On the Glacial Phenomena of the Long Island, or Outer Hebrides. Second Paper

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Summary of Results and Conclusion: 1. Primary or General Glaciation .—The evidence now brought forward demonstrates that the extent and thickness of the ice-sheet that buried Western Scotland during the climax of the glacial period was much greater than geologists have hitherto supposed. It has been shown that the whole of the Long Island from the Butt of Lewis to Barra Head has been overflowed from the Minch by ice that moved outwards from the Inner Islands and the mainland, and the actual thickness of this ice-sheet it is now possible to measure with some approach to exactness. We have seen that the only points in the Outer Hebrides which have escaped glaciation are those that exceed a height of 1600 feet. Taking this, therefore, as the thickness of the ice that overflowed the lowest ground of the Long Island, and 3000 feet as the probable upper limits of the ice-sheet in Western Ross and Sutherland, we readily arrive at the depth of the ice-sheet that filled up the Minch. Immediately off the east coast of Harris the thickness of the ice would he upwards of 2300 feet; for the sea has a depth there of 121 fathoms. In the centre of the North Minch the upper surface of the mer de glace would be 2300 feet above what is now the sea-level, the actual thickness of the ice being 2700 feet. Nearer the shores of the mainland, in the Inner Sound, the depth attained by the ice would be still greater, no less than 3700 feet. Measuring from the Cliseam in North Harris to the mountains of Torridon, we have a distance of 56 miles, so that the inclination of the surface of the mer de glace was very little, the fall not being more than 1400 feet, or about 1 in 211. But slight as that incline was, it was probably twice as great as the slope of the mer de glace that filled up the German Ocean. Of course no one can believe that such enormous masses of ice could ever have been nourished by the snowfields of the mainland alone. The thickness attained clearly points to excessive precipitation over the whole area covered by the mer de glace ; and we may be allowed to suppose that the ice was probably of the same peculiar laminated structure as that underneath which the Antarctic lands lie buried. If the surface of the mer de glace continued to fall at the same rate, we should find marks of glaciation in St. Kilda up to a height of about 200 feet; whether this is actually the case I hope to ascertain at an early opportunity. But whether it be so or not, it is clear that the facts brought forward in this and the preceding paper completely negative the notion of a great Atlantic glacier, which it has been supposed flowed in upon Scotland. It may have occurred to some as a difficulty that the moraine profonde , or till, of the ice-sheet contains so very few stones that do not occur in situ in the Outer Hebrides. If the ice actually flowed from the mainland and Skye across the Minch and over the Long Island, why should not many recognizable Skye rocks occur in the Hebridean till? The explanation appears to be as follows:—We must remember that although the upper surface of the ice-sheet flowed steadily towards the north-west, yet the bottom portion in the Minch would move in quite a different direction. The lower strata that impinged upon the foot-slopes of the Long Island, which are now below water, would be deflected to right and left, and two under-currents would set along the bottom of the Minch, one flowing north-east and the other south-west. These currents being due to obstruction would exert excessive erosive action upon the bed of the sea, and this would eventually result in the formation of deep rock-basins having the same trend as the present coast-line of the Outer Hebrides. Now, as I have pointed out elsewhere, such a set of rock-basins does actually exist in the very position which, from theoretical considerations, they ought to occupy. Such being the course followed by the under portions of the mer de glace in the Minch, it is evident that the débris dragged on underneath the ice would be rolled north-east and south-west along the bottom of the Minch, and would thus never invade the Outer Hebrides at all. Now and then a few boulders might be pushed beyond the reach of the under-currents, and these would be dragged over the Long Island; but this would be quite exceptional, and hence we need not be surprised at finding only an occasional stranger from Skye or the mainland in the till of the Outer Hebrides. Again, the very general absence of large angular erratics and perched blocks, derived from the lands beyond the Minch, is quite what we should expect. When the ice-sheet reached its greatest development, the Long Island was all but completely buried, only a few insignificant points in Harris and South Uist projecting above the surface of the mer de glace . So that the chances that any superficial erratics, travelling outwards from Skye or the mainland, should be caught on the tips of the Cliseam, or Beinn-mhor, or Hecla, were exceedingly small. The absence of such large angular erratics in the Outer Hebrides we may therefore look upon as an additional proof of the great depth attained by the ice that overflowed these islands. By far the larger number of erratics which are found lying loose at the surface must belong to the later stages of the glacial period, most of them having been stranded during the gradual dissolution of the ice-sheet, while others are the relics of local snowfields and glaciers. Consequently they need not have travelled any distance. Each cliff and mountain-top, as it appeared, would begin to break up under the influence of frosts much more intense than are now experienced in our islands; and thus numbers of large angular blocks and heaps of débris would be supplied to the surface of the dissolving ice. And this morainic rubbish would eventually be scattered over the islands, most abundantly in the hilly districts, and less plentifully over the low grounds at a distance from the mountains. 2. Shelly Boulder-clays and Interglacial Beds .