Fragments of Earth Lore: Sketches & Addresses Geological and Geographical
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Title: Fragments of Earth Lore: Sketches & Addresses Geological and Geographical
Author: James Geikie
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Transcriber's Note
Text emphasis is displayed as _Italics_ and =Bold= respectively.
Superscript characters are denoted with the carat character (i.e., 8^e).
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FRAGMENTS
OF
EARTH LORE
[Illustration: PLATE I
OROGRAPHIC MAP OF
SCOTLAND]
FRAGMENTS
OF
EARTH LORE
SKETCHES & ADDRESSES
Geological and Geographical
BY
JAMES GEIKIE, D.C.L., LL.D., F.R.S., &c.
MURCHISON-PROFESSOR OF GEOLOGY AND MINERALOGY IN THE
UNIVERSITY OF EDINBURGH
FORMERLY OF H.M. GEOLOGICAL SURVEY OF SCOTLAND
WITH MAPS AND ILLUSTRATIONS
EDINBURGH
JOHN BARTHOLOMEW & CO.
LONDON: SIMPKIN, MARSHALL, HAMILTON, KENT & Co., Ltd.
1893
PREFACE.
The articles in this volume deal chiefly with the history of Glacial
times and the origin of surface-features. As they were not written with
any view to their subsequent appearance in a collected form, each is so
far independent and complete in itself. Under these circumstances some
repetition was unavoidable, if the articles were not to be recast, and
I did not think it advisable to make such radical alteration. With the
exception of verbal changes and some excisions, therefore, the papers
remain substantially in their original state. Here and there a footnote
has been added to indicate where the views expressed in the text have
since been modified; but I have not been careful to insert such notes
throughout. Geologists, like other folk, live and learn, and the reader
will probably discover that the opinions set forth in some of the
later articles are occasionally in advance of those maintained in the
writer's earlier days.
I have to thank the Publishers of _Good Words_ for allowing me to
republish the articles on the Cheviot Hills and the Outer Hebrides. My
acknowledgments are also due to Mr. Bartholomew for the excellent maps
with which the volume is so well illustrated.
Edinburgh, _April 5th, 1893_.
LIST OF MAPS.
Plate I. PHYSICAL FEATURES OF SCOTLAND _Frontispiece_
" II. STRUCTURE OF MOUNTAINS 60
" III. PAST AND PRESENT GLACIATION OF THE WORLD 193
" IV. ICE AGE IN NORTHERN EUROPE 324
" V. THE GEOGRAPHICAL EVOLUTION OF CONTINENTS 348
" VI. BATHY-HYPSOMETRICAL MAP, ILLUSTRATING
DEVELOPMENT OF COAST-LINES 428
CONTENTS.
CHAP. PAGE
I. GEOGRAPHY AND GEOLOGY 1
II. THE PHYSICAL FEATURES OF SCOTLAND 14
III. MOUNTAINS: THEIR ORIGIN, GROWTH, AND DECAY 36
IV. THE CHEVIOT HILLS 62
V. THE LONG ISLAND, OR OUTER HEBRIDES 125
VI. THE ICE AGE IN EUROPE AND NORTH AMERICA 160
VII. THE INTERCROSSING OF ERRATICS IN GLACIAL
DEPOSITS 194
VIII. RECENT RESEARCHES IN THE GLACIAL GEOLOGY
OF THE CONTINENT 220
IX. THE GLACIAL PERIOD AND THE EARTH-MOVEMENT
HYPOTHESIS 248
X. THE GLACIAL SUCCESSION IN EUROPE 288
XI. THE GEOGRAPHICAL EVOLUTION OF EUROPE 326
XII. THE EVOLUTION OF CLIMATE 349
XIII. THE SCIENTIFIC RESULTS OF DR. NANSSEN'S
EXPEDITION 382
XIV. THE GEOGRAPHICAL DEVELOPMENT OF COAST-LINES 393
I.
Geography and Geology.[A]
[A] Portion of a lecture given in 1886 to the Class of Geology in the
University of Edinburgh.
The teaching of Geography naturally occupies a prominent place in
every school curriculum. It is rightly considered essential that we
should from an early age begin to know something of our own and other
countries. I am not sure, however, that Geography is always taught in
the most interesting and effective manner. Indeed, according to some
geographers, who are well qualified to express an opinion, the manner
in which their subject is presented in many of our schools leaves much
to be desired. But a decided advance has been made in recent years,
and with the multiplication of excellent text-books, maps, and other
appliances, I have no doubt that this improvement will continue. When
I attended school the text-books used by my teachers were about as
repellent as they could be. Our most important lesson was to commit to
memory a multitude of place-names, and the maps which were supposed
to illustrate the text-books were, if possible, less interesting and
instructive. Nowadays, however, teachers have a number of more or less
excellent manuals at their service, and the educational maps issued by
our cartographers show in many cases a very great advance on the bald
and misleading caricatures which did duty in my young days as pictures
of the earth's surface.
During the progress of some war we often remark that the task of
following the military operations compels us to brush up our Geography.
I am uncharitable enough to suspect that it would frequently be
truer to say that, before these campaigns commenced, we had no such
knowledge to brush up. The countries involved in the commotion were
probably mere names to many of us. We had no immediate interest in
them or their inhabitants, and had we been asked, before the outbreak
of hostilities, to indicate the precise positions of the places upon a
map, some of us perhaps might have been sorely puzzled to do so. Nor
is such ignorance always discreditable. One cannot know everything;
the land-surface of the globe contains upwards of 50 millions of
square miles, and one may surely be excused for not having a detailed
knowledge of this vast area. I have referred to the subject simply
because I think it gives us a hint as to how the teaching of Political
Geography might be made most instructive and interesting. Historical
narrative might often be interwoven with the subject in such a way
as to fix geographical features indelibly on the memory. Striking
and picturesque incidents, eventful wars, the rise and progress of
particular trades, the routes followed by commerce, the immigration
and emigration of races, the gradual development of the existing
political divisions of the Old World, the story of Columbus and the
early voyagers, the geographical discoveries of later times--all these,
and such as these, might be introduced into our lessons in Political
Geography. The wanderings of a Mungo Park, a Bruce, a Livingstone, a
Stanley, traced on a good map, could not fail to arrest the attention
of the youthful student of African geography. In like manner, the
campaigns of the great Napoleon might be made to do good service
in illustrating the geographical features of large portions of our
own continent. Then, as regards Britain, what a world of poetry and
romantic story clings to every portion of its surface--why, the very
place-names themselves might suggest to any intelligent teacher themes
and incidents, the deft treatment of which would make the acquisition
of Geography a delightful task to the dullest boy or girl.
The intimate relation that obtains between Political Geography and
History has indeed long been recognised, and is in fact self-evident.
And we are all well aware that in our school manuals of Geography it
has been usual for very many years to note the scenes of remarkable
events. Such notes, however, are of necessity extremely brief; and it
need hardly be said that to fully incorporate history in a text-book of
general Geography would be quite impracticable. It might be done to a
certain extent for our own and a few of the more important countries;
but similar detail need not be attempted in regard to regions which are
of less consequence from the political point of view. Indeed, I should
be inclined to leave the proper application of historical knowledge in
the teaching of Geography very much to the teacher himself, who would
naturally select such themes and incidents as seemed best adapted to
attract the attention of his pupils. Be that, however, as it may,
it is enough for my present purpose if I insist upon the fact that
the proper study of Political Geography involves the acquisition of
some historical knowledge. One can hardly conceive the possibility
of an intelligent student taking pains to become acquainted with the
political geography of a country without at the same time endeavouring
to learn something of its history--otherwise, his geographical
attainments would hardly surpass those of a commercial traveller, whose
geographical studies have been confined to the maps and tables of his
Bradshaw.
But if it be impossible to ignore History in the teaching of Political
Geography, it is just as impossible to exclude from our attention
great physical features and characteristics. Surface-configuration,
climate, and natural products all claim our attention. It is obvious,
in fact, that the proper study of Political Geography must give us
at least a general notion of the configuration, the river-systems,
and climatic conditions of many different lands. For has not the
political development of races depended most largely on the physical
conditions and natural resources of the countries occupied by them?
So far, then, as these have sensibly influenced the progress of
peoples, they come naturally under the consideration of Political
Geography. Thus, if Political Geography be closely connected and
interwoven, as it were, with History, not less intimate are its
relations to Physical Geography. It does not embrace all Physical
Geography, but it introduces us to many facts and phenomena, the
causes and mutual relations of which we cannot understand without
first mastering the teachings of Physical Geography. In the study of
this latter science we come more closely into contact with Nature;
we cease to think of the surface of the earth as parcelled out into
so many lots by its human occupants--we no longer contemplate that
surface from the limited point of view of the political geographer--we
are now not merely members of one particular community, but have
become true citizens of the world. To us north and south, east and
west are of equal interest and importance. Our desire now is to
understand, if haply we may, the complex system of which we ourselves
form a part. The distribution of land and water--the configuration of
continental areas and oceanic basins--the circulation of oceanic and
terrestrial waters--earth-movements and volcanoes--ice-formations--the
atmosphere--climatology--the geographical distribution of plants
and animals--in a word, _the world as one organic whole_ now forms
the subject of our contemplation. Such being the scope of Physical
Geography, it is satisfactory to know that its importance as a subject
of study in our schools has been fully recognised. This being admitted,
I shall now proceed to show that Physical Geography, although, like
Political Geography, it is a separate and distinct subject, yet, just
as the study of the latter involves some knowledge of History, so
the prosecution of Physical Geography compels us to make a certain
acquaintance with Geology. We cannot, in fact, learn much about the
atmosphere, about rain and rivers, glaciers and icebergs, earthquakes
and volcanoes, and the causes of climate, without at the same time
becoming more or less familiar with the groundwork on which geological
investigations are based. And just as a knowledge of history enables
us better to understand the facts of Political Geography, so some
acquaintance with the results of geological inquiry are necessary
before we can hope to comprehend many of the phenomena of which
Physical Geography treats. Let me try to make this plain. The physical
geographer, we shall suppose, is considering the subject of terrestrial
waters. He tells us what is meant by the drainage-system of a country,
points out how the various minor water-courses or brooks and streams
unite to form a river, describes for us the shape of the valley through
which a typical river makes its way--how the valley-slope diminishes
from the mountains onwards to the sea-coast--how, at first, in its
upper or mountain-track, the flow of the river is torrential--how,
as the slope of the valley decreases, the river begins to wind about
more freely, until it reaches the head of its plain-track or delta,
when, no longer receiving affluents, it begins to divide, and enters
the sea at last by many mouths. He tells us further what proportion of
the rainfall of the country passes seawards in our river, and he can
measure for us the quantity of water which is actually discharged. All
this is purely Physical Geography; but when we come to ask why some
rivers flow in deep cañons, like those of the Colorado--why valleys
should widen out in one part and contract, as it were, elsewhere--why
the courses of some rivers are interrupted by waterfalls and rapids,
and many other similar questions, the physical geographer must know
something of Geology before he can give an answer. He can describe
the actual existing conditions; without the aid of Geology, he can
tell us nothing of their origin and cause. So the political geographer
can map out for us the present limits of the various countries of
Europe, but History must be invoked if we would know how those
boundaries came to be determined. The moment, therefore, the physical
geographer begins to inquire into the origin of any particular
physical feature, he enters upon the domains of the geologist. And as
he cannot possibly avoid doing so, it is quite common now to find a
good deal of the subject-matter of Geology treated of in text-books
of Physical Geography. I state this merely to show how very closely
the two sciences are interlocked. Take, for example, the configuration
of river valleys just referred to. The physical geographer recognises
the fact that a river performs work; by means of the sediment which
it carries in suspension and rolls along its course, it erodes its
bed in many places, and undermines its banks, and thus its channel is
deepened and widened. He can measure the amount of sediment which it
carries down to the sea, and the quantity of saline matters which its
waters hold in solution: and knowing that all these substances have
been abstracted from the land, he is able to estimate approximately
the amount of material which is annually transferred from the surface
of the drainage-area involved. He discovers this to be so relatively
enormous that he has no difficulty in believing that the valleys
in which rivers flow might have been hollowed out by the rivers
themselves. But, without trespassing further into the geologist's
domains, he cannot go beyond this: and you will at once perceive that
something more is required to prove that any particular valley owes its
origin to the erosive action of running water. Suppose someone were
to suggest to him that his river-valley might be a minor wrinkle in
the earth's crust caused by earth-movements, or that it might indicate
the line of a fissure or dislocation, due to some comparatively recent
convulsion--how could his computation of the amount of material at
present carried seawards by the river prove such suggestions to be
erroneous? And what light could it throw upon the origin of the
varied configuration of the river-valley--how would it explain the
presence or absence of cascades and rapids, of narrow gorges and open
expanses? None of these phenomena can be interpreted and accounted
for without the aid of the geologist: without some knowledge of rocks
and rock-structures, the origin of the earth's surface-features
is quite inexplicable. To give an adequate explanation of all the
surface-features of a country in detail would of course require a
profound study of Geology; but a general acquaintance only with its
elementary facts is quite sufficient to enable us to form a reasonable
and intelligent view of the cause and origin of the main features
of the land as a whole. Thus a few lessons in elementary Geology
would make clear to any child how rivers have excavated valleys, why
cataracts and gorges occur here, and open valleys with gently-flowing
waters elsewhere.
Let me select yet another example to show how dependent Physical
Geography is upon Geology. The physical geographer, in describing the
features of the land, tells us how the great continental areas are
traversed in various directions by what he calls mountain-chains. Thus,
in speaking of America, he tells us that it may be taken as a type
of the continental structure--namely a vast expanse of land, low or
basin-like in the interior, and flanked along the maritime regions by
elevated mountain borders--the highest border facing the deepest ocean.
He points out further that the great continental areas are crossed from
west to east by well-marked depressions, to a large extent occupied
by water. Thus Europe is separated from Africa by the Mediterranean,
a depression which is continued eastward through the Black Sea into
the Aralo-Caspian area. South America is all but cut away from North
America, while Australia is separated from Asia by the East India Seas.
