Alfred Russel Wallace : Alfred Wallace : A. R. Wallace :
Russel Wallace : Alfred Russell Wallace (sic)

 
 
The Gorge of the Aar and Its Teachings
(S529: 1896)

 
Editor Charles H. Smith's Note: Printed in the Fortnightly Review issue of 1 August 1896. Original pagination indicated within double brackets. To link directly to this page, connect with: http://people.wku.edu/charles.smith/wallace/S529.htm


    [[p. 175]] There is perhaps no valley in Switzerland that offers to the tourist so much variety and grandeur, and to the glacialist so much instruction, as the Haslithal or valley of the Aar. I visited it for the first time last summer, walking over the Grimsel Pass to the Hospice and the Aar glacier, and thence along the old mule-track and fine new carriage-road to Meiringen; staying there three days to visit the Reichenbach Falls, the Kirchet Hill, the gorge of the Aar, and other interesting localities. It seemed to me at the time that the phenomena presented by this valley afforded a striking example of the vast amount of glacial erosion, and that some of the conclusions to which they point had been overlooked by English writers. They give us, in fact, a fresh and very powerful argument in support of the power of the ancient glaciers both to deepen valleys and to grind out lake-basins; and I now propose to lay before my readers the facts which seem to me to prove the correctness of this view.

    The Grimsel Pass is a low one, only a little over 7,000 feet, but for this reason, and because it lies directly between extensive areas of perpetual snow, which give rise to some of the finest glaciers in Switzerland, it has been extensively ice-ground and presents a scene of savage grandeur which is often absent from higher passes. Everywhere the rocks are ground into huge domes or smooth slopes or rounded hollows, and these ice-ground contours extend to at least a thousand feet higher, above which level the mountains rise in sharp peaks or serrated ridges. The descent towards the Grimsel Hospice is very grand, owing to the enormous surfaces of smooth ice-ground rocks of the hardest gneiss, which plunge down at a very high angle for nearly a thousand feet into the curious little enclosed valley, with its two small rock-basin lakes in which the hospice is situated. Here we see an example of the effects of a kind of eddy in the old ice streams to which I think sufficient attention has not been paid. The torrent from the Aar glacier comes in from the west, but before reaching the Hospice turns off abruptly through a narrow gorge into the main valley, running at first nearly north. But looked at from above, this gorge is invisible, and it seems as if the valley from the glacier continued through the two small lakes further to the east. It is evident that when this district was buried deep in ice very little of it could escape through the gorge, but must have flowed over the higher slopes, while the portion in the valley, fed by ice-streams from nearly opposite directions, would acquire a slow eddying motion which [[p. 176]] would greatly aid its grinding power, and thus account for the land-locked valley and the two small rock-basin lakes.

    Proceeding down the valley we see on all sides precipitous slopes of ice-worn rocks, some of which are so smooth and so extensive that steps have had to be cut in them to form the old mule-path, the new road here going on the opposite side of the valley. Usually the valley is narrow or V-shaped, but in several places, where it widens out, as a little below the Grimsel gorge, above the Handeck fall, and especially at Guttannen about half way to Meiringen, there is more or less of flat valley bottom, suggesting filled-up lake-basins, and showing that in these places the valley is of the U-form, which is held to be the characteristic result of glacial erosion. Why this form is not more general seems to me to be due to the character of the pre-existing valleys. Where these were narrow and precipitous, with the features of ordinary mountain gorges, the greater part of the weight of the ice would rest upon their slopes, which would be ground or split off as the ice moved downwards, while it would hardly penetrate at all into the narrow valley bottom, or if it did, would be so checked in its motion by the friction of the rugged sides, craggy projections, and constant change of direction as to cause very little erosion. For this reason the higher narrow and deep valleys exhibit only those indications of superficial grinding which have been thought to prove that glaciers, however large, have no great erosive power; but whenever we reach the lower levels, where the original valleys were themselves wider, we find proofs of much greater grinding power, both vertical and horizontal, leading to the characteristic U-forms, which are so well seen in the valley of Meiringen and in the lakes below it, in the Lauterbrunnen valley in the Rhone valley between Martigny and Brieg, and especially in the grand vertical precipices which bound the Lake of Lucerne throughout its eastern half and its south-western arm.

