Part 9

Some reference has already been made (see p. 64) to the general appearance presented by the valleys of the Cheviots. In their upper reaches they are often rough and craggy; narrow dells, in fact, flanked with steep shingle-covered slopes, and occasionally overlooked by beetling cliffs, or fringed with lofty scaurs of decomposing rocks. As we follow down the valleys they gradually widen out; the hill-slopes becoming less steep, and retiring from the stream so as to leave a narrow strip of meadow-land through which the clear waters canter gaily on to the low grounds of the Teviot. In their middle reaches these upland dales are not infrequently well cultivated to a considerable height, as in the districts between Hownam and Morebattle, and between Belford and Yetholm--the former in the valley of the Kale, and the latter in that of the Bowmont. It is noticeable that all the narrower and steeper reaches lie among Silurian strata and Old Red Sandstone porphyrites. No sooner do we leave the regions occupied by these tough and hard rock-masses than the whole aspect of the scenery changes. The surrounding hills immediately lose in height and fall away into a softly undulating country, through which the streams and rivers have dug for themselves deep romantic channels. Nevertheless, it is a fact, as we shall see by-and-by, that south-west of the region occupied by the igneous rocks of the Cheviot Hills, all the higher portions of the range (Hungry Law, Carter Fell, Peel Fell, etc.) are built up of sandstones. For the present, however, I confine attention to those valleys whose upper reaches lie either wholly or in part among igneous rocks or Silurian strata. A typical and certainly the most beautiful example is furnished us by the vale of the River Jed. This stream rises among the sandstone heights which have just been mentioned as composing the south-west portion of the Cheviot range. The first seven or eight miles of its course lead us through a broad open valley, which has been hollowed out almost exclusively in sandstones and shales; by-and-by, however, we are led into a Silurian tract, and thereupon the valley contracts and the hill-slopes descend more steeply to the stream. But we soon leave the grassy glades of this Silurian tract and enter all at once upon what may be termed the lower reaches of the Jed. No longer cooped up in the rocky gully, painfully worn for itself in the hard greywacké and shales, the stream now winds through a much deeper and broader channel which has evidently been excavated with greater ease. Precipitous banks and scaurs here overlook the river at every bend, the banks becoming higher and higher and retiring further and further from each other, as the water glides on its way, until at last they fairly open upon the broad vale of the Teviot. Sometimes the river flows along one side of its valley for a considerable distance, and whenever this is the case, it gives us a line of bold cliffs which are usually flanked on the opposite side by sloping ground. This is the general character of all valleys of erosion, and especially of the lower reaches of the Jed.

A glance at the cliffs and scaurs of the Jed shows that they consist of horizontal or gently undulating strata of soft earthy, friable, shaly sandstone, arranged in thin beds and bands, which alternate rapidly with crumbling, sandy, and earthy shales; the whole forming a loose and unconsolidated mass that readily becomes a prey to the action of the weather, rain, frost, and running water. The prevailing colour is a dull red, but pale yellow, white, green, and purple discolorations are visible when the strata are closely scanned. The finest sections occur between Glen Douglas and Inchbonnie, and at Mossburnford, but the cliffs throughout present the same general appearance, and are picturesque in the highest degree. Everywhere the banks are thickly wooded, and even the steep red scaurs are dashed and flecked with greenery, which droops and springs from every ledge and crevice in which a root can fix itself. How vivid and striking is the contrast between the fresh delicate green of early summer and the rich warm tint of these rocks, which when lit up by the setting sun seem almost to glow and burn! Well may the good folk of Jedburgh be proud of the lovely valley in which their lot is cast. In no similar district in Scotland will the artist meet with a greater number of such "delicious bits," in which all the charms of wood and water, of meadow and rock are so harmoniously combined. It is not with the scenic beauties of the Jed, however, that we have at present to do. I wish the reader to examine with me certain appearances visible at the base of the red beds, where these rest upon those older rocks which have formed the subject of the preceding papers. In the bed of the river at Jedburgh, we see the junction between the red beds and the Silurian strata, and may observe how the bottom portions of the former, which repose immediately upon the greywackés, are abundantly charged with well-rounded and water-worn stones. Many of these stones consist of greywacké, hardened grit, and other kinds of rock, and most of them undoubtedly have been derived from Silurian strata. In other districts where the old igneous rocks of the Cheviots form the pavement upon which the red beds repose, the stones in the lower portions of the latter are made up chiefly of rounded fragments of the underlying porphyrites. All which clearly shows that the red beds have been built out of the ruins of the older strata of the district. This is unquestionably the origin not only of the conglomerates, but of all the red beds through which the River Jed cuts its way from the base of the hills to the Teviot. When we trace out the boundary of these beds, we find that this leads us along the base of the hills, close to the hill-foot; and not only so, but it frequently takes us into the hill-valleys also. And this shows that the Cheviots had already been deeply excavated by streams before any portion of the red beds was deposited.

