Northern Ireland - Antrim Plateau

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This area is mainly within Antrim but extends westwards into Londonderry. It is largely an upland area, covered by thin glacial superficial deposits formed as a consequence of ice that built up around Lough Neagh. The steep-sided river valleys or Glens of Antrim are found on the eastern margin of the plateau. The main settlements are Londonderry, Coleraine, Ballymena, Antrim and Belfast.

Sedimentary bedrock

The Antrim Plateau is a unique geological area of the UK in that it contains an almost continuous sequence of flat-lying layers from 420 to 30 million years old. The uppermost layer of the plateau is composed of hard, black basalt lava that erupted from deep fissures in the Earth’s crust as the North Atlantic Ocean began to open about 60 million years ago. In places as much as 1 km of basalt is known to be present. Below the basalt, is a layer of the Chalk up to a maximum of 120 m thick, which was laid down in an ocean about 80 million years ago. Chalk is a fine-grained white rock composed of fragments and microfossils of calcium carbonate; it is a special type of limestone. The Chalk of Northern Ireland is unusual because it is extremely hard. The value of chalk lies in its purity which makes it much sought after as a raw material. Around the margins of the plateau, caves have been formed in the chalk where it has been dissolved by slightly acidic rain and stream waters.

Beneath the Chalk the rocks are 200 million year old mudstones and limestones that are up to 250 m thick, which formed in warm shallow seas that existed at a time when dinosaurs roamed the Earth and Northern Ireland lay south of the Equator. Because of their position below the harder chalk and basalt, some of the softer mudstone layers are responsible for often spectacular landslips that are seen around the coast of County Antrim. Deeper still lie red mudstones and sandstones comprising the Mercia Mudstone and Sherwood Sandstone. This sedimentary sequence is known to be up to 3 km thick in the vicinity of Larne and was laid down between 300 and 200 million years ago by rivers, lakes and sand dunes in desert environments. The sandstones contain pores, or spaces between the grains that can hold water, oil or gas so this layer is an important aquifer, meaning it can be used to supply drinking water, and is a potential reservoir for oil and gas resources.

Finally rocks, ranging between 420 to 300 million years old, are also found in places below the Antrim Plateau and indicate that shallow seas and deltaic environments existed here at a time when Northern Ireland lay in tropical latitudes. These rocks are rich in fossil plant material that is most obvious when seen as coal and less so when trapped in black mudstones. These carbon rich rocks, the Coal Measures, are economically vital in that they contain coal seams and are potential sources for oil and gas.

Basement rocks

The basement rocks are deeply buried throughout this area. In north-east Antrim they are known to be metamorphic rocks identical to those of the Sperrin Mountains.