reaches the islands from across the Atlantic, spreads out over the shelf and its ameliorating effect on the air over it is magnified. The effect of tidal movement is also increased by the shallowness of the water.
Despite their small area, the British Isles include rocks representing all the major geological periods. It is largely because of their long and complicated geological history that the British Isles have a range of scenery almost unrivalled in any area of comparable size anywhere in the world.
In the main island of Great Britain the hilly and mountainous areas lie to the north and west, so that it is possible to draw a broad distinction between Highland Britain and Lowland Britain. An irregular line joining the mouth of the Tyne in the north-east with the mouth of the Exe in the south-west marks the division between these two contrasted parts of Britain.
Highland Britain thus includes the whole of Scotland; the broad upland running from north to south through the north of England and known as the Pennines; the well-known Lake District in the north-west of England; practically the whole of Wales; and the south-western peninsula coinciding approximately with the counties of Devon and Cornwall. Highland Britain is built up almost entirely of rocks older in age than the Coal Measures and there are large tracts of the surface lying more than 1,000 feet above sea level. Many parts of the surface have only thin, poor soils, with the result that large stretches of moorland are found over the Highlands of Scotland, the Pennines, the Lake District and the mountains of Wales as well as on the higher ground in the south-west. The farmer has been able to use only the valley lands and the plains where soils are deeper and richer, so that human settlements are sometimes widely scattered, and villages and towns may be separated by considerable stretches with few if any habitations.
Lowland Britain, comprising south-east England and the Midlands, is built up almost entirely of rocks younger than the Coal Measures, which are less resistant to weathering and have broken down to form deep fertile soils. Scarcely any part of Lowland Britain reaches i,000 feet above sea level, so drat practically the whole, with the exception of a few patches of poor soil or rocky land, has been cultivated, and farmland stretches over the whole area except where interrupted by urban and industrial settlements. Elaborate land drainage systems have been developed through the centuries to bring under cultivation the fertile soil of the low-lying Fenland of Lincolnshire and other nearby areas in East Anglia.
Towards the end of its long geological history, when the ancestors of present-day man had already settled in Britain, came the great Ice Age, and at one period or another during this time the whole of Britain north of a line joining the Thames and the Bristol Channel was covered by ice caps and ice sheets. The ice naturally accumulated on the higher ground and swept from the mountains of Scotland, northern England and Wales any loose rock or soil which had previously been formed, so that when the ice eventually disappeared the hills stood out as barren rocky areas, while a thick mantle of glacial debris boulders, boulder-clay, sands and gravels lay distributed widely over the lower ground. The ice had blocked up previous drainage channels and left large lakes, which have since gradually dried up, leaving deposits of sand, silt and mud, often affording soils of great fertility.
In Ireland, where the solid rocks are often covered deeply by the debris left by ice sheets, the great central plain has large boggy areas, due to interruption of the previous natural drainage. The mountains and hill masses of Ireland are irregularly disposed round the fringes of the island, and in the higher parts the moorland cover resembles that of the higher parts of Highland Britain.
Because of the complex geology and the varied relief which results, Britain enjoys not only very attractive and contrasting scenery within short distances but a remarkable, ever different, coastline. The ancient rocks of Highland Britain often reach the coast in
