experienced some further modification. Considerable depression of the
land supervened, and the waters of our inland sea or lake rose high on
the slopes of the Cheviots. Mark now how the character of the sediment
changes. The prevailing red colour has disappeared, and white, yellow,
and pale greenish or grey sand begins to be poured over the bed of
the lake. Even yet, however, ferruginous matter exists in sufficient
quantity to tint the sediment red in some places. With the appearance
of these lighter-coloured sandy deposits, the conditions seem to have
become better fitted to sustain life. Fish of peculiar forms, which,
like the gar-pike of North American lakes, were provided with a strong
scaly armour of tough bone, began to abound, weeds grew in the water,
and the neighbouring land supported a vegetation now very meagrely
represented by the few remains of plants which have been preserved.
In some places fish-scales are found in considerable abundance.
They belong to several genera and species which are more or less
characteristic of the Old Red Sandstone formation. The most remarkable
form was the _Pterichthys_, or wing-finned fish. Its blunt-shaped head
and the anterior portion of its body were sheathed in a solid case
of bone, formed by the union of numerous bony scales or plates. Two
curious curved spine-like arms occupied the place of pectoral fins,
and may have been used by the creature in paddling along the bottom of
the sea or lake in which it lived. The posterior part of the body was
covered with bony scales, but these were not suturally united. Other
kinds of fish were the _Holoptychius_ and _Coccosteus_, both of which
were, like the Pterichthys, furnished with bony scales. The scales of
the former overlapped, and had a curious wrinkled surface. The head of
the Coccosteus was protected by a large bony shield or buckler, and a
similar bony armour covered the ventral region.
The organic remains of these fish-bearing strata are too scanty,
however, to enable us to form any idea of the kind of climate which
characterised the district at this long-past period; but if we rely
upon the fossils which have been met with in strata of the same or
approximately the same age elsewhere, we may be pretty sure the
climate was genial, and nourished on the land an abundant vegetation,
consisting of ferns, great reeds, and club-mosses, which attained the
dimensions of large trees, conifers, and other strange trees which have
no living analogues.
It seems most likely that when the land sank down in the Cheviot
district, so as to allow the old lake to reach as it were a higher
level, some communication with the outlying ocean was effected. Red
ferruginous mud would then cease to accumulate, or gather only now
and then; the deposits would for the most part be white or yellow,
or pale green; and fish would be able to come in from the sea. The
communication with the ocean, however, was probably never very free,
but liable to frequent interruption.
Here, then, ends the third great period of time represented by the
rocks of the Cheviot district. The first period, as we have seen,
closed with the deposition of the Silurian strata. Thereafter
supervened a vast lapse of time, not recorded in the Cheviots by the
presence of any rocks, but represented in other regions by younger
members of the Silurian system. During this unrecorded portion of past
time, the Silurian strata of the Cheviots were hardened, compressed,
folded, upheaved to the light of day, and worn into hills and valleys
by the action of the sub-aërial forces. Then began the second period
of rock-forming in our district. Volcanoes poured out successive beds
of molten matter and showers of stones and ashes, and so built up the
rock-masses of the highest parts of the Cheviot Hills. These eruptions
belong to the Old Red Sandstone age, and form a portion of what we term
the Lower Old Red Sandstone. After the extinction of the volcanoes,
another prolonged period elapsed, which is not accounted for in the
Cheviots by the presence of any rocks. Then it was, as we know, that
the great volcanic bank was denuded and worn into a system of hills
and valleys. Now, since it is evident that the red beds of the Jed and
other places are also of Old Red Sandstone age, it follows that they
must belong to a higher place in the Old Red Sandstone formation than
the much-denuded igneous rocks upon which they rest unconformably.
The reasonable conclusion seems to be that the denudation or wearing
away of the Lower Old Red Sandstone igneous rocks of the Cheviots was
effected during that period which is represented in other districts of
Scotland by what is called the Middle Old Red Sandstone, so that the
Jed beds will thus rank as Upper Old Red Sandstone.
