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.
The scratched 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."
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 produced
crag 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.
[Footnote 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.]
[Footnote 11: _Denudation_ is a geological term used to denote the
removal of rock by the wasting action of water, whereby the underlying
mineral masses are _denuded_ or 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. In Fig. 2, _a_ represents 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_; _c_ marks the hollow on the west side of the crag.
[Illustration: 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, an _anticlinal 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]
[Footnote 12: This remarkable example of denudation was first described
by Mr. M'Laren, in his _Sketch 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 successive _faults_, they must have covered
these hills completely to a depth of not less than 6000 feet.[13]
From this 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.
[Footnote 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. We sail 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 whose surface
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]
[Footnote 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 scenes have 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.
[Footnote 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.--(See Fig. 2.)
If my reader has ever visited the channel of a mountain-torrent--
"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 the plains,
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 changing scene, 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.