Regional Geology of the UK / Britain Continued
Scottish Highlands - Isle of Skye - The Lake District - English Midlands - Geology of the Southern Uplands - Geology of the Southwest
By considering the changes in the distribution of land and water over successive geological periods, we have traced in bare outline the evolution of the physical geography of these islands. In the present chapter we must analyse the physical geography as it is at the present day. Leaving on one side for the moment the island of Ireland, the island of Britain is broadly divisible into two parts: a Highland Zone on the north and west, a Lowland Zone on the south and east. It is possible to suggest more than one line which may be used to separate these two divisions, but the most satisfactory seems to be that used in Fig. 17 and elsewhere in this book (see Chapter XXIV) and which cuts across the country, following a somewhat irregular course, from the mouth of the River Exe in the south-west to near the mouth of the River Tees in the north-east. It is roughly the line separating the outcrop of the old Paheozoic rocks on the one hand and the younger Mesozoic and Tertiary rocks on the other. To the north and west of the line lie the remnants of the great mountain chains which were built up by successive earth building movements of Pre-Cambrian, Siluro-Devonian, and Carbo-Permian times. The mountains are but remn ants of their former mighty selves, but they still comprise the major mountain and hill masses of Britain. Generally the most ancient masses are those which occur, as in the case of the Highlands of Scotland, farthest to the north-west. Naturally the margins of the ancient rocks are not infrequently covered by strata of later ages. To the south and east of the line one finds first the broad plains or low-lying plateaus, built up mainly of Triassic rocks, which constitute the Midlands of England. Not infrequently small remnants of the ancient mountains stand up as islands in the midst of these plains of younger rocks. Farther southwards and eastwards the MIdland plains give place to what may be called, in the broadest possible sense, the scarplands of England. Indeed, it is possible to draw another line across England, again somewhat irregularly, from the Dorset coast to the north Yorkshire Coast. To the south and east of this line lie low ridges, separated by shallow valleys, which mark respectively the Outcrop of the harder or more resistant and softer or less resistant beds respectively of the geological sequence from the early Jurassic onwards. Like the Trjassjc rocks of the Midlands, these rocks rest upon an ancient platform which lies buried beneath them at a greater or less depth. Sometimes, as for example under London, the ancient platform is within a thousand feet of the surface. At other times the Paheozoic platform lies at such a depth that the full thickness of the overlying beds has never been penetrated by the boring tools of the well-engineer. The Jurassic and later rocks have themselves been gently tilted, usually towards the south-east, by the Alpine system of earth movements; but it is only in the extreme south that there are signs of what might be called severe folding. For the purposes of a preliminary account we may distinguish in Britain the broad physiographic and structural units described in the following pages.
THE HIGHLAND ZONE
(1) The Highlands of Scotland.—The Highlands of Scotland are built up for the most part of great masses of ancient metamorphic or crystalline rocks—gneiss, schists, slates, and quartzites. Some of the folding doubtless took place in Pre-Cambrian times, and it would seem that some of the great intrusions of granite and of other rockS are of (lie same date. The great period of earth movem ents which determined the major structures of the Scottish Highl ands was. however, the Caledoflian, or Siluro-Dcvonian. These movCfllcnts gave rise to great mountain chains with a general grcnd from south-wCSt to north-east, and this is still the dominant grain of the countrY. Bctwccn the great mountain chains were ihc deep basins in which the deposits of the Old Red Sandstone were laid down.
