144 



WORK OF SNOW AND ICE 



Fig. 144. Portion of the east face of Bowdoin Glacier, North Greenland, show- 

 ing oblique upward thrust, with shear. 



debris. It perhaps occurs also at the top of the basal zone of ice so loaded with 

 debris that it is incapable of ready movement 1 . 



It is probable, also, that sharp differential strains and shearing are developed 

 at the level where the surface water of the warm season, sinking into the ice, reaches 

 the zone of freezing; for the expansion which attends the freezing may cause the 

 expanding layer to shear over the part below. As the level of freezing descends 

 with the advance of the warm season, the zone of shearing sinks. 



Expansion at the zone where descending water freezes not only leads to shear, 

 but to the development of surface cracks, for the surface is stretched as the zone 

 below expands. In the course of years, the cracks developed in this way may 

 become wide crevasses, limited below by the depth of the zone of freezing in 

 summer. 



High temperature and water. Toward the lower end of a glacier, the higher 

 temperature and the greater abundance of water lend their aid to the fundamental 

 elements of movement. During the warm season, the ice here is bathed in water 

 all the time, so that the necessary changes in the crystals are facilitated. Under 

 these conditions, movement takes place more readily than in the drier, colder and 

 more open, granular ice of lower temperature, near the source of the glacier. 



Application. The co-operation of these several factors appears to explain the 

 peculiarities of glacial movement. In regions of intense cold, where a dry state 

 and low temperature prevail, as in the heart of Greenland, the snow-ice mass may 



1 The crystals of ice have a peculiar structure which has been thought by some 

 to be an important factor in shearing, and so in the motion of glaciers ice. See 

 the authors' Geologic Processes, p. 312; also (10) p. 323. 