—It is very remarkable that shelly boulder-clays occur nowhere in the Long Island, save only upon the low grounds in the extreme north of Lewis. I searched for them everywhere, but without finding a single trace in any of the islands further south. For this peculiar restriction of their range there must be some reason; and the explanation, as it seems to me, is not far to seek. I have shown how the general absence from the unfossiliferous till of erratics from Skye and the mainland is to be accounted for by the deflection of the lower strata of the ice-sheet at the bottom of the Minch. Two under-currents of ice set along the inner margin of the Long Island, one to south-west, the other to north-east. Now it is evident that these currents would eventually sweep round the obstruction caused by the great Hebridean ridge, and thereafter continue in the general direction of the ice-flow. If we examine the Admiralty charts of the Minch, we shall find that the ridge in question presents a steep face to the east, from a point opposite the Eye Peninsula to another point which lies 15 miles south of Barra Head. North of Eye Peninsula the sea-bottom rises with a gentle gradient to the coast of Lewis, so that there was nothing here to prevent the north-east under-current creeping up and over the low ground near the Butt. Let us note further that the most northerly rock-basin of any importance along the inner margin of the Long Island occurs just opposite the Eye Peninsula. Beyond this point the under-current met with little or no obstruction, and it ceased therefore to exert the same degree of erosion upon the sea-bottom. It now gradually turned away north and north-west, following the trend of the ridge, until—always creeping more and more to the west—it at last overflowed the northern end of Lewis. As its course lay along what had only recently been the sea-bottom, its moraine profonde necessarily contained shells and other marine exuviæ, and a much more considerable admixture of silt and clay than the unfossiliferous till throughout the rest of the Outer Hebrides. No shelly boulder-clay occurs at the southern termination of the Long Island. But a glance at the Admiralty charts at once explains its absence. The steep ridge of rock that opposed the passage of the mer de glace continues, as I have said, to a point 15 miles south of Barra Head. Here the great Hebridean ridge terminates, and here the under-current of the ice was first enabled to change its direction and flow westward, its course being mapped out on the sea-bottom by the broad hollows it scooped out. It is evident, therefore, that, as the ice at the bottom of the Minch could not invade Barra and the islands to the south, no shelly boulder-clay would be pushed over any part of the ridge that now remains above water. The interglacial deposits indicate a considerable change of climate; the ice-sheet which had overflowed the Long Island had now vanished, so far as the Outer Hebrides were concerned, and the sea once more occupied the Minch and rose to a height of 175 feet at least above its present level. The actual limits of submergence we cannot tell, for the reasons that follow; neither can we make any definite statement as to the character of the interglacial climate. The deposits are mere fragments, and their full meaning will probably not be realized until they are viewed in connexion with what is known of interglacial deposits in other regions. In the Upper shelly Boulder-clay we read the return of precisely the same conditions as those that witnessed the accumulation of the Lower. The ice-sheet again overflowed the Long Island, and its under-current stole over the Butt as before. The interglacial beds were now subjected to much denudation. We can hardly doubt that they must at one time have occupied very considerable areas, not only in Lewis, but in the low-lying districts of the other islands to the south; for it is hardly conceivable that the interglacial submergence of 175 feet was confined to Northern and Eastern Lewis. The only portions, however, that now remain are the small patches at Garrabost and near the Butt. In no other part of the Long Island did I find any trace of them, although I was continually on the look-out. Yet it is by no means improbable that, could we obtain good sections across the low flats of North and South Uist, patches of them might be obtained here and there underlying and probably overlying till. The interrupted and patchy nature of the interglacial beds, and their remarkable absence from so much of the low grounds, where they cannot but have at one time existed, are undoubtedly due to the erosive action of the latest ice-sheet, which, like the ice-sheets of earlier glacial periods, made a nearly clean sweep of the Hebridean ridge, leaving patches of the earlier deposits in sheltered nooks, and sparing them to some extent in areas where the ice had freedom to flow in one broad uninterrupted stream, and where consequently its erosive action was not so great. Before I had examined the glacial phenomena of the southern islands of the Outer Hebrides I was hardly prepared to believe that the latest ice-sheet attained so great a development. It seems, however, impossible now to resist the conclusion that the last mer de glace —that, namely, which cut out the interglacial deposits—actually overwhelmed the whole Outer Hebrides. Whether it reached to as great a height as the earlier ice-sheet, whose moraine profonde is represented by the Lower shelly Boulder-clay, I cannot say; but it is possible that a minute examination of the evidence, and careful mapping-out of all the details, may eventually enable us to settle this point. It is possible also that we may yet obtain evidence to show that there were more than two incursions of the mer de glace . We know, indeed, from the facts supplied in other parts of the country, that there really were more than two cold periods. But unfortunately the records of the earlier phases of the glacial epoch are always more or less fragmentary; and in a region like that under review we cannot expect to meet with early glacial and interglacial deposits in such abundance as in lower latitudes, and in places where there was less obstruction to the flow of the ice. As it is, we must look upon all the unfossiliferous till and morainic débris of the Long Island as pertaining to the incursion of the latest ice-sheet, although much of it may be merely that of earlier ice-periods reworked and redistributed. 3. Local Glaciers .