not a few lakes, of the Highlands have a north-east and south-west
trend. This trend corresponds to what geologists call the _strike_ of
the strata. The rocks of the Highlands have been compressed into a
series of folds or anticlines and synclines, which have the direction
just stated--namely, north-east and south-west. A careless observer
might therefore rashly conclude that these surface-features resembled
those of the Jura--in other words, that the long parallel hollows were
synclinal troughs, and that the intervening ridges and high grounds
were anticlinal arches or saddle-backs. Nothing could be further from
the truth. A geological examination of the ground would show that
the features in question were everywhere the result of denudation,
guided by the petrological character and geological structure of the
rocks. Several of the most marked hollows run along the backs of
anticlinal axes, while some of the most conspicuous mountains are
built up of synclinal or trough-shaped strata. Ben Lawers, and the
depression occupied by Loch Tay, are excellent examples; and since
that district has recently been mapped in detail by Mr. J. Grant
Wilson, of the Geological Survey, I shall give a section (Fig. 10) to
show the relation between the form of the ground and the geological
structure of the rocks. This section speaks for itself. Here evidently
is a case where "valleys have been exalted and mountains made low."
A well-marked syncline, it will be observed, passes through Ben
Lawers, while Loch Tay occupies a depression scooped out of an equally
well-defined anticline--a structure which is just the opposite of
that which we should expect to find in a true mountain-chain. It will
be also noted that Glen-Lyon coincides neither with a syncline nor a
fault; it has been eroded along the outcrops of the strata. Many of the
north-east and south-west hollows of the Highlands indeed run along
the base of what are really great escarpments--a feature which, as we
have seen, is constantly met with in every region where the strata
"strike" more or less steadily in one direction. In the Highlands the
strata are most frequently inclined at considerable angles, so that the
escarpments succeed each other more rapidly than would be the case if
the strata were less steeply inclined. In no case does any north-east
and south-west hollow coincide with a structural cavity. Loch Awe has
been cited as an example of a superficial depression formed by the
inward dip of the strata on either side. But, as was shown many years
ago by my brother, A. Geikie,[E] this lake winds across the _strike_
of the strata. Moreover, if it owed its existence to a great synclinal
fold, why, he asks, does it not run along the same line as far as the
same structure continues? It does not do so: it is not continuous with
the synclinal fold, while vertical strata appear in the middle of the
lake, where, as my brother remarks, they have clearly no business to be
if the sides of the lake are formed by the inward dip of the schists.
[E] _Trans. Edin. Geol. Soc._ vol. ii. p. 267.
The Great Glen, as I mentioned in the preceding article, coincides
with a fracture or dislocation--a line of weakness along which the
denuding agents had worked for many ages before the beginning of Old
Red Sandstone times; and it is possible that smaller dislocations
may yet be detected in other valleys. But in each and every case the
valleys as we now see them are valleys of erosion; in each and every
case the mountains are mountains of circumdenudation; they project
as eminences because the rock-masses which formerly surrounded them
have been gradually removed. We have only to protract the outcrops
of the denuded strata--to restore their continuations--to form some
faint idea of the enormous masses of rock which have been carried away
from the surface of the Highland area since the strata were folded
and fractured. All this erosion speaks to the lapse of long ages. The
mountains of elevation which doubtless at one time existed within
the Highland area had already, as we have seen, suffered extreme
erosion before the beginning of Old Red Sandstone times, much of
the area having been converted into an undulating plateau or plain,
which, becoming submerged in part, was gradually overspread by the
sedimentary deposits of the succeeding Old Red Sandstone period. Those
sediments were doubtless derived in large measure from the denudation
of the older rocks of the Highlands, and since they attain in places
a thickness of 20,000 feet, and cover many square miles, they help
us to realise in some measure the vast erosion the Highland area had
sustained before the commencement of the Carboniferous period. Nor
must we forget that the Old Red Sandstone formation which borders the
Highlands has itself experienced excessive denudation: it formerly had
a much greater extension, and doubtless at one time overspread large
tracts of the Highlands. Again, we have to remember that during the
Carboniferous and Permian periods, and the later Mesozoic and Cainozoic
eras, the Highlands probably remained more or less continuously in the
condition of land. Bearing this in mind, we need not be surprised that
not a vestige of the primeval configuration brought about by the great
earth-movements of late Silurian times has been preserved. Indeed, had
the Highland area, after the disappearance of the Old Red Sandstone
inland seas, remained undisturbed by any movement of elevation or
depression, it must long ago have been reduced by sub-aërial erosion to
the condition of a low-lying undulating plain. But elevation en masse
from time to time took place, and so running water and its numerous
allies have been enabled to carry on the work of denudation.
