Ogof Ffynnon Ddu National Nature Reserve

The grassy uplands opposite Craig-y-Nos Country Park and the Dan-yr-Ogof National Showcaves Centre hide a labyrinthine network of caverns and cascades, stalgamites and stalactites, passageways and potholes.

Ogof Ffynnon Ddu means Cave of the Black Spring, a reference to the vast cave system discovered beneath the moors in 1946. It’s a 300m deep underground world with an overall length of over 30 miles and the difference in level between its highest and lowest points is 308m, a record for the UK.

Above ground

The reserve, situated at over 350m above sea level, is one of Wales’ most exhilarating National Nature Reserves, with breathtaking views across South Wales and the Brecon Beacons. The terrain is a mixture of rocky outcrops and tufty moorland, with few obvious paths. But don’t be put off, for Ogof Ffynnon Ddu is immensely rewarding for those who take time to explore it fully and appreciate its compelling character.

The great divide

The reserve overlooks a deep fracture in the earth’s crust, known as the Swansea Valley Disturbance, which marks the dividing line between industrial and rural South Wales. The northern horizon is filled with smooth grasslands covering the old red sandstone rocks that dominate the Brecon Beacons National Park.

To the south, the geology is very different. First comes a broad band of limestone, then an even broader band of millstone grit – the two rocks that characterise the reserve. South again are the coal measures that have been exploited since the Industrial Revolution.

Dissolved by water, polished by ice

There is much to see on the surface, too. Gnarled, fissured beds of limestone occupy the lower slopes while the higher ridge to the south is capped by millstone grit.

The limestone is seen to best effect in a section of limestone pavement along the northern end of the reserve. Here, the rock was fractured by the stresses caused by the Swansea Valley Disturbance, and these cracks were opened by water dissolving the limestone to form classic features known as grykes.

Yet more evidence of the erosive effect of water can be seen across the landscape, which is pockmarked by swallow or sink holes, the result of the collapsed roofs of caverns and tunnels.

A plateau of millstone grit dominates much of the reserve, providing evidence of the last Ice Age, 10,000 years ago. On the ridge, the very hard sandstones have been smoothed and polished by the movement of the ice that once covered the surfaces. From a distance they gleam in the sunlight like a giant mirror, but closer inspection reveals scratch marks that indicate the ice sheet’s direction of travel. Other glacial evidence includes erratics, large rocks and boulders left stranded by the glacier as the ice retreated.


Herb-rich grasslands grow on the limestone, while heather and bilberry moorland covers the acid peat lying on the millstone grit. The limestone pavement is also rich in flora, especially in the sheltered, inaccessible grykes (this area is further protected form the effects of grazing sheep by being fenced off).

The flowers are at their best in early summer. The reserve contains rarities, including mountain everlasting, autumn gentian, mossy saxifrage and great burnet. Hairy greenweed, a low, yellow-flowering shrub, is one particularly rare species growing here.

Birds on the reserve include peregrine falcons, nightjars, ravens, ring ouzels, wheatears, red grouse and red kites. Below ground, the waters contain crustaceans that can only survive underground, and the cave is a winter habitat for moths and bats.

Visiting Ogof Ffynnon Ddu

Ogof Ffynnon Ddu is open to the public. Access is free, however access to the caves is confined to experienced cavers only. The terrain can be very rough underfoot in places.

For a gentle, easy walk you can follow the old railway line along the side of the valley. Although this just skirts the edge of the reserve, the views are splendid.

More experienced and enthusiastic walkers can access the heart of the reserve by following paths upwards from the car park. One particularly well-defined and convenient pathway is the old tramway, a steep incline that climbs to the reserve’s eastern boundary.

How to get there

Turn off the A4067 between Craig-y-Nos Country Park and Pen-y-Cae, following the minor road upwards for a mile or so until you come to the settlement of Penwyllt, where you bear right and park beside the large, abandoned quarry.

Nearest village


OS grid reference