Geology of South Wales
Encyclopedia
South Wales
is an area with many features of outstanding interest to geologist
s, who have for long used the area for University field trips.
This varied and accessible region has provided a written record of geological interest going back to the 1100s when Giraldus Cambrensis
noted pyritous shale
s near Newport
. Some of the first published representations of fossil
s were those of fossil plants taken from coal measures near Neath
(Gibson late 17th century).The great British geologists Adam Sedgwick
and Roderick Murchison
did fundamentally important work in South Wales on Old Red Sandstone
and the underlying rocks. In the first volume of memoirs (1846) published by the Geological Survey
was a conspectus of the geology of South Wales which set a template for all future work. Significant discoveries continue to be made; for instance, a small area of Precambrian
and Cambrian
rocks in Carmarthenshire
was detected by Professor John Cope in the late 1970s, while some parts of South Wales have not seen a Geological Survey
officer since the mid-19th century. The geology of south Wales was central to the massive contribution Wales made to the Industrial Revolution
.
Several successive periods are represented, including all of the Palaeozoic periods. Pembrokeshire
has outcrops of both Precambrian
and Cambrian
(542-488 million years ago) rocks. A notable feature of the Ordovician
system (488-444 million years old) is a major subsidence area, or basin, which later filled with substantial thicknesses of mostly sandy and muddy sediment. The line of the shallower continental shelf area, to the south-east, ran approximately from the Long Mynd
(in Shropshire
) to Haverfordwest
in Pembrokeshire
. Some volcanic action also occurred. This thick sedimentary deposition continued into the Silurian period (444-416 million years old). Silurian volcanicity led to production of lavas at localised sites such as Skomer Island.
Silurian
rocks are widely distributed in south Wales and are well diplayed on the Cardiganshire
coast. The Silurian and earlier rocks were folded, faulted, and distorted several times, notably during the late Ordovician
, and late Silurian
, by which time the area had begun to rise above sea level as mountain ranges were uplifted to the north-west. This was caused by the collision of a proto-North America ("Laurentia
") and the present southern Britain ("Eastern Avalonia
"). The intervening sea, the Iapetus Ocean
, closed as a result of plate tectonic movements, similar to thiose which are now closing the Mediterranean Sea
between Europe and Africa.
The succeeding Devonian
period (416-359 million years ago) saw mostly alluvial deposits ("Old Red Sandstone
"), which were interrupted by major uplift. Following this, the land subsided again in the Carboniferous
period (359-299 million years ago). In the early Carboniferous period, limestones were deposited on a continental shelf
which deepened towards the south, where there was a major sea area. During earth movements caused by the approach of a continent from the south, shallowing of the sea occurred which led to thick alluvial and coastal plain sediments (see below) containing economic developments of coal. Carboniferous Limestone
outcrops occur in south Pembrokeshire
, the Gower Peninsula
, the Vale of Glamorgan
and around the edge of the coalfield
.
At the end of the Carboniferous, during Permian
(299-251 my ago) time, Wales
is thought to have been a landmass. Only in late Triassic
(251-200 my ago) times did deposition return, with coarse-grained screes forming around a series of low but rugged hills. Continuing deposition led to the formation of the early Jurassic
deposits of mudstones and muddy limestones (the "Lias"). Deposits later than this are almost comple and tely absent, although the Chalk sea probably covered most of Wales between 100 and 66 million years ago. Any sucjh later deposits have been eroded away.
One of the geological rarities of south Wales is the coastline of Ogmore-by-Sea
and Southerndown
. Its cliff
s are composed partly of earliest Jurassic Sutton stone, a rare freestone
that is a marginal or coastal deposit derived in part from erosion of carboniferous
limestone
. Sutton stone has always been highly regarded: as well as being used in construction throughout the Vale of Glamorgan
, it was also shipped over the Bristol Channel
to North Devon
and North Cornwall
which are both deficient in limestone.
A major geological feature of the Upper Carboniferous rocks in south Wales is the south Wales coalfield
. The rocks comprising this important area were laid down during the later Carboniferous. This sedimentary succession includes a sequence with a thickness of more than 1800 m (5,906 ft) in the west. The coal measures
were laid down on a low-lying waterlogged plain with peat mires immediately south of an ancient and persistent geological feature known as the Wales-London-Brabant High. Burial converted the peat deposits to thinner coal seams. Later earth movements distorted the coal-bearing strata, and the difficulty of mining led to premature closure of many mines on economic grounds, especially in Pembrokeshire where seams were squeezed completely out of shape.
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...
is an area with many features of outstanding interest to geologist
Geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...
s, who have for long used the area for University field trips.
