Roxburghshire and Berwickshire, especially in the regions between
Kelso and Smailholm, and between Duns and Coldstream. The dwellers
along the banks of the Tweed are quite familiar with the fact that the
roads which run parallel with the river are smooth and level, for they
coincide with the trend of the ridges and hollows; whilst those that
cross the country at right angles to this direction must of course
traverse ridge after ridge, and are therefore exceedingly uneven. In
this low-lying district most of the ridges are composed of superficial
deposits of stony and gravelly clay and sand, and the same is the case
with those that sweep round the north-eastern spurs of the Cheviots
by Coldstream and Ancroft. Some ridges, however, consist either of
solid rock alone, as near Stichill, or of rock and overlying masses
of clay and stones. In the hilly regions, again, nearly all the ridges
are of rock alone, especially in the districts lying between Melrose
and Selkirk and between Selkirk and Hawick. Indeed, the hills drained
by the upper reaches of the Teviot and its tributaries are more or
less fluted and channelled, as it were--many long parallel narrow
hollows having been driven out along their slopes and even frequently
across their broad tops. This scolloped and ridged aspect of the
hills, however, disappears as we approach the upper reaches of the
hill-valleys. From Skelfhill Pen (1745 feet) by Windburgh Hill (1662
feet), on through the ridge of the Cheviot watershed, none of the hills
shows any appearance of a uniformly-wrinkled surface.
[Illustration: Fig. 10.--Rounded Rocks, with superficial deposits, _t_
_t_ _t_, heaped up against steep faces. The arrows indicate direction
followed by the smoothing agent.]
A close inspection of the rock-ridges satisfies one that they have
been smoothed off by some agent pressing upon them in a direction that
coincides with their own trend; and not only so, but the smoothing
agent, it is clearly seen, must have come from the watersheds and then
pressed outwards to the low-grounds which are now watered by the Teviot
and the Tweed. This is shown by the manner in which the rocks have
been smoothed off, for their smooth faces look towards the dominant
watersheds, while their rough and unpolished sides point away in the
opposite direction. Sometimes, however, we find that more or less
steeply projecting rocks _face_ the dominant watersheds. When such is
the case, there is usually a long sloping "tail" behind the crag--a
"tail" which is composed chiefly of superficial deposits. The hills
between Hume and Stichill afford some good examples. The two kinds of
appearances are exhibited in the accompanying diagram (Figs. 10, 11.)
The appearance shown in Fig. 10 is of most common occurrence in the
upland parts of the country, while "crag and tail" (as shown in Fig.
11) is seen to greatest advantage in the open low-grounds. In both
cases it will be observed that superficial deposits (_t_) nestle behind
a more or less steep face of rock.
[Illustration: Fig. 11.--"Crag and Tail"; boss of hard rock, _c_;
intersecting sandstones, _s_; superficial deposits heaped up in rear of
crag, _t_. The arrow indicates direction followed by smoothing agent.]
When the rocks have not been much exposed to the action of the
weather, they often show a polished surface covered with long parallel
grooves and striæ or scratches. Such polished and scratched surfaces
are best seen when the superficial deposits have been only recently
removed. Often, too, when we tear away the thick turf that mantles the
hill-slopes, we find the same phenomena. Indeed, wherever the rocks
have not been much acted upon by the weather, and thus broken up and
decomposed, we may expect to meet with more or less well-marked grooves
and stride. Now the remarkable circumstance about these scratches is
this--they agree in direction with the trend of the rock-ridges and the
hollows described above. Nor can we doubt that the superficial markings
have all been produced by one and the same agent. In the upper valleys
of the Cheviots, the scratches coincide in direction with the valleys,
which is, speaking generally, from south to north, but as we approach
the low-grounds they begin to turn more to the east (just, as we have
seen, is the case with the ridges and hollows), until we enter England
to the east of Coldstream, where the striæ point first nearly due east,
but eventually swing round to the south-east, as is well seen upon the
limestone rocks between Lowick and Belford. In Teviotdale the general
trend of the striæ is from south-west to north-east, a direction
which continues to hold good until the lower reaches of the Tweed
are approached, when, as we have just mentioned, they begin to turn
more and more to the east. Thus it becomes evident that the denuding
agent, whatever it was, that gave rise to these ridges and scratched
rock-surfaces must have pressed outwards from all the dominant
watersheds, and, sweeping down through the great undulating strath that
lies between the Cheviots and the Lammermuirs, must have gradually
turned away to the east and south as it rounded the northern spurs of
the former range, so as to pass south-east over the contiguous maritime
districts of Northumberland.
