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. 
