Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

by James Geikie

I have described the condition of the Cheviot district during the climax of the Ice Age as one of intense arctic cold, the whole ridge of hills being then completely smothered in snow and ice. This excessive climate, however, did not last continuously throughout the so-called glacial period, but was interrupted by more than one mild interglacial epoch. We have evidence in Scotland, as in other countries, to show that the great confluent ice-masses melted away so as to uncover all the low-grounds and permit the reappearance of plants and animals. Rivers again watered the land, and numerous lakes diversified the face of the country. Willows, hazels, and alders grew in the sheltered valleys, oak-trees flourished in the low-grounds, and Scots firs clustered upon the hill-slopes. A strong, grassy vegetation covered wide areas, and sedges and rushes luxuriated in marshy places and encroached upon the margins of the lakes. The mammoth, or woolly-coated elephant, roamed over the land, and among its congeners were the extinct ox, the horse, the Irish elk, and the reindeer. After such a temperate condition of things had continued for some time—perhaps for thousands of years—the land, during the last interglacial epoch, became gradually submerged to a depth of several hundred feet, and a cold, ungenial sea, in which flourished species of northern and arctic shells, covered the low-grounds of Scotland. The cold continuing to increase, our glaciers descended for the last time from the mountains and encroached upon the bed of the sea, until they became confluent, fairly usurping the floor of the German Ocean, and pushing back the western seas as far as, and even beyond, the islands of the Outer Hebrides. There is good reason to believe that such great changes of climate occurred several times during the glacial period, which thus seems to have consisted of an alternation of cold and genial epochs. But as the last phase in this extraordinary series of changes was a cold one, during which great glaciers scoured the face of the country, we now obtain only a few scattered traces of the genial conditions that characterised the preceding mild interglacial epochs. Vegetable accumulations, lake and river deposits with mammalian remains, marine beds and their shelly contents, were all ploughed up by the ice, and to a very large extent demolished. Here and there, however, we find in the till or boulder-clay that marks the last cold epoch, wasted fragments of trees, tusks of mammoths, and broken sea-shells; while underneath the till we occasionally come upon old lake deposits with vegetable and mammalian remains, or, as the case may be, beds of marine origin well stocked with sea-shells of arctic species. And these freshwater and marine beds repose, in many cases, upon an older accumulation of till, which belongs to an earlier cold epoch of the glacial period. In the Cheviot district proper, the traces of mild, interglacial conditions are very slight, but in the immediate neighbourhood we find them more strongly marked. Thus, in the valley of the Slitrig, near Hawick, we notice freshwater beds with peaty matter lying between a lower and an upper till or boulder-clay; and interglacial freshwater beds also appear in the neighbouring county of Peebles, particularly in the valley of the Leithan Water. Again, in the valley of the Tweed near Carham, there occur interglacial beds in which I detected numerous bones of water-rats and frogs. These interglacial remains acquire a peculiar interest when we come to view the “superficial deposits” of Scotland in connection with those of England and the Continent; for, as I have endeavoured to show elsewhere,[I] it is most likely that the ancient gravels of England, which contain the earliest traces of man, belong for the most part to interglacial times; and the extraordinary changes of climate described above may therefore have been actually witnessed by human eyes. Indeed, I believe it was the advent of the last cold epoch of the Ice Age that drove out the old tribes who used the rude flint implements that are now found in the gravel deposits and caves of England, and who occupied the British area along with hippopotami, rhinoceroses, elephants, lions, hyænas, and other animals. The men who entered Britain after the final disappearance of arctic conditions, were more advanced in civilisation, and were accompanied by a very different assemblage of animals—by a group represented by oxen, sheep, dogs, and other creatures, most of which are still indigenous to Britain.

[I] Great Ice Age.

But to return to the Cheviots. When the final cold epoch had reached its climax, and the ice-sheet began to melt away for the last time, the tops of the hills then once more became uncovered, and large blocks, detached by the action of the frost, fell upon the surface of the glaciers, and were borne down the valleys, some of them to become stranded here and there on hill-slopes, others to be carried far away from the Cheviot area and dropped at last over Northumberland and Durham, or even further south. As the melting of the ice continued, and the glacier of the Tweed ceased to reach the sea, great accumulations of gravel and sand were formed. Underneath the ice, sub-glacial streams ploughed out the till, and paved their hidden courses with gravel and sand. In summer-time, the whole surface of the Tweed glacier was abundantly washed with water, which, pouring down by clefts and holes in the ice, swelled all the sub-glacial streams and rivers. At the same time, floods descending from the Lammermuirs and the Cheviots, pushed with them vast quantities of shingle, gravel, and sand, part of which was swept upon the surface of the Tweed glacier, while much seems to have gathered along its flanks, forming banks and ridges running parallel with the course of the valley.

