Linear shingle reef of glacial deposits left by receding ice sheets at the end of the last ice age. It extends towards Ireland and is associated with the legend of Cantre'r Gwaelod.

The reef leaves the mainland at Shell Island/Mochras (see Mochras Spit NPRN 518505) and extends south-west into the shallow waters of Cardigan Bay. The reef has been recorded through shallow water during summer aerial reconnaissance by the Royal Commission on 12th July 2013. It is associated with Sarn Gynfelin reef (NPRN 518516 & 268148) to the south, which extends from Wallog inlet in Ceredigion.

The reef is dotted with numerous wrecks, the subject of marine geophysical survey by Wessex Archaeology for Cadw and in 2019 marine survey by the Geological Survey of Ireland for the EU-funded Ireland-Wales CHERISH Project.

Claire Parry, RCAHMW, 6 January 2011. Updated by Toby Driver, RCAHMW, Jan 2020.