Illustrations of the Huttonian Theory of the Earth

by John Playfair

Note xvi. § 100.

Rivers and Lakes.

315. Rivers are the causes of waste most visible to us, and most obviously capable of producing great effects. It is not, however, in the greatest rivers, that the power to change and wear the surface of the land is most clearly seen. It is at the heads of rivers, and in the feeders of the larger streams, where they descend over the most rapid slope, and are most subject to irregular or temporary increase and diminution, that the causes which tend to preserve, and those that tend to change the form of the earth's surface, are farthest from balancing one another, and where, after every season, almost after every flood, we perceive some change produced, for which no compensation can be made, and something removed which is never to be replaced. When we trace up rivers and their branches toward their source, we come at last to rivulets, that run only in time of rain, and that are dry, at other seasons. It is there, says Dr Hutton, that I would wish to carry my reader, that he may be convinced, by his own observation, of this great; fact, that the rivers have, in general, hollowed out their valleys. The changes of the valley of the main river are but slow; the plain indeed is wasted in one place, but is repaired in another, and we do not perceive the place from whence the repairing matter has proceeded. That which the spectator sees here, does not therefore immediately suggest to him what has been the state of things before the valley was hollowed out. But it is otherwise in the valley of the rivulet; no person can examine it without seeing, that the rivulet carries away matter which cannot be repaired, except by wearing away some part of the surface of the place upon which the rain that forms the stream is gathered. The remains of a former state are here visible; and we can, without any long chain of reasoning, compare what has been with what is at the present moment. It requires but little study to replace the parts removed, and to see nature at work, resolving the most hard and solid masses, by the continued influences of the sun and atmosphere.[161] We see the beginning of that long journey, by which heavy bodies travel from the summit of the land to the bottom of the ocean, and we remain convinced, that, on our continents, there is no spot on which a river may not formerly have run.[162]

[161] Theory of the Earth, vol. ii. 294.

[162] Ibid. p. 296.

316. The view thus afforded of the operations, in their nascent state, which have shaped out and fashioned the present surface of the land, is necessary to prepare us for following them to the utmost extent of their effects. From these effects, the truth of the proposition, that rivers have cut and formed, not the beds only, but the whole of the valleys, or rather system of valleys, through which they flow, is demonstrated on a principle which has a close affinity to that on which chances are usually calculated, [§ 99]. In order to conceive rightly the course of a great river, and the communication subsisting between the main trunk and its remotest branches, let us take the instance of the Danube and cast our eyes on one of the maps constructed by Marsigli, for illustrating the natural history of that great river.[163] When it is considered, that over all the vast and uneven surface, which reaches from the Alps to the Euxine, and from the mountains of Crapack to those of Hæmus, a regular communication is kept up between every point and the line of greatest depression, in which the river flows, no one can hesitate to acknowledge, that it is the agency of the waters alone which has opened them a free passage through all the intricacies of this amazing labyrinth. In effect, suppose this communication to be interrupted, and that some sudden operation of nature were to erect a barrier of mountains to oppose the Theise or the Drave, as they rolled their waters to the Danube. From this what could possibly result, but the damming up of those rivers till their waters were deep, or high enough to find a vent, either under the bases or over the tops of the opposing ridge. Thus there would be formed immense lakes and immense cataracts, which, by filling up what was too low, and cutting down what was too high, would in time restore such a uniform declivity of surface as had before prevailed. Just so in the times that are past, whatever may have been the irregularities of the surface at its first emerging from the sea, or whatever irregularities may have been produced in it by subsequent convulsions, the slow action of the streams would not fail in time to create or renew a system of valleys communicating with one another, like that which we at present behold. Water, in all circumstances, would find its way to the lowest point; though, where the surface was quite irregular, it would not do so till after being dammed up in a thousand lakes, or dashed in cataracts over a thousand precipices. Where neither of these is the case; and where the lake and the cataract are comparatively rare phenomena; there we perceive that constitution of a surface, which water alone, of all physical agents, has a tendency to produce; and we must conclude, that the probability of such a constitution having arisen from another cause, is, to the probability of its having arisen from the running of water, in such a proportion as unity bears to a number infinitely great.

[163] Histoire du Danube, tom. i. tab. 34.

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.

323. In order to give uniform declivities to the rivers, the lakes must not only be filled up or drained, but the cataract, wherever there is one, must be worn away. The latter is an operation in all cases visible. The stream, as it precipitates itself over the rocks, hurries along with it, not only sand and gravel, but occasionally large stones, which grind and wear down the rock with a force proportioned to their magnitude and acceleration. The smooth surface of the rocks in all waterfalls, their rounded surface, and curious excavations, are the most satisfactory proofs of the constant attrition which they endure; and, where the rocks are deeply intersected, these marks often reach to a great height above the level on which the water now flows. The phenomena, in such instances, are among the arguments best calculated to remove all incredulity respecting the waste which rivers have produced, and are continuing to produce. They suffer no doubt to remain, that the height and asperity of every waterfall are continually diminishing; that innumerable cataracts are entirely obliterated; that those which remain are verging toward the same end, and that the Falls of Montmorenci and Niagara must ultimately disappear.