A few words now as to the character of the superficial deposits which
enter so largely into the composition of the long parallel banks and
ridges in the low-grounds of Roxburghshire, Berwickshire, and the
northern part of Northumberland. The most conspicuous and noteworthy
deposit is a hard tough tenacious clay, which is always more or
less well-charged with blunted and sub-angular stones and boulders,
scattered pell-mell through the mass. This clay is as a rule quite
unstratified--it shows no lines of bedding, and although here and
there it contains irregular patches and beds of gravel and sand, yet
it evidently does not owe its origin to the action of water. Its
colour in the upper part of Teviotdale and the Cheviots is generally a
drab-brown, or pale grey and sometimes yellow, while here and there,
as in the upper reaches of the Jed valley, it is a dark dingy bluish
grey. In the lower parts of Teviotdale and in the Tweed district it
is generally red or reddish brown. The stones in the clay have all
been derived from the rocks of the region in which it occurs. Thus
in Teviotdale we find that in the higher reaches of the dale which
are Silurian the stones and boulders consist of various kinds of
greywacké, etc. In the lower reaches, however, when we pass into the
Red Sandstone area, we note that the clay begins to contain fragments
of red sandstone, while the clay itself takes on a reddish tinge, until
we get down to the vale of the Tweed, where not only is the clay very
decidedly red, but its sandstone boulders also are very numerous.
The same appearances present themselves in passing outwards from the
Cheviots. At first the clay contains only stones that have been derived
from the upper parts of the hills, but by-and-by, as we near the
low-grounds, other kinds begin to make their appearance, so that by the
time we reach the Tweed we may obtain from the clay specimens of every
kind of rock that occurs within the drainage-area of the Teviot and the
lower reaches of the River Tweed.
Look at the stones, and you shall observe that all the harder and
finer-grained specimens are well-smoothed and covered with striæ or
scratches, the best marked of which run parallel with the longer axis
of each stone and boulder. These scratches are evidently very similar
to those markings that cover the surface of the underlying solid rock,
and we may feel sure, therefore, that the denuding agent which smoothed
and scratched the solid rocks had also something to do with the stones
and boulders of the clay.
Underneath the stony clay, or _Till_, as it is called, we find here
and there certain old river gravels. We know that these gravels are
river-formations, because not only do they lie at the bottom of the
river-valleys, but the stones, we can see, have been arranged by water
running in one constant direction, and that direction is always _down_
the valley in which the gravels chance to occur. Frequently, however,
there is no trace of such underlying gravels, but the till rests
directly upon the solid rocks.
Now what do all these appearances mean? It is clear that there is no
natural agent in this country engaged in rounding and scratching the
rocks, or in accumulating a stony clay like till. In alpine regions,
however, we know that glaciers, as they slowly creep down their
valleys, grind and polish and scratch the rocks over which they pass,
and that underneath the moving ice one may detect smoothed and striated
stones precisely resembling those that occur in till. Frost in such
alpine regions splits up the rocks of the cliffs and mountain-slopes
that overlook a glacier, and immense masses of angular stones and
débris, thus loosened, roll down and accumulate along the flanks of
the ice-streams. Eventually such accumulations are borne slowly down
the valley upon the back of the glacier, and are dropped at last over
the terminal front of the ice, where they become intermingled with the
stones and rubbish, which are pushed or washed out from underneath the
ice. These heaps and masses of angular débris and stones are called
"moraines," and one can see that in Switzerland the glaciers must at
some time have been much larger, for ancient moraines occur far down
in the low-grounds of that country--the glaciers being now confined
to the uppermost reaches of the deep mountain-valleys. Moreover, we
may note how the mountain-slopes overlooking the present puny glaciers
have been rubbed by ice up to a height of sometimes a thousand feet and
more above the level of the existing ice-streams. Now since the aspect
presented by the glaciated rock-surfaces of Switzerland is exactly
paralleled by the rounded and smoothed rocks of Scotland, there can
be no doubt that the latter have had a similar origin. Again, we find
throughout the low-grounds of Switzerland a deposit of till precisely
resembling that which is so well developed in Teviotdale and the valley
of the Tweed. And as there can be no doubt that the Swiss till has been
produced by the action of glacier ice, we are compelled to believe the
same of the till in Scotland.
