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._