« ZurückWeiter »
T. Loch Treig.
ing in a northeasterly direction, and some ten miles in length. Across the mouth of this valley, at right angles with it, runs the valley of Glen Spean, trending from east to west, Glen Roy thus opening directly at its southern extremity into Glen Spean. Around the walls of the Glen Roy valley run three terraces, one above the other, at different heights, like so many roads artificially cut in the sides of the valley, and indeed they go by the name of the "parallel roads." These three terraces, though in a less perfect state of preservation, are repeated for a short distance at exactly the same levels on the southern wall of the valley of Glen Spean, just opposite the opening of the Glen Roy valley; that is, they make the whole circuit of Glen Roy, stop abruptly, on both sides, at its southern extremity, and reappear again on the op
G. Glen Gloy.
L. 0. Loch Lochy.
A. Loch Arkeig.
E. Loch Eil.
N. Ben Nevis.
1, 2, 3. The three parallel roads.
posite wall of Glen Spean. I should add, however, that all three do not come to this sudden termination ; for the lowest of these terraces turns eastward into the valley of Glen Spean, following the whole curve of the eastern half of the valley, while, of the two upper terraces, there is no trace whatever, nor is there any indication that either of the three ever existed in the western half of the valley. When I first visited the region, these phenomena had already been the subject of earnest discussion among English geologists. The commonly accepted explanation of the facts was that these terraces marked ancient sea-levels at a time when the ocean penetrated much farther into the interior, and Glen Roy and the adjoining valleys were as maxy fiords or estuaries. And though the present elevation of the locality made such an interpretation improbable at first sight, the first or highest of the terraces being eleven hundred and forty-four feet above the present sealevel, the second eighty-two feet below the first, and the third and lowest two hundred and twelve feet below the second, or eight hundred odd feet above the level of the sea, it was thought that the oscillations of the land, its alternate subsidences and upheavals, proved by the modern results of geology to have been so great and so frequent, might account even for so remarkable a change. There are, however, other objections to this theory not so easily explained away. There are no traces of organic life upon these terraces. If they were ancient sea-beaches, we should expect to find upon them the remains of marine animals, shells, Crustacea, and the like. All the explanations given to lessen the significance of this absence of organic remains are futile. Again, v. In should the lower terrace alone be continued into the eastern end of the valley of Glen Spean, while there are no terraces at all in its western part, since both must have been as fully open to the sea as Glen Roy valley itself? This seemed the more inexplicable since all the terraces exist on the valley-wall opposite the outlet of Glen Roy, showing that this sheet of water, wherever it came from, filled the valley itself and the space between it and the southern wall of Glen Spean, but failed to spread, on either side of that space, into the eastern and western extension of Glen Spean. It is evident, that, at the time the water filled Glen Roy, some obstruction blocked the valley of Glen Spean, both to the east and west, leaving, however, that space in the centre free into which Glen Roy opens, while, by the time the water had sunk to the level of the lowest terrace, one of these barriers, that to the east, must have been removed, for the lowest terrace, as I have said, is continuous throughout the eastern part of Glen Spean.*
* Hiving enumerated the characteristic features of the glacial phenomena in the preceding pages, I throw into this note some expla
Prepossessed as I was with the idea of glacial agency in times anterior to ours, these phenomena appeared to me under