Let us further note that in the deep mountain-valleys of Switzerland
the glacial deposits consist for the most part of coarse morainic
débris--of such materials, in short, as the terminal moraines of
existing glaciers are mainly composed. Not infrequently this morainic
débris has been more or less acted upon by the rivers that escape from
the glaciers, and the angular stones have been rounded and arranged in
bedded masses. It is only when we get out of the mountain-valleys and
approach the low-grounds that the till, or stony clay, begins to appear
abundantly. The same phenomena characterise the Cheviot district. In
the upper reaches of the mountain-valleys at the heads of the Teviot,
the Kale, the Bowmont, etc., either till does not occur or it is thin
and often concealed below masses of rude morainic débris and gravel.
Out in the low-grounds, however, till, as we have already remarked, is
the most conspicuous of all the superficial deposits. From these facts
it may be inferred that till indicates the former presence of great
confluent glaciers, while morainic débris in hill-valleys points to the
action of comparatively small local and isolated glaciers.
What, then, are the general conclusions which may be derived from a
study of the rock-ridges, flutings, and striæ, and the till of the
Cheviot district? Clearly this: that the whole country has at one time
been deeply buried under glacier ice. The evidence shows us that the
broad strath stretching between the Lammermuirs and the Cheviots must
have been filled to overflowing with a great mass of ice that descended
from the uplands of Peebles and Selkirk and the broad-topped heights
that overlook the sources of the Teviot. The Cheviots appear to have
been quite buried underneath this wide sea of ice, and so likewise
were the Lammermuirs. At the same time, as we know, all Scotland was
similarly enveloped in a vast sheet of snow and ice, which streamed out
from the main watersheds of the country, and followed the lines of the
chief straths--that is to say, the general slope of the ground. The
track of the ice in the Cheviot district is very distinctly marked. In
Teviotdale it followed the trend of the valley, and, grinding along
the outcrop of the Silurian strata, deepened old hollows and scooped
out new ones in the soft shaly beds, while the intervening harder
strata, which offered greater resistance to the denuding action of
the ice, did not wear so easily, and so were rounded off, and formed
a series of ridges running parallel to the eroded hollows. The stones
and rubbish, dragged along underneath the ice, necessarily increased
as the glacier mass crept on its way. The rocks were scratched and
grooved by the stones that were forced over them, and the polishing
was completed by the finer sand and clay which resulted from the
grinding process. Wherever a rock projected there would be a tendency
for the stones and clay and sand to gather behind it. One may notice
the same kind of action upon the bed of a stream, where the sediment
tends to collect in the rear of prominent stones and boulders. And we
can hardly fail to have observed further that the sediment of a river
often arranges itself under the action of the current in long banks,
which run parallel to the course of the water. Underneath the ice-sheet
the stones, sand, and clay behaved in the same way. Behind projecting
rocks in sheltered nooks and hollows, they accumulated, while in
places exposed to the full sweep of the ice-stream they were piled up
and drawn out into long parallel banks and ridges, the trend of which
coincided with that of the ice-flow. The presence of confused and
irregular patches and lenticular beds of sand, clay, and gravel in the
till is not difficult to understand when we know that there is always
more or less water flowing on underneath a glacier. Such streams must
assort the débris, and roll angular fragments into rounded stones and
pebbles; but the materials thus assorted in layers will ever and anon
be crushed up so as to be either partially or wholly obliterated by the
slowly moving glacier.
