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 resultinsr 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 surfoces 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. 



