CHAPTER III.

How the boulder came to be one "Crag and tail"—Scenery of central Scotland: Edinburgh—"Crag and tail" formerly associated in its origin with the boulder-clay—This explanation erroneous—Denudation an old process—Its results—Illustration from, the Mid-Lothian coal-field—The three Ross-shire hills—The Hebrides relics of an ancient land—Scenery of the western coast—Effects of the breakers—Denudation of the Secondary strata of the Hebrides—Preservative influence of trap-rocks—Lost species of the Hebrides—Illustration—Origin of the general denudation of the country—Illustrative action of streams—Denudation a very slow process—Many old land-surfaces may have been effaced—Varied aspect of the British Islands during a period of submergence—Illustration.

Thescratched and grooved surface of the boulder was produced when it was fast frozen in some iceberg, and driven gratingly across some submarine summit, or stranded on some rocky coast-line. But, from its rounded form, the stone had evidently undergone a long process of wear and tear previous to its glacial journey. Probably it had hitherto lain along a surf-beaten beach, where in the course of ages it had gradually been worn into its present rounded shape. But how came it there? It must originally have formed part of a flat sandstone bed, with many other beds piled above it. By what agency, then, was this great pile reduced to fragments?

The answer to these questions must be a somewhat lengthened one, for the subject relates not to a few beds of rock hastily broken up and dispersed, but to the physical changes of an entire country, carried on during a vast succession of geological periods.

A phenomenon, known familiarly as "crag and tail," has long been connected in its origin with the drift or boulder beds. Has my reader ever travelled through central Scotland? If so,he must often have noticed the abrupt isolated form of many of the hills, presenting a mural front to the west, and a long sloping declivity to the east. From the great number of isolated hard trap-rocks in this region, the phenomenon is much better seen than in most other parts of the kingdom. There is, for instance, the castle rock of Stirling, with its beetling crag and castellated summit, which present so imposing a front to the west. Many other examples are seen along the line of the Edinburgh and Glasgow Railway. The range of hills south of Linlithgow, the singularly abrupt basalt of Binny Craig, the long rounded ridge of Ratho, the double-peaked crag of Dalmahoy, the broad undulation of woody Corstorphine, are all examples more or less marked. Edinburgh itself is an excellent illustration. The Calton Hill shows a steep front to the town, while its eastern side slopes away down to the sea. Arthur's Seat, in like manner, has a precipitous western face, and a gentle declivity eastward. The Castle rock, too, shoots up perpendicularly from the valley that girdles it on the north, west, and south, sinking away to the east in a long slope—

"Whose ridgy back heaves to the sky,Piled deep and massy, close and high."

"Whose ridgy back heaves to the sky,

Piled deep and massy, close and high."

East-Lothian presents several well-marked instances; in particular, North Berwick Law and Traprain. A phenomenon so general must have had some general origin, and it was accordingly attributed to the same agency which produced the drift-clays and the striated rock-surfaces, when these were believed to be the results of great diluvial action. It would seem, however, that the phenomenon of crag and tail should not be associated with the boulder-clay. The latter is undoubtedly a newer Tertiary formation,[10]but the denudation[11]which producedcrag and tail must have been going on long ere the Tertiary ages had begun. There is satisfactory evidence that large areas of our country were planed down at a greatly more ancient period than that of even the oldest of the Tertiary series. Thus, the whole area of the county of Sussex suffered a very extensive denudation during the later Secondary ages. The Hebrides had undergone a similar process previous to the deposition of the Lias and Oolite, and the Greywacke hills of south Scotland, previous to the formation of the Old Red Sandstone. There seems thus to have been a general and continuous process of degradation at work during a long succession of geological ages.

[10]The reader is referred to the table of the geological formations at the end of the volume for the relative position of the beds described.

[11]Denudationis a geological term used to denote the removal of rock by the wasting action of water, whereby the underlying mineral masses aredenudedor laid bare.

The results of this long-continued action are of the most startling kind. I have referred to the phenomenon of crag and tail as perhaps the most readily observable. We must not fail to remember that the crag which now stands up so prominently above the level of the surrounding country, at one period lay buried beneath an accumulation of sandstone, shale, or other strata, all of which have been carried away, so as to leave the harder rock in bold relief, with a portion of the less coherent strata sloping as a long tail from its eastern side. The crag, too, is often breached in many places, worn down at one end, rounded on the summit, and sometimes well-nigh ground away altogether, whilst in front there is invariably a deep hollow scooped out by the current when arrested by the abrupt cliff. InFig. 2,arepresents a crag of greenstone worn away and bared of the shales which once covered it;b, the sloping "tail" of softer strata, protected from abrasion by the resistance of the trap-rock, and covered by a deep layer of drift,d;cmarks the hollow on the west side of the crag.

