Geological structure of Great Britain
Encyclopedia
The geological structure of Great Britain
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:
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
s and other forms of remote sensing
.
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.
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.
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. Within this composite terrane, 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 and parts of the Lleyn peninsula are deemed to be composed of numerous micro-terranes, known collectively as the Rosslare - Monian Terranes.
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
, 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.
(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.
took place between about 490 and 390 million years ago as the former micro-continent of Avalonia
collided obliquely with the former continent
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
northwards to the Shetland Islands
.
Key structures include:
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.
was a complex affair whereby the former micro-continent of Armorica
collided with Laurussia (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
to Devon
and Cornwall
.
Structures include:
began 200 million years ago and continues to the present day. It comprises a series of collisions between various micro-continents between northern Europe
and Africa
. Its effects are most evident in the Alps
, Pyrenees
, Carpathians and other mountain ranges of southern Europe
but the northernmost 'ripples' of this event have affected the structure of southern England.
Structures include;
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
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
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...
has experienced several episodes of mountain building or 'orogenies
Orogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
', 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 faultThrust faultA thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
s - foldsFold (geology)The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
- sedimentary basins
- grabenGrabenIn geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
s and horsts - unconformitiesUnconformityAn unconformity is a buried erosion 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 of time before deposition of the younger, but the term is used to describe...
Our understanding of Britain's large-scale structure has been gained over many decades by simple geological field survey
Geological survey
The term geological survey can be used to describe both the conduct of a survey for geological purposes and an institution holding geological information....
together with an increasing range of technological methods including gravity surveys
Earth's gravity
The gravity of Earth, denoted g, refers to the acceleration that the Earth imparts to objects on or near its surface. In SI units this acceleration is measured in metres per second per second or equivalently in newtons per kilogram...
, seismic surveys
Reflection seismology
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a...
, aeromagnetic survey
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 areas of the Earth's surface to be covered quickly for...
s and other forms of remote sensing
Remote sensing
Remote sensing is the acquisition of information about an object or phenomenon, without making physical contact with the object. In modern usage, the term generally refers to the use of aerial sensor technologies to detect and classify objects on Earth by means of propagated signals Remote sensing...
.
Terranes
A useful approach to considering Britain's geological structure is to examine the various terraneTerrane
A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...
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 TerraneHebridean 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...
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
Great Glen Fault
The Great Glen Fault is a long strike-slip fault that runs through its namesake the Great Glen in Scotland. However, the fault is actually much longer and over 400 million years old.-Location:...
. Similarly the Highland Boundary Fault
Highland Boundary Fault
The Highland Boundary Fault is a geological fault that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east...
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
Southern Uplands Fault
The Southern Uplands Fault is a fault in Scotland that runs from Girvan to Dunbar on the East coast. It marks the southern boundary of the Scottish Midland Valley....
. Each of these terranes form a part of the former continent of Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
whose southeastern margin is defined by the Iapetus Suture
Iapetus Suture
The Iapetus Suture is one of several major geological faults caused by the collision of several ancient land masses forming a suture. It represents in part the remains of what was once the Iapetus Ocean. Iapetus was the father of Atlas in Greek mythology, making his an appropriate name for what...
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 DistrictLake District
The Lake District, also commonly known as The Lakes or Lakeland, is a mountainous region in North West England. A popular holiday destination, it is famous not only for its lakes and its mountains but also for its associations with the early 19th century poetry and writings of William Wordsworth...
and North Pennines
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...
. The majority of the rest of England and Wales north of the Variscan Front
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
are considered to constitute the Avalon Composite Terrane. Within this composite terrane, the north-south aligned Malvern Line
Malvern Line
The Malvern Line or Malvern Lineament is the name applied to a north-south aligned lineament which runs through the Malvern Hills of western England and extends southwards towards Bristol and northwards past Stourport....
(or 'Malvern Lineament') divides the Wrekin Terrane in the west from the Charnwood Terrane in the east. The Isle of Anglesey 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 BlockAlston Block
The Alston Block is a term used by geologists to describe the geological structure of the North Pennines of northern England and which forms a part of the Pennine Block & Basin Province which originated during the Carboniferous period. It is defined by the Stublick and Ninety Fathom Faults to the...
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
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 Fault System to the south whilst to the north it is...
to the south by the Stainmore Trough. This latter block, coincident with the Yorkshire Dales
Yorkshire Dales
The Yorkshire Dales is the name given to an upland area in Northern England.The area lies within the historic county boundaries of Yorkshire, though it spans the ceremonial counties of North Yorkshire, West Yorkshire and Cumbria...
, is defined to the west by the Dent Fault and to the south by the Craven Fault
Craven Fault
The Craven Fault is the name applied by geologists to the group of crustal faults in the Pennines that form the south edge of the Askrigg Block. It is evident at the surface in the contrast of limestone with millstone grit. It is coincident with the south edge of the Yorkshire Dales National Park...
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 orogeniesOrogeny
Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...
(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 OrogenyCaledonian orogeny
The Caledonian orogeny is 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 occurred from the Ordovician to Early Devonian, roughly...
took place between about 490 and 390 million years ago as the former micro-continent of Avalonia
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...
collided obliquely with the former continent
Continent
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...
of Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
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
South Wales is an area of Wales bordered by England and the Bristol Channel to the east and south, and Mid Wales and West Wales to the north and west. The most densely populated region in the south-west of the United Kingdom, it is home to around 2.1 million people and includes the capital city of...
northwards to the Shetland Islands
Shetland Islands
Shetland is a subarctic archipelago of Scotland that lies north and east of mainland Great Britain. The islands lie some to the northeast of Orkney and southeast of the Faroe Islands and form part of the division between the Atlantic Ocean to the west and the North Sea to the east. The total...