Thus in the geological history of the Scottish Highlands we may trace
the successive phases through which many other elevated tracts
have passed. The Scandinavian plateau, and many of the mountains of
middle Germany--such, for example, as the Harz, the Erzgebirge, the
Thüringer-Wald, etc.--show by their structure that they have undergone
similar changes. First we have an epoch of mountain-elevation, when the
strata are squeezed and crushed laterally, fractured and shattered--the
result being the production of a series of more or less parallel
anticlines and synclines, or, in other words, a true mountain-chain.
Next we have a prolonged period of erosion, during which running water
flows through synclinal troughs, works along the backs of broken and
shattered anticlines, and makes its way by joints, gaping cracks,
and dislocations, to the low grounds. As time goes on, the varying
character of the rocks and the mode of their arrangement begin to tell:
the weaker structures are broken up; rock-falls and landslips ever and
anon take place; anticlinal ridges are gradually demolished, while
synclines tend to endure, and thus grow, as it were, into hills, by
the gradual removal of the more weakly-constructed rock-masses that
surround them. Valleys continue to be deepened and widened, while the
intervening mountains, eaten into by the rivers and their countless
feeders, and shattered and pulverised by springs and frosts, are
gradually narrowed, interrupted, and reduced, until eventually what
was formerly a great mountain-chain becomes converted into a low-lying
undulating plain. Should the region now experience a movement of
depression, and sink under the sea, new sedimentary deposits will
gather over its surface to a depth, it may be, of many hundreds or even
thousands of feet. Should this sunken area be once more elevated en
masse--pushed up bodily until it attains a height of several thousand
feet--it will form a plateau, composed of a series of horizontal strata
resting on the contorted and convoluted rocks of the ancient denuded
mountain-chain. The surface of the plateau will now be traversed by
streams and rivers, and in course of time it must become deeply cleft
and furrowed, the ground between the various valleys rising into
mountain-masses. Should the land remain stationary, its former fate
shall again overtake it; it will inevitably be degraded and worn down
by the sub-aërial agents of erosion, until once more it assumes the
character of a low-lying undulating plain.
Through such phases our Highlands have certainly passed. At a very
early epoch the Archæan rocks of the north-west were ridged up into
great mountain-masses, but before the beginning of the pre-Cambrian
period wide areas of those highly-contorted rocks had already been
planed across, so that when subsidence ensued the pre-Cambrian
sandstones were deposited upon a gently undulating surface of highly
convoluted strata. Another great epoch of mountain-making took place
after Lower Silurian times, and true mountain-ranges once more appeared
in the Highland area. We cannot tell how high those mountains may have
been, but they might well have rivalled the Alps. After their elevation
a prolonged period of erosion ensued, and the lofty mountain-land
was reduced in large measure to the condition of a plain, wide areas
of which were subsequently overflowed by the inland seas of Old Red
Sandstone times--so that the sediments of those seas or lakes now
rest with a violent unconformity on the upturned and denuded edges
of the folded and contorted Silurian strata. At a later geological
period the whole Highland area was elevated _en masse_, forming an
undulating plateau, traversed by countless streams and rivers, some of
which flowed in hollows that had existed before the beginning of Old
Red Sandstone times. Since that epoch of elevation the Highland area,
although subject to occasional oscillations of level, would appear to
have remained more or less continuously in the condition of dry land.
The result is, that the ancient plateau of erosion has been deeply
incised--the denuding agents have carved it into mountain and glen--the
forms and directions of which have been determined partly by the
original surface-slopes of the plateau, and partly by the petrological
character of the rocks and the geological structure of the ground.
