rocks, and of these the largest portions occur in the north-west of
Scotland.[59] Most of our mountains, however, belong to upheavals
dating from Palæozoic time, though the actual exposure and shaping of
them into their present forms must be referred to a far later period.
Two leading epochs of movement in Palæozoic time can be recognised.
Of these the older, dating from before the Lower Old Red Sandstone
and part at least of the Upper Silurian period, was distinguished by
the plication of the rocks in a dominant north-east and south-west
direction, and the effects of these movements can be traced in the
trend of the Lower Silurian ridges and hollows to the present day.
In Wales two types of mountain-form exist--the Snowdon type, and that
of the Breconshire Beacons. In the former, the greater prominence
of the high grounds arises primarily from the existence of masses
of volcanic rocks, which from their superior durability have been
better able to withstand the progress of degradation. In the latter
the heights are merely the remaining fragments of a once continuous
tableland of Old Red Sandstone.
The Lake District presents a remarkable radiation of valleys from a
central mass of high ground. It might be supposed that these valleys
have been determined by some radiating system of fractures in the
rocks; but an examination of the area shows them to be singularly
independent of geological structure. So much do they disregard the
strike, alternations, and dislocations of the rocks among which
they lie that the conclusion is forced upon us that they have been
determined by some cause independent of that structure, and before
the rocks now visible were exposed at or could affect the surface.
This could only have happened by the spread of a deep cover of later
rocks over the site of the Lake mountains. The former presence of
such a cover, which is demanded for the explanation of the valleys,
can be inferred from other evidence. The Carboniferous Limestone on
the flanks of the Lake District is so thick that it must have spread
nearly or entirely over the site of the mountains. But it was overlain
by the Millstone Grit and Coal-measures, so that the whole area was
probably buried under several thousand feet of Carboniferous strata
which stretched continuously across what is now the north of England.
At the time of the formation of the anticlinal fold of the Pennine
Chain, the site of the Lake District appears to have been upraised
as a dome-shaped eminence, the summit of which lay over the tract
now occupied by the heights from Scafell to Helvellyn. The earliest
rain that fell upon this eminence would gather into divergent streams
from the central watershed. In the course of ages, after possibly
repeated uplifts, these streams have cut down into the underlying
core of old Palæozoic rocks, retaining on the whole their original
trend. Meanwhile the whole of the overlying mantle of later formations
has been stripped from the dome, and is now found only along the
borders of the mountains. The older rocks, partly faulted down and
yielding to erosion, each in its own way, have gradually assumed that
picturesqueness of detail for which the area is so deservedly famous.
The Scottish Highlands likewise received their initial plications
during older Palæozoic time, their component rocks having been thrown
into sharp folds trending in a general north-east and south-west
direction.[60] But there is reason to believe that they were
subsequently in large measure buried under Old Red Sandstone, and
possibly under later accumulations. No positive evidence exists as to
the condition of this region during the vast interval between the Old
Red Sandstone and the older Secondary rocks. We can hardly believe
it to have remained as land during all that time, otherwise, the
denudation, vast as it is, would probably have been still greater. Not
improbably the region had become stationary at a base-level of erosion
beneath the sea; that is, it lay too low to be effectively abraded
by breaker-action, and too high to become the site of any important
geological formation. The present ridges and valleys of the Highlands
are entirely the work of erosion. When they began to be traced, the
area probably presented the aspect of a wide undulating tableland.
Since that early time the valleys have sunk deeper and deeper into
the framework of the land, the ridges have grown narrower, and the
mountains have arisen, not by upheaval from below, but by the carving
away of the rest of the block of which they formed a part. In this
evolution, geological structure has played an important part in guiding
the erosive tools. The composition of the rock-masses has likewise been
effective in determining the individuality of the mountain-forms.
The mountains of Ireland are distributed in scattered groups round the
great central plain, and belong to at least three geological periods.
The oldest groups probably took their rise at the time of the older
Palæozoic upheaval, those of the north-west being a continuation of the
Scottish Highlands, and those of the south-east being a prolongation of
those of Wales. Later in date as regards the underground movements that
determined their site, are the mountainous ridges of Kerry and Cork.
These are local uplifts which, though on a small scale, are by far
the best examples in Britain of true mountain-structure. The Old Red
Sandstone and Carboniferous rocks have there been thrown into broad
arches and troughs which run in a general east and west direction. In
some cases, as in the Knockmealdown Mountain, the arch is composed
entirely of Old Red Sandstone flanked with Carboniferous strata. But in
most instances an underlying core of Silurian rocks has been exposed
along the centre of the arch. As not only the Carboniferous Limestone,
but the rest of the Carboniferous system covered the south of Ireland
and participated in this plication, the amount of denudation from
these ridges has been enormous. On the Galty range, for example, it
can hardly have been less but may have been more than 12,000 feet.
