CROSBY : ORIGIN OF ESKERS. 
387 
ified with the till, the product possibly of superglacial streams fall¬ 
ing through the ice at these points, since crevassing would probably 
occur as early where the ice bends over the summit of a drumlin as 
anywhere. At the same time, the ice must have tended to draw 
away from the lee slopes and leave the vacant spaces in which gravel 
could be deposited by superglacial, but not possibly by subglacial 
streams. We do not discover, however, any such general intercala¬ 
tion of washed drift with the ground moraine, either in drumlins 
or elsewhere, as to suggest that crevasses were a common feature of 
the ice just before it ceased to move ; and of course none could be 
formed after it became stationary. Again, it appears probable that, 
when the ice was thick enough to override drumlins from one hundred 
to several hundred feet in height, its thickness was too great to permit 
extensive crevassing, especially in view of the fact that the thick¬ 
ness essential to flow must increase rapidly with diminishing slope 
and increasing roughness of the ground or basal friction, and the 
further fact that even alpine glaciers in relatively smooth and unob¬ 
structed valleys of high gradient, and seldom exceeding 500 to 1,000 
feet in thickness, are but little affected by crevasses, except in cas¬ 
cades and laterally where they feel the friction of the valley walls. 
In this connection we may profitably note once more that the mar¬ 
ginal portion of the Malaspina glacier, with a thickness of at least 
one thousand feet and a steep frontal slope, at the base of lofty 
mountains and feeling the thrust of powerful alpine glaciers, and 
with absolutely nothing to hinder its free motion seaward across the 
sloping coastal plain, is practically motionless. These considerations 
clearly point to the conclusion that the Pleistocene ice sheet on the 
highly dissected and rough peneplain surface of the greater part of 
the glaciated area, with only extremely low average gradients in any 
direction, and then often either to the northward or transverse to 
the glacial movement, must have been at least two thousand feet 
thick when it ceased to move, and too thick in general for effective 
crevassing, or the formation of crevasses extending from the top 
to the bottom of the ice. 
Crevasses are the product of tensile stresses; and it is obvious 
thut as, during the progressive cessation of its flow from the south 
northward, each portion or zone of the ice becomes stationary, the 
ice still in motion immediately to the northward will crowd forcibly 
against it and tend to obliterate by compressive stresses any 
