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*The Crust Around Britain

F17: The main crustal units of BritainF18: Younger cover time scale

Although the plates of the Earth's crust are in constant motion, the strongest effects of plate tectonics in any one area occur at peak times with longish periods of relative tranquillity in between. Rock strata accumulate in these tranquil periods and are disrupted and deformed by later plate collisions. This may cause thickening of the continental crust and its elevation as mountain ranges. The mountains are eroded down and deposition of sedimentary strata resumes. The crust is thus built up of units, some highly deformed, others hardly disturbed. These units are stacked above or against each other. The junctions of the units are either old erosion surfaces or large dislocations.

There are five major units building the crusts under British seas (F17). At the top is the Younger Cover, ranging in the age from Permian to Recent (F18). It fills the basins containing most of our oil and gas.

Below is the Older Cover comprising Devonian and/or Carboniferous strata between 400 and 300 million years old. In southern Britain, rocks of this age are strongly deformed and cut into slices which have been stacked above each other to form the Variscan fold-belt or Variscides. Under much of the North Sea, the Older Cover forms an 'underlay' resting on early Palaeozoic strata folded 410 million years ago (the late Caledonides) and mainly late Precambrian strata folded and metamorphosed 510 million years ago and earlier (the early Caledonides).

F19: The stucture of Britain and the adjacent continental shelf with, above, a section from the Western approaches to the hebrides

Precambrian rocks older than 600 million years form the foundations of the whole structure. In the north are ancient crystalline rocks with a flat cover of less ancient Precambrian strata involved in adjacent early Caledonian folding. In the south, the Precambrian basement was not formed until the very end of the Precambrian Era.

The map (F19) shows the outcrops of the main units of the continental crust in North-West Europe. It is immediately obvious that the British Isles are surrounded by seas mainly underlain by the Younger Cover formations. The map shows the location of the main depressions in which the Younger Cover was deposited. Some of these exceed 10km in depth. Over 'highs' such as the Mid-North Sea High and the London-Brabant Platform, the thickness diminishes to 2 km or less.

The Older Cover is extensively exposed on the sea floor only in the western Irish Sea and in the Orkney-Shetland region. Subsea outcrops of the Variscides border the Western Approaches and Celtic Sea basins. The Caledonides have a very restricted subsea outcrop consistent with their tendency to form upland areas of Britain and Ireland. The ancient Precambrian basement of North Britain - the 'Lewisian Gneiss' - is widely exposed whereas the young Precambrian of south Britain has almost as few subsea exposures as on land.

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