The third and latest group of Irish mountains is that of Mourne and
Carlingford, which may with some probability be referred to older
Tertiary time when the similar granitic and porphyritic masses in Mull
and Skye were erupted.
The Tablelands of Britain strictly include the mountains, which are in
general only prominences carved out of tablelands. There are still,
indeed, large areas in which the plateau character is well shown. Of
these the most extensive and in many respects the most interesting is
the present tableland or plain of Central Ireland. As now exposed,
this region lies upon an undulating eroded surface of Carboniferous
Limestone. But it was formerly covered by at least 3,000 or 4,000 feet
more of Carboniferous strata, as can be shown by the fragments that
remain.[61] The present system of drainage across the centre of Ireland
took its origin long before the ancient tableland had been reduced to
its present level, and before some of the ridges, now prominent, had
been exposed to the light.
The Moors and Wolds of Yorkshire present us with a fragment of a
tableland composed of nearly horizontal Jurassic and Cretaceous rocks.
The Lammermuir Hills and Southern Uplands of Scotland extend as a broad
tableland which has been formed on a deeply eroded surface of Lower and
Upper Silurian rocks.
The Scottish Highlands may also be looked upon as the relics of an
ancient tableland cut out of highly crumpled and plicated schists.
Among the eastern Grampians large fragments of the plateau exist at
heights of more than 3,000 feet, forming wide undulating plains that
terminate here and there at the edge of precipices. In the Western
Highlands, the erosion having been more profound, the ridges are
narrower, the valleys deeper, and isolated peaks more numerous (p.
112). It is the fate of a tableland to be eventually cut down by
running water into a system of valleys which are widened and deepened,
until the blocks of ground between are sharpened into ridges and
trenched into separate prominences. The Highlands present us with far
advanced stages of this process.
In the youngest of British tablelands--that of the volcanic region
of Antrim and the Inner Hebrides--we meet with some of the earlier
parts of the change. That interesting tract of our islands reveals a
succession of basaltic sheets which appear to have spread over the
wide valley between the Outer Hebrides and the mainland, and to have
reached southwards beyond Lough Neagh. Its original condition must have
resembled that of the lava-fields of Idaho and Oregon--a sea-like
expanse of black basalt stretching up to the base of the mountains.
What may have been the total thickness of basalt cannot be told; but
the fragment remaining in Ben More, Mull, is more than 3,000 feet
thick. So vast has been the erosion since older Tertiary time that the
volcanic plateau has been trenched in every direction by deep glens
and arms of the sea, and has been reduced to detached islands. It is
strange to reflect that all this revolution in the topography has
been effected since the soft clays and sands of the London Basin were
deposited.
THE VALLEYS.
The intimate relation of a system of valleys to a system of drainage
lines, first clearly enunciated by Hutton and Playfair, has received
ample illustrations from all parts of the world.[62] But the notion is
not yet extinct that, in some way or other, valleys have been as much,
if not more, determined by subterranean lines of dislocation than by
superficial erosion. Some favourite dogmas die hard, and though this
dogma of fracture has been demolished over and over again, it every
now and then reappears, dressed up anew as a fresh contribution to
scientific progress. We have only to compare the surface of a much
dislocated region with its underground structure, where that has been
revealed by mining operations, as in our coal fields, to see that
valleys comparatively seldom, and then only as it were by accident,
run along lines of dislocation, but that they everywhere cut across
them, and that faults rarely make a feature at the surface, except
indirectly by bringing hard and soft rocks against each other.
In Britain, as in other countries, there is a remarkable absence
of coincidence between the main drainage system and the geological
structure of the region. We may infer from this fact either that the
general surface, before the establishment of the present drainage
system, had been reduced to a base-level of denudation above or under
the sea, the original inequalities of configuration having been
planed off irrespective of structure; or at least, that the present
visible rocks were buried under a mass of later unconformable and
approximately level strata, on the unequally upraised surface of which
the present drainage system began to be traced. Where the existing
watershed coincides generally with the crest of an anticline, its
position has obviously been fixed by the form of the ground produced by
the plication, though occasionally an anticline may have been deeply
buried below later rocks, the subsequent folding of which along the
same line would renew the watershed along its previous trend. Where
drainage lines coincide with structure, they are probably, with few
exceptions, of secondary origin; that is, they have been developed
during the gradual denudation of the country. Since the existing
watershed and main drainage-lines of Britain are so independent of
structure, and have been determined chiefly by the configuration of the
surface when once more brought up within the influence of erosion, it
may be possible to restore in some degree the general distribution of
topography when they were begun.