We find, in fact, all over the world that well-marked natural features
are constantly being repeated. Not only do the great land-masses
of the globe bear certain resemblances to each other, but even in
their detailed structure similar parallelisms recur. The physical
geographer notes all these remarkable phenomena, but he can give us
no clue to their meaning. He may describe with admirable skill the
characteristic features of plains and plateaux, of volcanic mountains
and mountain-chains, but he cannot tell us why plains should occur
here and mountains there; nor can he explain why some mountains, such
as those of Scotland or Norway, differ so much in configuration from
the Alps and the Pyrenees. The answer to all these questions can only
be given by Geology. It is from this science we learn how continental
areas and oceanic basins have been evolved. The patient study of the
rocks has revealed the origin of the present configuration of the
land. There is not a hill or valley, not a plateau or mountain-region,
which does not reveal its own history. The geologist can tell you why
continents are bordered by coast-ranges, and why their interiors are
generally comparatively low and basin-shaped. The oceanic basins and
continental areas, we learn, are primeval wrinkles in the earth's
crust, caused by its irregular subsidence upon the gradually cooling
and contracting nucleus. The continents are immense plateau-like areas
rising more or less abruptly above those stupendous depressions
of the earth's crust which are occupied by the ocean. While those
depressions are in progress the maritime borders of the land-areas are
subjected to enormous squeezing and crushing, and coast-ranges are the
result--the elevation of those ranges necessarily holding some relation
to the depth of the contiguous ocean. For, the deeper the ocean the
greater has been the depression under the sea, and, consequently,
the more intense the upheaval along the continental borders. It is
for the same reason that destructive earthquakes are most likely to
occur in the vicinity of coast-ranges which are of comparatively
recent geological age. These, and indeed all, mountains of elevation
are lines of weakness along which earth-movements may continue from
time to time to take place. But all mountains are not mountains of
elevation; many elevated regions owe their mountainous character simply
to the erosive action of sub-aërial agents, such as rain, frost, ice,
and running water, the forms assumed by the mountains being due to
their petrological character and geological structure. There are,
for example, no true mountains of elevation in Scotland; hence to
write of the _chain of the Grampians_ or the _range of the Lowthers_
is incorrect and actually misleading. Without the aid of Geology the
geographer cannot, in fact, discriminate between mountains of elevation
and mountains of denudation; hence geographical terms so constantly in
use as _mountain-range_ and _mountain-chain_ are very often applied by
writers, ignorant of geological structure, to elevated regions which
have no claim to be described either as _chains_ or _ranges_. Some
knowledge of Geology, therefore, is essential to us if we would have
correct views of many of the grandest features of the globe. But it
will be said that, after all, the physical geographer deals with the
earth as we now find it; he does not need to trouble himself with the
origin of the phenomena he describes. Well, as I have just shown, he
cannot, even if he would, escape trenching on Geology; and if he could,
his subject would be shorn of much of its interest. He recognises that
the world he studies has in it the elements of change--the forces of
Nature are everywhere modifying the earth's surface--considerable
changes are sometimes brought about even in one's lifetime, while
within the course of historical ages still greater mutations have
taken place--he becomes conscious, in short, that the existing state
of things is but the latest phase of an interminable series of changes
stretching back into the illimitable past, and destined to be prolonged
into the indefinite future. Thus he gladly welcomes the labours of the
geologist, whose researches into the past have thrown such a flood of
light upon the present. In fact, he can no more divorce his attention
from the results of geological inquiry than the political geographer
can shut his eyes to the facts of History.
Let me, in conclusion, give one further illustration of the close
inter-dependence of the two sciences of which I am speaking. One of the
subjects treated of by Physical Geography is the present geographical
distribution of plants and animals. The land-surface of the globe has
been mapped out into so many biological regions, each of which is
characterised by its special fauna and flora. The greatest changes in
the flora and fauna of a continent are met with as we pass from south
to north, or _vice versa_. Proceeding in the direction of the latitude,
the changes encountered are much less striking. Now, these facts are
readily explained by the physical geographer, who points out that
the distribution is due chiefly to climatic conditions--a conclusion
which is obvious enough. But when we go into details we find that mere
latitude will not account for all the phenomena. Take, for example,
the case of the Scandinavian flora of our own Continent. It is true
that this flora is largely confined to northern latitudes; but isolated
colonies occur in our own mountains and in the mountains of middle
and southern Europe. How are these to be accounted for? The physical
geographer says that the plants grow there simply because they obtain
at high levels in low latitudes the favourable climatic conditions
underneath which they flourish at low levels in high latitudes. He
therefore concludes that the distribution of life-forms is due to
varying climatic and physical conditions. But if we ask him how those
curious colonies of foreigners come to be planted on our mountains,
he cannot tell. To get our answer we must come to the geologist; and
he will explain that they are, as it were, living fossils--monuments
of former great physical and climatic changes. He will prove to us
that the climate of Europe was at a recent geological period so cold
that the Scandinavian flora spread south into middle Europe, where it
occupied the low grounds. When the climate became milder, then the
northern invaders gradually retired--the main body migrating back
to the north--while some stragglers, retreating before the stronger
Germanic flora, took shelter in the mountains, whither the latter could
not or would not follow, and so there our Scandinavians remain, the
silent witnesses of a stupendous climatic revolution. Now, all the
world over, plants and animals have similar wonderful tales to tell of
former geographical changes. The flora and fauna of our country, for
example, prove that the British Islands formed part of the Continent at
a very recent geological period; and so, from similar evidence, we know
that not long ago Europe was joined on to Africa. On the other hand,
the facts connected with the present distribution of life demonstrate
that some areas, such as Australia, have been separated from the
nearest continental land for vastly prolonged periods of time.
It would be a very easy matter to adduce many further illustrations to
show how close is the connection between the studies of the physical
geographer and the geologist. I do not indeed exaggerate when I say
that no one can hope to become a geologist who is not well versed in
Physical Geography; nor, on the other hand, can the physical geographer
possibly dispense with the aid of Geology. The two subjects are as
closely related and interwoven, the one with the other, as History is
with Political Geography. I do not see therefore how educationists who
have admitted the great importance of Physical Geography as a branch
of general education, can logically exclude Geology as a subject of
instruction in schools. Already, indeed, it has been introduced by many
teachers, and I am confident that ere long it will be as generally
taught as Physical Geography. I would not, however, present the subject
to young people as a lesson to be learned from books. A good teacher
should be able to dispense with these helps, or rather hindrances--for
such they really are to a young beginner. His pupils ought to have
previously studied the subject of Physical Geography, and if they have
been well taught they ought to have already acquired no mean store of
geological knowledge. They ought, in fact, to have learned a good deal
about the great forces which are continually modifying the surface of
the globe, and what they have now to do is to study more particularly
the results which have followed from the constant operation of
those forces. We shall suppose, for example, that the teacher has
described how rivers erode their channels, and waves tend to cut back
a coast-line, and how the products of erosion, consisting of gravel,
sand, and mud, are distributed along river-valleys and accumulated in
lakes and seas. He now exhibits to his class good-sized fragments of
conglomerate, sandstone, and shale, and points out how each of these
rocks is of essentially the same character, and must therefore have
had the same origin, as modern sedimentary accumulations. His pupils
should be encouraged to examine the rocks of their own neighbourhood,
whether exhibited in natural sections or artificial exposures, and to
compare these with the products of modern geological action. One hour's
instruction in the field is, in fact, worth twenty hours of reading
or listening to lectures. Knowledge at first hand is what is wanted.
There are many excellent popular or elementary treatises dealing with
Historical Geology, and these have their uses, and may be read with
profit as well as pleasure. But the mere reading of such books, it is
needless to say, will never make us geologists. They help no doubt to
store the mind with interesting and entertaining knowledge, but they do
not cultivate the faculties of observation and reasoning. And unless
geology is so taught as to accomplish this result, I do not see why it
should enter into any school curriculum. Further, I would remark that,
however interesting a geological treatise may be, it cannot possibly
stimulate the imagination as the practical study of the science
is bound to do. One may put into the hands of a youth a clear and
well-written description of some particular fossiliferous limestone,
and he may by dint of slavish toil be able to repeat verbatim all
that he has read. That is how a good deal of book-knowledge of science
is acquired. Only think, however, of the drudgery it involves--the
absolute waste of time and energy. But let us illustrate our lesson
by means of a lump of the limestone itself; let us show him the
character of the rock and the nature of its fossil contents, and his
difficulties disappear. Better still--let us take him, if we can, into
a limestone quarry, and he will be a dull boy indeed if he fails fully
to understand what limestone is, or to realise the fact that the rock
he is looking at accumulated slowly, like existing oceanic formations,
at the bottom of a sea that teemed with animal life. It is unnecessary,
however, that I should illustrate this subject further. I would only
repeat that the beginner should be taught from the very first to use
his own eyes, and to draw logical conclusions from the facts which
he observes. Trained after this manner, he would acquire, not only a
precise and definite knowledge of what geological data really are,
but he would learn also how to interpret those data. He would become
familiar, in fact, with the guiding principles of geological inquiry.
How much or how little of Historical Geology should be given in schools
will depend upon circumstances. Great care, however, should be taken to
avoid wearying the youthful student with strings of mere names. What
good is gained by learning to repeat the names of fifty or a hundred
fossils, if you cannot recognise any one of these when it is put into
your hand? With young beginners I should not attempt anything of that
kind. If the neighbourhood chanced to be rich in fossils, I should
take my pupils out on Saturday to the sections where they were found,
and let them ply their hammers and collect specimens for themselves. I
should describe no fossils which they had not seen and handled. Of the
more remarkable forms of extinct animals and plants, which are often
represented by only fragmentary remains, I should exhibit drawings
showing the creatures as they have been restored by the labours of
comparative anatomists. Such restorations and ideal views of geological
scenes like those given by Heer, Dana, Saporta, and others, convey far
more vivid impressions of the life of a geological period than the
most elaborate description. In fine, the story of our earth should be
told much in the same manner as Scott wrote the history of Scotland
for his grandson. There is no more reason for requiring the juvenile
student to drudge through minute geological data before introducing him
to the grand results of geological investigation, than there is for
compelling him to study the manuscripts in our Record Offices before
allowing him to read the history which has been drawn from these and
similar sources of information. It is enough if at the beginning of
his studies he has already learned the general nature of geological
evidence and the method of its interpretation. Provided with such a
stock of geological knowledge as I have indicated, our youth would
leave school with some intelligent appreciation of existing physical
conditions, and a not inadequate conception of world-history.
II.
The Physical Features of Scotland.[B]
[B] _Scottish Geographical Magazine_, vol. i., 1885.
Scotland, like "all Gaul," is divided into three parts, namely, the
Highlands, the Central Lowlands, and the Southern Uplands. These, as a
correctly drawn map will show, are natural divisions, for they are in
accordance not only with the actual configuration of the surface, but
with the geological structure of the country. The boundaries of these
principal districts are well defined. Thus, an approximately straight
or gently undulating line taken from Stonehaven, in a south-west
direction, along the northern outskirts of Strathmore to Glen Artney,
and thence through the lower reaches of Loch Lomond to the Firth of
Clyde at Kilcreggan, marks out with precision the southern limits of
the Highland area and the northern boundary of the Central Lowlands.
The line that separates the Central Lowlands from the Southern Uplands
is hardly so prominently marked throughout its entire course, but it
follows precisely the same north-east and south-west trend, and may
be traced from Dunbar along the base of the Lammermoor and Moorfoot
Hills, the Lowthers, and the hills of Galloway and Carrick, to Girvan.
In each of the two mountain-tracts--the Highlands and the Southern
Uplands--areas of low-lying land occur, while in the intermediate
Central Lowlands isolated prominences and certain well-defined belts
of hilly ground make their appearance. The statement, so frequently
repeated in class-books and manuals of geography, that the mountains of
Scotland consist of three (some writers say five) "ranges" is erroneous
and misleading. The original author of this strange statement probably
derived his ignorance of the physical features of the country from
a study of those antiquated maps upon which the mountains of poor
Scotland are represented as sprawling and wriggling about like so many
inebriated centipedes and convulsed caterpillars. Properly speaking,
there is not a true mountain-range in the country. If we take this
term, which has been very loosely used, to signify a linear belt of
mountains--that is, an elevated ridge notched by cols or "passes"
and traversed by transverse valleys--then in place of "three" or
"five" such ranges we might just as well enumerate fifty or sixty, or
more, in the Highlands and Southern Uplands. Or, should any number
of such dominant ridges be included under the term "mountain-range,"
there seems no reason why all the mountains of the country should
not be massed under one head and styled the "Scottish Range." A
mountain-range, properly so called, is a belt of high ground which has
been ridged up by earth-movements. It is a fold, pucker, or wrinkle
in the earth's crust, and its general external form coincides more or
less closely with the structure or arrangement of the rock-masses of
which it is composed. A mountain-range of this characteristic type,
however, seldom occurs singly, but is usually associated with other
parallel ranges of the same kind--the whole forming together what
is called a "mountain-chain," of which the Alps may be taken as an
example. That chain consists of a vast succession of various kinds of
rocks, which at one time were disposed in horizontal layers or strata.
But during subsequent earth-movements those horizontal beds were
compressed laterally, squeezed, crumpled, contorted, and thrown, as it
were, into gigantic undulations and sharper folds and plications. And,
notwithstanding the enormous erosion or denudation to which the long
parallel ridges or ranges have been subjected, we can yet see that the
general contour of these corresponds in large measure to the plications
or foldings of the strata. This is well shown in the Jura, the parallel
ranges and intermediate hollows of which are formed by undulations of
the folded strata--the tops of the long hills coinciding more or less
closely with the arches, and the intervening hollows with the troughs.
Now folded, crumpled, and contorted rock-masses are common enough in
the mountainous parts of Scotland, but the configuration of the surface
rarely or never coincides with the inclination of the underlying
strata. The mountain-crests, so far from being formed by the tops of
great folds of the strata, frequently show precisely the opposite kind
of structure. In other words, the rocks, instead of being inclined away
from the hill-tops like the roof of a house from its central ridge,
often dip into the mountains. When they do so on opposite sides the
strata of which the mountains are built up seem arranged like a pile of
saucers, one within another.
There is yet another feature which brings out clearly the fact that the
slopes of the surface have not been determined by the inclination of
the strata. The main water-parting that separates the drainage-system
of the west from that of the east of Scotland does not coincide with
any axis of elevation. It is not formed by an anticlinal fold or
"saddleback." In point of fact it traverses the strata at all angles
to their inclination. But this would not have been the case had the
Scottish mountains consisted of a chain of true mountain-ranges. Our
mountains, therefore, are merely monuments of denudation, they are
the relics of elevated plateaux which have been deeply furrowed and
trenched by running water and other agents of erosion. A short sketch
of the leading features presented by the three divisions of the country
will serve to make this plain.
* * * * *
The Highlands.--The southern boundary of this, the most extensive of
the three divisions, has already been defined. The straightness of
that boundary is due to the fact that it coincides with a great line
of fracture of the earth's crust--on the north or Highland side of
which occur slates, schists, and various other hard and tough rocks,
while on the south side the prevailing strata are sandstones, etc.,
which are not of so durable a character. The latter, in consequence
of the comparative ease with which they yield to the attacks of the
eroding agents--rain and rivers, frost and ice--have been worn away to
a greater extent than the former, and hence the Highlands, along their
southern margin, abut more or less abruptly upon the Lowlands. Looking
across Strathmore from the Sidlaws or the Ochils, the mountains seem
to spring suddenly from the low grounds at their base, and to extend
north-east and south-west, as a great wall-like rampart. The whole area
north and west of this line may be said to be mountainous, its average
elevation being probably not less than 1500 feet above the sea.
