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

Abstract

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