[Illustration: Fig. 8.--_s_, Silurian strata; _i_, Cheviot Igneous
Rocks (Lower Old Red Sandstone); _r_, Upper Old Red Sandstone
series; _c_, Kelso Igneous Rocks (Lower Carboniferous); _d_, Lower
Carboniferous Sandstones, Shales, etc.]
I come now to speak of certain rocks which, although they are developed
chiefly beyond the limits of our district, yet require a little
consideration before we can complete our account of the geological
history of the Cheviots. The rocks referred to consist chiefly of old
lava-beds, which very closely resemble those of the Lower Old Red
Sandstone. They appear on the south side of the Tweed valley below
Kelso, whence they extend south-west and west, crossing the river at
Makerstoun, and sweeping north to form the hills about Smailholm,
Stichill, and Hume (Fig. 8). All to the east of these rocks, the valley
of the Tweed is occupied by a great thickness of grey sandstones,
and grey and blue shales and clays, with which are associated thin
cement-stone bands, and occasional coarse sandy limestones called
cornstone. These strata rest upon the outskirts of the Kelso igneous
rocks, and are clearly of later date than these, since in their lower
beds, which are often conglomeratic, we find numerous rounded fragments
of the igneous rocks upon which the sandstones and shales abut. The
latter have yielded a number of fossils, both animals and plants, to
which I shall refer presently. In the bed of the Teviot near Roxburgh,
and elsewhere, the Kelso igneous rocks are found reposing upon whitish
and reddish sandstones, which are evidently the upper members of the
red beds of the Jed Water and other localities.
Strata closely resembling the grey sandstones and shales of the Tweed
valley appear among the Cheviot Hills at the head of the Jed Water,
where they are marked by the presence of thick massive sandstones,
which form all the tops of the hills between Hungry Law and the heights
that overlook the sources of the Liddel Water--the greatest height
reached being at Carter Fell, which is 1815 feet above the sea-level.
The strata at this place contain some impure limestone and thin seams
of coal, while beds of lava and tuff appear intercalated in the series.
[Illustration: Fig. 9.--Section across old volcanic neck. The dotted
line above suggests the original form of the volcano; _b_, plug of
igneous rock which rose in a molten state and cooled in the vent.]
Now let us rapidly sum up what seem to be the inferences suggested
by these briefly-stated facts. We have seen that the Upper Old Red
Sandstone began to be deposited in a lake which, as time wore on,
probably communicated with the sea, while the land was undergoing a
process of depression, so that the area of deposition was thus widely
increased, and sediment gradually accumulated in places and at levels
which had existed as land when the ancient lake first appeared in the
Cheviot district. The old lava-beds of Kelso show that the volcanic
forces, which had long been quiescent, again became active. Great
floods of molten matter issued from the bowels of the earth, and poured
over the bottom of the inland sea. But all the larger volcanoes of this
period were confined to the centre of the Tweed valley. Not a few
little isolated volcanoes, however, seem to have dotted the sea-bottom
beyond the limits of the Kelso area. From these, showers of stones
were ejected, and sometimes also they poured out molten matter. Their
sites are now represented by rounded hills which stand up, more or
less abruptly, above the level of the undulating tracts in which they
occur (Fig. 9). Among the most marked are Rubers Law, Black Law, the
Dunian, and Lanton Hill. Of course it is only the plugged-up vents or
necks that now remain; all the loose ejectamenta by which these must
at one time have been surrounded have long since been worn and washed
away. At last the Kelso volcanoes became extinct, and the little ones
also probably died out at the same time. Another long period now
ensued, during which the inland sea disappeared, and its dried-up bed
was subjected to the denuding action of the sub-aërial forces. The
volcanic rocks of the Kelso district suffered considerable erosion,
while the softer sandy strata amongst which they were erupted no doubt
experienced still greater waste. Ere long, however, the scene again
changes; and what is now the vale of Tweed becomes a wide estuary,
the shores of which are formed at first by the Kelso igneous rocks.
