THE MECHANICS OF GLACIERS 9^7 



and would therefore have a tendency to rise, would be squeezed 

 up, so to speak. 



We cannot reason very definitely about the pressure above 

 the neve-line, for the much increased breadth of the reservoir, 

 the smaller density of the ice and the possibly greater slope of 

 the bed may result in either pressure or tension according to 

 their relations to each other and to the thickness of the glacier. 



The idea of a pressure on the lower reaches of a glacier due 

 to the ice above has been generally held by glacialists, but I 

 think without satisfactory reasons. It seems to have been 

 inferred generally from the diminishing velocity of the lower 

 parts of the glacier ; but it is not due to this, for it is quite con- 

 ceivable that by an increasing slope, and suitable melting, every 

 section might have its normal velocity ; there would be no 

 effective pressure upon the upper regions, and there might still 

 be a diminishing surface velocity. 



Indeed, the extra pressure is not due simply to diminishing 

 flow, but to the fact that the flow diminishes more rapidly than 

 the cross-section. This may be stated in algebraic language by 

 saying that the normal flow is not proportional to the linear 

 dimensions of the section ; it is not even proportional to the 

 square of the linear dimensions, but probably to the third power. 



In the dissipator of a circular ice-sheet, or of a spreading 

 glacier, successive sections increase in breadth with a more rapid 

 diminution of thickness than in the case of a linear glacier. 

 This causes a considerable pressure of the rear on the forward 

 sections, resulting in a more rapid radial movement of the ice 

 than would occur without this pressure and in the opening of 

 radial crevasses. It is not unlikely that such crevasses determined 

 the positions of the eskers formed during the glacial epoch. 



Direction of flow — Stratification. — In order that the general 

 volume of the glacier should be preserved we must have below 

 the neve-line, where there is melting, a component of the motion 

 towards the surface, and this component is strongest where the 

 melting is greatest, i. e., it gets larger as we descend the dissi- 

 pator. On account of the gradual thinning of the glacier this 