This varied and accessible region has provided a written record of geological interest going back to the 1100s when Giraldus Cambrensis
Giraldus Cambrensis
Gerald of Wales , also known as Gerallt Gymro in Welsh or Giraldus Cambrensis in Latin, archdeacon of Brecon, was a medieval clergyman and chronicler of his times...
noted pyritous shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s near Newport
Newport
Newport is a city and unitary authority area in Wales. Standing on the banks of the River Usk, it is located about east of Cardiff and is the largest urban area within the historic county boundaries of Monmouthshire and the preserved county of Gwent...
. Some of the first published representations of fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s were those of fossil plants taken from coal measures near Neath
Neath
Neath is a town and community situated in the principal area of Neath Port Talbot, Wales, UK with a population of approximately 45,898 in 2001...
(Gibson late 17th century).The great British geologists Adam Sedgwick
Adam Sedgwick
Adam Sedgwick was one of the founders of modern geology. He proposed the Devonian period of the geological timescale...
and Roderick Murchison
Roderick Murchison
Sir Roderick Impey Murchison, 1st Baronet KCB DCL FRS FRSE FLS PRGS PBA MRIA was a Scottish geologist who first described and investigated the Silurian system.-Early life and work:...
did fundamentally important work in South Wales on Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
and the underlying rocks. In the first volume of memoirs (1846) published by the Geological Survey
British Geological Survey
The British Geological Survey is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS headquarters are in Keyworth, Nottinghamshire, but other centres...
was a conspectus of the geology of South Wales which set a template for all future work. Significant discoveries continue to be made; for instance, a small area of Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
and Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
rocks in Carmarthenshire
Carmarthenshire
Carmarthenshire is a unitary authority in the south west of Wales and one of thirteen historic counties. It is the 3rd largest in Wales. Its three largest towns are Llanelli, Carmarthen and Ammanford...
was detected by Professor John Cope in the late 1970s, while some parts of South Wales have not seen a Geological 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....
officer since the mid-19th century. The geology of south Wales was central to the massive contribution Wales made to the Industrial Revolution
Industrial Revolution
The Industrial Revolution was a period from the 18th to the 19th century where major changes in agriculture, manufacturing, mining, transportation, and technology had a profound effect on the social, economic and cultural conditions of the times...
.
Several successive periods are represented, including all of the Palaeozoic periods. 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....
has outcrops of both Precambrian
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
and Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
(542-488 million years ago) rocks. A notable feature of the Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
system (488-444 million years old) is a major subsidence area, or basin, which later filled with substantial thicknesses of mostly sandy and muddy sediment. The line of the shallower continental shelf area, to the south-east, ran approximately from the Long Mynd
Long Mynd
The Long Mynd in Shropshire, England, is a part of the Shropshire Hills Area of Outstanding Natural Beauty. It is south of the county town Shrewsbury, and has an area of over 22 square kilometres , most of which takes the form of a heathland plateau. Most of the land on the Long Mynd is owned by...
(in Shropshire
Shropshire
Shropshire is a county in the West Midlands region of England. For Eurostat purposes, the county is a NUTS 3 region and is one of four counties or unitary districts that comprise the "Shropshire and Staffordshire" NUTS 2 region. It borders Wales to the west...
) to Haverfordwest
Haverfordwest
Haverfordwest is the county town of Pembrokeshire, Wales and serves as the County's principal commercial and administrative centre. Haverfordwest is the most populous urban area in Pembrokeshire, with a population of 13,367 in 2001; though its community boundaries make it the second most populous...
in 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....
. Some volcanic action also occurred. This thick sedimentary deposition continued into the Silurian period (444-416 million years old). Silurian volcanicity led to production of lavas at localised sites such as Skomer Island.
Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
rocks are widely distributed in south Wales and are well diplayed on the Cardiganshire
Ceredigion
Ceredigion is a county and former kingdom in mid-west Wales. As Cardiganshire , it was created in 1282, and was reconstituted as a county under that name in 1996, reverting to Ceredigion a day later...
coast. The Silurian and earlier rocks were folded, faulted, and distorted several times, notably during the late Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
, and late Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
, by which time the area had begun to rise above sea level as mountain ranges were uplifted to the north-west. This was caused by the collision of a proto-North America ("Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
") and the present southern Britain ("Eastern 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...
"). The intervening sea, the Iapetus Ocean
Iapetus Ocean
The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale . The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia...
, closed as a result of plate tectonic movements, similar to thiose which are now closing the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
between Europe and Africa.
The succeeding Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period (416-359 million years ago) saw mostly alluvial deposits ("Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
"), which were interrupted by major uplift. Following this, the land subsided again in the Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
period (359-299 million years ago). In the early Carboniferous period, limestones were deposited on a continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...
which deepened towards the south, where there was a major sea area. During earth movements caused by the approach of a continent from the south, shallowing of the sea occurred which led to thick alluvial and coastal plain sediments (see below) containing economic developments of coal. Carboniferous Limestone
Carboniferous limestone
Carboniferous Limestone is a term used to describe a variety of different types of limestone occurring widely across Great Britain and Ireland which were deposited during the Dinantian epoch of the Carboniferous period. They were formed between 363 and 325 million years ago...
outcrops occur in south Pembrokeshire
South Pembrokeshire
South Pembrokeshire was one of six local government districts of Dyfed, Wales from 1974 to 1996.It was formed on April 1, 1974 under the Local Government Act 1972, from the following parts of the administrative county of Pembrokeshire:...
