Fragments of Earth Lore: Sketches & Addresses Geological and Geographical

We see, then, that each kind of rock has its own particular mode of weathering, and that the configuration of a country depends primarily upon this and upon geological structure. Indeed, so close is the connection between the geology and the surface-outline of a country, that to a practised observer the latter acts as an unfailing index to the general nature of the underlying rocks, and tells him at a glance whether these are igneous like basalt and porphyrite, aqueous like sandstone and shale, or hardened and altered strata like greywacké. But while one cannot help noticing how in the Cheviot district the character of the scenery depends largely upon the nature and structure of the rocks, he shall, nevertheless, hardly fail to observe that flowing outlines are more or less conspicuous over all the region. And as he descends into the main valleys, he shall be struck with the fact that the hill-slopes seem to be smoothed off in a direction that coincides with the trend of these valleys. In short, he cannot help noticing that the varied configuration that results from the weathering of different rock-masses has been subsequently modified by some agent which seems to have acted universally over the whole country. In the upper reaches of the Cheviot valleys, the rocks have evidently been rounded off by some force pressing upon them in a direction coinciding with that of the valleys; but soon after entering upon their lower reaches, we notice that the denuding or moulding force must have turned gradually away to the north-east – the northern spurs of the Cheviots, and the low-grounds that abut upon these being smoothed off in a direction that corresponds exactly with the trend of that great strath through which flow the Teviot and the Tweed, from Melrose downwards. Throughout this broad strath, which extends from the base of the Lammermuirs to the foot of the Cheviots, and includes the whole of Teviotdale, the ground presents a remarkable closely-wrinkled surface, the ridges and intervening hollows all coinciding in direction with the general trend of the great strath, which is south-west and north-east; but turning gradually round to east, as we approach the lower reaches of the Tweed.

Passing round the north-eastern extremity of the Cheviot range into Northumberland, we observe that the same series of ridges and hollows continues to follow an easterly direction until we near the sea-board, when the trend gradually swings round to the south-east, as in the neighbourhood of Belford and Bamborough, where the ridges run parallel with the coast-line.

The ridges and hollows are most conspicuous in the low-grounds of Roxburghshire and Berwickshire, especially in the regions between Kelso and Smailholm, and between Duns and Coldstream. The dwellers along the banks of the Tweed are quite familiar with the fact that the roads which run parallel with the river are smooth and level, for they coincide with the trend of the ridges and hollows; whilst those that cross the country at right angles to this direction must of course traverse ridge after ridge, and are therefore exceedingly uneven. In this low-lying district most of the ridges are composed of superficial deposits of stony and gravelly clay and sand, and the same is the case with those that sweep round the north-eastern spurs of the Cheviots by Coldstream and Ancroft. Some ridges, however, consist either of solid rock alone, as near Stichill, or of rock and overlying masses of clay and stones. In the hilly regions, again, nearly all the ridges are of rock alone, especially in the districts lying between Melrose and Selkirk and between Selkirk and Hawick. Indeed, the hills drained by the upper reaches of the Teviot and its tributaries are more or less fluted and channelled, as it were – many long parallel narrow hollows having been driven out along their slopes and even frequently across their broad tops. This scolloped and ridged aspect of the hills, however, disappears as we approach the upper reaches of the hill-valleys. From Skelfhill Pen (1745 feet) by Windburgh Hill (1662 feet), on through the ridge of the Cheviot watershed, none of the hills shows any appearance of a uniformly-wrinkled surface.

A close inspection of the rock-ridges satisfies one that they have been smoothed off by some agent pressing upon them in a direction that coincides with their own trend; and not only so, but the smoothing agent, it is clearly seen, must have come from the watersheds and then pressed outwards to the low-grounds which are now watered by the Teviot and the Tweed. This is shown by the manner in which the rocks have been smoothed off, for their smooth faces look towards the dominant watersheds, while their rough and unpolished sides point away in the opposite direction. Sometimes, however, we find that more or less steeply projecting rocks face the dominant watersheds. When such is the case, there is usually a long sloping “tail” behind the crag – a “tail” which is composed chiefly of superficial deposits. The hills between Hume and Stichill afford some good examples. The two kinds of appearances are exhibited in the accompanying diagram. The appearance shown in is of most common occurrence in the upland parts of the country, while “crag and tail” is seen to greatest advantage in the open low-grounds. In both cases it will be observed that superficial deposits (t) nestle behind a more or less steep face of rock.

