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Thegeology of Great Britain is renowned for its diversity. As a result of its eventful geological history,Great Britain shows a rich variety oflandscapes across the constituent countries ofEngland,Wales andScotland. Rocks of almost all geological ages are represented atoutcrop, from theArchaean onwards.
Seismographic research shows that thecrust of theEarth belowGreat Britain is from 27 to 35 km (17 to 22 mi) thick. The oldest surface rocks are found in northwestScotland and are more than half asold 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 inBrittany and theChannel Islands. The youngest rocks are found in southeastEngland.
Thebedrock geology consists of a complex mix of generally oldermetamorphic rocks overlain by varying sequences ofsedimentary rocks into both of whichigneous rocks have been intruded at different times. The complexity of Great Britain's geology is due ultimately to its being subject to a variety ofplate 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 itsgeological structure with major faulting and folding being a legacy of eachorogeny (mountain-building period), often associated withvolcanic 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 anytectonic boundary or active volcanic region, nor are there other geological phenomena in the island group able to give rise to active volcanism.[1] 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 theerodedigneous rocks formed during theseorogenies (mountain-building periods).[1] TheBritish Geological Survey listsSnowdonia andthe 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.[1]
Overlain on thisbedrock 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 ofglacial 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".
This description of the geological history of Great Britain is based on that of P. Toghill.[2]
TheLewisian gneiss, the oldest rocks inGreat Britain, date from at least 2.7 billion years ago in theArchaean eon, theEarth itself being about 4.6 billion years old. They are found in the far north west ofScotland and in theHebrides, 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 andmetamorphosed into crystalline gneiss.
South of the gneisses are a complex mixture of rocks forming the North WestHighlands andGrampian Highlands in Scotland. These are essentially the remains of foldedsedimentary rocks that were originally 25 km thick, deposited over the gneiss on what was then the floor of theIapetus Ocean. The process started about 1,000 Ma, with a notable 7 km thick layer ofTorridon Sandstone being deposited about 800 Ma, as well as thedebris deposited by anice sheet 670 Ma.
Palaeomagnetic evidence indicates that 520 Ma, what is now Great Britain was split between twocontinents, separated by 7,000 km (4,300 mi) of ocean. The north of Scotland was located at about 20° south of theequator on the continent ofLaurentia near theTropic of Capricorn, while the rest of the country was at about 60° south on the continent ofGondwana near theAntarctic Circle.
In Gondwana, England andWales were near asubduction zone. Both countries were largely submerged under a shallow sea studded withvolcanic islands. The remains of these islands underlie much of central England with small outcrops visible in many places. Around 600 Ma, theCadomian Orogeny (mountain building period) caused the English and Welsh landscape to be transformed into a mountainous region, along with much of north westEurope.
In the earlyCambrian 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 ofsediment 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.
Five hundred million years ago, in theOrdovician period, southern Great Britain, the east coast ofNorth America and south-eastNewfoundland broke away from Gondwana to form the continent ofAvalonia, which by 440 Ma had drifted (by the mechanisms ofplate tectonics) to about 30° south.
During this period, north Wales was subject tovolcanic activity. The remains of these volcanoes are still visible, one example of which isRhobell Fawr dating from 510 Ma. Large quantities of volcaniclava and ash known as theBorrowdale Volcanics covered theLake District and this can still be seen in the form of mountains such asHelvellyn andScafell Pike.
The Ordovician also saw the formation of theSkiddawslate deposits around 500 Ma.
Deposition continued into the early part of theSilurian period, with mudstones and sandstones being laid down, notably in Wales.
Avalonia had now joined with the continent ofBaltica, and the combined landmass collided withLaurentia at about 20° south between 425 and 400 Ma, joining the southern and northern halves ofGreat Britain together. The resultingCaledonian Orogeny produced anAlpine-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 alongstrike-slip faults trending north-east to south-west across Scotland, theGreat 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 theMendip Hills and inPembrokeshire.
The collision between continents continued during theDevonian period, with continuing uplift, and more volcanic deposits such as those now formingBen 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 ofDevon gave the period its name, though deposits of this age are found in many other places, such as theBrecon Beacons, theMidland Valley of Scotland, and theOrkney Islands. Most of these are of terrestrial origin and are informally known as theOld Red Sandstone.
TheCaledonian mountains had largely beeneroded away by the end of the period during which the country would have experienced an ariddesertclimate as it was located close to theequator at between 10° and 15° south.