Into this estuary, rivers and streams carry the spoil of the Southern
Uplands, and strew its bed with sand and mud. Occasionally ferns and
large coniferous trees are floated down, and, getting water-logged,
sink to the bottom, where they become entombed in the slowly
accumulating sediment. The character of these buried plants shows that
the climate must have been genial. They belong to species which are
characteristic of the Carboniferous system, and we look upon them with
interest as the forerunners of that vast plant-growth which by-and-by
was to cover wide areas in Britain, and to give rise to our coal-seams,
the source of so much national wealth. In the waters of the estuary,
minute crustaceous creatures called _cyprides_ abounded, and with these
was associated a number of small molluscs, chiefly univalves. Here
and there considerable quantities of calcareous mud and sand gathered
on the bed of the estuary, and formed in time beds of cement-stone,
and impure limestone or cornstone. How long that condition of things
obtained in the Tweed valley we cannot tell; but we know that after
a very considerable thickness of sediment had accumulated, estuarine
conditions prevailed over the south-west end of what is now the Cheviot
range. This points to a considerable depression of the land. In this
same region volcanic action appeared, and streams of lava and showers
of fragmental materials were ejected--the remains of which are seen
in Hungry Law, Catcleugh Shin, and the head-waters of the Jed. Genial
climatic conditions continued; and here and there, along what were
either low islets or the flat muddy shores of the estuary, plants grew
in sufficient quantity to form masses of vegetation which, subsequently
buried under mud and sand, were compressed and mineralised, and so
became coal. The only place where these are now met with is on the
crest of the Cheviots at Carter Fell. The process of depression still
continuing, thick sand gradually spread over the site of the submerged
forests. To trace the physical history immediately after this, we must
go out of the Cheviot district; and it may suffice if I merely state
that these estuarine or lacustrine conditions, which prevailed for a
long time not only over the Tweed and Cheviot areas but in various
other parts of Scotland, at last gave place to the sea. In this sea,
corals, sea-lilies, and numerous molluscs and fishes abounded--all
pointing to the prevalence of genial climatic conditions. The organic
remains and the geological position of the estuarine beds of the
Tweed and the Cheviots--resting as they do upon the Upper Old Red
Sandstone--prove them to belong to the Lower series of the great
Carboniferous system.
It was some time during the Carboniferous period that wide sheets of
melted matter were forcibly intruded among the Old Red Sandstone and
the Lower Carboniferous strata of the Cheviot district; but although
these are now visible at the surface, as at Southdean, Bonchester,
etc., they never actually reached that surface at the time of their
irruption. They cooled in the crust of the earth amongst the strata
between which they were intruded, and have only been exposed to view by
the action of the denuding forces which have worn away the sedimentary
beds by which they were formerly covered.
A very wide blank next occurs in the geological history of the
Cheviots. We have no trace of the many great systems, comprising
vast series of strata and representing long eras of time, which we
know, from the evidence supplied by other regions, followed after the
deposition of the Lower Carboniferous strata. The Middle and Upper
Carboniferous groups are totally wanting, so likewise is the Permian
system; and all the great series of "Secondary" systems, of which the
major portion of England is composed, are equally absent. Nay, even
Tertiary accumulations are wanting. There is one very remarkable relic,
however, of Tertiary times, and that is a long dyke or vertical wall
of basalt-rock which traverses the country from east to west, crossing
the crest of the Cheviots near Brownhart Law, and striking west by
north through Belling Hill, by the Rule Water at Hallrule Mill, on
towards Hawick. This is one of a series of such dykes, common enough
in some parts of Scotland, which become more numerous as we approach
the west coast, where they are found associated with certain volcanic
rocks of Tertiary age, in such a way as to lead to the belief that they
all belong to the same period. The melted rock seems to have risen
and cooled in great cracks or fissures, and seldom to have overflowed
at the surface. Indeed it is highly probable that many or even most
of the dykes never reached the surface at all, but have been exposed
by subsequent denudation of the rocks that once overlaid them. Such
would appear to have been the case with the great dyke of the Cheviot
district.
