Pen y Fan (left in my photo) in the Brecon Beacons National Park (Parc Cenedlaethol Bannau Brycheiniog) is the highest peak in south Wales at 886 metres above sea-level. The twin peaks of Pen y Fan and Corn Du (on the right) is 873 m. The pair were formerly known as Cadair Arthur meaning Arthur’s Seat.
Pen y Fan comprises rocks of the Old Red Sandstone (which isn’t always red and isn’t always sandstone) laid down during the geological era known as the Devonian period (a 60-million year epoch spanning the end of the Silurian, 420 million years ago to the beginning of the Carboniferous, 360 Mya. The Old Red Sandstone describes a suite of sedimentary rocks deposited in a variety of environments in the North Atlantic region. It extends in the east across Britain, Ireland and Norway and in the west along the northeastern seaboard of North America. It also extends northwards into Greenland and Svalbard. The relatively large Anglo-Welsh basin of the ORS extends across much of South Wales and into Herefordshire, Worcestershire and Gloucestershire with outliers in Somerset and north Devon.
What lies beneath? Well, carboniferous limestone formed in shallow tropical seas during the Paleozoic era more than 300 Mya fromthe shells and skeletons of ancient sea creatures. More detail of the geology of the Brecon Beacons can be found here.