Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

by James Geikie

We have now seen that the whole of the Long Island has been ground, and rubbed, and scraped by land- or glacier-ice which has traversed the ground in a prevalent south-east and north-west direction. We have seen also that this ice attained so great a thickness that it was able to overflow all the hills up to a height of 1600 feet above the sea. It is needless to say that such a mass could not have been nurtured on the islands themselves. They have no gathering grounds of sufficient extent, and if they had, the ice would not have taken the peculiar direction it did. Instead of flowing across the islands it would have radiated outwards from the mountain-valleys. Where, then, did the ice come from?

Looking across the Minch we see Skye and the mountains of the north-west Highlands, and those regions, as we know, have also been subjected to extreme glaciation. From the appearances presented by the mountains of Ross-shire we are compelled to believe that all that region was buried in ice up to a height of not less than 3000 feet—the ice-sheet was probably even as much as 3500 feet in thickness. The evidence shows that the under portion of this vast ice-sheet flowed slowly off the country into the Minch by way of the great sea-lochs. Thus we know that an enormous mass crept down Loch Carron and united with another great stream stealing out from the mountains of Skye, to flow north through the hollows of Raasay Sound and the Inner Sound into the Minch. So deep was the ice that it completely smothered the island of Raasay (1272 feet high) and overflowed all the lofty trappean table-lands of Skye. From the Coolins, as a centre-point, another movement of the ice-sheet was towards the south-west, against the islands of Rum, Cannay, and Eigg. Further north similar vast masses of ice streamed out into the Minch, from Loch Torridon, Gairloch, Loch Ewe, and Loch Broom. The direction of the glaciation in the north of Skye, which is towards north-west, shows that the glacier-mass which overflowed that area must eventually have reached the shores of the Long Island. In short, there cannot be a reasonable doubt that the immense sheet of ice that streamed off the north-west Highlands must have filled up entirely the basin of the Minch, and thereafter streamed across the Outer Hebrides. But it may be objected that if the Outer Hebrides were overflowed by ice that streamed from the mainland across the north end of Skye, we ought to get many fragments of Skye rocks and Ross-shire rocks too in the sub-glacial débris or till of Lewis and Harris, and the north end of North Uist. But all such fragments are apparently wanting. True, there are bits of stone like the igneous rocks of Skye often met with in the Hebridean till, but as veins or dykes of precisely the same kind of rock occur in the Long Island itself, we cannot say that the stones referred to are other than native. A little reflection will show us, however, that it is extremely improbable indeed that stones derived from Skye and the mainland should ever have been dragged on under the ice, and deposited amongst the till of the Long Island. There is only one part of the whole Outer Hebrides where we might have anticipated that fragments from the mainland should occur; and there, sure enough, they put in an appearance.

But before I attempt to explain the non-occurrence of Skye rocks in the till of the Outer Hebrides, let me show in a few words what the glaciation of the Long Island, Skye, and the north-west Highlands teaches us as to the general aspect presented by the ice-sheet. The height reached by the surface of the ice in Ross-shire and the Long Island respectively indicates of course that the main movement was from the mainland. We must conceive of an immense sheet of solid ice filling up all the inequalities of the land, obliterating the glens, and sweeping across the hill-tops; and not only so, but occupying the wide basin of the Minch to the entire exclusion of the sea, the surface of the ice rising so high that it overtopped the whole of the Outer Hebrides, and left only the tips of a few of the higher mountains uncovered. The slope of the surface was persistently outwards from the mainland, and the striation of the Long Island indicates clearly that the dip or inclination of that surface was towards the north-west. Nay, more than this, we are now enabled for the first time to say with some approach to certainty what was the precise angle of that inclination. If we take the upper surface of the ice in Ross-shire to have been 3000 feet (and it was not less), then the slope between the mainland and the Outer Hebrides was only 25 feet in the mile, or about 1 in 210. It is quite possible, however, and even probable, that the actual height attained by the ice-sheet in the north-west Highlands was more than 3000 feet. I think it may yet turn out to have been 3500 feet, and if this were so it would give an inclination for the surface of the ice of about 35 feet in the mile. In either case the slope was so very gentle that to the eye it would have appeared like a level plain. Over the surface of this plain would be scattered here and there a solitary big erratic or two, while in other places long trains of large and small angular boulders would stream outwards. All these would be derived from such mountain in Skye and the mainland as were able to keep their heads above the level of the ice-flow; while a few also might be dislodged by the frost and rolled down upon the glacier from the tips of the Clisham and the Langa in Harris, and Hecla and Beinn Mhor in South Uist. Every such block, it is evident, would be carried across the buried Hebrides, out into the Atlantic in the direction indicated by the glaciation of the Long Island—that is, towards the north-west.

