Illustrations of the Huttonian Theory of the Earth

by John Playfair

317. The courses of many rivers retain marks that they once consisted of a series of lakes, which have been converted into dry ground, by the twofold operation of filling up the bottoms, and deepening the outlets. This happens, especially, when successive terraces of gravelly and flat land are found on the banks of a river, [§ 100]. Such platforms, or haughs as they are called in this country, are always proofs of the waste and detritus produced by the river, and of the different levels on which it has run; but they sometimes lead us farther, and make it certain, that the great mass of gravel which forms the successive terraces on each side of the river, was deposited in the basin of a lake. If, from the level of the highest terrace, down to the present bed of the river, all is alluvial, and formed of sand and gravel, it is then evident, that the space as low as the river now runs must have been once occupied by water; at the same time, it is dear, that water must have stood, or flowed as high at least, as the uppermost surface of the meadow. It is impossible to reconcile these two facts, which are both undeniable, but by supposing a lake, or body of stagnant water, to have here occupied a great hollow, (which by us must be held as one of the original inequalities of the globe, because we can trace it no farther back,) and that this hollow, in the course of ages, has been filled up by the gravel and alluvial earth brought down by the river, which is now cutting its channel through materials of its own depositing. There is no great river that does not afford instances of this, both in the hilly part of its course, and where it descends first from thence into the plain. Were there room here for the minuter details of topographical description, this might be illustrated by innumerable examples.

318. It is said above, that the water must have run or stood, in former times, as low as the present bottom of the river; but there is often clear evidence, that it has run or stood much lower, because the alluvial land reaches far below the present level of the river. This is known to hold in very many instances, where it has happened that pits have been sunk to considerable depths on the banks of large rivers. By that means, the depth of the alluvial ground, under the present bed of the river, has been discovered to be great; and from this arises the difficulty, so generally experienced, of finding good foundations for bridges that are built over rivers in large valleys, or open plains, the ground being composed of travelled materials to an unknown depth, without any thing like the native or solid strata. In such cases, it is evident, that formerly the water must have been much lower, as well as much higher, than its present level, and this is only consistent with the notion, that the place was once occupied by a deep lake.

319. If, following the light derived from these indications, we go back to the time when the river ran above the highest of those levels at which it has left any traces of its operations, we shall see it composed of a series of lakes and cataracts, from which, by the filling up of the one, and the wearing down of the other, the waters have at length worked out to themselves a quiet and uninterrupted passage to the ocean. We may, indeed, on good evidence, go back still farther than the succession of such meadows or terraces, as are above mentioned, will carry us, and may consider the whole valley, or trough of the river, as produced by its own operations. The original inequalities of the surface, and the disposition of the strata, must no doubt have determined the water courses at first; but this does not hinder us from considering the rivers as having modified and changed those inequalities, and as the proximate causes of the shape and configuration which the surface has now assumed.

320. From this gradual change of lakes into rivers, it follows, that a lake is but a temporary and accidental condition of a river, which is every day approaching to its termination; and the truth of this is attested, not only by the lakes that have existed, but also by those that continue to exist. Where any considerable stream enters a lake, a flat meadow is usually observed increasing from year to year. The soil of this meadow is disposed in horizontal strata: the meadow is terminated by a marsh; which marsh is acquiring solidity, and is soon to be converted into a meadow, as the meadow will be into an arable field. All this while the sediment of the river makes its way slowly into the lake, forming a mound or bank under the surface of the water, with a pretty rapid slope toward the lake. This mound increases by the addition of new earth, sand, and gravel, poured in over the slope; and thus the progress of filling up continually advances.

321. In small lakes, this progress may easily be traced; and will be found singularly conspicuous in that beautiful assemblage of lakes, which so highly adorns the mountain scenery of Westmoreland and Cumberland. Among these a great number of instances appear, in which lakes are either partially filled up, or have entirely disappeared. In the Lake of Keswick, we not only discover the marks of filling up at the upper end, which extend far into Borrowdale, from which valley a small river flows into the lake; but we have the clearest proof, that this lake was once united to that of Bassenthwaite, and occupied the whole valley from Borrowdale to Ouse-Bridge. These two lakes are at present joined only by a stream, which runs from the former into the latter, and their continuity is interrupted by a considerable piece of alluvial land, composed of beds of earth and gravel, without rock, or any appearance of the native strata. This separation, therefore, seems no other than a bar, formed by the influx of two rivers, that enter the valley here from opposite sides, the Greata from the east, and Newland's water from the west. The surface of this meadow is at present twelve or fifteen feet at least above the level of either lake and a quantity of water of that depth must therefore have been drawn off by the deepening of the issue at Ouse-Bridge, through which the water of both lakes passes, in its way to the ocean.

Many more examples, similar to this, may be collected from the same lakes; there are indeed few places from which, in this branch of geology, more information may be collected.

322. The larger lakes exemplify the same progress. Where the Rhone enters the Lake of Geneva, the beach has been observed to receive an annual increase; and the Portus Valesiæ, now Port Valais, which is at present half a league from the lake, was formerly close upon its bank. Indeed, the sediments of the Rhone appear clearly to have formed the valley through which it runs, to a distance of about three leagues at least from the place where the river now discharges itself into the lake. The ground there is perfectly horizontal, composed of sand and mud, little raised above the level of the river, and full of marshes. The deposition made by the Rhone after it enters the lake, is visible to the eye; and may be seen falling down in clouds to the bottom.

The great lakes of North America are undergoing the same changes, and, it would seem, even with more rapidity. As the rivers, however, which supply these vast reservoirs, are none of them very great, the filling up is much less remarkable than the draining off of the water, by the deepening of the outlet. An intelligent traveller has remarked, that in Lake Superior itself the diminution of the waters is apparent, and that marks can be discovered on the rocks, of the surface having been six feet higher than it is at present. In the smaller lakes this diminution is still more evident.[164] In some of those far inland, the ground all round appeared to the same traveller to be the deposit from the rivers, of which the lakes themselves may be considered as a mere expansion.[165]

[164] Mackenzie's Voyages through the Continent of North America to the Frozen and Pacific Oceans, p. xlii. and xxxvi.

[165] Ibid. p. 122.