Imágenes de páginas
PDF
EPUB

vails; for though in its course it curves to the southeast, it reaches the sea at a higher degree of north latitude than it had before attained in its most northeasterly bend in Tweedsmuir. It thus represents a wavy line of force steadily pursuing its way to an end or point. The confluent streams of north-west and south-east unite in harmony in its peaceful valley, and murmur in one full flow along its green hillsides and through its tranquil haughs —a fitting boundary-line between the old hostile kingdoms - the opposing forces of north and south.

When the Tweed bursts from the hills to the plain below Melrose, and then flows through a rich well-wooded strath to the sea, it touches on rocks much younger than the Silurian strata of the hills. These are the Upper Old Red Sandstone and the subsequent Carboniferous deposits. In the Cheviots we find felstone, besides a vast mass of volcanic material belonging to the Lower Old Red Sandstone; while Carboniferous strata stretch from the flanks of those hills into Liddesdale. The lowlands of Dumfriesshire are partly Carboniferous and partly of still younger date. But the Silurian system is the true ground-frame of the district of the Tweed and its tributaries; and this shows very little of the covering of subsequent geological strata. It appears, in truth, except for the grass, the heather, and the wild flowers of the hills, very much as it was left by the sea and the ice of innumerable ages ago. These bare greywacke heights and haughs, unblessed by aught of late geological bounty or luxuriousness, have had a great deal to do in making and moulding the hardy, sinewy men who have

lived among them for the many hundreds of years of British and Scottish story. But we may probably also be grateful that, while there was little to enrich the human nature there, the green hills and the grey rocks have helped at least to quicken and nourish the pathos and the quiet reverence of the heart.

Long ago this region was wholly, or in great part, under the ocean; and where the highest hills now catch the first glimmer of the early sun, the waves broke in foam, and sea-birds shrieked and flew amid the war of waters. “The long belt of high ground between St Abb's Head and St Patrick's Channel is an ancient sea-bottom; the broad green tops, dotted to-day with sheep and grouse and blackcock, took their levelled outline under the grinding power of the breakers, and partly, perhaps, of drift-ice borne by ocean currents.” 1 This submergence seems, according to the same authority, to have taken place so far back as during the lapse of the geological period known as the Old Red Sandstone. After this epoch there came an upheaval of the land, -probably very gradual,—the rocks having been already hardened, crushed, and elevated into ridges. There would doubtless be inequalities of surface even at this early period, but probably the general appearance was that of an extended table-land. This, exposed to the action of rain and frost, would gradually become shaped and carved into valleys and river-basins. Long after the land had been moulded into nearly its present form, there came a period when the whole surface down to the sea was covered with a great ice-sheet, sealing it as rigidly

1 Sir A. Geikie, Scenery of Scotland, chap. ix.

as Greenland is sealed from hill to sea at the present day.

During this great ice period there seems to have been again a submergence of the land under the sea, to the extent, as variously estimated, of from 500 to 2000 feet, and probably the consequent flow and action of icebergs which would touch and grind down at least the higher portions of the sunken area. At length there came a time, known as the second or subsequent glacial epoch, when the land, having been once more upheaved, was again covered by one unbroken ice-surface, until this began gradually to disappear, and glaciers only lay on the greater heights, and moved outwards down the valleys. We have evidence of this glacier action in numerous moraines, particularly towards the heads of the waters that come down from the greater heights of the district,—as Polmood Burn from Broadlaw (2754 feet), Manor Water from Notman and Dollar Law (2680), and Winterhope Burn from Loch Craig Head (2625), where a series of moraines stretching across the high valley bar the way and dam back on the east and south the waters of Loch Skene. Professor Young of Glasgow has shown in a most interesting paper the extent of the glacier remains in Upper Tweeddale, and that these are not found at a lower elevation than 1000 feet above sea-level. The point has also been treated with much skill and grace by Sir Archibald Geikie. Speaking of the moraines near Loch Skene, he says: “Everything around tells of the old glaciers. Mound after mound stretching in crescent shape across the valley, and coming

1 Scenery and Geology of Scotland, chap. xi.

2 Ibid.

down in irregular piles from the Midlaw Corry on the left, huges masses of rock still perilously poised on the summits of the ridges where they had been tumbled by the ice that bore them from yonder dark cliffs, and then the lake itself so impressively the result of the damming back of the water by the bars of detritus thrown across the glen.”

The enormous lapse of time since the Carboniferous period may well account for the denudation, rounding, smoothing, and the wavy sculpture of the hills which they now present to the eye; for while rains and frosts and water-flow may make and accentuate cleugh and scaur in the valleys, they certainly help to soften and smooth the higher tops and slopes of the Lowland heights. The working of the ice-sheets, the movements of the glaciers and their streams, the influence of rain, of frost and mountain-spring, of burn and water-flow, supervening through a countless series of years, upon the original table-land worn into a plain by the action of the sea, scooped and hollowed this old sea-plain into glens and hopes, rounded and smoothed the hill-tops and hillsides, left them here scaured red and deep, and there rich in pastoral green, gave us wavy lines of hills as if arrested in water-flow,—gave us, in a word, the district we live in as the product of the sculpture of the unseen powers of those long gone years.

These greywacke rocks of the southern uplands, called Lower Silurian, are simply hardened sea-sand and mud, and of softer texture than the rocks of the Highlands. They were thus more subject to the power of abrasion, -smoothing and rounding,—and hence we find in the

[ocr errors]

southern uplands quite different forms and outlines from those of the north of Scotland. We have nothing, for example, corresponding to the horizontal strata and battlemented fronts of the mountains of Torridon sandstone as they face the waves of the Atlantic on the western coast of Ross and Inverness—the rock-castles of nature. And the long, smooth, flowing, wavy line of the southern hills is seldom broken by the conical form which the quartzite mountains of the north generally assume,—though here and there where thicker and more indurated masses of greywacke form the summits we have conically pointed tops—as, for example, on the opposite sides of the Leithen Water near its source. The "gnarled craggy outline,” ruggedness, and “spiry summits” which characterise the more quartzose of the gneisses and schists are awanting-unless in some portions of the district to the west of Nithsdale. We have not, moreover, anything which we can set alongside the crags, precipices, and corries which strike and arrest the eye in the granite heights of Ben Aven, Ben Macdhui, or Cairn Toul. Yet in the inner recesses, the sacred heart, of the southern uplands, -as in Glenrath and Blackhouse, Polmood and Glenheurie, Gameshope and Talla, up Winterhope, on Donald's Cleugh, and by Loch Skene, in Carrifran and Blackhope, in the eastern dip of Hartfell, at the head of Ettrick, and still more in the wilds of Carrick and Galloway,—there are precipices steep and corries grey and deep, and crags and scaurs, and long down-spreading screes of wasted rock, which bespeak those limitless powers at work in a far past time that make and mar the world, and impress us with a sense of

[ocr errors]
« AnteriorContinuar »