MECHANICS OF GLA.CIERS. 67 



the steepness of the valley as a rule increasing as we approach its 

 head, is well known ; and it follows from this that the biting-power 

 of the glacier upon the rocks (which diminishes with the steepness 

 in proportion to the cosine of the angle of inclination) is less as we 

 ascend into the steeper slopes of the glacier-region. Moreover it is 

 self-evident that it is not on such steeper parts of the valley that 

 the advocates of the excavation-theory would call its action into 

 requisition. 



In the work of erosion, such as is imputed to glaciers, it is of 

 course well known that the work is mainly done by the stones and 

 sand which have found their way, either through crevasses or 

 between the glacier and the rocky sides of its channel, to the base. 

 These form the teeth of the file. We must recollect however (1) 

 that these and the rocks upon which they act are of about equal 

 hardness ; only at most, therefore, one half of the finer detritus 

 which comes away in the glacier- stream is produced by the grinding 

 away by these stones of the rocks themselves, the wearing of the 

 stones furnishing an equal amount of it ; (2) that the stones held in 

 the ice are only passive instruments, and can only do work upon 

 the rocks when they move — that is, when the resistance to their 

 motion against the rock is less than the yielding-power of the ice 

 which holds them. Moreover, much of the detritus which comes 

 away in the glacier-stream may well be derived directly from the finer 

 portions of the moraines which have fallen to the bed of the glacier. 

 It has been suggested that the freezing of water within the 

 crevices and pores of the rocky bed of the glacier must by its ex- 

 pansion break up the rock-surface, and thus furnish detritus for 

 the glacier to carry away, as the loosened materials are caught up 

 by the ice. This prima facie seems a sound argument in favour of 

 excavation ; we must therefore examine it. We must recollect (1) 

 that the water contained in this way within the rock is exposed to 

 subterranean heat passing up by conduction from below, and that, 

 if this is slow, owing to the low conductivity of the rock-materials, 

 the cooling effect of the ice of the glacier is, a fortiori, equally slow. 

 (2) The actual surface of the rock at any given point is in contact 

 with either (a) the water of the glacier-stream, which is not below 

 0° C, and therefore cannot freeze the water within the rock, or (b) 

 in contact with the ice (or a stone stuck in the ice and at the same 

 temperature as the ice), in which case the ice may be either at or 

 below 0°C, according to the pressure at the point of contact, as 

 Helmholtz's reasoning shows. Ice at 0° C. has no power to freeze 

 water at 0° C, since with equality of temperature there can be no 

 exchange of heat between the bodies ; and if the ice be below 0° C. 

 it can only be so at a pressure proportionately greater. This very 

 pressure must be exerted upon the rock, and so counteract the ex- 

 pansive force of the water within the rock. The hypothesis is thus 

 shown to be wholly inadmissible. Further, the actual appearance 

 of glaciated rocks shows that they have not been thus broken up by 

 freezing water while the glacier covered them. 



It is conceivable that some of the surface-portions of the glacier 



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