Geology of South Wales

This varied and accessible region has provided a written record of geological interest that dates to the 12th century, when Giraldus Cambrensis noted pyritous shales near Newport.

The British geologists Adam Sedgwick and Roderick Murchison did fundamentally important work in South Wales on Old Red Sandstone and the underlying rocks.

A notable feature of the Ordovician system (488-444 million years old) is a major subsidence area, or basin, which later filled with substantial thicknesses of mostly sandy and muddy sediment.

The line of the shallower continental shelf area, to the south-east, ran approximately from the Long Mynd (in Shropshire) to Haverfordwest in Pembrokeshire.

During earth movements caused by the approach of a continent from the south, sea shallowing led to thick alluvial and coastal plain sediments (see below) containing economic developments of coal.

Only in late Triassic (251-200 my ago) times did deposition return, with coarse-grained screes forming around a series of low but rugged hills.

The coal measures were laid down on a low-lying waterlogged plain with peat mires immediately south of an ancient and persistent geological feature known as the Wales-London-Brabant High.