CHAP. XIII. DUE TO GLACIER-LAKES. 261 glacier from Loch Treig then protruded itself across Glen Spean, and rested on the flank of the hill on the opposite side in such a manner as effectually to prevent any water from escaping over the Glen Glaster col.' The proofs of such a glacier having actually existed at the point in question consist, he says, in numerous cross striæ observable in the bottom of Glen Spean, and in the presence of moraine matter in considerable abundance on the flanks of the hill extending to heights above the Glen Glaster col.' When the ice shrank into less dimensions the second shelf would be formed, having its level determined by the col last mentioned, Glen Spean in the meantime being filled with a glacier. Finally, the ice blockage common to Glens Roy, Spean, and Laggan, which consisted probably of a glacier from Ben Nevis, gave rise to the lowest and most extensive lake, the waters of which escaped over the pass of Muckul or the 'col' at the head of Loch Laggan, which, as Mr. Jamieson has now ascertained, agrees precisely in level with the lowest of all the shelves, and where there are unequivocal signs of a river having flowed out for a considerable period. Dr. Hooker has described some parallel terraces, very analogous in their aspect to those of Glen Roy, as existing in the higher valleys of the Himalaya, of which his pencil has given us several graphic illustrations. He believes these Indian shelves to have originated on the borders of glacierlakes, the barriers of which were usually formed by the ice and moraines of lateral or tributary glaciers, which descended into and crossed the main valley, as we have supposed in the case of Glen Roy; but others he ascribes to the terminal moraine of the principal glacier itself, which had retreated during a series of milder seasons, so as to leave an interval between the ice and the terminal moraine. This interspace caused by the melting of ice becomes filled with water and forms a lake, the drainage of which usually takes place by 262 COMPARATIVELY MODERN DATE OF THE CHAP. XIII. percolation through the porous parts of the moraine, and not by a stream overflowing that barrier. Such a glacier-lake Dr. Hooker actually found in existence near the head of the Yangma valley in the Himalaya. It was moreover partially bounded by recently formed marginal terraces or parallel roads, implying changes of level in the barrier of ice and moraine matter.* It has been sometimes objected to the hypothesis of glacierlakes, as applied to the case of Glen Roy, that the shelves must have taken a very long period for their formation. Such a lapse of time, it is said, might be consistent with the theory of pauses or stationary periods in the rise of the land during an intermittent upward movement, but it is hardly compatible with the idea of so precarious and fluctuating a barrier as a mass of ice. But the reader will have seen that the permanency of level in such glacier-lakes has no necessary connection with minor changes in the height of the supposed dam of ice. If a glacier descending from higher mountains through a tributary glen enters the main valley in which there happens to be no glacier, the river is arrested in its course and a lake is formed. The dam may be constantly repaired and may vary in height several hundreds of feet without affecting the level of the lake, so long as the surplus waters escape over a 'col' or parting ridge of rock. The height at which the waters remain stationary is determined solely by the elevation of the 'col,' and not by the barrier of ice, provided the barrier is higher than the 'col.' But if we embrace the theory of glacier-lakes, we must be prepared to assume not only that the sea had nothing to do with the original formation of the 'parallel roads,' but that it has never, since the disappearance of the lakes, risen in any one of the glens up to the level of the lowest shelf, which * Hooker, Himalaya Journal, vol. i. p. 242; ii. pp. 119, 121, 166. I have also profited by the author's personal explanations. CHAP. XIII. PARALLEL ROADS OF GLEN ROY. 263 is about 850 feet high; for in that case the remarkable persistency and integrity of the roads and deltas, before described, must have been impaired. We have seen (p. 244) that fifty miles to the south of Lochaber, the glacier formations of Lanarkshire with marine shells of arctic character have been traced to the height of 524 feet. About fifty miles to the south-east in Perthshire are those stratified clays and sands, near Killiecrankie, which were once supposed to be of submarine origin, and which in that case would imply the former submergence of what is now dry land to the extent of 1,550 feet, or several hundred feet beyond the highest of the parallel roads. Even granting that these laminated drifts may have had a different origin, as above suggested (p. 246), there are still many facts connected with the distribution of erratics and the striation of rocks in Scotland which are not easily accounted for without supposing the country to have sunk, since the era of continental ice, to a greater depth than 525 feet, the highest point to which marine shells have yet been traced. After what was said of the pressure and abrading power of a general crust of ice, like that now covering Greenland, it is almost superfluous to say that the parallel roads must have been of later date than such a state of things, for every trace of them must have been obliterated by the movement of such a mass of ice. It is no less clear, that as no glacier-lakes can now exist in Greenland, so there could have been none in Scotland, when the mountains were covered with one great crust of ice. It may, however, be contended, that the parallel roads were produced when the general crust of ice first gave place to a period of separate glaciers, and that no period of deep submergence ever intervened in Lochaber after the time of the lakes. Even in that case, however, it is difficult not to suppose that the Glen Roy country participated in the downward movement which sank part of Lanarkshire 525 264 DATE OF GLEN ROY TERRACE LINES. CHAP. XIII. feet beneath the sea, subsequently to the first great glaciation of Scotland (p. 244). Yet that amount of subsidence might have occurred, and even a more considerable one, without causing the sea to rise to the level of the lowest shelf, or to a height of 850 feet above the present sea-level. This is a question on which I am not prepared at present to offer a decided opinion. Whether the horizontality of the shelves or terrace-lines is really as perfect as has been generally assumed, is a point which will require to be tested by a more accurate trigonometrical survey than has yet been made. The preservation of precisely the same level in the lowest line throughout the Glens of Roy, Spean and Laggan, for a distance of twenty miles east and west, and ten or twelve miles north and south, would be very wonderful if ascertained with mathematical precision. Mr. Jamieson, after making in 1862 several measurements with a spirit-level, has been led to suspect a rise in the lowest shelf of one foot in a mile in a direction from west to east, or from the mouth of Glen Roy to a point six miles east of it in Glen Spean. To confirm such observations, and to determine whether a similar rate of rise continues eastward as far as the pass of Muckul, would be most important. On the whole, I conclude that the Glen Roy terrace-lines and those of some neighbouring valleys, were formed on the borders of glacier-lakes, in times long subsequent to the principal glaciation of Scotland. They may perhaps have been nearly as late, especially the lowest of the shelves, as that portion of the post-pliocene period in which Man coexisted in Europe with the mammoth. CHAP. XIV, EXTINCT GLACIERS IN WALES. 265 CHAPTER XIV. CHRONOLOGICAL RELATIONS OF THE GLACIAL PERIOD AND Continued. SIGNS OF EXTINCT GLACIERS IN WALES GREAT SUBMERGENCE OF - STILL GREATER DEPRESSION INFERRED FROM STRATIFIED DRIFT - IN RELATION TO THE AGE OF THE EXISTING FAUNA AND FLORA. THE Extinct Glaciers in Wales. HE considerable amount of vertical movement in opposite directions, which was suggested in the last chapter, as affording the most probable explanation of the position of some of the stratified and fossiliferous drifts of Scotland, formed since the commencement of the glacial period, will appear less startling, if it can be shown that independent observations lead us to infer that a geographical revolution of still greater magnitude accompanied the successive phases of glaciation through which the Welsh mountains have passed. That Wales was once an independent centre of the dispersion of erratic blocks, has long been acknowledged. Dr. Buckland published in 1842 his reasons for believing that the Snowdonian mountains in Caernarvonshire were formerly |