Note 183 — Note xviii. § 112.

In Scotland there is one valley, of a kind that I believe is extremely rare in any part of the world, in accounting for which, the hypothesis of a torrent or debacle might, if any where, be employed to advantage. This is the valley which extends across the island, from Inverness to Fort William, or from sea to sea, being open at both ends, and very little elevated in the middle. It is nearly straight, and of a very uniform breadth, except that towards each end it widens considerably. The bottom, reckoning transversely, is flat, without any gradual slope from the sides towards the middle. From the sides the mountains rise immediately, and form two continued ridges of great height, like ramparts or embankments on each side of a large fossé. A great part of the bottom of this singular valley is occupied by lakes, namely, Loch Ness, Loch Oich, and Loch Lochy. Its length is about sixty-two miles, and the point of partition from which the waters run different ways, viz. north-east to the German Ocean, and south-west to the Atlantic, is between Loch Oich and Loch Lochy; and, by the estimation of the eye, I should hardly think that it is elevated more than ten or fifteen feet above the surface of either lake. The country on both sides is rugged and mountainous, and the streams which descend from thence into the valley, either fall directly into the lakes, or turn off almost at right angles when they enter the valley. Though the bottom of this valley, therefore, is every where alluvial, with the exception, perhaps, of a few rocks which appear at the surface, it is certainly not excavated by the rivers which now flow in it. The direction of the valley, it is to be observed, is the same with that of the vertical strata which compose the mountain on either side.

Here, then, we have a valley, not cut out by the working of any streams which now appear; and we may therefore make trial of the hypothesis of a debacle. This, however, will afford us no assistance; because, if we suppose what is now hollow to have been once occupied by the same kind of rock which is on either side, no force of torrents can have suddenly loosened and removed from its place a body of such vast magnitude. A greater column of water, than one having for its base a transverse section of the valley, could not act against it, and this would have to overcome the cohesion and inertia of a column of rock of the same section, and of the length of sixty-two miles. It is not hazarding much to affirm, that no velocity which could be communicated to water, not even that which it could acquire by falling from an infinite height, could give to it a force in any degree adequate to this great effect.

The explanation of this valley, which appears to me the most probable, is the following. It will be shown hereafter, that there is good reason to suppose, that, in most parts of our island, the relative level of the sea and land has been in past ages considerably higher than it is at present. In such circumstances, this valley may have been under the surface of the sea, the highest part of it being scarcely 100 feet above that level at present. It may have been a kind of sound, therefore, or strait, which connected the German Sea with the Atlantic; and the strong currents, which, on account of the different times of high water in these two seas, must have run alternately up and down this strait, may have produced that flatness of the bottom, and straightness of the sides, and that widening at the extremities, which are mentioned above. In this way, too, some difficulties are removed relative to Loch Ness, which is so deep as hardly to be consistent with the indefinite length of the period of waste that must be ascribed to the mountains on each side of it. Its depth is said, where greatest, not to be less than 180 fathoms. According to this hypothesis, it may, at no very distant period, have been a part of the bottom of the sea.