On the southeast side of Llynn y Foel are the remains of a crescent-shaped dam. The dam is 20m long and comprises two parrallel 1m-high drystone walls, with an overall width of 8m. The lake was probably used as a reservoir in connection with the Prince Llewelyn Slate Quarry, Dolwyddelan. A sluice is marked in this location of the 1894 Plan of the Gwydyr Estate held in the National Library of Wales. M.Railton, NPA, 9th December 2010.