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WALK 4

Cwm Sere and Cefn Cwm Llwch

Start	Pont y Caniedydd (SO 039 244)
Distance	9.5km (6 miles)
Total Ascent	560m (1841ft)
Map	OL12 Western Area

This is a fulfilling mountain route that explores one of the most spectacular and wildest valleys in the Beacons and includes sections which can be demanding in snow and ice conditions. The lower reaches of the valley are well wooded and lead into an amphitheatre created by the steep northern slopes of Cribyn and Pen y Fan. A mountain stream with small waterfalls forms a foreground to Cribyn and Pen y Fan – the two highest peaks in the Beacons. The upper parts of the route can be quite serious in bad or winter conditions but a number of less demanding variations are possible. The main features of interest are the geomorphology and glaciology, the panoramic views and some archaeological sites.

From the car parking area at Pont y Caniedydd, cross the bridge and head south up the tarmac road, passing a farm (Bailea) on the way. On the right of the road in Cwm Sere is a woodland nature reserve managed by the Brecknock Wildlife Trust (see ‘Cwm Sere woodland,’ Walk 7). At the head of this valley is the north-east face of Pen y Fan and to its left is Bryn Teg ridge.

Continue on past the turning on the left to Bailea Farm and follow the road up the hill to where it swings sharp left and through a gate. Ignore this turning and follow the stony track for 250m straight ahead to a gate in the hill fence on the far side of which is a National Trust sign for Cwm Cynwyn. The ridge on the right is the descent route leading towards the end of the walk.

The stony track you have just walked along is part of the Roman Road that leads to Bwlch ar y Fan (see Walk 11). The Scots pines (Pinus Sylvestris) immediately on the left as you cross the hill fence may have been planted as route indicators to drovers and, if this is the case, would have signalled that shelter and grazing could be obtained from the farm nearby. The Roman Road was probably used long before the Romans or even the Celts came to the Brecon Beacons.

Bear right at the gate and follow the stone wall around the front of Bryn Teg and into the valley of Cwm Sere. After about 400m the stone wall has collapsed (SN033 234) Just after this point drop gradually across the slope until the stream course is reached. Follow the stream course, either by scrambling along the bank or by staying on more even ground above, following a convenient sheep track.

Nant Sere

Once in the basin below Cribyn, strike across towards the foot of the north-east face of Pen y Fan and from here climb the headwall by the track which starts from bottom right and continues to top left. You will cross piles of stones brought down the gullies by the winter frosts. The headwall track rises at an easy angle, presenting no problems, and arrives at the unnamed col between Cribyn and Pen y Fan. There is a good view of Cribyn during the ascent.

From the col, climb steadily (W) following the line of Craig Cwm Sere to the summit of Pen y Fan. In early spring the vibrant colours of rare arctic-alpines may be seen on the most inaccessible crags.

FLORA OF NORTH-EAST FACE OF PEN Y FAN

This steep, impregnable face protects one of Britain’s true botanical treasures from grazing sheep. The combination of high altitude and a shaded northern aspect creates living conditions more akin to polar latitudes than to temperate southern Britain. Extensive ledges high up on the face are crammed full of interesting and unusual species which bring the otherwise bleak and foreboding crags alive with vibrant colour in spring. Interesting plants include roseroot (Sedum rosea), rock stonecrop (Sedum forsteranum), mossy saxifrage (Saxifraga hypnoides), purple saxifrage (Saxifraga oppositifolia), vernal sandwort (Minuartia verna), sea campion (Silena maritima), Wilson’s filmy fern (Hymenophyllum wilsonii), globe flower (Trollius europaeus), serrated wintergreen, green spleenwort (Asplenium viride), lesser meadow rue (Thalictrum minus), brittle bladder-fern (Cystopteris fragilis) and northern bedstraw (Galium boreale). These ledges also support an unusual collection of bryophytes and several upland invertebrate species, including a rare arctic aphid. These plants and animals are highly specialised to be able to survive in these extreme conditions.

Looking back down Cwm Sere from the approach to Pen y Fan, there is an ideal view of its geomorphology. Cwm Sere was carved by ice into a U-shape but this has been altered slightly since the last Ice Age. On the left there is a distinct step in the valley side, a post-glacial feature known as an ‘antiplanation terrace’, while Nant Sere has been eroding away a notch in its base (see ‘Glacial origins of U-shaped valleys,’ Introduction).

An antiplanation terrace is formed when a snowbed develops on a sheltered step in a valley side. This snowbed erodes into the hillside by freeze-thaw action (see ‘Glacial origins of Llyn Cwm Llwch,’ Walk 3), depositing material further downslope. In the case of the western side of Cwm Sere, a minor platform was formed due to differences in resistance to erosion of underlying rock types and, in fact, this antiplanation terrace may be related to the change from Brownstones to the underlying Senni Beds (see ‘Geology of the Brecon Beacons,’ Introduction).