One of the most curious aspects of the denudation of Britain is its
extraordinary inequality. In one region the framework of the land
has been cut down into the very Archæan core, while in the immediate
vicinity there may be many thousands of feet of younger strata which
have not been removed. This inequality must result from difference in
total amount of upheaval above the base-line of denudation, combined
with difference in the length of exposure to denudation. As a rule
the highest and oldest tracts will be most deeply eroded. Much of the
denudation of Britain appears to have been effected in the interval
between the close of the Carboniferous and end of the Triassic
period.[63] This was a remarkable terrestrial interval, during part
of which the climate was so arid that salt lakes were formed over
the centre of England. Yet the denudation ultimately accomplished
was enormous, thousands of feet of Carboniferous rock being entirely
removed from certain areas, such as the site of the present Bristol
Channel. An interesting analogy to this condition of things is
presented by the Great Basin and adjoining tracts of Western America,
where at the present time marked aridity and extensive salt-lakes are
accompanied by great erosion.
The deeply-eroded post-Carboniferous land of Britain was eventually
screened from further degradation, either by being reduced through
denudation to a base-level or by being protected by submergence. It
was to a large extent covered with Secondary rocks, though the covering
of these may have been but thin over what are now the higher grounds.
The present terrestrial areas emerged at some period later than the
Chalk.[64] In England there were three chief tracts of land--Wales, the
Pennine Chain, and the Lake District. The eastern half of the country,
covered with Secondary rocks, was probably the last portion to be
uplifted above the sea; hence the watersheds and drainage lines in that
tract may be regarded as the youngest of all.
The history of some of the valleys of the country tells the story of
the denudation. The Thames is one of the youngest rivers, dating from
the time when the Tertiary sea-bed was raised into land. Originally its
source probably lay to the west of the existing Jurassic escarpment
of the Cotswold Hills, and it flowed eastward before the Chalk
escarpment had emerged. By degrees the Chalk downs have appeared, and
the escarpment has retreated many miles eastward. The river, however,
having fixed its course in the Chalk, has cut its way down into it, and
now seems as if it had broken a path for itself across the escarpment.
As all the escarpments are creeping eastward, the length and drainage
area of the Thames are necessarily slowly diminishing.
The Severn presents a much more complex course; but its windings across
the most varied geological structure are to be explained by its having
found a channel on the rising floor of Secondary rocks between the base
of the Welsh hills and the nascent Jurassic escarpments. The Wye and
Usk afford remarkable examples of the trenching of a tableland. The Tay
and Nith are more intricate in their history. The Shannon began to flow
over the central Irish plain when it was covered with several thousand
feet of strata now removed. In deepening its channel it has cut down
into the range of hills north of Limerick, and has actually sawn it
into two.[65]
THE LAKES.[66]
The Lakes of Britain present us with some of the most interesting
problems in our topography. It is obvious that the existence of
abundant lakes in the more northern and more rocky parts of the
country points to the operation of some cause which, in producing
them, acted independently of and even in some measure antagonistically
to the present system of superficial erosion. It is likewise evident
that as the lakes are everywhere being rapidly filled up by the
daily action of wind, vegetation, rain, and streamlets, they must be
of geologically recent origin, and that the lake-forming process,
whatever it was, must have attained a remarkable maximum of activity
at a comparatively recent geological epoch. Hardly any satisfactory
trace is to be found of lakes older than the present series. How
then have our lakes arisen? Several processes have been concerned in
their formation. Some have resulted from the solution of rock-salt or
of calcareous rocks and a consequent depression of the surface. The
'meres' of Cheshire, and many tarns or pools in limestone districts,
are examples of this mode of origin. Others are a consequence of the
irregular deposit of superficial accumulations. Thus, landslips have
occasionally intercepted the drainage and formed lakes. Storm-beaches,
thrown up by the waves along the sea-margin, have now and then ponded
back the waters of an inland valley or recess. The various glacial
deposits--boulder-clays, sands, gravels, and moraines--have been
thrown down so confusedly on the surface that vast numbers of hollows
have thereby been left which, on the exposure of the land to rain, at
once became lakes. This has undoubtedly been the origin of a large
proportion of the lakes in the lowlands of the north of England,
Scotland, and Ireland, though they are rapidly being converted by
natural causes into bogs and meadow-land. Underground movements may
have originated certain of our lakes, or at least may have fixed the
direction in which they have otherwise been produced.[67]
A large number of British lakes lie in basins of hard rock, and have
been formed by the erosion and removal of the solid materials that
once filled their sites. The only agent known to us by which such
erosion could be effected is land-ice. It is a significant fact that
our rock-basin lakes occur in districts which can be demonstrated to
have been intensely glaciated. The Ice-Age was a recent geological
episode, and this so far confirms the conclusion already enforced,
that the cause which produced the lakes must have been in operation
recently, and has now ceased. We must bear in mind, however, that it is
probably not necessary to suppose that land-ice excavated our deepest
lake-basins out of solid rock. A terrestrial surface of crystalline
rock, long exposed to the atmosphere, or covered with vegetation and
humus, may be so deeply corroded as, for two or three hundred feet
downward, to be converted into mere loose detritus, through which the
harder undecomposed veins and ribs still run. Such is the case in
Brazil, and such may have been also the case in some glaciated regions
before the glaciers settled down upon them. This superficial corrosion,
as shown by Pumpelly, may have been very unequal, so that when the
decomposed material was removed, numerous hollows would be revealed.
