Geology of Great Britain

Source: Wikipedia, the free encyclopedia.

The geology of Great Britain is renowned for its diversity. As a result of its eventful geological history, Great Britain shows a rich variety of landscapes across the constituent countries of England, Wales and Scotland. Rocks of almost all geological ages are represented at outcrop, from the Archaean onwards.

Geology of Great Britain
Quaternary (Alluvium)
  Paleogene / Neogene (Tertiary)
  Cretaceous
 
Lower Cretaceous
  middle/
upper Jurassic
 
lower Jurassic
 
upper Triassic
 
lower Triassic
 
upper Permian
 
lower Permian
  upper Carboniferous (Coal Measures)
  middle Carboniferous
 
lower Carboniferous (limestone
)
  Devonian
  Ordovician / Silurian
  Cambrian
  Neoproterozoic
  Proterozoic (upper Precambrian)
  Lewisian (lower Precambrian)
  granite
  Paleogene volcanics

Overview

old 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 in Brittany and the Channel Islands. The youngest rocks are found in southeast England
.

Bedrock and past volcanism

The

plate 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 its geological structure with major faulting and folding being a legacy of each orogeny (mountain-building period), often associated with volcanic
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 any

the 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]

Superficial deposits

Overlain on this

glacial 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".

Geological history

This description of the geological history of Great Britain is based on that of P. Toghill.[2]

Precambrian

Archaean eon

The

Lewisian gneiss, the oldest rocks in Great Britain, date from at least 2.7 billion years ago in the Archaean eon, the Earth itself being about 4.6 billion years 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.

Proterozoic eon

South of the gneisses are a complex mixture of rocks forming the North West

Torridon Sandstone being deposited about 800 Ma, as well as the debris deposited by an ice sheet
670 Ma.

Palaeomagnetic evidence indicates that 520 Ma, what is now Great Britain was split between two continents, separated by 7,000 km (4,300 mi) 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.

Palaeozoic 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. The first animals with hard shells evolved at this time, consequently, fossils become much more common in rocks formed during this and later periods.

Ordovician period

Five hundred million years ago, in the Ordovician period, southern Great 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.

During this period, north Wales was subject to

Borrowdale Volcanics covered the Lake District and this can still be seen in the form of mountains such as Helvellyn and Scafell Pike
.

The Ordovician also saw the formation of the 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.

Caledonian Orogeny produced an Alpine-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 along strike-slip faults trending north-east to south-west across Scotland, the Great 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 the Mendip Hills and in Pembrokeshire.

Devonian period

The Old Red Sandstone Continent in the Devonian

The collision between continents continued during the

Orkney Islands. Most of these are of terrestrial origin and are informally known as the Old Red Sandstone
.

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 as it was located close to the equator
at between 10° and 15° south.

Carboniferous period

Around 360 Ma, at the start of the

organic acids in rainwater and groundwater
.

These were followed by dark marine

English Midlands
, northern England and Wales.

Throughout the period, southwest England in particular was affected by the collision of

tectonic pressure was from the south or south-east, and there developed 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 Carboniferous period, the various continents of the Earth had fused to form the super-continent of

sedimentary rock, somewhat similar to the later, Triassic New Red Sandstone
.

Permian period

The

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

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 Great Britain, prior to the breakup of the super-continent in the Jurassic
period.

Rock fragments found near

Manicouagan Impact Crater in Canada
, although this is still being debated.

Jurassic period

As the

Isle of Portland
.

Cretaceous period

Sedimentation would retreat from most of England beginning in the Cretaceous, with the deposition of the lagoonal

Seven Sisters, and also forming Salisbury 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.

Cenozoic era

Palaeogene period

In the early

intrusions of the Inner Hebrides
of Scotland.

An early phase of the

Weald-Artois Anticline
to the south, the North Downs, South Downs and Chiltern Hills.

During the period, the

grasses
.

Neogene period

Miocene and Pliocene epochs

In the

subtropical mixed forest once grew on Anglesey.[6] The climate of the UK cooled and became drier throughout the Neogene [7][8]

Quaternary period

Pleistocene epoch
The Merton Stone, one of the largest glacial erratics in England

The major changes during the

Anglian Glaciation, with ice up to 1,000 m (3300 ft) thick that reached as far south as London and Bristol. This took place between about 478,000 to 424,000 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

Clactonian Man
, also date from this period.

The Wolstonian 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 of Wolston southeast of Coventry which is thought to mark the southern limit of the ice.

The Wolstonian Stage was followed by the

Ipswichian Stage, 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 icesheet itself reaching south to Wolverhampton and Cardiff. The oldest human remains in Great Britain, 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

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

The last twelve thousand years are known as the Holocene Epoch. Amongst the most significant geological features created during this period are the peat deposits of Scotland, and of coastal and upland areas of England and Wales. Many of the lowland deposits, such as the Somerset Levels, The Fens and Romney Marsh have recently been artificially drained.

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. 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 continuing compaction 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.

Great Britain continues to be subject to several very minor earthquakes 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.

Geological features

Geological resources

Great Britain's complex geology has provided it with a wide range of geological resources including abundant supplies of coal, the initial extraction of which powered the Industrial Revolution. During the 20th century, the exploitation of North Sea oil and gas provided further energy supplies for the nation.[10] Geothermal energy and onshore oil have also been exploited to a lesser extent.

A wide variety of

copper and tin) or other uses (e.g. china clay
).

Events

Institutions

Government agencies

Learned societies

Pioneers of British geology

Awards

Geology of the UK: Section from Snowdon to Harwich showing underlying strata.
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.

See also

References

  1. ^ a b c "Why no volcanoes? - British Geological Survey". Bgs.ac.uk. Retrieved 3 November 2018.
  2. ISBN 1-85310-890-1
    .
  3. ISSN 2296-701X
    .
  4. S2CID 134737967
    .
  5. S2CID 129857989
    .
  6. S2CID 234861204
    .
  7. S2CID 245937316
    .
  8. ^ Gibson, Martha; Pound, Matthew. "We reconstructed Britain of millions of years ago to see what climate breakdown will involve". The Conversation.
  9. ^ "British earthquakes". Archived from the original on 2005-04-07. Retrieved 2004-05-20.
  10. ^ Shepherd, Mike (2015). Oil Strike North Sea: A first-hand history of North Sea oil. Luath Press.

Further reading

  • Bennison, G.M.; Wright, A.E. (1969). The Geological History of the British Isles. London: Edward Arnold. pp. 406 + x.
    ISBN 0-7131-2226-9
    .

External links

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