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Namurian Rocks of Great Britain

The Namurian is the name given in northwest Europe to a stage of the Carboniferous period. It is named for the Belgian city and province of Namur where strata of this age occur. It is a chronostratigraphic term which equates with the regional lithostratigraphic term, Millstone Grit series.

The Namurian stage lasted from 326 to 313 million years ago. It is preceded by the Visean stage/age (which corresponds to the upper Carboniferous Limestone) and succeeded by the Westphalian stage/age (which corresponds to the lower and middle Coal Measures). The Namurian straddles the boundary between the Mississippian epoch (359-318mya) and the Pennsylvanian epoch (318-299mya) of the Carboniferous period.

Biostratigraphy Across South Wales, the top of the Namurian is defined as the base of the Gastrioceras subcrenatum marine band.

The Namurian is divided into several NW European sub-stages, all of which were formerly considered to be ‘stages’:

Yeadonian

Marsdenian

Kinderscoutian

Alportian

Chokierian

Arnsbergian

Pendleian

Many of these are named from areas of the Peak District and southern Pennines region of England where rocks of this age are found in abundance.

The Namurian Provinces of Great Britain

Rocks of Namurian age survive in several different provinces around Britain.

Pennines (inc N & S Cumbria, Peak District and northeast Wales)

South Wales, Bristol & Forest of Dean area

Culm of South West England

English Midlands

Midland Valley of Scotland

The Namurian sequence of rocks in south Wales comprises a varying thickness of quartz-rich sandstones and conglomerates, overlying which are mudstones within which further sandstone beds occur. The sandstones are interpreted as having been laid down in a series of major river deltas formed where rivers flowing from the Wales-Brabant Massif to the north entered an arm of the sea. The depositional basin extended over time and the sandstones were overlaid by mud settling out o the water column. From time to time there were distinct rises in sea-level and these events are marked by the occurrence of marine bands within the rock sequence. These include: Gastrioceras subcrenatum MB Cancelloceras (Gastrioceras) cumbriense MB Carbonicola MB Cancelloceras (Gastrioceras) cancellatum MB Anthracoceras (Anthracoceratites?) MB Bilinguites superbilinguis (Reticuloceras superbilingue?) MB Bilinguites bilinguis (Reticuloceras reticulatum?) MB Reticuloceras circumplicatile MB V henkei MB (?) Isohomoceras subglobosum MB N stellarum MB

Stratigraphy of the Namurian rocks of South Wales

In south Wales, the former Millstone Grit Series has been re-named as the Marros Group. The name-change arose because these sediments were laid down in a depositional basin which was firmly separated by the Wales-Brabant landmass from the Pennine basins of northern England where the term ‘Millstone Grit’ originates. The Marros Group comprises the Twrch Sandstone Formation, the overlying Bishopston Mudstone Formation and, at the top of the sequence, the Telpyn Point Sandstone Formation. These are named from the localities where they are best expressed ie Marros and Telpyn Point near Amroth, both in south Pembrokeshire and Bishopston near Swansea.

The sequence is thickest in Gower where it reaches 7-800m though it attains 350m thickness in the Black Mountain area. In the latter area, more than half of this figure is due to the Twrch Sandstone. The sequence thins considerably in the far east where it is only 30m thick at Brynmawr.

The Twrch Sandstone Formation

This lowest formation of the Namurian sequence of rocks in the south Wales province is comprised of a quartz-rich sandstone which attains a thickness of nearly 200m in the area from which it derives its name – the valley of the Afon Twrch in the Black Mountain. The formation was previously known as the Basal Grit.

The Bishopston Mudstone Formation

The sequence is named from the village of Bishopston at the west end of the Gower peninsula.

Extent of the Namurian rocks of South Wales

They occur at outcrop in a more or less continuous band extending from Pembrokeshire east through Carmarthenshire and southern Powys to the western edge of Monmouthshire. A further outcrop stretches east-west across Gower. Rocks of Namurian age generally occur at depth beneath the South Wales Coalfield basin whose margins are roughly defined by these outcrops. For the most part, the Twrch Sandstone formation forms positive relief features in the landscape whilst the Bishopston Mudstones underlie areas of subdued relief.

The West

St. Brides Bay, Pembrokeshire in the west eastwards to Haverfordwest and then dip under the waters of Carmarthen Bay at Amroth. At the east side of the bay, the outcrop continues under Kidwelly the east-northeast, passing north of Cross Hands and Llandybie to Carreg Dwfn at the western extremity of the Brecon Beacons National Park where the Twrch Sandstone/Basal Grit begins to assert itself as a major landform shaping element.

The Black Mountain

The Twrch Sandstone/Basal Grit forms a series of hills in the Black Mountain straddling the Carmarthenshire/Powys border. From west to east these include Carreg Dwfn, Tair Carn Uchaf and Tair Carn Isaf, Garreg Lwyd, Foel Fraith, Cefn Carn Fadog, Garreg Las, Carreg-lem and Carreg Goch. The outcrop wraps around the flanks of Cribarth and underlies the village of Pen-y-cae in the upper Swansea valley.

Fforest Fawr

The hills of Carreg-lwyd and Carreg Cadno on the southern slopes of Fforest Fawr are formed from the Twrch Sandstone/Basal Grit. Eastwards it becomes increasingly broken by a series of north-northwest to south-southeast oriented faults which run through the Waterfalls Country around Ystradfellte. The formation reasserts itself at Gwaun Cefnygarreg and at Cefn Cadlan. The outcrop is affected by the Neath Disturbance and the rock occurs to the south at Mynydd-y-glog and east through Penmoelallt to the Taff Valley at Cefn Coed-y-cymmer.

The East

The Twrch Sandstone/Basal Grit forms the southern slopes of Cefn Cil-sanws and continues north of Merthyr Tydfil to form the ridge of Cefn yr Ystrad and much of the plateau of Mynydd Llangynidr. From Mynydd Llangatwg eastwards, the outcrop narrows and the rock occurs only in a narrow band around the Clydach Gorge and Gilwern Hill and Blorenge.

The East and South Crops

The so-called ‘east crop’ extends south from Blorenge through Mynydd Garnclochdy and Mynydd Garn-wen to Pontypool. Namurian strata continue south as a thin band outcropping to the west of Cwmbran, southwest through Risca, passing to the south of Caerphilly and then westwards to the north of Bridgend. Nowhere south of Pontypool do these rocks contribute significantly to the landscape.

Gower

The outcrop extends from the estuary of the River Loughor under Pengwern Common east-southeastwards to Swansea Bay at Mumbles. However exposures are limited due to a cover of estuarine deposits in the west and of glacial till further east.

Northeast Wales

During Namurian times, a part of the Pennine Basin extended across to what is now northeast Wales. The Wales-Brabant landmass lay to its south and divided it from the South Wales/Bristol basin. Namurian rocks now outcrop between Prestatyn and Oswestry.

Stratigraphy of the Namurian rocks of Northeast Wales

The Namurian succession is ascribed to the Millstone Grit Group and the underlying Craven Group. The greater thickness of the sequence is provided by the Cefn y Fedw Sandstone Formation which interleaves with the mudstones of the Bowland Shale Formation. It is underlain by the Pentre Chert Formation and overlain by the Gwespyr Sandstone Formation.

The upper part of the Gwespyr Sandstone is of lower Westphalian age. The Gwespyr Sandstone and the Holywell Shales were formerly ascribed to the Lower Coal Measures.