British Isles - Geography

Geography

See also: Geology of Great Britain, Geography of Ireland, Geography of the Isle of Man, Geography of the Channel Islands, and List of islands in the British Isles

The British Isles lie at the juncture of several regions with past episodes of tectonic mountain building. These orogenic belts form a complex geology which records a huge and varied span of earth history. Of particular note was the Caledonian Orogeny during the Ordovician Period, c. 488–444 Ma and early Silurian period, when the craton Baltica collided with the terrane Avalonia to form the mountains and hills in northern Britain and Ireland. Baltica formed roughly the north western half of Ireland and Scotland. Further collisions caused the Variscan orogeny in the Devonian and Carboniferous periods, forming the hills of Munster, south-west England, and south Wales. Over the last 500 million years the land which forms the islands has drifted northwest from around 30°S, crossing the equator around 370 million years ago to reach its present northern latitude.

The islands have been shaped by numerous glaciations during the Quaternary Period, the most recent being the Devensian. As this ended, the central Irish Sea was de-glaciated and the English Channel flooded, with sea levels rising to current levels some 4,000 to 5,000 years ago, leaving the British Isles in their current form. Whether or not there was a land bridge between Great Britain and Ireland at this time is somewhat disputed, though there was certainly a single ice sheet covering the entire sea.

The west coasts of Ireland and Scotland that directly face the Atlantic Ocean are generally characterised by long peninsulas, and headlands and bays; the internal and eastern coasts are "smoother".

There are about 136 permanently inhabited islands in the group, the largest two being Great Britain and Ireland. Great Britain is to the east and covers 216,777 km2 (83,698 sq mi),. Ireland is to the west and covers 84,406 km2 (32,589 sq mi). The largest of the other islands are to be found in the Hebrides, Orkney and Shetland to the north, Anglesey and the Isle of Man between Great Britain and Ireland, and the Channel Islands near the coast of France.

The islands are at relatively low altitudes, with central Ireland and southern Great Britain particularly low lying: the lowest point in the islands is Holme, Cambridgeshire at −2.75 m (−9.02 ft). The Scottish Highlands in the northern part of Great Britain are mountainous, with Ben Nevis being the highest point on the islands at 1,343 m (4,406 ft). Other mountainous areas include Wales and parts of Ireland, however only seven peaks in these areas reach above 1,000 m (3,281 ft). Lakes on the islands are generally not large, although Lough Neagh in Northern Ireland is an exception, covering 381 square kilometres (147 sq mi). The largest freshwater body in Great Britain (by area) is Loch Lomond at 71.1 square kilometres (27 sq mi), and Loch Ness, by volume whilst Loch Morar is the deepest freshwater body in the British Isles, with a maximum depth of 1,017 ft (310 m). There are a number of major rivers within the British Isles. The longest is the Shannon in Ireland at 360.5 km (224 mi). The river Severn at 354 km (220 mi) is the longest in Great Britain. The isles have a temperate marine climate. The North Atlantic Drift ("Gulf Stream") which flows from the Gulf of Mexico brings with it significant moisture and raises temperatures 11 °C (20 °F) above the global average for the islands' latitudes. Winters are cool and wet, with summers mild and also wet. Most Atlantic depressions pass to the north of the islands, combined with the general westerly circulation and interactions with the landmass, this imposes an east-west variation in climate.