318 



GEOLOGY, 



ing is observed to occur chiefly where the ice below the plane of shearing 

 is protected more or less from the force of the thrust. It perhaps also 

 occurs where the basal ice becomes so overloaded with debris that it 

 is incapable of ready movement. 



It is also probable that sharp differential strain and shearing are 

 developed at the level where the surface-water of the warm season, 



descending into the ice, reaches 

 the zone of freezing. The expand- 

 ing of the freezing water at the 

 upper limit of the cold zone may 

 cause the layer expanded by it to 

 shear over that below. As the 

 level of freezing is lowered with 

 the advance of the warm season, 

 the zone of shearing also sinks. 

 This may be regarded as an 

 auxiliary agency of shearing, of 

 application to a special horizon. 



High temperature and water. — 

 In the zone of waste, a higher tem- 

 perature and more water lend 

 their aid to the fundamental 

 agencies of movement, and there 

 is need for these aids to promote a 

 proportionate movement, for here 

 the granules are more intimately 

 interlocked and the ice more 

 compact and inherently more solid 

 and rigid. The average tempera- 

 ture is, however, near the melting- 

 point (p. 276), and during the 

 warm season the ice is bathed in water so that the necessary changes 

 in the crystals are facilitated, and movement apparently takes place 

 even more readily than in the more open granular portion of lower 

 temperature and dryer state. The extraordinary movements of cer- 

 tain tongues of ice in some of the great fiords of Greenland are probably 

 due to the convergence of very thick slow-moving ice from the interior 

 into basins leading down to the fiords. Into the same basins a large 

 amount of surface-water is concentrated at the same time, with the 



Fig. 294. — Portion of the lateral margin 

 of a North Greenland glacier. Shows 

 upturning of the layers at the base, the 

 cleanness of the ice above the bottom, 

 and, possibly, shearing. 



