The geological structure of

Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...

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. I ...

s * folds * sedimentary basins *

graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

s ''and'' horsts *

unconformities An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

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 surveys and other forms of

remote sensing Remote sensing is the acquisition of information about an physical object, 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 inform ...

Terranes

A useful approach to considering Britain's geological structure is to examine the various terranes 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 is defined to the east by the Moine Thrust, 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 separates the Central Highlands terrane from the Midland Valley terrane, which is itself separated from the Central Southern Uplands terrane by the Southern Uplands Fault. Each of these terranes form a part of the former continent of Laurentia whose southeastern margin is defined by the Iapetus Suture 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 which includes the

Lake District The Lake District, also known as ''the Lakes'' or ''Lakeland'', is a mountainous region and National parks of the United Kingdom, national park in Cumbria, North West England. It is famous for its landscape, including its lakes, coast, and mou ...

and North Pennines. 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; within it, the north–south aligned Malvern line (or 'Malvern lineament') divides the Wrekin terrane in the west from the Charnwood terrane in the east. The

Isle of Anglesey Anglesey ( ; ) is an island off the north-west coast of Wales. It forms the bulk of the Principal areas of Wales, county known as the Isle of Anglesey, which also includes Holy Island, Anglesey, Holy Island () and some islets and Skerry, sker ...

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

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

Yorkshire Dales The Yorkshire Dales are a series of valleys, or Dale (landform), dales, in the Pennines, an Highland, upland range in England. They are mostly located in the Ceremonial counties of England, ceremonial county of North Yorkshire, but extend into C ...

, is defined to the west by the

Dent Fault The Dent Fault is a major fault (geology), fault zone on the boundary between the counties of Cumbria and North Yorkshire in northern England. It is named after the village of Dent, South Lakeland, Dent in Dentdale, on the western margin of the ...

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.

Caledonian progeny

The Caledonian orogeny took place between about 490 and 390 million years ago as the former micro-continent of Avalonia collided obliquely with the former

continent A continent is any of several large geographical regions. Continents are generally identified by convention (norm), convention rather than any strict criteria. A continent could be a single large landmass, a part of a very large landmass, as ...

of Laurentia 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 South Wales ( ) is a Regions of Wales, loosely defined region of Wales bordered by England to the east and mid Wales to the north. Generally considered to include the Historic counties of Wales, historic counties of Glamorgan and Monmouthshire ( ...

northwards to the Shetland Islands. Key structures include: :* Moine Thrust :* Great Glen Fault :* Walls Boundary Fault :* Highland Boundary Fault :* Midland Valley graben :* Southern Uplands Fault :* Menai Strait fault zone :* Bala Fault :* Welsh Borderland fault system :* Cribarth Disturbance :* Neath Disturbance 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.

Variscan orogeny

The Variscan orogeny was a complex affair whereby the former micro-continent of Armorica collided with

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

(otherwise known as

Euramerica 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 Pa ...

or the Old Red Continent), followed by a further collision between

Gondwana Gondwana ( ; ) was a large landmass, sometimes referred to as a supercontinent. The remnants of Gondwana make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia (continent), Australia, Zea ...

and the enlarged Laurussia. In Britain it resulted in a variety of geological structures across the southwest from

Pembrokeshire Pembrokeshire ( ; ) is a Principal areas of Wales, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and otherwise by the sea. Haverfordwest is the largest town and ...

and South

Glamorgan Glamorgan (), or sometimes Glamorganshire ( or ), was Historic counties of Wales, one of the thirteen counties of Wales that existed from 1536 until their abolishment in 1974. It is located in the South Wales, south of Wales. Originally an ea ...

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

Devon Devon ( ; historically also known as Devonshire , ) is a ceremonial county in South West England. It is bordered by the Bristol Channel to the north, Somerset and Dorset to the east, the English Channel to the south, and Cornwall to the west ...

and

Cornwall Cornwall (; or ) is a Ceremonial counties of England, ceremonial county in South West England. It is also one of the Celtic nations and the homeland of the Cornish people. The county is bordered by the Atlantic Ocean to the north and west, ...

. 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 251.902 Mya. It is the s ...

and

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

times. The Worcester Basin links to the Stafford Basin which in turn links to the Cheshire Basin 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.

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 continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

and

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

. Its effects are most evident in the

Alps The Alps () are some of the highest and most extensive mountain ranges in Europe, stretching approximately across eight Alpine countries (from west to east): Monaco, France, Switzerland, Italy, Liechtenstein, Germany, Austria and Slovenia. ...

, Pyrenees,

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

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
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