364 Culver—The Erosive Action oj Ice. 
sistance of friction is all on one side and opposes the movement 
of the ice. It thus appears that the retardation of motion is. 
more than that resulting from the expansion of the ice. 
In the deepest portion of the basin the ice is of course thick¬ 
est and presses most heavily on its bed. It is upon this un¬ 
doubted fact that the chief argument for differential erosion is 
based. Let this be examined closely. 
The reduction of motion due to vertical and horizontal expan¬ 
sion has already been noted, as has the further retardation due 
to friction on the slopes. 
The increased pressure of the thicker part of the ice greatly 
increases the friction‘on that part of the bed, but does not in¬ 
crease the tendency to onward movement. The only force urg¬ 
ing the ice onward is the pressure of the ice on the lee slope. 
This is more than balanced by the resistance already noted, and 
if the ice were a perfectly rigid body I do not see how motion 
would be possible. As it is, differential motion must inevitably 
result which still further reduces the motion of the lower layers 
of ice if indeed actual shearing does not occur with resulting 
stoppage of the bottom portion of the ice. 
If this reasoning is correct the ice in a rock basin must be re¬ 
garded as moving very slowly at the surface, and with increas¬ 
ing slowness toward the bottom where it may possibly cease to 
move altogether. If this be granted the result so far as erosion 
is concerned must be to cut down the rim of the basin faster 
than the bottom, so that, without the filling which is known to 
occur in scores of cases, the tendency of ice-action is rather to 
destroy than to create rock basins. 
5. The great excavating power of ice is thought by many to be 
proven by the thickness and great extent of the glacial drift. 
This material was evidently deposited by the ice; it must have- 
been transported by the ice; therefore, the probability is great 
that it was torn up and furnished for transportation by the same- 
agency. That the till might be composed largely of pre-glacial 
accumulations of decomposed rock was early suggested. To- 
this it was replied that the presence of carbonates in consid- 
able quantities in the till was proof that it could not be com. 
posed of residual material, since such soils are always and every- 
