ICE-EROSION IN THE CUILLIN HILLS, SKYE. 233 



that this tendency will become effective in varying degree, depending inversely upon 

 the magnitude of the tributaries. The smallest of these will be completely blocked ; 

 the larger may be only partially checked, or at least they will surrender their freedom 

 of exit later in the waxing stage of glaciation and recover it sooner in the waning stage ; 

 the largest will be able to assert themselves throughout. For these reasons — apart from 

 the consideration of the varying thickness of the ice as affecting the rate of erosive 

 action — we must look for much more erosion in the main valley than in the smaller 

 tributary glens. Moreover, while the bottom of one of these latter is occupied by ice 

 which is not effectively eroding, the upper parts of its bounding slopes may be reduced 

 by the action of ice moving athwart the direction of the glen. 



These points seem to be illustrated in the Cuillin district. In the first place the 

 sides of the main valleys are formed in some cases by long straight mountain-slopes 

 broken only by narrow gullies. The west face of Blath-bheinn and the south-west 

 slope of Druim nan Ramh are good examples, in Strath na Creitheach and Coruisk 

 respectively. The ice-worn surfaces inside these gullies prove that they are not of post- 

 Glacial age ; and they seem to be the relics of larger glens which have been almost 

 obliterated, just as an inscription cut in a stone slab is obliterated by grinding away the 

 surface.* The walls of the gully never curve towards the wall of the main valley to 

 form a continuous surface with it. The pre-Glacial glens in these places, though larger 

 than the gullies which now represent them, were evidently of very small dimensions. 

 They must have been wholly aborted as channels during the great glaciation. In this 

 connection it is instructive to contrast the slope of Druim nan Ramh overlooking 

 Coruisk with the opposite side of the same main valley. 



A different and very interesting case occurs where a tributary glen of larger 

 dimensions than the preceding, but still small in comparison with the trunk valley, has 

 come steeply down to join the trunk in a direction making a high angle with it. Here 

 the lower part of the tributary glen may have been greatly reduced or wholly 

 obliterated by the planing process, while its head has been on the other hand 

 developed into a cirque in the manner to be discussed later. Of this, numerous 

 examples, with various modifying circumstances, are found in the Cuillins, the most 

 remarkable being Coir' an Lochain, overlooking Coruisk. Here we have a corrie 500 

 yards across and going back 1000 yards, the tarn on the floor of the corrie being at an 

 altitude of over 1800 feet. The stream draining this, on emerging from the corrie, 

 plunges abruptly over a steep slope, fully 1000 feet high, consisting of smooth glaciated 

 rocks in which there is, for the most part, no sort of channel. The strise which 

 everywhere mark this steep slope, and may be seen through the water which 

 cascades over it, axe parallel to the direction of Coruisk and at right angles to the 



* Something comparable with this has been described by Dr W. T. Blanford in the Great Glen of Scotland 

 (" On a Particular Form of Surface, apparently the Result of Glacial Erosion, seen on Loch Lochy and Elsewhere," 

 Quart. Joum. Geol. Soc, vol. lvi. pp. 198-203, pi. ix., 1900). There are. however, considerable differences between 

 the two cases. 



