- Geology of Great Britain
The Geology of Great Britain is hugely varied and complex, and gives rise to the wide variety of landscapes found across the islands. This varied
geologyhas also meant that the island has been an important source for the formation of many geological concepts.
Seismographical research shows that the crust of the
Earthbelow Great Britainis between 27 and 35 km (17 to 22 miles) thick. The oldest rocks are found at the surface in north west Scotlandand are more than half as old as the planet. They are thought to underlie much of Great Britainand Ireland(although boreholes have only penetrated the first few kilometres), but next appear extensively at the surface in Brittanyand the Channel Islands. The youngest rocks are found in south east England.
bedrockconsists of many layers formed over vast periods of time. These were laid down in various climates as the global climate changed, the landmasses moved due to Plate Tectonics, and the land and sea levels rose or fell. From time to time horizontal forces caused the rock to undergo considerable deformation, folding the layers of rock to form mountains which have since been eroded and overlain with other layers. To further complicate the geology, the land has also been subject to periods of earthquakes and volcanic activity.
Deposits by glaciers
Overprinted on this
bedrockgeology ("solid geology" in the terminology of the maps) is a somewhat variable distribution of soils and fragmental material deposited by glaciers ( boulder clay, and other forms of glacial drift in the recent past. Maps showing the distribution of this "drift" geology are frequently produced as either separate maps, or as literal overprints on the solid geology maps. When ordering maps, this distinction should be kept in mind. Catalogues often distinguish them as "S", "D" or "S+D" maps. "Drift" geology is often more important than "solid" geology when considering building works, drainage, siting water boreholes, sand & gravel resources, soil fertility, and many other issues. 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".
cite book |title=The Geology of Britain: An Introduction |last=Toghill |first=Peter |authorlink= |coauthors= |year=2000 |publisher=Swan Hill Press |location= Shrewsbury|isbn=1 85310 890 1 ]
Gneisses, the oldest rocks in Great Britainor Ireland, date from at least 2,700 Ma (Ma = millions of years ago) in the Archaeanperiod of this era, the Earthitself being about 4,600 Ma old. They are found in the far north west of Scotlandand 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.
South of the Gneisses are a complex mixture of rocks forming the North West Highlands and
GrampianHighlands in Scotland, as well as the Connemara, Donegaland Mayo mountains of Ireland. 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 in about 1,000 Ma, with a notable 7 km thick layer of Torridon Sandstonebeing deposited about 800 Ma, as well as the debris deposited by an ice sheet670 Ma.
Palaeomagnetic evidence indicates that 520 Ma, what is now the UK was split between two
continents, separated by 7000 km (4500 miles) of ocean. The north of Scotland was located at about 20° south of the equatoron the continent of Laurentianear the Tropic of Capricorn, while the rest of the country was at about 60° south on the continent of Gondwananear the Antarctic Circle.
In Gondwana, England and
Waleswere near a subductionzone. 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.
In the early
Cambrianperiod 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 sedimentwere 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. As this is when the first hard shells evolved, fossils become much more common from this period onwards.
500 million years ago, in the
Ordovicianperiod, southern Britain, the east coast of North Americaand 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 and south Mayo experienced volcanic activity. The remains of these volcanoes are still visible, for example
Rhobell Fawr, dating from 510 Ma. Large quantities of volcanic lavaand ash known as the Borrowdale Volcanicscovered both Wales and the Lake District, still seen in the form of mountains such as Helvellynand Scafell Pike.
The Ordovician also saw the formation of the Welsh
Skiddaw slatedeposits around 500 Ma.
Deposition continued into the early part of the
Silurianperiod, with mudstones and sandstones being laid down, notably in Wales. Avaloniahad now joined with the continent of Baltica, and the combined landmass collided with Laurentiaat about 20° south between 425 and 400 Ma, joining the southern and northern halves of Great Britaintogether. The resulting Caledonian Orogenyproduced an Alpine-style mountain range in much of north and west 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 Faultbeing 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 Hillsand in Pembrokeshire.
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 Old Red Sandstoneof Devongave the period its name, though deposits are found in many other places, such as the Brecon Beacons, the Midland Valley of Scotland, and the Orkney Islands.
Caledonian mountainshad largely been eroded away by the end of the period during which the country would have experienced an arid desert climateand been located between 10° and 15° south of the equator.
Around 360 Ma during the
Carboniferousperiod 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 Districtof Derbyshire, north Lancashire, the northern Penninesand southeast Scotland. Caves have developed more recently in the limestone in some of these areas.