Much of the Highlands of Scotland has probably remained land from those very remote ages to the present day, and consequently sub-aerial denudation has gone on almost continuously, culminating with the work of the great ice caps which covered Scotland during the last glacial epoch. As a result, the Highlands of Scotland of to-day no longer present the highly accidented scenery of the younger mountain belts of the world, but rather the rounded outlinea which betoken the results of eons of sub-aerial denudation and the work of ice. On the whole, then, the Highlands form an irregularly suifaced P!ateau with its greatest elevation along the western margin, sloping On the whole towards the east. The plateau surface has in general an average elevation of between 2,000 and 3,000 feet; some of the higher points often marking the outcrops of granite, of both Pre-Cambrian and Sjluro-Devonjan ages. The highest points may reach over 4,000 feet, and included amongst them is Ben Nevjs, the highest mountain in the British Isles, of 4,400 feet. The Scottish Highlands are divided into the North-West Highlands and the Central Highlands or Grampians by the great cleft of Glen More. The North-West Highlands are by far the more rugged and the grander, and meet the Atlantic Ocean in the intricate forded and island-bounded coast of western Scotland. It seems possible that the Alpine earth movements, being unable to fold the old stable block, tended to fracture it ins tead, and the belts of crushed rock which were formed along the fractures have been more easily excavated by rivers and by ice, and by the waves of the ocean, thus giving rise to the fords with their remarkable rectangular bends.1 Through some of the major cracks, The system of major cracks developed along the western coasts of Scotland may be due to pressure exerted from below by masses of molten rock attempting to find an outlet. Just as when one presses with the point of a stick on a sheet of ice covering a pond, or when a stone is thrown through a window, both conc entric and radial cracks would be developed, and it would seem that this explains the varied directions followed by the fords and valleys of the area. See J. W. Gregory, “The Origin and Nature of Fiords”; and J. W. Gregory, “The Scottish Lochs and their Origin,” Proc. R. Phil. Soc. Glasgow, XLV. 1914, 18,3—196.
Rocks of Tertiary age have welled up and give rise to the marked lava plateaus of Skye and SOniC other parts of the western coast of Scotland, comparable in character to those found in Icel and. The Old Red Sandstone, originally deposited in valleys, still tcnds to occur in valley or lowland situations, but the more resistant rocks of the period may cover considerable stretches, as they do over CaithflC5 The ancient rocks of the Highlands on the whole give rise to a poor tyPe of soil, whilst much of the soil which must previously have been formed has been swept away from most of the higher areas by the ice sheets of the glacial epoch. The tracts occurring on the Old Red Sandstone are, on the whole, more fertile. The same is true of the lower eastern margins of the ancient rocks themselves, as for example in the area known as the Buchan Plateau. GeograPhicY the eastern margins, whether they are on the ancient rocks or on Old Red Sandstone, are somewhat distinct from the main mass of the Highlands and are frequently considered as a separate region under the title of North-Eastern Scotland. It should be noted that the Orkney Islands form a detached portion of this area, whilst the Shetland Islands, farther north, resemble more closely the central part of the Highlands themselves. The southern limit of the Highlands of Scotland is remarkably well defined by the great Highland boundary fault, actually a succession of faults, which runs across the country with a Caledonian trend, from the mouth of the Clyde to the east coast in the neighbourhood of Stonehaven. The faults seem to have been initiated at the same time as the Caledonian earth movements, but intermittent movem ents along them have undoubtedly occurred from that time until the present.
|(2) The Southern Uplands.—The Southern Uplands
consist of the denuded remains of a great mountain chain of Siluro-Devonian
age and which runs across the south of Scotland from the southw est to
the north-east, that is with a characteristically Caledonian trend.
There is, however, a marked difference between the Southern Uplands and
the Highlands, in that the rocks of which the Southern Uplands consist
are sediments, mainly of Ordovician and Silurian age, very highly
folded. In the south-west, particularly in Gallow ay, there are large
granitic intrusions, but ancient metamorphic rocks, such as those
constituting the greater part of the Highlands, are absent.
Consequently all the higher parts of the Southern Upl ands are formed of moorland country with rounded outlines, passi ng on lower ground in the east, particularly in the Tweed basin, to quiet rolling pastoral, and often well-wooded country. In the south-west, cutting across the main mass of the Southern Uplands, are the well-known dales. These dales are comparatively straight clefts running from north-north-west to south-south-east and afford important routeways. Fringing the Southern Uplands, along the shores of the Irish Sea, are stretches of low ground, occupied by smiling, well-watered pasture. The northern limit of the Southern Uplands is formed by a great zone of faulting, comparable in charact er to the fault-line which bounds the Highlands, though not giving rise to such a marked feature.
