The available evidence seems to point not to one catastrophic phase of uplift, but to a slow and intermittent emergence, involving not only the old blocks, but also the younger folded areas of southern and eastern England. Many of those interested in the landforms of Britain have brought forward evidence for an intermittently falling base-level in the shape of successions of terraces and terrace-like forms extending to elevations as high as 2,000 ft or more, though often confined to lower elevations. Opinion has not been unanimous that these features are, in fact, terraces formed by erosion, neither is it finally agreed, assuming that these are terraces, by what particular processes of erosion they have been formed. The arguments as to the responsible processes are inconclusive, in view of the lack of fossiliferous deposits and the dangers of arguing from form alone, but for present purposes they are not of great significance. Whether a terrace at 1,500 ft is interpreted as subaerial or marine in origin does not affect the general conclusion that base-level must have been some 1,500 ft higher than at present during the formation of that terrace. Thus the idea of a sea-level falling since the middle of the Tertiary era from a level high up our present mountains, or of the intermittent and almost uniform uplift of the whole of Britain in the same period, cannot be escaped. As the terraces and terrace-like forms appear to be virtually horizontal, the former interpretation has been the favoured one in Britain.
The intermittently falling base-level has had two important effects on the relief of Britain. It has encouraged down-cutting at the expense of lateral erosion and back-wearing, thus ensuring the preservation of a youthful appearance in much of the relief. In addition, the probable emergence of more and more of Britain from beneath the sea is a useful hypothesis to account for the superimposed nature of the drainage of many parts of Britain, for example the Lake District, South Wales, and the Hampshire Basin.
The evolution of the relief of Britain was disturbed violently by the advent of the Ice Age. Although opinion on the number and relative importance of glacial and interglacial periods is open at present and awaits evidence provided by deep borings, it seems to be beyond question that Britain was glaciated more than once, although there is considerable doubt whether the classic pattern of four glaciations can be read into the available evidence. But this is a stratigraphical rather than a geographical problem.
The presence on the western side of the land of the uplifted plateaux of old rocks directly in the path of winds from the Atlantic led to the accumulation of the main ice caps on the Highlands of Scotland, the Lake District and Wales. Thus Britain shows a repetition on a smaller scale of the pattern of ice caps in Europe, with the largest one in Scandinavia and smaller ones to the east on the various mountains of the U.S.S.R. where the precipitation was insufficient for their best development. The location of the ice caps is presumptive evidence for the continuation in the Ice Age of an atmospheric circulation not vastly different from the present one and against the assumption of a vast, permanent glacial anticyclone with outblowing easterly winds.
The ice flowed, with velocity depending on its surface gradient, from these centres of accumulation into the eastern and southern lowlands where it spread out, slowed down and finally halted, when wastage equalled supply. The structural and relief divisions have thus been accentuated by an agent of erosion, the distribution of which they helped to control. The western highlands not only possessed ice caps for the longest periods, but were also subjected to the greatest glacial erosion for ice was moving rapidly over them. Ice erosion was dominant here and has left a characteristic imprint which subsequent subaerial erosion has had little effect in modifying.
The lowlands of Britain are areas of glacial deposition rather than glacial erosion with the form of the deposits varying to some extent with the age of the glaciation and their proximity to the highlands. The areas nearest to the highlands are characterized by those forms depending on the moulding of drift by rapidly moving ice, notably drumlins, while areas near the ice margins are commonly buried beneath featureless sheets of boulder clay. Well-preserved depositional forms are found mainly in the area covered by the last glaciation, the area of the Newer Drift, and so are not found south of York except in coastal Lincolnshire and north Norfolk.