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 Measureswere formed, the source of the majority of Britain's extensive coalreserves that powered the Industrial Revolution. Coal can be found in many areas of Britain and Ireland, as far north as the Midland Valley of Scotland, as far south as Kentand in Ireland, though it has largely been mined in the English midlands, northern England and Wales.
Throughout the period, southwest England in particular was affected by the collision of
continental plates. The Variscan orogeny(mountain building period) around 280 Ma caused major deformation in south west England. Towards its end granitewas formed beneath the overlying rocks of Devonand Cornwall, now exposed as Dartmoorand Bodmin Moor, giving rise to mineralised deposits of copperand tin. The general region of Variscan folding was south of an east–west line roughly from south Pembrokeshireto Kent. The main tectonicpressure was from the south or south-east, and may have resulted in dextral strike-slip faulting. The Devon- Cornwallmassif may originally have been some distance further east, then to be moved westwards. Lesser Variscan folding took place as far north as Derbyshireand Berwick-upon-Tweed.
By the end of the period the various continents of the World had fused to form one super-continent of
Pangaea, with Britain in the interior, where it was again subject to a hot arid desert climate, with frequent flash floods leaving deposits that formed red beds, somewhat similar to the later, Triassic New Red Sandstone.
Permianwas characterised for 30 million years by arid desert and erosion of the areas uplifted in the Variscan Orogeny (southwest England and adjacent areas in the present-day English Channel). Later, much of Great Britain was submerged in shallow waters as the polar ice sheets melted and the Tethys Oceanand Zechstein Seaformed, depositing shale, limestone, gravel, and marl, before finally receding to leave a flat desert with salt pans.
As Pangaea drifted during the
Triassic, Great Britain moved away from the equator until it was between 20° and 30° north. Red beds, including sandstonesand red mudstones form the main sediments of the New Red Sandstone. The remnants of the Variscan uplands in Franceto the south were eroded down, resulting in layers of the New Red Sandstone being deposited across central England, and in faulted basins in Cheshireand the Irish Sea. A basin developed in the Hampshireregion around this time. Rifting occurred within and around Britain and Ireland, prior to the breakup of the super-continent in the Jurassic period.
Rock fragments found near
Bristolappear to indicate that in 214 Ma Great Britain was showered with a fine layer of debris from an asteroid impact at the Manicouagan Impact Craterin Canada, although this is still being debated.
Jurassicstarted, Pangaeabegan to break up and sea levels rose, as Britain and Ireland drifted on the Eurasian Plateto between 30° and 40° north. With much of the Isles under water again, sedimentary rocks were deposited and can now be found underlying much of southern England from the Cleveland Hillsof Yorkshireto the Jurassic Coastin Dorset, including clays, sandstones, greensands, oolitic limestoneof the Cotswold Hills, corallian limestoneof the Vale of White Horseand the Isle of Portland.
The burial of
algaeand bacteria below the mud of the sea floor during this time resulted in the formation of North Sea oiland natural gas, much of it trapped in overlying sandstone by saltdeposits formed as the seas fell to form the swamps and salty lakes and lagoons that were home to dinosaurs. Fact|date=August 2008
The modern continents having formed, the
Cretaceoussaw the formation of the Atlantic Ocean, gradually separating northern Scotland from North America. The land underwent a series of uplifts to form a fertile plain.
After 20 million years or so, the seas started to flood the land again until much of Britain and Ireland were again below the sea, though sea levels frequently changed.
Chalkand flints were deposited over much of Great Britain, now notably exposed at the White Cliffs of Doverand the Seven Sisters, and also forming Salisbury Plain. The high sea levels left only small areas of land exposed. This caused the general lack of land-origin sand, mud or clay sediments around this time - some of the late Cretaceous strata are almost pure chalk.
In the early
Palaeogeneperiod between 63 and 52 Ma, the last volcanic rocks in Great Britain were formed, with the major eruptions that formed the Antrim Plateau and the basaltic columns of the Giant's Causeway. The volcanic Lundy Islandin the Bristol Channelalso dates from this period.
Alpine Orogenythat took place about 50 Ma was responsible for the shaping of the London Basin syncline, the Weald-Artois Anticlineto the south, and also the North Downs, South Downs and Chiltern Hills.
During the period the
North Seaformed, Britain was uplifted. Some of this uplift was along old lines of weakness from the Caledonian and Variscan Orogenies long before. The uplifted areas were then eroded, and further sediments were deposited over southern England, including the London Clay, while the English Channelconsisted of 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 lost them through erosion. By 35 Ma the landscape included beech, oak, redwood and palm trees, along with grassland.
