The Geology of Great Britain
Here are three sections: A glossary and etymology of commonly used terms; a time line which includes degrees north or south to indicate the position of Britain at a given time; and a general review of Britain’s geology.
This is followed by geological notes on specific counties, on the sub-tab.
AFTER THESE, CERTAIN ENGLISH COUNTIES ARE DISCUSSED IN GENERAL GEOLOGICAL REVIEW.
1. GLOSSARY & ETYMOLOGY
1. GLOSSARY & ETYMOLOGY
Anglian: the Great & 1st of last three Ice Ages. The ice sheet reached south to the modern Thames. It forced the Thames to its present course; formerly it crossed East Anglia, (from which this ice age takes its name). The Anglian Ice Age was in the the Cenozoic era, Quaternary period and Pleistocene epoch: between 480,000 to 425,000 years ago
Anticline: associated strata which slope away from each other as they descend into the crust- often an eroded dome, as seen in the Weald-Artois Anticline which is flanked by the North and South Downs .
Archaean Eon: the earliest Eon in British geology; “archaios”: Gk: ancient. The geology is not well understood. 4000-520 MYA.
Avalonia: Ordovician sub-continent which included Southern Britain- it broke from Gwondana, was in the south of the Devonia Old Red Sandstone continent- with Laurentia and Baltica- and later found itself in the centre of Pangaea.
Baltica: The sub continent which merged with Avalonia in the late Ordovician or early Silurian Age- the combined land mass then collided with Laurentia which included North Britain- thus the landmass of today was united.
Batholith: a large metamorphic or ignious rock mass reachging down to unknown depths: the best example being that in the West Country of England which is granite. The meaning is Bathos: Depth and Lithos: Stone
Boulder Clay: glacial clay typically that deposited by the Anglian Ice Age over Eastern England.
Cambrian period: Period typified by rocks such as those of the Cambrian Mountains in Wales. It is the 1st period of the Palaeozoic era. 520-430 MYA.
Caledonian: an orogeny (mountain building event) of the Silurian period caused by Laurentia (of which North Britain was a part) colliding obliquely with Baltica and Avalonia which contained South Britain.
Carboniferous period: Period typified by limestone epoch (Lower)and coal measure epoch (Upper) . It means “ Coal forming”. Britain was on the Equator. It is the 5th period of the Paleozoic era. 430-205 MYA.
Cene: a suffix in certain epoch names which comes from “kainos” and means “recent”.
Cenozoic: the present era, subdivided into Tertiary and Quaternary periods. It means “recent animal life”. 66 MYA- Present day.
Cretaceous: A period, it means “chalk”, finished with the K Pg Mass extinction (allegedly caused by an impact event near Yucatan). It has 2 epochs: Upper (late) and Lower (early). It is the 3rd period of the Mesozoic era. 130-66 MYA.
Devensian: the last Ice age in the the Cenozoic era, Quaternary period and Pleistocene epoch: This age diverted the Severn south & saw the breaking of the Continental land bridge. The flood-water Lake Lapworth changed the geology of the West Midlands as it broke out to the south. Name uncertain, possibly from Deventer- a town in Overijssel Netherlands- which may have marked the southern limit of the ice. It finished 10,000 years ago.
Devonian Period: typified by Old Red Sandstone from Devon to The Brecon Beacons & Ben Nevis & Orkneys. Named after the rocks of Devon. It is the 4th period of the Paleozoic era. 315-255 MYA.
Drift: late loose material deposited over the solid geology by glaciers, seas, rivers, or wind blown: boulder clay, sands & gravels are examples. “Drift” includes glacial deposits, meltwater deposits, alluvia, & river terraces. Geological maps cite regions of “bedrock” & “superficial deposits” rather than “solid” & “drift”.
Eocene: 2nd epoch of the Tertiary period of the Cenozoic era- It means “Recent Dawn” ( Eos: dawn, Kainos: recent) & is typified by the rise of flowering plants. 66-45 MYA.
