The geological history of the British Isles is complex, which can be seen in they irregular landscape. This is due to the fact that Britain and Ireland lie at the Earth's geological crossroads. Seven large portions of continental crust influenced the geological history of this area, even before the Mesozoic formation of the Atlantic Ocean. Since the Paleozoic era, there have been sequence of crustal shortening and extension events. These events have determined the succession and distribution of rock units in the British Isles.
The northern portion of the British Islands had been geographically separated from the southern half until the Silurian period (Paleozoic era). Thus, Scotland and northwest Ireland were part of Laurentia (North American craton) in low southern latitudes. These areas had a complex early history that dates back to two billion years ago, before the Cambrian period (the first of the Paleozoic). The Grampian orogeny was merely the last of a series of Precambrian events, which deformed and metamorphosed the Laurentian crust. The Athollian orogeny, on the other hand, further heated and uplifted the Laurentian continental margin in Ordovician time.
England, Wales, and southeast Ireland were part of Gondwana continent in late Precambrian time, when they were affected by the Cadomian orogeny. During the Cambrian, this part of Gondwana lay in high southern latitudes, 40 degrees away from northern Britain, across the intervening Iapetus Ocean. In early Ordovician time, southern Britain started to rift away from Gondwana as part of eastern Avalonia crustal fragment, probably together with western Avalonia, Iberia, and Armorica. Eastern Avalonia drifted northwards.
During the late Devonian time (fourth period of Paleozoic), eastern and western Avalonia and Baltica were welded to Laurentia. However, they were still separated from Gondwana by the Rheic Ocean. After the Carboniferous extension, the closure of this ocean was accompanied by the Variscan orogeny. This affected the southern fringe of Ireland, Wales, and England.
At the beginning of the Mesozoic, the British Isles began to drift across the Equator as it continued to move forward as part of the global supercontinent Pangaea. However, this continent started to break up in Permo-Triassic time, giving way to the formation of new oceans, such as the Atlantic. The associated crustal extension dominated the British areas throughout the Triassic, particularly in the North Sea and on the Atlantic margin.
The weak Alpine shortening affected southern England in the Paleocene (Tertiary period/Cenozoic era). This was a remote effect of continental collision in southern Europe. Subsequent crustal movements have been dominantly vertical.
Below, schematic drawing of the main continental fragments, which are relevant to the geological history of the British Isles.
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