But while the upper strata of the ice doubtless followed that particular course, it is obvious that this could not be the case with the under portion of the great sheet, the path of which would be controlled in large measure by the form of the ground over which the ice moved. The upper strata that overflowed the Outer Hebrides, as we have seen, were locally deflected again and again by important obstacles, and it is quite certain that the same would take place with the deeper portions of the ice-flow.

It is well known that the sea along the inner margin of the Long Island is very deep. In many places it reaches a depth of 600 feet, and occasionally the sounding-lead plunges down for upwards of 700 feet. It would seem, however, that these great depths did not exist before the advent of the ice-sheet, but that the bottom of the Minch along the eastern borders of the Long Island was then some 250 or 300 feet shallower than now, the floor of the sea having since been excavated in the manner I shall presently describe. It is quite apparent, therefore, that the long ridge of the Outer Hebrides must have offered an insuperable obstacle to the direct passage of the bottom-ice out to the Atlantic. Here was a great wall of rock shooting up from the floor of the Minch, at a high angle, to a height ranging in elevation from 400 feet to upwards of 3000 feet. It is simply impossible that the lower strata of the ice that occupied the bed of the Minch could climb that precipitous barricade. They were necessarily deflected, one portion creeping to north-east and another to south-west, but both hugging the great wall of rock all the way. We see precisely the same result taking place in the bed of every stream. Let us stand upon an almost submerged boulder, and note how the water is deflected to right and left, and we shall observe at the same time that the boulder, by obstructing the current, forces the water downwards upon the bed of the stream, the result being that a hollow is dug out in front. Now, in a similar manner, the ice, squeezed and pressed against the Hebridean ridge by the steady flow of the great current that crossed the Minch, necessarily acted with intense erosive force upon its bed. Hence in the course of time it scooped out a series of broad deep trenches along the whole inner margin of the Long Island, the amount of the excavation reaching from 200 to 300 feet. Similar excavated basins occur in like positions opposite all the precipitous islands of the Inner Hebrides. Wherever, indeed, the ice-sheet met with any great obstruction to its flow, there excessive erosion took place, and a more or less deep hollow was dug out in front of the opposing cliff, or crag, or precipitous mountain. While, therefore, the upper strata of the ice-sheet overflowed the Outer Hebrides from south-east to north-west, the under portions of the same great ice-flow were compelled by the contour of the ground to creep away to north-east and south-west, until they could steal round the ridge and so escape outwards to the Atlantic.

This being the case, we have a very simple and obvious explanation of the absence of Skye rocks in the till of the Long Island. One sees readily enough that the sub-glacial débris dragged across the Minch would naturally be carried away to south-west and north-east by the “under-tow” or deflected ice. It is quite impossible that any Skye fragments or bits of rock from the mainland could travel over the bed of the Minch, and then be pushed up the precipitous rock wall of the Long Island. There is only one place in all the Outer Hebrides where we might expect to meet with extraneous boulders in the till, and that is in the north of Lewis, where the land shelves gently into the sea, and the great rocky ridge terminates. Here the under-strata of the ice would begin to steal up upon the land, favoured by its gentle inclination, and in that very place accordingly we meet with a deposit of till in which are found many boulders of a hard red sandstone, and some of various porphyries which are quite alien to the Long Island. Moreover, the till itself in that locality is much more of a clay than the usual sub-glacial débris in other parts of Lewis, and contains numerous fragments of sea-shells. All this is quite in keeping with the other evidence. The extreme north end of Lewis was overflowed by the under-current that crept up the bed of the Minch, hugging the Hebridean ridge, and dragging along with it a muddy mass interspersed with the shells and other marine exuviæ that lay in its path, and numerous stones, some of which may have come from Skye, while others were derived from the mainland.

