Geology of England

The geology of England is mainly sedimentary. The youngest rocks are in the south east around London, progressing in age in a north westerly direction.Southampton University
retrieved 21/1/07 The Tees–Exe line marks the division between younger, softer and low-lying rocks in the south east and the generally older and harder rocks of the north and west which give rise to higher relief in those regions. The geology of England is recognisable in the landscape of its counties, the building materials of its towns and its regional extractive industries.

Bedrock

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.

Superficial deposits

Overlain on this bedrock or "solid" geology is a somewhat variable distribution of soils and fragmental material deposited by glaciers (boulder clay, and other forms of glacial drift in the geologically recent past. "Drift" geology is often more important than "solid" geology when considering building works, drainage, siting water boreholes, soil fertility, and many other issues.

Glaciation and the resulting glacial and fluvio-glacial deposition has had a major impact on the landscape of England covering many areas with a veneer of glacial till in the lower lying areas north of a line running from Bristol to London. In the Ribble valley, Lancashire in north west England the resulting drumlins are clearly visible. Cromer Ridge in East Anglia is a terminal moraine. Indeed, most of East Anglia is covered with glacial till which has produced its rich loamy soils. This unconsolidated material (it is not stuck tightly together) is very easily eroded hence the rapid rate of retreat of the coastline of this region.

A similar situation exists in east Yorkshire in the Holderness district. The chalk outcrop at Flamborough Head in the north produces a headland relatively resistant to coastal erosion whilst the coastline south of this at such places as Mappleton and Hornsea with their soft glacial deposits are vulnerable.

Former ice caps did not reach south of the line running from Bristol to London, so this area has only been impacted by fluvio-glacial deposition which is represented in gravel beds around rivers such as the Thames. As the ice caps retreated northwards, more fluvio-glacial deposition occurred for example in the Vale of York

Geological history

Precambrian supereon

No rocks earlier than the Proterozoic occur at surface within England.

Proterozoic eon

The Proterozoic lasted from (2,500–541 Ma). The early geological development of the Avalonia terrane, including England, is believed to have been in volcanic arcs near a subduction zone on the margin of the Gondwana continent. Some material may have accreted from volcanic island arcs which formed further out in the ocean and later collided with Gondwana as a result of plate tectonic movements. The igneous activity had started by 730 million years ago and continued until around 570 million years ago, resulting in a region of volcanic islands within a shallow sea. The remains of these islands underlie much of central England with small outcrops visible in various places.

Around 600 million years ago, the Cadomian Orogeny (mountain building period) created mountains in what would subsequently become England, along with much of north west Europe.

Phanerozoic eon

The Phanerozoic comprises the Palaeozoic, Mesozoic and Cenozoic eras, each of which are represented in English geology.

Palaeozoic era

The Palaeozoic comprises six periods from the Cambrian to the Permian (539–252 Ma).

= Cambrian period

=
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.

= Ordovician period

=
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.

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.

= Silurian period

=
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.

= Devonian period

=
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.

= Carboniferous period

=

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 cyclic coal 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.

= Variscan Orogeny

=
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, Triassic New 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.

= Permian period

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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.

Mesozoic era

The Mesozoic comprises the Triassic, Jurassic and Cretaceous periods (252–66 Ma).

= Triassic period

=
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.

= Jurassic period

=
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, oolitic limestone 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.

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

=

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.

Cenozoic era

The Cenozoic comprises the Palaeogene, Neogene and Quaternary periods (66 Ma–present).

= Palaeogene

=
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 Basin syncline and the Weald anticline to the south. This orogeny also led to the development of the North Downs, South Downs and Chiltern Hills escarpments, 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. 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.

= Neogene

=
In the Miocene and Pliocene

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