Pleistocene Ice-sheet in N. America. 321 



of the ice has retreated far to tlie north through ablation, developing 

 a bold and as-o-ressive front, it is obvious that the succeeding cold 

 period must have caused a rapid extension of sedentary ice south- 

 ward from the front of the moving sheet, and the former would 

 inevitably be progressively overridden and absorbed by the latter. 



Basal Belations 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 considerable 

 depth, possibly nearly, if not quite, to the bottom of the residuary 

 soil or other surface detritus; that is, down to the firm rocks. 

 There could have been no original plane of separation or move- 

 ment 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. 



With increased thickness the ice-sheet became a more and more 

 efficient protection to the ground against the climate of the Ice Age, 

 and the steady efflux of heat from the earth's interior would thus 

 tend to gradually loosen the hold of the frost upon the rocky 

 substratum. 



Observations in the Arctic regions and at higher altitudes show 

 that the ground may become frozen to a depth of several hundred 

 feet ; but it is well known that glaciated areas are not in general 

 those of most exti-eme cold. A humid climate is the first essential, 

 whereas extremely frigid areas, such as the interior of Alaska and 

 northern Siberia, are relatively dry and non-glaciated. A mean 

 annual temperature a few degrees only below freezing is all that is 

 required for active glaciation ; and as the increasing thickness of 

 a sedentary ice-sheet would tend to neutralize the downward 

 penetration of frost, we need not suppose that the ground beneath 

 the ice would be frozen to any great depth or far below the detrital 

 layer. 



In considering the most probable plane of shearing when the 

 ice-sheet begins to move — whether in obedience to its own weight 

 or through the overriding thrust of a thicker northern sheet — we 

 have to consider (1) the ice-sheet proper; (2) the frozen soil beneath 

 it, to which the ice-sheet is still firmly united ; (3) the unfrozen soil 

 resting upon or passing downward into the solid rocks. At the in- 

 ception of movement the most probable plane of slipping or shearing 

 would be in the unfrozen soil, the frozen soil and overlying ice 

 moving en masse, and the movement being lubricated by the un- 

 frozen soil, which at most points would be of an argillaceous and 

 plastic character. 



Observations heretofore made on modern glaciers and ice-sheets 

 are of little value in this connection, because nowhere in the field of 

 observation are realized the conditions that must obtain at the base 

 of a sedentary or recently sedentary ice-sheet. The true glaciers or 

 ice-rivers of Alpine districts, Greenland, etc., are mere lobes of ice 

 descending under the influence of gravity from the edges of neve 



DEOADE IT. VOL. lY. — NO. VII. 21 



