564 ,R. M. Deeley — -Glacier Motion and Erosion. 



This is the explanation I would also apply here. The Arch^an 

 mass thrust over the quartzite during a period of intense folding, has 

 been sheared at its base, so as to give rise to the underlying green. 

 schists with their foliation parallel to the thrust-plane. 



The anticlinal arrangement of the rocks is perhaps due to a 

 subsequent movement, analogous to that which appears to have 

 aifected the thrust-plane in Eaggedstone Hill, as I hope to show 

 very shortly. 



This hypothesis clears the way for the acceptance of the Cambrian 

 age of the Hartley quartzite. 



IX. — Glacier Motion and Erosion. 

 By R. M. Deeley, F.G.S. 



ALTHOUGH the ice of many glaciers, more especially those 

 which move over massive impervious rocks, is clean and 

 almost free from rock fragments, in many cases towards the bottom 

 of the glacier, boulders, pebbles, and even masses of clay, mud, 

 or sand abound. The rock-masses have evidently been derived from 

 the floor over which the glaciers move, for the fragments become 

 more numerous as we descend in the ice, the upper portion of 

 the glacier being often quite free from foreign matter. 



The process by which the boulders rise and become imbedded in 

 the glacier would seem to be somewhat in conflict with our notions 

 of viscous flow, for the stream lines in viscous fluids are always 

 regarded as being direct. It has, however, been pointed out that 

 when a glacier encounters an obstacle a portion of it may pass over 

 the top, and flow over the streams of ice working round the sides. 

 In this way boulders from near the summit of the crag may be 

 trailed over the side streams and thus become imbedded in the mass 

 of the glacier. This explanation may account for some of the 

 intrusions met with, but scarcely seems sufficient to explain the 

 presence of the great quantities of material sometimes found in the 

 lower portions of some large glaciers or ice lobes. 



I have pointed out in a pi'evious paper ^ that a viscous substance 

 may be either adherent to its bed or slide slowly over it. Pitch, for 

 instance, adheres firmly to the sides of its channel, and the flow results 

 wholly from the shear of layers of pitch over layers of pitch, the mole- 

 cules actually in contact with the sides being for all practical purposes 

 stationary. The ice of a glacier, on the other hand, is frequently 

 separated from the rock upon which it rests by a film of water, and 

 moves bodily as well as by the diff'erential motion of its particles. 

 But in both cases the flow is direct and could not cause boulders to 

 work their way from the sides or bottom into the mass. Such 

 a result, however, could be produced if the viscous body were 

 sliding over its bed in some places and adhering in others. The 

 possibility of this will be seen from the Figure. Here the ice, 

 which is moving from left to right, is sliding over the rocky floor 

 between A and B, and adhering to it from B to C. Between 



1 Geol, Mag., Dec. IV, Vol. IV, pp. 388-397. 



