still finer panoramic views are to be had from the top of Blackhall
Hill, Hownam Law, the Cheviot, as also from various points on the line
of the Roman Road and other paths across the hills into England),
the observer will hardly fail to be struck by the great variety of
outlines exhibited. Some of the hills, especially those to the west and
north-west, are grouped in heavy masses, and present for the most part
a soft, rounded contour, the hills being broad atop and flowing into
each other with long, smooth slopes. Other elevations, such as those
to the east and north-east of Carter Fell, while showing similar long
gentle slopes, yet are somewhat more irregular in form and broken in
outline, the hills having frequently a lumpy contour. Very noteworthy
objects in the landscape also are the little isolated hills of the
low grounds, such as Rubers Law, and the Dunian, above Jedburgh. They
rise, as I have said, quite suddenly out of that low gently undulating
country that sinks softly into the vales of the Teviot and the Tweed.
This variety arises from the geological structure of the district. The
hills vary in outline partly because they are made up of different
kinds of rock, and partly owing to the mode in which these rocks have
been arranged. But notwithstanding all this variety of outline, one
may notice a certain sameness too. Flowing outlines are more or less
conspicuous all over the landscape. Many of the hills, especially as
we descend into Teviotdale, seem to have been smoothed or rounded off,
as it were, so as to present their steepest faces as a rule towards
the south-west. And if we take the compass-bearing of the hill-ridges
of the same district, we shall find that these generally trend from
south-west to north-east So much, then, at present for the surface
configuration of the Cheviot region. When we come to treat of the
various rock-masses, and to describe the superficial accumulations
underneath which these are often concealed, we shall be in a better
position to give an intelligible account of the peculiar form of the
ground, and the causes to which that configuration must be ascribed.
The solid rocks which enter into the composition of the Cheviots
consist mainly of (1) hard grey and blue rocks, called _greywacké_ by
geologists, with which are associated blue and grey shale; (2) various
old igneous rocks; and (3) sandstones, red and white, interbedded
with which occur occasional dark shales. Now, before we can make any
endeavour towards reconstructing in outline the physical geography of
the Cheviot Hills during past ages, it is necessary that we should
discover the order in which the rock-masses just referred to have been
amassed. I shall first describe, therefore, some sections where the
members of the different series are found in juxtaposition, for the
purpose of pointing out which is the lowest-lying, and consequently the
oldest, and which occupy the uppermost and intermediate positions.
[Illustration: FIG 1.--Conglomerate and Red Sandstone, etc., _c_,
resting on Greywacké and Shale, _g_.]
The first section to which reference may be made is exposed in the
course of the River Jed, at Allars Mill, a little above Jedburgh.
This section is famous in its way as having been described and
figured by Dr. Hutton, who may be said to have founded the present
system of physical geology. In the bed of the stream are seen certain
confused ridges of a greyish blue rock running right across the river
course--that is, in a direction a little north of east and south
of west. These ridges are the exposed edges of beds of greywacké
and shale, which are here standing on end. The beds are somewhat
irregular, being inclined from the vertical, now in one direction
and now in another, or, as a geologist would say, the "dip" changes
rapidly, sometimes being up the valley and sometimes down. The same
beds continue up the steep bank of the river for a yard or two, and
are there capped by another set of rocks altogether, namely, by soft
red sandy beds which at the bottom become _conglomeratic_--that is
to say, they are charged with water-worn stones. The annexed diagram
(Fig. 1) will show the general appearances presented: _g_ represents
the vertical greywacké and shale, and _c_ the overlying deposits of
conglomerate and red sandy beds. Now let us see what this section
means. What, in the first place, is greywacké? The term itself has
really no meaning, being a name given by the miners in the Harz
Mountains to the unproductive rocks associated with the vein-stones
which they work. When we break the rock we may observe that it is a
granular mixture of small particles of quartz, to which sometimes
felspar and other minerals are added. The grains are bound together
in a hardened matrix of argillaceous or clayey and silicious matter,
blue, or grey, or green, or brown and yellow, as the case may be.