324. Though there can be no doubt of the justness of the preceding conclusions, when applied to lakes in general, some apparent exceptions occur, in which the progress of draining and filling up seems to have been suspended, or even to have gone in a contrary direction. These exceptions consist of the lakes which appear to have received a greater quantity of materials than was sufficient to have filled them up. Such, for example, is the Lake of Geneva, which receives the Rhone descending from the Valais, one of the deepest and longest valleys on the surface of the earth. Now, if this valley, or even a large proportion of it, had been excavated by the Rhone itself, as our theory leads us to suppose, the lake ought to have been entirely filled up, because the materials brought down by the river seem to be much greater than the lake, on any reasonable supposition concerning its original magnitude, can possibly have received. What, then, it may be said, has become of all that the Rhone has brought down and deposited in it? The lake, at this moment, retains, in some places, the depth of more than 1000 feet; and yet, of all that the Rhone carries into it, nothing but the pure water issues. If it has been continuing to diminish, both in superficial extent and in depth, from the time when the Rhone began to run into it, what must have been its original dimensions?

I cannot pretend to remove entirely the difficulty which is here stated; yet I think the following remarks may go some length in doing so.

325. It is certain, that from the present state of the Lake of Geneva, and of the ground round it, we can hardly draw any inference as to its original dimensions. Saussure has traced, with his usual skill, the marks of the course of the Rhone, on a level greatly above the present; and, by observations on the side of Mont Saleve, has found proofs of the running of water, at least 200 toises above the present superficies of the lake. But, if ever the superficies of the lake stood at this height, or at this height nearly, though we can conjecture but little concerning the state of the adjacent country, which no doubt was also on a higher level, the lake may very well be supposed to have been of far greater dimensions than it is now. It may have occupied the whole space from Jura to Saleve, and included the Lake of Neufchâtel; so that it may have been of magnitude sufficient to receive the spoils of the Valais, which, as the surface of its waters lowered, may have been washed away and carried down to the sea. Thus it may have afforded a temporary receptacle for the debris of the Alps, and may have served for an entrepot, as it were, where those debris were deposited, before they were carried to the place of their ultimate destination.

326. But the great depth which the lake has at present, still remains to be explained, because no mud or gravel could be carried beyond the gulf, of a thousand feet deep, which was here ready to receive it. The reality of this difficulty must be acknowledged; and some cause seems to act, if not in the generation, yet certainly in the preservation of lakes, with which we are but little acquainted We can indeed imagine some causes of that kind to occur in the course of the degradation of the land, which may produce new lakes, or increase the dimensions of the old. The wearing away of a stratum, or body of strata, may lay bare, and render accessible to the water, some beds of mineral substances soluble in that fluid. The district, for instance, in Cheshire, which contains rock-salt, extends over a tract of fourteen or fifteen miles, and is covered by a thick stratum of clay, more or less indurated, which defends the salt from the water at the surface, and preserves the whole mass in a state of dryness. Should this covering be broke open by any natural convulsion, or should it be worn away, as it must be in the progress of the general detritus, the water would gain admission to the saline strata, would gradually dissolve them, and form of course a very deep and extensive lake, where all was before dry land. This event is not only possible, but it should seem, that in the course of things it must necessarily happen.

327. Something of this kind may have taken place in the track of the Rhone, and may have produced the Leman Lake. It is not impossible, that, at a very remote period, the Rhone descended from the Alps without forming any lake, or at least any lake of which the remains are now existing; and this supposition, which is more probable than that of [§ 325], we shall soon find to be conformable to appearances of another kind. The river may have wore away the secondary limestone strata over which it took its course after it left the schistus of the mountains; and, in doing so, may have reached some stratum of a saline nature, and this being washed out, may have left behind it a lake, which is but modern compared with many of the revolutions that have happened on the surface of the earth.[166]

[166] There are salt springs at Bex, near Aigle, about ten miles from the head of the lake: saline strata, therefore, are probably at no great distance.

This explanation is no doubt hypothetical; but it is proposed in one of those cases, in which hypothetical reasonings are warranted by the strictest rules of philosophical investigation. It is proposed in a case, where the causes visible to man seem inadequate to the effect, and where we must therefore have recourse to an agent that is invisible. If the operations ascribed to this agent are conformable to the analogy of nature, it is all that can in reason be required.

