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The Marros Group is the name given to a suite of rocks of Namurian age laid down during the
Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
Period in South
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 ...
. 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 The Pennines (), also known as the Pennine Chain or Pennine Hills, are a range of highland, uplands mainly located in Northern England. Commonly described as the "Vertebral column, backbone of England" because of its length and position, the ra ...
and
Peak District The Peak District is an Highland, upland area in central-northern England, at the southern end of the Pennines. Mostly in Derbyshire, it extends into Cheshire, Greater Manchester, Staffordshire, West Yorkshire and South Yorkshire. It is subdivi ...
of northern
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...
and northeast Wales by this new name.


Stratigraphy

The Group comprises a thick unit of coarse
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
known as the Twrch Sandstone (formerly the ‘Basal Grit’) which is overlain by the Bishopston Mudstone and the Telpyn Point Sandstone. The
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...
s of these latter two formations was formerly known as the ‘Middle Shales’, a name reflecting the position of this sequence 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’. Likewise the Twrch Sandstone contains bands of mudstone, often correlating with marine bands. The sequence as developed along the north crop of the South Wales Coalfield varies along its length but within the mudstone which forms the greater thickness of the Bishopston Mudstone Formation, the following are observed: *Subcrenatum Sandstone *Cumbriense Quartzite *Sub-Cumbriense Sandstone *Sub-Carbonicola Sandstone *Sub-Cancellatum Sandstone *Twelve-foot Sandstone Note that the names of many of the sandstone units are derived from the marine bands (themselves named for diagnostic fossils within them) beneath which they typically occur. Not all of the sandstones occur within any one place, the Subcrenatum thins out to the east whilst the typically double-layered Cumbriense Sandstone thins out to the west. Further west in Pembrokeshire the sequence above and including the Sub-Carbonicola is absent and is replaced by the Telpyn Point Sandstone Formation.


Landscapes

The Twrch Sandstone Formation gives rise to numerous positive landscape features such as Carreg Dwfn, Tair Carn Uchaf, Gwaun Cefnygarreg in the west of the
Brecon Beacons National Park Brecon Beacons National Park, officially named Bannau Brycheiniog National Park (), is a National parks of the United Kingdom, national park in Wales. It is named after the Brecon Beacons (), the mountain range at its centre. The national park ...
and the plateau surfaces of Mynydd Llangatwg and
Mynydd Llangynidr Mynydd Llangynidr is a mountain in the Brecon Beacons National Park largely in the county of Powys, south Wales. Its southern slopes extend into the northernmost parts of the county borough of Blaenau Gwent. It is named from the village of Llang ...
in the east. Faulted blocks of the Twrch Sandstone and Bishopston Mudstone formations are responsible for much of the dramatic scenery within the national park's Waterfall Country. The Marros Group is underlain by the Carboniferous Limestone, the boundary being
unconformable An unconformity is a buried erosional or non-depositional 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 ...
. In contrast, the contact with the overlying Farewell Rock which lies at the base of the South Wales Coal Measures is conformable.British Geological Survey Research Report RR/07/01: Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore) Waters, C.N. et al 2007


Origin of names

The Marros Group is named from the locality ( Marros) in southwest
Carmarthenshire Carmarthenshire (; or informally ') is a Principal areas of Wales, county in the South West Wales, south-west of Wales. The three largest towns are Llanelli, Carmarthen and Ammanford. Carmarthen is the county town and administrative centre. ...
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 The Gower Peninsula (), or simply Gower (), is a peninsula in the South West Wales, south-west of Wales. It is the most westerly part of the historic county of Glamorgan, and is now within the City and County of Swansea. It projects towards th ...
, southwest of
Swansea Swansea ( ; ) is a coastal City status in the United Kingdom, city and the List of urban areas in the United Kingdom, second-largest city of Wales. It forms a Principal areas of Wales, principal area, officially known as the City and County of ...
and the Telpyn Point Sandstone from the coastal locality of that name near Marros.


References

{{reflist Carboniferous System of Europe Geology of Wales