CHAPTER X

THE HIGHLAND LAKES

ONE of the great charms of Highland landscape is the gleam of still water that so often gives the one element of repose in a scene of broken cliff and tumbled crag, of noisy cascade and driving cloud. No casual tourist can fail to notice what a wonderful variety of lakes he meets with in the course of any traverse he may take across the country. Among the higher mountains, there is the little tarn nestling in a dark sunless corry, and half encircled with grim snowrifted crags. In the glen, there is the occasional broadening of the river into a lake that narrows again to let the stream rush down a rocky ravine. In the wider strath, there is the broad, still expanse of water, with its fringe of wood and its tree-covered islets. In the gneiss region of the north-west, there is the little lochan lying in its basin of bare rock, and surrounded with scores of others, all equally treeless and desolate.

While alive to all their charm and variety, the geologist experiences in sight of these lakes a peculiar interest, for he recognises in them one of the great problems of his science to which no completely satisfactory solution has yet been found. Many lakes indeed present no difficulty,

but have their history plainly written on every surrounding lineament of the ground. Others, in spite of all that has been done to extract from them the secret of their origin, persistently refuse to tell it.

It must not be supposed, however, that there is anything specially mysterious about the Scottish lakes. In considering their history, we are bound to remember that they are only a local exhibition of a feature that characterises the whole of the northern part of the northern hemisphere. They find their counterparts in Scandinavia and Finland on this side of the Atlantic, and in British North America on the other. They may be arranged in four classes, each of which has its own peculiar scenery, and has been formed in a different way: (1) Lakes of the plains; (2) Moraine

FIG. 49.-Section showing the structure of the basins of the Lakes of the Plains, lying in hollows of the superficial covering of drift.

tarns; (3) Rock-tarns; (4) Glen-lakes. About the first two of these classes there is no difference of opinion, but much discussion has arisen as to the history of the last two.

1. The Lakes of the Plains, as their name denotes, do not properly belong to the Highlands, and I will therefore reserve the description of them until I come to the consideration of the Midland Valley, where they are so well developed. But, for the sake of completeness of narrative, I may say here that these lakes lie in hollows of the covering of detritus left on the surface of country when the icesheets and icebergs retreated (Fig. 49). These superficial materials were thrown down very irregularly, and when water began once more to flow over the land, it gathered into the depressions and formed lakes.

2. Moraine-Tarns are small sheets of water ponded back by some of the last moraines shed by the retreating glaciers. They may be counted by hundreds in the Highlands, generally at the heads of glens or at the mouths of corries (Fig. 50; see also Fig. 37). Probably the most southerly in the Highland region are those in the western part of the granite hills of Arran, of which the most picturesque is one which lies at the foot of the corry on the seaward face of the northern end of Beinn Bhreac (spotted hill). The granite corries of Aberdeenshire furnish still more striking examples, such as those around the Ben Macdhui, Braeriach, Cairn Gorm, Ben Aven, Ben a' Bhuird, and Lochnagar.

FIG. 50.-Section showing the structure of a Moraine Lake, where the water is ponded back by moraine mounds in a valley. a, Rock of the district. b, Moraine mounds.

As a rule, lakes of this class are only to be seen among the high grounds. But in the north and west, where the glaciers came down to the sea, moraine-tarns are to be found at much lower levels. Loch Brora, for example, in the east of Sutherland, to which further allusion will be made in the next chapter, is only separated from the North Sea by a series of moraine mounds and the raised beach which has been levelled out of them.

3. Rock-Tarns are small lakes lying in rock-basins on the sides of mountains or the summits of ridges, and on rocky plateaux or plains. They have no necessary depend

ence upon lines of valley.

On the contrary, they are

badcast over the districts in which by far the most abundant of all the ispersed over all parts of the Highnerous in the north-west, especially nd in the west of Ross-shire and surface of the Archæan gneiss is so with them that many tracts consist almost water as of land (Fig. 52). They almost invariably Their sides, and

rongly ice-worn platforms of rock.

rocky islets which diversify their surface, have been powerfully glaciated. They cannot be due directly to either fracture or subsidence, but are obviously hollows produced by erosion. They have accordingly, with much

FIG. 51.--Section showing the structure of a Rock-Tarn.

probability, been assigned to the gouging action of the sheets of land-ice by which the general glaciation of the country was effected.

The most striking example of this class of lakes in the Highlands is undoubtedly Coruisk, in the Isle of Skye. Lying only a few hundred yards from the end of the Atlantic inlet, Loch Scavaig, and not many feet above the level of the sea, it is almost surrounded by an array of the blackest and most jagged precipices in Britain. The rock (gabbro) of which they consist is of volcanic origin, and is endowed with singular toughness and durability. Along the crests and upper parts of the cliffs, it has been split by

the weather, acting along its joints and dykes, until it presents a notched and splintered sky-line, to which there is elsewhere no equal within these islands. But lower down, where the ice that once filled the corry has been able to act upon its sides, this obdurate rock has been ground smooth, polished, and striated. Its very obduracy, which

06

FIG. 52. Part of the island of Lewis, illustrating the remarkable abundance of rock-tarns in the districts of Archæan gneiss.

must have made the task of the glacier a more than usually laborious one, has enabled it to retain the impress of the ice-work with a freshness and perfection truly astonishing. Dome rises above dome, hummock beyond hummock, so smooth and shorn that it is difficult to realise that the ice