    It is, however, after passing through the narrow gorge between the hamlets of Inner and Outer Urweid, with its picturesque waterfall which the coach road passes by tunnelling under it, that we approach the most remarkable feature of the district. The valley rapidly widens, with a perfectly flat bottom, till at Innertkirchen it becomes nearly a mile wide. Here, on the right, the united waters of the Gadmen and Genthal valleys form a junction with the Aar, issuing out of a narrow gorge between lofty rocks; but the most striking object is the barrier of the Kirchet which extends quite across the valley, rising abruptly to the height of 500 feet above it, and appearing entirely to dam the course of the foaming torrent which has now become a good sized river. Passing along the high road, which by three great zigzags mounts up the steep ascent, the entrance to the gorge is imperceptible; and it is not till having crossed the hill and descended on the other side into the wide alluvial plain of Meiringen that the stream is again [[p. 177]] seen as a considerable and rapidly flowing river. It has passed through the hill by the celebrated Aarschlucht, one of the most remarkable gorges in Europe, which is now rendered easily accessible by a wooden platform suspended a few feet above the surface of the water, and extending for about two-thirds of the total length of the gorge, or nearly half a mile.

    On entering the gorge we are struck by its extreme narrowness, usually not more than six or eight feet, often not more than four, and in some places even less; its great depth, from 200 to 300 feet, and its remarkable uniformity of width not perceptibly increasing upwards. The sides are cut away in curves or hollows such as would be caused by tumultuous eddying waters; the walls frequently overhang and then recede again, so that it is only here and there that they are sufficiently vertical to enable us to catch a glimpse of the sky, and wherever we do so we see that the upper edges of the chasm are little, if any, farther apart than are the rock-walls between which we walk. The whole surface of the rock--a hard crystalline limestone--is evidently water-worn, never presenting surfaces due to fracture except perhaps where a lateral stream enters by a picturesque cascade falling over a vertical rock, and where the gorge opens out so that daylight and sunlight freely enter it. The artificial causeway finishes where a dry lateral gorge, with a steeply rising floor of earth and vegetable débris, affords an exit to the plateau and the road from Meiringen. This short lateral gorge is of especial interest, because it reproduces in almost every detail the features of the main gorge, being about the same average width, having similar walls of hollow curvilinear form, and being equally narrow to the very top. This lateral gorge is, however, quite dry, and even in the wettest seasons can hardly have more than a trickle of water because it has no catchment basin, opening out as it does on the top of the bossy limestone rocks of the narrow ridge of the Kirchet. Hence we reach the important conclusion that this gorge could not have been formed by water derived from ordinary streams, unless at a period so remote that the whole surface contours of the district were very different from what they are now. The only explanation that seems to accord with the facts is, that we have here the result of the action of sub-glacial torrents acting throughout the whole period during which the area was buried in ice. Thus only are we able to explain the fact of the almost uniform narrowness of the gorge from bottom to top, since during the process of its formation the rock-walls would be preserved from ordinary denuding agencies, and be kept at a nearly uniform temperature. Hence we have the actual surface as it was left by the glacial waters, and its extreme narrowness together with the luxuriant vegetation which covers the plateau and fringes the edges of the chasm, aided also by the comparatively mild climate of the lower valley, have [[p. 178]] preserved for us one of the most curious and instructive products of the great ice-age. This view of the origin of the gorge is adopted by Professor Bonney, who, in his paper read before the Royal Geographical Society in 1893, says, "This chasm has been sawn by the sub-glacial torrent, while the ice itself has moulded every rock on the barrier into billowy undulations." I may, therefore, take it for granted that this view is held by most geologists who have attended to the subject, and it appears to me to be the only reasonable one; yet it is so important, and leads to conclusions so entirely opposed to the views expressed by Professor Bonney in the same paper, that it seems advisable to ascertain whether any collateral evidence can be obtained in support of it.

