South Wales is an area with many features of outstanding interest to
geologist
A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althoug ...
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 that dates to the 12th century, when
Giraldus Cambrensis
Gerald of Wales ( la, Giraldus Cambrensis; cy, Gerallt Gymro; french: Gerald de Barri; ) was a Cambro-Norman priest and historian. As a royal clerk to the king and two archbishops, he travelled widely and wrote extensively. He studied and taugh ...
noted pyritous
shales near
Newport.
Some of the first published representations of
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s were of fossil plants from
coal measure
In lithostratigraphy, the coal measures are the coal-bearing part of the Upper Carboniferous System. In the United Kingdom, the Coal Measures Group consists of the Upper Coal Measures Formation, the Middle Coal Measures Formation and the Lower Coal ...
s near
Neath (Gibson late 17th century). The British geologists
Adam Sedgwick and
Roderick Murchison did fundamentally important work in South Wales on
Old Red Sandstone
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...
and the underlying rocks. The first volume of memoirs (1846) published by the
Geological Survey
A geological survey is the systematic investigation of the geology beneath a given piece of ground for the purpose of creating a geological map or model. Geological surveying employs techniques from the traditional walk-over survey, studying outc ...
contained a conspectus of the geology of South Wales that set a template for all future work.
Significant discoveries continue to be made. For instance, in
Carmarthenshire
Carmarthenshire ( cy, Sir Gaerfyrddin; or informally ') is a county in the south-west of Wales. The three largest towns are Llanelli, Carmarthen and Ammanford. Carmarthen is the county town and administrative centre. The county is known as ...
, in the late 1970s, John Cope detected a small area of
Precambrian and
Cambrian rocks. Other parts of South Wales, however, have not seen a
Geological Survey
A geological survey is the systematic investigation of the geology beneath a given piece of ground for the purpose of creating a geological map or model. Geological surveying employs techniques from the traditional walk-over survey, studying outc ...
officer since the mid-19th century. The geology of south Wales was central to the massive contribution Wales made to the
Industrial Revolution
The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
.
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
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
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
, photo =
, photo_alt =
, photo_caption = View down Townbrook Valley toward Burway Hill
, country_type =
, country = England
, subdivision1_type = County
, subdivision1 = Shropshire
, border ...
(in
Shropshire
Shropshire (; alternatively Salop; abbreviated in print only as Shrops; demonym Salopian ) is a landlocked historic county in the West Midlands region of England. It is bordered by Wales to the west and the English counties of Cheshire to ...
) to
Haverfordwest
Haverfordwest (, ; cy, Hwlffordd ) is the county town of Pembrokeshire, Wales, and the most populous urban area in Pembrokeshire with a population of 14,596 in 2011. It is also a community, being the second most populous community in the county, ...
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
Skomer () or Skomer Island is an island off the coast of Pembrokeshire, in the community of Marloes and St Brides in west Wales. It is well known for its wildlife: around half the world's population of Manx shearwaters nest on the island, the ...
.
Silurian rocks are widely distributed in south Wales and are well displayed on the
Cardiganshire coast. The Silurian and earlier rocks were folded, faulted, and distorted several times, notably during the late
Ordovician
The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya.
T ...
, 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
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 ...
") and the present southern Britain ("Eastern
Avalonia"). The intervening sea, the
Iapetus Ocean, closed as a result of plate tectonic movements, similar to those that now close the
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...
between Europe and Africa.
The succeeding
Devonian period (416-359 million years ago) saw mostly alluvial deposits ("
Old Red Sandstone
The Old Red Sandstone is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the northeastern seaboard of North America. It also exte ...
"), 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, sea shallowing led to thick alluvial and coastal plain sediments (see below) containing economic developments of coal.
Carboniferous Limestone
Carboniferous Limestone is a collective term for the succession of limestones occurring widely throughout Great Britain and Ireland that were deposited during the Dinantian epoch (geology), Epoch of the Carboniferous period (geology), Period. T ...
outcrops occur in
south Pembrokeshire
South Pembrokeshire ( cy, De Sir Benfro) was one of six local government districts of Dyfed, Wales from 1974 to 1996.
Creation
The district was formed on 1 April 1974 under the Local Government Act 1972, from the following parts of the administra ...
, the
Gower Peninsula
Gower ( cy, Gŵyr) or the Gower Peninsula () in southwest Wales, projects towards the Bristol Channel. It is the most westerly part of the historic county of Glamorgan. In 1956, the majority of Gower became the first area in the United Kingdom ...
, the
Vale of Glamorgan
The Vale of Glamorgan ( cy, Bro Morgannwg ), often referred to as The Vale, is a county borough in the south-east of Wales. It borders Bridgend County Borough to the west, Cardiff to the east, Rhondda Cynon Taf to the north, and the Bristol ...
and around the edge of
the coalfield.
At the end of the Carboniferous, 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 ...
(299-251 my ago) time,
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 ...
is thought to have been a landmass. Only in late
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 ...
(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
The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
deposits of mudstones and muddy limestones (the "Lias"). Deposits later than this are almost completely absent, although the Chalk sea probably covered most of Wales between 100 and 66 million years ago. Any such later deposits have been eroded away.
One of the geological rarities of south Wales is the coastline of
St Brides Major at
Ogmore-by-Sea
Ogmore-by-Sea ( Welsh: ''Aberogwr'', meaning "Mouth of the River Ogmore") is a seaside village in St Brides Major community in the Vale of Glamorgan, Wales. It lies on the western limit of the Glamorgan Heritage Coast of south Wales. The pop ...
and
Southerndown. Its
cliff
In geography and geology, a cliff is an area of rock which has a general angle defined by the vertical, or nearly vertical. Cliffs are formed by the processes of weathering and erosion, with the effect of gravity. Cliffs are common on co ...
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
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
.
[www.ccgc.gov.uk sssi sites: southerndown coast]
accessed 5 November 2013 Sutton stone has always been highly regarded: as well as being used in construction throughout the
Vale of Glamorgan
The Vale of Glamorgan ( cy, Bro Morgannwg ), often referred to as The Vale, is a county borough in the south-east of Wales. It borders Bridgend County Borough to the west, Cardiff to the east, Rhondda Cynon Taf to the north, and the Bristol ...
, it was also shipped over the
Bristol Channel to
North Devon and
North Cornwall
North Cornwall ( kw, An Tiredh Uhel) is an area of Cornwall, England, United Kingdom. It is also the name of a former local government district, which was administered from Bodmin and Wadebridge . Other towns in the area are Launceston, Bude, P ...
, which are both deficient in limestone.
A major geological feature of the Upper Carboniferous rocks in south Wales is the
south Wales coalfield
The South Wales Coalfield ( cy, Maes glo De Cymru) extends across Pembrokeshire, Carmarthenshire, Swansea, Neath Port Talbot, Bridgend, Rhondda Cynon Taf, Merthyr Tydfil, Caerphilly, Blaenau Gwent and Torfaen. It is rich in coal deposits, espe ...
. 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 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.
References
See also
*
Geology of Wales
The geology of Wales is complex and varied; its study has been of considerable historical significance in the development of geology as a science. All geological periods from the Cryogenian (late Precambrian) to the Jurassic are represented at o ...
*
Geography of Wales
{{DEFAULTSORT:Geology Of South Wales
Geology of Wales