Great Britain, although hardly exceeding 750 miles in latitudinal extent and 375 miles in longitudinal extent, possesses a wide diversity of landforms. In general terms they may be assigned to the operation of the factors of structure, process and time, but it is probably true to say that structure is the dominant one of the three because of the great variety of structure and rock type displayed in Great Britain. Added to this is the fact that Britain as a whole is in a stage of late youthful dissection, if one may use the terms of the Davisian cycle for an area with so complex a recent geological history. At such a stage in the cycle of erosion the effects of structural and lithological variety should be most apparent in the relief. Yet process, too, has played its part: most of the landforms have probably been developed under the so-called normal subaerial cycle of erosion and modified by glaciation, but the nature of the glaciation varies from severe erosion in the highlands through deposition in the lowlands to periglacial activity beyond the margins of the ice-sheets. In some ways glaciation has accentuated differences due to structure and lithology and so increased the basic importance of these two in the relief of Great Britain.
The dominant structural trend in Great Britain is north-east to south-west, a reflexion partly of the fact that the oldest rocks are in the north-west and the youngest in the south-east and partly of the importance of the Caledonian fold-trend in the structure. The Highlands of Scotland, the Southern Uplands, the Lake District and most of Wales show this Caledonian trend, partly in their folding, but often more obviously in their fracturing. Renewal of Caledonian trends during the later Hercynian folding is responsible for structures such as the down-faulted Central Lowlands of Scotland and the Rossendale and Bowland Forests, west of the Pennines. A comparable north-east to south-west graining of the country is shown in the southeastern scarplands, although this is not due to Caledonian folding but to greater uplift of the north-west than of the south-east. Yet there are considerable areas where the trend is different, especially in the whole of southern England, which is dominated by east-west structures and relief, and in the Pennines with their north-south alinement.
Just as Europe may be regarded as a series of structural zones with Alpine structures south of Hercynian structures and both south of the Caledonian regions, so may Britain be considered as a series of structural belts flanking an ancient crystalline nucleus in the far northwest. In the Outer Hebrides and along parts of the north-west coast of Scotland between Skye and Cape Wrath are exposed the oldest rocks in Britain: these are the Lewisian, a series of heavily glaciated metamorphic rocks, mostly coarse gneisses, resembling in structure and relief the Laurentian shield of Canada, to which the area has been likened.
Flanking the Lewisian and generally occupying the next most north-westerly position in Great Britain are the Caledonian mountains: the Highlands and Southern Uplands of Scotland, the Lake District, and Wales north of the Brecon Beacons. The continuity of the zone is broken by the down-faulted Central Lowlands of Scotland, by the Eden valley, and by the Triassic lowland of Cheshire and Lancashire. The mountains resemble each other not only in structure, but also to a great extent in lithology. Within each area lithological diversity is so great as almost to constitute geomorphological homogeneity. The Highlands of Scotland differ from the other areas in being composed almost entirely of crystalline metamorphic rocks, a great series of gneisses, schists, granulites and quartzites included in the Moine and Dalradian series. The rocks are coarse, micaceous and flaggy, shattered by fractures and scored by the Pleistocene glaciations. In the other Caledonian areas the rocks are less altered Lower Palaeozoic sediments and include predominantly shales, slates, mudstones, greywackes, and flags with variety supplied, at least in the Lake District and North Wales, by important accumulations of ancient volcanic rocks. Fracture and glaciation are again prominent, though probably to a lesser degree than in the Highlands of Scotland.
South and east of the Caledonian zone lies the Hercynian zone, involving rocks of Devonian and Carboniferous ages. North of Pembrokeshire, Britain was outside the zone of strong Hercynian folding, which runs east-west across the centre of Europe, and in this region the Hercynian rocks form plateaux, cuestas and lowlands of relatively simple structure and relief. These include the low plateaux of Caithness and the Orkneys, the Pennines, the plain of Hereford and the Black Mountains. But even in this more northern region exception must be made of the down-faulted Central Lowlands of Scotland, where the rocks are shattered by faults and where igneous activity, especially volcanic activity, has been extremely important. In Pembrokeshire and in Devon and Cornwall the rocks have been much more intensely folded and these areas form exceptions to the generalization that the Hercynian areas, unlike the Caledonian areas, are regions where structure and relief seem to be fairly simply related. They are much more like the highly folded Ardennes and Rhine Block to which they are connected beneath the Mesozoic and Tertiary rocks of south-eastern England.
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