A dam on the eastern side of Llyn Cerrigllwydion Isaf controlling water into leat NPRN 261660. The drystone dam was placed across the point where the Nant Cerrigyrhelyg exits Llyn Cerrigllwydion Isaf. It consists of stone laid up to a height of 0.50m above the ground surface. The southern section is approximately 35m south southeast to north northwest, then at the northern end it runs to the north northeast for a further 10m. It is at least 3m wide, but was partially obscured by vegetation in October 2009. Water trickles through it to the stream, but it no longer supplies water to the leat. It was built to raise or maintain the water level of the lake so that sufficient water was available for leat NPRN 261660. It presumably dates to 1876 when the leat was constructed by the New Cwm Elan mine company, according to David Bick in his 1991 book The Old Metal Mines of mid-Wales, Part 6 A Miscellany, page 21. J.J. Hall, Trysor, 29 May 2009