[Illustration:
PLATE II.
INFLUENCE OF ROCK STRUCTURE ON THE FORM OF THE GROUND.
FIG. 1. PLATEAU OF ACCUMULATION: HORIZONTAL STRATA, DENUDED.
FIG. 2. SYNCLINAL (S.O.) AND ANTICLINAL (E.) STRATA, DENUDED.
FIG. 3. FAULTED STRATA, SHOWING DENUDATION.
FIG. 4. MOUNTAIN OF ACCUMULATION,--VOLCANO.
FIG. 5. DIAGRAMMATIC SECTION OF A TYPICAL MOUNTAIN CHAIN,
OR MOUNTAINS OF UPHEAVAL.
FIG. 6. TYPES OF ROCK STRUCTURE IN THE ALPS (AFTER PROF. HEIM)
_The dotted lines show portions of strata, denuded._
FIG. 7. ESCARPMENTS (e) AND DIP SLOPES (d).
FIG. 8. EROSION OF ANTICLINAL MOUNTAINS.
FIG. 9. PLATEAU OF EROSION, SHOWING MOUNTAINS OF CIRCUMDENUDATION (aa).
FIG 10. SECTION ACROSS BEN LAWERS AND LOCH TAY, SHOWING MOUNTAINS
OF CIRCUMDENUDATION.
The Edinburgh Geographical Institute J. G. Bartholomew F.R.G.S.
]
Thus, in the evolution of the surface-features of the earth, the
working of two great classes of geological agents is conspicuous--the
subterranean and the sub-aërial. The sinking down of the crust upon the
cooling nucleus would appear to have given rise to the great oceanic
depressions and continental ridges, just as the minor depressions
within our continental areas have originated many mountain-chains. In
the area undergoing depression the strata are subjected to intense
lateral pressure, to which they yield along certain lines by folding
up. The strata forming the Alps, which are 130 miles broad, originally
occupied a width of 200 miles; and similar evidence of enormous
compression is conspicuous in the structure of all mountains of
elevation. Great elevation, however, may take place with little or
no disturbance of stratification: wide continental areas have been
slowly upheaved _en masse_, and sea-bottoms and low-lying plains
have in this way been converted into lofty plateaux.[F] Many of the
most conspicuous features of the earth's surface, therefore, are due
directly to subterranean action. All those features, however, become
modified by denudation, and eventually the primeval configuration
may be entirely destroyed, and replaced by contours which bear no
direct relation to the form of the original surface. (See Fig. 9.)
In the newer mountain-chains of the globe the surface-features are
still largely those due directly to upheaval; so in some recently
elevated plateaux the ground has not yet been cut up and converted into
irregular mountain-masses. Many of the more ancient mountain-chains
and ranges, however, have been exposed so long to the abrading action
of the denuding agents that all trace of their original contour has
vanished. And in like manner plateaux of great age have been so
highly denuded, so cut and carved by the tools of erosion, that their
plateau character has become obscured. They have been converted into
undulating mountainous and hilly regions. Everywhere throughout the
world we read the same tale of subsidence and accumulation, of upheaval
and denudation. The ancient sedimentary deposits which form the major
portion of our land-surfaces, are the waste materials derived from
the demolition of plains, plateaux, and mountains of elevation. In
some mountain-regions we read the evidence of successive epochs of
uplift, separated by long intervening periods of erosion, followed
by depression and accumulation of newer sediments over the denuded
surface. Thus the Alps began to be elevated towards the close of
Palæozoic times. Erosion followed, and subsequently the land became
depressed, and a vast succession of deposits accumulated over its
surface during the long-continued Mesozoic era into early Cainozoic
times. Again, a great upheaval ensued, and the Mesozoic and Eocene
strata were violently contorted and folded along the flanks of the
chain. Then succeeded another period of erosion and depression, which
was again interrupted by one or more extensive upheavals. Away from
those lines of weakness which we call mountain-chains, we constantly
encounter evidence of widespread movements of elevation, during
which broad areas of sea-bottom have been upheaved to the light of
day, and, after suffering extensive denudation have subsided, to be
again overspread with the spoils of adjacent lands, and then upheaved
once more. And such oscillations of level have occurred again and
again. Looking back through the long vista of the past, we see each
continental area in a state of flux--land alternating with sea, and
sea with land--mountains and plateaux appearing and disappearing--a
constant succession of modifications, brought about by the antagonistic
subterranean and sub-aërial agents.
