450 LEM RAINIKE EHIME SILIG IE ICAI VAL OU 
edge. In Canada some of the path of the ice was rather rough, 
but not in any sense comparable in this respect with most of the 
coast of Greenland, and it was all deeply overridden at the time 
the ice reached across Lake Huron to points farther south. It 
follows that substantially all the northern drift of Michigan, 
Ohio, Indiana and Illinois was carried forward from Canada 
englacially. But the larger portion of the drift south of the 
lakes is of local origin, derived from rocks near by. This region, 
as the last ice-sheet found it, was probably a comparatively 
smooth drift plain, made so by earlier Pleistocene ice-sheets 
Almost the only way that local débris could become englacial 
was by being taken up, absorbed or incorporated directly into 
the lower layers of the ice as the glacier moved along. Under 
the great pressure of ice, hundreds or perhaps thousands of feet 
deep, this process must have become more or less effective. It 
seems almost certain that substantially all the drift south of the 
lakes was transported englacially, and in only the lower layers 
of the ice-sheet. The maximum load which can be carried in 
the bottom layers of the ice without overthrust cannot exceed 
a certain amount for a given pressure and rate of glacial flow, 
and that amount is probably not large. When the bottom lay- 
ers become overloaded they clog beneath the ice and cease to 
move, while the cleaner upper ice forms a plane of shear above 
the clogged layers and overrides them. Professor Salisbury, 
writing of the glaciers of Greenland, says: 
Professor Russell has called attention to the fact that the movement of 
ice is influenced by the amount of débris which it carries. This doctrine finds 
abundant confirmation in the north. The lower part of the ice, which is well 
charged with débris, or altogether full of it, seems to virtually lose its motion 
and to become the bed over which the upper ice passes. It is not possible 
to say that its motion is absolutely lost, but many phenomena seem to make 
it certain that the upper portion of the ice of a glacier passes over the lower 
débris-charged portion in the same way that it passes over a rock bed. The 
lower part of the ice in such cases becomes virtually an ice conglomerate, the 
mobility of which is certainly slight.’ 
r« Salient Points Concerning the Glacial Geology of North Greenland,” by R 
D. SALISBURY. JOUR. GEOL., Vol. IV, No. 7, pp. 800-801. 