Old Red Sandstone, and was later occupied by a shallow arm of the Carbonifeb01s sea, so shallow that at an early stage it was suitable for the growth of the swamp forests which have left their traces at the present day in seams of coal. At the same period volcanic activity was rife, and as a general result at the present day neither the relief nor the geology of the Midland Valley of Scotland can be described as simple. It is only in the broadest possible sense a valley. It is possible to distinguish a northern fringing corridor or broad valley, then a line of volcanic hills, then the central lowlands wherein lie the great Lanarkshire or Central Coalfields and the Midlthiaifeshire Coalfield, then a line of hills along the south and an ill-defined valley separating them in turn from the Southern Uplands. These sub-divisions of the valley and their correlation with the geology may be seen in Figs. 23 and 24.
The District has some of the finest rock scenery and, us the Lake I Isles. Gec. atiVClY low altitude, rugged mountains in the Britisl despite their graphicaflY there is a rcma1kalcontrast between the area of ancient rocks in the heart of the Lake District and the surrounding ring of oungr strata Only on the south-east do the Shapflnines. the Lake District proper with the hill masses of the Pc Fells connect (5) WaleS or the Welsh ft if.—For the purposes of a general account the whole of the Welsh massif may be considered together.
|For this purpose the massif may be described as embracing all the hill masses which lie to the west of the English Midlands, excluding only the southern part of the county of Glamorgan, which actually ought to be considered as part of the English plain or scarplands. The eastern margins of the area So defined can be quite clearly traced from any physical map. I the north the Welsh hills abut quite abruptly on the Cheshire and north ShroP shire plain; along the margin lies the North Wales Coalfield. The hills of central and southern Shropshire belong structurally to the Welsh massif, and from central Shropshire southwards to the mouth of the Severn the eastern limit is defined by the line of hills running southwards from the Wrekin in Shropshire to the Abberley Hills and the Malvern Hills. The oldest part of the Welsh massif is in the north-west, where the ancient crystalline rocks of the isle of Anglesey underwent folding in Pre-Cambrian times. The folding was continued during the Caledonian earth movements and postC arboniferous foldings followed along the same lines, so that one finds narrow bands of Carboniferous Limestone pinched in amongst the ancient rocks. The whole of Anglesey, as the result of later denudation, has been worn down to a low plateau, almost to a sea- level plain, and its complicated geological history is scarcely sugg ested by the somewhat uninteresting surface features of the island.||
The mainland of north Wales is still, on the other hand, a land of rugged mountains. On the whole the grain of the country is from south-west to north-east, indicating that the mountains owe their origin in the main to the Caledonian earth movements. The rocks involved in the folding are, for the most part, Cambrian, Ordovician and Silurian, but the mountains owe their rugged character of to-day to the large masses of contemporaneous lava which were extruded as well as to other igneous masses which were intruded into the rocks before and during the folding. The igneous rocks have proved themselves more resistant to weathering, and most of the higher points, such for example as Snowdon and Cader Idris, mark the outcrop of one or more masses of igneous rock. As one passes from north into central Wales, the igneous masses become less numerous, and this is one reason for the less rugged relief of central Wales. The age of the folding of the rocks, too, becomes successively younger as one goes towards the south. Whilst north and central Wales, which at that time were probably continuous with south-eastern Ireland, formed the land mass which we have already called St. George’s Land in Carboniferous times, south Wales was occupied first by the Carboniferous Limestone sea, then by the fringing swampy lands on which grew the Coal Measure forests. Coal Measures were laid down over a huge area in South Wales, and at the end of the Carboniferous period were folded by the Carbo-Permian or Armorican earth movements. These earth movements resulted there in folds with an east—west trend, thus causing the great coal basin of the South Wales Coalfield. It would seem that the Armorican earth movements were unable to fold the already highly complicated and hardened masses of North Wales, and that the earth-building waves broke against this resistant mass both along its southern and eastern sides. Thus there are east—west folds in the south, but north—south folds in the east. Some of the latter are remarkably sharp, and one forms that curious line of hills, the Malvern Hills. The Malvern Hills folds are
noted, coincide with low uplands, the Keuper
Mans with low ground, gently undulating but otherwise rather
featureless. It must be remembered that over very many areas the
underlying solid rocks are masked by the glacial drifts.
