204 ALASKA GLACIERS 



pressure ; the abrasion is more rapid as the pressure of the 

 glacier against the bed-rock is greater. 1 The resistance 

 which the moving ice, through its viscosity, 2 opposes to 

 change of form, causes it to press unequally on different 

 parts of an uneven bed, and to abrade most rapidly those 

 parts whose prominence compels the ice to change its 

 direction. Thus in a second way there is a tendency to 

 reduce the profile of the bed to simple forms. 



The amount of resistance developed by viscosity de- 

 pends on the rate of deformation; more force is necessary 

 to deflect the ice quickly than to deflect it slowly. The 

 parts of the bed which cause the most abrupt turns are 

 therefore subjected to greatest pressure and to greatest 

 wear, with the result that the profiles of the bed eventually 

 become curves of large radius, adjusted to slow bending 

 of the moving ice. 



If the general motion of the ice is very slow, the resist- 

 ance developed by viscosity is small and the resulting 

 sculpture curves have comparatively small radius. If the 

 ice moves rapidly, the sculpture curves have large radius. 



1 There are two theoretic limits to the law that abrasion increases with pres- 

 sure. It has been argued by N. S. Shaler that because the melting temperature of 

 ice is lowered by pressure, the basal part of a thick glacier must consist of water 

 instead of ice (Outlines of the Earth's History, pages 237-239, 1898); and such 

 'pressure-molten' water would manifestly be powerless to grind rock waste 

 against the rock bed. G. F. Becker has suggested to me in conversation that 

 where the pressure is great there also the elastic limit of the ice is far exceeded 

 and the ice should be expected to flow about rock fragments so as to incorporate 

 them in the glacier and reduce or destroy their effectiveness as tools of abrasion. 

 These considerations are not included in the above analysis because, while I do 

 not see my way to their satisfactory discussion, I fail to perceive that they help 

 to explain the phenomena of ice erosion as seen in Alaska. Whatever their in- 

 fluence may be, it has not prevented exceptionally great erosion in places where 

 the Pleistocene ice was exceptionally deep. 



2 In untechnical usage the word viscosity is ambiguous, being applied to a 

 property of liquids opposed to mobility and to a property of solids opposed to 

 rigidity. In the present paper it has the technical meaning given by the physi- 

 cist, and is the property of fluids and solids in virtue of which internal dif- 

 ferential movement, or shear, consumes time. The greater the viscosity the 

 slower the yielding to a given shearing force. 



