Geology of Great Britain

The geology of Great Britain is renowned for its diversity. As a result of its eventful geological history, Great Britain shows a rich variety of landscapes across the constituent countries of England, Wales and Scotland. Rocks of almost all geological ages are represented at outcrop, from the Archaean onwards.

Overview Seismographic research shows that the crust of the Earth below Great Britain is from 27 to 35 km (17 to 22 mi) thick. The oldest surface rocks are found in northwest Scotland and are more than half as old as the planet. These rocks are thought to underlie much of Great Britain (although boreholes have only penetrated the first few kilometres), but next appear extensively at the surface in Brittany and the Channel Islands. The youngest rocks are found in southeast England.

Bedrock and past volcanism The bedrock geology consists of a complex mix of generally older metamorphic rocks overlain by varying sequences of sedimentary rocks into both of which igneous rocks have been intruded at different times. The complexity of Great Britain's geology is due ultimately to its being subject to a variety of plate tectonic processes over a very extended period of time. Changing latitude and sea levels have been important factors in the nature of sedimentary sequences, while successive continental collisions have affected its geological structure with major faulting and folding being a legacy of each orogeny (mountain-building period), often associated with volcanic activity and the metamorphism of existing rock sequences. Great Britain does not have any active volcanoes now, but has had an active volcanic past. The last active volcanoes stopped erupting about 60 million years ago and the islands are no longer located upon any tectonic boundary or active volcanic region, nor are there other geological phenomena in the island group able to give rise to active volcanism. However, on a geological timescale, a considerable degree of active and large-scale volcanism occurred in Great Britain, and many famous landmarks are formed of the eroded igneous rocks formed during these orogenies (mountain-building periods). The British Geological Survey lists Snowdonia and the Lake District as having extremely large volcanic eruptions around 450 Ma (million years ago), Edinburgh Castle lying upon the remains of a volcano dating back 350 Ma, and some islands of western Scotland as being remnants of volcanoes from around 60 Ma.

Superficial deposits Overlain on this bedrock geology ("solid geology" in the terminology of maps) is a varied distribution of unconsolidated material of more recent origin. It includes material deposited by glaciers (boulder clay and other forms of glacial drift such as sand and gravel). "Drift" geology is often more important than "solid" geology when considering building works, drainage, siting water boreholes, sand and gravel resources and soil fertility. Although "drift" strictly refers to glacial and fluvio-glacial deposits, the term on geological maps has traditionally included other materials including alluvium, river terraces, etc. Recent maps use the terms "bedrock" and "superficial" in place of "solid" and "drift".

Geological history This description of the geological history of Great Britain is based on that of P. Toghill.

Precambrian

Archaean eon The Lewisian gneiss, the oldest rocks in Great Britain, date from at least 2.7 billion years ago in the Archaean eon, the Earth itself being about 4.6 billion years old. They are found in the far north west of Scotland and in the Hebrides, with a few small outcrops elsewhere. Formed from rock originally deposited at the surface of the planet, the rocks were later buried deep in the Earth's crust and metamorphosed into crystalline gneiss.

Proterozoic eon South of the gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands in Scotland. These are essentially the remains of folded sedimentary rocks that were originally 25 km thick, deposited over the gneiss on what was then the floor of the Iapetus Ocean. The process started about 1,000 Ma, with a notable 7 km thick layer of Torridon Sandstone being deposited about 800 Ma, as well as the debris deposited by an ice sheet 670 Ma. Palaeomagnetic evidence indicates that 520 Ma, what is now Great Britain was split between two continents, separated by 7,000 km (4,300 mi) of ocean. The north of Scotland was located at about 20° south of the equator on the continent of Laurentia near the Tropic of Capricorn, while the rest of the country was at about 60° south on the continent of Gondwana near the Antarctic Circle. In Gondwana, England and Wales were near a subduction zone. Both countries were largely submerged under a shallow sea studded with volcanic islands. The remains of these islands underlie much of central England with small outcrops visible in many places. Around 600 Ma, the Cadomian Orogeny (mountain building period) caused the English and Welsh landscape to be transformed into a mountainous region, along with much of north west Europe.

Palaeozoic era

Cambrian period In the early Cambrian period, the volcanoes and mountains of England and Wales were eroded as the land became flooded by a rise in sea level, and new layers of sediment were laid down. Much of central England formed a stable block of crust, which has remained largely undeformed ever since. Sandstones were deposited in the north of Scotland. The first animals with hard shells evolved at this time, consequently, fossils become much more common in rocks formed during this and later periods.

Ordovician period Five hundred million years ago, in the Ordovician period, southern Great Britain, the east coast of North America and south-east Newfoundland broke away from Gondwana to form the continent of Avalonia, which by 440 Ma had drifted (by the mechanisms of plate tectonics) to about 30° south. During this period, north Wales was subject to volcanic activity. The remains of these volcanoes are still visible, one example of which is Rhobell Fawr dating from 510 Ma. Large quantities of volcanic lava and ash known as the Borrowdale Volcanics covered the Lake District and this can still be seen in the form of mountains such as Helvellyn and Scafell Pike. The Ordovician also saw the formation of the Skiddaw slate deposits around 500 Ma.

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