Looking up the slope the flat-capped summit of Corn Du can be seen, and to the left is Bwlch Duwynt, which means ‘Windy Gap’. The final ascent to the summit of Pen y Fan is up a very stony area and The National Trust has built a zigzag path up this face to the top. The crags here are made of Plateau Beds that form a distinctive cap to the summits of Pen y Fan and Corn Du (see ‘Geology of the Brecon Beacons,’ Introduction).

Walk across the flat surface of Pen y Fan, which is also badly eroded, to the trig point. This can be found in bad visibility by carefully following the north-east crag line. The trig point is at the end of this to the left. The north-east face is very steep and care must be taken not to stray too near the edge, especially in strong south-westerly winds or when corniced in winter. The summit is the site of an Iron Age cairn and there are good views and photographic opportunities from here. For details of the geomorphology of Pen y Fan, see ‘Geology and geomorphology of Pen y Fan,’ Walk 5.

Cribyn and Pen y Fan from above Cwm Llwch

The slope of the summit surface is the key to the dip of the resistant Plateau Beds which cap the summit (see ‘Geology of the Brecon Beacons,’ Introduction). The summit is, in fact, the dip slope of this rock formation which lies unconformably on the Brownstones, but the general trend in dip of all the rock strata in this area is to the south. A slight component of the dip controls the drainage in the valley sides, favouring the eastern facing slopes. A close look at the map reveals this to be true for the majority of the gullies in Cwm Sere and Cwm Cynwyn.

Leave the summit of Pen y Fan by carefully scrambling (NNE) down the exposed Plateau Beds. The rocks can be slippery but the route drops quickly to the fine ridge of Cefn Cwm Llwch. As you scramble down from the summit, look carefully at the upper surfaces of the near horizontal Plateau Beds for ripple marks (see Walk 1).

Below to the left is the valley of Cwm Llwch and the lake of Llyn Cwm Llwch (see ‘Glacial origins of Llyn Cwm Llwch,’ Walk 3). Looking down to the right you will see a large gully which holds a frothy white cataract after heavy rain.

Cwm Sere from the summit of Pen y Fan, showing the antiplanation terrace on the eastern flank of Cefn Cwm Llwch

Once on the flat section of the ridge, look across to the east for a magnificent view over the ridges of the Beacons and the triangular profile of Cribyn. Looking through the gap between Cribyn and Pen y Fan, you can see the bluff of the ridge which leads down on the right-hand side of the Neuadd Valley. Turning to the entrance of Cwm Sere, there is a pleasant woodland area which is managed by the Brecknock Wildlife Trust (see Walk 7).

Follow the path along the flat ridge, leaving it when it drops gradually to the valley of Cwm Gwdi. Keep to the eastern edge of the ridge, following a path through some boggy areas with mainly heather, past the disused quarries and onwards to Allt Ddu.

From the spur of Cefn Cwm Llwch, which is very well populated with skylarks in spring and summer, there is a good view looking back to Pen y Fan with Cribyn on the left and Corn Du sticking through the gap in the ridge. As you descend this route further you can see the town of Brecon over to the left beyond the end of the ridge. There is an area of heather and bilberry with the odd pool in the peat. At the end of this ridge is a number of hummocks and hollows, part of an old quarry, and you can look down past a rowan tree growing out of the crags to Cwmcynwyn Farm. There is a rock outcrop here with bedding showing clearly.

From the pools on the summit of Allt Ddu drop down past the stones of the quarry towards the Plas-y-gaer settlement. Just on the edge of this hill there is a distinctive furrow in the hillside which is an old quarry path. Continue down this path (marked with a dotted line on the map) towards the settlement, descending the front of the ridge. The descent of this ridge provides a good view of the settlement with the earth bank being now planted with a line of large trees.

Plas-y-gaer is an Iron Age settlement some 2000 years old, gaer meaning fortress and plas meaning place. The site is unexcavated but was probably built to defend the surrounding fertile land. There are surviving earthworks forming an oval shape and the height of the main rampart varies between 2 and 3.2m. As there is no sign of an entrance, access was probably gained from the north. No ancient features are visible in the interior.

At the bottom of the slope turn left in front of the settlement. As you come round the corner of the settlement follow the path diagonally down to the left, arriving at a stone wall, where there is a finger post and gate. A track leads to the farm (Plas-y-gaer). At the farm turn left through a gate and right into a field at the blue waymark post.

From here head across the field to a line of pylons reached through a gap in the trees. Turn right at a line of trees and walk up to a gate and stile in the right corner of the field. After crossing the stile, descend a shallow valley to another stile which you will see in the field about 60m in front of you. Follow the wire fence on the right-hand side downslope to the derelict buildings. Walk to the left of the farm buildings to a stile in a fence and onto a track. At the end of the farm track is a gate beyond which is a tarmac road. Turn right on the road and drop down the hill to the start at Pont Caniedydd.