Wales (Cymru) - South Wales
The South Wales area extends eastwards from Pembrokeshire, south of the Welsh Borderland Fault, then runs along the northern flank of the South Wales Coalfield to the Welsh Border in the east. Much of the area comprises an upland plateau deeply incised by the rivers of the South Wales Valleys. The contrasting low-lying coastal plain of the Gwent Levels, Newport and Vale of Glamorgan are also included within this area. The main towns and cities in this area include Cardiff, Swansea, Neath, Bridgend, Newport and Merthyr Tydfil.
Sedimentary bedrock
The youngest Sedimentary Bedrock comprises mudstones and limestones (Plate P662435) deposited between 200 to 150 million years ago that underlie much of the Vale of Glamorgan. Along the northern margin of the Severn Estuary, a complex landscape is developed of older sedimentary bedrock layers, including red pebbly sandstones, and mudstone deposited in rivers and deserts between 300 and 200 million years ago. These layers are exposed between Chepstow and Barry and also around Porthcawl.
The main part of the South Wales Coalfield forms a large basin-like down-fold of sedimentary rock layers with the youngest and highest layers coming to the surface in its central part and the progressively older layers exposed around the rim of the basin. In general the Coal Measures succession comprises an upper division dominated by thick sandstone beds with subordinate mudstones and only thin coal seams and lower layers dominated by mudstone with scattered sandstones and abundant coal seams. These pass down into a sequence of mudstones and sandstones (Millstone Grit). Layers of Carboniferous Limestone form a distinctive rim around the Coalfield and are also present beneath it, including parts of south Pembrokeshire, much of the Gower Peninsula, and parts of Monmouthshire.
This area is cut by numerous faults of which those aligned north-east to south-west along the lines of the Swansea and Neath valleys are the most prominent. The rock layers exposed in south Pembrokeshire and the Gower Peninsula have been more strongly affected by earth movements and are contorted into a series of fold structures that are strongly disrupted by west-east trending geological faults. As well as widespread deep and shallow coal mining, historic iron ore mining and lime processing took place in the northern parts of the coalfield. Being soluble, the Carboniferous Limestone has been dissolved by rainwater percolating through it to form extensive cave systems, such as those in the Ystradfellte area. The Carboniferous Limestone is a major aquifer in this area, with most water flow along caves and fractures and it provides a significant public supply. Millstone Grit and Coal Measures sandstones also represent minor aquifers. Although abstraction for private supply is not uncommon, this is limited in the Coal Measures, partly due to contact with the abandoned mine workings. The oldest rocks exposed at surface in the area comprise the Old Red Sandstone formed of red mudstone and sandstone exposed in a narrow belt in south Pembrokeshire but occupying a more extensive tract eastward, and continuous with that of the south-east Wales area.