A ruined dam dating to the late nineteenth century when Llyn y Mynydd reservoir was created, presumably to service the Penrhyn Slate Quarries. The eastern overflow sluice measures 15m long by 5m wide. The dam bank itself is constructed of earth and stone with a stone-face internal bank and measures up to 7m wide by 2.5m high with a break of slope of approximately 120 degrees. The main central sluice is brick faced and shows in section an central internal brick wall along the extent of the dam which is breached in the center of the main sluice. It is shown on both the 1st Edition and current OS mapping. P.J. Schofield, OA North, 22nd January 2008