A glance at the contoured or the shaded sheets of the Ordnance Survey's
map of Scotland will show better than any verbal description the
manner in which our Highland mountains are grouped. It will be at once
seen that to apply the term "range" to any particular area of those
high grounds is simply a misuse of terms. Not only are the mountains
not formed by plications and folds, but they do not even trend in
linear directions. It is true that a well-trained eye can detect
certain differences in the form and often in the colouring of the
mountains when these are traversed from south-east to north-west. Such
differences correspond to changes in the composition and structure of
the rock-masses, which are disposed or arranged in a series of broad
belts and narrower bands, running from south-west to north-east across
the whole breadth of the Highlands. Each particular kind of rock gives
rise to a special configuration, or to certain characteristic features.
Thus, the mountains that occur within a belt of slate, often show a
sharply cut outline, with more or less pointed peaks and somewhat
serrated ridges--the Aberuchill Hills, near Comrie, are an example. In
regions of gneiss and granite the mountains are usually rounded and
lumpy in form. Amongst the schists, again, the outlines are generally
more angular. Quartz-rock often shows peaked and jagged outlines;
while each variety of rock has its own particular colour, and this in
certain states of the atmosphere is very marked. The mode in which the
various rocks yield to the "weather"--the forms of their cliffs and
corries--these and many other features strike a geologist at once; and
therefore, if we are to subdivide the Highland mountains into "ranges,"
a geological classification seems the only natural arrangement that can
be followed. Unfortunately, however, our geological lines, separating
one belt or "range" from another, often run across the very heart
of great mountain-masses. Our "ranges" are distinguished from each
other simply by superficial differences of feature and structure. No
long parallel hollows separate a "range" of schist-mountains from
the succeeding "ranges" of quartz-rock, gneiss, or granite. And no
degree of careful contouring could succeed in expressing the niceties
of configuration just referred to, unless the maps were on a very
large scale indeed. A geological classification or grouping of the
mountains into linear belts cannot therefore be shown upon any ordinary
orographical map. Such a map can present only the relative heights
and disposition of the mountain-masses, and these last, in the case
of the Highlands, as we have seen, cannot be called "ranges" without
straining the use of that term. Any wide tract of the Highlands, when
viewed from a commanding position, looks like a tumbled ocean in
which the waves appear to be moving in all directions. One is also
impressed with the fact that the undulations of the surface, however
interrupted they may be, are broad--the mountains, however they may
vary in detail according to the character of the rocks, are massive,
and generally round-shouldered and often somewhat flat-topped, while
there is no great disparity of height amongst the dominant points of
any individual group. Let us take, for example, the knot of mountains
between Loch Maree and Loch Torridon. There we have a cluster of eight
pyramidal mountain-masses, the summits of which do not differ much in
elevation. Thus in Liathach two points reach 3358 feet and 3486 feet;
in Beinn Alligin there are also two points reaching 3021 feet and
3232 feet respectively; in Beinn Dearg we have a height of 2995 feet;
in Beinn Eighe are three dominant points--3188 feet, 3217 feet, and
3309 feet. The four pyramids to the north are somewhat lower--their
elevations being 2860 feet, 2801 feet, 2370 feet, and 2892 feet. The
mountains of Lochaber and the Monadhliath Mountains exhibit similar
relationships; and the same holds good with all the mountain-masses of
the Highlands. No geologist can doubt that such relationship is the
result of denudation. The mountains are monuments of erosion--they
are the wreck of an old table-land--the upper surface and original
inclination of which are approximately indicated by the summits of the
various mountain-masses and the direction of the principal water-flows.
If we in imagination fill up the valleys with the rock-material which
formerly occupied their place, we shall in some measure restore the
general aspect of the Highland area before its mountains began to be
shaped out by Nature's saws and chisels.
It will be observed that while streams descend from the various
mountains to every point in the compass, their courses having often
been determined by geological structure, etc., their waters yet tend
eventually to collect and flow as large rivers in certain definite
directions. These large rivers flow in the direction of the average
slope of the ancient table-land, while the main water-partings that
separate the more extensive drainage-areas of the country mark out,
in like manner, the dominant portions of the same old land-surface.
The water-parting of the North-west Highlands runs nearly north and
south, keeping quite close to the western shore, so that nearly all the
drainage of that region flows inland. The general inclination of the
North-west Highlands is therefore easterly towards Glenmore and the
Moray Firth. In the region lying east of Glenmore the average slopes of
the land are indicated by the directions of the rivers Spey, Don, and
Tay. These two regions--the North-west and South-east Highlands--are
clearly separated by the remarkable depression of Glenmore, which
extends through Loch Linnhe, Loch Lochy, and Loch Ness, and the further
extension of which towards the north-east is indicated by the straight
coast-line of the Moray Firth as far as Tarbat Ness. Now, this long
depression marks a line of fracture and displacement of very great
geological antiquity. The old plateau of the Highlands was fissured and
split in two--that portion which lay to the north-west sinking along
the line of fissure to a great but at present unascertained depth. Thus
the waters that flowed down the slopes of the north-west portion of the
broken plateau were dammed by the long wall of rock on the "up-cast,"
or south-east side of the fissure, and compelled to flow off to
north-east and south-west along the line of breakage. The erosion thus
induced sufficed in the course of time to hollow out Glenmore and all
the mountain-valleys that open upon it from the west.
The inclination of that portion of the fissured plateau which lay to
the south-east is indicated, as already remarked, by the trend of
the principal rivers. It was north-east in the Spey district, nearly
due east in the area drained by the Don, east and south-east in that
traversed by the Tay and its affluents, westerly and south-westerly in
the district lying east of Loch Linnhe.[C] Thus, a line drawn from Ben
Nevis through the Cairngorm and Ben Muich Dhui Mountains to Kinnaird
Point passes through the highest land in the South-east Highlands, and
probably indicates approximately the dominant portion of the ancient
plateau. North of that line the drainage is towards the Moray Firth;
east of it the rivers discharge to the North Sea; while an irregular
winding line, drawn from Ben Nevis eastward through the Moor of Rannoch
and southward to Ben Lomond, forms the water-parting between the North
Sea and the Atlantic, and doubtless marks another dominant area of the
old table-land.
[C] The geological reader hardly requires to be reminded that many
of the minor streams would have their courses determined, or greatly
modified, by the geological structure of the ground. Thus, such
streams often flow along the "strike" and other "lines of weakness,"
and similar causes, doubtless, influenced the main rivers during the
gradual excavation of their valleys.
That the valleys which discharge their water-flow north and east
to the Moray Firth and the North Sea have been excavated by rivers
and the allied agents of erosion, is sufficiently evident. All the
large rivers of that wide region are typical. They show the orthodox
three courses--namely, a torrential or mountain-track, a middle or
valley-track, and a lower or plain-track. The same is the case with
some of the rivers that flow east from the great north-and-south
water-parting of the North-west Highlands, as, for example, those that
enter the heads of Beauly Firth, Cromarty Firth, and Dornoch Firth.
Those, however, which descend to Loch Lochy and Loch Linnhe, and the
sea-lochs of Argyllshire, have no lower or plain-track. When we
cross the north-and-south water-parting of the North-west Highlands,
we find that many of the streams are destitute of even a middle or
valley-track. The majority are mere mountain-torrents when they reach
the sea. Again, on the eastern watershed of the same region, a large
number of the valleys contain lakes in their upper and middle reaches,
and this is the case also with not a few of the valleys that open upon
the Atlantic. More frequently, however, the waters flowing west pass
through no lakes, but enter the sea at the heads of long sea-lochs or
fiords. This striking contrast between the east and west is not due to
any difference in the origin of the valleys. The western valleys are as
much the result of erosion as those of the east. The present contrast,
in fact, is more apparent than real, and arises from the fact that
the land area on the Atlantic side has been greatly reduced in extent
by subsidence. The western fiords are merely submerged land-valleys.
Formerly the Inner and Outer Hebrides were united to themselves and the
mainland, the country of which they formed a part stretching west into
the Atlantic, as far probably as the present 100 fathoms line. Were
that drowned land to be re-elevated, each of the great sea-lochs would
appear as a deep mountain-valley containing one or more lake-basins of
precisely the same character as those that occur in so many valleys
on the eastern watershed. Thus we must consider all the islands lying
off the west coast of the Highlands, including the major portions of
Arran and Bute, as forming part and parcel of the Highland division of
Scotland. The presence of the sea is a mere accident; the old lands
now submerged were above its level during a very recent geological
period--a period well within the lifetime of the existing fauna and
flora.
The old table-land of which the Highlands and Islands are the denuded
and unsubmerged relics, is of vast geological antiquity. It was
certainly in existence, and had even undergone very considerable
erosion, before the Old Red Sandstone period, as is proved by the
fact that large tracts of the Old Red Sandstone formation are found
occupying hollows in its surface. Glenmore had already been excavated
when the conglomerates of the Old Red Sandstone began to be laid
down. Some of the low-lying maritime tracts of the Highland area in
Caithness, and the borders of the Moray Firth, are covered with the
sandstones of that age; and there is evidence to show that these
strata formerly extended over wide regions, from which they have since
been removed by erosion. The fact that the Old Red Sandstone deposits
still occupy such extensive areas in the north-east of the mainland,
and in Orkney, shows that the old table-land shelved away gradually
to north and east, and the same conclusion may be drawn, as we have
seen, from the direction followed by the main lines of the existing
drainage-system. We see, in short, in the table-land of the Highlands,
one of the oldest elevated regions of Europe--a region which has been
again and again submerged either in whole or in part, and covered with
the deposits of ancient seas and lakes, only to be re-elevated, time
after time, and thus to have those deposits in large measure swept away
from its surface by the long-continued action of running water and
other agents of denudation.
* * * * *
The Central Lowlands.--The belt of low-lying ground that separates the
Highlands from the Southern Uplands is, as we have seen, very well
defined. In many places the Uplands rise along its southern margin as
abruptly as the Highlands in the north. The southern margin coincides,
in fact, for a considerable distance (from Girvan to the base of the
Moorfoots) with a great fracture that runs in the same direction as the
bounding fracture or fault of the Highlands. The Central Lowlands may
be described, in a word, as a broad depression between two table-lands.
A glance at the map will show that the principal features of the
Lowlands have a north-easterly trend--the same trend, in fact, as the
bounding lines of the division. To this arrangement there are some
exceptions, the principal being the belt of hilly ground that extends
from the neighbourhood of Paisley, south-east through the borders of
Renfrewshire and Ayrshire, to the vicinity of Muirkirk. The major part
of the Lowlands is under 500 feet in height, but some considerable
portions exceed an elevation of 1000 feet, while here and there the
hills approach a height of 2000 feet--the two highest points (2352 and
2335 feet) being attained in Ben Cleugh, one of the Ochils, and in
Tinto. Probably the average elevation of the Lowland division does not
exceed 350 or 400 feet. Speaking generally, the belts of hilly ground,
and the more or less isolated prominences, are formed of more durable
rocks than are met with in the adjacent lower-lying tracts. Thus the
Sidlaws, the Ochil Hills, and the heights in Renfrewshire and Ayrshire,
are composed chiefly of more or less hard and tough volcanic rocks; and
when sandstones enter into the formation of a line of hills, as in the
Sidlaws, they generally owe their preservation to the presence of the
volcanic rocks with which they are associated. This is well illustrated
by the Lomond Hills in Fifeshire, the basal and larger portion of which
consists chiefly of somewhat soft sandstones, which have been protected
from erosion by an overlying sheet of hard basalt-rock. All the
isolated hills in the basin of the Forth are formed of knobs, bosses,
and sheets of various kinds of igneous rock, which are more durable
than the sandstones, shales, and other sedimentary strata by which they
are surrounded. Hence it is very evident that the configuration of the
Lowland tracts of Central Scotland is due to denudation. The softer
and more readily disintegrated rocks have been worn away to a greater
extent than the harder and less yielding masses.
Only in a few cases do the slopes of the hill-belts coincide with folds
of the strata. Thus, the northern flanks of the Sidlaws and the Ochils
slope towards the north-west, and this also is the general inclination
of the old lavas and other rocks of which those hills are composed.
The southern flanks of the same hill-belt slope in Fifeshire towards
the south-east--this being also the dip or inclination of the rocks.
The crest of the Ochils coincides, therefore, more or less closely,
with an anticlinal arch or fold of the strata. But when we follow the
axis of this arch towards the north-east into the Sidlaws, we find it
broken through by the Tay valley--the axial line running down through
the Carse of Gowrie to the north of Dundee. From the fact that many
similar anticlinal axes occur throughout the Lowlands, which yet give
rise to no corresponding features at the surface, we may conclude that
the partial preservation of the anticline of the Ochils and Sidlaws is
simply owing to the greater durability of the materials of which those
hills consist. Had the arch been composed of sandstones and shales it
would most probably have given rise to no such prominent features as
are now visible.
Another hilly belt, which at first sight appears to correspond roughly
to an anticlinal axis, is that broad tract of igneous rocks which
separates the Kilmarnock coal-field from the coal-fields of the Clyde
basin. But although the old lavas of that hilly tract slope north-east
and south-west, with the same general inclination as the surface, yet
examination shows that the hills do not form a true anticline. They
are built up of a great variety of ancient lavas and fragmental tuffs
or "ashes," which are inclined in many different directions. In short,
we have in those hills the degraded and sorely denuded fragments of an
ancient volcanic bank, formed by eruptions that began upon the bottom
of a shallow sea in early Carboniferous times, and subsequently became
sub-aërial. And there is evidence to show that after the eruptions
ceased the volcanic bank was slowly submerged, and eventually buried
beneath the accumulating sediments of later Carboniferous times.
The exposure of the ancient volcanic bank at the surface has been
accomplished by the denudation of the stratified masses which formerly
covered it, and its existence as a dominant elevation at the present
day is solely due to the fact that it is built up of more resistant
materials than occur in the adjacent low-lying areas. The Ochils and
the Sidlaws are of greater antiquity, but have a somewhat similar
history. Into this, however, it is not necessary to go.
