Marros Group
Encyclopedia
The Marros Group is the name given to a suite of rocks of Namurian
age laid down during the Carboniferous
period in South Wales
. These rocks were formerly known as the Millstone Grit Series
but are now distinguished from the similar but geographically separate rock sequences of the Pennines and Peak District
of northern England
and northeast Wales by this new name.
The Group comprises a thick unit of coarse sandstone
known as the Twrch Sandstone (formerly the ‘Basal Grit’) which is overlain by the Bishopston Mudstone. The mudstone
was formerly known as the ‘Middle Shales’, a name reflecting its position sandwiched between the Basal Grit below and the Farewell Rock
, the lowermost sandstone of the South Wales Coal Measures
, above. The mudstone itself contains a few bands of sandstone such as the ‘Twelve Foot Sandstone’ and locally the ‘Cumbriense Sandstone’.
The Marros Group is underlain by the Carboniferous Limestone
, the boundary being unconformable
. In contrast, the contact with the overlying Farewell Rock which lies at the base of the South Wales Coal Measures
is conformable.
The Marros Group is named from the locality (Marros
) in southwest Carmarthenshire
where these rocks are well exposed in spectacular coastal cliffs. The Twrch Sandstone derives its name from the vicinity of the Afon Twrch
where these beds reach their maximum thickness. The mudstones derive their name from Bishopston
on the Gower
peninsula, southwest of Swansea
.
Namurian
The Namurian is a stage in the regional stratigraphy of northwest Europe with an age between roughly 326 and 313 Ma . It is a subdivision of the Carboniferous system or period and the regional Silesian series. The Namurian is named for the Belgian city and province of Namur where strata of this age...
age laid down during 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 in South 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...
. These rocks were formerly known as the Millstone Grit Series
Millstone Grit
Millstone Grit is the name given to any of a number of coarse-grained sandstones of Carboniferous age which occur in the Northern England. The name derives from its use in earlier times as a source of millstones for use principally in watermills...
but are now distinguished from the similar but geographically separate rock sequences of the Pennines and Peak District
Peak District
The Peak District is an upland area in central and northern England, lying mainly in northern Derbyshire, but also covering parts of Cheshire, Greater Manchester, Staffordshire, and South and West Yorkshire....
of northern England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
and northeast Wales by this new name.
The Group comprises a thick unit of coarse sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
known as the Twrch Sandstone (formerly the ‘Basal Grit’) which is overlain by the Bishopston Mudstone. The mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
was formerly known as the ‘Middle Shales’, a name reflecting its position sandwiched between the Basal Grit below and the Farewell Rock
Farewell Rock
The Farewell Rock is the name given to a series of sandstones which lie at the boundary of the Coal Measures with the underlying Marros Group in South Wales. Once thought to be a single sandstone, it is now accepted that the same name has been applied to several different sandstones of similar age...
, the lowermost sandstone of the South Wales 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...
, above. The mudstone itself contains a few bands of sandstone such as the ‘Twelve Foot Sandstone’ and locally the ‘Cumbriense Sandstone’.
The Marros Group is underlain by the 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...
, the boundary being unconformable
Unconformity
An 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...
. In contrast, the contact with the overlying Farewell Rock which lies at the base of the South Wales Coal Measures
Coal Measures Group
The Coal Measures Group is a lithostratigraphical term coined to refer to the coal-bearing succession of rock strata which occur in Great Britain within the Westphalian Stage of the Carboniferous Period. The Coal Measures Group is preceded by the Millstone Grit Group which is of Namurian age...
is conformable.
The Marros Group is named from the locality (Marros
Eglwyscummin
Eglwyscummin is a Community Ward about southeast of Whitland in Carmarthenshire. It incorporates Marros, Cyfig and the village of Red Roses. It consists mostly of farmland and is crossed by a main road route to the once popular holiday resort of Pendine Sands and the main route to south...
) in southwest 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...
where these rocks are well exposed in spectacular coastal cliffs. The Twrch Sandstone derives its name from the vicinity of the Afon Twrch
Afon Twrch
The Afon Twrch is a river which rises in the Black Mountain in south Wales. It forms the boundary between the counties of Powys and Carmarthenshire and, downstream of Ystradowen, between Powys and Neath Port Talbot county borough.The headwaters arise on the southern slopes of Bannau Sir Gaer and...
where these beds reach their maximum thickness. The mudstones derive their name from Bishopston
Bishopston, Swansea
Bishopston is a village and community situated on the Gower Peninsula, some west south west of the centre of Swansea in South Wales. Forming part of the Bishopston ward of the City and County of Swansea, it is one of the largest villages on Gower. Bishopston has its own rugby club, South Gower...
on the Gower
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"...
peninsula, southwest of Swansea
Swansea
Swansea is a coastal city and county in Wales. Swansea is in the historic county boundaries of Glamorgan. Situated on the sandy South West Wales coast, the county area includes the Gower Peninsula and the Lliw uplands...
.