Fig. 2"Crag and tail."

But when we come to measure the actual amount of material that has been carried away, we are lost in conjecture as to the vastness of the time which such a process must have occupied. For instance, the coal-bearing strata of Mid-Lothian must at one period have been connected with those of Linlithgow and Stirling. At a subsequent date, the western area subsided to form the Stirlingshire coal-basin, and the eastern area, in like manner, sank down to form the coal-basin of Mid-Lothian, while the intermediate portion stretched from east to west as a great arch, or, as it is termed geologically, ananticlinal axis. Now, the whole of this arch has been worn away, not a vestige of it remains, and yet its upper or coal-bearing part was fully 3000 feet thick.[12]

[12]This remarkable example of denudation was first described by Mr. M'Laren, in hisSketch of the Geology of Fife and the Lothians, a work in which the author showed himself to be in advance of the science of his time.

Let us take a small portion of this district, and endeavour to calculate the amount of matter thus removed. The Pentland hills form a chain stretching from near Edinburgh for some fourteen miles southward, and having an average breadth of about two miles and a half. They are formed chiefly of felspathic trap-rocks, resting upon and interstratified with conglomerate apparently of Old Red age, which in turn lies upon vertical Silurian slates. Before the Carboniferous strata were thrown down by successivefaults, they must have covered these hills completely to a depth of not less than 6000 feet.[13]Fromthis small area, therefore, stratified sandstones, shales, limestones, and coal, must have been removed to the enormous extent of one billion, eight hundred and fifty-four thousand, four hundred and sixty-four millions of cubic feet.

[13]The actual depth of the Mid-Lothian coal-field, to the base of the carboniferous limestone, is rather more than 3000 feet. It is, perhaps, rather under than over the truth to allow 3000 feet for the total thickness of beds from the limestone to the conglomerate of Liberton, though, owing to the curved and contorted position of the strata from Edinburgh to Stirlingshire, it is impossible to obtain a measurement of their real thickness. I have attributed the isolation of the Falkirk and Mid-Lothian coal-fields to the effect of faults and general depressions of their areas. This was assuredly the case in the latter coal-field, and probably in the former also. The trap which occurs between them, though in great abundance, has certainly not acted as an elevating agent. It occurs in beds among the strata, and, judging from the number of associated tufas, appears to have been to a considerable extent erupted while the lower carboniferous series was forming. Mr. M'Laren, in his excellent work, p. 100, states his opinion that the traps may have materially contributed to push up the coal strata. A careful and extended examination of the district has convinced me that this view is incorrect.

But, perhaps, the most striking instances of denudation in the British Islands are the three famous Ross-shire hills—Suil Veinn, Coul Mor, and Coul Bheig. They are formed of piles of sandstone beds like tiers of regular masonry, and reach a height of 3000 feet over the sea. The sandstone of which they are composed must once have formed a bed or set of beds fully 2000 feet thick, that covered the whole district for many miles around. Yet of this extensive deposit there now exist only a few isolated fragments. I have watched the sunshine and shadow of an autumn sky resting alternately on these strange pyramidal hills, as they towered in their giant proportions like the last remnants of a mighty rampart that had stood the brunt of a long siege, and, breached at last in many places, had been all but levelled to the ground. How long-continued and how potent must that agency have been which could cut down and disperse the massive barrier that flanked the western coast of Ross-shire to a height of 2000 feet!

The Hebrides are but the shattered relics of an old land that had its mountain-peaks and its glens, its streams and lakes, and may have nursed in its solitude the red-deer and the eagle, but was never trodden by the foot of man. A glance at the map is enough to convince us of this. We there see islands, and peninsulas, and promontories, and deep bays, and long-retiring inlets, as though the country had been submerged and only its higher points remained above water. The conviction is impressed more strongly upon us by a visit to these shores. Wesail through the windings of one of the "sounds," and can scarcely believe that we are on the bosom of the salt sea. Hills rise on all sides, and the water, smooth as a polished mirror, shows so pure and limpid that in the sunshine we can see the white pebbles that strew its bed many fathoms down. The eastern shore is often abruptly interrupted by long-receding lochs edged round with lofty mountains, and thus, where we had looked to see a deep heathy glen, with, perchance, a white tree-shaded mansion in the far distance, and a few dun smoking cottages in front, we are surprised to catch a glimpse of the white sails of a yacht, or the darker canvas of the herring-boats. We sail on, and soon a sudden turn brings us abruptly to the mouth of the sound. A bold headland, studded around with rocky islets, rises perpendicularly from the sea, bleak and bare, without a bush or tree, or the faintest trace of the proximity of man. The broad swell of the Atlantic comes rolling in among these rocks, and breaks in foam against the grey cliffs overhead. In tempests, such a scene must be of the most terrific kind. Wo to the hapless vessel that is sucked into the vortex of these breakers, whose roar is sometimes heard at the distance of miles! Even in the calmest weather the white surf comes surging in, and a low sullen boom is ever reverberating along the shore. We see the harder rocks protruding far into the sea, and often pierced with long twilight caves, while the softer ones are worn into deep clefts, or hollowed out into open bays strewed over with shingle. The sunken rocks and islets, scarcely showing their tops above water, were all evidently at one time connected, for, as we recede from the shore, we can mark how the process of demolition goes on. There is first the projecting ness or promontory, well-nigh severed from the mainland, but still connected by a rude arch, through which the swell ever gurgles to and fro. Then, a little farther from the shore, a huge isolated crag, washed on all sides by the surge, raises its grey lichen-clothed summit. A short way beyond, there is the well-worn islet whosesurface shelters neither lichen nor sea-weed, but is ever wet with the dash of the waves. Further to the sea, the white gleam of the breakers marks the site of the sunken rock. Thus, in the space of a hundred yards, we may sometimes behold the progress of change from land to sea, and see before us a specimen of that action which slowly but yet steadily has narrowed and breached the outline of our western shores.[14]