The principal hills of the Lowlands form two interrupted belts,
extending north-east and south-west, one of them, which we may call the
Northern Heights, facing the Highlands, and the other, which may in
like manner be termed the Southern Heights, flanking the great Uplands
of the south. The former of these two belts is represented by the
Garvock Hills, lying between Stonehaven and the valley of the North
Esk; the Sidlaws, extending from the neighbourhood of Montrose to the
valley of the Tay at Perth; the Ochil Hills, stretching along the south
side of the Firth of Tay to the valley of the Forth at Bridge-of-Allan;
the Lennox Hills, ranging from the neighbourhood of Stirling to
Dumbarton; the Kilbarchan Hills, lying between Greenock and Ardrossan;
the Cumbrae Islands and the southern half of Arran; and the same line
of heights reappears in the south end of Kintyre. A well-marked hollow,
trough, or undulating plain of variable width, separates these Northern
Heights from the Highlands, and may be followed all the way from near
Stonehaven, through Strathmore, to Crieff and Auchterarder. Between the
valleys of the Earn and Teith this plain attains an abnormal height
(the Braes of Doune); but from the Teith, south-west by Flanders
Moss and the lower end of Loch Lomond to the Clyde at Helensburgh,
it resumes its characteristic features. It will be observed also
that a hollow separates the southern portion of Arran from the much
loftier northern or Highland area. The tract known as the Braes of
Doune, extending from Glen Artney south-east to Strath Allan, although
abutting upon the Highlands, is clearly marked off from that great
division by geological composition and structure, by elevation and
configuration. It is simply a less deeply eroded portion of the long
trough or hollow.
Passing now to the Southern Heights of the Lowlands, we find that these
form a still more interrupted belt than the Northern Heights, and that
they are less clearly separated by an intermediate depression from the
great Uplands which they flank. They begin in the north-east with the
isolated Garleton Hills, between which and the Lammermoors a narrow
low-lying trough or hollow appears. A considerable width of low ground
now intervenes before we reach the Pentland Hills, which are in like
manner separated from the Southern Uplands by a broad low-lying tract.
At their southern extremity, however, the Pentlands merge more or less
gradually into a somewhat broken and interrupted group of hills which
abut abruptly on the Southern Uplands, in the same manner as the
Braes of Doune abut upon the slate hills of the Highland borders. In
this region the greatest heights reached are in Tinto (2335 feet), and
Cairntable (1844 feet), and, at the same time, the hills broaden out
towards north-west, where they are continued by the belt of volcanic
rocks already described as extending between the coal-fields of the
Clyde and Kilmarnock. Although the Southern Heights abut so closely
upon the Uplands lying to the south, there is no difficulty in drawing
a firm line of demarcation between the two areas--geologically and
physically they are readily distinguished. No one with any eye for
form, no matter how ignorant he may be of geology, can fail to see
how strongly contrasted are such hills as Tinto and Cairntable with
those of the Uplands, which they face. The Southern Heights are again
interrupted towards the south-east by the valleys of the Ayr and the
Doon, but they reappear in the hills that extend from the Heads of Ayr
to the valley of the Girvan.
Betwixt the Northern and Southern Heights spread the broad Lowland
tracts that drain towards the Forth, together with the lower reaches
of the Clyde valley, and the wide moors that form the water-parting
between that river and the estuary of the Forth. The hills that occur
within this inner region of the Central Lowlands are usually more or
less isolated, and are invariably formed by outcrops of igneous rock.
Their outline and general aspect vary according to the geological
character of the rocks of which they are composed--some forming more
or less prominent escarpments like those of the Bathgate Hills and the
heights behind Burntisland and Kinghorn, others showing a soft rounded
contour like the Saline Hills in the west of Fifeshire. Of the same
general character as this inner Lowland region is the similar tract
watered by the Irvine, the Ayr, and the Doon. This tract, as we have
seen, is separated from the larger inner region lying to the east by
the volcanic hills that extend from the Southern Heights north-west
into Renfrewshire.
The largest rivers that traverse the Central Lowlands take their rise,
as might be expected, in the mountainous table-lands to the north and
south. Of these the principal are the North and South Esks, the Tay
and the Isla, the Earn, and the Forth, all of which, with numerous
tributaries, descend from the Highlands. And it will be observed
that they have breached the line of the Northern Heights in three
places--namely, in the neighbourhood of Montrose, Perth, and Stirling.
The only streams of importance coming north from the Southern Uplands
are the Clyde and the Doon, both of which in like manner have broken
through the Southern Heights. Now, just as the main water-flows of
the Highlands indicate the average slope of the ancient land-surface
before it was trenched and furrowed by the innumerable valleys that
now intersect it, so the direction followed by the greater rivers that
traverse the Lowlands mark out the primeval slopes of that area. One
sees at a glance, then, that the present configuration of this latter
division has been brought about by the erosive action of the principal
rivers and their countless affluents, aided by the sub-aërial agents
generally--rain, frost, ice, etc. The hills rise above the average
level of the ground, not because they have been ridged up from below,
but simply owing to the more durable nature of their component rocks.
That the Northern and Southern Heights are breached only shows that the
low grounds, now separating those heights from the adjacent Highlands
and Southern Uplands, formerly stood at a higher level, and so allowed
the rivers to make their way more or less directly to the sea. Thus,
for example, the long trough of Strathmore has been excavated out of
sandstones, the upper surface of which once reached a much greater
height, and sloped outwards from the Highlands across what is now the
ridge of the Sidlaw Hills. Here then, in the Central Lowlands, as in
the Highlands, true mountain- or hill-ranges are absent. But if we
are permitted to term any well-marked line or belt of high ground a
"range," then the Northern and Southern Heights of the Lowlands are
better entitled to be so designated than any series of mountains in the
Highlands.
* * * * *
The Southern Uplands.--The northern margin of this wide division having
already been defined, we may now proceed to examine the distribution
of its mountain-masses. Before doing so, however, it may be as well
to point out that considerable tracts in Tweeddale, Teviotdale, and
Liddesdale, together with the Cheviot Hills, do not properly belong
to the Southern Uplands. In fact, the Cheviots bear the same relation
to those Uplands as the Northern Heights do to the Highlands. Like
them they are separated by a broad hollow from the Uplands, which they
face--a hollow that reaches its greatest extent in Tweeddale, and
rapidly wedges out to south-west, where the Cheviots abut abruptly
on the Uplands. Even where this abrupt contact takes place, however,
the different configuration of the two regions would enable any
geologist to separate the one set of mountains from the other. But for
geographical purposes we may conveniently disregard these geological
contrasts, and include within the Southern Uplands all the area lying
between the Central Lowlands and the English Border.
If there are no mountains in the Highlands so grouped and arranged as
to be properly termed "ranges," this is not less true of the Southern
Uplands. Perhaps it is the appearance which those Uplands present when
viewed from the Central Lowlands that first suggested the notion that
they were ranges. They seem to rise like a wall out of the low grounds
at their base, and extend far as eye can reach in an approximately
straight line. It seems more probable, however, that our earlier
cartographers merely meant, by their conventional hill-shading, to mark
out definitely the water-partings. But to do so in this manner now,
when the large contour maps of the Ordnance Survey may be in any one's
hands, is inexcusable. A study of those maps, or, better still, a visit
to the tops of a few of the dominant points in the area under review,
will effectually dispel the idea that the Southern Uplands consist of a
series of ridges zigzagging across the country. Like the Highlands, the
area of the Southern Uplands is simply an old table-land, furrowed into
ravine and valley by the operation of the various agents of erosion.
Beginning our survey of these Uplands in the east, we encounter
first the Lammermoor Hills--a broad undulating plateau--the highest
elevations of which do not reach 2000 feet. West of this come the
Moorfoot Hills and the high grounds lying between the Gala and the
Tweed--a tract which averages a somewhat higher elevation--two points
exceeding 2000 feet in height. The next group of mountains we meet
is that of the Moffat Hills, in which head a number of important
rivers--the Tweed, the Yarrow, the Ettrick, and the Annan. Many points
in this region exceed 2000 feet, others approach 2500 feet; and some
reach nearly 3000 feet, such as Broad Law (2754 feet), and Dollar
Law (2680 feet). In the south-west comes the group of the Lowthers,
with dominant elevations of more than 2000 feet. Then follow the
mountain-masses in which the Nith, the Ken, the Cree, the Doon, and
the Girvan take their rise, many of the heights exceeding 2000 feet,
and a number reaching and even passing 2500 feet, the dominant point
being reached in the noble mountain-mass of the Merrick (2764 feet).
In the extreme south-west the Uplands terminate in a broad undulating
plateau, of which the highest point is but little over 1000 feet. All
the mountain-groups now referred to are massed along the northern
borders of the Southern Uplands. In the south-west the general surface
falls more or less gradually away towards the Solway--the 500 feet
contour line being reached at fifteen miles, upon an average, from
the sea-coast. In the extreme north-east the high grounds descend in
like manner into the rich low grounds of the Merse. Between these low
grounds and Annandale, however, the Uplands merge, as it were, into the
broad elevated moory tract that extends south-east, to unite with the
Cheviots--a belt of hills rising along the English Border to heights of
1964 feet (Peel Fell), and 2676 feet (the Cheviot).
The general configuration of the main mass of the Southern
Uplands--that is to say, the mountain-groups extending along the
northern portion of the area under review, from Loch Ryan to the coast
between Dunbar and St. Abb's Head--is somewhat tame and monotonous. The
mountains are flat-topped elevations, with broad, rounded shoulders
and smooth grassy slopes. Standing on the summits of the Higher hills,
one seems to be in the midst of a wide, gently undulating plain, the
surface of which is not broken by the appearance of any isolated peaks
or eminences. Struggling across the bogs and peat-mosses that cover
so many of those flat-topped mountains, the wanderer ever and anon
suddenly finds himself on the brink of a deep green dale. He discovers,
in short, that he is traversing an elevated undulating table-land,
intersected by narrow and broad trench-like valleys that radiate
outwards in all directions from the dominant bosses and swellings of
the plateau. The mountains, therefore, are merely broad ridges and
banks separating contiguous valleys; in a word, they are, like the
mountains of the Highlands, monuments of erosion, which do not run in
linear directions, but form irregular groups and masses.
The rocks that enter into the formation of this portion of the Southern
Uplands have much the same character throughout. Consequently there
is less variety of contour and colour than in the Highlands. The
hills are not only flatter atop, but are much smoother in outline,
there being a general absence of those beetling crags and precipices
which are so common in the Highland regions. Now and again, however,
the mountains assume a rougher aspect. This is especially the case
with those of Carrick and Galloway, amongst which we encounter a
wildness and grandeur which are in striking contrast to the gentle
pastoral character of the Lowthers and similar tracts extending along
the northern and higher parts of the Southern Uplands. Descending to
details, the geologist can observe also modifications of contour even
among those monotonous rounded hills. Such modifications are due to
differences in the character of the component rocks, but they are
rarely so striking as the modifications that arise from the same cause
in the Highlands. To the trained eye, however, they are sufficiently
manifest, and upon a geologically coloured map, which shows the
various belts of rock that traverse the Uplands from south-west to
north-east, it will be found that the mountains occurring within
each of those separate belts have certain distinctive features. Such
features, however, cannot be depicted upon a small orographical map.
The separation of those mountains into distinct ranges, by reference
to their physical aspect, is even less possible here than in the
Highlands. Now and again, bands of certain rocks, which are of a
more durable character than the other strata in their neighbourhood,
give rise to pronounced ridges and banks, while hollows and valleys
occasionally coincide more or less closely with the outcrops of the
more readily eroded strata; but such features are mere minor details
in the general configuration of the country. The courses of brooks
and streams may have been frequently determined by the nature and
arrangement of the rocks, but the general slope of the Uplands and the
direction of the main lines of water-flow are at right angles to the
trend of the strata, and cannot therefore have been determined in that
way. The strata generally are inclined at high angles--they occur, in
short, as a series of great anticlinal arches and synclinal curves, but
the tops of the grand folds have been planed off, and the axes of the
synclinal troughs, so far from coinciding with valleys, very often run
along the tops of the highest hills. The foldings and plications do
not, in a word, produce any corresponding undulations of the surface.
Mention has been made of the elevated moory tracts that serve to
connect the Cheviots with the loftier Uplands lying to north-west.
The configuration of these moors is tamer even than that of the
regions just described, but the same general form prevails from
the neighbourhood of the Moffat Hills to the head-waters of the
Teviot. There, however, other varieties of rock appear, and produce
corresponding changes in the aspect of the high grounds. Not a few of
the hills in this district stand out prominently. They are more or less
pyramidal and conical in shape, being built up of sandstones often
crowned atop with a capping of some crystalline igneous rock, such as
basalt. The Maiden Paps, Leap Hill, Needs Law, and others are examples.
The heights draining towards Liddesdale and lower reaches of Eskdale,
composed chiefly of sandstones, with here and there intercalated
sheets of harder igneous rock, frequently show escarpments and
terraced outlines, but have a general undulating contour; and similar
features are characteristic of the sandstone mountains that form the
south-west portion of the Cheviots. Towards the north-east, however,
the sandstones give place to various igneous rocks, so that the hills
in the north-east section of the Cheviots differ very much in aspect
and configuration from those at the other extremity of the belt. They
have a more varied and broken outline, closely resembling many parts of
the Ochils and other portions of the Northern and Southern Heights of
the Central Lowlands.
The low-lying tracts of Roxburghshire and the Merse, in like manner,
present features which are common to the inner region of the Central
Lowlands. Occasional ridges of hills rise above the general level of
the land, as at Smailholm and Stitchell to the north of Kelso, while
isolated knolls and prominences--some bald and abrupt, others smooth
and rounded--help to diversify the surface. Bonchester Hill, Rubers
Law, the Dunian, Penielheugh, Minto Hills, and the Eildons may be
mentioned as examples. All of these are of igneous origin, some being
mere caps of basalt resting upon a foundation of sandstone, while
others are the stumps of isolated volcanoes.
In the maritime tracts of Galloway the low grounds repeat, on a smaller
scale, the configuration of the lofty Uplands behind, for they are
composed of the same kinds of rock. Their most remarkable feature is
the heavy mountain-mass of Criffel, rising near the mouth of the Nith
to a height of 1800 feet.
Everywhere, therefore, throughout the region of the Southern Uplands,
in hilly and low-lying tracts alike, we see that the land has been
modelled and contoured by the agents of erosion. We are dealing, as
in the Highlands, with an old table-land, in which valleys have been
excavated by running water and its helpmates. Nowhere do we encounter
any linear banks, ridges, or ranges as we find described in the
class-books, and represented upon many general maps of the country.
In one of those manuals we read that in the southern district "the
principal range of mountains is that known as the Lowther Hills, which
springs off from the Cheviots, and, running in a zigzag direction to
the south-west, terminates on the west coast near Loch Ryan." This
is quite true, according to many common maps, but unfortunately the
"range" exists upon those maps and nowhere else. The zigzag line
described is not a range of mountains, but a water-parting, which is
quite another matter.
