The south and east of Britain

The south and east of Britain, farthest away from the Lewisian shield, is formed of a series of less resistant rocks, ranging in age from Permian to Pleistocene, and consisting mainly of clays, sandstones and limestones. North of London the prevailing general dip towards the south-east is reflected in the dominant scarpland pattern. South of London, the trend is mainly east-west, a reflexion in part of the effects of the buried Hercynian structures on the Alpine folding, which produced here a wealth of minor east-west asymmetric folds and fractures. The folds are usually simple except in the far south, where the rocks are vertical in the northern limb of the Isle of Wight monocline, while local thrusting has occurred on the northern side of the Weymouth anticline. But structures on the whole are much simpler than those of Caledonian and Hercynian Britain, a fact reflected in the less complicated pattern of relief, while the lesser resistance of the rocks is the cause of the highest elevations rarely exceeding 1,000 ft.

The geomorphological development of these various zones of Britain has been very similar except in so far as they themselves have caused differences in glaciation, through their position and elevation, and been affected by differences in glaciation, owing to their position.

Whatever the age of the rocks and of the folding, the whole of Britain appears to have been worn down and flooded by the sea by the end of the Cretaceous, according to the widely held hypothesis that a cover of Cretaceous rocks formerly extended over most of Britain. The Tertiary era, on the other hand, was dominantly a period of uplift, not in one single movement, but in a series of movements which gave Britain the main outlines of its present form.

Only in the south-eastern parts of Britain may the phases of uplift and their extents be judged with any degree of confidence, for it is in this region that Tertiary sediments have been laid down and later affected by folding and uplift. At the end of the Cretaceous a series of gentle earth-movements defined for the first time the major units of an uplifted Weald and down-warped Hampshire and London basins, though not in their present extent and form. But this does represent, at least, the beginning of modern relief for, at earlier dates, the London Basin had stood up, often above the sea, while the Weald had been down-warped. Other evidence for the emergence of Britain at the beginning of the Tertiary comes from the other end of Britain, from the north-west of Scotland, which was part of a great province of Eocene igneous activity including Antrim, the Faeroes, Iceland and Jan Mayen Island. The associated lavas, the products of subaerial eruptions, imply that, at the time of their formation, this whole area was dry land. Yet, today, the originally more extensive areas of igneous rocks have been separated by fracturing and foundering into a series of isolated remnants, notably in Skye, Rhum, Mull, Arran and the Ardnamurchan peninsula. The date of this second phase of movement is most likely to have been the middle of the Tertiary era, either the late Oligocene or the early Miocene.

The evidence for this again comes principally from southern England, where the Eocene and Oligocene sediments of the Hampshire Basin were involved in the Alpine folding to produce the series of structures already referred to. Outside these parts of Britain, where the earth-movements may be dated by the sediments affected, the blocks of older rocks are often held to have been uplifted as plateaux. Such uplift cannot be proved, because of the lack of young sediments to show it, but an argument by analogy may be derived from the uplifted Hercynian blocks of Europe. The fracturing and uplift of such areas as the Central Plateau of France, where there are Lower Tertiary sediments to demonstrate it, were completed at the time of the Alpine folding. Some difficulty arises from the distribution of the intensity of uplift, which, judging from the Central Plateau and other European features, was greatest nearest to the Alps. Yet in Britain the hypothesis of mid-Tertiary uplift requires gentle folding in the south and strong uplift in the north farthest from the Alps.

Whatever the difficulties, however, it is likely that the emergence and uplift of Great Britain was accomplished in two main phases in the Tertiary, though it must not be presumed that all was quiet between, for geological evidence can be advanced to show the probable development of a tilt towards the east in southern Britain in the intervening period. The period between the two phases of uplift may well have been one of predominant erosion and the formation of surfaces of faint relief, which are perhaps now just detectable as the approximately accordant summits of the highlands of western and northern Britain.