194 BULLETIN OF THE 
About one fifth of the surface is occupied by the pits which were left by 
the removal of these larger fragments, the remainder being, except for 
the post-glacial wear to which it has been subjected, in the planed down 
condition resulting from the attrition of the fragments embedded in the 
moving ice. We have therefore to conclude that at least four fifths of 
the down-wearing of this mass during the later stages of the.glacier’s 
action was accomplished by the scoring and scratching action of the 
glacier, and not by the plucking out of large masses such as constitute 
the boulders in the great train. The portion of the eroded matter re- 
moved from the grooves was, at the time of its formation, broken into 
the state of sand, the grains of which, like the larger fragments, were 
borne on by the glacial movement. 
For reasons which will appear hereafter it is important to consider 
whether or no this relative excess in the quantity of the fine-grained 
material removed by the ice was limited to the closing stages of the 
Glacial Period. At first sight it seems likely that the thicker the ice 
moving over a rock surface the greater would be its tendency to rend 
the rock over which it flowed. The pressure of an ice sheet upon its 
base is directly as its depth, and up to a certain point the abrading 
power of a glacier must increase with its vertical section. When, how- 
ever, the ice has attained a certain thickness, it must attain the maxi- 
mum effect which it can exercise upon the surface over which it moves. 
After that, the shearing action of the upper upon the lower parts of the 
ice must cause the superior part to flow over the lower without propor- 
tionately increasing the erosive action. 
- In corroboration of the view that there was probably no great diminu- 
tion in the plucking action of the glacier in the later stages of its work, 
we find that the boulders of the train which lie nearest its source are the 
largest which appear anywhere in its path, and that from its origin to 
its extremity the fragments in the train gradually, and rather uniformly, 
diminish in size through the process of rending by attrition to which 
erratics are commonly subjected in the process of glacial carriage. We 
furthermore note the fact that almost everywhere within the glaciated 
district where we can examine a large surface of any of our mass- 
ive rocks, we find, as here, that the proportion of the scored away 
or ground down to the rent surfaces is generally very great. From 
these considerations I am disposed to assume that the material removed 
from Iron Hill in the form of boulders was very much less than that 
which was carried away in the form of sand or yet more finely divided 
matter. 