At Allars Mill, and generally throughout the Cheviot district, the
prevailing colour is a pale greyish blue or bluish grey; but shades of
green and brown often occur. The component particles of the rock are
usually rounded or water-worn. Again, we notice that the ridges and
bands of rock that traverse the course of the Jed at Allars Mill are
merely the outcrops of successive _strata_ or beds. It is clear then
that greywacké and the grey shales that accompany it are _aqueous_
rocks--that is to say, they consist of hardened sediment, which has
undoubtedly been deposited in successive layers of variable thickness
by water in motion. But since the sediments of rivers and currents
are laid down in approximately horizontal planes, it is evident that
if the greywacké and shale be sedimentary deposits they have suffered
considerable disturbance since the time of their formation; for, as
we have seen, the beds, instead of being horizontal or only gently
inclined, actually approach the vertical. The fact is, that the
outcrops which we see are only the truncated portions of what were
formerly rapid undulations or folds of the strata, the tops of the
folds or arches having been cut away by geological agencies, to which
I shall refer by-and-by. What were at one time horizontal strata have
been crumpled up into great folds, the folds being squeezed tightly
together, and their upper portions planed away before the overlying red
sandy beds were laid down. The accompanying diagram (Fig. 2) may serve
to make all this clearer. Let A A represent the present surface of the
ground, and B B a depth of say fifty feet or a hundred feet from the
surface. The continuous lines between A and B represent the greywacké
beds as we now see them in section; the dotted lines above A A indicate
the former extension of the strata, and the dotted lines below B B
their continuation below that datum line. Hence it is obvious that in
a succession of vertical or highly inclined beds, we may have the same
strata repeated many times, the same beds coming again and again to the
surface. Thus the stratum at S is evidently the same bed as that at W,
X, Y, and Z.
[Illustration: Fig 2.]
Such great foldings or redoublings of strata are most probably
originated during subsidence of a portion of the earth's crust. While
the ground is slowly sinking down, the strata underneath are perforce
compelled to occupy less space laterally, and this they can only do by
yielding amongst themselves. All folding or contortion on the large
scale--that, namely, which has affected areas of strata extending
over whole countries--seems to have taken place under great pressure;
in other words, to have been produced at considerable depths from
the earth's surface. We can conceive, therefore, of a wide tract of
land sinking down for hundreds of feet, and producing at the surface
comparatively little change. But a depression of a few hundred feet
at the surface implies a considerably greater depression at a depth
of several thousand feet from the surface, and it is at great depths,
therefore, that the most violent folding must take place. Consequently
considerable contortion, and much folding, and lateral crushing
and reduplication of strata may occur, and yet no trace of this be
observable at the surface, save only a gentle depression. For example,
in Greenland, a movement of subsidence has been going on for many
years--the land has been slowly sinking down. The rocks at the surface
are of course quite undisturbed by this widely-extended movement, but
the strata at great depths may be undergoing much compression and
contortion. It follows from such considerations, that if we now get
highly contorted strata covering wide areas at the surface, we suspect
that very considerable _denudation_ has taken place. That is to say,
large masses of rock have been removed by the geological agents of
change, so as to expose the once deeply-buried tops of the arched or
curved and folded strata. We may therefore infer from a study of the
phenomena in the Jed at Allars Mill, first, that the red sandy beds are
younger than the greywacké and shale, seeing that they rest upon them;
and, second, that a very long period of time must have elapsed between
the deposition of the older and the accumulation of the younger set
of strata; for it is obvious that considerable time was required for
the consolidation and folding of the greywacké, and an incalculable
lapse of ages was also necessary to allow of the gradual wearing away
by rain, frost, and running water of the great thickness of rocks
underneath which the greywacké was crumpled. And all this took place
before the horizontally-bedded red sandstone and conglomerate gathered
over the upturned ends of the underlying strata. The succession of
rocks at Allars Mill is seen in many other places in the Cheviot
district, but enough has been said to prove that the greywacké beds are
the older of the two sets of strata.
There is another class of rocks, the relative position of which we
must now ascertain, for no one shall wander much or far among the
Cheviots without becoming aware of the existence of other kinds of
rock than greywacké and sandstone. Many of the hills east of Oxnam
and Jed Waters, for example, are composed of igneous masses--of
rocks which have had a volcanic origin. As we shall afterwards see,
the whole north-eastern section of the Cheviots is built up of such
rocks. At present, however, we are only concerned with the relation
which these bear to the greywacké and the red sandy beds. Now at
various localities--for example, in Edgerston Burn, on the hill-face
south of Plenderleith, and again along the steep front of Hindhope
and Blackball Hills, which are on the crest of the Cheviots--we find
that the igneous rocks rest upon the greywacké and shale (see Fig. 3)
precisely in the same way as do the red sandy beds. They therefore
belong to a later date than the greywacké. In other places, again, we
meet with the conglomerates and red sandstones (_c_, Fig. 4) resting
upon and wrapping round the igneous rocks, _i_, and thus it becomes
quite obvious that the latter occupy an intermediate position between
the greywacké and shale on the one hand, and the conglomerate and red
sandstone upon the other.
[Illustration: Fig. 3.--Igneous rocks (_i_, _a_) resting on Greywacké
and Shale, _g_.]
[Illustration: Fig. 4.--_c_, Conglomerate and Sandstones, resting on
Igneous rocks, _i_.]
