The correlation between these different loose materials and the position in which they are found helps us also to detect their origin. The loose materials bearing glacier-marks are always found resting upon surfaces which have been worn, abraded, and engraved in the same manner, while the water-worn pebbles are everywhere found resting upon rocks the abrasion of which may be traced to water. It is true that in some localities, as, for instance, in the gravel-pit of Mount Auburn, near Cambridge, large masses of glacier-worn pebbles alternate with beach-shingle; but it is easy to show that there was here a glacier advancing into the sea, crowding its front moraine and the materials carried under it over and into the shingle washed up by the waves upon the beach. Not infrequently, also, river-pebbles may be found among glacial materials. This is especially the case where, after the disappearance of large glaciers, rivers have occupied their beds. Examples of this kind may be seen in all the valleys of the Alps.
But, besides the special character of the individual fragments, the true origin of any accumulation of glacier-débris, commonly called drift, may be detected by the total absence of stratification, so essential a feature in all water-deposits. This absence of stratification throughout its mass is, after all, the great and important characteristic of the drift; and though I have alluded to it before, I reiterate it here, as that which distinguishes it from all like accumulations under water. I may be pardoned for dwelling upon this point, because the great controversy among geologists respecting the nature and origin of the sheet of loose materials scattered over a great part of the globe turns upon it. The débris of which the drift consists are thrown together pell-mell, without any arrangement according to size or weight, larger and smaller fragments being mixed so indiscriminately that the heaviest materials may be on the very summit of the mass, and the lightest at the bottom in immediate contact with the underlying rock, or the larger pieces may stand at any level in the mass of finer ones. Impalpable powder, coarse sand, rounded, polished, and scratched fragments of every size are mixed together in a homogeneous paste, in which the larger materials are imbedded, to use a homely, but expressive comparison, like raisins and currants in a pudding. The adhesive paste holding all these fragments together is, no doubt, the result of the friction to which the whole was subjected under the glacier, and which has worked some of the softer materials into a kind of cement.
The mode of aggregation of water-worn materials is very different. Examine the shingle along our beaches: we find it so distributed as to show that the fading tide-wave has carried the lighter materials farther than the heavier ones, and the successive deposits exhibit an imperfect cross-stratification resulting from changes in the height of the tide and the direction of the wind. Moreover, in any materials collected under water we find the heavier ones at the bottom, the lighter on the top. It is true that large angular boulders may occasionally be found resting upon beach-shingle, but their presence in such a connection is easily explained. They may have been dropped there by floating icebergs, or have fallen from crumbling drift-cliffs.
I should add, in speaking of drift-materials, that, while we find the large angular boulders resting above them, we occasionally find boulders of unusual size mingled with them; but, when this is the case, such massive fragments are more or less rounded, polished, and marked in the same way as the smaller pebbles, or as the surfaces over which the glacier has passed. This is important to remember, because, when we examine the drift in countries where the ice, during the glacier-period, overtopped nearly all the mountains, so that few fragments could fall from them upon its surface, we find scarcely any angular boulders, while the drift is interspersed with larger fragments of this character, carried under the ice, instead of on its back. Another distinction between water-worn deposits and drift consists in the fact that the former are washed clean, while the latter always retains the mud gathered during its journey and spread throughout its mass.
In summing up the glacial evidences, I must not omit the moraines, though I have described them so fully in a previous article that I need not do more than allude to them here; but any argument for the glacial theory which did not include these characteristic walls erected by glaciers would be most imperfect. We need hardly discuss the theory of currents with reference to the formation of terminal moraines, extending across the valleys from side to side. Any current powerful enough to bring the boulders and débris of all sorts of which these walls are composed to the places where they are found would certainly not build them up with such regularity, but would sweep them away or scatter them along the bottom of the valley. That this is actually the case is seen in the lower course of the valley of the Rhone, where there are no transverse moraines, while they are frequent and undisturbed in the upper part of the valley. This is no doubt owing to the fact, that, when the main glacier had already retreated considerably up the valley, the lateral glaciers from the chains of the Combin and the Diablerets still reached the valley of the Rhone at a lower point, and barred the outlet of the waters from the glaciers above. A lake was thus formed, which, when the lower glaciers retreated up the lateral valleys, swept away all the lower transverse moraines, and formed the flat bottom of Martigny. In this case, the moraines were totally obliterated; but there are many other instances in which the materials have been only broken up and scattered over a wider surface by currents. In such remodelled moraines, the glacier-mud has, of course, been more or less washed away. We have here a blending of the action of water with that of the glacier; and, indeed, how could it be otherwise, when the colossal glaciers of past ages gradually disappeared or retreated to the mountain-heights? The wasting ice must have occasioned immense freshets, the action of which we shall trace hereafter, when examining the formation of our drift-ponds, of our river-beds and estuaries, as well as the river-terraces standing far above the present water-level.
And now, if it be asked how much of this evidence for the former existence of glaciers is to be found in Great Britain, I answer, that there is not a valley in Switzerland where all these traces are found in greater perfection than in the valleys of the Scotch Highlands, or of the mountains of Ireland and Wales, or of the lake-region in England. Not a link is wanting to the chain. Polished surfaces, traversed by striæ, grooves, and furrows, with a sheet of drift resting immediately upon them, extend throughout the realm,—the roches moutonnées raise their rounded backs from the ground there as in Switzerland,—transverse moraines bar their valleys and lateral ones border them, and the boulders from the hill-sides are scattered over the plains as thickly as between the Alps and the Jura, and are here and there perched upon the summits of isolated hills. This being the case, let us examine a little more closely the local phenomena connected with the ancient extension of glaciers in this region, and especially the parallel roads of Glen Roy.
Among the Grampian Hills, a little to the northeast of Ben Nevis, lies the valley of Glen Roy, a winding valley trending 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 opposite 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 many 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 sea-level, 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, why 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.[B]
Prepossessed as I was with the idea of glacial agency in times anterior to ours, these phenomena appeared to me under a 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 finest examples of the roches moutonnées I have seen in Scotland are to be found at the entrance of the valley of Loch 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 moutonnées 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, so 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 drift-period. All the currents that have swept 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 moutonnées in Scotland, especially about the neighborhood of Loch Awe, Loch Fyne, 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 east 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.