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Geography

The British Isles lie at the juncture of several regions with past episodes of tectonic mountain building. These orogenic belts form a complex geology that records a huge and varied span of Earth's history. Of particular note was the Caledonian orogeny during the Ordovician and early Silurian periods, 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 southern Wales. Over the last 500 million years the land that forms the islands has drifted north-west 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 deglaciated and the English Channel flooded, with sea levels rising to current levels some 8,000 years ago, leaving the British Isles in their current form.

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 83,700 sq mi (217,000 km). Ireland is to the west and covers 32,590 sq mi (84,400 km). 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 most densely populated island is Portsea Island, which has an area of 9.5 sq mi (25 km) but has the third highest population behind Great Britain and Ireland.

The islands are at relatively low altitudes, with central Ireland and southern Great Britain particularly low-lying: the lowest point in the islands is the North Slob in County Wexford, Ireland, with an elevation of −3.0 metres (−9.8 ft). The Scottish Highlands in the northern part of Great Britain are mountainous, with Ben Nevis being the highest point on the islands at 1,345 m (4,413 ft). Other mountainous areas include Wales and parts of Ireland, although 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 150 square miles (390 km). The largest freshwater body in Great Britain (by area) is Loch Lomond at 27.5 square miles (71 km), and Loch Ness (by volume) whilst Loch Morar is the deepest freshwater body in the British Isles, with a maximum depth of 310 m (1,017 ft). There are a number of major rivers within the British Isles. The longest is the Shannon in Ireland at 224 mi (360 km). The river Severn at 220 mi (354 km) is the longest in Great Britain.