The hills are shadows, and they flow
From form to form, and nothing stands;
They melt like mists, the solid lands,
Like clouds they shape themselves and go.
[F] This is the generally accepted view of modern geologists. It is
very difficult, however, to understand how a wide continental area can
be vertically upheaved. It seems more probable that the upheaval of the
land is only apparent. The land seems to rise because the sea retreats
as the result of the subsidence of the crust within the great oceanic
basins. See Article xiv. (1892.)
IV.
The Cheviot Hills.[G]
[G] From _Good Words_ for 1876.
I.
The ridge of high ground that separates England from Scotland is not,
like many other hilly districts, the beloved of tourists. No guide-book
expatiates upon the attractiveness of the Cheviots; no cunningly-worded
hotel-puffs lure the unwary vagrant in search of health, or sport,
or the picturesque, to the quiet dells and pastoral uplands of the
Borders. Since the biographer of Dandie Dinmont, of joyous memory,
joined the shades, no magic sentences, either in verse or prose,
have turned any appreciable portion of the annual stream of tourists
in the direction of the Cheviots. The scenery is not of a nature to
satisfy the desires of those who look for something piquant--something
"sensational," as it were. It is therefore highly improbable that
the primeval repose of these Border uplands will ever be disturbed
by inroads of the "travelling public," even should some second Burns
arise to render the names of hills and streams as familiar as household
words. And yet those who can spare the time to make themselves well
acquainted with that region should do so; they will have no reason
to regret their visit, but very much the reverse. For the scenery is
of a kind which grows upon one. It shows no clamant beauties--you
cannot have its charms photographed--the passing stranger may see
nothing in it to detain him; but only tarry for a while amongst these
green uplands, and you shall find a strange attraction in their soft
outlines, in their utter quiet and restfulness. For those who are
wearied with the crush and din of life, I cannot think of a better
retreat. One may wander at will amongst the breezy hills, and inhale
the most invigorating air; springs of the coolest and clearest water
abound, and there are few of the brooks in their upper reaches which
will not furnish natural shower-baths. Did the reader ever indulge in
such a mountain-bath? If not, then let him on a summer day seek out
some rocky pool, sheltered from the sun, if possible, by birch and
mountain-ash, and, creeping in below the stream where it leaps from the
ledges above, allow the cool water to break upon his head, and he will
confess to having discovered a new aqueous luxury. Then from the slopes
and tops of the hills you have some of the finest panoramic views to
be seen in this island. Nor are there wanting picturesque nooks, and
striking rock scenery amongst the hills themselves: the sides of the
Cheviot are seamed with some wild, rugged chasms, which are just as
weird in their way as many of the rocky ravines that eat into the
heart of our Highland mountains. The beauty of the lower reaches of
some of the streams that issue from the Cheviots is well known; and
few tourists who enter the vale of the Teviot neglect to make the
acquaintance of the sylvan Jed. But other streams, such as the Bowmont,
the Kale, the Oxnam, and the Rule will also well repay a visit. In
addition to all these natural charms, the Cheviot district abounds in
other attractions. Those who are fond of Border lore, who love to seek
out the sites of old forays, and battles, and romantic incidents, will
find much to engage them; for every stream, and almost every hill, is
noted in tale and ballad. Or if the visitor have antiquarian tastes, he
may rival old Monkbarns, and do his best to explain the history of the
endless camps, ramparts, ditches, and terraces which abound everywhere,
especially towards the heads of the valleys. To the geologist the
district is not less interesting, as I hope to be able, in the course
of these papers, to show. The geological history of the Cheviots might
be shortly summed up, and given in a narrative form, but it will
perhaps be more interesting, and, at the same time more instructive,
if we shall, instead, go a little into detail, and show first what
the nature of the evidence is, and, second, how that evidence may be
pieced together so as to tell its own story. I may just premise that
my descriptions refer almost exclusively to the Scottish side of the
Cheviots--which is not only the most picturesque, but also the most
interesting, both from an antiquarian and geological point of view.
