Illustrations of the Huttonian Theory of the Earth

by John Playfair

362. Longitudinal valleys, with the water bursting out transversely from their sides, like the preceding, are by no means confined to mountains of the first order. We have a very good example, though on a small scale, of a valley of this sort, within a few miles of Edinburgh. The Pentland Hills form a double ridge, separated by a small longitudinal valley, that runs from N. E. to S. W., the water of which issues from an opening almost in the middle, and directed towards the south. This, therefore, is not the work of any great torrent, which overwhelmed the country; for no one direction, which it is possible to assign to such a torrent, will afford an explanation, both of the valley and its outlet.[183]

[183] 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.

363. They who maintain the existence of the debacle, will no doubt allege, that though these valleys were not cut out by means of it, yet others may. But it must be recollected, that if some of the greatest and deepest valleys on the face of the earth, such as that just mentioned, on the east side of Mont Blanc, are thus shown to be the work of the daily wasting of the surface, what other inequalities can be great enough to require the interposition of a more powerful cause? If a dignus vindice nodus does not exist here, in what part of the natural history of the earth is it likely to be found?

364. The large masses of rock so often met with at a distance from their original place, are one of the arguments used for the debacle. It has, however, been shown, that, supposing a form of the earth's surface considerably different from the present, especially, supposing the absence of the valleys which the rivers have gradually cut out, the transportation of such stones is not impossible, even by such powers as nature employs at present. Now, without the supposition that the surface was more continuous, and that its present inequalities did not exist, no force of torrents, whatever their velocity and magnitude may have been, could have produced this transportation. No force of water could raise a stone like the pierre de goutté from the bottom of a valley, to the top of a steep hail. Indeed, if we suppose a great fragment of rock to be hurried along on a horizontal or an inclined plane, by the force of water, the moment it comes to a deep valley, and has to rise up over an ascent of a certain steepness, it will remain at rest; the water itself will lose its velocity, and the heavy bodies which it carried with it will proceed no farther. Thus, therefore, we have the following dilemma. If the surface is not supposed to have had a certain degree of uniformity in past times, a debacle is insufficient for the transportation of stones: If it is supposed to have had that uniformity, a debacle is unnecessary.

365. Another fact, which has been supposed favourable to the opinion of the action of great torrents at some former period, is, that in countries like that round Edinburgh, where whinstone hills rise up from among secondary strata, a remarkable uniformity is observed in the direction of their abrupt faces. Thus, in the country just mentioned, the steep faces generally front the west, whiles in the opposite direction, the slope is gentle, and the hills decline gradually into the plain. Hence it is supposed, that a torrent, sweeping from west to east, has carried off the strata from the west side of these hills, but, being obstructed by the whinstone rock, has left the strata on the east side in their natural place.

But, besides that no force which can ever be ascribed to a torrent could have removed at once bodies of strata 300 or 400 feet, nay even 800 or 1000 in thickness, which must have been the case if this were the true explanation of the fact, there is a circumstance which may perhaps enable us to explain these phenomena without the assistance of any extraordinary cause. The secondary strata in which the whinstone hills are found in this part of Scotland, are not horizontal, but rise or head towards the west, dipping towards the east. The side, therefore, of the whinstone hills which is precipitous, is the same with that towards which the strata rise. Now, from the manner in which these hills are supposed to have been elevated, the strata are likely to have been most broken and shattered towards that side, while, on the opposite, they had the support of the whinstone rock. They would become a prey, therefore, more easily to the common causes of erosion and waste on the upper side than on the lower. The streams that flowed from the higher grounds would wear them on the former most readily; and the action of these streams would be resisted by the superior hardness of the whinstone, just as the great torrent of the debacle is supposed to have been.

It should also be observed, that this fact of the uniform direction of the abrupt faces of mountains, is often too hastily generalized. In primitive countries, it is no farther observed than by the steep faces of the mountains being most frequently turned toward the central chain. In Scotland, as soon as you leave the flat country, and enter the Highlands, the scarps of the hills face indiscriminately all the points of the compass, and are directed as often to the east as to the west.

366. Where the strata are nearly horizontal, they afford the most distinct information concerning the direction and progress of the wasting of the land. The inclined position of the strata, which in all other cases must enter for so much into our estimate of the causes which have produced the present inequality of the earth's surface, disappears there entirely; and the whole of that inequality is to be ascribed to the operations at the surface, whether they have been sudden or gradual. A very important fact from a country of this sort, is related by Barrow, in his Travels into Southern Africa. The mountains about the Cape of Good Hope, and as far to the north as that ingenious traveller prosecuted his journey, are chiefly of horizontal strata of sandstone and limestone, exhibiting the appearance, on their abrupt sides, of regular layers of masonry, of towers, fortifications, &c. Now, among all these mountains, he observed, that the high or steep sides look constantly down the rivers, while the sloping or inclined sides have just the opposite direction. When, in travelling northward, he passed the line of partition, where the waters from running south take their direction to the north, he found, that the gradual slope, which had hitherto been turned to the north, was now turned to the south: The abrupt aspect of the mountains, in like manner, from facing the south, was directed to the north; so that, in both cases, the hills turned their backs on the line of greatest elevation.[184]