Around 360 Ma, at the start of theCarboniferous period, Great Britain was lying at the equator, covered by the warm shallow waters of theRheic Ocean, during which time the Carboniferous Limestone was deposited, as found in theMendip Hills, north and south Wales, in thePeak District ofDerbyshire, northLancashire, the northernPennines and southeast Scotland.Caves developed more recently in the limestone of some of these areas by the action ofcarbonic acid and otherorganic acids in rainwater andgroundwater.
These were followed by dark marineshales,siltstones, and coarse sandstones of theMillstone Grit. Later,river deltas formed and the sediments deposited were colonised byswamps andrain forest. It was in this environment that thecyclicCoal Measures were formed, the source of the majority of Great Britain's extensivecoal reserves that powered theIndustrial Revolution. Coal can be found in many areas of Great Britain, as far north as the Midland Valley ofScotland, as far south asKent, 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 ofcontinental plates. The mountain building period known as theVariscan orogeny, which occurred around 280 Ma, caused major deformation in south west England. Towards the end of this periodgranite was formed beneath the overlying rocks ofDevon andCornwall, now exposed atDartmoor andBodmin Moor, giving rise to mineralised deposits ofcopper andtin. The general region of Variscan folding was south of an east–west line roughly from southPembrokeshire toKent. The maintectonic pressure was from the south or south-east, and there developeddextral strike-slip faulting. TheDevon-Cornwall massif may originally have been some distance further east, then to be moved westwards. Lesser Variscan folding took place as far north asDerbyshire andBerwick-upon-Tweed.
By the end of the Carboniferous period, the various continents of the Earth had fused to form the super-continent ofPangaea. 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 redsedimentary rock, somewhat similar to the later,TriassicNew Red Sandstone.
ThePermian 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 theTethys Ocean andZechstein Sea formed, depositingshale,limestone,gravel, andmarl, before finally receding to leave a flat desert withsalt pans.
As Pangaea drifted during theTriassic, Great Britain moved away from the equator until it was between 20° and 30° north. Red beds, includingsandstones and redmudstones form the main sediments of theNew Red Sandstone. The remnants of the Variscan uplands inFrance to the south were eroded down, resulting in layers of the New Red Sandstone being deposited across central England, and in faulted basins inCheshire and the Irish Sea. A basin developed in theHampshire region around this time.Rifting occurred within and around Great Britain, prior to the breakup of the super-continent in theJurassic period.
Rock fragments found nearBristol appear to indicate that, in 214 Ma, Great Britain was showered with a fine layer of debris from anasteroid impact at theManicouagan Impact Crater inCanada, although this is still being debated.
As theJurassic period began,Pangaea began to break up, sea levels rose and Great Britain drifted on theEurasian 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 theCleveland Hills ofYorkshire to theJurassic Coast inDorset. They also underlie younger strata to the east. In Southern England, the Jurassic rocks are subdivided upwards as theLias,Inferior Oolite,Great Oolite,Ancholme (interfingering withCorallian) andPortland groups. These rock units includesandstones,greensands,ooliticlimestone of theCotswold Hills,corallian limestone of theVale of White Horse and theIsle of Portland.
Sedimentation would retreat from most of England beginning in the Cretaceous, with the deposition of the lagoonalPurbeck and primarily freshwaterWealden groups being largely confined to theWessex andWeald basins, with marine units, including theSpeeton Clay, occurring in eastern England. The Aptian saw amarine transgression that flooded most areas of southern England, into shallow seas, with the successive deposition of theLower Greensand andGault andUpper Greensand occurring in the Aptian-Albian. At the beginning of the Late Cretaceous, around 100 million years ago, deposition changed to that ofchalk, with the deposition of theChalk Group continuing until the end of the Cretaceous. Chalk was deposited over much of Great Britain, now notably exposed at theWhite Cliffs of Dover and theSeven Sisters, and also formingSalisbury 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.
In the earlyPalaeogene period, between 63 and 52 Ma, the last volcanic rocks in Great Britain were formed. The major eruptions at this time produced theAntrim Plateau, thebasaltic columns of theGiant's Causeway and the lavas and igneousintrusions of theInner Hebrides of Scotland.
An early phase of theAlpine Orogeny, about 50 Ma, was responsible for the shaping of theLondon Basinsyncline, theWeald-Artois Anticline to the south, the North Downs, South Downs and Chiltern Hills.