We can only conjecture what the condition of this part of southern
Scotland was in the long ages that elapsed between the termination of
the Lower Carboniferous period and the close of the Tertiary ages. It
is more than likely that it shared in some of the submergences that
ensued during the deposition of the upper group of the Carboniferous
system; but after that it may have remained, for aught we can tell, in
the condition of dry land all through those prolonged periods which
are unrecorded in the rocks of the Cheviot Hills, but have left behind
them such noteworthy remains in England and other countries. Of one
thing we may be sure, that during a large part of those unrecorded ages
the Cheviot district could not have been an area of deposition. Rather
must it have existed for untold eras as dry land; and this explains
and accounts for the enormous denudation which the whole country has
experienced; for there can be little doubt that the Lower Carboniferous
strata of Carter Fell were at one time continuous with the similar
strata of the lower reaches of the Tweed valley. Yet hardly a trace of
the missing beds remains in any part of the country between the ridge
of the hills at the head of the Jed Water and the Tweed at Kelso. Only
little patches are found capping the high ground opposite Jedburgh, as
at Hunthill, etc. Thus more than a thousand feet of Lower Carboniferous
strata, and probably not less than five hundred or six hundred feet of
Old Red Sandstone rocks, have been slowly carried away, grain by grain,
from the face of the Cheviot district since the close of the Lower
Carboniferous period.
IV.
In the first of these papers some reference was made to the
configuration of the ground in the Cheviot district. We have seen
that the outlines assumed by the country have been determined in
large measure by the nature of the rocks. Thus where igneous masses
abound, the hills present a more or less irregular, and broken or lumpy
contour, while the valleys are frequently narrow and deep. In the
tracts occupied by Silurian strata, we have, as a rule, broad-topped
hill-masses with a smoothly-rounded outline, whose slopes generally
fall away with a long gentle sweep into soft green valleys, along the
bottoms of which the streams often flow in deep gullies and ravines.
Where the country is formed of sandstones, and other associated strata,
the hills are generally broad and well-rounded, but the outline is
not infrequently interrupted by lines of cliff and escarpment. These
strata, however, are confined chiefly to the low-grounds, where they
form a gently-undulating country, broken here and there, as in Dunian
Hill, Bonchester Hill, Rubers Law, etc., by abrupt cones and knobs of
igneous rock.
It is evident, then, that the diversified character of the Cheviot
Hills and the adjoining low-grounds depends on the character of the
rocks and also, as we shall see presently, upon geological structure.