[14]I have endeavoured to illustrate the process of denudation by a reference to breaker-action on the existing coast-line of the Hebrides; but a strong current must have materially increased the force of the ancient waves, and produced abrasion to some depth below them.

If we attempt to trace the connexions of strata among the Hebrides, we shall be more fully impressed with the magnitude of the changes which have been effected. Thus the Lias and Oolite occur in patches along the shores of Mull, Morven, Ardnamurchan, Eigg, Skye, Raasay, and Applecross. But though now only in patches, these formations must once have extended over a considerable area, for they seem to form the under-rock of the whole of the northern part of Skye, and are seen in almost every lone island from Ardnamurchan Point to the Shiant Isles. These scattered portions, often many miles distant from each other, are the remnants of a great sheet of liassic and oolitic strata, now almost entirely swept away, and are extant from having been covered over with hard trap-rocks. But for these it may be doubted whether we should ever have known that corals once gleamed white along the shores of Skye, that the many-chambered ammonite swam over the site of the Coolin Hills, that the huge reptilian monsters of these ancient times, icthyosaurs and plesiosaurs, careered through the waters that laved the grey hills of Sleat, and that forests of zamia and cycas, and many other plants indicative of a warm climate, bloomed green and luxuriant along the site of that strange mist-clad cliff-line, that shoots up into the pinnacles of the Storr and Quiraing. It is curious to reflect, that the records of these peaceful sceneshave been preserved to us by the devastating eruptions of volcanic forces; that the old lava-streams which spread death through the waters along whose bed they travelled, have yet been the means of protecting the districts which they wasted, while those parts where they did not reach have been long since swept away. It is allowable to believe, that in the portions of liassic strata which have been destroyed there existed the remains of not a few species, perhaps some genera, to be found nowhere else, and of whose former existence there is now, by consequence, no trace. In the small island of Pabba—a relic of the Scottish Lias—I found thirty-one species, of which Dr. Wright has pronounced four to be new.[15]A subsequent visit to the adjacent island of Raasay has increased the list. In short, every patch of these Secondary rocks, if thoroughly explored, might be found to yield its peculiar organisms. And in the far larger area that has been carried away there existed, doubtless, many more. We are accustomed to see individuals perish and their remains crumble away, but the species still holds on. In the stratified portion of the earth's crust, however, we mark how not merely individuals have perished, but whole genera and species; but of these the remains are still before us in the rocks; we can study their forms, and, from a comparison with recent species and genera, can arrive at some idea of their nature and functions. In this way, we are able to picture the various conditions of the earth when these organisms lived in succession upon its surface. Yet, we may readily conjecture, that in ancient eras many tribes and genera of plants and animals lived for ages, and then passed away without leaving any record of their existence. Many circumstances might concur to prevent the preservation of their remains. The species of the Hebrides were preserved in the usual manner, but the cemetery in which their remains were entombed has been washed away, and they can be seen nowhere else. It is as if on some isolated country there had lived a race of men,tall Patagonians, or swarthy Hottentots, or diminutive Laplanders, with a civilisation of their own; owing to some change of climate the race gradually dwindled down until it died out; eventually, too, the land settled down beneath the sea with all its ruined cities and villages, which, as they reached in succession the level of the waves, were torn up and dispersed, and other races at last voyaged over the site of that old land, dreaming not, that in bygone years fellow-mortals of an extinct type had pastured their herds where now there rolled a widespread sea.

[15]Quart. Jour. Geol. Soc., vol. xiv. p. 26.

