Einjldcinl Drift. — Crosby. 213 
to my mind rectilinear stria', often scores of feet in length, 
are an impossibility unless we conceive the entire mass of the 
ground moraine as enclosed in the moving ice. 
Professor Chamberlin's observations in Greenland show 
that any number of horizontal shearing planes may be postu- 
lated, with a corresponding reduction of the basal velocity; 
but I see, with Upham, no escape from the conclusion that 
during the period of active and efficient glaciation of the bed- 
rock surfaces, the ground moraine was very scanty or wholly 
wanting, being incorporated with the moving ice. Imagine a 
modern glacier or ice-sheet as launched upon a plain covered 
by 25 to 50 feet of sedentary detritus passing gradually down- 
ward into firm rocks, and consider how little chance there 
would be for the development on the underlying bed-rock of a 
typical glaciated surface, so long as the ice and soil are es- 
sentially distinct. The preliminary removal of the detritus 
by simple thrust and drag is seen not to be a valid explana- 
tion when we consider the great breadth of the glaciated area, 
and the enormous marginal accumulation which would inev- 
itably result. A true ground moraine between bed-rock and 
ice, and distinct from both, belongs to the waning stage of 
the ice-sheet and to lobes of ice (glaciers) descending from 
an ice-cap or neve field into a climatic zone where permanent 
ice cannot form. 
All this appears to be quite consistent with the local origin 
of the greater part of the till or ground moraine of the Pleis- 
tocene ice-sheet and the well-established fact that the total 
i 
movement of the ice amounted to hundreds of miles, if we 
simply suppose that through the granular plasticity of the 
ice, or a series of shear planes, tlie basal, drift-laden layer 
moves much more slowly than the overlying clear ice. 
Absorption of Drift by the Pleistocene Ice-sheet. 
Glacialists are sharply divided in opinion as to the power 
of an ice-sheet to absorb or incorporate with its mass the de- 
tritus over which it moves ; and the arguments pro and con 
have been stated with much fullness and force in recent pa- 
pers by Upham, Chamberlin, and others.* It is clearly in- 
cumbent upon the advocate of the englacial theory to prove 
*Uphani has recently cited all the more important of the later ( tri 
tuitions to tins discussion. (Bull. Geol. Sue. Ainer.. v. 72, 7.'!. i 
