Geology of England

In the early Cambrian period the volcanoes and mountains of England were eroded as the land became flooded by a rise in sea level, and new layers of sediment were laid down. Cambrian shales laid down in a shallow sea are exposed in the Midlands at Nuneaton. Much of central England formed a stable block of crust which has remained largely undeformed ever since.

Five hundred million years ago, in the Ordovician period, southern Britain, the east coast of North America and south-east Newfoundland broke away from Gondwanaland to form the continent of Avalonia. The Skiddaw slates of the Lake District consist of metamorphosed marine sediments laid down on the northern margin of Avalonia.^[4]

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

In the Silurian period, sandstones and mudstones were deposited in some parts of England. Volcanic ashes and lavas deposited during the period are still found in the Mendip Hills.

Avalonia had now joined with the continent of Baltica, and the combined landmass collided with Laurentia around 425 million years ago, joining the southern and northern halves of the British Isles together. The resulting Caledonian Orogeny produced an Alpine-style mountain range. England lay on the southern fringe of this range.

In the Devonian period, northern England was a region uplifted by the Caledonian Orogeny. The uplifted regions were gradually eroded down, resulting in the deposition of numerous sedimentary rock layers in lowlands and seas. The Old Red Sandstone was deposited across much of central and southern England. Sea levels varied considerably at this time with the coastline advancing and retreating from north to south across England. The Old Red Sandstone of Devon gave the period its name.^[5]

Around 360 million years ago during the Carboniferous period, England was lying at the equator, covered by the warm shallow waters of the Rheic Ocean. During this time Carboniferous Limestone was deposited, as found in the Mendip Hills, in the Peak District of Derbyshire, north Lancashire and the northern Pennines. The erosion of this landscape by carbonation has led to very distinctive scenery. Particularly notable is the area around Malham in the Yorkshire Dales with its limestone pavements, sink holes and shake holes. Gaping Gill contains a waterfall disappearing underground into the Carboniferous limestone.

The formation of Carboniferous Limestone was followed by the deposition of dark marine shales, siltstones and coarse sandstones of the Millstone Grit, notably in the area later uplifted to form the Pennine anticline. This sequence can be seen in the Yorkshire Dales with Ingleborough protruding up above the Carboniferous Limestone landscape below.

Later, river deltas formed and the sediments deposited were colonised by swamps and rain forest. It was in this environment that the cycliccoal measures were formed. Coal can be found as far south as Kent with deposits stretching northwards to Tyne and Wear; though coal has largely been mined in the Midlands and northern England. One of the best exposures of the Coal Measures is on the north east coast between Whitley Bay and Seaton Sluice.^[6]

Around 280 million years ago, at the close of the Carboniferous period, the Variscan orogeny (mountain building period) caused major deformation in south west England. The general region of Variscan folding was south of an east–west line roughly from Avon to Kent, though lesser folding took place as far north as Derbyshire and Berwick-upon-Tweed. The main tectonic pressure was from the south or south-west.

Towards the end of the orogeny the emplacement of a large batholith of granite occurred below what is now Devon and Cornwall. This granite is now exposed as Dartmoor, Bodmin Moor and several other exposures in Cornwall, including the Isles of Scilly. Granite tors form important elements in the landscape in these areas. Metamorphism of the country rocks and hydrothermal circulation of metal-rich fluids in the cooling rock provided the source of the extensive metalliferous deposits of the region (primarily copper and tin). Later weathering of the granite led to deposits of kaolin which has been excavated as it is an important source of china clay and ball clay used in the production of such products as porcelain and shiny printing paper.

By the end of the period, England had a hot arid desert climate, with frequent flash floods leaving deposits that formed red beds, somewhat similar to the later, TriassicNew Red Sandstone.

After the end of the Carboniferous period, an intrusion of quartz dolerite formed the Whin Sill. The River Tees flows over this at High Force on the Alston Block. Whin Sill is also seen again at Hadrian's Wall. The country rock is Lower Carboniferous limestone and shales.^[1]

The Permian period 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 England 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.^[7]

As Pangaea drifted during the Triassic period, England 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. A basin developed in the Hampshire region around this time. Rifting occurred within and around England, prior to the breakup of the super-continent in the Jurassic period.

Rock fragments found near Bristol appear to indicate that in 214 million years ago England was showered with a fine layer of debris from an asteroid impact at the Manicouagan Impact Crater in Canada (when both areas were parts of the Pangaea supercontinent), although this is still being debated.

As the Jurassic period started, Pangaea began to break up and sea levels rose, as England drifted on the Eurasian Plate to between 30° and 40° north. With much of England 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, greensands, ooliticlimestone of the Cotswold Hills, corallian limestone of the Vale of White Horse and the Isle of Portland. A particularly interesting Jurassic site is on the North Yorkshire coast between Staithes and Port Mulgrave.^[8]

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.

After 20 million years during the Cretaceous period, the seas started to flood the land again until much of England was again below the sea, though sea levels frequently changed. Chalk and flints were deposited over much of England, 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.

In the early Palaeogene period between 63 and 52 million years ago, the last igneous rocks in England were formed. The granite Lundy Island in the Bristol Channel dates from this period.

The Alpine Orogeny that took place from about 50 million years ago was responsible for the shaping of the London Basinsyncline and the Wealdanticline to the south. This orogeny also led to the development of the North Downs, South Downs and Chiltern Hillsescarpments, and the near-vertical folds in south Dorset and the Isle of Wight.

During the period England was uplifted leading to erosion and the formation of the Sub-Paleogene surface.^[9] 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 Palaeogene, but lost them through erosion. By 35 million years ago, the landscape included beech, oak, redwood and palm trees, along with grassland.

In the Miocene and Pliocene epochs of the Neogene period, further uplift and erosion occurred, particularly in the Pennines. 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 Pleistocene epoch have 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 in the northwest which thinned considerably as it reached as far south as London and Bristol. This glaciation 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 Stage period that followed the glaciation of the Anglian Stage. 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 these islands also date from this time, including the skull of Swanscombe Man from 250,000 years ago, and the earlier Clactonian Man.

Multiple glaciations occurred during Wolstonian Stage, between about 352,000 to 132,000 years ago. Thought to have peaked around 150,000 years ago, it was named after the town of Wolston south of Birmingham which is considered to mark the southern limit of the ice.

The Wolstonian Stage was followed by interglacial climate of the Ipswichian Stage, during which hippopotamus are known to have lived as far north as Leeds.

The most recent glaciations occurred during the Devensian Stage, which is thought to have started around 75,000 years ago, peaked around 20,000 years ago and ended a mere 11,500 years ago. The valleys of the Lake District and parts of the Pennines were further eroded by glaciers, with the ice sheet itself reaching south to Birmingham. 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 glaciated U-shaped valleys of the Lake District and erratics (blocks of rock) that have been transported by ice, including some from the Oslo region and deposited on the coast of Yorkshire.

Over the last twelve thousand years during the Holocene epoch the most significant new geological features have been the deposits of peat, as well as in coastal areas that have recently been artificially drained such as the Somerset Levels, The Fens and Romney Marsh.

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, due to Scotland's continuing to rise as a result of the weight of Devensian ice being lifted, England is sinking. This is 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. A contributory factor is the draining of many stretches of land.

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.

Recent flooding events leave geological evidence such as the Bristol Channel floods in 1607.

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,^[10] many of them originating within England itself. Notable was the Colchester earthquake in 1884 and the 2002 Dudley earthquake.