—While the latest ice-sheet slowly melted away, local ice was enabled to flow down the valleys in all directions. At first these local glaciers coalesced with the general mer de glace ; but eventually this connexion was severed, and many of them continued to exist probably to late glacial times, to that period, namely, which is represented on the mainland of Scotland by those shelly clays which are found resting upon the youngest accumulations of boulder-clay. None of these local glaciers, in their independent condition, was of any size, as one may judge not only from the position of their moraines, but from the size of the valleys and the capacity of the feeding-grounds. In leaving the subject of glaciation, a word or two may be added on the very notable absence of kames. Not a trace of these was met with between the Butt and Barra Head. All this is quite in keeping with the view that our kames owe their origin to torrential waters flowing under and upon, and escaping at the margins of, the melting mer de glace . Their remarkable distribution on the mainland of Scotland shows that this was their origin; they were deposited and heaped up by water flowing from higher to lower levels. Hence their contents become finer-grained and better water-worn the further they are traced from their sources. In the lower reaches of our broad valleys they are generally composed of well-bedded sand and water-worn gravel; in the upper parts of the valleys they are not so well bedded, and their materials are generally much coarser. Traced nearer to the mountains, they gradually pass into irregular spreads and heaps of morainic gravels and débris . Now the broad flats of Lewis would not be traversed during the melting of the ice-sheet by any great diluvial torrents; for none of the large valleys opens to the north. Nor is the configuration of the ground in South Lewis and Harris a whit more favourable. The valleys are short and steep, and not such as kames are ever found in. We get the coarse morainic gravel and débris ; but those materials have not travelled far enough to be sufficiently water-worn, and the valleys are too narrow and steep to permit the heaping up of banks of such incoherent materials. It is precisely the same with the other islands of the Outer Hebrides. The conditions for the formation of kames did not exist in the Long Island any more than in the upper and central part of any mountain district of the Northern Highlands. If kames were formed by diluvial torrents derived from the melting of the Hebridean ice, they probably now lie submerged at a considerable distance from the land, where they will occupy the same position relatively to the mountainous Hebrides as the kames of the Lowlands do to the Highlands and Southern Uplands. Judging from all analogy, indeed, one can hardly doubt that the submarine plateau which extends beyond the Outer Hebrides to the 100-fathom line must be more or less plentifully covered with drift sand and gravel, laid down during the final retreat of the mer de glace . And I am thus led to believe that much of the sand which is driven in such vast quantities chiefly upon the western shores of the Outer Hebrides, and which is gradually silting up the channels between the various islands, is derived from drift-deposits corresponding in nature and origin to the great diluvial sands of Northern Germany. 4. Postglacial or recent submergence .—In concluding, I would draw attention to the striking fact that there has been no considerable submergence of the Long Island since the close of the last glacial period. No high-level beaches occur anywhere. No trace of any undoubted postglacial occupation by the sea is found above a height of 80 feet. At the Butt some shelly sand, which appears to be of aqueous origin, overlies the glacial deposits, and reaches to a height of 80 feet or so above the sea. It is just possible, however, that this accumulation may be merely a rearranged portion of the fossiliferous glacial beds; at all events, no recent marine deposit attained an equal elevation in any of the islands. Shelly sands do occur at higher levels, especially in the little islands south of Barra; but these are simply blown in by the strong westerly winds from the sandy beaches, the shelly materials consisting chiefly of finely comminuted fragments of the common cockle. What appear to be old sea-levels occur along the shores of West Loch Tarbert, and here and there upon the west coasts of several of the islands; but none of these reaches higher than a dozen feet or so above the sea; and I should have been surer of their marine origin had I found beach-deposits resting upon them, but I was not so fortunate. Nevertheless one can hardly doubt that much of the low flat ground that borders on the Atlantic has been reclaimed from the sea in postglacial times. The absence of recent marine deposits at high levels has struck me also in not a few of the islands of the Inner Hebrides, the raised beaches which I have seen occurring mostly below a height of 100 feet above the present sea-level. From the aspect presented by some of these “raised beaches,” they seemed to indicate no long persistent submergence, but rather to have been the result of storm-waves. Not unfrequently they occur on low spits of land exposed to the full swell of the Atlantic, and they appear also upon the borders of narrow bays facing in the same manner the open sea. Along the steeper shores of the same islands, however, one usually looked in vain for ledges excavated in the solid rock at levels corresponding to those occupied by the shingle-beds or raised beaches, and the most distinct ledges occur at low levels. According to my colleague, Mr. J. Horne, there is a similar absence in the Shetland Islands of high-level recent marine deposits, and my brother has shown that this is likewise true of the Orkneys. Although very much has been done towards working out the history of the postglacial submergence of Scotland, there is yet a great deal more to be learned, and not a little, as I believe, to be unlearned. At present I have no evidence to indicate that the mainland of Scotland experienced a greater degree of submergence in postglacial times than 100 feet. The marine deposits at higher levels than this belong, as I have brought forward evidence to show, not to postglacial, but to interglacial times. At various points along the shores of the Long Island occur beds of peat, with remains of trees, in positions which prove that these outer islands in late postglacial and recent times were of larger dimensions than now. Precisely the same phenomena reappear on the opposite coasts of the mainland, as geologists have long been aware.

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