Trail Running > Routes > Brecon Beacons Routes > Craig Cerrig Gleisiad & Fan Frynych
A beautiful 12 km trail run across the lush glacial clifftops and panoramic moorland summits of this hidden nature reserve.
Why run this trail?
This fell run circles the northern edge of Fforest Fawr, one of the quieter and more atmospheric corners of the Brecon Beacons. The sister peaks of Craig Cerrig Gleisiad (629 m) and Fan Frynych (629 m) form a natural amphitheatre of glacial cliffs and rugged moorland, much loved by local runners and hikers. Designated a National Nature Reserve, the area is rich in rare arctic-alpine plants and provides an important refuge for nesting birds such as peregrine falcon and ring ouzel. The fact that all this wilderness sits just a few hundred metres from the busy A470 adds a touch of magic.
Compact but constantly surprising, the route delivers spectacular views into hidden valleys and across the remote uplands of Fan Gyhirych and the Black Mountain beyond. The return leg follows a short but evocative section of the Roman road Sarn Helen, before a final ascent of Fan Frynych rewards the effort with stunning, high-definition views of Pen y Fan and Corn Du.
The opening climb up Craig Cerrig Gleisiad makes for a lung-busting entrée – the first kilometre averages a punishing 26% gradient. The route can be run in reverse to save this section for the descent, but ground conditions are important — it would be hazardous in wet or icy weather.
A470 to Craig Cerrig Gleisiad
The run begins from a large lay-by on the western side of the A470. Pass through the metal gate and follow the path into the trees, skirting the National Nature Reserve information board. The trail climbs gently at first and, after around 300 metres, reaches a stone wall and gate marking the entrance to the steep-sided bowl of the reserve. Cross the stream to your left and follow the obvious path as it begins to weave up the lower slopes of Craig Cerrig Gleisiad.
The gradient ramps up quickly and the trail becomes relentlessly steep. I was glad I’d brought poles — several loose, muddy sections would otherwise have required an undignified all-fours approach! With my focus fixed firmly on the ground, I gradually became aware of the dense carpet of colourful flora cloaking the hillside — strikingly different from anywhere else I’ve run in the Brecon Beacons.
It’s worth stopping occasionally on this climb, not only to catch your breath, but to look back and appreciate just how rapidly the A470 disappears from view, shrinking to an inconsequential thread in the valley below. There’s some towering close-up views of Pen y Fan and Corn Du to marvel at too.
Eventually the trail eases and begins to circle the rim of the crag, opening out to reveal stunning views along the entire escarpment. On a bright winter morning, I found myself stopping repeatedly to photograph thick clouds tumbling over the cliffs through the sunlight.
Craig Cerrig Gleisiad to Craig Cwm-du
As the rocky edge of the ridge begins to dissipate, bear left over a stile onto rough grazing land and head west, keeping the fence and broken wall to your right. The path remains obvious but is very boggy in places — after weeks of heavy rain, numerous watercourses criss-crossed the trail during my visit.
The route descends gradually, revealing impressive views of the river threading its way through the cwm below, with the rounded peaks of Fan Nedd and Fan Gyhirych rising ahead. The open, grassy moorland here feels markedly different from the lush slopes of Craig Cerrig Gleisiad — quieter, bleaker, and wonderfully remote. Buzzards, red kites and crows circled overhead as the wide green sweep of the Senni Valley basked in the winter sun.
Craig Cwm-du to Sarn Helen
After about 2 km, the trail veers sharply right and plunges steeply into the valley below. I zig-zagged across the wet grass in a valiant attempt to control gravity, but still managed to complete part of the descent seated firmly on my backside. With only a couple of unimpressed ponies to testify, I dusted myself off and emerged at the foot of the slope onto the ancient Roman road of Sarn Helen.
I was genuinely pleased to discover that the ‘road’ had effectively become a shallow, rocky stream bed — perfect for washing some of the accumulated crud from my shoes and restoring a bit of traction before the next climb.
Sarn Helen to Fan Frynych
A pleasant – if bumpy – transitional kilometre on Sarn Helen leads towards the climb of Fan Frynych. The rocky track winds over the Nant Cwm-du and offers great views back to the crags above before curving around the flank of the mountain. Look out for a grassy path branching off to the right through the bracken — this leaves Sarn Helen around 600 metres after the trail pulls away from the river.
The broad grassy track climbs steadily to a small stand of pine trees at a junction of paths. Turn right to continue uphill on what appears to be the bed of an old industrial track (there are faint remains of quarry workings dotted across the hillside). The gradient increases and the trail veers left onto open moorland as the ridge of Craig Cwm-du looms into view once more.
From here, it’s a steady ascent to the stone cairn on Fan Frynych. The views towards Pen y Fan and Corn Du are exceptional and beautifully defined. It’s a further 600 metre hop to the summit trig point, which offers full 360-degree panoramas and yet another perfect excuse to pause.
Fan Frynych to A470
Leaving the summit, the trail undulates through heather before joining a broad, stony track that leads towards the lesser peak of Twyn Dylluan-ddu. At the junction of paths, take the first wooden gate on the right and follow the trail south to traverse the front of the mountain, crossing several streams and minor waterfalls along the way.
After roughly 1.5 km, the path delivers you back to the stone wall and wooden gate marking the Craig Cerrig Gleisiad trailhead. From here, simply retrace your steps through the trees to the lay-by on the A470.
With cobwebs — and mind — thoroughly blown away, I reflected on what is a compact yet incredibly varied trail run through one of the Brecon Beacons’ lesser-visited corners. Ups and downs, a bit of everything underfoot and mud and glory in equal measure.
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Disclaimer
All routes and GPX files are provided as a guide only. Conditions, rights of way, and landmarks may change. Always carry an up‑to‑date map and use your own judgement. See the Trail Run Disclaimer page for details.