Eon: Longest measure of geological time, an eon contains several era and an era has several periods, which can have several epochs. Eon> Era> Period> Epoch.
Epoch: a subdivision of a period of geological time, which is in turn shorter than an era or eon. It may be subdivided into “stages”. Eon> era> period> epoch
Era: a geological division of time shorter than an eon and longer than a period. An era contains several periods. Periods can contain several epochs. Eon> Era> Period> Epoch
Erratic: large rock transported by glaciation, or water. Example: Scottish Pink Granite glacial erratic are seen on the North Devon Coast. 50,000 tons of erratics were moved very recently by inundation in the North Devon valleys in the 1950s.
Flint: categorised with Churts, flint is a grey or grey black nodular form of quartzite or silicone found in layers in the Upper Cretaceous chalks. Flints have an outer shell which is rougher and whiter than the inner material- presumably here chalk and silicone combines. It comes in exotic organic shapes which look as if they are remnants of soft bodied creatures such as jellies or sponges or the holes bored by creatures. The source of the silicone which filled these cavities is also open to speculation. It was possibly from the hard parts (spines)of certain silicous sponges. Formerly the chalk was overlaid by Sarsen which is a silcrete of sandstone. As the sarsen eroded or dissolved that would release silicone to permiate the upper chalk and inhabit the oids created by the sponges, jellies or boring creatures. However the silicone must have had an origin prior to being the accretion material in the sarsen- and again that could have been sponges.
Gneiss: hard crystalline rock containing quartz, felspar and mica. Same material as granite but with a schist like styucture. The oldest British rocks are gneiss.The word is slavonic in origin, means rotten or decomposed, and was used by the miners of the Erzgebirge to describe a local rock.
Gondwana: the very early super continent, South of the Equator,- greatest in the Carboniferous period & persisted through the Mesozoic era. Named by the geologist Suess after a region of Central India where he studied Upper Paleolithic and Mesolithic formations.
Granite: a metamorphic rock with quartz ,feldspar and mica crystals. The great granite batholith of the West Country stretches west from Dartmoor Forest through Cornwall to the Isles of Scilly.
Greensand: there is a rock called greensand which is a sandstone heavily mixed with glauconite. But in English geology it means certain sub-divisions of the Cretaceous- usually Upper (later) and Lower Earlier) Greensand. They are not particularly green and are in proximity with other formations, such as gault clay, iron stone, fuller’s earth and Rag & Hassock. Typically they are found within chalk-edged anticlines such as the Weald and are therefore earlier in the Cretaceous than the chalk. Sometimes the name is used for soft sandstones from the earlier Jurassic.
Holocene: the modern epoch, our own recent epoch of the Quaternary. “Holo”: “All”; “Kainos”: “Recent”; the last 12,000 years since the last Ice Age: the second epoch of the Quaternary Period of which the first was the Pleistocene. Some have advocated beginning a new epoch due to global warming and man made changes: Homocene might be appropriate.
Hoxnian: a warm interglacial stage after the Anglian Ice Age, in which early man appeared in Britain at Swanscombe, Clacton and particularly at Hoxne in Suffolk, after which this stage is named. Here evidence of Paleolithic Man of the Acheulean culture has been found.
Iapetus Ocean: an ancient ocean, in the Proterozoic eon; 1000 to 700 MYA
Jurassic period: typified by Cotswold limestone, Portland stone, Dorset coast and York. Sandstone: named from Jura of Eastern France-Switzerland Britain 35° north. It usually has 2 epochs: Upper (late) and Lower (early) but can have 3 including “Middle”. Rocks are mainly marine and so fossils would also be of marine organisms. It is the 2nd period of the Mesozoic era. 180-130 MYA.
Kimmeridge Clay: a Jurassic marine clay rich in the fossils of marine dinosaurs, crocodiles, turtles etc- found above the oolithic limestone (Cotswold type) of the Jurassic.
Laurentia: Ordovician sub-continent which included North Britain. It collided with Avalonia to form Britain in the Silurian period. Perhaps named from the St Lawrence region of eastern North America?