We have now cleared the way so far, preparatory to an attempt to trace
the geological history of the Cheviots. The three sets of rocks,
whose mutual relations we have been studying, are those of which the
district is chiefly composed; but, as we shall see in the sequel,
there are others, not certainly of much extent, but nevertheless
having an interesting story to tell us. Nor shall we omit to notice
the superficial accumulations of clay, gravel, sand, silt, alluvium,
and peat; monuments as they are of certain great changes, climatic and
geographical, which have characterised not the Cheviots only, but a
much wider area.
II.
If we draw a somewhat straight line from Girvan, on the coast of
Ayrshire, in a north-east direction to the shores of the North Sea,
near Dunbar, we shall find that south of that line, up to the English
border, nearly the whole country is composed of various kinds of
greywacké and shale like the basement beds of the Cheviot district.
Here and there, however, especially in certain of the valleys and some
of the low-lying portions of this southern section of Scotland, one
comes upon small isolated patches and occasional wider areas of younger
strata, which rest upon and conceal the greywackés and shales. Such is
the case in Teviotdale, the Cheviot district, and the country watered
by the lower reaches of the Tweed, in which regions the bottom beds
are hidden for several hundreds of square miles underneath younger
rocks. Indeed, the greywacké and shale form but a very small portion of
the surface in the Cheviots, appearing upon a coloured geological map
like so many islands or fragments, as it were, which have somehow been
detached from the main masses of greywacké of which the Lammermuirs and
the uplands of Dumfries and Selkirk shires are composed. Although the
bottom rocks of the Cheviot Hills are thus apparently separated from
the great greywacké area, there can be no doubt that they are really
connected with it, the connection being obscured by the overlying
younger strata. For if we could only strip off these latter, if we
could only lift aside the great masses of igneous rock and sandstone
that are piled up in the Cheviot Hills and the adjoining districts,
we should find that the bottom upon which these rest is everywhere
greywacké and shale. In part proof of this it may be mentioned that
at various places in those districts which are entirely occupied by
sandstone and igneous rock, the streams have cut right down through
the younger rocks so as to expose the bottom beds, as in Jed Water at
Allars Mill. Again, when we trace out the boundaries of any detached
areas of greywacké we invariably find these bottom beds disappearing on
all sides underneath the younger strata by which they are surrounded.
One such isolated area occurs in the basin of the Oxnam Water, between
Littletonleys and Bloodylaws, a section across which would exhibit
the general appearance shown in the accompanying diagram (Fig. 5).
Another similarly isolated patch is intersected by Edgerston Burn and
the Jed Water between Paton Haugh and Dovesford. But the largest of
these detached portions appears, forming the crest of the Cheviots,
at the head of the River Coquet. There the basement beds occupy the
watershed, extending westward, some three or four miles, as far as
the sandstones of Hungry Law, while to the north and east they plunge
under the igneous rocks of Brownhart Law and the Hindhope Hills. Now
it is evident that all those detached and isolated areas of greywacké
and shale are really connected underground, and not only so, but they
also piece on in the same way to the great belt of similar strata
that stretches from sea to sea across the whole breadth of Scotland.
Indeed, we may observe in the Cheviot district how long and massive
promontories of greywacké jut out from that great belt, and extend
often for miles into the areas that are covered with younger strata,
as, for example, in the Brockilaw and Wolfelee Hills. A generalised
section across the greywacké regions of the Cheviot Hills would
therefore present the appearances shown in the annexed diagram (Fig.
6), in which G represents the basement beds, I the igneous rocks, and C
the red sandstones, etc.
[Illustration: Fig. 5.--Section across Greywacké area of Oxnam Water;
G, Greywacké and Shale; C, Sandstone, etc.]
[Illustration: Fig. 6.--Diagram section across Greywacké districts of
Cheviot Hills.]
Throughout the whole of the district under review the bottom beds
are observed to dip at a high angle--the strata in many places being
actually vertical--and the edges or crops of the strata run somewhat
persistently in one direction, namely, from south by west to north
by east; or, as a geologist would express it, the beds have an
approximately south-west and north-east "strike." Now as the dip is
sometimes to north-west and sometimes to south-east, it is evident that
the rocks have been folded up in a series of rapid convolutions, and
that some of the beds must be often repeated.
From the character of the fossils which the bottom beds have yielded
we learn that the strata belong to that division of past time
which is known as the Silurian age. These fossils appear to be of
infrequent occurrence, and the creatures of which they are the relics
occupied rather a humble place in the scale of being. They are called
_graptolites_ (from their resemblance to pens), an extinct group of
hydroid zoophytes, apparently resembling the sertularians of our own
seas.
