ALPINE SCULPTURE. 187 
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 " 