    There are in Switzerland a number of other gorges which have the same general characters, of being decidedly water-worn throughout, very narrow and deep, and of approximately the same width from bottom to top; but they are all formed by lateral streams where entering into a main valley, not in course of the main valley itself. Such are the gorges of the Trient at its outlet to the Rhone, that of Pfœffers on the Rhine, and many others of less importance on the tributaries of other Alpine rivers. In many of the larger valleys almost every lateral tributary enters the main stream either by a cascade or by a gorge of this character, these gorges usually being the outlet of the drainage of considerable valleys, the stream appearing to have cut through a rocky barrier similar to that of the Aar, but on a less impressive scale. Now all the gorges which have these special characters have almost certainly been formed in the same way--by sub-glacial torrents, and, if so, the presence of such gorges would be an indication of geologically recent glaciation. It is true that similar gorges may have been sometimes formed, without the intervention of an ice-covering, in rainless districts where sub-aerial denudation would not eat away the upper lips of the chasm as it was being formed. This is to some extent the case in the cañons of the Colorado; but although the lower portions may exhibit the effects of water erosion only, being by their great depth kept at a nearly uniform temperature, the upper portions of these cañons have been greatly widened and everywhere exhibit surfaces of fracture and sub-aerial erosion, largely due to the great and rapid alternations of temperature to which they are exposed.

    If this view of the formation of these water-worn gorges is correct they ought to be found only in regions which have been recently glaciated, and it will be interesting to inquire if this is the case. In our own country we have many small gorges of this character, in Wales, the Lake District, and Scotland, that of Dungeon Gill, in Westmoreland, being an example; but none are to be found in decidedly non-glaciated areas, such as Devonshire, though narrow [[p. 179]] ravines are common enough. So in the Northern United States there are many such gorges, the Ausable Chasm in the Adirondacks, and Watkin Glen near Seneca Lake are well-known tourist resorts; but in the Southern States, beyond the glaciated area, there are no similar gorges, although the southern Alleghanies are loftier than farther north, and contain much grand and picturesque mountain scenery and many waterfalls and deep ravines, but these are all of the rugged and weathered type.

    The best test, however, of these vertical gorges being really produced by sub-glacial torrents, will be to ascertain whether they occur in any extensive tropical mountain-region where there has certainly been no recent glaciation. Brazil seemed to me to be such a country, of great extent, mountainous throughout a large part of its area, and very thoroughly explored. I therefore wrote to my friend, Professor Branner, of the Stanford University, California, who spent three years in making a geological survey of Brazil, visiting every part of the country and paying special attention to all the physical features of the surface.

    In reply to my inquiry, Professor Branner writes as follows:--

    "What you say of certain channels in Switzerland is certainly most interesting. . . . I recall the fact that there are just such gorges at Grindelwald and at Zermatt opening from beneath the glaciers. I have turned over in my mind all the facts I have in regard to stream-channels in unglaciated areas, and I cannot recall a single instance of such a gorge. Those most nearly approaching it are some of the mountain gorges in the diamond regions of Brazil, but those belong to an entirely different type." (Sketches are given showing deep valleys vertical on one side and sloping on the other.) "There are no such gorges as you describe in any part of Brazil that I have seen. In the United States there are such in the glaciated area, such as Watkin Glen in the State of New York. South of the glaciated area I have no recollection of ever having seen such gorges."

