Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

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.

By-and-by, however, a great change took place, and the big ice-sheet melted off the Long Island and vanished from the Minch. We read the evidence for this change of climate in certain interesting deposits which occur in considerable bulk at the northern extremity of Lewis, and in smaller patches in the Eye peninsula of the same island. In those districts the old sub-glacial débris or till is covered with beds of clay and sand in which many marine exuviæ are found – shells of molluscs, entomostraca, foraminifera, etc. They clearly prove, then, that after the ice-sheet had vanished Lewis was submerged in the sea to a depth of not less than 200 feet, and they also prove that the temperature of the sea was much the same then as now, for the shells all belong to species that are still living in these northern waters. It is very remarkable that the marine deposits in question seem to occur nowhere else in any part of the Long Island. We cannot believe that the submergence was restricted to the very limited areas where the shell-beds are met with: it must, on the contrary, have affected a very large portion, if not the whole, of the Outer Hebrides. Why, then, do not we meet with shelly sands and clays, with raised beaches and other relics of the former occupation of these islands by the sea, covering wide areas in the low-grounds? How can we explain the absence of such relics from all those districts which, being much under the level of 200 feet, must necessarily have at one time formed part of the sea-floor? The explanation is not difficult to discover.

Resting upon the surface of the shell-beds at Ness and Garabost we find an upper or overlying accumulation of sub-glacial débris or till. At Ness this upper till closely resembles, in general appearance, the lower deposit that rests directly upon the rocks. It is a pell-mell accumulation of silty clay, crammed with glaciated stones, amongst which are many fragments of red sandstone and some extra-Hebridean rocks, and interspersed through it occur also broken fragments of sea-shells. The marine deposits lying below are usually much confused and contorted, and here and there they are even violently commingled with the upper till. They show, generally, a most irregular surface under that accumulation, and are evidently only the wreck of what they must at one time have been. Now the presence of this upper till proves beyond doubt that the intense arctic conditions of climate once more supervened. A big ice-sheet again filled up the basin of the Minch and flowed over the Long Island – its under-tow creeping along the inner margin of the lofty rock-barrier as before, and eventually stealing over the low-ground at the Butt, where its bottom-moraine or till was dragged over the marine deposits, and confusedly commingled with them. The upper strata of the ice that streamed across the islands renewed the work of abrasion, and succeeded in scraping away all traces of the late occupation by the sea. If any such now exist they must lie buried under the till that cloaks the low-ground on the western margins of the islands. Hence it is that we find not a vestige of shelly beds in any part of the Long Island which was exposed to the full brunt of the ice-flow. At Garabost they have been ploughed through in the most wonderful manner, and only little patches remain. At Ness, however, they are more continuous. This is owing to the circumstance that the ground in that neighbourhood is low-lying and offered no obstacle to the passage of the ice out to sea. Hence the shell-beds were not subjected to such excessive erosion as overtook them along the whole eastern border of the Long Island.

Eventually, however, this later advance of the ice-sheet ceased. The climate grew less arctic, and the great glacier began to melt away, until the time came that its upper strata ceased to overflow the islands. They then passed away to north and south, along the hollow now occupied by the Minch, following the same path as the bottom-ice. Considerable snow-fields, however, still covered the Outer Hebrides, and large local glaciers occupied all the mountain-valleys, and, descending to low levels, piled up their terminal moraines. Some of these local glaciers appear to have gone right out into the Minch, as in South Uist, and may have coalesced with the great glacier that still filled that basin. It was during this condition of things that most of the great perched blocks that are scattered so profusely over the islands began to be dropt into their present positions. During the climax of glacial cold, when the upper strata of the ice-sheet streamed across the Hebrides, large fragments of rock would certainly be wrenched off and carried on underneath the ice; but as only a few of the Hebridean mountain-tops were then exposed, there would be a general absence of such enormous erratics as are detached by frost and rolled down upon the surface of a glacier, and any such superficially-borne erratics would be transported, of course, far beyond the Long Island into the Atlantic. When the ice had ceased to overflow the islands, boulders derived from Skye and the mainland would no longer be carried so directly out to the Atlantic, but would travel thither by the more circuitous route, which the now diminished ice-sheet was compelled to follow.

