ICE-EROSION IN" THE CUILLIN HILLS, 8KYE. 



235 



N.hjW. 



have already remarked, the efficiency of erosive action at the lower surface of ice well 

 supplied with rock-debris may be expected to increase, cseteris paribus, with the pres- 

 sure, and therefore with the thickness of the superincumbent mass. A glacier, unlike 

 a river, comes into being full-grown ; and the ice-cap which covered the Skye mountains 

 during the great glaciation was, as we have urged, thickest towards the centre. 



As we pass up any of the valleys of the Cuillins, we find that the U-shape becomes 

 more pronounced and the concave sweep of the transverse section more regular (fig. 5). 

 Further, the valley does not contract 

 towards its head, but shows a decided 

 expansion (figs. 3 and 4). These points 

 may be made out to some extent on 

 the contoured Ordnance Map; and even 

 in our map it is noticeable that the 

 concave portions of the 2000-feet line 

 have the sweeping curve of bays. In 

 some places these contrast with sharper 

 indentations of the 1000-feet line, 

 but this point would come out more clearly if the line corresponding with 1500 feet 

 were drawn. 



The expansion, becoming more marked towards the head of the valley, culminates 

 in a cirque or typical corrie in the strict sense.* In vertical section the simple cirque 

 presents a flowing concave curve up to the actual crest-lines of the bounding ridges, 

 and the form is the same in longitudinal as in transverse section (fig. 6). More 

 accurately, these terms cease to have any meaning ; for there is no longer any 



Fig. 5. — Two transverse sections of Coir' a' Ghreadaidh : 

 scale, about 2J inches to a mile. 



S.S.w. 



N.N.E. 



jfie. g, — Longitudinal profile of the floor of Coir' a' Ghrunnda ; scale, about 2J inches to a mile. Also another section across 

 the cirque which forms the head of the valley. 



L is the tarn at the bottom of the cirque ; M the crescentic moraine opposite the mouth of the valley, described 

 below. 



' Thalweg,' or rather the whole surface of the cirque may be regarded as the Thalweg. 

 If part of the water which courses down the slopes collects into gullies or other 

 channels, these are to be regarded rather as incidents not essential to the typical cirque. 

 This presents the same sweeping concave form in horizontal as in vertical section, and 

 may be pictured simply as the half, or rather more than the half, of a hemispherical bowl. 



* Gaelic coire, a cauldron : the term is, however, loosely applied in common usage to the whole valley. 



