Height (of the submerged dam) – 20m
Length – 180m
The submerged dam can only be seen at low reservoir levels. Garreg Ddu has two purposes: to hold back water to enter the Foel Tower, and to provide access to the Claerwen valley. The Foel Tower is where drinking water is extracted to start its three day journey to Birmingham. Garreg Ddu means Black Stone.
On the bank opposite Garreg Ddu is where Cwm Elan House stood. This belonged to Mr Thomas Grove, the uncle of Percy Bysshe Shelley. Cwm Elan House, Nantgwyllt House and Dol y Mynach House were all demolished, leaving only the remains of a few garden walls.