The area around Pwll Byfre is one of the most spectacular examples in the whole of the Brecon Beacons National Park, indeed the whole of the UK for that matter, of 'foundered strata'. The Millstone Grit which here overlies the Carboniferous Limestone, has subsided as the limestone beneath it has dissolved away in circulating groundwater. The overall thickness of rock which has been removed in this way may amount to several hundred feet locally!
The shattered gritstone has since been quarried as 'silica sand' for use at a local refactory brick-making works
SN8515 - an industry now confined to history.