A large stone dam retaining the Grwyne Fawr reservoir, completed in 1928. The dam was built of rough stone blocks up to 5 tons in weight lowered into place by crane onto a bed of concrete. The gaps in between were packed with smaller stones and the whole layer capped with concrete, and the whole process repeated again and again. The outer face of the dam was faced with carefully dressed stone. Source: Rev DA Tipper in his 1975 book "Stones and Steam in the Black Mountains
The dam was still in use and intact when recorded in September 2007.
J.J.Hall, Trysor, 9 February 2008