Blaen Grwyne Fawr

Beautiful views of the Brecon Beacons.

The Grwyne Fawr is a river in the Brecon Beacons National Park in south Wales. A section of it forms the administrative border between Powys and Monmouthshire and also of the historic counties of Brecon and Monmouth. The river and its major tributary the Grwyne Fechan flow into the River Usk at Glangrwyney.

The name probably originates from gweryn or gwerynau and mawr/fawr meaning 'large river at the wet place'.


The southern part of the Grwyne Fechan valley in the Black Mountains
The Gwryne Fawr rises at Blaen Grwyne Fawr on the southern slopes of Rhos Dirion in the Black Mountains and follows a southeastward course for several miles, its flow interrupted by the presence of Grwyne Fawr Reservoir, the only waterbody within this range of hills. Some way below the reservoir, Mynydd Du Forest clothes the sides of the valley. Though a public road penetrates the valley as far as the north end of the forest, the valley is very sparsely populated. It is only approaching the hamlet of Partrishow that the valley takes on a farmed appearance. The river now heads west along a valley whose alignment is guided by the presence of a geological line of weakness known as the Neath Disturbance.


Rhos Dirion in the Black Mountains
To the west, the Grwyne Fechan takes a parallel course to that of it larger neighbour. It rises on the southern slopes of Waun Fach and gathers a number of tributary streams from the broad ridge which separates it from the Grwyne Fawr. Though there are numerous conifer plantations, the valley has a more open aspect than that of the Grwyne Fawr.

The two rivers join near the village of Llanbedr and head south as the Grwyne Fawr through the hamlet of Llangenny to their confluence with the River Usk at Glangrwyney.

By the end of the 19th-century there were serious water shortages in the western parts of the historic county of Monmouthshire. The area had seen a massive population increase as a result of the steel and coal industries but the supply of water to the area was complicated due to its high altitude. A site for a new reservoir was chosen at the head of the Grwyne Fawr valley because at 1725 ft above sea level it was high enough to allow water to be gravity fed to Abertillery and the surrounding areas.[3]

Building works started in 1912, but almost immediately hit problems.[4] The road constructed up the valley could not take the weight of the heavy traction engines used for haulage. It was decided that a new railway would be required, however negotiations with local land owners failed and the line remained incomplete. With the outbreak of the first world war proceedings were put on hold.

Work recommenced in January 1919, the railway was completed and connected to the main line at Llanvihangel Crucorney. A village of construction workers and their families grew in the valley at Blaen-y-cwm. A community of some 300 people lived there with a hostel, canteen, day school, police station and hospital.[5] The workmen quarried 200,000 tons of sandstone for the construction of the dam. Large boulders weighing up to 5 tons each were set in concrete to form the dam, which is 130 feet thick at its base, the walls were then faced with dressed stone. A 16-inch diameter steel pipeline was tunnelled through Coity Mountain carrying water to a holding reservoir in Cwmtillery. The dam was finally completed in February 1928, after which the railway and structures that served its construction were quickly dismantled including the workers village.[4] The total cost of construction was reported to be £1 million.