At last the time came when the ice had fairly vanished from the lower reaches of the Tweed, and we now walk over its bed and mark the long ridges and banks of shingle and gravel that were formed by the sub-glacial streams and rivers, and the somewhat similar accumulations that gathered along the sides of the glacier at the foot of the Lammermuir Hills. Here and there, also, we note the heaps (i.e. moraines) of shingle, earth, clay, and débris, with large erratics which travelled on the surface of the ice, and were dropped upon the ground as that ice melted away. All the loose erratics that lie at the surface in the lower reaches of the Tweed valley have come from the west. Some of them rest upon hard rock, others upon till, and yet others crown the tops and slopes of gravel and sand hillocks, or appear in low mounds of morainic origin.

In the valleys of the Cheviot Hills one traces the footsteps of the retiring glaciers in mounds and hummocks of rude earthy débris, blocks, and rock-rubbish. These are terminal moraines, and they indicate certain pauses in the recession of the ice. The most remarkable examples occur in the valley of the Kale Water at Blinkbonny, a mile or so above the village of Eckford. At that place a bank of morainic matter at one time blocked up the valley of the Kale, and thus formed a wide and extensive lake that stretched up to and beyond Morebattle. Numerous curious hillocks of gravel and sand are banked against the moraine, and point to the action of the flood-waters that escaped from the melting glacier. Other gravelly moraine mounds occur higher up the same valley, as near Grubbit Mill. These last tell us of a time when the Kale glacier had retreated still further, so as to have its terminal front near where Morebattle now is. Wreaths and hummocks of gravel and sand, extending from Grubbit to the north-east, along the hollow in the hills that leads to Yetholm Loch, indicate the course taken by a portion of the torrents that escaped from the ice in summer-time. In other hill-valleys, similar indications of ancient local glaciers may be seen. Some of the most conspicuous of these appear upon the slopes and in the high valleys within the drainage-areas of the Jed and the Kale. They consist chiefly of mounds and hillocks, made up of coarse earthy débris and rock-rubbish; sometimes these are solitary and rest in the throat of a valley, at other times they are scattered all over the hill-slopes and valley-bottom. One can have no doubt as to what they mean: they indicate clearly the presence of insignificant glaciers that were soon to vanish away. The larger and better-defined mounds are true terminal moraines, while the scattered heaps of rubbish point out for us the beds in which the glaciers lay. Thus, from the sea-coast up to the highest ridge of this border country, we follow the spoor of the melting ice; passing from massive and wide-spread deposits of till, gravel, and sand, and angular débris in the low-grounds, up to insignificant heaps and scatterings of rock-rubbish and angular boulders at the higher levels of the country.

Several more or less extensive flats in the hill-valleys indicate the former presence of lakes which have become obliterated by the action of the streams. But by far the most conspicuous example of such silted-up lakes is that of the Kale valley, to which reference has already been made. In the later stages of the Ice Age that river-valley must have existed as a lake from Marlfield up to and beyond Morebattle. Indeed, there is evidence to show that even within historical times a considerable lake overspread the flat grounds in this neighbourhood. The name Morebattle is supposed to mean the “village by the lake,” and, up to a few years ago, there was a sheet of water called Linton Loch a little to the east of Morebattle. But this has been drained by the proprietor, and is now represented by only two insignificant pools. The present course of the Kale between Marlfield and Kalemouth is of post-glacial age—the old pre-glacial and interglacial course being filled up with drifted materials. As the appearances at this place are somewhat typical of many of the valleys of the Cheviot district, I may briefly summarise the history of the Morebattle lake.

