384 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
been powerfully augmented by the overriding of the sedentary 
margin of newly formed ice, during every advance of the ice sheet, 
and the overriding of the stagnant margin of the old and wasted ice 
during every recession of the ice sheet. The essentially stagnant 
condition of the outer portion of the Malaspina glacier, with a thick¬ 
ness 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 northward could induce forward movement in 
a comparatively thin sheet of ice resting upon a strongly dissected 
but approximately 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 cor¬ 
responding elevation of the -englacial drift. In view of these con¬ 
siderations, it may, perhaps, reasonably be affirmed that observations 
on existing ice masses do not afford a safe criterion for 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 subglacial 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 thick¬ 
ness, well below 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 striation of the bed rock sur¬ 
face must have ceased at the same time, for the striae are every¬ 
where 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 