.
Key structures include:
- Moine Thrust
- Great Glen FaultGreat Glen FaultThe Great Glen Fault is a long strike-slip fault that runs through its namesake the Great Glen in Scotland. However, the fault is actually much longer and over 400 million years old.-Location:...
- Walls Boundary Fault
- Highland Boundary FaultHighland Boundary FaultThe Highland Boundary Fault is a geological fault that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east...
- Midland Valley grabenCentral LowlandsThe Central Lowlands or Midland Valley is a geologically defined area of relatively low-lying land in southern Scotland. It consists of a rift valley between the Highland Boundary Fault to the north and the Southern Uplands Fault to the south...
- Southern Uplands FaultSouthern Uplands FaultThe Southern Uplands Fault is a fault in Scotland that runs from Girvan to Dunbar on the East coast. It marks the southern boundary of the Scottish Midland Valley....
- Menai Strait Fault Zone
- Bala FaultBala FaultBala Fault is a SW-NE trending geological fault in Wales that extends offshore into Cardigan Bay. In the offshore area it is a major normal fault and forms the bounding structure to the Cardigan Bay Basin, with a fill including about 2500 m of Lias Group. At its northeastern end it links to the...
- Welsh Borderland Fault SystemWelsh Borderland Fault SystemThe 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...
- Cribarth DisturbanceCribarth DisturbanceThe 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 comprises both a series of faults and associated folds which were active during the mountain-building period...
- Neath DisturbanceNeath DisturbanceThe Neath Disturbance is a geological structure which stretches across south Wales from Swansea Bay northeastwards as far as Hereford in western England. It comprises a series of both faults and associated folds which were active during the mountain-building period known as the Variscan orogeny...
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 OrogenyVariscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
was a complex affair whereby the former micro-continent of Armorica
Armorica
Armorica or Aremorica is the name given in ancient times to the part of Gaul that includes the Brittany peninsula and the territory between the Seine and Loire rivers, extending inland to an indeterminate point and down the Atlantic coast...
collided with Laurussia (otherwise known as Euramerica
Euramerica
Euramerica was a minor supercontinent created in the Devonian as the result of a collision between the Laurentian, Baltica, and Avalonia cratons .300 million years ago in the Late Carboniferous tropical rainforests lay over the equator of Euramerica...
or the Old Red Continent), followed by a further collision between Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
and the enlarged Laurussia. In Britain it resulted in a variety of geological structures across the southwest from Pembrokeshire
Pembrokeshire
Pembrokeshire is a county in the south west of Wales. It borders Carmarthenshire to the east and Ceredigion to the north east. The county town is Haverfordwest where Pembrokeshire County Council is headquartered....
and South Glamorgan
Glamorgan
Glamorgan or Glamorganshire is one of the thirteen historic counties and a former administrative county of Wales. It was originally an early medieval kingdom of varying boundaries known as Glywysing until taken over by the Normans as a lordship. Glamorgan is latterly represented by the three...
in Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
to Devon
Devon
Devon is a large county in southwestern England. The county is sometimes referred to as Devonshire, although the term is rarely used inside the county itself as the county has never been officially "shired", it often indicates a traditional or historical context.The county shares borders with...
and Cornwall
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
.
Structures include:
- Dodman-Start Thrust
- Lizard Thrust
- Carrick Thrust
The Alpine Orogeny
The Alpine OrogenyAlpine orogeny
The Alpine orogeny is an orogenic phase in the Late Mesozoic and Tertiary that formed the mountain ranges of the Alpide belt...
began 200 million years ago and continues to the present day. It comprises a series of collisions between various micro-continents between northern Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and Africa
Africa
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
. Its effects are most evident in the Alps
Alps
The Alps is one of the great mountain range systems of Europe, stretching from Austria and Slovenia in the east through Italy, Switzerland, Liechtenstein and Germany to France in the west....
, Pyrenees
Pyrenees
The Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...
, Carpathians and other mountain ranges of southern Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
but the northernmost 'ripples' of this event have affected the structure of southern England.
Structures include;
- Wealden AnticlineWeald-Artois AnticlineThe Weald–Artois anticline is a large anticline, a geological structure running between the regions of the Weald in southern England and the Artois in northeastern France. The fold formed during the Alpine orogeny, from the late Oligocene to middle Miocene as an uplifted form of the Weald basin...
- Purbeck MonoclinePurbeck MonoclineThe Purbeck Monocline is a geological fold. 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 deformation. A monocline is a step-like fold, in which one limb is roughly horizontal...
See also
- Geology of Great Britain
- Gravity anomalies of Britain and IrelandGravity anomalies of Britain and IrelandVariations in the strength of gravity occur from place to place according to the density distribution of the rocks beneath the surface. Such gravity anomalies have been mapped across the British Isles and the surrounding seas and they reveal aspects of these islands’ geological structure.-Negative...
- List of geological faults of England
- List of geological faults of Northern Ireland
- List of geological faults of Scotland
- List of geological folds in Great Britain