The table-land of the Southern Uplands, like that of the Highlands, is
of immense antiquity. Long before the Old Red Sandstone period, it had
been furrowed and trenched by running water. Of the original contour of
its surface, all we can say is that it formed an undulating plateau,
the general slope of which was towards south-east. This is shown by the
trend of the more important rivers, such as the Nith and the Annan,
the Gala and the Leader; and by the distribution of the various strata
pertaining to the Old Red Sandstone and later geological periods. Thus,
strata of Old Red Sandstone and Carboniferous age occupy the Merse
and the lower reaches of Teviotdale, and extend up the valleys of the
Whiteadder and the Leader into the heart of the Silurian Uplands.
In like manner Permian sandstones are well developed in the ancient
hollows of Annandale and Nithsdale. Along the northern borders of the
Southern Uplands we meet with similar evidence to show that even as
early as Old Red Sandstone times the old plateau, along what is now its
northern margin, was penetrated by valleys that drained towards the
north. The main drainage, however, then as now, was directed towards
south-east.
Many geological facts conspire to show that the Silurian table-land of
these Uplands has been submerged, like the Highlands, in whole or in
part. This happened at various periods, and each time the land went
down it received a covering of newer accumulations--patches of which
still remain to testify to the former extent of the submergences. From
the higher portions of the Uplands those accumulations have been almost
wholly swept away, but they have not been entirely cleared out of the
ancient valleys. They still mantle the borders of the Silurian area,
particularly in the north-east, where they attain a great thickness
in the moors of Liddesdale and the Cheviot Hills. The details of the
evolution of the whole area of the Southern Uplands form an interesting
study, but this pertains rather to Geology than to Physical Geography.
It is enough, from our present point of view, to be assured that the
main features of the country were chalked out, as it were, at a very
distant geological period, and that all the infinite variety in the
relief of our land has been brought about directly, not by titanic
convulsions and earth-movements, but by the long-continued working of
rain and rivers--of frost and snow and ice, supplemented from time to
time by the action of the sea.
The physical features more particularly referred to in this paper are
of course only the bolder and more prominent contours--those namely
which can be expressed with sufficient accuracy upon sheets of such
a size as the accompanying orographical map of Scotland (Plate I.).
With larger maps considerably more detail can be added, and many
characteristic and distinguishing features will appear according to
the care with which such maps are drawn. In the case of the Ordnance
Survey map, on the scale of 1 inch to a mile, the varying forms of the
surface are so faithfully delineated as frequently to indicate to a
trained observer the nature of the rocks and the geological structure
of the ground. The artists who sketched the hills must indeed have had
good eyes for form. So carefully has their work been done, that it is
often not difficult to distinguish upon their maps hills formed of such
rocks as sandstone from those that are composed of more durable kinds.
The individual characteristics of mountains of schist, of granite, of
quartz-rock, of slate, are often well depicted: nay, even the varieties
of igneous rock which enter into the formation of the numerous hills
and knolls of the Lowlands can frequently be detected by the features
which the artists have so intelligently caught. Another set of features
which their maps display are those due to glaciation. These are
admirably brought out, even down to the smallest details. A glance at
such maps as those of Teviotdale and the Merse, for example, shows at
once the direction taken by the old _mer de glace_. The long parallel
flutings of the hill-slopes, _roches moutonnées_, projecting knolls and
hills with their "tails," the great series of banks and ridges of stony
clay which trend down the valley of the Tweed--these, and many more
details of interest to specialists, are shown upon the maps. All over
Scotland similar phenomena are common, and have been reproduced with
marvellous skill on the shaded sheets issued by the Ordnance Survey.
And yet the artists were not geologists. The present writer is glad
of this opportunity of recording his obligations to those gentlemen.
Their faithful delineations of physical features have given him many
valuable suggestions, and have led up to certain observations which
might otherwise not have been made.
III.
Mountains: Their Origin, Growth, and Decay.[D]
[D] _Scottish Geographical Magazine_, vol. ii., 1886.
Mountains have long had a fascination for lovers of nature. Time was,
however, when most civilised folk looked upon them with feelings akin
to horror; and good people, indeed, have written books to show that
they are the cursed places of the earth--the ruin and desolation of
their gorges and defiles affording indubitable proof of the evils
which befell the world when man lapsed from his primitive state of
innocence and purity. All this has changed. It is the fashion now to
offer a kind of worship to mountains; and every year their solitudes
are invaded by devotees--some, according to worthy Meg Dods, "rinning
up hill and down dale, knapping the chuckie-stanes to pieces wi'
hammers, like sae mony roadmakers run daft--to see, as they say, how
the warld was made"--others trying to transfer some of the beauty
around them to paper or canvas--yet others, and these perhaps not the
least wise, content, as old Sir Thomas Browne has it, "to stare about
with a gross rusticity," and humbly thankful that they are beyond the
reach of telegrams, and see nothing to remind them of the _fumun et
opes strepitumque Romæ_. But if the sentiment with which mountains are
regarded has greatly changed, so likewise have the views of scientific
men as to their origin and history. Years ago no one doubted that all
mountains were simply the result of titanic convulsions. The crust of
the earth had been pushed up from below, tossed into great billows,
shivered and shattered--the mountains corresponding to the crests of
huge earth-waves, the valleys to the intervening depressions, or to
gaping fractures and dislocations. This view of the origin of mountains
has always appeared reasonable to those who do not know what is meant
by geological structure, and in some cases it is pretty near the
truth. A true mountain-chain, like that of the Alps, does indeed owe
its origin to gigantic disturbances of the earth's crust, and in such
a region the larger features of the surface often correspond more or
less closely with the inclination of the underlying rocks. But in many
elevated tracts, composed of highly disturbed and convoluted strata,
no such coincidence of surface-features and underground structure can
be traced. The mountains do not correspond to great swellings of the
crust--the valleys neither lie in trough-shaped strata, nor do they
coincide with gaping fractures. Again, many considerable mountains are
built up of rocks which have not been convoluted at all, but occur in
approximately horizontal beds. Evidently, therefore, some force other
than subterranean action must be called upon to explain the origin of
many of the most striking surface-features of the land.
Every geologist admits--it is one of the truisms of his science--that
corrugations and plications are the result of subterranean action. Nor
does any one deny that when a true mountain-chain was first upheaved
the greater undulations of the folded strata probably gave rise to
similar undulations at the surface. Some of the larger fractures and
dislocations might also have appeared at the surface and produced
mural precipices. So long a time, however, has elapsed since the
elevation of even the youngest mountain-chains of the globe that the
sub-aërial agents of erosion--rain, frost, rivers, glaciers, etc.--have
been enabled greatly to modify their primeval features. For these
mountains, therefore, it is only partially true that their present
slopes coincide with those of the underlying strata. Such being the
case with so young a chain as the Alps, we need not be surprised to
meet with modifications on a still grander scale in mountain-regions of
much greater antiquity. In many such tracts the primeval configuration
due to subterranean action has been entirely remodelled, so that
hills now stand where deep hollows formerly existed, while valleys
frequently have replaced mountains. And this newer configuration is the
direct result of erosion, guided by the mineralogical composition and
structural peculiarities of the rocks.
It is difficult, or even impossible, for one who is ignorant of
geological structure to realise that the apparently insignificant
agents of erosion have played so important a _rôle_ in the evolution
of notable earth-features. It may be well, therefore, to illustrate
the matter by reference to one or two regions where the geological
structure is too simple to be misunderstood. The first examples I shall
give are from tracts of horizontal strata. Many readers are doubtless
aware of the fact that our rock-masses consist for the most part of the
more or less indurated and compacted sediments of former rivers, lakes,
and seas. Frequently those ancient water-formed rocks have been very
much altered, so as even sometimes to acquire a crystalline character.
But it is enough for us now to remember that the crust of the globe, so
far as that is accessible to observation, is built up mostly of rocks
which were originally accumulated as aqueous sediments. Such being the
case, it is obvious that our strata of sandstone, conglomerate, shale,
limestone, etc., must at first have been spread out in approximately
horizontal or gently inclined sheets or layers. We judge so from what
we know of sediments which are accumulating at present. The wide
flats of our river valleys, the broad plains that occupy the sites of
silted-up lakes, the extensive deltas of such rivers as the Nile and
the Po, the narrow and wide belts of low-lying land which within a
recent period have been gained from the sea, are all made up of various
kinds of sediment arranged in approximately horizontal layers. Now,
over wide regions of the earth's surface the sedimentary strata still
lie horizontally, and we can often tell at what geological period they
became converted into dry land. Thus, for example, we know that the
elevated plateau through which the river Colorado flows is built up
of a great series of nearly horizontal beds of various sedimentary
deposits, which reach a thickness of many thousand feet. It is
self-evident that the youngest strata must be those which occur at the
surface of the plateau, and they, as we know, are of lacustrine origin
and belong to the Tertiary period. Now, American geologists have shown
that since that period several thousands of feet of rock-materials have
been removed from the surface of that plateau--the thickness of rock so
carried away amounting in some places to nearly 10,000 feet. Yet all
that prodigious erosion has been effected since early Tertiary times.
Indeed, it can be proved that the excavation of the Grand Cañon of
the Colorado, probably the most remarkable river-trench in the world,
has been accomplished since the close of the Tertiary period, and is
therefore a work of more recent date than the last great upheaval
of the Swiss Alps. The origin of the cañon is self-evident--it is a
magnificent example of river-erosion, and the mere statement of its
dimensions gives one a forcible impression of the potency of sub-aërial
denudation. The river-cutting is about 300 miles long, 11 or 12 miles
broad, and varies from 3000 to 6000 feet in depth.
Take another example of what denuding agents have done within a recent
geological period. The Faröe Islands, some twenty in number, extend
over an area measuring about 70 miles from south to north, and nearly
50 miles from west to east. These islands are composed of volcanic
rocks--beds of basalt with intervening layers of fine fragmental
materials, and are obviously the relics of what formerly was one
continuous plateau, deeply trenched by valleys running in various
directions. Subsequent depression of the land introduced the sea to
these valleys, and the plateau was then converted into a group of
islands, separated from each other by narrow sounds and fiords. Were
the great plateau through which the Colorado flows to be partially
submerged, it would reproduce on a larger scale the general phenomena
presented by this lonely island-group of the North Atlantic. The
flat-topped "buttes" and "mesas," and the pyramidal mountains of
the Colorado district would form islands comparable to those of the
Faröes. Most of the latter attain a considerable elevation above the
sea--heights of 1700, 2000, 2500, and 2850 feet being met with in
several of the islands. Indeed, the average elevation of the land in
this northern archipelago can hardly be less than 900 feet. The deep
trench-like valleys are evidently only the upper reaches of valleys
which began to be excavated when the islands formed part and parcel
of one and the same plateau--the lower reaches being now occupied by
fiords and sounds. It is quite certain that all these valleys are the
work of erosion. One can trace the beds of basalt continuously across
the bottoms, and be quite sure that the valleys are not gaping cracks
or fractures. Now, as the strata are approximately horizontal, it is
obvious that the hollows of the surface have nothing whatever to do
with undulations produced by earth-movements. The sub-aërial erosion
of the islands has resulted in the development of massive flat-topped
and pyramidal mountains. These stand up as eminences simply because the
rock-material which once surrounded them has been gradually broken up
and carried away. Nothing can well be more impressive to the student
of physical geology than the aspect presented by these relics of
an ancient plateau (Plate II. Fig. 1). Standing on some commanding
elevation, such as Nakkin in Suderöe, one sees rising before him great
truncated pyramids--built up of horizontal beds of basalt rising tier
above tier--the mountains being separated from each other by wide and
profound hollows, across which the basalt-beds were once continuous.
Owing to the parallel and undisturbed position of the strata, it is
not hard to form an estimate of the amount of material which has been
removed during the gradual excavation of the valleys. In order to do so
we have simply to measure the width, depth, and length of the valleys.
Thus in Suderöe, which is 19 miles long and 6 miles broad, the bottoms
of the valleys are 1000 feet at least below the tops of the mountains,
and some of the hollows in question are a mile in width. Now, the
amount of rock worn away from this one little island by sub-aërial
erosion cannot be less than that of a mass measuring 10 miles in
length by 6 miles in breadth, and 800 feet in thickness. And yet the
Faröe Islands are composed of rocks which had no existence when the
soft clays, etc., of the London Basin were being accumulated. All the
erosion referred to has taken place since the great upheaval of the
Eocene strata of the Swiss Alps.
But if the evidence of erosion be so conspicuous in regions composed
of horizontal strata, it is not less so in countries where the rocks
are inclined at various angles to the horizon. Indeed, the very fact
that inclined strata crop out at the surface is sufficient evidence of
erosion. For it is obvious that these outcrops are merely the truncated
ends of beds which must formerly have had a wider extension. But while
the effects produced by the erosion of horizontal strata are readily
perceived by the least-informed observer, it requires some knowledge
of geological structure to appreciate the denudation of curved or
undulating strata. And yet there is really no mystery in the matter.
All we have to do is by careful observation to ascertain the mode of
arrangement of the rocks--this accomplished, we have no difficulty
in estimating the minimum erosion which any set of strata may have
experienced. An illustration may serve to make this plain. Here, for
example, is a section across a region of undulating strata (Fig. 2).
Let the line _A B_ represent the surface of the ground, and _C D_ be
any datum line--say, the sea-level. An observer at _A_, who should
walk in the direction of _B_, would cross successively eight outcrops
of coal; and, were he incapable of reading the geological structure
of the ground, he might imagine that he had come upon eight separate
coal-seams. A glance at the section, however, shows that in reality he
had met with only two coals, and that the deceptive appearances, which
might be misread by an incautious observer, are simply the result of
denudation. In this case the tops of a series of curved or arched beds
have been removed (as at _E_), and, by protracting the lines of the
truncated beds until they meet, we can estimate the minimum amount of
erosion they have sustained. Thus, if the strata between _o_ and _p_
be 300 feet thick, it is self-evident that a somewhat greater thickness
of rock must have been removed from the top of the anticlinal arch or
"saddleback" at _E_.