Lewisian: Earliest British rocks typified by those from Lewis in the Outer Hebrides. Associated with the Lower (early) Pre-Cambrian. C. 2,700 MYA
Limestone: an alkaline rock which was partly or wholly formed by the deposition of matter from organisms, being at least 50% calcium carbonate. It must also have been a submarine deposition. Limestones can be recycled from older limestones and so the link with life need not be direct. As they range into dolomites the calcium carbonate is replaced by magnesium carbonate. Chalk is a limestone. Limestones are not metamorphosed.
Loess: a soft porous rock usually calcareous and strictly deposited by the action of wind- generally it is seen on the margins of former ice sheets and deserts.
Lower Cretaceous: Old Cretaceous period (Chalk bearing), but this period is often typified by greensands, and gault clays.
Lower Triassic: Early Triassic typified by New Red Sandstone.
Mesozoic: An era which means “Middle animal life”. It contains the Triassic, Jurassic and Cretaceous periods. The old geological name was “Secondary” which with “Primary” is not used now, but Tertiary and Quaternary are still used, not as era, but as periods of the Cenozoic era.
Miocene: 4th epoch of the Tertiary period of the Cenozoic era. (once considered the 3rd) Typified by the development of whales, apes & mega-fauna; it means “less recent” as opposed to Pliocene which means “more recent”. 35-12 MYA.
Neogene: a period, it means “New Creation”. It contained the Miocene and Pliocene epochs.
Neolithic: New Stone Age- analytical division in human development rather than a purely geological one.
Neopaleogene: “Late Old Creation”, associated with old volcanics.
Neoproterozoic: latest period of the Proterozoic: it means “late, before animal life”.
Oligocene: the 3rd epoch of the Tertiary period of the Cenozoic era. “Oligos-kainos”: “few-recent”. Typified by the appearance of large browsing mammals. 45-35 MYA
Ordovician period: Period typified by volcanic rocks of North Wales and Lake District; named after the British tribe the Ordovici- from North Wales. Ordovician and Silurian are sometimes put together. It is the 2nd period of the Paleozoic era. (510-420 MYA)
Orogeny: Mountain Creation (episode)
Oxford Clay: a Jurassic marine clay which is subdivided in the Peterborough (lower), Stewartby (middle, and Weymouth (upper). It underlies most of South East England but comes to the surface erratically from Wiltshire to South Yorkshire and further south. The lower measured are fossil rich, the middle and upper measures are fossil poor. It can be quite solid and tends towards a mudstone.
Palaeozoic era: It means the era of ancient animal life. The old name was “Primary” which with “Secondary” is not now used but “Tertiary” and “Quaternary” are still used as periods in the Cenozoic era.
Paleocene: 1st epoch of the Tertiary Period in the Cenozoic era. Palaios -kainos”: “ancient-recent”. It is typified by the 1st placental animals.
Paleogene: “old creation”, a period in the Cenozoic era, 60-30 MYA.
Paleolithic: Old Stone Age; anthropological rather than geological, describing a stage of human tool development.
Pangaea: The late super-continent, mainly north of the Equator. It began to break up as the Jurassic period started.
Period: a measure of geological time shorter than an era and longer than an epoch. An era is shorter than an eon. Eon> Era> Period> Epoch.
Permian Period: a period of arid desert, shallow sea then desert again at 10 degrees north. Very few British deposits; it is named after Perm in Russia. It is the 6th period of the Paleozoic era. It has Upper (late) and Lower (early) epochs. 205-180 MYA.
Plate tectonics: Continents, sub continents and super continents float on plates which collide break up and merge, causing orogenies and volcanic episodes. A region where a plate sinks beneath another is a subduction zone. Plate boundaries do not stay fixed in relation to a given landmass such as Britain.
Pleistocene: The 1st epoch of the Quaternary period of the Cenozoic era, the last 1,000,000 years excluding the last 12,000 years which is the Holocene epoch, typified by glacial boulder clays. It meaning is “pleistos-kainos”: “most-recent.”
