Pen y Gadair Fawr and Grwyne Fawr Reservoir

The walk starts from the Mynydd Du Forest car park at Blaen-y-cwm, at the head of the Grwyne Fawr valley road. A forest track runs up the valley for nearly 3 km to Grwyne Fawr Reservoir, a masonry dam built from 1912 to 1928 for the Abertillery and District Water Board, which no longer supplies drinking water. A path then climbs the open hillside above the reservoir, about 240 m in 1.5 km, to the ridge at around 785 m, and the way follows the ridge to the summit of Pen y Gadair Fawr, 800 m, marked by a large round cairn of small stones, about 15 m across, that the national record says could be a burial mound. The descent is by a path down the east side of the hill to the edge of the forest, and a lane and a short path lead back to the car park.

It is 9.1 km with 448 m of ascent; allow about 3½ hours. The grade is Moderate.

The top is open country with no public right of way to it: it is access land, and the paths are not waymarked. The ridge is exposed, and the ground is peat that is boggy after rain, so the path is hard to follow in mist and cloud: keep this for a clear day, with a map and compass, warm clothes and boots.

Welsh Water has said that a road closure on Cwm Road is due for pipe works in the Grwyne Fawr area from early September to early November 2026, Monday to Friday, 8am to 4pm: it is not clear that this is the valley road to the car park, so check before you drive up. The Mynydd Du Forest car park is run by Natural Resources Wales and is shown as free in OpenStreetMap.

Route drawn by Global Adventurer along OpenStreetMap paths (© OpenStreetMap contributors, ODbL), with figures worked out from it. Check the forecast and the ground on the day: paths change.