Great Britain Geological Structure and Relief

First of all it must be emphasized that the British Isles have always been a part of the continent. They stand on the continental shelf; most of the North Sea is less than 300 feet deep; and both that sea and the Strait of Dover are of comparatively recent origin.

The north and west of Great Britain comprise the old blocks. They belong mainly to the old Caledonian mountain system, a continuation of the Norwegian mountains. In the north the Scottish Highlands and the coastal islands consist mainly of gneiss; farther south in the southern highlands of Scotland, as well as in Cumberland and Wales, sedimentary Silurian rocks predominate. Both the gneiss and the sedimentaries, however, are combined with granite.

Débris from these old Caledonian Mountains was deposited in a sea which occupied most of the area which now forms England, and even spread over the lower parts of the Caledonian system into Scotland. Then these deposits, Devonian and Carboniferous in age, were folded into a second mountain system. These Hercynian mountains surround the Caledonian system in the south of Wales and in Cornwall, and form the Pennine anticline in north central England. Rocks of this same age are also still preserved in certain depressions of the Caledonian system such as the central Scottish Lowlands and the northeastern coast of Scotland. Next, during Mesozoic times, the Hercynian mountains were worn down and their débris formed marine sediments where southeastern England is now located.

The present relief of Great Britain is due to pressure from the south and consequent uplift during the Alpine period. The old eroded mountain blocks were raised, and in many places, such as parts of the Scotch Highlands and Wales, their present surface still shows the former peneplain, only slightly affected by the later erosion of running water and glaciers. Here, as in Sweden, new fault lines appeared, or old ones were renewed, and to such fractures are due several transverse depressions of which the narrow cleft of Glenmore extending straight across Scotland is an instance.

The Scottish Lowland may be lower than the Highlands partly because the blocks north and south of it were uplifted along the old fault lines, but the main reason is that it consists of limestones and sandstones which are eroded more easily than the older rocks on the two sides. They now present a typical erosion landscape with hills representing the harder outcrops, and valley eroded on softer rock between them.

In the Scotch Highlands the maximum elevation is Ben Nevis, with an altitude of 4,406 feet; the southern uplands of Scotland with their softer, rounded forms, are much lower, their highest part reaching only 2,680 feet. The glaciated and dissected Lake District, or Cumberland Mountains so celebrated for their scenery, achieve 3,210 feet. Very like the Scottish Highlands in appearance is the mountain system of Wales, where Mount Snowdon attains 3,560 feet. In the Cornish peninsula a few plateaus, notably Exmoor and Dartmoor, are as much as 2,000 feet high, with structural depressions between them. These highlands of Devon are a revival of the Pennine anticline; but in some other sections, where old formations come to the surface, as in the Weald south of London, it is uncertain whether the dome-like uplift belongs solely to Alpine times or is a revival of an older one.

The Pennine Chain, too, shows the old peneplain surface dissected by gaps which are important passages for transportation. Here the plateau rises to 2,000 feet in the south and as high as 3,500 feet in the north, with typical karst erosion on the Carboniferous limestone. On both sides the Pennines fall steeply to the Midland plains. In the central part, south of the cross-section, the limestone is replaced by the overlying millstone-grit, which assures a supply of soft water and thus has played an important and long-unappreciated part, in making it easy to use steam engines, and in the development of the textile industry.

During the Alpine disturbances the Mesozoic deposits (Triassic, Jurassic, and Cretaceous) were also slightly uplifted on the flanks of the Pennine anticline, and were exposed to erosion. They are responsible for the typical scenery of the English lowlands. Taking the Pennine anticline with its Carboniferous limestone as a center, and going toward the North Sea or London, one finds first a land of coal-bearing strata near Sheffield. Then one crosses a succession of younger and younger geological formations. The first is hard Permian limestone. This thin layer is at once followed by Triassic sandstone and clay, which, on account of their lack of resistance to erosion, form the vales, or broad, level valleys of the Weaver, Trent, and Ouse river systems with outlets to the Mersey and Humber. Next comes a steep escarpment due to relatively resistant Jurassic limestone and forming Lincoln edge (600 to 800 feet), in the cross-section. Much farther south along the line of the western escarpment in A288 it forms the Cotswold Hills, while north of the Humber depression, it produces the North York moors. Then follows a band of soft clay forming the depression of the upper Thames and the Ouse River, and extending toward the Wash. The other escarpment represents the resistance of the Cretaceous chalk to erosion. It includes the Chiltern Hills (800 feet) and the East Anglian Ridge in the south, and continues into the Lincoln and Yorkshire Wolds in the north. In the dome of the Weald, this formation comes to the surface again in both the North and South Downs. It also appears in Salisbury Plain, and along the coast of the Isle of Wight. The last low area includes the East Anglian plain in the north and the Southhampton section in the south, with the uplift of the Weald between them.

In the Alpine period Great Britain was still connected with the European continent, and the North Sea did not exist in its present form. During the Ice Age, Great Britain was covered partly by local glaciers and partly by the western extension of the great Scandinavian glacier which moved as far south as the Thames River. The mountains were given their typical glaciated features and the lowlands were partly covered by a mantle of fertile drift. Only after the Ice Age did Great Britain assume its present form. The rise of the ocean because of the melting of the ice partially explains the formation of the North Sea and of the drowned coastline which in western Scotland is accompanied by numerous coastal islands like those that border the shore of Norway. The land connection between England and France disappeared, and Great Britain first experienced the insular isolation that so strongly influenced its eventual development.

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