Again, let us draw a section across strata which have been fractured
and dislocated, and we shall see how such fractures likewise enable us
to estimate the minimum amount of erosion which certain regions have
experienced. In Fig. 3 we have a series of strata containing a bed of
limestone _L_, and a coal-seam _a_. The present surface of the ground
is represented by the line _A B_. At _F_ the strata are traversed by a
fault or dislocation--the beds being thrown down for say 500 feet on
the low side of the fault--so that the coal at _a^2_ occurs now at a
depth of 500 feet below its continuation at _a^1_. At the surface of
the ground there is no inequality of level--the beds overlying the coal
(_a²_) having been removed by denudation. Were the missing rocks to
be replaced, they would occupy the space contained within the dotted
lines above the present surface _A B_. Such dislocations are of common
occurrence in our coal-fields, and it is not often that they give rise
to any features at the surface. We may thus traverse many level or
gently-undulating tracts, and be quite unconscious of the fact that
geologically we have frequently leaped up or dropped down for hundreds
of feet in a single step. Nay, some Scottish streams and rivers flow
across dislocations by which the strata have been shifted up or down
for thousands of feet, and in some places one can have the satisfaction
of sitting upon rocks which are geologically 3000 yards below or
above those on which he rests his feet. In other words, thousands of
feet of strata have been removed by denudation from the high sides of
faults. These, as I have said, often give rise to no feature at the
surface; but, occasionally, when "soft" rocks have been shifted by
dislocations, and brought against "hard" rocks, the latter, by better
resisting denudation than the former, cause a more or less well-marked
feature at the surface, and thus betray the presence of a fault to the
geologist. The phenomena presented by faults, therefore, are just as
eloquent of denudation as is the truncated appearance of our strata;
and only after we have carefully examined the present extension and
mutual relations of our rock-masses, their varied inclination, and
the size of the dislocations by which they are traversed, can we
properly appreciate the degree of erosion which they have sustained.
Before we are entitled to express any opinion as to the origin of
the surface-features of a country, we must first know its geological
structure. Until we have attained such knowledge, all our views as
to the origin of mountains are of less value than the paper they are
written upon.
I have spoken of the evidence of denudation which we find in our
truncated and dislocated rock-masses; there is yet another line of
evidence which I may very shortly point out. As every one knows, there
exist in this and many other countries enormous masses of igneous
rocks, which have certainly been extruded from below. Now, some of
these rocks, such as granite, belong to what is called the _plutonic_
class of rocks; they are of deep-seated origin--that is to say, they
never were erupted at the surface, but cooled and consolidated at great
depths in the earth's crust. I need not go into any detail to show
that this is the case--it is a conclusion based upon incontrovertible
facts, and accepted by every practical geologist. When, therefore, we
encounter at the actual surface of the earth great mountain-masses of
granite, we know that in such regions enormous denudation has taken
place. The granite appears at the surface simply because the thick
rock-masses under which it solidified have been gradually removed by
erosion.
The facts which I have now briefly passed in review must convince us
that erosion is one of the most potent factors with which the geologist
has to deal. We have seen what it has been able to effect in certain
tracts composed of strata which date back to a recent geological
period, such as the plateau of the Colorado and the pyramidal mountains
of the Faröe Islands. If in regions built up of strata so young as
the rocks of those tracts the amount of erosion be so great, we may
well expect to meet with evidence of much more extensive denudation in
regions which have been subjected for enormously longer periods to the
action of the eroding agents.
The study of geological structure, or the architecture of the earth's
crust, has enabled us to group all mountains under these three
principal heads:--
1. _Mountains of Accumulation._
2. _Mountains of Elevation._
3. _Mountains of Circumdenudation._
1. Mountains of Accumulation.--Volcanoes may be taken as the type
of this class of mountains. These are, of course, formed by the
accumulation of igneous materials around the focus or foci of eruption,
and their mode of origin is so generally understood, and, indeed,
so obvious, that I need do no more than mention them. Of course,
they are all subject to erosion, and many long-extinct volcanoes are
highly denuded. Some very ancient ones, as those of our own country,
have been so demolished that frequently all that remains are the now
plugged-up pipes or flues through which the heated materials found a
passage to the surface--all those materials, consisting of lavas and
ashes, having in many cases entirely disappeared. In former times
volcanic eruptions often took place along the line of an extensive
fissure--the lava, instead of being extruded at one or more points,
welled-up and overflowed along the whole length of the fissure, so as
to flood the surrounding regions. And this happening again and again,
vast plateaux of igneous rock came to be built up, such as those of the
Rocky Mountains, Iceland, the Faröes, Antrim and Mull, Abyssinia and
the Deccan. These are called _plateaux of accumulation_ (see Fig. 1),
and all of them are more or less highly denuded, so that in many cases
the plateaux have quite a mountainous appearance. Of course, plateaux
of accumulation are not always formed of igneous rocks. Any area of
approximately horizontal strata of aqueous origin, rising to a height
of a thousand feet or more above the sea, would come under this class
of plateau--the plateau of the Colorado being a good example. Although
that plateau is of recent origin, yet its surface, as we have seen,
has been profoundly modified by superficial erosion; and this is true
to a greater extent of plateaux which have been much longer exposed
to denudation. It is obvious that even mountains and plateaux of
accumulation often owe many of their present features to the action of
the surface-agents of change.
2. Mountains of Elevation.--We have seen that the strata which enter
most largely into the composition of the earth's crust, so far as that
is open to observation, consist of rocks which must originally have
been disposed in horizontal or approximately horizontal layers. But,
as every one knows, the stratified rocks are not always horizontally
arranged. In Scotland they rarely are so. On the contrary, they are
inclined at all angles from the horizon, and not infrequently they even
stand on end. Moreover, they are often traversed by dislocations, large
and small. No one doubts that these tilted and disturbed rocks are
evidence of wide-spread earth-movements. And it has been long known to
geologists that such movements have happened again and again in this
and many other countries where similar disturbed strata occur. Some of
these movements, resulting in the upheaval of enormous mountain-masses,
have taken place within comparatively recent geological times. Others
again date back to periods inconceivably remote. The Pyrenees, the
Alps, the Caucasus, the Himalaya, which form the back-bone of Eurasia,
are among the youngest mountains of the globe. The Highlands of
Scotland and Scandinavia are immeasurably more ancient; they are, in
point of fact, the oldest high grounds in Europe, nor are there any
mountain-masses elsewhere which can be shown to be older. But while the
Alps and other recent mountains of elevation still retain much of their
original configuration, not a vestige of the primeval configuration of
our own Highlands has been preserved; their present surface-features
have no direct connection with those which must have distinguished them
in late Silurian times. Our existing mountains are not, like those of
the Alps, mountains of elevation.
The structure of a true mountain-chain is frequently very complicated,
but the general phenomena can be readily expressed in a simple diagram.
Let Fig. 5 be a section taken across a mountain-chain, _i.e._ at right
angles to its trend or direction. The dominant point of the chain is
shown at _B_, while _A_ and _C_ represent the low grounds. Now, an
observer at _A_, advancing towards _B_, would note that the strata, at
first horizontal, would gradually become undulating as he proceeded on
his way--the undulations getting always more and more pronounced. He
would observe, moreover, that the undulations, at first symmetrical,
as at _a_, would become less so as he advanced--one limb of an arch
or _anticline_, as it is termed, being inclined at a greater angle
than the other, as at _b_. Approaching still nearer to =B=, the arches
or anticlines would be seen eventually to bend over upon each other,
so as to produce a general dip or inclination of the strata towards
the central axis of the chain. Crossing that axis (_B_), and walking
in the direction of the low grounds (_C_), the observer would again
encounter the same structural arrangement, but of course in reverse
order. Thus, in its simplest expression, a true mountain-chain consists
of strata arranged in a series of parallel undulations--the greater
mountain ridges and intervening hollows corresponding more or less
closely to the larger undulations and folds of the strata. Now, could
these plicated strata be pulled out, could the folds and reduplications
be smoothed away, so as to cause the strata to assume their original
horizontal position, it is obvious that the rocks would occupy a
greater superficial area. We see, then, that such a mountain-chain
must owe its origin to a process of tangential or lateral thrusting
and crushing. The originally horizontal strata have been squeezed
laterally, and have yielded to the force acting upon them by folding
and doubling up. It seems most probable that the larger contortions
and foldings which are visible in all true mountain-chains, owe their
origin to the sinking down of the earth's crust upon the cooling and
contracting nucleus. During such depressions of the crust the strata
are necessarily subjected to enormous lateral compression; they are
forced to occupy less space at the surface, and this they can only do
by folding and doubling-back upon themselves. If the strata are equally
unyielding throughout a wide area, then general undulation may ensue;
but should they yield unequally, then folding and contortion will take
place along one or more lines of weakness. In other words, the pressure
will be relieved by the formation of true mountain-chains. Thus,
paradoxical as it may seem, the loftiest mountains of the globe bear
witness to profound depression or subsidence of the crust. The Andes,
for example, appear to owe their origin to the sinking down of the
earth's crust under the Pacific; and so in like manner the Alps would
seem to have been ridged up by depression of the crust in the area of
the Mediterranean. Mountain-chains, therefore, are true wrinkles in the
crust of the earth; they are lines of weakness along which the strata
have yielded to enormous lateral pressure.
A glance at the geological structure of the Alps and the Jura shows us
that these mountains are a typical example of such a chain; they are
mountains of elevation. In the Jura the mountains form a series of long
parallel ridges separated by intervening hollows; and the form or shape
of the ground coincides in a striking manner with the foldings of the
strata. In these mountains we see a succession of symmetrical flexures,
the beds dipping in opposite directions at the same angle from the axis
of each individual anticline. There each mountain-ridge corresponds
to an _anticline_, and each valley to a _syncline_, or trough-shaped
arrangement of strata. But as we approach the Alps the flexures become
less and less symmetrical, until in the Alps themselves the most
extraordinary convolutions and intricate plications appear, the strata
being often reversed or turned completely upside down.
Though it is true that the slopes of this great mountain-chain
not infrequently correspond more or less closely to the slope or
inclination of the underlying rocks, it must not be supposed that this
correspondence is often complete. Sometimes, indeed, we find that the
mountains, so far from coinciding with anticlines, are in reality
built up of synclinal or basin-shaped strata; while in other cases
deep and broad valleys run along the lines of anticlinal axes (Fig.
6). All this speaks to enormous erosion. A study of the geological
structure of the Alps demonstrates that thousands of feet of rock have
been removed from those mountains since the time of their elevation. A
section drawn across any part of the chain would show that the strata
have been eroded to such an extent, and the whole configuration so
profoundly modified, that it is often difficult, or even impossible,
to tell what may have been the original form of the surface when the
chain was upheaved. And yet the Alps, it must be remembered, are of
comparatively recent age, some of their highly-confused and contorted
rocks consisting of marine strata which are of no greater antiquity
than the incoherent clays and sands of the London Tertiary basin.
Now, when we reflect upon the fact that, in the case of so young a
mountain-chain, the configuration due to undulations of the strata
has been so greatly modified, and even in many places obliterated, it
is not hard to believe that after sufficient time has elapsed--after
the Alps have existed for as long a period, say, as the mountains
of middle Germany--every mountain formed of anticlinal strata shall
have disappeared, and those synclines which now coincide with valleys
shall have developed into hills. The reader who may have paid little
or no attention to geological structure and its influence upon the
form of the ground, will probably think this a strange and extravagant
statement; yet I hope to show presently that it is supported by all
that we know of regions of folded strata which have been for long
periods of time subjected to denudation.
* * * * *
3. Mountains of Circumdenudation.--In countries composed of undulating
and folded strata which have been for long ages exposed to the
action of eroding agents, the ultimate form assumed by the ground
is directly dependent on the character of the rocks, and the mode
of their arrangement. The various rock-masses which occur in such a
neighbourhood as Edinburgh, for example, differ considerably in their
power of resisting denudation. Hence the less readily eroded rocks
have come in time to form hills of less or greater prominence. Such
is the case with the Castle Rock, Corstorphine Hill, the Braids, the
Pentlands, etc. These hills owe their existence, as such, to the fact
that they are composed of more enduring kinds of rock than the softer
sandstones and shales by which they are surrounded, and underneath
which they were formerly buried to great depths. Some hills, again,
which are for the most part built up of rocks having the same character
as the strata that occur in the adjacent low grounds, stand up as
prominences simply because they have been preserved by overlying caps
or coverings of harder rocks--rocks which have offered a stronger
resistance to the action of the denuding agents. The Lomond Hills are
good examples. Those hills consist chiefly of sandstones which have
been preserved from demolition by an overlying sheet of basalt-rock.
But the mode in which rocks are arranged is a not less important factor
in determining the shape which the ground assumes under the action of
the agents of erosion. Thus, as we have already seen, flat-topped,
pyramidal mountains, and more or less steep-sided or trench-like
valleys, are characteristic features in regions of horizontal strata.
When strata dip or incline in one general direction, then we have a
succession of escarpments or dip-slopes, corresponding to the outcrops
of harder or less readily eroded beds, and separated from each other
by long valleys, hollows, or undulating plains, which have the same
trend as the escarpments (Fig. 7). This kind of configuration is
well exemplified over a large part of England. The general dip or
inclination of the Mesozoic or Secondary strata throughout that
country, between the shores of the North Sea and the English Channel,
is easterly and south-easterly--so that the outcrops of the more
durable strata form well-defined escarpments that face the west and
north-west, and can be followed almost continuously from north to
south. Passing from the Malvern Hills in a south-easterly direction,
we traverse two great escarpments--the first coinciding with the
outcrop of the Oolite, and forming the Cotswold Hills; and the second
corresponding to the outcrop of the Chalk, and forming the Chiltern
Hills. The plains and low undulating tracts that separate these
escarpments mark the outcrops of more yielding strata--the low grounds
that intervene between the Cotswolds and the Malvern Hills being
composed of Liassic and Triassic clays and sandstones. In Scotland
similar escarpments occur, but owing to sudden changes of the dip,
and various interruptions of the strata, the Scottish escarpments are
not so continuous as those of the sister-country. Many of the belts
of hilly ground in the Scottish Lowlands, however, exemplify the
phenomena of escarpment and dip-slope. Thus, the Sidlaws in Forfarshire
consist of a series of hard igneous rocks and interbedded sandstones
and flags--the outcrops of which form a succession of escarpments with
intervening hollows. The same appearances recur again and again all
over the Lowlands. Wherever, indeed, any considerable bed of hard rock
occurs in a series of less enduring strata--the outcrop of the harder
rock invariably forms a well-marked feature or escarpment. As examples,
I may refer to Salisbury Crags, Craiglockhart Hill, Dalmahoy Crags, the
Bathgate Hills, King Alexander's Crag, etc. All these are conspicuous
examples of the work of denudation--for it can be demonstrated
that each of these rock-masses was at one time deeply buried under
sandstones and shales, and they now crop out at the surface, and form
prominent features simply because the beds which formerly covered and
surrounded them have been gradually removed.
