ALPINE SCULPTURE. 18? 



scoop out valleys. But the glacier does more than abrade. 

 Rocks are not homogeneous; they are intersected by joints 

 and places of weakness, which divide them into virtually 

 detached masses. A glacier is undoubtedly competent to 

 root such masses bodily away. Indeed the mere a priori 

 consideration of the subject proves the competence of a 

 glacier to deepen its bed. Taking the case of a glacier 

 1,000 feet deep (and some of the older ones were probably 

 three times this depth), and allowing 40 feet of ice to an 

 atmosphere, we find that on every square inch of its bed 

 such a glacier presses with a weight of 375 Ibs., and on 

 every square yard of its bed with a weight of 486,000 Ibs. 

 With a vertical pressure of this amount the glacier 

 is urged down its valley by the pressure from behind. 

 We can hardly, I think, deny to such a tool a power of 

 excavation. 



The retardation of a glacier by its bed has been referred 

 to as proving its impotence as an erosive agent: but this 

 very retardation is in some measure an expression of the 

 magnitude of the erosive energy. Either the bed must 

 give way, or the ice must slide over itself. We get indeed 

 some idea of the crushing pressure which the moving 

 glacier exercises against its bed from the fact that the 

 resistance, and the effort to overcome it, are such as to 

 make the upper layers of a glacier move bodily over the 

 lower ones a portion only of the total motion being due 

 to the progress of the entire mass of the glacier down its 

 valley. 



The sudden bend in the valley of the Rhone at Martigny 

 has also been regarded as conclusive evidence against the 

 theory of erosion. " Why," it has been asked, " did not 

 the glacier of the Rhone go straight forward instead of 

 making this awkward bend?" But if the valley be a 

 crack, why did the crack make this bend? The crack, I 

 submit, had at least as much reason to prolong itself in a 

 straight line as the glacier had. A statement of Sir John 

 Herschel with reference to another matter is perfectly 

 applicable here: "A crack once produced has a tendency 

 to run for this plain reason, that at its momentary limit, 

 at the point at which it has just arrived, the divellent 

 force on the molecules there situated is counteracted only 

 by half of the cohesive force which acted when there was 

 no crack, viz., [the cohesion of the uncracked portion alone" 