I have said that the red beds are approximately horizontal; sometimes, however, they have a decided _dip_ or inclination, and when this is continuous, it is invariably in a direction away from the hills. Thus as we traverse the ground from the hill-foot to the Teviot, we pass over the outcrops of the red beds and slowly rise from a lower to a higher geological position. The strata, however, are generally so flat that their dip is often not greater than the average slope or inclination of the ground. Hence when we ascend the valley-slopes from the stream, we soon reach the higher beds of the series, as, for example, in the undulating heights that overlook the Jed in the neighbourhood of Jedburgh. In that district a number of quarries have been opened, in which the upper beds of the red series are well exposed, as at Ferniehirst, Tudhope, etc. These consist of thick beds of greyish white, yellowish, and reddish sandstones, which, unlike the crumbling earthy deposits below, are quite suitable for building purposes. Scales of fish and plant remains are often met with in the thick sandstones, but the underlying earthy, friable red beds appear to be quite destitute of any organic remains.

Let us now briefly recapitulate the main facts we have just ascertained. They are these:--1. All the low grounds that abut upon the hills are composed of horizontal or nearly horizontal strata, which consist chiefly of red earthy beds, passing down into conglomerates, and up into whitish and reddish sandstones. 2. The conglomeratic portion forms the boundary of the series, fringing the outskirts of the hills, and resting sometimes upon Silurian strata and sometimes upon Old Red Sandstone igneous rocks. 3. Fossils occur in the white and red sandstones, but seem to be wanting in the underlying red earthy beds.

The accompanying diagram (Fig. 7) gives a generalised view of the relation borne by the red beds to the older rocks of the Cheviots. It will be seen that the former rest _unconformably_ upon the Old Red Sandstone igneous rocks, and also, of course, upon the Silurian strata. The section shows that the red beds lie upon a worn and denuded surface. Now this speaks to the lapse of a long period of time. It may be remembered that we had some grounds for believing that the latest eruptions of the Cheviot volcanoes were sub-aërial. The evidence now enables us to advance further, and to state that after the close of the volcanic period, the whole Cheviot district existed as an elevated tract of dry land, from which streams flowed north and south. And for so long a time did these conditions endure, that the rivulets and streams were enabled to scoop out many channels and broad valleys before any of the outlying red beds had come into existence. Before the conglomerate beds were laid down, the ancient volcanic bank of the Cheviots had thus suffered great erosion. This is what "unconformability" means. It points to the prolonged continuance of a land-surface, subject as that must always be to the wearing action of the sub-aërial forces. Rain and frost disintegrate the rocks, and running water rolls the débris from higher to lower levels, and piles it up in the form of gravel, sand, and mud in lakes and the sea. While the old volcanic country of the Cheviots was being thus denuded, it would appear that a wide extent of land existed in the Northern Highlands and Southern Uplands of Scotland, and also in what are now the lake districts of England and the hilly tracts of Wales. And in all these regions valleys were formed, which at a subsequent time were more or less filled up with newer deposits.

The presence of the red beds that sweep round the base of the Cheviot Hills shows unmistakably that a period of submergence followed these land conditions. All the low grounds of Southern Scotland disappeared beneath a wide sheet of water, which stretched from the foot of the Lammermuirs up to the base of the Cheviots, and here and there entered the valleys, and so extended into the hills. This water, however, does not seem to have been that of an open sea; rather was it portion of a great freshwater lake, brackish lagoon, or inland sea. The lowest beds of the red series are merely hardened layers and masses of gravel and rolled shingle, which would seem at first sight to indicate the former action of waves along a sea-beach. There are certain appearances, however, which lead one to suspect that these ancient shingle beds may have had quite another origin. In some places the stones exactly resemble those which are found so abundantly in glacial deposits. They are sub-angular and blunted, and, like glaciated stones, occasionally show striæ or scratches. This, however, is very rarely the case. Most of the stones appear subsequently to have been rolled about in water, and in this process they must have lost any ice-markings they may have had, and become smoothed and rounded like ordinary gravel stones. The same appearances may be noted in the glacier valleys of Norway and Switzerland, where at the present day the glaciated stones which are pushed out at the lower ends of the glaciers are rolled about in the streams, and soon lose all trace of ice-work. It is impossible, however, to enter here into all the details of the evidence which lead one to suspect that glaciers may have existed at this early period among the Cheviot and Lammermuir Hills. In the latter district, the conglomerates occur in such masses and so exactly resemble the morainic débris and ice-rubbish of modern glacial regions, that the late Sir A. C. Ramsay long ago suggested their ice-origin.