328. Another circumstance may also influence the generation and preservation of lakes; but it is also one with which we are but little acquainted. The strata, and indeed the whole body of mineral substances which forms the basis of our land, have been raised up from the bottom of the sea, by a progress that should seem in general to have been gradual and slow. Appearances, however, are not wanting, which show, that this progress is not uniform; and that both rising and sinking in the surface of the land, or in the rocks which are the base of it, have happened within a period of time, which is by no means of great extent. In this progress, the elevations and depressions may not be the same for every spot. They may be partial, and one part of a stratum, or body of strata, may rise to a greater height, or be more depressed, than another. It is not impossible, that this process may affect the depth of lakes, and change the relative level of their sides and bottom.

329. All lakes, however, do not involve the difficulty which the preceding conjectures are intended to remove. The great lakes of North America do not, for instance, receive their supply from very large rivers. Of course, it is not from a tract great in comparison of themselves, that the waste and detritus is brought down into them; and it seems not at all wonderful, that, without being filled up, they have been able to receive it. The same, in a degree at least, is true of many other lakes.

It should also be considered, that we may err greatly in the estimate we make of the materials actually carried down and deposited in any lake. To judge of their entire amount, we should know the original form of the inequalities on the earth's surface; of the quantity of depression which existed, independently of the rivers; and though, in general, these original inequalities may be overlooked, and the present considered as made by the running of water, yet, in particular instances, this may be far from true. The Valais, for example, which we consider as the work of the Rhone, may, when the Alps rose out of the sea, have included many depressions of the surface, which the river joined together, and, from being a series of lakes, formed into one great valley.

330. The mouths by which rivers on bold rocky coasts discharge their waters into the sea, afford a very striking confirmation of the conclusions concerning the general system of waste and degradation which have been drawn above. At these mouths we usually see, not only the bed of the river, but frequently a considerable valley, cut out of the solid rock, while that rock preserves its elevation, and its precipitous aspect, wherever it is not intersected by a run of water. No convulsion that can have torn asunder the rocks; no breach that can have been made in them, antecedent to the running of the waters, will account for the circumstance of every river finding a corresponding opening, by which it makes its way to the sea; for that opening being so nearly proportional to the magnitude of the river, and for such breaches never occurring but where streams of water are found.

331. The actual survey of any bold and rocky coast, will make this clearer than any general statement can possibly do. Let us take, for an example, the coast of the British Channel, from Torbay to the Land's End, which is faced by a continued rampart of high cliffs, formed of much indurated and primeval rock. If we consider the breaches in this rampart, at the mouths of the Dart, of the Plym and Tamer, of the river at Fowey, of the Fal, the Hel, &c. it will appear perfectly clear, that they have been produced by their respective streams. Where there is no stream, there is no breach in the rock, no softening in the bold and stern aspect which this shore every where presents to the ocean. If we look at the smaller streams, we find them working their way through the cliffs at the present moment; and we see the steps by which the larger valleys of the Dart and the Tamer have been cut down to the level of the sea. If we would have still clearer evidence, that no breaches made antecedently to the running of the rivers have opened a way for them, we need only look to the opposite side, or northern shore, of the same promontory, where we also find a series of outlets, all originating in the ridge of the country, and becoming deeper as they approach the sea, but altogether unconnected with the openings on the south side; and this could hardly have been the case, had they been the effects of previous concussions, or of any peculiarity in the original structure of the rocks.

332. In contemplating such coasts as these, when we go back to the time when the rivers ran upon a level as high as the highest of the cliffs on the sea shore, we must suppose, that the land then extended many miles farther into what is now occupied by the sea. When at Plymouth, for instance, the Tamer and the Plym flowed on the level of Mount Edgecombe or of Staten Heights, if the rivers ran with a moderate declivity into the sea, the coast must have advanced many miles beyond its present line. Thus the land, when higher, was also more extended, and the limits of our island in that ancient state, were doubtless very different from these by which it is at present circumscribed.

If with the same views we consider any other of the bold coasts which the map of the world presents us with, we shall quickly remark, that wherever a deep intersection of the sea is made into the land, as on the western shores of our own island, or on those of Norway, a river runs in at the head of it, and points out by what means such inlets are formed, viz. by the united powers of the sea and of the land, the waters of the latter having opened the way by which those of the former have penetrated so far into the country.

333. It is not meant assuredly to deny the irregularities of the sea coast, as it may have originally existed; these irregularities no doubt determined the initial operations of that waste and decay, by which, in process of time, they were themselves entirely effaced. The line of our coasts may be compared to one of those curves, which are sometimes treated of in the higher geometry, where the ordinates are functions, not only of, their abscissæ, but also of the time elapsed since a certain epocha. The form of the curve at that epocha, or when the time began to flow, corresponds to the original form of the sea coast, on its emerging from the ocean, and before the powers of wasting and decay had begun to act upon it. To speak strictly, the original figure, in both cases, influences all the subsequent; but the farther removed from it in point of time, the less is that influence; so that, in physical questions, and for the purpose of such approximations as suit the imperfection of our knowledge, the consideration of the original figure may be wholly left out.