When the rocks have not been much exposed to the action of the weather, they often show a polished surface covered with long parallel grooves and striæ or scratches. Such polished and scratched surfaces are best seen when the superficial deposits have been only recently removed. Often, too, when we tear away the thick turf that mantles the hill-slopes, we find the same phenomena. Indeed, wherever the rocks have not been much acted upon by the weather, and thus broken up and decomposed, we may expect to meet with more or less well-marked grooves and stride. Now the remarkable circumstance about these scratches is this – they agree in direction with the trend of the rock-ridges and the hollows described above. Nor can we doubt that the superficial markings have all been produced by one and the same agent. In the upper valleys of the Cheviots, the scratches coincide in direction with the valleys, which is, speaking generally, from south to north, but as we approach the low-grounds they begin to turn more to the east (just, as we have seen, is the case with the ridges and hollows), until we enter England to the east of Coldstream, where the striæ point first nearly due east, but eventually swing round to the south-east, as is well seen upon the limestone rocks between Lowick and Belford. In Teviotdale the general trend of the striæ is from south-west to north-east, a direction which continues to hold good until the lower reaches of the Tweed are approached, when, as we have just mentioned, they begin to turn more and more to the east. Thus it becomes evident that the denuding agent, whatever it was, that gave rise to these ridges and scratched rock-surfaces must have pressed outwards from all the dominant watersheds, and, sweeping down through the great undulating strath that lies between the Cheviots and the Lammermuirs, must have gradually turned away to the east and south as it rounded the northern spurs of the former range, so as to pass south-east over the contiguous maritime districts of Northumberland.

A few words now as to the character of the superficial deposits which enter so largely into the composition of the long parallel banks and ridges in the low-grounds of Roxburghshire, Berwickshire, and the northern part of Northumberland. The most conspicuous and noteworthy deposit is a hard tough tenacious clay, which is always more or less well-charged with blunted and sub-angular stones and boulders, scattered pell-mell through the mass. This clay is as a rule quite unstratified – it shows no lines of bedding, and although here and there it contains irregular patches and beds of gravel and sand, yet it evidently does not owe its origin to the action of water. Its colour in the upper part of Teviotdale and the Cheviots is generally a drab-brown, or pale grey and sometimes yellow, while here and there, as in the upper reaches of the Jed valley, it is a dark dingy bluish grey. In the lower parts of Teviotdale and in the Tweed district it is generally red or reddish brown. The stones in the clay have all been derived from the rocks of the region in which it occurs. Thus in Teviotdale we find that in the higher reaches of the dale which are Silurian the stones and boulders consist of various kinds of greywacké, etc. In the lower reaches, however, when we pass into the Red Sandstone area, we note that the clay begins to contain fragments of red sandstone, while the clay itself takes on a reddish tinge, until we get down to the vale of the Tweed, where not only is the clay very decidedly red, but its sandstone boulders also are very numerous. The same appearances present themselves in passing outwards from the Cheviots. At first the clay contains only stones that have been derived from the upper parts of the hills, but by-and-by, as we near the low-grounds, other kinds begin to make their appearance, so that by the time we reach the Tweed we may obtain from the clay specimens of every kind of rock that occurs within the drainage-area of the Teviot and the lower reaches of the River Tweed.

Look at the stones, and you shall observe that all the harder and finer-grained specimens are well-smoothed and covered with striæ or scratches, the best marked of which run parallel with the longer axis of each stone and boulder. These scratches are evidently very similar to those markings that cover the surface of the underlying solid rock, and we may feel sure, therefore, that the denuding agent which smoothed and scratched the solid rocks had also something to do with the stones and boulders of the clay.

Underneath the stony clay, or Till, as it is called, we find here and there certain old river gravels. We know that these gravels are river-formations, because not only do they lie at the bottom of the river-valleys, but the stones, we can see, have been arranged by water running in one constant direction, and that direction is always down the valley in which the gravels chance to occur. Frequently, however, there is no trace of such underlying gravels, but the till rests directly upon the solid rocks.

