The geological structure of

Great Britain Great Britain is an island in the North Atlantic Ocean off the northwest coast of continental Europe. With an area of , it is the largest of the British Isles, the largest European island and the ninth-largest island in the world. It i ...

is complex, resulting as it does from a long and varied geological history spanning more than two billion years. This piece of the Earth's crust has experienced several episodes of mountain building or ' orogenies', each of which has added further complexity to the picture. A wide range of geological structures occur across Britain and include examples at a variety of scales of: * faults *

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

s * folds *

sedimentary basins Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsiden ...

* grabens ''and'' horsts *

unconformities An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

Our understanding of Britain's large-scale structure has been gained over many decades by simple geological field survey together with an increasing range of technological methods including gravity surveys, seismic surveys,

aeromagnetic survey An aeromagnetic survey is a common type of geophysical survey carried out using a magnetometer aboard or towed behind an aircraft. The principle is similar to a magnetic survey carried out with a hand-held magnetometer, but allows much larger ar ...

s and other forms of

remote sensing Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Eart ...

Terranes

A useful approach to considering Britain's geological structure is to examine the various

terrane In geology, a terrane (; in full, a tectonostratigraphic terrane) is a crust (geology), crust fragment formed on a tectonic plate (or broken off from it) and Accretion (geology), accreted or "Suture (geology), sutured" to crust lying on another pla ...

s from which it is composed. These are essentially continental fragments whose boundaries are generally defined by faults. Individual terranes typically contain suites of structures, the histories and form of which differ from those of neighbouring terranes.

Terranes of Scotland

The

Hebridean Terrane The Hebridean Terrane is one of the terranes that form part of the Caledonian orogenic belt in northwest Scotland. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt. The basement is formed by Archa ...

is defined to the east by the

Moine Thrust The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast south-west to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of t ...

, beyond which lies the Northern Highlands Terrane. This area in turn abuts against the Central Highlands (or Grampian) Terrane along the Great Glen Fault. Similarly the

Highland Boundary Fault The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two different geological terranes which give rise to two distinct physiographic terr ...

separates the Central Highlands Terrane from the

Midland Valley Terrane Midland may refer to: Places Australia * Midland, Western Australia Canada * Midland, Albert County, New Brunswick * Midland, Kings County, New Brunswick * Midland, Newfoundland and Labrador * Midland, Ontario India * Midland Ward, Kohima, Naga ...

, which is itself separated from the

Central - Southern Uplands Terrane Central is an adjective usually referring to being in the center of some place or (mathematical) object. Central may also refer to: Directions and generalised locations * Central Africa, a region in the centre of Africa continent, also known as ...

by the

Southern Uplands Fault The Southern Uplands Fault (or occasionally Southern Upland Fault) is a fault in Scotland that runs from Girvan (or more specifically from the Rhins of Galloway) to Dunbar on the East coast. It marks the southern boundary of the Scottish Midland ...

. Each of these terranes form a part of the former continent of

Laurentia Laurentia or the North American Craton is a large continental craton that forms the ancient geological core of North America. Many times in its past, Laurentia has been a separate continent, as it is now in the form of North America, althoug ...

whose southeastern margin is defined by the

Iapetus Suture The Iapetus Suture is one of several major Fault (geology), geological faults caused by the collision of several ancient land masses forming a suture (geology), suture. It represents in part the remains of what was once the Iapetus Ocean. Iapet ...

Zone, which runs parallel to the English-Scottish border, though some miles to its south.

Terranes of England and Wales

The Central - Southern Uplands Terrane extends across the northernmost part of England. Its southern margin is the Iapetus Suture, south of which lies the

Leinster - Lakesman Terrane Leinster ( ; ga, Laighin or ) is one of the provinces of Ireland, situated in the southeast and east of Ireland. The province comprises the ancient Kingdoms of Meath, Leinster and Osraige. Following the 12th-century Norman invasion of Ir ...

which includes the Lake District and

North Pennines The North Pennines is the northernmost section of the Pennine range of hills which runs north–south through northern England. It lies between Carlisle to the west and Darlington to the east. It is bounded to the north by the Tyne Valley and ...

