An Act of Parliament in 1910 allowed for the creation of the Abertillery and District Water Board and the construction of the Grwyne Fawr reservoir. The reservoir was needed to supply fresh water to the growing settlements in Monmouthshire. The construction of the reservoir began in 1912 and was finally completed in 1928.

The stone-built dam is 52.7 metres (173 ft) high and the mean reservoir height is 525.8 metres (1725 ft) above sea level with a 35 kilometre (22 mile) switchback to Abertilleray and Cross Keys. The contractors 90 centimetre (3 ft) gauge railway from Llanfihangel Crucorny operated the highest adhesion traction railway in the UK, rising to 548.6 metres (1,800 ft). The track bed has been re-surfaced to form a road. The reservoir is still in use.

Claire Parry, RCAHMW, 21 June 2011.

Sources:
Tipper Rev DA, 1975, Stone and Steam in the Black Mountains
J.J.Hall, Trysor, 4 February 2008
A Guide to the Industrial Archaeology of South East Wales, AIA, 2003