Now what do all these appearances mean? It is clear that there is no natural agent in this country engaged in rounding and scratching the rocks, or in accumulating a stony clay like till. In alpine regions, however, we know that glaciers, as they slowly creep down their valleys, grind and polish and scratch the rocks over which they pass, and that underneath the moving ice one may detect smoothed and striated stones precisely resembling those that occur in till. Frost in such alpine regions splits up the rocks of the cliffs and mountain-slopes that overlook a glacier, and immense masses of angular stones and débris, thus loosened, roll down and accumulate along the flanks of the ice-streams. Eventually such accumulations are borne slowly down the valley upon the back of the glacier, and are dropped at last over the terminal front of the ice, where they become intermingled with the stones and rubbish, which are pushed or washed out from underneath the ice. These heaps and masses of angular débris and stones are called “moraines,” and one can see that in Switzerland the glaciers must at some time have been much larger, for ancient moraines occur far down in the low-grounds of that country – the glaciers being now confined to the uppermost reaches of the deep mountain-valleys. Moreover, we may note how the mountain-slopes overlooking the present puny glaciers have been rubbed by ice up to a height of sometimes a thousand feet and more above the level of the existing ice-streams. Now since the aspect presented by the glaciated rock-surfaces of Switzerland is exactly paralleled by the rounded and smoothed rocks of Scotland, there can be no doubt that the latter have had a similar origin. Again, we find throughout the low-grounds of Switzerland a deposit of till precisely resembling that which is so well developed in Teviotdale and the valley of the Tweed. And as there can be no doubt that the Swiss till has been produced by the action of glacier ice, we are compelled to believe the same of the till in Scotland.

Let us further note that in the deep mountain-valleys of Switzerland the glacial deposits consist for the most part of coarse morainic débris – of such materials, in short, as the terminal moraines of existing glaciers are mainly composed. Not infrequently this morainic débris has been more or less acted upon by the rivers that escape from the glaciers, and the angular stones have been rounded and arranged in bedded masses. It is only when we get out of the mountain-valleys and approach the low-grounds that the till, or stony clay, begins to appear abundantly. The same phenomena characterise the Cheviot district. In the upper reaches of the mountain-valleys at the heads of the Teviot, the Kale, the Bowmont, etc., either till does not occur or it is thin and often concealed below masses of rude morainic débris and gravel. Out in the low-grounds, however, till, as we have already remarked, is the most conspicuous of all the superficial deposits. From these facts it may be inferred that till indicates the former presence of great confluent glaciers, while morainic débris in hill-valleys points to the action of comparatively small local and isolated glaciers.

What, then, are the general conclusions which may be derived from a study of the rock-ridges, flutings, and striæ, and the till of the Cheviot district? Clearly this: that the whole country has at one time been deeply buried under glacier ice. The evidence shows us that the broad strath stretching between the Lammermuirs and the Cheviots must have been filled to overflowing with a great mass of ice that descended from the uplands of Peebles and Selkirk and the broad-topped heights that overlook the sources of the Teviot. The Cheviots appear to have been quite buried underneath this wide sea of ice, and so likewise were the Lammermuirs. At the same time, as we know, all Scotland was similarly enveloped in a vast sheet of snow and ice, which streamed out from the main watersheds of the country, and followed the lines of the chief straths – that is to say, the general slope of the ground. The track of the ice in the Cheviot district is very distinctly marked. In Teviotdale it followed the trend of the valley, and, grinding along the outcrop of the Silurian strata, deepened old hollows and scooped out new ones in the soft shaly beds, while the intervening harder strata, which offered greater resistance to the denuding action of the ice, did not wear so easily, and so were rounded off, and formed a series of ridges running parallel to the eroded hollows. The stones and rubbish, dragged along underneath the ice, necessarily increased as the glacier mass crept on its way. The rocks were scratched and grooved by the stones that were forced over them, and the polishing was completed by the finer sand and clay which resulted from the grinding process. Wherever a rock projected there would be a tendency for the stones and clay and sand to gather behind it. One may notice the same kind of action upon the bed of a stream, where the sediment tends to collect in the rear of prominent stones and boulders. And we can hardly fail to have observed further that the sediment of a river often arranges itself under the action of the current in long banks, which run parallel to the course of the water. Underneath the ice-sheet the stones, sand, and clay behaved in the same way. Behind projecting rocks in sheltered nooks and hollows, they accumulated, while in places exposed to the full sweep of the ice-stream they were piled up and drawn out into long parallel banks and ridges, the trend of which coincided with that of the ice-flow. The presence of confused and irregular patches and lenticular beds of sand, clay, and gravel in the till is not difficult to understand when we know that there is always more or less water flowing on underneath a glacier. Such streams must assort the débris, and roll angular fragments into rounded stones and pebbles; but the materials thus assorted in layers will ever and anon be crushed up so as to be either partially or wholly obliterated by the slowly moving glacier.