As the stones and clay were derived from the underlying rocks, it is no
wonder that the colour of the till should vary. In the Silurian tracts
it is pale yellowish, or bluish grey, and the stones consist chiefly
of fragments of Silurian rocks, all blunted and smoothed, and often
beautifully polished and striated. When we get into the Red Sandstone
region of the low-grounds the colour of the clay begins by-and-by to
change, and fragments of red sandstone become commingled with the
Silurian stones, until ere long the colour of the deposit is decidedly
red, and sandstone fragments abound. Everywhere the stones show that
they have been carried persistently in one direction, and that is _out
from the watershed, and down the main valleys_.
The direction of the ice-marks upon the solid rocks, and the trend of
the "drums," as the parallel ridges of till are termed, show that the
ice-sheet of Teviotdale and Tweed gradually turned away to the east and
south-east as it swept round the north-eastern spurs of the Cheviots.
Now we may well ask why the ice did not go right out into the North
Sea, which is apparently the course it ought to have followed. The
same curious deflection affected the great ice-stream that occupied
the basin of the Forth. When it got past North Berwick, that stream,
instead of flowing directly east into the North Sea, turned away to
the south-east and overflowed the northern spurs of the Lammermuirs,
bringing with it into the valley of the Tweed stones and boulders which
had travelled all the way from the Highlands. It is obvious there
must have been some impediment to the flow of the Scottish ice into
the basin of the North Sea. What could have blocked its passage in
that direction? At the very time that Scotland lay concealed beneath
its ice-sheet, Norway and Sweden were likewise smothered in ice which
attained a thickness of not less than five or six thousand feet.
The whole basin of the Baltic was occupied by a vast glacier which
flowed south into Northern Germany, and this sheet was continuous with
glacier-ice that crossed over Denmark. When we consider how shallow
the North Sea is (it does not average more than forty fathoms between
Scotland and the Continent), we cannot doubt that the immense masses
of ice descending from Norway could not possibly have floated off,
but must actually have crept across the bottom of that sea until they
abutted upon and coalesced with the Scottish ice, so as to form one
vast _mer de glace_.
Thus it was that the Scandinavian ice blocked up the path of the
Scottish glaciers into the basin of the North Sea, and compelled them
to flow south-east into England.[H] Had there been no such obstruction
to the passage of the Scottish glaciers, it is impossible to believe
that snow and ice could ever have accumulated to such a depth in
Scotland. The Scottish ice reached a thickness of some three thousand
feet in its deeper parts. It is evident, however, that had there been
a free course for the glaciers, they would have moved off before they
could have attained this thickness. And we can hardly doubt, therefore,
that it was the damming-up of their outlet by the great Scandinavian
ice-sheet that enabled them to deepen to such an extent in the valleys
and low-grounds of Scotland.
[H] In the extreme north of Scotland we find that the Scottish ice was,
in like manner, compelled to turn aside and overflow Caithness from
south-east to north-west.
When the ice-sheet was at its thickest, the Cheviots were completely
covered, nevertheless they served to divide the ice-flow between
Scotland and England, although here and there one finds that the ice
passed over some of the lower summits, carrying with it boulders and
stones. This is by no means an uncommon circumstance in Scotland and
other glaciated countries. Thus we note that Highland boulders have
been brought into the vale of the Tweed across the Lammermuirs; and in
the same way boulders from the heights overlooking Eskdale have been
carried over some of the lower hill-tops into the vale of the Teviot.
In like manner the Swedish ice occasionally overflowed the lower
mountain-tops of the dividing ridge or watershed into Norway.
What wonder now that the Cheviot area should exhibit so many flowing
outlines, that the hills should be so smoothed and rounded and fluted,
that the low-grounds should be cumbered with such heaps of clay and
striated stones? Long before the great glaciers appeared, the rocks
were weathered and worn by the action of the usual atmospheric forces,
and each had assumed its own peculiar outline; but how greatly has
this been modified by the grinding action of the ice-sheet! To what
an extent have projecting rocks been rubbed, and how great is the
destruction that has befallen the loose accumulations of river gravel,
sand, and clay that gathered in the valleys before the advent of the
Ice Age! All that now remains of these are a few patches preserved here
and their underneath the till. The Cheviots can tell us nothing of the
kinds of plants and animals that clothed and peopled the country in
pre-glacial times. All we learn is that streams and rivers flowed as
they flow now, and that by-and-by everything was changed, and the land
disappeared underneath a vast covering of snow and ice.