During the period, theNorth Sea formed and Great Britain wasuplifted. 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 theLondon Clay, were deposited over southern England, while theEnglish Channel was characterised bymud flats and river-depositedsands. 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 asbeech,oak,redwood andpalm, along withgrasses.
In theMiocene andPliocene epochs of theNeogene, further uplift and erosion occurred, particularly in Wales, the Pennines, and the Scottish Highlands. The only significant Miocene deposit in Great Britain is theBrassington Formation in the Peak District. Evidence from the Brassington Formation suggests that, in the middle and late Miocene (12 to 7 million years ago) a diverse warm-temperate forest grew in the UK.[3][4][5] The oldest Neogene sediments have been reported fromWales and include fossil pollen that shows asubtropical mixed forest once grew onAnglesey.[6] The climate of the UK cooled and became drier throughout the Neogene[7][8]
The major changes during thePleistocene were brought about by several recentice ages. The most severe was theAnglian Glaciation, with ice up to 1,000 metres (3,300 feet) thick that reached as far south asLondon andBristol. This took place between about 478,000 to 424,000 years ago, and was responsible for the diversion of theRiver Thames onto its present course.
There is extensive evidence in the form of stone tools that southern England was colonised byhuman populations during the warmHoxnian Stage that followed the Anglian Glaciation. It is possible that theEnglish Channel repeatedly opened and closed during this period, causing Great Britain to become an island from time to time. The oldest humanfossils found to date in the British Isles, which include the skull ofSwanscombe Man from 400,000 years ago, and the earlierClactonian Man, also date from this period.
TheWolstonian Glaciation, between about 352,000 to 130,000 years ago, which is thought to have peaked around 150,000 years ago, was named after the village ofWolston southeast ofCoventry which is thought to mark the southern limit of the ice.
The Wolstonian Stage was followed by theIpswichian Stage, during whichhippopotamus are known to have lived as far north asLeeds.
During the most recentDevensian glaciation, which is thought to have started around 115,000 years ago, peaked around 20,000 years ago and ended a mere 10,000 years ago, theUsk valley andWye valley were eroded byglaciers, with the icesheet itself reaching south toWolverhampton andCardiff. The oldest human remains in Great Britain, theRed Lady of Paviland (29,000 years old), date from this time. It is thought that the country was eventually abandoned as the ice sheet reached its peak, being recolonised as it retreated. By 5,000 years ago, it is thought that Great Britain was warmer than it is at present.
Among the features left behind by the ice are thefjords of the west coast of Scotland, theU-shaped valleys of theLake District anderratics (blocks of rock) that have been transported from theOslo region ofNorway and deposited on the coast ofYorkshire.
The last twelve thousand years are known as theHolocene Epoch. Amongst the most significant geological features created during this period are thepeat deposits of Scotland, and of coastal and upland areas of England and Wales. Many of the lowland deposits, such as theSomerset Levels,The Fens andRomney Marsh have recently been artificially drained.
Since humans began clearing the forest during theNew Stone Age, most of the land has now beendeforested, speeding the natural processes oferosion. Large quantities of stone, gravel and clay are extracted each year, and by 2000 11% of England was covered byroads orbuildings.
At the present time, Scotland is continuing to rise as a result of the weight of Devensian ice being lifted. Southern and eastern England is sinking, generally estimated at 1 mm (1⁄25 inch) per year, with the London area sinking at double the speed partly due to the continuingcompaction of the recent clay deposits.
In addition, rises in sea level thought to be due toglobal warming appear likely to make low-lying areas of land increasingly susceptible to flooding, while in some areas the coastline continues to erode at a geologically rapid rate.
Great Britain continues to be subject to several very minorearthquakes each month, and more occasional light to moderate ones. During the 20th century, 25 earthquakes with a moment magnitude of 4.5 to 6.1 were felt,[9] many of them originating within the Isles themselves.
Great Britain's complex geology has provided it with a wide range of geological resources including abundant supplies ofcoal, the initial extraction of which powered theIndustrial Revolution. During the 20th century, the exploitation ofNorth Sea oil and gas provided further energy supplies for the nation.[10]Geothermal energy andonshore oil have also been exploited to a lesser extent.
A wide variety ofstone has been worked for building and general construction needs (e.g.sand and gravel) while other minerals have been mined or quarried for the chemical industry (e.g.salt), the production of metals (e.g.copper and tin) or other uses (e.g.china clay).
| Geology of England |
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| Geology of Wales | |
| Geology of Scotland | |
| Geology of Ireland | |
| Crown Dependencies | |
