The Reserve

The great divide

The NNR 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 determine the character of the reserve. South again are the coal measures that have been exploited since the Industrial Revolution.

Dissolved by water, polished by ice

Apart from the internationally important cave system, 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 "pavement" along the northern edge 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 tunnel.

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.


The differences in the rocks produce varying vegetation. The limestone is associated with herb-rich grasslands, while heather and bilberry moorland covers the acid peat lying on the millstone grit. The grassland contains rarities, including mountain everlasting, autumn gentian, mossy saxifrage and great burnet. The limestone pavement has a particularly rich flora, especially in the sheltered inaccessible grykes. This area is further protected from the effects of grazing sheep by being fenced off. The flowers are at their best in early summer. Hairy greenweed, a low, yellow-flowering shrub, is one particularly rare species growing here but plants like lily-of-the-valley, lesser meadow-rue and small scabious are also common here.


Birds on the reserve include peregrine, nightjar, raven, ring ouzel, wheatear, red grouse and red kite (the latter, once almost extinct in Wales has made a dramatic comeback in these parts).

Visiting the Reserve

Visiting the Reserve

Access to the caves is confined to experienced cavers only. Information for cavers can be found by following the "Cave Access" link above. There is underground access for non-cavers just across the valley at the National Showcaves Centre, Dan-Yr-Ogof (, one of Wales' most popular tourist attractions and part of another NNR. For anyone wanting to get into caving, more information can be found at

The surface of Ogof Ffynnon Ddu NNR is suitable for most visitors. 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. There's open access across the entire reserve, but please bear in mind that it can be very rough underfoot in parts. One particularly well defined and convenient pathway is the old tramway, a steep incline that climbs to the reserve's eastern boundary. There is currently no wheelchair access to the reserve.


·         Take ALL your litter home

·         avoid all risks of fire

·         leave flowers for others to enjoy

·         do not disturb wildlife or research equipment

·         keep dogs under control



How to get here

 How to get to the reserve

turn off the A4067 between Craig-y-nos Country Park and the village of Pen-y-cae, following the minor road upwards for a mile or so until you reach the settlement of Penwyllt, where you bear right and park beside the large, abandoned quarry. If you use Sat Nav, the nearest postcode is SA9 1GQ.

Bus routes 63, X63 and X64 operated by New Adventure Travel run between Brecon and Swansea along the A4067. Various minor roads and footpaths lead from the main road into the reserve.

A public footpath which is part of the Beacons Way, crosses the reserve. There are other permissive paths and most of the reserve is access land. The reserve is covered by Ordnance Survey Maps Landranger 160 (Brecon Beacons) and Explorer OL12 (Brecon Beacons National Park - Western Area)