As the snow and ice melted off the Hebrides, the rocks would begin to be exposed to the action of intense frost, and many fragments, becoming dislodged and falling upon névé, small local ice-sheets, and glaciers, would be stranded on hill-slopes and sprinkled over the low-grounds, along with much broken débris and rock-rubbish. Eventually all the lower-grounds would be deserted by the ice, glaciers would die out of the less elevated valleys, and linger in only a few of the glens that drain the higher mountain-masses. Such local glaciers have flowed often at right angles to the direction followed by the great ice-sheet. Thus, the ice-markings in the glens that come down from the Forest of Harris to West Loch Tarbert, run from north to south, while the trend of the older glaciation on the intervening high-grounds is from south-east to north-west.

The morainic rubbish and erratics of this latest phase in the glacial history of the Long Island may be traced down almost to the water’s edge, showing plainly that there has been no great submergence of that region since the disappearance of glacial conditions. This is somewhat remarkable, because along the shores of central and southern Scotland we have indisputable evidence to show that the land was drowned to the depth of at least fifty feet in post-glacial times. In the Outer Hebrides, however, there are no traces of any post-glacial submergence exceeding a dozen feet or so; that is to say, there is no proof that the Outer Hebrides have been of much less extent than they are now. On the contrary, we have many reasons for believing that they were within comparatively recent times of considerably larger size, and were even in all probability united to the mainland. The abundance of large trees in the peat-mosses, and the fact that these ancient peat-covered forests extend out to sea, are alone sufficient to convince one that the Outer Hebrides have been much reduced in area since the close of the glacial period. These now bleak islands at one time supported extensive forests, although nowadays a tree will hardly grow unless it be carefully looked after. That old forest period coincided in all probability with the latest continental condition of the British Islands – when the broad plains which are now drowned under the German Ocean formed part of a great forest-land, that included all the British Islands, and extended west for some distance into tracts over which now roll the waves of the Atlantic. The palmy days of the great British forests, however, passed away when the German Ocean came into existence. The climatic conditions were then not so favourable for the growth of large trees; and in the uplands of our country, and what are now our maritime districts, the forests decayed, and were gradually overgrown by and buried under peat-mosses. The submergence of the land continued after that, until central and southern Scotland were reduced to a considerably smaller size than now, and then by-and-by the process was reversed, and the sea once more retreated, leaving behind it a number of old raised beaches to mark the levels at which it formerly stood.

The greatest submergence that overtook central and southern Scotland in times posterior to the latest continental condition of Britain did not exceed fifty feet, or thereabout; and the extreme limits reached by the sea in the period that supervened between the close of the glacial epoch and the “age of forests” was not more than one hundred feet. The Outer Hebrides, however, were certainly not smaller in post-glacial times than they are now, and we have no evidence to show that after the “age of forests” had passed away the sea rose higher than a dozen feet or so above its present level. Now there are only two ways in which all this can be accounted for. Either the Hebrides remained stationary, or stood at a level higher than now, while the central and southern parts of Scotland were being submerged; or else there has been a very recent depression within the Hebridean area, which has carried down below the sea all traces of late glacial and post-glacial raised beaches. All we know for certain is, that the only raised beaches in the Long Island are met with in low maritime regions at only a few feet above the present high-water mark. My own impression is that the whole district has been submerged within comparatively recent times; for if the present coast-line had endured since the close of the glacial period, or even since the last continental condition of Britain, I should have expected the sea to have done more than it has in the way of excavation and erosion.

In a former article I have spoken of the sand-dunes and sandy flats of the west coast of the Long Island. These receive their greatest development in North Uist, Benbecula, and South Uist. Along the whole western margin of these islands stretch wide shoals and banks of yellow sand and silt, and similar shoals and banks cover the bed of the shallow sounds or channels. In the middle of the Sound of Harris one may often touch the bottom with an oar, and even run one’s boat aground. It is the same in the Sound of Barra, while, as I have already mentioned, one may walk at low-water from Benbecula into the adjacent islands of North and South Uist. Where does all this sand come from? Certainly not from the degradation of the islands by the sea, for the sounds appear to be silting up, and the general appearance of the sandy flats along the west coast indicates that the land is upon the whole gaining rather than losing. I have no doubt at all that this sand and silt are merely the old sub-glacial débris which the ice-sheet spread over the low shelving plateau that extends west under the Atlantic to the 100-fathoms line. That plateau must have been thickly covered with till, and with heaps and sheets of gravel and sand and silt, and it is these deposits, sifted and winnowed by the sea, which the tides and waves sweep up along the Atlantic margin of the islands.

