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 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, 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 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 the east. Faulted blocks of the Bishopston Mudstone and Telpyn Point Sandstone 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 is conformable.
Origin of names
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 the coastal locality of that name near Marros.
- British Geological Survey Research Report RR/07/01: Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore) Waters, C.N. et al 2007