| Marros Group | |
|---|---|
| Stratigraphic range:Namurian | |
 Sandstones and interbedded mudstones of the Telpyn Point Sandstone Formation, Haroldston Chins,St Bride's Bay | |
| Type | Group |
| Sub-units | Aberkenfig Formation, Bishopston Formation, Twrch Sandstone Formation, Telpyn Point Sandstone Formation |
| Underlies | Coal Measures Group |
| Overlies | Pembroke Limestone Group |
| Thickness | 20 m to 750 m |
| Lithology | |
| Primary | mudstonesandstone |
| Other | chert,limestone |
| Location | |
| Country | Wales |
| Extent | South Wales Coalfield,Pembrokeshire |
TheMarros Group is the name given to a suite of rocks ofNamurian age laid down during theCarboniferousPeriod in SouthWales. These rocks were formerly known as theMillstone Grit Series but are now distinguished from the similar but geographically separate rock sequences of thePennines andPeak District of northernEngland and northeast Wales by this new name.
The Group comprises a thickunit of coarsesandstone known as theTwrch Sandstone (formerly the ‘Basal Grit’) which is overlain by theBishopston Mudstone and theTelpyn Point Sandstone. Themudstones 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 theFarewell Rock, the lowermost sandstone of the South WalesCoal 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 withmarine 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:
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.[1]
The Twrch Sandstone Formation gives rise to numerous positive landscape features such asCarreg Dwfn,Tair Carn Uchaf,Gwaun Cefnygarreg in the west of theBrecon Beacons National Park and the plateau surfaces ofMynydd Llangatwg andMynydd Llangynidr 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'sWaterfall Country.
The Marros Group is underlain by theCarboniferous Limestone, the boundary beingunconformable. In contrast, the contact with the overlying Farewell Rock which lies at the base of theSouth Wales Coal Measures is conformable.[2]
The Marros Group is named from the locality (Marros) in southwestCarmarthenshire where these rocks are well exposed in spectacular coastal cliffs. The Twrch Sandstone derives its name from the vicinity of theAfon Twrch where these beds reach their maximum thickness. The mudstones derive their name fromBishopston on theGower Peninsula, southwest ofSwansea and the Telpyn Point Sandstone from thecoastal locality of that name near Marros.