24 8 MR ALFRED HARKER ON 



(xi.) Later Glaciers and Frost-Erosion. 



In numerous parts of Scotland geologists have recognised two more or less distinct 

 glaciations, the first a general one and the second a local. In the central part of Skye 

 also there is clear evidence of two glaciations. though — since foreign ice never obtained 

 a footing here — they cannot be distinguished in those terms. That the second 

 glaciation here corresponds with the local glaciation in adjacent parts of Scotland, and 

 was in a general sense contemporaneous with it, is evident from the map given below, 

 on which the flow of the ice during this later stage is indicated by a second set of 

 arrows. These have been inserted only in places where the evidence was quite clear ; 

 but they suffice to show that the movement was very different on many parts of the 

 lower ground from that during the principal glaciation, and that the difference was due 

 to the withdrawal of the Scottish ice-sheet. Instead of the general diversion westward, 

 upon which we commented before, there is now a simple radiate outflow from the 

 mountains to the sea. This reversion to what we may consider the natural direction 

 of ice-drainage for the district, consequent upon the removal of the constraint from 

 without, involved, at many places outside the mountain area, considerable departures 

 from the former directions of flow. Thus, to the north-west of the Cuillins, the new 

 line of movement, directed towards Loch Bracadale, was at right angles to the old. 

 In the valley of Allt Dearg M5r the direction of flow was directly reversed. In the 

 earlier glaciation the ice had moved up the valley, bringing boulders from Glen 

 Sligachan ; in the later glaciation it moved in the natural direction, carrying down the 

 rhyolitic and other rocks of Fionn-choire and Fhinn-choire, which are found in 

 abundance along the burn and to heights of 60 or 70 feet above it. The large extent 

 of country overrun by the later glaciers and the way in which, in certain cases, they 

 overflowed some of the lower watersheds, prove that they were of very' considerable 

 magnitude, but it is clear that they were greatly inferior to the ice-sheet of the earlier 

 glaciation. 



The erosive action exerted by these glaciers was mainly confined to the valleys 

 of the mountain area, and was almost negligible at a distance from the mountains. 

 Here it has often failed to obliterate the scorings made by the earlier glaciation, and 

 has usually caused but little disturbance in the older drift accumulations over which 

 it passed. Even quite near to the mountains, patches of hummocky drift have retained 

 most of their characteristic appearance in places where they have certainly been over- 

 ridden by the later glaciers. In the lower parts of the mountain valleys themselves, 

 however, the earlier drift accumulations seem to have suffered erosion in many places. 

 In certain cases, too, there are drift-ridges between the mouths of adjacent valleys, 

 which seem to have been originally of the nature of drumlins, but to have been 

 scarped and moulded by the later glaciers. 



The accumulations referable to this later glaciation are represented on the lower 