In my concluding paper I will show how this ice period passed away, and
how the present condition of things succeeded.
V.
I have described the condition of the Cheviot district during the
climax of the Ice Age as one of intense arctic cold, the whole ridge of
hills being then completely smothered in snow and ice. This excessive
climate, however, did not last continuously throughout the so-called
glacial period, but was interrupted by more than one mild interglacial
epoch. We have evidence in Scotland, as in other countries, to show
that the great confluent ice-masses melted away so as to uncover all
the low-grounds and permit the reappearance of plants and animals.
Rivers again watered the land, and numerous lakes diversified the face
of the country. Willows, hazels, and alders grew in the sheltered
valleys, oak-trees flourished in the low-grounds, and Scots firs
clustered upon the hill-slopes. A strong, grassy vegetation covered
wide areas, and sedges and rushes luxuriated in marshy places and
encroached upon the margins of the lakes. The mammoth, or woolly-coated
elephant, roamed over the land, and among its congeners were the
extinct ox, the horse, the Irish elk, and the reindeer. After such a
temperate condition of things had continued for some time--perhaps
for thousands of years--the land, during the last interglacial epoch,
became gradually submerged to a depth of several hundred feet, and a
cold, ungenial sea, in which flourished species of northern and arctic
shells, covered the low-grounds of Scotland. The cold continuing to
increase, our glaciers descended for the last time from the mountains
and encroached upon the bed of the sea, until they became confluent,
fairly usurping the floor of the German Ocean, and pushing back the
western seas as far as, and even beyond, the islands of the Outer
Hebrides. There is good reason to believe that such great changes of
climate occurred several times during the glacial period, which thus
seems to have consisted of an alternation of cold and genial epochs.
But as the last phase in this extraordinary series of changes was a
cold one, during which great glaciers scoured the face of the country,
we now obtain only a few scattered traces of the genial conditions
that characterised the preceding mild interglacial epochs. Vegetable
accumulations, lake and river deposits with mammalian remains, marine
beds and their shelly contents, were all ploughed up by the ice, and to
a very large extent demolished. Here and there, however, we find in the
till or boulder-clay that marks the last cold epoch, wasted fragments
of trees, tusks of mammoths, and broken sea-shells; while underneath
the till we occasionally come upon old lake deposits with vegetable and
mammalian remains, or, as the case may be, beds of marine origin well
stocked with sea-shells of arctic species. And these freshwater and
marine beds repose, in many cases, upon an older accumulation of till,
which belongs to an earlier cold epoch of the glacial period. In the
Cheviot district proper, the traces of mild, interglacial conditions
are very slight, but in the immediate neighbourhood we find them more
strongly marked. Thus, in the valley of the Slitrig, near Hawick, we
notice freshwater beds with peaty matter lying between a lower and
an upper till or boulder-clay; and interglacial freshwater beds also
appear in the neighbouring county of Peebles, particularly in the
valley of the Leithan Water. Again, in the valley of the Tweed near
Carham, there occur interglacial beds in which I detected numerous
bones of water-rats and frogs. These interglacial remains acquire a
peculiar interest when we come to view the "superficial deposits" of
Scotland in connection with those of England and the Continent; for,
as I have endeavoured to show elsewhere,[I] it is most likely that the
ancient gravels of England, which contain the earliest traces of man,
belong for the most part to interglacial times; and the extraordinary
changes of climate described above may therefore have been actually
witnessed by human eyes. Indeed, I believe it was the advent of the
last cold epoch of the Ice Age that drove out the old tribes who used
the rude flint implements that are now found in the gravel deposits
and caves of England, and who occupied the British area along with
hippopotami, rhinoceroses, elephants, lions, hyænas, and other animals.
The men who entered Britain after the final disappearance of arctic
conditions, were more advanced in civilisation, and were accompanied by
a very different assemblage of animals--by a group represented by oxen,
sheep, dogs, and other creatures, most of which are still indigenous to
Britain.
[I] _Great Ice Age._