Let us conceive, then, that when the ancient lake or inland sea of which I have spoken reached the base of the Cheviots, glaciers may have nestled in the valleys. Streams issuing from the lower ends of these would sweep great quantities of gravel down the valleys to the margin of the lake, and it is quite possible that there might be enough wave-action to spread the gravel out along the shores. It is evident, however, that the main heaps of shingle would gather opposite what were at that time the mouths of glacier valleys; and it is just in such positions that we now meet with the thickest masses of conglomerate. Ere long, however, the supposed glaciers would seem to have melted away, and only fine sand and mud, with here and there small rounded stones and grit, accumulated round the shores of the ancient lake. Of course, during all this time fine-grained sediment gathered over the deeper parts of the lake-bottom.

We have no evidence to show what kind of creatures, if any, inhabited the land at this time; nor do any fossils occur in the red earthy beds to throw light upon the conditions of life that may have obtained in the lake. If glaciers really existed and sent down ice-cold water, the conditions would hardly be favourable to life of any kind; for glacial lakes are generally barren. But the absence of fossils may be due to other causes than this. It is a remarkable fact, that red strata are, as a rule, unfossiliferous, and the few fossils which they do sometimes yield are generally indicative rather of lacustrine and brackish-water, than marine conditions. The paucity or absence of organic remains seems to have been often due to the presence in the water of a superabundance of salts. Now this excessive salinity may have arisen in either of two ways. First, we may suppose some wide reach of the sea to have been cut off from communication with the open ocean by an elevation of a portion of its bed; and in this case we should have a lagoon of saltwater, which evaporation would tend to concentrate to such a degree, that by-and-by nothing would be able to live in its waters. Or, again, we may have a lake so poisoned by the influx of springs and streams, carrying various salts in solution, as to render it uninhabitable by life of any kind, either animal or vegetable. Many red sandstone deposits, as Sir A. C. Ramsay has pointed out, are evidently lagoon-formations, which is proved by the presence of associated beds of rock-salt, gypsum, and magnesian limestone. They have slowly accumulated in great inland seas or lakes having no outlet, whose waters were subject to evaporation and concentration, although now and then they seem to have communicated more or less freely with the ocean. The red earthy beds of the Jed, however, though unfossiliferous, yet contain no trace of rock-salt or magnesian limestone. The only character they have in common with the salt-bearing strata of the New Red Sandstone of England is their colour, due to the presence of peroxide of iron, which we can hardly conceive could have been deposited in the mud of a sea communicating freely with the ocean. But a quiet lake, fed by rivulets and streams that drained an old volcanic district, is precisely the kind of water-basin in which highly ferruginous mud and sand might be expected to accumulate. Such a lake, tainted with the various salts, etc., carried into it by streams and springs (some of which may have been thermal; for, as we shall see presently, the volcanic forces, although quiescent, were yet not extinct), might well be unfitted for either animal or plant, and probably this is one reason why the red earthy beds of the Jed are so unfossiliferous.

After some time, the physical conditions in the regions under review experienced some further modification. Considerable depression of the land supervened, and the waters of our inland sea or lake rose high on the slopes of the Cheviots. Mark now how the character of the sediment changes. The prevailing red colour has disappeared, and white, yellow, and pale greenish or grey sand begins to be poured over the bed of the lake. Even yet, however, ferruginous matter exists in sufficient quantity to tint the sediment red in some places. With the appearance of these lighter-coloured sandy deposits, the conditions seem to have become better fitted to sustain life. Fish of peculiar forms, which, like the gar-pike of North American lakes, were provided with a strong scaly armour of tough bone, began to abound, weeds grew in the water, and the neighbouring land supported a vegetation now very meagrely represented by the few remains of plants which have been preserved. In some places fish-scales are found in considerable abundance. They belong to several genera and species which are more or less characteristic of the Old Red Sandstone formation. The most remarkable form was the _Pterichthys_, or wing-finned fish. Its blunt-shaped head and the anterior portion of its body were sheathed in a solid case of bone, formed by the union of numerous bony scales or plates. Two curious curved spine-like arms occupied the place of pectoral fins, and may have been used by the creature in paddling along the bottom of the sea or lake in which it lived. The posterior part of the body was covered with bony scales, but these were not suturally united. Other kinds of fish were the _Holoptychius_ and _Coccosteus_, both of which were, like the Pterichthys, furnished with bony scales. The scales of the former overlapped, and had a curious wrinkled surface. The head of the Coccosteus was protected by a large bony shield or buckler, and a similar bony armour covered the ventral region.