. The majority of the rest of England and Wales north of the Variscan Front are considered to constitute the Avalon Composite Terrane. Central to this composite terrane is the triangular-shaped

Midlands Microcraton The Midlands Microcraton is a triangular block of late Neoproterozoic crust of igneous and volcaniclastic origin which underlies the English Midlands. Its northern tip is at the southern edge of the English Peak District and its other two corners ...

; within it, the north–south aligned Malvern Line (or 'Malvern Lineament') divides the

Wrekin Terrane The Wrekin Terrane is one of five inferred fault bounded terranes that make up the basement rocks of the southern United Kingdom. The other notable geological terranes in the region are the Charnwood Terrane, Fenland Terrane, Cymru Terrane and ...

in the west from the Charnwood Terrane in the east. The

Isle of Anglesey Anglesey (; cy, (Ynys) Môn ) is an island off the north-west coast of Wales. It forms a principal area known as the Isle of Anglesey, that includes Holy Island across the narrow Cymyran Strait and some islets and skerries. Anglesey island, ...

and parts of the Lleyn peninsula are deemed to be composed of numerous micro-terranes, known collectively as the Rosslare - Monian Terranes.

Pennine Block & Basin Province

Northern England is characterised by a series of fault-bound blocks separated by sedimentary basins whose origin can be traced back to the Carboniferous period. The North Pennines are formed on the Alston Block which is defined to the west by the Pennine Fault and to the north by the Stublick and Ninety Fathom Faults. It is separated from the

Askrigg Block The Askrigg Block is the name applied by geologists to the crustal block forming a part of the Pennines of northern England and which is essentially coincident with the Yorkshire Dales. It is defined by the Dent Fault to the west and the Craven ...

to the south by the Stainmore Trough. This latter block, coincident with the

Yorkshire Dales The Yorkshire Dales is an upland area of the Pennines in the historic county of Yorkshire, England, most of it in the Yorkshire Dales National Park created in 1954. The Dales comprise river valleys and the hills rising from the Vale of York w ...

, is defined to the west by the Dent Fault and to the south by the Craven Fault System. The Northumberland and Gainsborough Troughs lie to the north and south of these two blocks.

Structural legacy of mountain-building episodes

Another approach to the study of the geological structure of the area is through consideration of the variety of structures resulting from each of several orogenies (or 'mountain-building' episodes) which have taken place over geological history. Structures originating in one event may play a part in subsequent orogenic events and be modified by them. Thus lines of crustal weakness commonly associated with, for example, the Caledonian Orogeny will often predate this particular mountain-building period, much as some of those created during this phase were reactivated during later events.

The Caledonian Orogeny

The

Caledonian Orogeny The Caledonian orogeny was a mountain-building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that ...

took place between about 490 and 390 million years ago as the former

micro-continent Continental crustal fragments, partly synonymous with microcontinents, are pieces of continents that have broken off from main continental masses to form distinct islands that are often several hundred kilometers from their place of origin. Caus ...

of Avalonia collided obliquely with the former

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions In geography, regions, otherwise referred to as zones, lands or territories, are areas t ...

along a line approximating to the modern English-Scottish border. This long drawn-out, multi-phase event resulted in innumerable geological structures, many of which have persisted to the present day and help to shape the landscapes of much of Britain, from South Wales northwards to the Shetland Islands. Key structures include: :*

:* Great Glen Fault :* Walls Boundary Fault :*

:* Midland Valley graben :*

:* Menai Strait Fault Zone :* Bala Fault :*

Welsh Borderland Fault System The Welsh Borderland Fault System is a zone of faulting and associated folding which runs northeastwards through Wales from Pembrokeshire through Carmarthenshire and Powys into Shropshire in England. It comprises the Tywi Lineament, Pontesford ...

