222 BULLETIN OF THE 



the supposition that the ice throughout the field rests upon the bed rock. 

 Under these conditions it appears necessary for its surface to have a 

 slope towards the margin of some degi'ees of declivity in order that the 

 sheet may be impelled downward with sufficient energy to overcome the 

 great resistance due to its friction on the bed rock. A slope sufficient to 

 accomplish this purpose would require an inconceivable thickness of ice 

 in the central part of the Xorth American glacier. The hypothesis of 

 pressure melting shows us a way out of this difficulty. We have only 

 to conceive the central parts of the area of the glacier to be freed from 

 the basal friction, to avoid the need of hypothecating a considerable 

 slope of the surface except near the margin of the ice. In this view, 

 the element of friction on the bed rock is substantially reduced to a belt 

 of limited width into which the ice is fed from the areas where pressure 

 melting occurs. 



The sudden advances and recessions in the position of the glacial 

 front can be better accounted for on this hypothesis than in any other 

 way. A slight increase in the pressure in the central portions of the 

 field, such as might be brought about by an increased snowfall extending 

 over a term of a few years, would probably lead to the discharge of 

 water rendered more or less fluid by compression into the marginal 

 portions of the area. This would naturally be attended by a sudden 

 outward march of the ice. In this way we may explain the prevailingly 

 wide fringe of territory in the Mississippi Valley which lies to the south- 

 ward of the southernmost distinct moraine, and which appears to have 

 been temporarily occupied by the ice sheet. This district is covered 

 by a layer of glacial waste, but at its outer margin we find none of 

 those accumulations of detritus wfiich indicate the permanent occupation 

 of a line by a "glacial front. 



It appears to me that we may by the hypothesis of pressure melting 

 explain the formation of those very thick deposits of till which occur in 

 certain parts of the glaciated area, and this in the following manner. 

 Until a glacial sheet has accumulated to such a depth as to bring 

 about pressure melting, the combined erosion of the bed rock and the 

 irregular movement of the ice near the surface over which it moves 

 bring about the admixture of rocky material with the frozen water to 

 the depth, it may be, of some hundred feet above the earth. If now 

 pressure melting begins, the debris will gradually drop upon the surface, 

 and this action will continue until perhaps all the detritus previously 

 intermingled with the ice has become separated from it. If from time 

 to time the glacier became so far thinned that its solid parts again 



