Geology of the British Isles

The Geology of the British Isles is hugely varied and complex, and gives rise to the wide variety of landscapes found across the islands. This varied geology has also meant that the country has been an important source for the formation of many geological concepts.

Seismographical research shows that the crust of the Earth below the British Isles is between 27 and 35 km (17 to 22 miles) thick. The oldest rocks are found at the surface in north west Scotland and are more than half as old as the planet. They are thought to underlie much of Britain and Ireland (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.

The bedrock consists 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 continental drift, 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.

Overprinted on this bedrock geology ("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, soil fertility, and many other issues.

Geological history

Proterozoic Era

The Gneisses, the oldest rocks in Britain or Ireland, date from at least 2,700 Ma (Ma = millions of years ago) in the Archean period of this era, the Earth itself being only about 4,600 Ma 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.

South of the Gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands in Scotland, as well as the Connemara, Donegal and Mayo mountains of northern 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 Sandstone being deposited about 800 Ma, as well as the debris deposited by an ice sheet 670 Ma.

Paleomagnetic 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 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.

Paleozoic 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. As this is when the first hard shells evolved, fossils become much more common from this period onwards.

Ordovician period

500 million years ago, in the Ordovician period, southern 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.

Having joined with the continent of Baltica, the combined landmass collided with Laurentia at about 20° south around 425 Ma, joining the southern and northern halves of the British Isles together.

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 lava and ash known as the Borrowdale Volcanics covered both Wales and the Lake District, still seen in the form of mountains such as Helvellyn and Scafell Pike.

The Ordovician also saw the formation of the Welsh 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. Later in the period, as the two halves of the British Isles collided between 425 and 400 Ma, the Caledonian fold mountains formed (the Caledonian Orogeny), covering much of the Isles perhaps 8000 feet (2500 m) thick. 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 (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 Old Red Sandstone of Devon gave 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.

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 and been located between 10° and 15° south of the equator.

Carboniferous period

Around 360 Ma during the Carboniferous period the British Isles were 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 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 Measures were formed, the source of the majority of Britain's extensive coal reserves 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 Kent and 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 granite was formed beneath the overlying rocks of Devon and Cornwall, now exposed as 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 may have resulted in 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 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.

Permian period

The Permian was 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 the British Isles were 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, the British Isles moved away from the equator until they were 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 Britain and Ireland, 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 started, Pangaea began to break up and sea levels rose, as Britain and Ireland drifted on the Eurasian Plate to 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 Hills of Yorkshire to the Jurassic Coast in Dorset, including clays, sandstones, and the oolitic limestone of the Cotswold Hills and the Isle of Portland.

The burial of algae and bacteria below the mud of the sea floor during this time resulted in the formation of North Sea oil and natural gas, much of it trapped in overlying sandstone by salt deposits formed as the seas fell to form the swamps and salty lakes and lagoons that were home to dinosaurs.

Cretaceous period

The modern continents having formed, the Cretaceous saw 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. Chalk and flints were 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. 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.

Cenozoic Era

Tertiary period

In the early Tertiary period between 63 and 52 Ma, the last volcanic rocks in the British Isles were formed, with the major eruptions that formed the Antrim Plateau and the basaltic columns of the Giant's Causeway. The volcanic Lundy Island in the Bristol Channel also dates from this period.

The Alpine Orogeny that took place about 50 Ma was responsible for the shaping of the Weald south of London, and also the North Downs, South Downs and Chiltern Hills.

During the period the North Sea formed, Britain was uplifted, particularly in Wales, the Pennines, and the Scottish Highlands. 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 Channel consisted 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 Tertiary, but lost them through erosion.

By 35 Ma the landscape included beech, oak, redwood and palm trees, along with grassland. By the end of the period, just 2 million years ago, the landscape would have been broadly recognisable today.

Quaternary period

The major changes during the Quaternary period have been brought about by several recent ice ages.

The most severe was the Anglian glaciation, with ice up to 1,000 m (3300 ft) thick that reached as far south as London and Bristol, took place between about 500,000 to 400,000 years ago, and was responsible for the diversion of the River Thames onto its present course.

There is extensive evidence in the form of stone tools that southern England was colonised by human populations during the warm Hoxnian interglacial period that followed the Anglian Glaciation. It is possible that the English Channel repeatedly 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 Man from 250,000 years ago, and the earlier Clactonian Man.

The Wolstonian glaciation, between about 200,000 to 130,000 years ago, and thought to have peaked around 150,000 years ago, was named after the town of Wolston south of Birmingham which is thought to mark the southern limit of the ice.

The Wolstonian was followed by the Ipswichian interglacial, during which hippopotamus are 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 the British Isles were warmer than they are 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 District and erratics (blocks of rock) that have been transported from the Oslo region of Norway and deposited on the coast of Yorkshire.

=Holocene Epoch

= Over the last twelve thousand years (the Holocene Epoch) the most significant new geological features have been the deposits of peat in Ireland and Scotland, as well as in coastal areas that have recently been artificially drained such as the Somerset Levels, The Fens and Romney Marsh in 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 2000 11% 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 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.

The British Isles continue 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 ^*, many of them originating within the Isles themselves.

Geological features

Cheddar Gorge - the largest gorge in the British Isles
Jurassic Coast - a UNESCO World Heritage Site
Great Glen Fault
Highland Boundary Fault
Southern Uplands Fault
Salisbury Plain
Tees-Exe line
Whin Sill

Geological resources

Events

Bristol Channel floods, 1607 (possible tsunami)
Colchester earthquake, 1884
2002 Dudley earthquake

Institutions

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