As the stones and clay were derived from the underlying rocks, it is no wonder that the colour of the till should vary. In the Silurian tracts it is pale yellowish, or bluish grey, and the stones consist chiefly of fragments of Silurian rocks, all blunted and smoothed, and often beautifully polished and striated. When we get into the Red Sandstone region of the low-grounds the colour of the clay begins by-and-by to change, and fragments of red sandstone become commingled with the Silurian stones, until ere long the colour of the deposit is decidedly red, and sandstone fragments abound. Everywhere the stones show that they have been carried persistently in one direction, and that is out from the watershed, and down the main valleys.

The direction of the ice-marks upon the solid rocks, and the trend of the “drums,” as the parallel ridges of till are termed, show that the ice-sheet of Teviotdale and Tweed gradually turned away to the east and south-east as it swept round the north-eastern spurs of the Cheviots. Now we may well ask why the ice did not go right out into the North Sea, which is apparently the course it ought to have followed. The same curious deflection affected the great ice-stream that occupied the basin of the Forth. When it got past North Berwick, that stream, instead of flowing directly east into the North Sea, turned away to the south-east and overflowed the northern spurs of the Lammermuirs, bringing with it into the valley of the Tweed stones and boulders which had travelled all the way from the Highlands. It is obvious there must have been some impediment to the flow of the Scottish ice into the basin of the North Sea. What could have blocked its passage in that direction? At the very time that Scotland lay concealed beneath its ice-sheet, Norway and Sweden were likewise smothered in ice which attained a thickness of not less than five or six thousand feet. The whole basin of the Baltic was occupied by a vast glacier which flowed south into Northern Germany, and this sheet was continuous with glacier-ice that crossed over Denmark. When we consider how shallow the North Sea is (it does not average more than forty fathoms between Scotland and the Continent), we cannot doubt that the immense masses of ice descending from Norway could not possibly have floated off, but must actually have crept across the bottom of that sea until they abutted upon and coalesced with the Scottish ice, so as to form one vast mer de glace.

Thus it was that the Scandinavian ice blocked up the path of the Scottish glaciers into the basin of the North Sea, and compelled them to flow south-east into England.8 Had there been no such obstruction to the passage of the Scottish glaciers, it is impossible to believe that snow and ice could ever have accumulated to such a depth in Scotland. The Scottish ice reached a thickness of some three thousand feet in its deeper parts. It is evident, however, that had there been a free course for the glaciers, they would have moved off before they could have attained this thickness. And we can hardly doubt, therefore, that it was the damming-up of their outlet by the great Scandinavian ice-sheet that enabled them to deepen to such an extent in the valleys and low-grounds of Scotland.

When the ice-sheet was at its thickest, the Cheviots were completely covered, nevertheless they served to divide the ice-flow between Scotland and England, although here and there one finds that the ice passed over some of the lower summits, carrying with it boulders and stones. This is by no means an uncommon circumstance in Scotland and other glaciated countries. Thus we note that Highland boulders have been brought into the vale of the Tweed across the Lammermuirs; and in the same way boulders from the heights overlooking Eskdale have been carried over some of the lower hill-tops into the vale of the Teviot. In like manner the Swedish ice occasionally overflowed the lower mountain-tops of the dividing ridge or watershed into Norway.

What wonder now that the Cheviot area should exhibit so many flowing outlines, that the hills should be so smoothed and rounded and fluted, that the low-grounds should be cumbered with such heaps of clay and striated stones? Long before the great glaciers appeared, the rocks were weathered and worn by the action of the usual atmospheric forces, and each had assumed its own peculiar outline; but how greatly has this been modified by the grinding action of the ice-sheet! To what an extent have projecting rocks been rubbed, and how great is the destruction that has befallen the loose accumulations of river gravel, sand, and clay that gathered in the valleys before the advent of the Ice Age! All that now remains of these are a few patches preserved here and their underneath the till. The Cheviots can tell us nothing of the kinds of plants and animals that clothed and peopled the country in pre-glacial times. All we learn is that streams and rivers flowed as they flow now, and that by-and-by everything was changed, and the land disappeared underneath a vast covering of snow and ice.

In my concluding paper I will show how this ice period passed away, and how the present condition of things succeeded.

V

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,9 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.