The ice may thus have had much of its work already done for it, and
would be mainly employed in clearing out the corroded debris, though
likewise finally deepening, widening, and smoothing the basins in the
solid rock.
THE HILLS AND ESCARPMENTS.
The Hills and Hill-groups of Britain have all emerged during the
gradual denudation of the country, and owe their prominence to the
greater durability of their materials as compared with those of the
surrounding lower grounds. They thus represent various stages in the
general lowering of the surface. In many cases they consist of local
masses of hard rock. Such is the structure of the prominent knobs of
Pembrokeshire and of Central Scotland, where masses of eruptive rock,
formerly deeply buried under superincumbent formations, have been laid
bare by denudation. In connection with such eruptive bosses, attention
should be given to the 'dykes' so plentiful in the north of England and
Ireland, and over most of Scotland. In numerous instances, the dykes
run along the crests of hills, and also cross wide and deep valleys.
Had the present topography existed at the time of their protrusion,
the molten basalt would have flowed down the hill-slopes and filled up
the valleys. As this never occurs, and as there is good evidence that
a vast number of the dykes are not of higher antiquity than the older
Tertiary periods, we may conclude that the present configuration of the
country has, on the whole, been developed since older Tertiary time--a
deduction in harmony with that already announced from other independent
evidence.
Escarpments are the prominent outcrops of flat or gently inclined
strata, exposed by denudation. They may be regarded as the steep edges
of hills in retreat. The British islands abound in admirable examples
of all ages from early Palæozoic rocks down to Tertiary deposits,
and of every stage of development, from an almost unbroken line of
cliff to scattered groups of islet-like fragments. The retreat of
our escarpments can be well studied along the edge of the Jurassic
belt from Dorsetshire to the headlands of Yorkshire, likewise in the
course of the edge of the Chalk across the island. Not less suggestive
are some of the escarpments of more ancient rocks, such as those of
the older Palæozoic limestones, the Old Red Sandstone of Wales, the
Carboniferous Limestone and Millstone Grit of Yorkshire, and the Coal
Measures of the Irish plain. Our volcanic escarpments are likewise
full of interest, as displayed in those of the Lower Old Red Sandstone
along both sides of the Tay, in those of the Carboniferous system in
Stirlingshire, Ayrshire, Bute, and Roxburghshire, and in those of the
Tertiary series in Antrim and the Inner Hebrides.
THE PLAINS.
The Plains of Britain, like those elsewhere, must be regarded as
local base-levels of denudation, that is, areas where, on the whole,
denudation has ceased, or at least has become much less than deposit.
Probably in all cases the areas they occupy have been levelled by
denudation. Usually a greater or less depth of detrital material has
been spread over them, and it is the more or less level surface of
these superficial accumulations that generally forms the plain. But
in some instances, such as the flats of the Weald Clay and the Chalk
of Salisbury Plain, there is hardly any such cover of detritus, the
denuded surface of underlying rock forming the actual floor on which
the vegetable soil rests.
Our plains, if classed according to the circumstances of their origin,
may be conveniently regarded as (1) river plains--strips of meadow-land
bordering the streams, and not infrequently rising in a succession
of terraces to a considerable height above the present level of the
water; (2) lake plains--tracts of arable ground occupying the sites of
former lakes, and of which the number is ever on the increase, owing
to the filling-up of the basins with sediment; (3) plains consisting
of portions of upraised sea-floors--partly eroded rock-platforms, but
mostly flat selvages of alluvial ground, formed of littoral materials
deposited when the land lay below its present level: in the northern
estuaries these raised beaches spread out as broad carse-lands, such as
those of the Tay, Forth, and Clyde; (4) glacial drift plains--tracts
over which the clays, sands, and gravels spread out during the Ice-Age
form the existing surface; (5) plains of subaërial denudation which
have been levelled by rain and other atmospheric agents, especially
upon tracts of rock of fairly uniform resistance, such as the soft
clays and sands of the Secondary and Tertiary formations; (6) submarine
plains--the present floor of the North Sea and of the Irish Sea, which
must be regarded as essentially part of the terrestrial area of Europe.