The Cheviots extend from the head of the Tyne in Northumberland, and
of the Liddel in Roxburghshire, to Yeavering Bell and the heights
in its neighbourhood (near Wooler), a distance of upwards of thirty
miles. Some will have it that the range goes westward so as to include
the heights about the source of the Teviot, but this is certainly a
mistake, for after leaving Peel Fell and crossing to the heights on the
other side of the Liddel Water, we enter a region which, both in its
physical aspect and its geological structure, differs considerably from
the hilly district that lies between Peel Fell and the high-grounds
that roll down to the wide plains watered by the Glen and the Till.
The highest point in the range is that which gives its name to the
hills--namely, the Cheviot--a massive broad-topped hill, which reaches
an elevation of 2767 feet above the sea, and from which a wonderful
panorama can be scanned on a clear day. The top of the hill is coated
with peat, fifteen to twenty feet thick, in some places. A number of
deep ravines trench its slopes, the most noted of which are Hen Hole
and the Bizzle. Peel Fell, at the other extremity of the range, is
only 1964 feet high, while the dominant points between Peel Fell and
the Cheviot are still lower--ranging from 1500 feet to 1800 feet. The
general character of the hills is that of smooth rounded masses, with
long flowing outlines. There are no peaks, nor serrated ridges, such as
are occasionally met with in the northern Highlands; and the valleys
as a rule show no precipitous crags and rocky precipices, the most
conspicuous exceptions being the deep clefts mentioned as occurring in
the Cheviot. The hills fall away with a long gentle slope into England,
while on the Scottish side the descent is somewhat abrupt; so that
upon the whole the northern or Scottish portion of the Cheviots has
more of the picturesque to commend it than the corresponding districts
in England. Indeed, the opposite slopes of the range show some rather
striking contrasts. The long, flat-topped elevations on the English
side, that sweep south and south-west from Carter Fell and Harden Edge,
and which are drained by the Tyne, the Rede Water, and the Coquet,
are covered for the most part with peat. Sometimes, however, when the
slope is too great to admit of its growth, the peat gives place to
rough scanty grass and scrubby heath, which barely suffice to hide the
underlying barren sandstone rocks. One coming from the Scottish side
is hardly prepared, indeed, for the dreary aspect of this region as
viewed from the dominant ridge of the Cheviots. If in their physical
aspect the English slopes of these hills are for the most part less
attractive than the Scottish, it is true also that they offer less
variety of interest to the geologist. Those who have journeyed in
stagecoaching times from England into Scotland by Carter Fell, will
remember the relief they felt when, having surmounted the hill above
Whitelee, and escaped from the dreary barrens of the English border,
they suddenly caught a sight of the green slopes of the Scottish hills,
and the well-wooded vales of Edgerston Burn and Jed Water. On a clear
day the view from this point is very charming. Away to the west stretch
in seemingly endless undulations the swelling hills that circle round
the upper reaches of Teviotdale. To east and north-east the eye glances
along the bright-green Cheviots of the Scottish border, and marks how
they plunge, for the most part somewhat suddenly, into the low grounds,
save here and there, where they sink in gentler slopes, or throw out a
few scattered outposts--abrupt verdant hills that somehow look as if
they had broken away from the main mass of the range. From the same
standpoint one traces the valleys of the Rule and the Jed--sweetest
of border streams--stretching north into the well-clothed vale of the
Teviot. Indeed, nearly the whole of that highly-cultivated and often
richly-wooded country that extends from the base of the Cheviots to
the foot of the Lammermuirs, lies stretched before one. Here and there
abrupt isolated hills rise up amid the undulating low grounds, to hide
the country behind them. Of these the most picturesque are dark Rubers
Law, overlooking the Rule Water; Minto Crags, and Penielheugh with its
ugly excrescence of a monument, both on the north side of the Teviot;
and the Eildon Hills, which, as all the world knows, are near Melrose.