ICE-EROSION IN THE CUILLIN HILLS, SKYE. 245 



away, at the upper bridge over the Brittle Kiver, which point they have reached by 

 way of Bealach a' Mhaim. To the right of the line the reverse is observed. Two 

 miles away, about the head of the Eynort River, the granite boulders are about ten 

 times as numerous as those of gabbro, and beyond this the latter are rare. Granite is 

 plentiful between Loch Eynort and Loch Harport, boulders up to 2 feet in diameter 

 occurring, for instance, on the top of Preshal Beg (1160 feet) near Talisker. It is, of 

 course, not to be assumed that all the granite necessarily comes from Glen Sligachan 

 and the hills overlooking it ; for that part of the ice from the central and eastern Red 

 Hills which found an outlet to the north must have been forced westward by the 

 Scottish ice-sheet before the latter itself effected a landing on the northern part of 

 Skye. That the Sligachan ice spread over a very wide tract is nevertheless proved by 

 boulders of certain easily recognised rocks ; e.g. , a pitchstone from Glamaig, a grano- 

 phyre crowded with ovoid patches of a basic rock from Glamaig and Srbn a' Bhealain, 

 and a granite enclosing gabbro debris from Marsco. 



There is no need here to trace out in the above fashion all the other branches of 

 the ice from the Cuillins. Their course cannot usually be followed with the same 

 precision as above over the low ground ; unless, indeed, boulders of some local and 

 distinctive rock be available for the purpose, such as the rhyolite, etc., of Fionn-choire 

 and the picrite of Coir' a' Ghrunnda. Wherever the test can be applied, we find a very 

 close correspondence between the transport of boulders and the direction of striae ; 

 which would not always be the case if the boulders had been to any important extent 

 carried on the ice or in its upper part. The manner in which the basaltic boulders 

 always become increasingly abundant as soon as we pass from the gabbro to basalt in 

 place is especially striking. Since it is certain that no basalt stood out above the ice, 

 this proves that, at least in the belt of country bordering the mountains, a very con- 

 siderable amount of erosion of the basaltic ground went on beneath the ice. Further, 

 this erosion, in so far as we have direct evidence of it, was effected not merely by 

 abrasion but by fracture of the surface rocks. 



This last point calls for further remark, for the importance of ice-action in tearing 

 away pieces of the subjacent rocks is one of the most salient facts of glacial erosion in 

 our area. It is emphasized by the large proportion which boulders bear to matrix in 

 all the glacial accumulations in the vicinity of the mountains. To such action also we 

 must ascribe the rough craggy surface (certainly not the pre-Glacial surface) seen often 

 on the lee side of a roche moutonnee just outside the mountains. In the interior of 

 the mountain-area, as already remarked, this is not usually seen, and the inference 

 suggested is that fracture of rocks is not so readily effected under a great pressure of 

 ice. If there be truth in this, it is still only one of the factors which determine 

 whether a rock shall yield by grinding down or by tearing away. The nature of the 

 rock is doubtless another factor, and an important one, especially as regards the 

 presence or absence of joints or other lines of weakness. In this connection it is to be 

 noticed that in the drift of the mountain-area proper boulders from the basic dykes and 



VOL. XL. PART II. (NO. 12). 2 



