Origin of Eskcrs. — Crosby. ii 
recession of the ice-sheet. The essentially stagnant condition 
of the outer portion of the Malaspina glacier, with a thickness 
of at least a thousand feet and a fairly steep frontal slope, and 
feeling the thrust of the powerful alpine glaciers behind it, is 
an instructive fact, suggesting that during the waning of the 
Pleistocene ice-sheet by active ablation over a breadth of one 
to several hundred miles back from the margin, this wasted 
marginal zone must have gradually ceased to move ; for it is 
inconceivable that the thrust of the thicker ice to the north- 
ward could induce forward movement in a comparatively thin 
sheet of ice resting upon a strongly dissected but approxi- 
mately level peneplain. Overriding, or at least a vertical 
thickening of the ice along the northern edge of the stagnant 
zone, appears inevitable ; and obviously this could not occur 
without a corresponding elevation of the englacial drift. In 
view of these considerations, it may, perhaps, reasonablv be 
affirmed that observations on existing ice masses do not af- 
ford a safe criterion for a judgment as to the amount or the 
depth of the englacial drift in that portion of the ice-sheet 
which was the locus of esker formation. 
During the period of maximum glaciation, when the drift 
was all englacial and glacial erosion of the bed-rock (surface 
was most severe, there could have been no important sub- 
glacial drainage ; for the basal contact of the ice was perfect 
and continuous, as indicated by the universality of striation 
beneath the till or ground moraine ; and the temperature of the 
ice must have been, throughout its entire thickness, well be- 
low the freezing point, even after making allowance for the 
lowering of the freezing point by pressure ; this reduction 
amounting approximately to one degree Cent, under a mile of 
ice. When, through the rise of the isogeotherms in the earth's 
crust, the basal temperature rose above the melting point of 
ice, the deposition of the ground moraine began ; and the stri- 
ation of the bed-rock surface must have ceased at the same 
time, for the stri?e are everywhere essentially rectilinear, which 
is inconceivable as due to the movement of the ice over a bed 
of loose material. We thus reach the conclusion that effective 
basal melting did not begin until after the ice had so far wasted 
by superficial ablation that its flow began to be influenced by 
topographic reliefs of relatively slight value, after, for instance. 