    I have endeavoured, by means of collateral evidence, to support the generally accepted view that the gorge of the Aar and others of like nature have been wholly formed by sub-glacial torrents, because, if they have been so formed, they afford a distinct class of evidence in favour of the large amount of glacial erosion in general, and of the theory of the glacial origin of the Swiss valley-lakes in particular, but which has, so far as I know, been overlooked by most writers on the subject. The nature and bearing of this evidence I will now briefly discuss.

    The large-scale Swiss Government Map shows us that the valley above the gorge of the Aar is less than 20 feet above the level of the valley below the gorge. Both are flat alluvial plains having every appearance of being filled-up lake basins. The Kirchet hill, pierced by the [[p. 180]] gorge, is about 450 feet high, the highest ice-ground bosses being nearly 600 feet, while the lowest point over which the road passes is a little more than 300 feet, but this notch is apparently the continuation of the lateral gorge already described. During the long time occupied in the cutting of the great gorge by the sub-glacial torrent, the Kirchet itself must have been greatly lowered, and was probably, when the ice first reached it, at least 150 feet higher than its present highest bosses, or about 800 feet above the present level of the upper plain. What is the depth of the alluvial deposit in this plain we do not know, but as it extends more than two miles up the valley, with a rise of about 30 feet in a mile, it is probably not less than 200 feet deep. But if we go back to the pre-glacial period before the gorge was cut, the valley-bottom must have sloped upward from the level of the Kirchet, and must therefore have been about 800 feet higher than it is now. It follows, that while the gorge was being cut, the floor of the valley above it was being ice-ground, resulting in a basin about 1,000 feet deep if we add 200 feet for the supposed depth of alluvial deposits. The cause of the exceptional grinding power of the glacier in this part is very clear. From the Grimsel downwards all the lateral tributaries are short, but just where the valley widens above Innertkirchen, the Urbach valley opens from the south, bringing the outflow of the Gauli glacier, and a little farther on a large valley opens from the north-east, having a drainage area about equal to that of the Aar itself, and which must have nearly doubled the size of the main icestream by the tributaries from the Trift, the Susten, and the Titlis glaciers. The effect of these great inflows from different directions must have been to cause a heaping up of the ice, and to give it an eddying motion, thus producing the powerful grinding tool which hollowed out the rock-basin above the Kirchet. The influx of this great icestream from the north-east will also explain the curious abruptness of the Kirchet hill, which, almost like a wall, blocks up the valley, and which Professor Bonney has advanced as an argument against the theory I am advocating. He says: "This grassy basin might well be claimed as an instance of glacial erosion--by which indeed it possibly may have been deepened; but if we attribute to this agency the removal of all the rock between the summit level of the barrier and the present meadow-floor, how are we to explain the existence of the steep rocky slope down which the road to Imhof descends in zigzags? The slopes, in the teeth of an advancing glacier, are always comparatively gentle, and very unlike those which are presented by this rocky rib." This would be a valid objection if the Aar glacier had continued its course in a straight, or nearly straight, line to Meiringen; but the influx of a large glacier-stream from the north-east must have so diverted that of the Aar, that the resultant flow would have been across the lower valley and almost along the [[p. 181]] steep face of Kirchet instead of directly across it. This would have been the case, because the glacier stream from the north-east was not only equal in size to that of the Aar valley, but had a more rapid descent, and, therefore, a quicker flow. In the last five miles the Aar valley has a fall of about 1,500 feet, while the two north-eastern valleys have an average fall of about 2,000 feet; and they are also much wider, which would still further facilitate rapidity of outflow.

    The influx of this powerful glacier stream from the north-east well explains the main physical features of the district, especially the nearly uniform plateau of the Geissholz Alp, at about the same level as the top of the Kirchet, bounded by the line of precipices extending to the Reichenbach falls, and continued along the south side of the valley. On the north side the plateaux are at a higher level, indicating that the motion of the glacier was there less rapid, and its grinding power correspondingly diminished. While the main body of the glacier thus moved onward towards Thun, the meeting of three glacier streams, of unequal size and velocity, and from different directions, would produce a vast ice-eddy at their junction, accompanied by a heaping up of the surface and great grinding power, thus leading to those curiously embayed hollows around Eppigen and Im Hof, which are not found on the opposite side of the valley, where the glacier had a direct onward motion.