Pliocene: the 5th epoch of the Tertiary period of the Cenozoic era, typified by large mammalian carnivores. The meaning is “pleion- kainos”: “more recent”. 12-1 MYA.
Precambrian Period: a very early period in the Archaean Eon- the first eon in British Geology: “before the Cambrian Period.” Up to 520 MYA.
Proterozoic eon: 2nd eon in British geology: means “before animals”. 1,000 to 500 MYA. Associated with the Upper Pre Cambrian.
Quaternary period: the latest period in which are two epochs, the Pleistocene and the Holocene- characterised by alluvia, and man. It is in the Cenozoic era and means “4th” because in old geology they noticed 4 distinct layers of fossils which they named primary, secondary, tertiary and quaternary.
Sandstone: generally a water deposited stratum- the corollary of erosion of high land. In Britain often descrived as “Old Red” (Devonian), or “New Red” (Triassic). Corallian Sandstones contain material from corals or shells in sand like form.
Sarsen: Above the Chalk lie Sarsen Stones; large slaps of field stone used extensively by the builders of stone monuments such as Stonehenge and Avebury; a particularly fine field of them lies on the Marlborough Downs just north of the Avebury-Marlborough Road. Sarsen means “Saracen” but the implication is generally “pagan”, with the buildings of the chalk down monuments seeming “Un Christian” to the people who coined the term. Sarsen stones are sandstone rocks found as field stone in quantity on the Marlborough Downs, Salisbury Plain and in Kent, Berkshire, Surrey, Essex, Oxfordshire, Dorset and Hampshire. They are the post-glacial remains of a cap of silcrete (sandstone glued together by silicone that once covered South East England- but seems to break up and erode quickly- even more so than the chalk. I wonder if the erosion of the Sarsen did not release silicone to form the layers of silicate flint in the Upper Cretaceous chalk.
Shale: a sedimentary rock which was clay-like when deposited but is now harder, laminated and does not slake in water. It has undergone change chemically or environmentally since deposition to harden it but is not metamorphosed. Originally it must have been a water born sediment.
Silurian period: period typified by mud & sandstones of South Wales. Laurentia collided with Avalonia. Named after Silures tribe of South Wales. Ordovician and Silurian are sometimes put together. It is the 3rd period of the Paleozoic era. 435-405 MYA.
Solid Geology: bed rocks are old metamorphic types overlaid by later sedimentary rocks; igneous rocks have intruded into both at different times.
Stage: a short measure of geological time: such as the Hoxian Warm Stage between the Anglian and Wolstonian glaciations.
Syncline: associated strata which converge as they enter the earths crust, as seen in the Thames Basin chalk. A landscape corrugated by a nearby orogeny will present anticlines and cynclines across a region.
Tertiary: late post glacial period. Originally it was the 3rd of 4 eras- Primary, Secondary Tertiary and Quaternary, but that usage is no longer followed. It is the 1st period of the Cenozoic era and has 5 epochs: Paleocene, Eocene, Oligocene, Miocene and Pliocene. post 66 MYA.
Triassic period: typified by “New Red Sandstone” deposited from erosion of the Vascian Mountains. 10° to 30° north. It has Upper (late) and Lower (early) epochs. In Germany rocks of this age had three strata : terrestrial- marine- terrestrial and this tripartite form led to the name “Triassic”. The 1st period of the Mesozoic era. 225-180MYA.
Upper Cretaceous: Late Cretacious period (chalk bearing). To 66 MYA.
Upper Triassic: Late Triassic
Variscan: a mountain building episode (orogeny) in France from the Permian period.
Wolstonian: Ice age, 2nd great recent Ice age, after the Anglian, before the Devensian in the Cenozoic era, Quaternary period and Pleistocene epoch: named after a village near Coventry on the southern edge of its ice sheet. 350,000-130,000 years ago.