From what has now been said it will be readily understood that in
regions composed of strata the inclination or dip of which is not
constant but continually changing in direction, the surface-features
must be more or less irregular. If the strata dip east the outcrops
of the harder beds will form escarpments facing the west, and the
direction of the escarpments will obviously change with the direction
of the dip. Undulating strata of variable composition will, in short,
give rise to an undulating surface, but the superficial undulations
will not coincide with those of the strata. On the contrary, in
regions consisting of undulating strata of diverse consistency the
hills generally correspond with synclinal troughs--or, in other words,
trough-shaped strata tend to form hills; while, on the other hand,
arch-shaped or anticlinal strata most usually give rise to hollows
(see Fig. 2). This remarkable fact is one of the first to arrest the
attention of every student of physical geology, and its explanation
is simple enough. An anticlinal arrangement of strata is a weak
structure--it readily succumbs to the attacks of the denuding agents; a
synclinal arrangement on the contrary, is a strong structure, which is
much less readily broken up. Hence it is that in all regions which have
been exposed for prolonged periods to sub-aërial denudation synclinal
strata naturally come to form hills, and anticlinal strata valleys or
low grounds. In the case of a mountain-chain so recently elevated as
that of the Alps, the mountain-ridges, as we have seen, often coincide
roughly with the greater folds of the strata. Such anticlinal mountains
are weakly built, and consequently rock-falls and landslips are of
common occurrence among them--far more common, and on a much larger
scale, than among the immeasurably older mountains of Scandinavia and
Scotland. The valleys of the Pyrenees, the Alps, and the Apennines,
are cumbered with enormous chaotic heaps of fallen rock-masses. From
time to time peaks and whole mountain-sides give way, and slide into
the valleys, burying hamlets and villages, and covering wide tracts of
cultivated land. Hundreds of such disastrous rock-falls have occurred
in the Alps within historical ages, and must continue to take place
until every weakly-formed mountain has been demolished. The hills
and mountains of Scotland have long since passed through this phase
of unstable equilibrium. After countless ages of erosion our higher
grounds have acquired a configuration essentially different from that
of a true mountain-chain. Enormous landslips like that of the Rossberg
are here impossible, for all such weakly-constructed mountains have
disappeared.
A little consideration will serve to show how such modifications and
changes have come about. When strata are crumpled up they naturally
crack across, for they are not elastic. During the great movements
which have originated all mountains of elevation, it is evident
that the strata forming the actual surface of the ground would often
be greatly fissured and shattered along the crests of the sharper
anticlinal ridges. In the synclinal troughs, however, although much
fissuring would take place, yet the strata would be compelled by the
pressure to keep together. Now, when we study the structure of such a
region as the Alps, we find that the tops of the anticlines have almost
invariably been removed, so as to expose the truncated ends of the
strata--the ruptured and shattered rock-masses having in the course
of time been carried away by the agents of erosion. Such mountains
are pre-eminently weak structures. Let us suppose that the mountains
represented in the diagram (Fig. 8) consist of a succession of strata,
some of which are more or less permeable by water, while others are
practically impermeable. It is obvious that water soaking down from the
surface will find its way through the porous strata (_p_), and come out
on the slopes of the mountains along the joints and cracks (_c_) by
which all strata are traversed. Under the influence of such springs and
the action of frost, the rock at the surface will eventually be broken
up, and ever and anon larger and smaller portions will slide downwards
over the surface of the underlying impermeable stratum. The undermining
action of rivers will greatly intensify this disintegrating and
disrupting process. As the river deepens and widens its valley (_v_),
it is apparent that in doing so it must truncate the strata that are
inclined towards it. The beds will then crop out upon the slopes of the
valley (as at _b_, _b_), and so the conditions most favourable for a
landslip will arise. Underground water, percolating through the porous
beds (p), and over the surface of the underlying impermeable beds (_i_,
_i_, _i_), must eventually bring about a collapse. The rocks forming
the surface-slopes of the mountain will from time to time give and
slide into the valley, or the whole thickness of the truncated strata
may break away and rush downwards; and this process must continue so
long as any portion of the anticlinal arch remains above the level of
the adjacent synclinal troughs.
Thus it will be seen that an anticlinal arch is a weak structure--a
mountain so constructed falls a ready prey to the denuding agents; and
hence in regions which have been exposed to denudation for as long a
period as the Scottish or Scandinavian uplands, a mountain formed of
anticlinally arranged strata is of very exceptional occurrence. When
it does appear, it is only because the rocks of which it is composed
happen to be of a more enduring character than those of the adjacent
tracts. The Ochil Hills exemplify this point. These hills consist of a
great series of hard igneous rocks, which are arranged in the form of a
depressed anticlinal arch--the low grounds lying to the north and south
being composed chiefly of sandstones and shales. Here it is owing to
the more enduring character of the igneous rocks that the anticlinal
arch has not been entirely removed. We know, however, that these
igneous rocks were formerly buried under a great thickness of strata,
and that their present appearance at the surface is simply the result
of denudation.
If an anticlinal arch be a weak structure, a synclinal arrangement of
strata is quite the opposite. In the case of the former each bed has
a tendency to slip or slide away from the axis, while in a syncline
it is just the reverse--the strata being inclined towards and not
away from the axis. Underground water, springs, and frost are enabled
to play havoc with anticlinal strata, for the structure is entirely
in their favour. But in synclinal beds the action of these powerful
agents is opposed by the structure of the rocks--and great rock-falls
and landslips cannot take place. Synclinal strata therefore endure,
while anticlinal strata are worn more readily away. Even in a true
mountain-range so young as the Alps, denudation has already demolished
many weakly-built anticlinal mountains, and opened up valleys along
their axes; while, on the other hand, synclinal troughs have been
converted into mountains. And if this be true of the Alps, it is still
more so of much older mountain-regions, in which the original contours
due to convolutions of the strata have entirely disappeared (see Fig.
9).
The mountains of such regions, having been carved out and modelled by
denuding agents, are rightly termed _mountains of circumdenudation_,
for they are just as much the work of erosion as the flat-topped and
pyramidal mountains which have been carved out of horizontal strata.
The Scottish Highlands afford us an admirable example of a mountainous
region of undulating and often highly-flexed strata, in which the
present surface-features are the result of long-continued erosion. As
already remarked, this region is one of the oldest land-surfaces in the
world. In comparison with it, the Pyrenees, the Alps, and the Himalayas
are creations of yesterday. The original surface or configuration
assumed by the rocks composing our Highland area at the time when these
were first crushed and folded into anticlines and synclines had already
been demolished at a period inconceivably more remote than the latest
grand upheaval of the Alps. Even before the commencement of Old Red
Sandstone times, our Archæan, Cambrian, and Silurian rocks had been
planed down for thousands of feet, so that the bottom beds of the Old
Red Sandstone were deposited upon a gently undulating surface, which
cuts across anticlines and synclines alike. In late Silurian and early
post-Silurian times the North-west Highlands probably existed as a true
mountain-chain, consisting of a series of parallel ranges formed by the
folding and reduplication of the strata. The recent observations of my
friends, Professor Lapworth and Messrs. Peach and Horne, in Sutherland,
have brought to light the evidence of gigantic earth-movements, by
which enormous masses of strata have been convoluted and pushed
for miles out of place. We see in that region part of a dissected
mountain-chain. The mountain-masses which are there exposed to view
are the basal or lower portions of enormous sheets of disrupted rock,
the upper parts of which have been removed by denudation. In a word,
the mountains of Sutherland are mountains of circumdenudation--they
have been carved out of elevated masses by the long-continued action
of erosion. To prove this, one has only to draw an accurate section
across the North-west Highlands, when it becomes apparent that the
form or shape of the ground does not correspond or coincide with the
convolutions of the strata, and that a thickness of thousands of
feet of rock has been denuded away since those strata were folded and
fractured. All over the Highlands we meet with similar evidence of
enormous denudation. The great masses of granite which appear at the
surface in many places are eloquent of the result produced by erosion
continued for immeasurable periods of time. Every geologist knows that
granite is a rock which could only have been formed and consolidated at
great depths. When, therefore, such a rock occurs at the surface, it is
evidence beyond all doubt of prodigious erosion. The granite has been
laid bare by the removal of the thick rock-masses underneath which it
cooled and consolidated.
A glance at any map of Scotland will show that many river-valleys, and
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.
After he has sated himself with the rare beauty of this landscape (and
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.
These consist chiefly of numerous varieties of a rock called porphyrite
by geologists, piled in more or less irregular beds, one on top of
another, in a somewhat confused manner. The colour of the freshly
fractured rocks is very variable, being usually some shade of blue or
purple; but pink, red, brown, greenish, and dark grey or almost black
varieties also occur. Some of the rocks are finely crystalline; others,
again, are much coarser, while many are compact, or nearly so, a lens
being required to detect a crystalline texture. The mineral called
felspar is usually scattered more or less abundantly through the matrix
or base, which itself is composed principally of felspathic materials.
Besides distinct scattered crystals of felspar, other minerals often
occur in a similar manner; mica and hornblende being the commonest.
Occasionally the rocks contain numerous circular, oval, or flattened
cavities, which are sometimes so abundant as to give the appearance
of a kind of coarse slag to the porphyrite. These little cavities,
however, are usually filled up with mineral matter--such as calcspar,
calcedony, jasper, quartz, etc. Sometimes also cracks, crannies, and
crevices of some size have been sealed up with similar minerals. Now
nearly all these appearances are specially characteristic of rocks
which have at one time been in a state of igneous fusion; nor can
there be any doubt that the Cheviot porphyrites are merely solidified
lava-beds, which have been poured out from the bowels of the earth.
In modern lavas we may notice not only a crystalline texture, but
frequently also we observe those in our porphyrites. Such cavities are
due to the expansive force of the vapours imprisoned in the molten mass
at the time of eruption. They form chiefly towards the upper surface of
a lava stream, and are often drawn out or flattened in the direction
in which the lava flows. Thus a stream of lava, as it creeps on its
way, becomes slaggy and scoriaceous or cindery above and in front, and
as the molten mass within continues to flow, the slags and cinders
that cover its face tumble down before it, and form the pavement
upon which the stream advances. In this way slags and cinders become
incorporated with the bottom of the lava, and hence it is that so many
volcanic rocks are scoriaceous, as well below as above. The vapours
which produce the cavities usually contain minerals in solution, and
these, as the lava cools, are frequently deposited, partially filling
up the vesicles, so as to form what are called geodes. But many of
the cavities have been filled in another way--by the subsequent
infiltration of water carrying mineral matter in solution. And since
we know that all rocks are so permeated by water, it is clear that
the cavities may have received their contents during many successive
periods, after the solidification of the rock in which they occur. It
is in this manner that the jaspers, calcedony, and beautiful agates of
commerce have been formed. Rocks abundantly charged with cavities are
said to be _vesicular_, and when the vesicles are filled with mineral
matter, then the mass becomes, in geological language, _amygdaloidal_,
from the almond-like shape assumed by the flattened vesicles.
Now all the appearances described above, and many others hardly
less characteristic of true lavas, are to be met with amongst those
porphyrites which, as I have said, form the major portion of the
Cheviot Hills. From the valley of the Oxnam, east by Cessford,
Morebattle, and Hoselaw, and south by Edgerston, Letham, Browndeanlaws,
and Hindhope, the porphyrites extend over the whole area, sweeping
north-east across the border on to the heights above the Rivers Glen
and Till. In the hills at Hindhope we notice a good display of the
oldest beds of the series. At the base occurs a very peculiar rock
resting upon the Silurian, and thus forming the foundation of the
porphyrites. It varies in colour, being pink, grey, green, red, brown,
or variously mottled. Sometimes it is fine-grained and gritty, like
a soft, coarse-grained sandstone; at other times it is not unlike a
granular porphyrite; but when most typically developed it consists of a
kind of coarse angular gravel embedded in a gritty matrix. The stones
sometimes show distinct traces of arrangement into layers; but they are
often heaped rudely together with little or no stratification at all.
They consist chiefly of fragments of porphyrites; but bits of Silurian
rocks also occur amongst them. This peculiar deposit unquestionably
answers to the heaps of dust, sand, stones, and bombs which are shot
out of modern volcanoes; it is a true tuff--that is, a collection of
loose volcanic ejectamenta.
Upon what kind of surface did it fall? Long before the eruptions began,
the Silurian rocks had been sculptured into hills and valleys by the
action chiefly of the sub-aërial forces, and it was upon these hills
and in these valleys that the igneous materials accumulated. It is
difficult to say, however, whether at this period the Cheviot district
was above or under water. The traces of bedding in the tuff would seem
to indicate the assorting power of water; but the evidence is too
slight to found upon, because we know that in modern eruptions, loose
ejectamenta frequently assume a kind of irregular bedded arrangement.
For aught we can say to the contrary, therefore, dry land may have
extended across what is now southern Scotland and northern England
when the first rumblings of volcanic disturbance shook the Cheviot
area. Be that as it may, we know that the volcanic outbursts began in
those old times, as they almost invariably commence now, by a discharge
of sand, small stones, blocks, and cinders. These, we may infer,
covered a wide area round the centre of dispersion--the chief focus
of eruption being probably in the vicinity of the big Cheviot, where
a mass of granite seems to occupy the core or deep-seated portion of
the old volcanic centre. The locality where the tuff occurs is some
nine miles or so distant from this point, and the intervening ground
could hardly have escaped being more or less thickly sprinkled with
the same materials. The whole of that intervening ground, however, now
lies deeply buried under the massive streams of once-molten rock that
followed in succession after the first dispersion of stones and débris.
Although, as I have said, it may be doubted whether at the beginning of
their activity the Cheviot volcanoes were sub-aqueous, yet there are
not a few facts that lead to the inference that the eruption of the
porphyrites took place for the most part, if not exclusively, under
water. The beds are occasionally separated by layers of sandstone,
grit, and conglomerate; but such beds are rare, and true tuffs are
rarer still. If the outbursts had been sub-aërial, we ought surely to
have met with these latter in greater abundance, while we should hardly
have expected to find such evidently water-arranged strata as do occur
here and there. The porphyrites themselves present certain appearances
which lead to the same conclusion. Thus we may observe how the bottoms
of the beds frequently contain baked or hardened sand and mud, showing
that the molten rock had been poured out over some muddy or sandy
bottom, and had caught up and enclosed the soft, sedimentary materials,
which now bear all the marks of having been subjected to the action
of intense heat. Sometimes, indeed, the old lava-streams seem to have
licked up beds of unconsolidated gravel, the water-worn stones being
now scattered through their under portions. As no fossils occur in any
of the beds associated with the porphyrites, one cannot say whether
the latter flowed into the sea or into great freshwater lakes. Neither
can we be certain that towards their close the eruptions were not
sub-aërial. They may quite well have been so. The porphyrites attain
a thickness of probably not less than fifteen hundred or two thousand
feet, and the beds which we now see are only the basal, and therefore
the older portions of the old volcanoes. The upper parts have long
since disappeared, the waste of the igneous masses having been so great
that only the very oldest portions now remain, and these, again, are
hewn and carved into hill and valley. Any loose accumulation of stones
and débris, therefore, which may have been thrown out in the later
stages of the eruptions, must long ere this have utterly disappeared.