    It appears, therefore, that the singular phenomenon of a great valley barred across by a precipitous rocky ridge, which is pierced only by a narrow water-worn gorge, admittedly sawn down by the débris-laden water of the sub-glacial torrent, does afford a most striking additional proof of the power of the old glaciers to grind out rock-basins. The only escape from this conclusion is to call in the aid of hypothetical local subsidences or elevations of which no direct evidence has yet been found. And when we see that, besides all the existing valley-lakes of glaciated countries, we have also to account for these gorge-pierced rocky dams, and the filled-up lake-basins above them, and for the number of other filled-up rock basins which occur in the course of all the great Alpine valleys, the explanation by subsidence becomes more and more difficult and improbable. And the difficulty is still further increased by the consideration, that all these earth-movements must have been comparatively recent, that in all cases they must have been so rapid that the erosion of the valley did not keep pace with it, and that, in all non-glaciated areas, either no earth-movements have occurred within the recent period, or, if they have occurred, they have failed, in every single case, to produce a lake-basin, because erosion has fully kept pace with elevation. A theory which involves such a series of improbable assumptions, and which is not supported by any direct evidence, stands self-condemned.

    But besides this body of evidence, in itself almost conclusive, I adduced in my article on the "Erosion of Lake Basins" (in the [[p. 182]] Fortnightly Review, December, 1893), a new and independent argument founded on the characteristic contours of all these valley-lakes, which were shown to be such as could be produced only by ice-erosion in a pre-existing valley, while any form of damming up by subsidence above or elevation below the lake would necessarily result in a very different contour.

    I refer to this argument, because, since my articles appeared, I have received from two American geologists copies of papers showing that similar features have been observed in some of the American valley-lakes, and the same general conclusion has been drawn from them. In a paper on "Glaciation in the Finger Lake region of New York," by Dr. D. F. Lincoln (in the American Journal of Science, vol. xliv., October, 1892), it is shown that Seneca Lake is bordered through a great part of its circumference by vertical cliffs of rock, that it undulates in moderate curves, and that inflowing streams with rocky beds enter it, either near the level of the water or at a considerable elevation. In some cases a lateral stream flows over nearly level ground to the cliff-edge without any valley banks, suggesting that the main valley occupied by the lake was formerly at this high level. The conclusion is reached that this and other lakes with similar characteristics have been "deeply gouged by ice." In a later paper (dated August, 1893) he notices the absence of side-valleys along Seneca and Cayuga Lakes, and points out the remarkable difference in this respect between these lakes and valleys situated south of the great moraine, which latter are "notched down to the flood-plain level every mile or two," while "the lake-side for stretches of ten to fifteen miles seems like an artificially smoothed garden slope."

    In the Bulletin of the Geological Society of America, vol. v., pp. 339-356 (March, 1894), Mr. Ralph S. Tarr has a paper on "Lake Cayuga a Rock-basin," illustrated by a map and section. He maintains that Lake Cayuga is situated in a rock basin, about 435 feet deep, and formed by glacial erosion; the proof being that the pre-glacial tributaries of the valley are rock-enclosed, and that their lowest points are above the present lake-surface. The map shows all the lakes of this region to have the very same kind of contour as that on which I founded my argument.

    I venture to hope that the brief sketch I have here given of one of the grandest and most interesting valleys in Switzerland and its bearing on that most curious and fascinating problem--the Great Ice Age, its causes, its duration, and its effects, may attract the attention of some readers who are about to visit the district, and enable them, when they come in sight of the Kirchet or visit the celebrated gorge, to appreciate the remarkable and almost unique character of the natural phenomena before them, and the marvellous episode in the recent geological history of the north-temperate zone which has produced them.


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