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.
13-728: 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 and the Telpyn Point Sandstone . The mudstones 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 ,
26-484: A formation in another region and a formation may reduce in rank for member or bed as it "pinches out". A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in the case of a marker horizon . A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of
39-418: A unit may be defined by terms such as "when the sandstone component exceeds 75%". Sequences of sedimentary and volcanic rocks are subdivided on the basis of their shared or associated lithology . Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank , higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank,
52-598: Is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. A sequence of fossil -bearing sedimentary rocks can be subdivided on the basis of the occurrence of particular fossil taxa . A unit defined in this way is known as a biostratigraphic unit, generally shortened to biozone . The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones. Type locality (geology) Type locality , also called type area ,
65-563: Is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in the North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. A supergroup
78-624: 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 and the Telpyn Point Sandstone from
91-456: The coastal locality of that name near Marros. Rock unit A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic , lithologic or paleontologic features ( facies ) that characterize it. Units must be mappable and distinct from one another, but the contact need not be particularly distinct. For instance,
104-616: 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 . In contrast, the contact with the overlying Farewell Rock which lies at the base of the South Wales Coal Measures
117-437: The formation. A member need not be mappable at the same scale as a formation. Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although the boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area. A group
130-557: 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
143-415: The main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by geological surveys . Units of formation or higher rank are usually named for the unit's type location , and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to
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#1732802377618156-595: The mudstone which forms the greater thickness of the Bishopston Mudstone Formation, the following are observed: 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
169-727: 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. 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 and the plateau surfaces of Mynydd Llangatwg and Mynydd Llangynidr in
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