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| Marros Group | |
|---|---|
| Name | Marros Group |
| Type | Geological group |
| Age | Namurian to Westphalian (Carboniferous) |
| Period | Carboniferous |
| Region | South Wales |
| Country | Wales, United Kingdom |
| Subunits | Twrch Sandstone, Bishopston Mudstone, Farewell Rock (local) |
| Overlies | South Wales Coal Measures Group |
| Underlies | Pembroke Limestone, Warwickshire Group |
Marros Group The Marros Group is a Carboniferous lithostratigraphic group cropping out across South Wales, with exposures in Carmarthenshire, Swansea, Pembrokeshire, and Glamorgan. It records fluvial, deltaic and marginal marine environments deposited during Namurian to Westphalian times and is a key unit for interpreting Carboniferous palaeogeography in the British Isles, Ireland, and adjacent offshore basins such as the Bristol Channel Basin. The group has been the subject of mapping and study by institutions including the British Geological Survey, Geological Society of London, and university departments at Cardiff University, University of Oxford, and University of Cambridge.
The Marros Group sits within the regional Carboniferous succession of the Pennine Basin, forming an intermediate unit between older marine limestone sequences such as the Pembroke Limestone Group and younger coal-bearing strata like the South Wales Coal Measures Group. In outcrop it forms escarpments and uplands near Mynydd y Gwair, Brecon Beacons National Park, and the Gower Peninsula. The group is equivalent in part to Namurian and Westphalian sequences correlated with units in the English Midlands, Derbyshire, and the Welsh Borders.
Lithologies within the Marros Group include thick-bedded sandstones, siltstones, mudstones, and subordinate seatearths and coals. Prominent sandstone units such as the Twrch Sandstone exhibit cross-bedding, channel-fill architecture and palaeocurrent indicators comparable to ancient fluvial systems documented in the Permian Basin and Appalachian Basin analogues. Mudstone intervals preserve root horizons and palaeosols similar to those described from the Westphalian Series in Northern England and the Midlands. Localised ironstone and sandstone facies have affinities with deposits in the Coal Measures Supergroup.
The Marros Group comprises multiple formations and members recognised in regional mapping. Key subunits traditionally include the Twrch Sandstone Formation, the Bishopston Mudstone Formation, and laterally equivalent units correlated with the Farewell Rock in parts of South Wales Coalfield stratigraphy. Correlations extend to Namurian sequences recognised in the Market Weighton Block and the Bristolian Basin, with biostratigraphic ties to assemblages documented from the Holkerian and Arnsbergian reference sections. Lateral facies changes link the group to contemporaneous units in the Clwydian Range, Shropshire and offshore to the Cardigan Bay Basin.
Fossil assemblages preserved in marros-type strata include plant macrofossils such as sigillariacean and lycopsid remains comparable to collections from Scotland and the Mersey Basin, and palynological records yielding spores correlated with the Namurian spore zones. Trace fossils in sandstones record arthropod and vertebrate activity analogous to ichnofabrics from Westphalia II localities in Germany and France. Marine interbeds and marginal marine shales within the group have produced brachiopod and bivalve faunas with affinities to faunas from the Pembroke Limestone Group and the Rhaetian–Hettangian boundary successions noted by workers at Natural History Museum, London.
Historically, sandstone units within the Marros Group have served as local building stone used in Swansea and Carmarthen vernacular architecture, similar to exploitation of Pennsylvanian sandstones in the Midlands. Mudstone and seatearth horizons host thin coals that were exploited in small-scale mines across the South Wales Coalfield fringe communities such as Neath and Pontardawe. Aggregates derived from Marros sandstones supply local construction; ironstone bands have been trialled for iron smelting analogous to historic workings in Mansfield and Derbyshire. Hydrogeological studies by Environment Agency (England and Wales) and the British Geological Survey assess the group’s role as an aquifer or aquiclude in catchments draining into the River Tawe and Bristol Channel.
Early descriptions of Marros-type strata appear in 19th-century surveys by pioneering geologists such as Roderick Murchison, Adam Sedgwick, and later regional mapping by Sir Archibald Geikie. Systematic lithostratigraphic definition and mapping were advanced by the British Geological Survey and academic studies at University College London and Imperial College London. Key modern contributions include basin analysis comparing Marros Group deposition with Carboniferous sequences in the Rhenish Massif and the Appalachian Basin, palaeoclimatic work using palynology performed at University of Birmingham, and sedimentological studies by teams at University of Leeds and University of Manchester.
Outcrops and landscapes underlain by the Marros Group occur in conservation areas such as the Brecon Beacons National Park, Gower AONB, and near sites managed by Natural Resources Wales. Quarrying, recreational walking along escarpments like those at Mynydd Llangatwg, and agricultural use impact exposures and fossil preservation; mitigation and planning policies are guided by local authorities including Swansea Council and Carmarthenshire County Council. Geological Sites of Special Scientific Interest (SSSIs) and educational geotrails highlight Marros exposures for outreach and scientific study, with collaboration between Natural England, Geopark》 organisations, and university outreach units.
Category:Carboniferous geology of Wales Category:Geologic groups of the United Kingdom