ALPINE SCULPTURE 259 



away and leaves the rock clean for further abrasion. Con- 

 fining the action of glaciers to the simple rubbing away of 

 the rocks, and allowing them sufficient time to act, it is not 

 a matter of opinion, but a physical certainty, that they will 

 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 d 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 pounds, and 

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

 pounds. With a vertical pressure of this amount the gla- 

 cier 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 gla- 

 cier exercises against its bed from the fact that the resist- 

 ance, 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 Ehone at Martigny 



