ICE-EROSION IN THE CUILLIN HILLS, SKYE. 227 



II. Physical Features of the Cuillin Hills as a typical district of 



Ice-Erosion. 



(iv.) General Considerations. 



Viewing ice-work in its dual aspect — destructive and constructive — we may expect 

 to find beneath an independent ice-cap, an inner area of glacial erosion and an outer 

 area of glacial accumulation. Allowing for a broad intermediate belt, in which the two 

 processes have operated either successively or simultaneously side by side, we may 

 recognise an approximate partition of this kind in the central part of Skye. The 

 presence of ice-worn and striated surfaces does indeed show that no part of the area 

 surveyed was quite beyond the province of glacial erosion ; but with increasing distance 

 from the mountains these signs become less frequent, while the mantle of drift becomes 

 more persistent and uniform. In the mountain district itself, on the other hand, the 

 drift disappears, and the evidence of important glacial erosion is displayed in the most- 

 striking manner. In this generalised statement, and still more in the detailed features 

 to be described, we have a further confirmation of our supposition that the ice attained 

 its greatest thickness over the mountain area proper. It cannot be doubted that, other 

 conditions being the same, erosion would be most active at places where the pressure at 

 the base of the ice was greatest, and the pressure at different places would bear approxi- 

 mately a direct ratio to the thickness of the ice. 



In discussing the phenomena of glacial erosion it is, then, to the Cuillins that we 

 must turn as the principal theatre of operations. Several causes contribute to make 

 this group of mountains a model of a well-marked type of glaciated topography. 

 Highly complex as it is in detailed structure, it may be broadly regarded as carved out 

 of a single unbroken rock-mass — viz., a great laccolite of gabbro. Another element of 

 simplicity arises from the independence of this centre of glaciation. The ground has 

 not been over-ridden, as in some other districts, by a foreign ice-sheet crossing ridge 

 and valley indifferently. The pre-Glacial surface-relief was strongly marked, and the 

 movement of the ice was guided, with few exceptions, by the form of the ground, so 

 that it exercised throughout the whole time a cumulative effect as regards developing 

 the characteristic forms in their simplest expression. It is a further advantage that the 

 actual shape of the ground is everywhere clearly exhibited. The mountains themselves 

 are of perfectly naked rock ; the same is true of all the higher corries, except in so far 

 as they are encumbered with screes ; and even in the lower corries and main valleys the 

 drift is never so thick as to obscure the true form. Again, it is to be remarked that the 

 effects of ice and frost-action remain practically without modification by later agencies, 

 the sum total of post-Glacial erosion being almost a negligible quantity.* Finally, it is 



* See Harkee, "Notes on Subaerial Erosion in the Isle of Skye," Geol. Mag. 1899, pp. 485-491. 



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