nations which may render my views of the parallel roads more intelligible, not to interrupt again the exposition with details. It would be desirable, however, that the reader should first make himself thoroughly familiar with the localities concerned, before proceeding any farther. I would therefore state here, that, in the wood-cut opposite, G. R. indicates the valley of Glen Roy, with the three parallel roads marked 1, 2, 3. Glen Spean is designated by G. S., and the river flowing at its bottom by S. Loch Laggan, out of which the River Spean rises, is marked V G- indicates Glen Gloy, a little valley to the northwest of Glen Roy, with a single terrace. Loch Treig is designated by T., Loch Lochy by L. 0., Loch Arkeig by A., and Mocldhu Hill by M., whilt E. indicates Loch Eil. The Great Glen of Scotland, through which the Caledonian Canal runs, extends in the direction of L. O. and E. The position of Ben Nevis is designated by N. The dotted area betwen X. and M. marks the place occupied by the great glacier of Ben Nevis, when it extended as far as Moeldhu; while the close continuous lines in front of Loch Treig indicate the direction of the glacial scratches left across Glen Spean by the glacier of Loch Treig, when it extended as far as the eastern termination of the two upper terraces. It ought to be remembered, in this connection, that the bottom of the valley of the Spean, as well as that of Glen Roy, is occupied by loose materials, partly drift, that is, materials acted upon by glaciers, and partly decomposed fragments of rocks brought down by the torrents, greatly impeding the observation of the polished surfaces. The river-bed is cut through thjs deposit, and here and there through the underlying rock. Besides the parallel roads, there are also peculiar accumulations of loose materials in Glen Roy and Glen Spean, more particularly connected with the lowest terrace, which Mr. Darwin and Professor Jnmieson have shown to be little deltas formed during the existence of the lake of Glen Roy at the bottom of the gullies intersecting the shelves of the upper roads. The outlet for the water at the period during which the second terrace was formed, not known when I visited Glen Roy, has been discovered by Mr. MilneHolme, and also observed by Professor Jamiesou. During the formation of the upper terrace, the waters escaped through the westernmost tributary of the River Spey, in the direction of the northeast corner of the woodcat, and during that of the lowest terrace, at the eastern end of Loch Laggan, also through 6. new aspect. I found the bottom of Glen Spean so worn by glacial action as to leave no doubt in my mind that it must have been the bed of a great glacier, and Dr. Buckland fully concurred with me in this impression. Indeed, the face of the country throughout that region presents not only the glacier - marks in great perfection, but other evidences of the ancient presence of glaciers. There are moraines at the lower end of Glen Spean, remodelled, it is true, by the action of currents, but still retaining enough of their ancient character to be easily recognized; and some of the finesf%xamples of the roches motttonne"es I have seen in Scotland are to be found at the entrance of the valley K Loeh Treig, a lateral valley opening into Glen Spean on its southern side, and, as we shall see hereafter, intimately connected with the history of the parallel roads of Glen Roy. These roches moutonne"es may very fairly be compared with those of the Grimsel, and exhibit all the characteristic features of the Alpine ones. One of them, lying on the western side of the valley where it opens into Glen Spean, is crossed by a trap-dike. The general surface of the hill, consisting of rather soft mica, has been slightly worn down by atmospheric agencies, Bo that the dike stands out some three - quarters of an inch above it. On the dike, however, the glacier - marks extend for its whole length in great perfection, while they have entirely disappeared from the surrounding surfaces, so as to leave the dike thus standing out in full relief. This is an instructive case, showing how little disintegration has gone on since the driftperiod. All the currents that have swept
the valley of the Spey. The state of preservation of the parallel roads ia such as to prove that no disturbance of any importance can have taken place in the country since they were formed. Far from believing, therefore, that these remarkable shelves are anclent seabeaches, I am prepared to maintain, that, had the area occupied by them been submerged only for a few days, under an ocean rising and falling for several feet with every tide, no vestige would have been left of their former exist
over it, all the rains that have beaten upon it, have not worn away one inch from the original surface of the hill. I have observed many other roches moutonntfes in Scotland, especially about the neighborhood of Loch Awe, Loch Pyne, and Loch Etive. In fact, they may be found in almost all the glens of Scotland, in the lake-region of England, and in the valleys of Wales and Ireland.
