Ogof Ffynnon Ddu is one of the most extensive cave networks in Europe. Above ground are limestone pavements and scarps.
OS grid ref SN 856155 (car park) to SN 868160
In and around Penwyllt are the remains of a former firebrick works. Former quarry-workings and tramroads are scattered across the landscape. Just a few buildings survive from the time when there was a population of 350 here at the ‘head of the wild’. A terrace which once housed workers and their families is now the headquarters of the South Wales Caving Club. Beyond it and at the southern end of a long tramway from Pwll Byfre silica sand pit are the remains of the brickwork kilns.
Above the former village are outcrops of Carboniferous Limestone and, above these, a line of Twrch Sandstone crags form the eastern skyline. Calcareous soils over the limestone give rise to a varied flora especially where plants are protected from the mouths of sheep. Areas of limestone pavement offer plants protection against both sheep and exposure to the worst of the elements.
There are areas of gritstone pavement (Twrch Sandstone) on the higher ground, their surfaces worn smooth by the passage of ice some 20,000 years ago. Striations gouged into the sometimes polished surface of this very hard rock have survived to tell us in which direction the ice moved across these hills.
- Carboniferous: Limestone, Twrch Sandstone (Millstone Grit)
- OS Landranger 160, Explorer map OL12
- Geological — BGS 1:50,000 sheet: 231 ‘Merthyr Tydfil’
- Childhood memories of former residents of Penwyllt available to download.
- Some interpretive panels around reserve
- Free car parking at Penwyllt at SN 856155
- Entry: free
- Open country; some rough ground; mountain environment — take care!
- By car — on minor road off A4067
- By train — nearest stations at Neath and Merthyr Tydfil – see Traveline Cymru
- By bus — see Traveline Cymru; 63 service (Stagecoach) stops at Glyntawe & Pen-y-cae