To the south-east the Keuper Marls are succeeded by the Rhetic and Liassic rocks. Unless the Rhetic or lower Lias contain hard or resistant beds sufficiently important to give rise to a scarp there is little to mark the junction. Thus the Vale of Evesham is partly on Keuper, partly on Lias; so also is the Vale of Berkeley.
The Plain of Lancastria.—The plain of Lancastria forms a cont inuation of the Midland Plain through the Midland Gap or Midl and Gate. It occupies the northern half of Shropshire, nearly the whole of Cheshire and that part of Lancashire which lies between the Pennines and the Irish Sea. There are two broad tongues of moorland, called respectively Bowland “Forest” and Rossendale “Forest,” which extend westwards from the Pennines into Lancas hire and which coincide with outcrops of Carboniferous Rocks (Millstone Grit and Lower Coal Measures). Elsewhere Lancashire is an undulating lowland: again, where Bunter Sandstones outcrop there may be low hills, where the Keuper Mans are at the surface true lowland prevails. The actual character of the surface, however, depends largely on the thickness and type of the mantle of glacial deposits. Physically the plain of Lancastria lies between the Welsh massif and the Pennines: on the west the undulating country of the North Wales Coalfield forms a transitional belt; in the east the coal measure country of the North Staffordshire Coalfield forms another transitional belt.
Bristol-Mendip Region.—To the south-west the Midland Plain narrows and
passes first into the Vale of Gloucester and then into the Vale of
Berkeley between the Forest of Dean or the Severn on the west, and the fine
scarp of the Cotswolds on the east. But farther southwards, in what may be
called the Bristol-Mendip Region, the plain disappears. Its place is taken by
country of varied rel ief lying between the Severn and the westernmost of the
Jurassic scarps. This region repeats, on a smaller scale, the features of parts
of the Midlands. It really consists of “islands” of old rocks wrapped round by
the softer Triassic and Liassic deposits. But there are several points of
difference: the islands are relatively larger and more numerous, the amount of
low ground correspondingly small. The islands, too, are of rocks of varied age;
there are the large Carboniferous Limestone masses of the Mendip Hills, and the
imi portant coal basins, as well as quite tiny patches of Silurian, Old Red
Sandstone and Carboniferous Limestone. The “islands” are
)remnants of Armorican folds, and it is in this region that the north—
• south Malvernian folds cross the more normal east—west folds. iThus some of the old blocks are elongated in a north—south direct 1on, such as the Tortworth Ridge north of Bristol and the Carboni‘ .. Limestone edges of the Kingswood coal basin. In the Mendips a succession of east—west folds en echelon has resulted in a d upland trending from west—north—west to east—south—east.
‘- Plain of Somerset.—In some ways this plain resembles that Midlands from which it is separated by the Bristol-Mendip The Vale of Taunton Deane is thus a Keuper Marl lowland,
the great feature of Somerset is the very extensive plain, almost sea-level and liable to extensive floods, which lies between the lantock Hills and the Mendip Hills, and which is interrupted only the narrow Liassic ridge of the Polden Hills.
L Jurassic Scarplands.—The Jurassic rocks of Britain crop over a belt of varying width extending from the Dorset coast the north Yorkshire coast. Over large areas the beds dip to South-east or east, and so give rise to a succession of hills or es where the harder or more resistant beds crop out, and valleys