208 
The American Geologist. 
April, 1896 
2. At a distance from mountains and independently ‘ of 
piedmont glaciers, by the steepening of the marginal gradient, 
chiefly through increased ablation due to climatic ameliora¬ 
tion. 
3. The cold period following each period of marked climatic 
amelioration and consequent recession of the margin of the 
ice-sheet must have spread a new sedentary ice-sheet over the 
deglaciated area, which would be progressive^ overridden 
and absorbed by the re-advance of the older sheet. 
Basal Relations of a Sedentary Ice-sheet. 
During the slow accumulation of the ice-sheet, and before 
it began to move, the ground beneath it, which must have been 
saturated with water, was probably frozen solid to a consid¬ 
erable depth, possibly nearly if not quite to the bottom of the 
residuary soil or other surface detritus; that is, down to the 
Arm rocks. There could have been no original plane of sepa¬ 
ration or movement between this frozen soil and the overlying 
ice-sheet; for the ice did not merely rest on the detritus , but 
extended down through it to the lower limit of frost. This 
point will probably be conceded by all who have noted the te¬ 
nacity with which ice in winter adheres to the ground, pave¬ 
ments, etc., when the temperature is below freezing. 
Ice, in the thin sheets with which we are familiar, separates 
readily from the underlying soil in mild weather, while the 
subsoil still remains frozen, for the simple reason that the 
solar heat passing downward through the ice is arrested by 
the surface of the ground, causing a local rise of temperature 
along the contact between the ice and frozen soil. The opera¬ 
tion of this principle evidently depends upon the thermal dia¬ 
phaneity of the ice; and hence it does not, in general, admit 
of application in the case of snow, which is relatively opaque 
to both light and heat. Therefore this cause of separation can 
not be invoked in the case of the growing ice-sheet of the 
Glacial period, since it was a necessary condition of its for¬ 
mation that the winter snows remained unmelted upon its 
surface, accumulating thus to a great thickness and slowly 
changing downward through neve to glacial ice. We must 
suppose, then, that from the top of the ice-sheet to the lower 
limit of frost in the soil was one solid mass,—ice above, and 
ice and soil below. 
