many. An obvious cause is an abrupt increase of 

 gradient in the bed (Fig. 137). If the change of gradient is con- 

 siderable, an ice-fall or cascade results, and the ice may be greatly 

 riven. Some of the transverse crevasses at the margins appear 

 to be the result of tension developed on curves. Oblique crevasses 

 on the surface near the sides are commonly ascribed to the tension 

 between the faster-moving center and the slower-moving margins, 

 and in like manner cracks that rise obliquely from the bottom are 

 attributed to the tension between the faster-moving parts above 

 and the slower-moving parts below. All crevasses indicate strains. 

 Liquids, whose pressures are equal in all directions, show nothing 

 analogous to crevassing. Longitudinal crevasses may affect both 

 the narrow part of a glacier and its deploying end, and are the result 

 of tension developed by movement within the ice itself, to which, 