But to return. We have seen that the long-continued action of the sea has been sufficient to breach and waste away the existing coast-line of western Scotland. When, therefore, such results are produced by so ordinary a cause, need we go to seek the agency of great debacles to explain the denudation of other parts of the country? It is known that at great depths currents have little effect upon the rocks which they traverse, and that their action is greater as it nears the surface. To account for the phenomena of crag and tail, and the general denudation of the country, we may suppose the land to have been often submerged and re-elevated. As hill after hill rose towards or sank below the sea-level, it would be assailed by a strong current that flowed from the west and north-west, until, in its slow upward or downward progress, it got beyond the reach of the denuding agencies. In this way the general contour of the land would be greatly though very gradually changed. Hills of sandstone, or other material of feeble resistance, would be swept away, the harder trap-rocks would stand up bared of the strata which once covered them, deep hollows would be excavated in front of all the more prominent eminences, and long declivities would be left behind them.—(SeeFig. 2.)

If my reader has ever visited the channel of a mountain-torrent—

"ImbresQuern super notas aluere ripas"—

"Imbres

Quern super notas aluere ripas"—

he must have noticed an exact counterpart to these appearances. When the waters have subsided, the overflowed parts are seen to be covered in many places with sand. Wherever a pebble occurs along the surface of this sand, it has invariably a hollow before it on the side facing the direction whence the stream is flowing, and a long tail of sand pointing down the channel. If we watch the motion of the water along its bed, the denuding agency may be seen actively at work. Every pebble that protrudes above the shallow streamlet arrests the course of the current, which is then diverted in three directions. One part turns off to the right hand of the pebble, and cuts away the sand from its flank; another part strikes off to the left, and removes the sand from that side; while a middle part descends in front of the pebble, and, by a kind of circular or gyratory movement, scoops out a hollow in the sand in front. Behind the pebble the water is pretty still, so that the sand remains undisturbed, and is further increased by the accumulation above it of sediment swept round by the lateral currents. Now, in place of the supposed stream, let us substitute the ocean with its westerly current—for the pebble, a great trap-hill—for the sand, easily friable shales and sandstones, and we have exactly the condition of things which produced crag and tail.

This process of destruction must have been in progress during many geological ages. We may suppose, that in that time the land often changed level, sometimes rising far above the sea, and sometimes sinking deep below it. We can well believe that the surface would often be covered with vegetation; that plants, widely differing from those which are now indigenous, clothed its hill-sides and shaded its valleys; and that animals of long extinct forms roamed over its plains or prowled amid its forests. When the country, in the lapse of centuries, sank beneath the sea-level, all trace of these scenes would eventually be effaced. The westerly currents would soon recommence the process of degradation, uprooting the forests, devastating theplains, wearing down the hills, and scooping out the valleys; and so, when the ocean-bed, in the course of ages, became again dry land, it would arise "another and yet the same." The little valley, where once, perchance, the mastodon used to rest his massive bulk amid a rich growth of ferns, shaded by the thick umbrage of coniferous trees, would emerge a deep glen with bare and barren rocks on either side; the site of the hill whereon herds of the gazelle-like anoplothere were wont to browse, might reappear a level plain; the low-browed rock, under whose shadow the ungraceful palæothere used of old to rest from the heat of the noon-tide sun, might emerge a beetling crag shooting up several hundred feet over the valley. It is by this repeated elevation and submergence, carried on for many ages, that our country has acquired its present configuration.

We can easily picture to ourselves the appearance which the British Islands would thus at different periods present. At one time, nearly the whole of England would be under water, with, however, a few islands representing the higher peaks of Cornwall; others scattered over the site of the West Riding of Yorkshire; and a hilly tract of land over what is now Wales. Scotland must have existed in a sorely mutilated state. A thick-set archipelago would represent the Cheviot Hills, and the country south of the Forth and the Clyde; north of which there would intervene a broad strait, with a comparatively large area of undulating land beyond, stretching across what is now the area of the Grampian Hills. A narrow fiord would run along the site of the Caledonian Canal, cutting the country into two parts, and running far into it on either side as deep lochs and bays. I have had such a condition of things vividly recalled when on the summit of a lofty hill in early morning, while the mists were still floating over the lower grounds, and only the higher hill-tops, like so many islands, rose above the sea of cloud. It was not a little interesting to cast the eye athwart this changingscene, and mark how each well-known peak and eminence looked when deprived of its broad sweep of base. What before had always seemed an abrupt precipitous summit, now took the form of a lonely rock or deep-sea stack, that might have served as a haunt for the gull and the gannet. The long swelling hill rose above the mist as a low undulating island, treeless and barren. It was easy to think of that wide expanse of mist as the veritable domain of ocean, to picture the time when these were veritable islands lashed by the surge, and to conjure up visions of ice-floes drifting through the narrows, or stranding on the rocks, amid a scene of wide-spread nakedness and desolation.

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