Yol. 56.] OF GLACIAL EROSION AT LOCH LOCHY. 199 



taken up by three lakes — Loch Ness, which is by far the longest, 

 to the north-east, Loch Oich in the middle, and Loch Lochy to 

 the south-west : the water-parting of the Glen being between Loch 

 Oich, which runs out north-eastward into Loch Ness, and Loch 

 Lochy, which drains south-westward into the arm of the sea 

 known as Loch Eil. The sides of the Great Glen show abundant 

 evidence of glacial action throughout, and it is of course an 

 elementary fact in the history of the Glacial Epoch that this Glen 

 was occupied by a large glacier during the whole time that the 

 Highlands of Scotland were wholly or partly covered with ice. 



The sides of the Glen have, I think, been planed — if the expres- 

 sion is permissible — by glacier-action to a greater extent than is usual 

 in glaciated slopes. This is especially the case on the south-east side 

 of the Glen, near the head (or north-eastern end) of Loch Lochy, 1 and 

 in the interval of 1 J miles between Loch Lochy and Loch Oich, near 

 Laggan. Here the sides of the Glen, up to a height of about 1000 

 feet above the sea (Loch Lochy itself is only 93 feet above tide- 

 level), form a singularly regular and flat slope of about 35°, as 

 shown in Plate IX. The numerous channels cut by the streams 

 that drain the slope are, on an average, not more than 10 or 15 

 feet deep. Occasionally a deeper channel is seen ; some may be 

 perhaps 50 feet deep, but this is quite exceptional. The channels, 

 where they are deepest and most numerous, near Laggan, occupy 

 less than a fourth of the surface. 



At intervals deep glens intersect the crests of the hills, which 

 rise to a height of about 2000 feet above the sea. These glens, in 

 the higher hills, are frequently 500 feet deep or even more, 2 but 

 the streams from the glens run out in comparatively shallow ravines 

 cut in the sloping plane that forms the side of the Great Glen. 



The rocks exposed in Loch Lochy and Loch Oich are chiefly 

 crystalline schists and gneissose rocks. But the surface of the 

 hill-range to the south-east of Loch Oich and of upper Loch Lochy 

 is shown on Geikie's Geological Map of Scotland to be formed by 

 a narrow band of Devonian strata, cut off to the north-west by the 

 fault which runs throughout the Great Glen. It is probable that 

 the peculiarly even slope of the hillside in this part of the Glen is 

 connected with the lithological character of the Devonian. The 

 hill south and south-east of Laggan, represented in the left half 

 of Plate IX, is composed of a hard sandstone that appears to dis- 

 integrate on exposure much more rapidly than the schistose and 

 gneissose formations of the neighbourhood. To this disintegration, 

 probably, may be attributed the great amount of freshwater erosion 

 exhibited, the hill south of Laggan being traversed by far more 

 watercourses than the remainder of the smoothed area, or than the 



1 Glen Roy, with its famous Parallel Roads, is within 4 or 5 miles to the 

 south-east, being just beyond the range of bills that here forms the south- 

 eastern side of the Great Glen. 



2 These measurements are taken from the Ordnance Survey map. 



