190 Correspondence — R. M. Deeley. 



to heights of more than a thousand feet, he suggests what seems far 

 more difficult to grasp, i.e. that the lower layers of ice, owing to the 

 formation of internal shear-planes, rose locally to the surface and 

 earned with them portions of the ground moraine. 



Being an advocate of the view that an ice-sheet may drag along 

 with it portions of the floor over which it moves, especially if the 

 floor be frozen, and carry such portions up slopes of more than 

 a thousand feet, I do not resent the rendering Mr. Crook gives of 

 such views ; neither do I dispute the fact that, as he suggests, 

 upward thrusts and contortions may be seen in glaciers. My 

 •experience, however, is that such signs of ' upward thrust ', by which 

 I understand him to mean shear, are confined to portions of the 

 glacier where the surface is rapidly melting. In such places the 

 ice is in compression and the glacier surface is rising to make up for 

 1 lie loss. 



Thrust is credited with far too much importance as a cause of 

 glacier flow. Rather should we attribute the flow to the absence of 

 support in frout. A glacier moves much as does a river. Where the 

 slope is small the velocity of the ice or water is slow and the river or 

 glacier deep ; where the river or glacier is wide the flow is slow and 



Fig. 1. 

 the ice or water shallow. Gravity is acting upon every portion of the 

 mass, and the speed and thickness everywhere adjust themselves so as 

 to pass the volume of ice or water to be got rid of. It has been 

 suggested that the forces due to gravity acting on a glacier could not 

 cause shear in its mass ; for a similar mass of soft clay would not 

 shear, and clay is much less capable of resisting shear than is ice. 

 Clay, however, is not a viscous substance, and on a slope the material 

 in front acts as a permanent buttress to prevent motion. With ice 

 the front gives way, either by suffering longitudinal compression or 

 by flow, and the rear portions follow. 



When large glaciers are concerned the flow is quite regular; there 

 may be shear where the stresses are sufficiently great, but such shear- 

 planes are strictly related to the floor upon or against which the ice 

 rests. Any local upthrust would soon raise a mound on the glacier 

 surface, the weight of which would check further upiise. Many 

 seem to experience difficulty in understanding how a glacier can move 

 up a slope and drag rock masses along with it. Now the movement 

 of a glacier is not necessarily due to the slope upon which it rests. 

 The motion is due to the slope of the upper surface of the ice, which 

 may be in the opposite direction to that of the floor upon which 

 it rests. 



We will suppose that the glacier rests in a hollow as in Fig. 1. 