There are many other points of interest to that I might touch upon, but I have said enough perhaps to indicate to any intelligent observer the kind of country he may be led to expect in the Long Island. Of course the history of the glacial period is very well illustrated in many parts of the mainland, which are much easier of access than the Outer Hebrides. But these islands contain, at least, one bit of evidence which does not occur anywhere else in Britain. In them we obtain, for the first time, data for measuring the actual slope of the ice-sheet. It does not follow, however, that the inclination of the surface towards the Atlantic was the same all over the area covered by the ice-sheet. The slope of the sheet that flowed east into the basin of the German Ocean, for example, may have been, and probably was, less than that of the Hebridean ice-flow. But apart altogether from this particular point, I think there is no part of the British Islands where the evidence for the former action of a great ice-sheet is more abundant and more easily read, or where one may realise with such vividness the conditions that obtained during that period of extraordinary climatic vicissitudes, which geologists call the Glacial Epoch.

Leaving these old arctic scenes, and coming down to the actual present, no one, I think, can wander much about the Outer Hebrides without pondering over the fate of the islanders themselves. Many writers have asserted that the Celt of these rather out-of-the-way places is a lazy, worthless creature, whom we Saxons should do our best to weed out. One cannot help feeling that this assertion is unfair and cruel. The fact is, we judge him by a wrong standard. He is by nature and long-inherited habits a fisherman, and has been wont to cultivate only so much land as should suffice for the sustenance of himself and those immediately dependent upon him. In old times he was often enough called upon to fight, wrongly or rightly, and thus acquired that proud bearing which it has taken so many long years of misery to crush out. He is, as a rule, totally unfit for the close confinement and hard work which are the lot of the great mass of our mechanics – does not see the beauty of that, and has rather a kind of contempt for the monotonous drudgery of large manufacturing towns. One of the few situations in town that he cares to fill is that of police-constable. Give him a life in the open air, however trying it may be, and he will be quite content if he can make enough to feed himself and family. If the fishing chance to be very profitable he does not, as a rule, think of saving the surplus he has made, but looks forward rather to a spell of idleness, when he can smoke his pipe and talk interminable long talks with his neighbours. No doubt this, judged by our own standard, is all very shocking. Why doesn’t he put his money in the savings-bank, and by-and-by die and leave it to those who come after him? Simply because he is a Celt, and not a Saxon.

Of course one knows how it will all end. Ere long the unadulterated Celt will be driven or improved out of these islands, and will retire to other lands, where, mingling and intermarrying with Teutons, he will eventually disappear, but not without leavening the races amongst which he is destined to vanish. And who will take his place in the Long Island? Probably a few farmers, a few shepherds, and a sprinkling of gamekeepers; and it is just possible that a few fishermen also may be allowed to settle down here and there upon the coast. One may see the process going on at present. Large tracts that once supported many villages are now quite depopulated. The time will come when somebody in Parliament will move for the reduction of the Civil Service estimates by the amount of the sheriff-substitute’s salary, and when the jail at Lochmaddy will have nothing higher in the scale of being to imprison than some refractory ram. One may be pardoned for wishing that he could foretell for the islands another fate than this. It is sad to think that a fine race of people is thus surely passing away from amongst us, for, despite all that can be urged against them, they are what I say. The fishermen of Lewis and Barra are bold, stalwart fellows, whom it would be difficult to peer amongst any similar class of men on the mainland. And all through the island one meets with equally excellent specimens of our kind. Many a brave soldier who fought our battles in the great French wars hailed from these outer islands. Pity it is that no feasible plan to prevent the threatened scattering of the race has yet been brought forward. Some day we may regret this, and come to think that though mutton and wool in the Long Island are desirable, yet islanders would have been better.

[Postscript. – On pages 153.4 I have described the second general ice-sheet that overflowed the Outer Hebrides as having eventually become resolved into a series of local ice-sheets and glaciers. Subsequent research, however, has since led me to believe that the district ice-sheets and local glaciers referred to were not the direct descendants of the last great ice-sheet. They appear to have come into existence long after that ice-sheet had entirely disappeared. See Article X.]

VI.