The organic remains of these fish-bearing strata are too scanty, however, to enable us to form any idea of the kind of climate which characterised the district at this long-past period; but if we rely upon the fossils which have been met with in strata of the same or approximately the same age elsewhere, we may be pretty sure the climate was genial, and nourished on the land an abundant vegetation, consisting of ferns, great reeds, and club-mosses, which attained the dimensions of large trees, conifers, and other strange trees which have no living analogues.

It seems most likely that when the land sank down in the Cheviot district, so as to allow the old lake to reach as it were a higher level, some communication with the outlying ocean was effected. Red ferruginous mud would then cease to accumulate, or gather only now and then; the deposits would for the most part be white or yellow, or pale green; and fish would be able to come in from the sea. The communication with the ocean, however, was probably never very free, but liable to frequent interruption.

Here, then, ends the third great period of time represented by the rocks of the Cheviot district. The first period, as we have seen, closed with the deposition of the Silurian strata. Thereafter supervened a vast lapse of time, not recorded in the Cheviots by the presence of any rocks, but represented in other regions by younger members of the Silurian system. During this unrecorded portion of past time, the Silurian strata of the Cheviots were hardened, compressed, folded, upheaved to the light of day, and worn into hills and valleys by the action of the sub-aërial forces. Then began the second period of rock-forming in our district. Volcanoes poured out successive beds of molten matter and showers of stones and ashes, and so built up the rock-masses of the highest parts of the Cheviot Hills. These eruptions belong to the Old Red Sandstone age, and form a portion of what we term the Lower Old Red Sandstone. After the extinction of the volcanoes, another prolonged period elapsed, which is not accounted for in the Cheviots by the presence of any rocks. Then it was, as we know, that the great volcanic bank was denuded and worn into a system of hills and valleys. Now, since it is evident that the red beds of the Jed and other places are also of Old Red Sandstone age, it follows that they must belong to a higher place in the Old Red Sandstone formation than the much-denuded igneous rocks upon which they rest unconformably. The reasonable conclusion seems to be that the denudation or wearing away of the Lower Old Red Sandstone igneous rocks of the Cheviots was effected during that period which is represented in other districts of Scotland by what is called the Middle Old Red Sandstone, so that the Jed beds will thus rank as Upper Old Red Sandstone.

I come now to speak of certain rocks which, although they are developed chiefly beyond the limits of our district, yet require a little consideration before we can complete our account of the geological history of the Cheviots. The rocks referred to consist chiefly of old lava-beds, which very closely resemble those of the Lower Old Red Sandstone. They appear on the south side of the Tweed valley below Kelso, whence they extend south-west and west, crossing the river at Makerstoun, and sweeping north to form the hills about Smailholm, Stichill, and Hume (Fig. 8). All to the east of these rocks, the valley of the Tweed is occupied by a great thickness of grey sandstones, and grey and blue shales and clays, with which are associated thin cement-stone bands, and occasional coarse sandy limestones called cornstone. These strata rest upon the outskirts of the Kelso igneous rocks, and are clearly of later date than these, since in their lower beds, which are often conglomeratic, we find numerous rounded fragments of the igneous rocks upon which the sandstones and shales abut. The latter have yielded a number of fossils, both animals and plants, to which I shall refer presently. In the bed of the Teviot near Roxburgh, and elsewhere, the Kelso igneous rocks are found reposing upon whitish and reddish sandstones, which are evidently the upper members of the red beds of the Jed Water and other localities.

Strata closely resembling the grey sandstones and shales of the Tweed valley appear among the Cheviot Hills at the head of the Jed Water, where they are marked by the presence of thick massive sandstones, which form all the tops of the hills between Hungry Law and the heights that overlook the sources of the Liddel Water--the greatest height reached being at Carter Fell, which is 1815 feet above the sea-level. The strata at this place contain some impure limestone and thin seams of coal, while beds of lava and tuff appear intercalated in the series.