I have already said enough, perhaps, about the abrasion of the Hebrides, but I may add a few words upon the origin of the freshwater lakes. Many of these rest in complete rock-basins; others, again, seem to lie partly upon solid rock and partly upon till; while yet others appear to occupy mere shallow depressions in the surface of the till. All of them thus owe their origin to the action of the ice-sheet. As one might have expected, the great majority lie along the outcrop of the gneissic strata, which, as a rule, corresponds pretty closely to the flow of the ice. Hence the general trend of the lakes is from south-east to north-west. In many cases in fashioning these rock-basins the ice has merely deepened in an irregular manner previously existing hollows, which are now, of course, filled with water. In not a few places, however, the lakes are drawn out in other directions—this being due usually to changes in the strike or outcrop of the strata. For example, over a considerable district in the south of Lewis many lake-hollows extend from south-west to north-east, or at right angles to the direction of the ice-flow. Such lakes are usually dammed up at one or both extremities by glacial débris.

Thus most of the features characteristic of the Outer Hebrides owe their origin directly or indirectly to the action of that great sheet of ice which swept over the islands during what is called the Glacial Period. And there is no region in northern Europe where the immensity of the abrading agent can be more vividly realised. From a study of the phenomena there exhibited we for the first time obtain a definite idea of the surface-slope, and are able to plumb the old ice-sheet, and ascertain with some approach to accuracy its exact thickness. In the deeper parts of the area, between the mainland and the Long Island, its thickness was not less than 3800 feet. Of course this great depth of ice could not have been derived exclusively from the snow that fell on the mountains of the north-west Highlands. Doubtless the precipitation took place over its whole surface, just as is the case in Greenland and over the Antarctic continent. The winter cold must have been excessive, but the precipitation necessary to sustain such a mass of ice implies great evaporation; in other words, the direct heat of the sun per diem in summer-time was probably considerably in excess of what it is now in these latitudes. The west and south-west winds must have been laden with moisture, the greater portion of which would necessarily fall in the form of snow. We see something analogous to this taking place in the Antarctic regions at the present day. That quarter of the globe has its summer in perihelion, and, therefore, must be receiving then more heat per diem than our hemisphere does in its summer season, which, as every one knows, happens when the earth is furthest removed from the sun. But, notwithstanding this, the summer of the Antarctic continent is cold and ungenial—the presence of the great ice-sheet there cooling the air and causing most of the moisture to fall as snow. Paradoxical as it may seem, therefore great summer heat is almost, if not quite, as necessary as excessive winter cold for the production and maintenance of a wide continental glacier.

III.

When we last took a peep at the Outer Hebrides we found those luckless islands all but obliterated under an immense sheet of ice extending from the mainland out into the Atlantic. How far west the great glacier spread itself we cannot as yet positively say; but if the known slope of its surface between the north-west Highlands and the Long Island continued, as there is every reason to believe it would, then it is extremely probable that the ice flowed out to the edge of the great Scottish submarine plateau. Here the sudden deepening of the Atlantic would arrest its progress and cause it to break up into icebergs. In those old times, therefore, a steep wall of ice would extend all along the line of what is now the edge of the 100-fathoms plateau. From this wall large tabular masses would ever and anon break away and float off into the Atlantic—a condition of things which is closely paralleled at present along the borders of the ice-drowned Antarctic continent.