The Ice Age in Europe and North America. 11

In casting about for a subject upon which to address you this evening, I thought I could hardly do better than give you the result of a comparison which I have recently been able to make between the glacial phenomena of Europe and North America. The subject of glaciation seems to be now somewhat worn; but I gather from the fact that writers can still be found who see in our superficial deposits strong evidence of the Deluge, that a short outline of what we really do know may not be unacceptable. In the short time at our disposal, it is obvious that I cannot enter into much detail, and that many interesting questions must remain untouched. It will be as well, therefore, that I should at the outset define the limits of the present inquiry, and state clearly what are the chief points to which I wish to direct your attention. My main object, then, will be to bring into prominence such evidence as seems to betoken in a special manner the uniformity of conditions that obtained in the northern hemisphere during the Ice Age. In other words, I shall confine myself to a description of certain characteristic and representative phenomena which are common to Europe and North America, with the view of showing that the physical conditions of the glacial period were practically the same in both continents.

The phenomena which might be considered under this head embrace nearly all the facts with which glacialists are familiar, but I purpose restricting myself to three questions only, viz.: —

1st. The extent of glaciation2nd. Changes of climate during the Ice Age3rd. The results of fluvio-glacial action

The consideration of these questions, even if it were exhaustive (which it cannot be on this occasion), would still leave the general subject very incomplete, for we must forego the discussion of all such interesting topics as the “connection between glaciation and submergence,” "the formation of rock-basins," and the “origin of the geographical distribution of our faunas and floras.” Confining my inquiry within the limits just specified, I shall begin by sketching broadly the general results obtained by glacialists in Europe, and thereafter I shall proceed to give an outline of the corresponding conclusions arrived at by American observers.

I.

The Extent of Glaciation in Europe

To what extent, then, let us ask, has Europe been glaciated? What areas have been covered with perennial snow and ice? Owing to the fulness and clearness of the evidence, we are able to give a very definite answer to this question. It is hardly too much to say that we are as well acquainted with the distribution of glacier-ice in Europe during the Ice Age as we are with that of existing snow-fields and glaciers.

The nature of the evidence upon which our knowledge is based is doubtless familiar to many whom I have the pleasure of now addressing, but for the sake of those who have not such familiarity with the subject I may be allowed to indicate very briefly its general character. A rock-surface over which ice has flowed for any considerable time exhibits either an abraded, worn, and smoothed appearance, or the rocks are disrupted and broken, and larger or smaller fragments are found to have been removed and carried forward in the direction followed by the ice. Now, ice-worn and shattered rock-surfaces of this description, such as can be seen underneath existing glaciers, occur more or less abundantly over vast regions in Europe. They are met with from the North Cape south as far as Leipzig, and from the Outer Hebrides east to the valley of the Petchora and the foot-slopes of the Ural Mountains. Nor are they confined to northern Europe. They appear again and again in France and Spain and Italy, and in the low-grounds of middle Europe, where they occupy positions now far removed from the influence of glacial action. Such ice-worn and disrupted rock-surfaces not only prove that glacier-ice formerly covered large portions of our Continent, but they also indicate for us the directions in which that enveloping ice moved. The smoother surfaces in question are very frequently marked with coarse and fine parallel scratches and grooves of precisely the same nature and origin as the scratches and grooves which characterise the rocky bed of a modern glacier. And these markings, having been produced by the sand, grit, and stones which are pushed and dragged over the rocks by flowing ice, necessarily discover for us the path of glacial movement. But all rocks subjected to glacial action are not necessarily smoothed and polished. Sometimes, owing to structural peculiarities, and for various other reasons, rocks cannot resist the pressure of the ice, but are crushed and broken, and the resulting fragments are rolled and dragged forward in the direction of ice-flow. In this manner the path of a glacier becomes strewed with débris which has from time to time been forced from its rocky bed. There is really no mystery, therefore in tracking the spoor of extinct glaciers; for we have two sets of facts to aid us, either of which might suffice to indicate the extent and direction of glaciation. Consider, however, for a moment, what one observes in connection with rock-striation. We have, in the first place, the rounding and smoothing, and the parallel ruts and striæ. Not only so, but we frequently find that one side of prominent projecting knolls and hills is more highly worn and abraded than the other. Often, indeed, one side may show no trace whatsoever of abrasion. Here, again, we have clear evidence of the direction of ice-flow. Who can doubt that the worn and abraded rocks look towards the point whence the ice came, and that the non-glaciated rocks in the rear have been sheltered by the rocks in front? It is for this reason that in the mountainous regions of northern Europe the striated and smoothed rock-surfaces invariably look up the valleys, while the broken and unworn rock-ledges face in the opposite direction.