Geology of South Wales

South Wales is an area with many features of outstanding interest to geologists, who have for long used the area for University field trips.

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. Some of the first published representations of fossils were of fossil plants from coal measures near Neath (Gibson late 17th century). The British geologists Adam Sedgwick and Roderick Murchison did fundamentally important work in South Wales on Old Red Sandstone and the underlying rocks. The first volume of memoirs (1846) published by the Geological Survey contained a conspectus of the geology of South Wales that set a template for all future work.

Significant discoveries continue to be made. For instance, in Carmarthenshire, in the late 70s, John Cope detected a small area of Precambrian and Cambrian rocks. Other parts of South Wales, however, have not seen a Geological Survey officer since the mid-19th century. The geology of south Wales was central to the massive contribution Wales made to the Industrial Revolution.

Several successive periods are represented, including all of the Palaeozoic periods. Pembrokeshire has outcrops of both Precambrian and Cambrian (542-488 million years ago) 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. Some volcanic action also occurred. This thick sedimentary deposition continued into the Silurian period (444-416 million years old). Silurian volcanicity led to production of lavas at localised sites such as Skomer Island.

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