2. POSITION OF BRITAIN IN TIME
Precambria Time: 3500- 550 MYA. Not well understood, generally before life, except biota.
Proterozoic Eon: 60° south- near Antarctic circle (1000-500 MYA)
Cambrian: 550-480 MYA . It is the 1st period of the Paleozoic era. 45° south
Ordovician :480-435 MYA. It is the 2nd period of the Paleozoic era. 30° south
Silurian: It is the 3rd period of the Paleozoic era. 20° south
Devonian: 405-340 MYA. It is the 4th period of the Paleozoic era. 12 degrees south- approaching equator.
Carboniferous Period: 340–260 MYA. It is the 5th period of the Paleozoic era. 0° on the equator or just north of it.
Permian: 260-225 MYA. It is the 6th period of the Paleozoic era. 10° north.
Triassic period: 225-180 MYA. Britain in Pangaea, 1st period of the Mesozoic era, drifts from equator to 20°&30° north.
Lias epoch: 200-180 MYA: Britain 30-32° north.
Jurassic period:180 -130 MYA. 2nd period of the Mesozoic era. 35° north.
Cretaceous period: 130-66MYA. 3rd period of the Mesozoic era. 40° north.
Tertiary: late post glacial period- post 66 MYA : Originally it was the “third” of the old 4 eras. 40-50° north.
Paleocene: 66-60 MYA, 1st epoch of Tertiary period of the Cenozoic era. 42° north
Eocene: 60-45 MYA, 2nd epoch of Tertiary period of the Cenozoic era. 44° north
Oligocene: 45-35 MYA, 3rd epoch of Tertiary period of the Cenozoic era. 46° north
Miocene: 30-12 MYA, 4th epoch of Tertiary period of the Cenozoic era. 48° north
Pliocene: 12-1 MYA, 5th epoch of Tertiary period of the Cenozoic era. 49° north
Quaternary period: 1,000,000 to today later period containing the Pleistocene and Holocene epochs. It originally meant “4th” of the old eras noticed by early geologists. 50° north.
Pleistocene epoch: Last 1,000,000 years excluding last 12,000 years. 1st epoch of the Quaternary period: 51° north
Anglian Ice Age was between 480,000 to 425,000 years ago. Almost modern position
Wolstonian: Ice age, 350,000-130,000 years ago. Almost modern Position
Devensian Ice Age, from110,000 to 10,000 years ago. Almost modern Position
Holocene: last 12,000 years, 2nd epoch of the latest period- the Quaternary: 52 degrees north
3. A REVIEW OF THE GEOLOGY OF BRITAIN
In the geology of Britain most geological ages are represented from the Archaean forwards. The Permian period is the least represented. The Earth‘s crust under Britain averages 19 miles thick. The oldest rocks (Lewisian gneiss) are on the surface in North West Scotland & are half the age of the planet. These rocks underlie much of Britain & also surface in the Channel Isles. The youngest rocks tend to be in the South East.
In the early years of geology 4 eras were seen in the fossil record: Primary, Secondary, Tertiary and Quaternary. Primary became the Paleozoic era. Secondary became the Mezozoic era and Tertiary and Quaternary were retained as periods of the modern Cenozoic era.
Britain’s geology was subject to plate tectonics in many periods; Britain changed latitude; & sea levels varied; both affected the deposition of sedimentary sequences. Several continental collisions changed geological structure causing faults & folding as a consequence of orogenies or mountain-building episodes, with their associated volcanic activity and the metamorphic change of earlier rocks.
Lewisian gneiss (from the Outer Hebridean island of that name), the oldest rock in Britain, dates from 2,700 MYA in the Archaean eon; the Earth being about 4,600 ,000,000 years old. Laid down as strata on the surface, these were later buried in the Earth’s crust & metamorphosed into crystalline gneiss which contains quartz, feldspar and mica, compositionally similar to granite.