Following the glacial indications wherever we could find them in the country about Glen Roy, it became evident to me that the whole western range of the Grampian Hills had once been a great centre of glaciers, that they had come down toward Glen Spean through all the valleys on the mountain-slopes to the north and south of it, so that this valley had become, as it were, the great drainage-bed for the masses of ice thus poured into it laterally, and moving down the valley from cast to west as one immense glacier. It is natural to suppose, that, at the breaking-up of the great sheet of ice which, if my view of the case is correct, must have covered the whole country at this time, the ice would yield more readily in a valley like that of Glen Roy, lying open to the south and receiving the full force of the sun, than in those on the opposite side of Glen Spean, opening to the north. At all events, it is evident that at some time posterior to this universal glacial period, when the ice began to retreat, Glen Roy became the basin of a glacial lake such as we now find in the Alps of Switzerland, where occasionally a closed valley becomes a trough, as it were, into which the water from the surrounding hills is drained. In such a lake no animals are found, such as exist in any other sheet of fresh water, and this would account for the absence of any organic remains on the terraces of Glen Roy. But at first sight it seemed that this theory was open in one respect to the same objection as the other. What prevented this sheet of water from spreading east and west in Glen Spean? If it not only filled Glen Roy, but extended to the southern side of Glen Spean immediately opposite the opening of Glen Roy, what prevented it from filling the whole of that valley also? In endeavoring to answer this question, I found the solution of the mystery.
The bed of Glen Spean, through its whole extent from east to west, is marked, as I have said, by glacial action, in rectilinear scratches and furrows. This westward track of the main glacier is crossed transversely near the centre of the valley by two other glacier-tracks cutting it at right angles. Upon tracing these cross-tracks carefully, I became satisfied, that, after the surrounding ice bad begun to yield, after the masses of ice which descended from the northern and southern slopes of the mountains into Glen Spean had begun to retreat, and to form local limited glaciers, two of those lateral glaciers, one coming down from Ben Nevis on the southwest, the other from Loch Treig on the southeast, extended farther than the others and stretched across Glen Spean.* These two glaciers for a long
• The wood-cut on p. 730 is a reproduction of the little map accompanying a paper of mine upon " The Glacial Theory and its Recent Progress," printed in the "Edinburgh New Philosophical Journal " for October, 1842. I might have greatly improved the topography, and represented more accurately the details of the phenomenon, by availing myself of the much larger and very minute map recently published by Professor Thomas 1-. Jainieson, of Aberdeen; but I thought it advisable to leave my first sketch as I presented it twenty-two years ago, in order to show that Sir Charles Lyell is mistaken in ascribing (se« " Antiquity of Man," pp. 260, 281) the discovery of the glacier of Loch Treig to Professor Jamieson. A comparison of his statements with mine will show that the solution of the problem offered by him is identical with that proposed by me, as he himself candidly admits (" Quarterly Journal of the Geological Society" for August, 1863, p. 239). I have only one fault to flnd with his observations, and, as 1 have never revisited the locality since, this remark may satisfy him that my examination of its features was not so hurried ;is he supposes. Professor Jamieson confounds the effects of two distinct glaciers moving in different valleys as the action of one and the same glacier. lu my paper, it is true, I
time formed barriers across the western and eastern extension of this valley, damming back the waters which filled
made no allusion to the great glacier of Glen Spean, the existence of which I had recognized along the river from Loch Laggan nearly to the Caledonian Canul. I publish my observations upon this great central glacier for the fint time in the present article, having omitted them in my contributions upon this subject to the scientific periodicals of the day simply because I thought best not to complicate my exposition of the facts concerning the parallel roads by considerations foreign to their origin, convinced as I waa, from the manner in which the glacial theory was then received, that they would not be understood, and still less admitted. But now that all the geologists of Great Britain seem to have given their adhesion to it, I may be permitted to state that I already knew then, what Professor Jamieson has overlooked in his latest paper, that a separate glacier had occupied the valley of the Spean prior to the formation of the parallel roads, and that at that time the glacier of Loch Treig was only a lateral tributary of the same, just as the glacier of the Thierberg is a tributary of the glacier of the Aar. It was not until the Glen Spean glacier had retreated to the hills east of Loch Laggan that the glacier of Loch Treig could form a barrier across Glen Spean, and thus dam the waters in Glen Roy which produced the parallel roads. The marks left by the great Glen Spean glacier in the valley are mistaken by Professor Jamieson for indications, that, in its greatest extension, the glacier of Loch freig not only advanced across Glen Spean, but divided into two branches, one moving westward down Glen Spean, the other eastward up Glen Spean, as far as Loch Laggan. Any one sufficiently familiar with existing glaciers to compare their action with the phenomena referred to above will at once sec the impossibility of such a course for any glacier coming down from Loch Treig. At the time the Grampians had become a separate centre of glacial action a great glncier must have moved down, towards the Caledonian Canal, through Glen Spean, receiving as tributaries lateral glaciers nut only from Loch Treig and Glen Rny, but also from all the other minor lateral valleys emptying into Glen Spean, the largest of which must have come from the range of Ben Nevis,—just as the great glacier of the valley of the Rhone once received as tributaries all the glaciers coming down into that valley from the southern slope of the Bernese Oberland, and from the northern slope of the Valerian Alps, and at one time also from the eastern slopes of the range of Mont Blanc. And when the large glacier Glen Roy and the central part of Glen Spean.