Cribarth Disturbance The Cribarth Disturbance is a geological structure forming a lineament which stretches across south Wales from Swansea up the Swansea Valley then northeastwards to Brecon and beyond. It consists of both a series of faults and associated folds wh ...

Neath Disturbance The Neath Disturbance is a geological structure which stretches across south Wales from Swansea Bay northeastwards as far as Hereford in western England. It consists of a series of both faults and associated folds which were active during the mo ...

Each of these structures is aligned northeast - southwest, albeit with the more northerly of them trending closer to NNE - SSW. A map or satellite photo readily reveals these major trends. There are hundreds of other lesser faults and folds which follow a similar alignment - a trend known as the Caledonoid trend.

The Variscan Orogeny

The

Variscan Orogeny The Variscan or Hercynian orogeny was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Variscan'', comes f ...

was a complex affair whereby the former micro-continent of

Armorica Armorica or Aremorica (Gaulish: ; br, Arvorig, ) is the name given in ancient times to the part of Gaul between the Seine and the Loire that includes the Brittany Peninsula, extending inland to an indeterminate point and down the Atlantic Coast ...

collided with

Laurussia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...

(otherwise known as Euramerica or the Old Red Continent), followed by a further collision between Gondwana and the enlarged Laurussia. In Britain it resulted in a variety of geological structures across the southwest from Pembrokeshire and South Glamorgan in

Wales Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...

Devon Devon ( , historically known as Devonshire , ) is a ceremonial and non-metropolitan county in South West England. The most populous settlement in Devon is the city of Plymouth, followed by Devon's county town, the city of Exeter. Devo ...

and

Cornwall Cornwall (; kw, Kernow ) is a historic county and ceremonial county in South West England. It is recognised as one of the Celtic nations, and is the homeland of the Cornish people. Cornwall is bordered to the north and west by the Atlantic ...

. Structures include: :* Dodman-Start Thrust :* Lizard Thrust :* Carrick Thrust

Post-Variscan crustal extension

An east-west extensional regime affecting the crust of England led to the formation of a series of structural basins developing through England between the English Channel and the Irish Sea during

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

and

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

times. The

Worcester Basin The Worcester Basin or Worcester Graben is a sedimentary basin in central England, filled with mainly Permian and Triassic rocks. It trends roughly north-south and lies between the East Malverns Fault in the west and the Inkberrow Fault in the eas ...

links to the Stafford Basin which in turn links to the

Cheshire Basin The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin extending under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire. The basin possesses something of t ...

and thence to the East Irish Sea Basin. The Needwood and Knowle basins are two smaller basins in the North Midlands associated with this rift complex.

The Alpine Orogeny

The Alpine Orogeny began 200 million years ago and continues to the present day. It comprises a series of collisions involving various micro-continents between northern

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a subcontinent of Eurasia and it is located entirel ...

and

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

. Its effects are most evident in the

Alps The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Swi ...

Pyrenees The Pyrenees (; es, Pirineos ; french: Pyrénées ; ca, Pirineu ; eu, Pirinioak ; oc, Pirenèus ; an, Pirineus) is a mountain range straddling the border of France and Spain. It extends nearly from its union with the Cantabrian Mountains to ...

Carpathians The Carpathian Mountains or Carpathians () are a range of mountains forming an arc across Central Europe. Roughly long, it is the third-longest European mountain range after the Urals at and the Scandinavian Mountains at . The range stretches ...

and other mountain ranges of southern

, but the northernmost 'ripples' of this event have affected the structure of southern England. Structures include;
:* Wealden Anticline :*

Purbeck Monocline The Purbeck Monocline is a geological fold in southern England. The term 'fold' is used in geology when one or more originally flat sedimentary strata surfaces are bent or curved as a result of plastic (i.e. permanent) deformation. A monocline is ...

Brenchley P.J & Rawson P.F. (eds) 2006, ''The Geology of England and Wales''. The Geological Society, London

References

{{reflist Carboniferous geology Geology of Wales Paleozoic England Stratigraphy of the United Kingdom