Each kind of rock has its own peculiar mode of weathering. All do
not crumble away under the action of rain, frost, and running water
in precisely the same manner. Some which yield equally and uniformly
give rise to smooth outlines, others of more irregular composition,
such as many igneous rocks, break up and crumble unequally in a
capricious and eccentric way, and these in the course of time present
a hummocky, lumpy, and rough irregular configuration. And as soft and
readily-weathered rocks must wear away more rapidly than indurated and
durable masses, it follows that the former will now be found most
abundantly at low levels, while the latter will enter most extensively
into the composition of the hills. But the contour of a country depends
not only upon the relative durability of the rocks, but also upon the
mode of their occurrence in the crust of the earth. Strata, as we
have seen, do not all lie in one way; some are horizontal, others are
inclined to the horizon, while yet others are vertical. Again, many
rocks are amorphous; that is to say, they occur in somewhat thick
masses which show no trace of a bedded arrangement. Such differences of
structure and arrangement influence in no small degree the weathering
and denudation of rocks, and cannot be left out of account when we are
seeking to discover the origin of the present configuration of our
hills and valleys. Thus, escarpments and the terraced aspect of many
hill-slopes are due to inequalities in the strata of which such hills
are built up. The softer strata crumble away more rapidly under the
touch of the atmospheric forces than the harder beds which rest upon
them, and hence the latter are undermined, and their exposed ends or
crops, losing support, fall away and roll down the slopes. The igneous
rocks of the Cheviots are arranged in beds; but so massive are these,
that frequently a hill proves to be composed from base to summit of
one and the same sheet of old lava. Hence there is a general absence
of that terraced aspect which is so conspicuous in hills that are
built up of bedded rock-masses. Here and there, however, the beds are
not so massive, several cropping out upon a hill-side; and whenever
this is the case (as near Yetholm) we find the hill-slopes presenting
the usual terraced appearance--a series of cliffs and escarpments,
separated by intervening slopes, rising one above the other. In the
Silurian districts no such terraces or escarpments exist, the general
high dip of the strata, which often approaches the vertical, precluding
any such contour. In a region composed of highly-inclined greywacké
and shale, however, we should expect to find that where the strata are
of unequal durability, the harder beds will stand up in long narrow
ridges, separated by intervening hollows, which have been worn out
along the outcrops of the softer and more easily-denuded beds. And such
appearances do show themselves in some parts of the Silurian area. As a
rule, however, the Silurian strata are not thick-bedded, and harder and
softer bands alternate so rapidly that they yield on the whole a smooth
surface under the action of the atmospheric forces. In the low-lying
districts, which, as I have said, are mostly occupied by sandstones and
shaly beds, all the abrupt isolated hills are formed of igneous rocks,
which are much harder and tougher than the strata that surround them.
It is quite evident that these hills owe their present appearance to
the durable nature of their constituent rocks, which now project above
the general level of the surface, simply because they have been better
able to resist the denuding agents than the softer rocks that once
covered and concealed them.
We see, then, that each kind of rock has its own particular mode of
weathering, and that the configuration of a country depends primarily
upon this and upon geological structure. Indeed, so close is the
connection between the geology and the surface-outline of a country,
that to a practised observer the latter acts as an unfailing index to
the general nature of the underlying rocks, and tells him at a glance
whether these are igneous like basalt and porphyrite, aqueous like
sandstone and shale, or hardened and altered strata like greywacké.
But while one cannot help noticing how in the Cheviot district the
character of the scenery depends largely upon the nature and structure
of the rocks, he shall, nevertheless, hardly fail to observe that
flowing outlines are more or less conspicuous over all the region.
And as he descends into the main valleys, he shall be struck with the
fact that the hill-slopes seem to be smoothed off in a direction that
coincides with the trend of these valleys. In short, he cannot help
noticing that the varied configuration that results from the weathering
of different rock-masses has been subsequently modified by some agent
which seems to have acted universally over the whole country. In the
upper reaches of the Cheviot valleys, the rocks have evidently been
rounded off by some force pressing upon them in a direction coinciding
with that of the valleys; but soon after entering upon their lower
reaches, we notice that the denuding or moulding force must have turned
gradually away to the north-east--the northern spurs of the Cheviots,
and the low-grounds that abut upon these being smoothed off in a
direction that corresponds exactly with the trend of that great strath
through which flow the Teviot and the Tweed, from Melrose downwards.
Throughout this broad strath, which extends from the base of the
Lammermuirs to the foot of the Cheviots, and includes the whole of
Teviotdale, the ground presents a remarkable closely-wrinkled surface,
the ridges and intervening hollows all coinciding in direction with the
general trend of the great strath, which is south-west and north-east;
but turning gradually round to east, as we approach the lower reaches
of the Tweed.
Passing round the north-eastern extremity of the Cheviot range into
Northumberland, we observe that the same series of ridges and hollows
continues to follow an easterly direction until we near the sea-board,
when the trend gradually swings round to the south-east, as in the
neighbourhood of Belford and Bamborough, where the ridges run parallel
with the coast-line.