Fragments of Earth Lore: Sketches & Addresses Geological and Geographical. Geikie James

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СКАЧАТЬ which traverses the country from east to west, crossing the crest of the Cheviots near Brownhart Law, and striking west by north through Belling Hill, by the Rule Water at Hallrule Mill, on towards Hawick. This is one of a series of such dykes, common enough in some parts of Scotland, which become more numerous as we approach the west coast, where they are found associated with certain volcanic rocks of Tertiary age, in such a way as to lead to the belief that they all belong to the same period. The melted rock seems to have risen and cooled in great cracks or fissures, and seldom to have overflowed at the surface. Indeed it is highly probable that many or even most of the dykes never reached the surface at all, but have been exposed by subsequent denudation of the rocks that once overlaid them. Such would appear to have been the case with the great dyke of the Cheviot district.

      We can only conjecture what the condition of this part of southern Scotland was in the long ages that elapsed between the termination of the Lower Carboniferous period and the close of the Tertiary ages. It is more than likely that it shared in some of the submergences that ensued during the deposition of the upper group of the Carboniferous system; but after that it may have remained, for aught we can tell, in the condition of dry land all through those prolonged periods which are unrecorded in the rocks of the Cheviot Hills, but have left behind them such noteworthy remains in England and other countries. Of one thing we may be sure, that during a large part of those unrecorded ages the Cheviot district could not have been an area of deposition. Rather must it have existed for untold eras as dry land; and this explains and accounts for the enormous denudation which the whole country has experienced; for there can be little doubt that the Lower Carboniferous strata of Carter Fell were at one time continuous with the similar strata of the lower reaches of the Tweed valley. Yet hardly a trace of the missing beds remains in any part of the country between the ridge of the hills at the head of the Jed Water and the Tweed at Kelso. Only little patches are found capping the high ground opposite Jedburgh, as at Hunthill, etc. Thus more than a thousand feet of Lower Carboniferous strata, and probably not less than five hundred or six hundred feet of Old Red Sandstone rocks, have been slowly carried away, grain by grain, from the face of the Cheviot district since the close of the Lower Carboniferous period.

      IV

      In the first of these papers some reference was made to the configuration of the ground in the Cheviot district. We have seen that the outlines assumed by the country have been determined in large measure by the nature of the rocks. Thus where igneous masses abound, the hills present a more or less irregular, and broken or lumpy contour, while the valleys are frequently narrow and deep. In the tracts occupied by Silurian strata, we have, as a rule, broad-topped hill-masses with a smoothly-rounded outline, whose slopes generally fall away with a long gentle sweep into soft green valleys, along the bottoms of which the streams often flow in deep gullies and ravines. Where the country is formed of sandstones, and other associated strata, the hills are generally broad and well-rounded, but the outline is not infrequently interrupted by lines of cliff and escarpment. These strata, however, are confined chiefly to the low-grounds, where they form a gently-undulating country, broken here and there, as in Dunian Hill, Bonchester Hill, Rubers Law, etc., by abrupt cones and knobs of igneous rock.

      It is evident, then, that the diversified character of the Cheviot Hills and the adjoining low-grounds depends on the character of the rocks and also, as we shall see presently, upon geological structure. Each kind of rock has its own peculiar mode of weathering. All do not crumble away under the action of rain, frost, and running water in precisely the same manner. Some which yield equally and uniformly give rise to smooth outlines, others of more irregular composition, such as many igneous rocks, break up and crumble unequally in a capricious and eccentric way, and these in the course of time present a hummocky, lumpy, and rough irregular configuration. And as soft and readily-weathered rocks must wear away more rapidly than indurated and durable masses, it follows that the former will now be found most abundantly at low levels, while the latter will enter most extensively into the composition of the hills. But the contour of a country depends not only upon the relative durability of the rocks, but also upon the mode of their occurrence in the crust of the earth. Strata, as we have seen, do not all lie in one way; some are horizontal, others are inclined to the horizon, while yet others are vertical. Again, many rocks are amorphous; that is to say, they occur in somewhat thick masses which show no trace of a bedded arrangement. Such differences of structure and arrangement influence in no small degree the weathering and denudation of rocks, and cannot be left out of account when we are seeking to discover the origin of the present configuration of our hills and valleys. Thus, escarpments and the terraced aspect of many hill-slopes are due to inequalities in the strata of which such hills are built up. The softer strata crumble away more rapidly under the touch of the atmospheric forces than the harder beds which rest upon them, and hence the latter are undermined, and their exposed ends or crops, losing support, fall away and roll down the slopes. The igneous rocks of the Cheviots are arranged in beds; but so massive are these, that frequently a hill proves to be composed from base to summit of one and the same sheet of old lava. Hence there is a general absence of that terraced aspect which is so conspicuous in hills that are built up of bedded rock-masses. Here and there, however, the beds are not so massive, several cropping out upon a hill-side; and whenever this is the case (as near Yetholm) we find the hill-slopes presenting the usual terraced appearance – a series of cliffs and escarpments, separated by intervening slopes, rising one above the other. In the Silurian districts no such terraces or escarpments exist, the general high dip of the strata, which often approaches the vertical, precluding any such contour. In a region composed of highly-inclined greywacké and shale, however, we should expect to find that where the strata are of unequal durability, the harder beds will stand up in long narrow ridges, separated by intervening hollows, which have been worn out along the outcrops of the softer and more easily-denuded beds. And such appearances do show themselves in some parts of the Silurian area. As a rule, however, the Silurian strata are not thick-bedded, and harder and softer bands alternate so rapidly that they yield on the whole a smooth surface under the action of the atmospheric forces. In the low-lying districts, which, as I have said, are mostly occupied by sandstones and shaly beds, all the abrupt isolated hills are formed of igneous rocks, which are much harder and tougher than the strata that surround them. It is quite evident that these hills owe their present appearance to the durable nature of their constituent rocks, which now project above the general level of the surface, simply because they have been better able to resist the denuding agents than the softer rocks that once covered and concealed them.

      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, СКАЧАТЬ