10 The Aiiicricaii Geologist. "^^^^^ ^^*^^- 
this border area was rapidly disappearing through super- 
glacial and subglacial ablation, are propositions accepted by 
all and requiring no argument here. Further, we may post- 
ulate, with Davis and other able students in this field, the 
stagnant condition of this marginal zone during the esker- 
forming period. In fact, a stationary ice-margin is a con- 
dition highly favorable, if not absolutely essential, to every 
theory of esker formation ; and, as Davis has insisted, it is de- 
manded by the highly irregular and fragmentary border of 
the ice of which we have conclusive proof in the distribution 
and outlines of the deposits formed upon or against it, and in 
the absence of evidence of glacial thrust. 
In my paper on englacial drift,* I have accepted and 
elaborated Upham's idea that over the vast plain or peneplain 
tracts of the glaciated area the ice-sheet was developed pri- 
marily by accumulation and not by invasion, existing at first 
as a sedentary ice-cap, which gradually acquired motion as it 
gained in thickness and also as it was progressively overridden 
by the older and already active ice to the northward. Again, 
I see now, as five years ago, no escape from the conclusion, 
also first enunciated by Upham, that during the period of ice 
accumulation and of maximum glaciation, the entire volume 
of the drift, including both the preglacial detritus and all that 
due to glacial rending and abrasion, was englacial, or firmly 
frozen in the basal portion of the ice. This conclusion is ab- 
solutely demanded by the universal rectilinear striation of the 
bed-rock surface, and is inconsistent, so far as I can see. with 
no established facts. I have also shown, in the paper cited 
above, that the observations of Chamberlin and others in 
Greenland and elsewhere, indicate that an ice-sheet, in its prog- 
ress across even a comparatively smooth surface, is subject 
to oblique shearing movements which tend to transfer the en- 
glacial drift to higher levels in the ice. 
I hold now, even more strongly than when writing the 
paper on englacial drift, that the tendency to the elevation of 
the drift in the ice must have 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 
* Amer. Geol., 17. pp. 203-234. 
