A drystone dam placed across the point where the Ffrwd Wen stream exits Llyn Cerrigllwydion Isaf. It consists of stone piled up to a height of 0.30m above the ground surface. It measures at least 15m east northeast to west southwest by at least 1.5m wide, but was partially obscured by vegetation. Water trickles through it to the stream. It was presumably built to raise, or maintain, the water level of the lake so that sufficient water was available for leat NPRN 260661 running from dam NPRN 503507. J.J. Hall, Trysor, 23 January 2010