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The Afon Pyrddin is the major tributary of the Nedd Fechan. Its name may be an oblique reference to St David, the pure man, or y pyr dyn in Welsh. The two significant falls on this short river are Sgwd Gwladus, formed where the river drops over the Twelve Foot sandstone and Sgwd Einion Gam where the river crosses a geological fault which brings the Farewell Rock into play. In the latter's case, just as at the neighbouring Henrhyd Falls and at Sgwd yr Eira, gullies in the sides of the gorge downstream of the falls betray the whereabouts of the fault which originally gave rise to the feature.

Though the quality of the landscape is similar, it is the river which forms the boundary of both Brecon Beacons National Park and Fforest Fawr Geopark. Most walkers approaching from Pontneddfechan are unaware that until they cross the footbridge at Pwll Du ar Byrddin, they have not set foot in either. This situation is the result of administrative convenience when the national park boundary was decided in the 1950's.