The general appearance of the Silurian strata of the Cheviots is
indicative of deposition in comparatively quiet water, but how deep
that water was one cannot say. Upon the whole, the beds look not
unlike the sediments that gather in calm reaches of the sea, such as
estuaries, betokening the presence of some not distant land from which
fine mud and sand were washed down. Another proof that some of the
strata at all events were accumulated not far from a shore-line, is
found in certain coarse bands of grit and pebbles, which are not likely
to have been formed in deep water. This evidence, however, cannot be
considered decisive, and in the present state of our knowledge all
that we can assert with anything like confidence is simply this:--That
during the deposition of the Silurian strata the whole of the Cheviot
area lay under water--existed, in short, as a muddy sea-bottom, in the
slime of which flourished here and there, in favourable spots, those
minute hydroid animals called _graptolites_.
Between the deposition of the Silurian and the formation of the rocks
that come next in order a long interval elapsed, during which the mud,
sand, and grit that gathered on the floor of the ancient sea were
hardened into solid masses, and eventually squeezed together into great
folds and undulations. It has already been pointed out that these
changes could hardly have been effected save under extreme pressure,
and this consideration leads us to infer that a great thickness of
strata has been removed entirely from the Cheviot district, so as to
leave no trace of its former existence. Long before the deposition
of the younger strata that now rest upon and conceal the Silurian
rocks, the action of the denuding forces--the sea, frosts, rain, and
rivers--had succeeded in not only sweeping gradually away the strata
underneath which the bottom beds were folded, but in deeply scarping
and carving these bottom beds themselves. Can we form any reasonable
conjecture as to the geological age of the strata underneath which the
bottom beds of the Cheviots were folded, and which, as we have seen,
had entirely disappeared before the younger rocks of the district were
accumulated? Well, it is obvious that the missing strata must have been
of later formation than the bottom beds, and it is equally evident that
they must have been of much more ancient date than the igneous rocks of
the Cheviot Hills. Now, as we shall afterwards see, these igneous rocks
belong to the Old Red Sandstone age, that is to say, to the age that
succeeded the Silurian. How is it then, if the bottom beds be really
of Silurian and the igneous rocks of Old Red Sandstone age, that a
gap is said to exist between them? The explanation of this apparent
contradiction is not far to seek. When we compare the fossils that
occur in the Silurian strata of the Cheviot Hills and the districts
to the west, with the organic remains disinterred from similar strata
elsewhere, as in Wales for example, we find that the bottom beds of
the Cheviots were in all probability accumulated at approximately the
same time as certain strata that occur in the middle division of the
Upper Silurian. In Wales and in Cumberland the strata that approximate
in age to the Silurian of the Cheviots are covered by younger strata
belonging to the same formation which reach a thickness of several
thousand feet. It may quite well be, therefore, that the succession of
Silurian strata in the Cheviots was at one time more complete than it
is now. The upper portions of the formation which are so well developed
in Wales and Cumberland, and which are likewise represented to a small
extent in Scotland, had in all probability their equivalents in what
are our border districts. In other words, there are good grounds for
believing that the existing Silurian rocks of the Cheviots were in
times preceding the Old Red Sandstone age covered with younger strata
belonging to the same great system. The missing Silurian strata of the
Cheviots may have attained a thickness of several thousand feet, and
underneath such a mass of solid rock the lower-lying strata might well
have been consolidated and subsequently squeezed into folds.
We now pass on to consider the next chapter in the geological history
of the Cheviot Hills. As we proceed in our investigations it will
be noticed that the evidence becomes more abundant, and we are thus
enabled to build up the story of the past with more confidence,
and with fuller details. For it is with geological history as with
human records--the further back we go in time the scantier do the
facts become. The rocks upon which Nature writes her own history
are palimpsests, on which the later writing is ever the most easily
deciphered. Nay, she cannot compile her newer records without first
destroying some of those compiled in earlier times. The sediments
accumulating in modern lake and sea are but the materials derived
from the degradation of the rocks we see around us, just as these in
like manner have originated from the demolition of yet older strata.
Thus the further we trace back the history of our earth, the more
fragmentary must we expect the evidence to be; and conversely, the
nearer we approach to the present condition of things the more abundant
and satisfactory must the records become. Accordingly, we find that
the igneous rocks of the Cheviot Hills tell us considerably more than
the ancient Silurian deposits upon which they rest. The surface of the
latter appears to be somewhat irregular underneath the igneous rocks,
showing that hills and valleys, or an undulating table-land, existed in
the Cheviot district prior to the appearance of the younger formation.
But before we attempt to summarise the history of that formation, it is
necessary to give some description, however short, of the rocks that
compose it.