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

Seismographical research shows that the crust of the Earth below Great Britain is from 27 to 35 km (17 to 22 miles) thick. The oldest surface rocks are found in north west 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 south east 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 material 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 2,700 Ma (million years ago) in the Archaean eon, the Earth itself being about 4,600 million 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 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.

Silurian period

Deposition continued into the early part of the Silurian period, with mudstones and sandstones being laid down, notably in Wales.

Avalonia had now joined with the continent of Baltica, and the combined landmass collided with Laurentia at about 20° south between 425 and 400 Ma, joining the southern and northern halves of Great Britain together. The resulting Caledonian Orogeny produced an Alpine-style mountain range in much of north and west Great Britain. The continental collision was probably at an oblique angle rather than a head-on collision, and this probably led to movement along strike-slip faults trending north-east to south-west across Scotland, the Great Glen Fault being the best example (some of these fault zones may have been old lines of weakness from earlier earth movements).

Volcanic ashes and lavas deposited during the Silurian are still found in the Mendip Hills and in Pembrokeshire.

Devonian period

The collision between continents continued during the Devonian period, with continuing uplift, and more volcanic deposits such as those now forming Ben Nevis. Sea levels varied considerably, with the coastline advancing and retreating from north to south across England. The uplifted region was gradually eroded down, resulting in the deposition of numerous sedimentary rock layers in lowlands and seas. The rocks of marine origin in the county of Devon gave the period its name, though deposits of this age are found in many other places, such as the Brecon Beacons, the Midland Valley of Scotland, and the Orkney Islands. Most of these are of terrestrial origin and are informally known as the Old Red Sandstone.

The Caledonian mountains had largely been eroded away by the end of the period during which the country would have experienced an arid desert climate as it was located close to the equator at between 10° and 15° south.

Carboniferous period

Around 360 Ma, at the start of the Carboniferous period, Great Britain was lying at the equator, covered by the warm shallow waters of the Rheic Ocean, during which time the Carboniferous Limestone was deposited, as found in the Mendip Hills, north and south Wales, in the Peak District of Derbyshire, north Lancashire, the northern Pennines and southeast Scotland. Caves developed more recently in the limestone of some of these areas by the action of carbonic acid and other organic acids in rainwater and groundwater.

These were followed by dark marine shales, siltstones, and coarse sandstones of the Millstone Grit. Later, river deltas formed and the sediments deposited were colonised by swamps and rain forest. It was in this environment that the cyclic Coal Measures were formed, the source of the majority of Great Britain's extensive coal reserves that powered the Industrial Revolution. Coal can be found in many areas of Great Britain, as far north as the Midland Valley of Scotland, as far south as Kent, although coal mining has largely been concentrated in the Scottish Midlands, English Midlands, northern England and Wales.

Throughout the period, southwest England in particular was affected by the collision of continental plates. The mountain building period known as the Variscan orogeny, which occurred around 280 Ma, caused major deformation in south west England. Towards the end of this period granite was formed beneath the overlying rocks of Devon and Cornwall, now exposed at Dartmoor and Bodmin Moor, giving rise to mineralised deposits of copper and tin. The general region of Variscan folding was south of an east–west line roughly from south Pembrokeshire to Kent. The main tectonic pressure was from the south or south-east, and there developed dextral strike-slip faulting. The Devon-Cornwall massif may originally have been some distance further east, then to be moved westwards. Lesser Variscan folding took place as far north as Derbyshire and Berwick-upon-Tweed.

By the end of the Carboniferous period, the various continents of the Earth had fused to form the super-continent of Pangaea. Great Britain was located in the interior of Pangea, where it was again subject to a hot arid desert climate with frequent flash floods leaving deposits that formed beds of red sedimentary rock, somewhat similar to the later, Triassic New Red Sandstone.

Permian period

The Permian was characterised for 30 million years by arid desert conditions and the erosion of the land that had uplifted in the Variscan Orogeny, southwest England and adjacent areas of the present-day English Channel. Later, much of Great Britain was submerged in shallow waters as the polar ice sheets melted and the Tethys Ocean and Zechstein Sea formed, depositing shale, limestone, gravel, and marl, before finally receding to leave a flat desert with salt pans.

Mesozoic era

Triassic period

As Pangaea drifted during the Triassic, Great Britain moved away from the equator until it was between 20° and 30° north. Red beds, including sandstones and red mudstones form the main sediments of the New Red Sandstone. The remnants of the Variscan uplands in France to the south were eroded down, resulting in layers of the New Red Sandstone being deposited across central England, and in faulted basins in Cheshire and the Irish Sea. A basin developed in the Hampshire region around this time. Rifting occurred within and around Great Britain, prior to the breakup of the super-continent in the Jurassic period.

Rock fragments found near Bristol appear to indicate that, in 214 Ma, Great Britain was showered with a fine layer of debris from an asteroid impact at the Manicouagan Impact Crater in Canada, although this is still being debated.

Jurassic period

As the Jurassic period began, Pangaea began to break up, sea levels rose and Great Britain drifted on the Eurasian Plate to between 30° and 40° north. With much of Great Britain under water again, sedimentary rocks were deposited and can now be found outcropping across much of England from the Cleveland Hills of Yorkshire to the Jurassic Coast in Dorset. They also underlie younger strata to the east. In Southern England, the Jurassic rocks are subdivided upwards as the Lias, Inferior Oolite, Great Oolite, Ancholme (interfingering with Corallian) and Portland groups. These rock units include sandstones, greensands, oolitic limestone of the Cotswold Hills, corallian limestone of the Vale of White Horse and the Isle of Portland.

Cretaceous period

Sedimentation would retreat from most of England beginning in the Cretaceous, with the deposition of the lagoonal Purbeck and primarily freshwater Wealden groups being largely confined to the Wessex and Weald basins, with marine units, including the Speeton Clay, occurring in eastern England. The Aptian saw a marine transgression that flooded most areas of southern England, into shallow seas, with the successive deposition of the Lower Greensand and Gault and Upper Greensand occurring in the Aptian-Albian. At the beginning of the Late Cretaceous, around 100 million years ago, deposition changed to that of chalk, with the deposition of the Chalk Group continuing until the end of the Cretaceous. Chalk was deposited over much of Great Britain, now notably exposed at the White Cliffs of Dover and the Seven Sisters, and also forming Salisbury Plain. The high sea levels left only small areas of land exposed, which accounts for the general lack of land-origin sand, mud or clay sediments found from around this time.

Cenozoic era

Palaeogene period

In the early Palaeogene period, between 63 and 52 Ma, the last volcanic rocks in Great Britain were formed. The major eruptions at this time produced the Antrim Plateau, the basaltic columns of the Giant's Causeway and the lavas and igneous intrusions of the Inner Hebrides of Scotland.

An early phase of the Alpine Orogeny, about 50 Ma, was responsible for the shaping of the London Basin syncline, the Weald-Artois Anticline to the south, the North Downs, South Downs and Chiltern Hills.

During the period, the North Sea formed and Great Britain was uplifted. Some of this uplift was along old lines of weakness left from the Caledonian and Variscan Orogenies long before. The uplifted areas were then eroded, and further sediments, such as the London Clay, were deposited over southern England, while the English Channel was characterised by mud flats and river-deposited sands. Much of the midlands and north of England may have been covered by Jurassic and Cretaceous deposits at the start of the Palaeogene, but these were lost through erosion. By 35 Ma, the landscape was colonised by trees such as beech, oak, redwood and palm, along with grasses.

Neogene period

=Miocene and Pliocene epochs

=
In the Miocene and Pliocene

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