Miocene and Pliocene epochs
Mioceneand Plioceneepochs of the Neogene, further uplift and erosion occurred, particularly in Wales, the Pennines, and the Scottish Highlands. Plant and animal types developed into their modern forms, and by about 2 million years ago the landscape would have been broadly recognisable today.
The major changes during the
Pleistocenehave been brought about by several recent ice ages.
The most severe was the
Anglian Stage, with ice up to 1,000 m (3300 ft) thick that reached as far south as Londonand Bristol, took place between about 478,000 to 424,000 years ago, and was responsible for the diversion of the River Thamesonto its present course.
There is extensive evidence in the form of stone tools that southern England was colonised by
humanpopulations during the warm Hoxnian Stagethat followed the Anglian Glaciation. It is possible that the English Channelrepeatedly opened and closed during this period, causing Britain to become an island from time to time. The oldest human fossils in the Isles also date from this time, including the skull of Swanscombe Manfrom 250,000 years ago, and the earlier Clactonian Man.
Wolstonian Stage, between about 352,000 to 130,000 years ago, and thought to have peaked around 150,000 years ago, was named after the town of Wolstonsouth of Birminghamwhich is thought to mark the southern limit of the ice.
The Wolstonian Stage was followed by the
Ipswichian interglacial, during which hippopotamusare known to have lived as far north as Leeds.
During the most recent
Devensian 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, the Usk valley and Wye valley were eroded by glaciers, with the ice sheet itself reaching south to Birmingham. The oldest human remains in Britain or Ireland, the Red 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 the
fjords of the west coast of Scotland, the U shaped valleys of the Lake Districtand erratics (blocks of rock) that have been transported from the Osloregion of Norwayand deposited on the coast of Yorkshire.
Over the last twelve thousand years (the
HoloceneEpoch) the most significant new geological features have been the deposits of peatin Ireland and Scotland, as well as in coastal areas that have recently been artificially drained such as the Somerset Levels, The Fensand Romney Marshin England.
Since humans began clearing the forest during the new stone age, most of the land has now been deforested, speeding the natural processes of
erosion. Large quantities of stone, gravel and clay are extracted each year, and by 200011% of England was covered by roads or buildings.
At the present time Scotland is continuing to rise as a result of the weight of Devensian ice being lifted. The rest of Britain 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 continuing compression of the recent clay deposits.
In addition, rises in sea level thought to be due to
global warmingappear 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 minor
earthquakes each month, and occasional light to moderate ones. During the 20th century 25 earthquakes with a magnitude of 4.5 to 6.1 on the Richter scale were felt [http://www.quakes.bgs.ac.uk/hazard/eqlst.htm] , many of them originating within the Isles themselves.
Cheddar Gorge- the largest gorge in Great Britain
Jurassic Coast- a UNESCO World Heritage Site
Great Glen Fault
Highland Boundary Fault
Southern Uplands Fault
*Midland Valley of Scotland
Siccar Pointin Berwickshireprovided early proof of the immense age of the Earth.
*The Moine Thrust was the first thrust belt to be discovered by geologists.
Dartmoor- one of a series of Moors in the South West of England developing Tors on a granitic batholith
North Sea oil
*Copper and Tin
Storegga Slides, caused a tsunami circa 6100 BC
Bristol Channel floods, 1607was caused by a tsunami
1755 Lisbon earthquakecaused a tsunami that hit Cornwall.
2002 Dudley earthquake
Geological Society of London
Edinburgh Geological Society
British Geological Survey
Pioneers of British Geology
Thomas George Bonney1833-1923
James Hutton1726-1797 the "Father of modern geology"
*Sir Charles Lyell 1797-1875
*Sir Roderick Murchison 1792–1871
*John Phillips 1800–1874
*William Smith 1769-1839 the "Father of English geology"
----This cross section shows what would be seen in a deep cutting nearly E. and W. across England and Wales. It shows also how, in consequence of the folding of the strata and the cutting off of the uplifted parts, old rocks which should be tens of thousands of feet down are found in borings in East Anglia only 1000 feet or so below the surface.]
List of natural disasters in the United Kingdom
Rock formations in the United Kingdom
* [http://www.quakes.bgs.ac.uk/ UK Earthquakes]
* [http://www.thepeakdistrict.info/fast/html/peak_district_geology.html UK Peak District Geology]
* [http://www.geographyinaction.co.uk/Geology%20files/Geol_index.html Northern Ireland Geology]
* [http://www.discoveringfossils.co.uk UK Geology/Fossil locations]
* [http://www.soton.ac.uk/~imw/jpg/eurogy.jpgGeology map of Europe]
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