Evidently the glacier descending from Loch Treig was the first to yield, for, by the time the Glen Roy lake had sunk to the level of the lowest terrace, the entrance to the eastern extension of the Talley must have been free, otherwise the water could not have spread throughout that basin as we find it did; but it would seem that by the time the western barrier, or the glacier from Ben Nevis, was removed, the sheet of water was too far reduced to have left permanent marks of its outflow into the Great Glen, except by disturbing and remodelling the large moraines of the older Glen Spean glacier. There are faint indications of other terraces in Glen Roy, even at a higher level than* the uppermost parallel road, owing their origin probably to the short duration of a higher level of the glacier-lake, when the great general glacier had not yet been lowered to a more permanent level determined by a limited circumscription within the walls of the valleys. There are other terraces in neighboring valleys at still different levels, — in Glen Gloy, for instance, where the one horizontal road was no doubt formed in consequence of the damming of the valley by a glacier from Loch Arkeig. Mr. Darwin has seen another in Glen Kinfillen, which I would explain by the presence of a glacier in the Great Glen, the marks of which are particularly distinct about the eastern end of Glen Garry.
The evidence of the ancient presence of glaciers is no less striking in other parts of the Scotch Highlands. Between the southeastern range of the Grampian Hills, in Forfarshire and Perthshire, and the opposite ridge of Sidlaw Hills, stretches
occupying the lower, and therefore warmer, level gradually disappeared and retreated far away to levels where it could maintain itself against the effect of a returning milder climate, the opening spring of our era, as we may call it, the lateral glaciers, arising from the nearer high grounds, could extend across the valleys, but not before.
the broad valley of Strathmore. At the time when Glen Spean received the masses of ice from the slopes of the western Grampian range, the glaciers descended from the valleys on the southern slope of the southeastern range and from those on the northern slope of Sidlaw Hills into the capacious bed of the valley which divides them. The glacial phenomena of this region present a striking resemblance in their general relations to those of the Alps and the Jura. The Grampian range on the northern side of Strathmore valley occupies the same position in reference to that of the Sidlaw Hills opposite, as does the range of the Alps (o that of the Jura, while the intervening valley may be compared to the plain of Switzerland. As from the Bernese Oberland and from the valleys of the Reuss and Limmath gigantic glaciers came down and stretched across the plain of Switzerland to the Jura, scattering their erratic boulders over its summit and upon its slopes at the time of their greater extension, and, as they withdrew into the higher Alpine valleys, leaving them along their retreating track at the foot of the Jura and over the whole plain, so did the glaciers from Glen Prossen and parallel valleys on the Grampian Mountains extend across the valley of Strathmore, dropping their boulders" not only on the slopes and along the base of the Sidlaw Hills, but scattering them in their retreat throughout the valley, until they were themselves reduced to isolated glaciers in the higher valleys. At the same time other glaciers came down from the heights of Schihallion on the west, and, descending through the valley of the Tay, joined the great masses of ice in the valley of Strathmore, thus combining with the eastern ice-field, just as the glacier from Mont Blanc and the valley of the Rhone formerly combined in the western part of Switzerland with those of the Bernese Oberland. The relations are identical, though the geographical position is reversed, — the higher range, or the Grampian Hills, lying to