South of the Lewisian gneiss, a mixture of rocks are seen in the North West Highlands; laid down as folds of sedimentary rock originally 15 miles thick & deposited over the gneiss on the floor of the Iapetus Ocean. The process started about 1,000 MYA. A 5 mile thick layer of Torridon sandstone was deposited 800 MYA, with drift deposits from an ancient ice sheet 670 MYA .
520 MYA , Britain was on two different continents, separated by over 4,000 miles of ocean. The north of Scotland was 20° south, on the continent of Lauentia; England & Wales were 60° south on the continent of Gondwana near the Antarctic.
In Gondwana, England & Wales were near a subduction zone & under a shallow sea in which volcanos protruded as islands; their remains underlie much of Central England & appear as outcrops. Around 600 MYA, the Cadomian Orogeny raised up England, Wales & North West Europe into a highland region.
In the early Cambrian period mountains in England & Wales were eroded & the land flooded by a rise in sea level; sediment was laid down. Central England formed a stable crust which has remained largely intact & un-deformed. Sandstones were deposited in Northern Scotland. The first animals with shells evolved; their fossils become common in rocks.
500 MYA, in the Ordovician period, Southern Britain, with other regions, broke from Gondwana & formed the continent of Avalonia, which by 440 MYA had drifted to 30° south.
North Wales was volcanically active. Some of these Ordovician volcanoes remain, one is Rhobell Fawr. Large quantities of volcanic lava & ash from the Borrowdale Volcanics, covered the Lake District: & is seen in fells such as Helvellyn & Scafell Pike. The Ordovician Skiddaw Slate was formed at this time. (Skiddaw is in the north of the Lake District).
Early Silurian strata included mudstones & sandstones which are seen notably in South Wales. Avalonia had joined the continent of Baltica, & the combined landmass collided with Laurentia at 20° south about c400 MYA, thus joining North & South Britain. The collision caused the Caledonian Orogeny: Alpine like mountains in North & West Britain. Collision was at an oblique angle which led to movement along slip faults aligned north-east to south-west across Scotland, one such is the Great Glen. This distinct alignment characterises Scotland today.
Silurian volcanic ash & lava are found in the Mendip Hills of North Somerset & in Pembrokeshire, South West Wales.
Britain in the Devonian was on a continent characterised by Old Red Sandstone, comprising the former sub-continents of Laurentia to the north west, Baltica to the north east and Avalonia- much smaller than the other two- to the south.
Plate collisions continued in the Devonian, with uplift & volcanic deposits. Sea levels varied; coastlines advanced & retreated across England. Uplifted regions eroded, causing many sedimentary strata in lowlands & seas. The marine rocks of Devon gave the period its name; deposits are also found in the Brecon Beacons, Central Lowlands of Scotland & the Orkneys. They are collectively known as “Old Red Sandstone.”
The Caledonian Mountains had largely been eroded down by the end of the Devonian & the country was arid desert, located close to the equator: roughly 12° south.
360 MYA, as the Carboniferous began, Britain was on the Equator & covered by the warm shallow waters of the Theic Ocean; carboniferous limestone from marine creatures was deposited, as found in the Mendip Hills, Wales, the Peaks, North Lancashire, the North Pennines & South-East Scotland. Limestone can develop cave systems by the action of carbonic & other acids in rain & ground water.
Carboniferous limestone was followed by marine shale, siltstone, & coarse sandstones such as millstone grit. Later, river deltas laid down sediment & then Upper Carboniferous swamps & rain forest developed from which organic material formed coal measures.
Throughout the Carboniferous, South West England was affected by nearby plate collision, the corollary of which was the Variscan Orogeny of 280 MYA. This caused deformation seen in the region south of a west to south-east line from Pembroke to Kent where strike slippage is seen in faults. Later granite formed beneath the surface rocks of Devon & Cornwall, now exposed by erosion in Dartmoor Forest & on Bodmin Moor; they contain deposits of tin, lead and copper. This large Cornubian granite batholith was originally further east, but moved westwards. Lesser Variscan folding took place in Derbyshire and as far north as Berwick. By the late Carboniferous, continents had fused into the super-landmass Pangaea, with Britain in its interior: a hot arid desert with flash floods depositing beds of red sedimentary sandstone.