Before the advent of the last great age of ice the Kale would seem to have flowed from Marlfield, close to the line now followed by the turnpike road as far as Easter Wooden, after which it passed near the present sites of Blinkbonny and Mosstower, and so on to the Teviot, which it joined some little distance above Kalemouth. During the Ice Age many of the old river-courses were completely choked up with clay, stones, and gravel, so that when the ice melted away the rivers did not always or even often regain their old channels. Thus, in the case of the Kale, we find that the present course of the river below Marlfield is of recent or post-glacial age, having been excavated by the river since the close of the glacial epoch. The old or pre-glacial course lies completely choked up and concealed under the rubbish shot into it at a time when glacier-ice filled all the valley of the Kale down to Marlfield. At this latter place the Kale glacier seems to have made a considerable pause—it ceased for some time to retreat—and thus a heavy bank of gravel, sand, shingle, earth, blocks, and angular rubbish gathered in front of it, and obliterated the old river-course into which they were dropped. When the glacier at last disappeared, a lake was formed above the morainic dam that closed the valley below Marlfield, and the outflow of the lake took place at a point lying some little distance to the north of the old or pre-glacial course of the Kale. By slow degrees the river excavated a new channel for itself in the Old Red Sandstone rocks, and in doing so gradually lowered the level of the waters. This and the silting action of the Kale and its feeders slowly converted the lake-hollow into a broad alluvial flat through which the river now winds its way.

Another extensive lake seems to have occupied the vale of the Teviot between Jedfoot and Eckford, and similar old lake-beds occur in several of the hill-valleys. One good example is seen in the valley of the Oxnam Water, where the flat tract that extends from the old village of Oxnam up to the foot of the Row Hill indicates the former presence of a lake which has been drained by the stream cutting for itself a gorge in Silurian greywackés and shales. In many other valleys it is easy to see that the streams do not always occupy their pre-glacial courses, and some of the old forsaken courses are still patent enough. Thus, a glance at the hollow that extends from Mossburnford on the Jed to Hardenpeel on the Oxnam is enough to convince one that in pre-glacial, and probably in early post-glacial times also, a considerable stream has flowed from what is now the vale of the Jed into the valley of the Oxnam.

In all the valleys we meet with striking evidence to show that the streams and rivers must formerly have been larger than they are now. Certain banks and ridges of gravel fringe the valley-slopes at considerable heights, and indicate the action of deeper and broader currents than now make their way towards the sea. It is probable that these high-level gravel terraces date their existence back to the close of the Ice Age, when local glaciers still lingered in some of the mountain-valleys, and when in summer-time great floods and torrents descended from the hills.

An extremely humid climate seems to have characterised Scotland even in post-glacial times, as may be gathered from the phenomena of her peat-mosses. Very little peat occurs on the Scottish side of the Cheviots, and it is conspicuous chiefly on the very crest of the hills, where it attains a thickness that varies from a foot or two up to five or six yards. Here and there we detect the remains of birch under the peat, but the peat itself is composed chiefly of bog-moss and heather. The evidence so abundantly supplied by the peat-mosses in other parts of Scotland shows that after the Ice Age had passed away the Scottish area became clothed with luxuriant forests of oak, pine, and other trees. At that time the British Islands appear to have been joined to themselves and the Continent across the upraised beds of the Irish Sea and the German Ocean. Races of men who used polished stone implements and sailed in canoes that were hollowed out of single oaks inhabited the country, together with certain species of oxen (now either extinct or domesticated), the elk, the beaver, the wolf, and other animals, such as the dog and the sheep, which are still indigenous. The climate was more excessive then than it is now—the summers being warmer and the winters colder. By-and-by, however, submergence ensued, the great wooded plain that seems once to have extended between Britain and the Continent disappeared below the waves, and the climate of this country became more humid. The old forests began to decay and the peat-mosses to increase, until by-and-by large areas in the low-grounds passed into the condition of dreary moor and morass, and even the brushwood and stunted trees of the hills died down and became enveloped in a mantle of bog-moss. A study of the present condition of the Scottish peat-mosses leads one to believe that the rate of increase is now much exceeded by the rate of decay, and that the eventual disappearance of the peat that clothes hill-tops and valley-bottoms is only a question of time. Draining and other agricultural operations have no doubt influenced to some extent this general decay of the peat-mosses; but there is reason to suspect that the change of climate, to which the decay of the peat is due, may really be owing to some cosmical cause. Quite recently an accomplished Norwegian botanist has come to similar conclusions regarding the peat-mosses of the Scandinavian peninsula.