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 had 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.[C] These two glaciers for a long time formed barriers across the western and eastern extension of this valley, damming back the waters which filled 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 valley 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 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 to 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 the north in Scotland, and the lower one, or the Sidlaw Hills, to the south, while in Switzerland, on the contrary, the higher range lies to the south and the lower to the north. I have alluded especially to Glen Prossen because the glacial marks in that valley are remarkably distinct, the whole bed of the valley being scratched, polished, and furrowed by the great rasp which has moved over it, while the concentric moraines at its lower extremity are very striking. But these signs, so perfectly preserved in Glen Prossen, recur with greater or less intensity in all the corresponding valleys, leaving no doubt that the same phenomena existed over the whole region.
Among the localities of Scotland where the indications of glacial action are most marked is the region about Stirling. Near Stirling Castle the polished surfaces of the rocks with their distinct grooves and scratches show us the path followed by the ice as it moved down in a northeasterly direction toward the Frith of Forth from the mountains on the northwest. To the west of Edinburgh, also, there is a broad glacier-track, showing that here also the ice was ploughing its way eastward to find an outlet on the shore.
The western slope of the great Scotch range is no less remarkable for its glacier-traces. The heads of Loch Long, Loch Fyne, Loch Awe, and Loch Leven everywhere show upon their margins the most distinct glacial polish and furrows, while from the trend of these marks and the distribution of the moraines, especially about Ben Cruachan, it is obvious that in this part of the country the glaciers moved westward and southward. About Aberdeen, on the contrary, they moved eastward, while in the vicinity of Elgin they advanced toward the north.
It thus appears that the whole range of the Grampians formed a great centre for the distribution of glaciers, and that a colossal ice-field spread itself over the whole country, extending in every direction toward the lower lands and the sea-shore. As the glaciers which now descend through all the valleys of the Alps, along their northern as well as their southern slopes, and in their eastern as well as their western prolongation, though limited, in our days, within the valley-walls, nevertheless once covered the plain of Switzerland and that of Northern Italy, so did the ice-fields of the Grampians during the greatest extension of the Scotch glaciers spread over the whole country. They also were, in course of time, reduced to local glaciers, circumscribed within the higher valleys of the more mountainous parts of the country, until they totally disappeared, as those of Switzerland would also have done, had it not been for the greater elevation of that country above the level of the sea. Scotland nowhere rises above the present level of perpetual snow, while in Switzerland the whole Alpine range has an altitude favorable to the preservation of glaciers. In the range of the Jura, however, which had at one time its local glaciers also, but which nowhere now rises above the line of perpetual snow, they have disappeared as completely as in the Grampian Hills.
It would lead me too far, were I to give here a special account of all the investigations I made in 1840 upon the distribution of glaciers in Great Britain. I will therefore only point out a few of the more distinct areas of distribution. The region surrounding Ben Wyvis formed such a centre of dispersion from which glaciers radiated, and we have another in the Pentland Hills about Edinburgh. In Northumberland, the Cheviot Hills present a glacial centre of the same kind, and in the Westmoreland Hills we have still another. In the last-named locality, the glacial tracks can be followed in various directions, some of them descending toward the northwest from the heights of Helvellyn, others moving southward toward Ambleside. In Wales the same kind of glacial distribution has been observed; but, as Professor Ramsay has treated this subject in full, I would refer my readers to his masterly work for a further account of the ancient Welch glaciers. In Ireland I had also opportunities of making extensive local investigations of glacial action. I observed the centres of distribution in the neighborhood of Belfast, in the County of Wicklow, and in Cavan.
But nowhere are these phenomena more striking than in Fermanagh County about the neighborhood of Enniskillen, and more especially in the immediate vicinity of Florence Court, the seat of the Earl of Enniskillen. On the northern slope of Ben Calcagh are five valleys lying parallel with each other and opening into the valley of Loch Nilly, which runs from east to west at the base of the mountain. A road now passes through this valley, and, where it crosses the mouth of either of the five valleys rising towards the mountain-slope, it cuts alternately through the two horns of a crescent-shaped wall which bars the lower end of every one of them. These crescent-shaped mounds are so many terminal moraines, built up by the five glaciers formerly descending through these lateral valleys into the valley of Loch Nilly. They bore the same relation to each other as the glaciers de Tour and d'Argentière, the Glacier des Bois with the Mer de Glace, the Glacier des Bossons and the Glacier de Taconet, now bear to each other in the valley of Chamouni; and were it not for the smaller dimensions of the whole, any one familiar with the tracks of ancient glaciers might easily fancy himself crossing the ancient moraines at the foot of the northern slope of the range of Mont Blanc, through which the Arve has cut its channel, the valley of Chamouni standing in the same relation to Mont Blanc as the valley of Loch Nilly does to Ben Calcagh.
I have dwelt thus at length on the glaciers of Great Britain because they have been the subject of my personal investigations. But the Scotch Highlands and the mountains of Wales and Ireland are but a few of the many centres of glacial distribution in Europe. From the Scandinavian Alps glaciers descended also to the shores of the Northern Ocean and the Baltic Sea. There is not a fiord of the Norway shore that does not bear upon its sides the tracks of the great masses of ice which once forced their way through it, and thus found an outlet into the sea, as in Scotland. Indeed, under the water, as far as it is possible to follow them through the transparent medium, I have noticed in Great Britain and in the United States the same traces of glacial action as higher up, so that these ancient glaciers must have extended not only to the sea-shore, but into the ocean, as they do now in Greenland. Nor is this all. Scandinavian boulders, scattered upon English soil and over the plains of Northern Germany, tell us that not only the Baltic Sea, but the German Ocean also, was bridged across by ice, on which these masses of rock were transported. In short, over the whole of Northern Europe, from the Arctic Ocean to the northern borders of its southern promontories, we find all the usual indications of glacial action, showing that a continuous sheet of ice once spread over nearly the whole continent, while from all the mountain-ranges descended those more limited glacial tracks terminating frequently in transverse moraines across the valleys, showing, that, as the general ice-sheet broke up and contracted into local glaciers, every cluster or chain of hills became a centre of glacial dispersion, such as the Alps are now, such as the Jura, the Highlands of Scotland, the mountains of Wales and Ireland, the Alps of Scandinavia, the Hartz, the Black Forest, the Vosges, and many others have been in ancient times.