We can point to the beds which mark the beginning of volcanic activity
in the Cheviots; we can prove that volcanoes continued in action there
for long ages, great streams of lava being poured out--the eruptions of
which were preceded and sometimes succeeded by showers of stones and
débris; we can show, also, that periods of quiescence, more or less
prolonged, occasionally intervened, at which times water assorted the
sand and mud, and rounded the stones, spreading them out in layers.
But whether this water action took place in the sea or in a lake we
cannot tell. Indeed, for aught one can say, some of the masses of
rounded stones I refer to may point to the action of mountain torrents,
and thus be part evidence that the volcanoes were sub-aërial. If we
are thus in doubt as to some of the physical conditions that obtained
in the Cheviot district during the accumulation of the porphyrites
and their associated beds, we are left entirely to conjecture when
we seek to inquire into the conditions that prevailed towards the
close of the volcanic period. For just as we have proof that before
this period began the Silurian strata had been subjected to the most
intense denudation--had, in short, been worn into hill and valley--so
do we learn from abundant evidence that the rocks representing the old
volcanoes of the Cheviots are merely the wrecks of formerly extensive
masses. Not only have the upper portions of these volcanoes been swept
away, but their lower portions, likewise, have been deeply incised, and
thousands of feet of solid rock have been carried off by the denuding
forces. And by much the greater part of all this waste took place
before the accumulation of those sandstones which now rest upon the
worn outskirts of the old volcanic region.
III.
Some reference has already been made (see p. 64) to the general
appearance presented by the valleys of the Cheviots. In their upper
reaches they are often rough and craggy; narrow dells, in fact, flanked
with steep shingle-covered slopes, and occasionally overlooked by
beetling cliffs, or fringed with lofty scaurs of decomposing rocks. As
we follow down the valleys they gradually widen out; the hill-slopes
becoming less steep, and retiring from the stream so as to leave a
narrow strip of meadow-land through which the clear waters canter gaily
on to the low grounds of the Teviot. In their middle reaches these
upland dales are not infrequently well cultivated to a considerable
height, as in the districts between Hownam and Morebattle, and between
Belford and Yetholm--the former in the valley of the Kale, and the
latter in that of the Bowmont. It is noticeable that all the narrower
and steeper reaches lie among Silurian strata and Old Red Sandstone
porphyrites. No sooner do we leave the regions occupied by these tough
and hard rock-masses than the whole aspect of the scenery changes.
The surrounding hills immediately lose in height and fall away into a
softly undulating country, through which the streams and rivers have
dug for themselves deep romantic channels. Nevertheless, it is a fact,
as we shall see by-and-by, that south-west of the region occupied by
the igneous rocks of the Cheviot Hills, all the higher portions of
the range (Hungry Law, Carter Fell, Peel Fell, etc.) are built up of
sandstones. For the present, however, I confine attention to those
valleys whose upper reaches lie either wholly or in part among igneous
rocks or Silurian strata. A typical and certainly the most beautiful
example is furnished us by the vale of the River Jed. This stream
rises among the sandstone heights which have just been mentioned as
composing the south-west portion of the Cheviot range. The first seven
or eight miles of its course lead us through a broad open valley, which
has been hollowed out almost exclusively in sandstones and shales;
by-and-by, however, we are led into a Silurian tract, and thereupon
the valley contracts and the hill-slopes descend more steeply to the
stream. But we soon leave the grassy glades of this Silurian tract and
enter all at once upon what may be termed the lower reaches of the
Jed. No longer cooped up in the rocky gully, painfully worn for itself
in the hard greywacké and shales, the stream now winds through a much
deeper and broader channel which has evidently been excavated with
greater ease. Precipitous banks and scaurs here overlook the river at
every bend, the banks becoming higher and higher and retiring further
and further from each other, as the water glides on its way, until at
last they fairly open upon the broad vale of the Teviot. Sometimes the
river flows along one side of its valley for a considerable distance,
and whenever this is the case, it gives us a line of bold cliffs which
are usually flanked on the opposite side by sloping ground. This is
the general character of all valleys of erosion, and especially of the
lower reaches of the Jed.
A glance at the cliffs and scaurs of the Jed shows that they consist of
horizontal or gently undulating strata of soft earthy, friable, shaly
sandstone, arranged in thin beds and bands, which alternate rapidly
with crumbling, sandy, and earthy shales; the whole forming a loose
and unconsolidated mass that readily becomes a prey to the action of
the weather, rain, frost, and running water. The prevailing colour is
a dull red, but pale yellow, white, green, and purple discolorations
are visible when the strata are closely scanned. The finest sections
occur between Glen Douglas and Inchbonnie, and at Mossburnford, but
the cliffs throughout present the same general appearance, and are
picturesque in the highest degree. Everywhere the banks are thickly
wooded, and even the steep red scaurs are dashed and flecked with
greenery, which droops and springs from every ledge and crevice in
which a root can fix itself. How vivid and striking is the contrast
between the fresh delicate green of early summer and the rich warm
tint of these rocks, which when lit up by the setting sun seem almost
to glow and burn! Well may the good folk of Jedburgh be proud of the
lovely valley in which their lot is cast. In no similar district in
Scotland will the artist meet with a greater number of such "delicious
bits," in which all the charms of wood and water, of meadow and rock
are so harmoniously combined. It is not with the scenic beauties of
the Jed, however, that we have at present to do. I wish the reader
to examine with me certain appearances visible at the base of the
red beds, where these rest upon those older rocks which have formed
the subject of the preceding papers. In the bed of the river at
Jedburgh, we see the junction between the red beds and the Silurian
strata, and may observe how the bottom portions of the former, which
repose immediately upon the greywackés, are abundantly charged with
well-rounded and water-worn stones. Many of these stones consist of
greywacké, hardened grit, and other kinds of rock, and most of them
undoubtedly have been derived from Silurian strata. In other districts
where the old igneous rocks of the Cheviots form the pavement upon
which the red beds repose, the stones in the lower portions of the
latter are made up chiefly of rounded fragments of the underlying
porphyrites. All which clearly shows that the red beds have been
built out of the ruins of the older strata of the district. This is
unquestionably the origin not only of the conglomerates, but of all
the red beds through which the River Jed cuts its way from the base
of the hills to the Teviot. When we trace out the boundary of these
beds, we find that this leads us along the base of the hills, close to
the hill-foot; and not only so, but it frequently takes us into the
hill-valleys also. And this shows that the Cheviots had already been
deeply excavated by streams before any portion of the red beds was
deposited.
I have said that the red beds are approximately horizontal; sometimes,
however, they have a decided _dip_ or inclination, and when this is
continuous, it is invariably in a direction away from the hills. Thus
as we traverse the ground from the hill-foot to the Teviot, we pass
over the outcrops of the red beds and slowly rise from a lower to a
higher geological position. The strata, however, are generally so
flat that their dip is often not greater than the average slope or
inclination of the ground. Hence when we ascend the valley-slopes
from the stream, we soon reach the higher beds of the series, as,
for example, in the undulating heights that overlook the Jed in the
neighbourhood of Jedburgh. In that district a number of quarries
have been opened, in which the upper beds of the red series are well
exposed, as at Ferniehirst, Tudhope, etc. These consist of thick beds
of greyish white, yellowish, and reddish sandstones, which, unlike
the crumbling earthy deposits below, are quite suitable for building
purposes. Scales of fish and plant remains are often met with in the
thick sandstones, but the underlying earthy, friable red beds appear to
be quite destitute of any organic remains.
Let us now briefly recapitulate the main facts we have just
ascertained. They are these:--1. All the low grounds that abut upon the
hills are composed of horizontal or nearly horizontal strata, which
consist chiefly of red earthy beds, passing down into conglomerates,
and up into whitish and reddish sandstones. 2. The conglomeratic
portion forms the boundary of the series, fringing the outskirts of the
hills, and resting sometimes upon Silurian strata and sometimes upon
Old Red Sandstone igneous rocks. 3. Fossils occur in the white and red
sandstones, but seem to be wanting in the underlying red earthy beds.
[Illustration: Fig. 7.--S, Silurian strata; _i_, Old Red Sandstone
Igneous Rocks; _a^1_, Conglomerate; _a^2_, Red earthy beds; _a^3_,
White and Red Sandstones.]
The accompanying diagram (Fig. 7) gives a generalised view of the
relation borne by the red beds to the older rocks of the Cheviots.
It will be seen that the former rest _unconformably_ upon the Old
Red Sandstone igneous rocks, and also, of course, upon the Silurian
strata. The section shows that the red beds lie upon a worn and
denuded surface. Now this speaks to the lapse of a long period of
time. It may be remembered that we had some grounds for believing that
the latest eruptions of the Cheviot volcanoes were sub-aërial. The
evidence now enables us to advance further, and to state that after
the close of the volcanic period, the whole Cheviot district existed
as an elevated tract of dry land, from which streams flowed north and
south. And for so long a time did these conditions endure, that the
rivulets and streams were enabled to scoop out many channels and broad
valleys before any of the outlying red beds had come into existence.
Before the conglomerate beds were laid down, the ancient volcanic
bank of the Cheviots had thus suffered great erosion. This is what
"unconformability" means. It points to the prolonged continuance of
a land-surface, subject as that must always be to the wearing action
of the sub-aërial forces. Rain and frost disintegrate the rocks, and
running water rolls the débris from higher to lower levels, and piles
it up in the form of gravel, sand, and mud in lakes and the sea. While
the old volcanic country of the Cheviots was being thus denuded,
it would appear that a wide extent of land existed in the Northern
Highlands and Southern Uplands of Scotland, and also in what are now
the lake districts of England and the hilly tracts of Wales. And in all
these regions valleys were formed, which at a subsequent time were more
or less filled up with newer deposits.
The presence of the red beds that sweep round the base of the Cheviot
Hills shows unmistakably that a period of submergence followed these
land conditions. All the low grounds of Southern Scotland disappeared
beneath a wide sheet of water, which stretched from the foot of the
Lammermuirs up to the base of the Cheviots, and here and there entered
the valleys, and so extended into the hills. This water, however, does
not seem to have been that of an open sea; rather was it portion of a
great freshwater lake, brackish lagoon, or inland sea. The lowest beds
of the red series are merely hardened layers and masses of gravel and
rolled shingle, which would seem at first sight to indicate the former
action of waves along a sea-beach. There are certain appearances,
however, which lead one to suspect that these ancient shingle beds
may have had quite another origin. In some places the stones exactly
resemble those which are found so abundantly in glacial deposits. They
are sub-angular and blunted, and, like glaciated stones, occasionally
show striæ or scratches. This, however, is very rarely the case. Most
of the stones appear subsequently to have been rolled about in water,
and in this process they must have lost any ice-markings they may have
had, and become smoothed and rounded like ordinary gravel stones.
The same appearances may be noted in the glacier valleys of Norway
and Switzerland, where at the present day the glaciated stones which
are pushed out at the lower ends of the glaciers are rolled about in
the streams, and soon lose all trace of ice-work. It is impossible,
however, to enter here into all the details of the evidence which lead
one to suspect that glaciers may have existed at this early period
among the Cheviot and Lammermuir Hills. In the latter district, the
conglomerates occur in such masses and so exactly resemble the morainic
débris and ice-rubbish of modern glacial regions, that the late Sir A.
C. Ramsay long ago suggested their ice-origin.
Let us conceive, then, that when the ancient lake or inland sea of
which I have spoken reached the base of the Cheviots, glaciers may
have nestled in the valleys. Streams issuing from the lower ends of
these would sweep great quantities of gravel down the valleys to the
margin of the lake, and it is quite possible that there might be enough
wave-action to spread the gravel out along the shores. It is evident,
however, that the main heaps of shingle would gather opposite what were
at that time the mouths of glacier valleys; and it is just in such
positions that we now meet with the thickest masses of conglomerate.
Ere long, however, the supposed glaciers would seem to have melted
away, and only fine sand and mud, with here and there small rounded
stones and grit, accumulated round the shores of the ancient lake. Of
course, during all this time fine-grained sediment gathered over the
deeper parts of the lake-bottom.
We have no evidence to show what kind of creatures, if any, inhabited
the land at this time; nor do any fossils occur in the red earthy beds
to throw light upon the conditions of life that may have obtained in
the lake. If glaciers really existed and sent down ice-cold water,
the conditions would hardly be favourable to life of any kind; for
glacial lakes are generally barren. But the absence of fossils may
be due to other causes than this. It is a remarkable fact, that red
strata are, as a rule, unfossiliferous, and the few fossils which
they do sometimes yield are generally indicative rather of lacustrine
and brackish-water, than marine conditions. The paucity or absence of
organic remains seems to have been often due to the presence in the
water of a superabundance of salts. Now this excessive salinity may
have arisen in either of two ways. First, we may suppose some wide
reach of the sea to have been cut off from communication with the
open ocean by an elevation of a portion of its bed; and in this case
we should have a lagoon of saltwater, which evaporation would tend to
concentrate to such a degree, that by-and-by nothing would be able to
live in its waters. Or, again, we may have a lake so poisoned by the
influx of springs and streams, carrying various salts in solution,
as to render it uninhabitable by life of any kind, either animal or
vegetable. Many red sandstone deposits, as Sir A. C. Ramsay has pointed
out, are evidently lagoon-formations, which is proved by the presence
of associated beds of rock-salt, gypsum, and magnesian limestone.
They have slowly accumulated in great inland seas or lakes having no
outlet, whose waters were subject to evaporation and concentration,
although now and then they seem to have communicated more or less
freely with the ocean. The red earthy beds of the Jed, however, though
unfossiliferous, yet contain no trace of rock-salt or magnesian
limestone. The only character they have in common with the salt-bearing
strata of the New Red Sandstone of England is their colour, due to
the presence of peroxide of iron, which we can hardly conceive could
have been deposited in the mud of a sea communicating freely with the
ocean. But a quiet lake, fed by rivulets and streams that drained an
old volcanic district, is precisely the kind of water-basin in which
highly ferruginous mud and sand might be expected to accumulate. Such
a lake, tainted with the various salts, etc., carried into it by
streams and springs (some of which may have been thermal; for, as we
shall see presently, the volcanic forces, although quiescent, were yet
not extinct), might well be unfitted for either animal or plant, and
probably this is one reason why the red earthy beds of the Jed are so
unfossiliferous.
After some time, the physical conditions in the regions under review
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
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