, the Gower Peninsula
Gower Peninsula
Gower or the Gower Peninsula is a peninsula in south Wales, jutting from the coast into the Bristol Channel, and administratively part of the City and County of Swansea. Locally it is known as "Gower"...
, the Vale of Glamorgan
Vale of Glamorgan
The Vale of Glamorgan is a county borough in Wales; an exceptionally rich agricultural area, it lies in the southern part of Glamorgan, South Wales...
and around the edge of the coalfield
South Wales Coalfield
The South Wales Coalfield is a large region of south Wales that is rich with coal deposits, especially the South Wales Valleys.-The coalfield area:...
.
At the end of the Carboniferous, during Permian
Permian
The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...
(299-251 my ago) time, 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...
is thought to have been a landmass. Only in late Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
(251-200 my ago) times did deposition return, with coarse-grained screes forming around a series of low but rugged hills. Continuing deposition led to the formation of the early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
deposits of mudstones and muddy limestones (the "Lias"). Deposits later than this are almost comple and tely absent, although the Chalk sea probably covered most of Wales between 100 and 66 million years ago. Any sucjh later deposits have been eroded away.
One of the geological rarities of south Wales is the coastline of Ogmore-by-Sea
Ogmore-by-Sea
Ogmore-by-Sea is a seaside village in the Vale of Glamorgan, Wales. It lies on the western limit of the Glamorgan Heritage Coastline of south Wales....
and Southerndown
Southerndown
Southerndown is a village in South Wales to the southwest of Bridgend, close to St Brides Major, Llantwit Major and Ogmore-by-Sea. It is mostly known for its beach , which is a popular tourist destination during the summer months and since 1972 has been designated a Heritage Coast and is part of...
. Its cliff
Cliff
In geography and geology, a cliff is a significant vertical, or near vertical, rock exposure. Cliffs are formed as erosion landforms due to the processes of erosion and weathering that produce them. Cliffs are common on coasts, in mountainous areas, escarpments and along rivers. Cliffs are usually...
s are composed partly of earliest Jurassic Sutton stone, a rare freestone
Freestone
A freestone is a stone used in masonry for molding, tracery and other replication work required to be worked with the chisel. The freestone must be fine-grained, uniform and soft enough to be cut easily without shattering or splitting. Some sources say that the stone has no grain, but this is...
that is a marginal or coastal deposit derived in part from erosion of carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
. Sutton stone has always been highly regarded: as well as being used in construction throughout the Vale of Glamorgan
Vale of Glamorgan
The Vale of Glamorgan is a county borough in Wales; an exceptionally rich agricultural area, it lies in the southern part of Glamorgan, South Wales...
, it was also shipped over the Bristol Channel
Bristol Channel
The Bristol Channel is a major inlet in the island of Great Britain, separating South Wales from Devon and Somerset in South West England. It extends from the lower estuary of the River Severn to the North Atlantic Ocean...
to North Devon
North Devon
North Devon is the northern part of the English county of Devon. It is also the name of a local government district in Devon. Its council is based in Barnstaple. Other towns and villages in the North Devon District include Braunton, Fremington, Ilfracombe, Instow, South Molton, Lynton and Lynmouth...
and North Cornwall
North Cornwall
North Cornwall was the largest of the six local government districts of Cornwall, United Kingdom. Its council was based in Wadebridge . Other towns in the district included Bude, Bodmin, Launceston, Padstow, and Camelford....
which are both deficient in limestone.
A major geological feature of the Upper Carboniferous rocks in south Wales is the south Wales coalfield
South Wales Coalfield
The South Wales Coalfield is a large region of south Wales that is rich with coal deposits, especially the South Wales Valleys.-The coalfield area:...
. The rocks comprising this important area were laid down during the later Carboniferous. This sedimentary succession includes a sequence with a thickness of more than 1800 m (5,906 ft) in the west. The coal measures
Coal Measures
The Coal Measures is a lithostratigraphical term for the coal-bearing part of the Upper Carboniferous System. It represents the remains of fluvio-deltaic sediment, and consists mainly of clastic rocks interstratified with the beds of coal...
were laid down on a low-lying waterlogged plain with peat mires immediately south of an ancient and persistent geological feature known as the Wales-London-Brabant High. Burial converted the peat deposits to thinner coal seams. Later earth movements distorted the coal-bearing strata, and the difficulty of mining led to premature closure of many mines on economic grounds, especially in Pembrokeshire where seams were squeezed completely out of shape.