The Pennines (Carboniferous Limestone Hills): This is the dominant single feature of English geology touching on 10 counties. They are an upfold of Carboniferous limestone. The Tyne Gap separates them from the Cheviots, and the Eden & Lune rivers separate them from the Lake District. At their most southerly they reach Nottinghamshire, and are generally termed the Peak District in their southern parts. They are the watershed of every major Northern River in England, excerpt, arguably, the North Tyne. They are broader to the north and generally escarpe to the west falling gently to the east. Cross Fell is the highest point at 2,930 ft. The Aire Gap divides the Northern and Southern Pennines, those of the south being generally lower with their highest point, over 2,400 ft, at Kinder Scout and The Peak, in Derbyshire. Some high points are pavemented by erosion and glaciation; fissures in these are called grykes. The Pennines are composed of Carboniferous Limestone, Millstone Grit and local shale. The limestone is eroded by organic acids in rain and ground water, producing extensive cave systems, underground rivers and sink holes or pots. The limestone was laid down when England was on the Equator in the Lower Carboniferous period c430-300 MYA; then this region must have been a sea-floor. The most likely uplift period was that of the Early Cretaceous, the Variscan orogeny being too early and too far south, (though some Variscan folding certainly effected the entire length of the Pennines). Millstone Grit, a course sandstone, was formed later than the limestone. The western escarping may in part have been due to the edge of the Irish Sea Ice Stream from the glaciation events of the Pleistocene.
The Permian was, for 30 million years, arid desert; erosion of the Variscan Orogeny continued. Later, much of Britain was submerged in shallow waters as polar ice sheets melted & the Tethys Ocean & Zechstein Sea formed, depositing shale, limestone, gravel and marl. Waters receded leaving a flat desert with salt pans. There are few Permian deposits in Britain. The period is named after typical rocks found near Perm in Russia.
Pangaea drifted during the Triassic away from the equator until it was between 20° & 30° north. “New Red Sandstone” formed. The remnants of the Variscan uplands in France eroded, causing layers of this New Red Sandstone to be deposited across Central England, the Cheshire Basin, & the Irish Sea. A lowland depression developed in Hampshire. Rifts occurred as the super-continent prepared to break up (which it did in the Jurassic period.)
Rock fragments found near Bristol show that c210 MYA Britain was showered with debris from the Canadian Manicouagab Impact. This period is typified by German rocks showing tripartite strata of land-sea-land deposits, hence the name “Triassic”.
The Jurassic began as Pangaea began breaking up; sea levels rose & Britain drifted on the Eurasian Plate to 35° north. Much of Britain was under water & sedimentary rocks were deposited; these underlie England from the Yorkshire Cleveland Hills to the Dorset Coast. These include sandstones, greensands, oolithic limestone (Cotswolds), corallian limestone (Vale of the White Horse) & Portland stone. Coralline sandstone was thought to be coral based but often comprises the exoskeletal remains of species
Buried organic matter in the mud of the sea floor formed hydrocarbons, often trapped by salt deposits as sea levels fell. Swamps & salt lagoons spread across the previously marine environment.
Modern continents took shape & the Atlantic Ocean formed, dividing Britain (the north of which was formerly on the Laurentia plate) from North America. Britain was uplifted; fertile plains developed, then, after 20 million years, the seas flooded the land again. Sea levels changed frequently. Chalk with flint strata was deposited, as exposed at Dover, the Seven Sister, Sussex, & forming Salisbury Plain. The sea left few pieces of land exposed, so little river deposited sand, mud or clay is found & late Cretaceous strata are mainly chalk.
In the early Palaeogene period, 60 to 50 MYA, Britain’s last volcanic rocks were active, producing the Antrim Hills, the basalt of the Giant’s Causeway, & the Staffa igneous intrusions of the Inner Hebrides.
The Alpine Orogeny that took place about 50 MYA caused the London Basin syncline, Chiltern Hills, & the Weald-Artois Anticline, buttressed now by the North Downs & South Downs. The North Sea formed, Britain was uplifted, often along old fault lines from the ancient Caledonian & Variscan Orogenies). As high lands eroded, sediment, such as London Clay, was deposited over Southern England & the English Channel region was characterised by mud flats with river-deposited sand. Much of North England, covered by Jurassic & Cretaceous deposits in the early Palaeogene, was then eroded down. By 35 MYA the land was colonised by grasslands, modern British hardwoods, redwood & palms.
In the Miocene & Pliocene epochs of the Neogene period, further uplift & erosion occurred, in Wales, the Pennines, & the Highlands. By about 2 MYA plants, animals & the land were broadly like those of today.
The great changes of the Pleistocene were caused by ice ages. The most severe was the Anglian, in which ice up to 3000 ft thick reached a line from London to Bristol. The Anglian Ice Age was between 480,000 to 425,000 years ago; it diverted the Thames which had formerly followed Chilterns and crossed East Anglia. Stone tools show that Southern England was colonised by humans during the warm Hoxnian Stage between the Anglian & Wolstonian Glaciation. Finds at Happisburgh on the Norfolk Coast show that hominids lived on the banks of the Thames there prior to its re-channelling. It has been suggested that Britain become an island from time to time, but that would have required a breaking of the chalk land bridge prior to the post Devensian Age. The oldest human fossils yet found include the skull of Swanscombe Man from Kent 250,000 years ago, & Clacton Man, which was earlier but from the same general period. The Wolstonian Glaciation between 350,000 & 130,000 years ago, peaked about 150,000 years ago. Named after Wolston south east of Coventry which marks the limit of the ice.
The Wolstonian was followed by the Ipswichian warm stage, during which fauna now found in Africa lived as far north as Yorkshire.
The recent Devensian Glaciation, from 110,000 years ago, peaked around 20,000 years ago & ended 10,000 years ago; the Usk and Wye valleys (Wales & Monmouth) were eroded by glaciers, with the ice sheet‘s southern edge lying from Glamorgan to Scarborough with a coastal tongue south along the Lincolnshire. The oldest nearly complete human remains in Britain, the Red Lady of Paviland (actually male and 30,000 years old) date from the mid Devensian. He lived on the plain south of the ice hunting on lands now under the Bristol Channel. It is thought that the country was eventually abandoned as the ice sheet reached its peak (10,000 years after Paviland) to be recolonised later. In the Holocene 5,000 years ago Britain was warmer than today. It will be noticed that ice age peaks much are nearer their end than their beginning: Start: 110,000 years ago; Peak: 20,000 years ago; End: 10,000 years ago.
Among the features left by the ice are fjord-lochs of West Scotland, U shaped valleys of the Lake District & erratics, transported from South Norway, deposited on the Yorkshire coast.
The last 12,000 years are the Holocene Epoch. Peat deposits were laid down in wetlands bogs. Many lowland, such as the Somerset Levels. Fens & Romney have been artificially drained, but usually this accompanied natural retreat of the sea caused by post glacial silting. Some landscapes were changed by digging- such as the Broads. Since the Neolithic Age most forest has been cleared, speeding natural erosion. By 2000 10% of England was built upon.
Post glacial adjustment was fast in eastern rivers which are sediment heavy and very slow in some western rivers which were sediment light: On the Taw it is not completed
Scotland continues to rise (Isostatic rebound) following the Devensian ice sheet’s disappearance. South & East England sinks at 1/25 of an inch annually; London sinks at double this speed due to the compaction of recent clay deposits.
Modern rises in sea level of about 3mm a year may make low-lying areas of land increasingly susceptible to inundation. In places land increase and sea level rise goes hand in hand with sea born deposition at ever higher levels. In some areas (Yorkshire Coast, Walton in Essex) the coastline erodes rapidly.
SEE COUNTY GEOLOGIES ON THE SUB-TAB OF THIS SUBJECT