The Dyffryn Burial Chamber is a two-period Neolithic chambered tomb, sited on the west-facing coastal strip of Ardudwy and excavated in 1961-2 by T.G.E. Powell. The visible monument comprises an earlier, small portal-dolmen on the west side, blocked with twin portal stones and a closing slab and set within a small oval cairn. Although the tomb had been rifled, a pit containing fragments of five Neolithic pottery vessels was found sealed by the cairn in front of the tomb. This tomb has a small prehistoric cup-mark carved on one of its portal stones. The second, larger megalithic tomb was built to the east and was set within a large, rectangular cairn which incorporated the earlier tomb. This later tomb produced both Neolithic and Bronze Age pottery. Other finds included a fine, broken, stone pendant.

The site is the finest of a group of portal dolmens which survive on the west-facing slopes of Ardudwy and include examples at Cors-y-gedol (NPRN 93724), Gwern Einion (NPRN 302696), Bron-y-foel (NPRN 302754), and the south cairn at Carneddau Hengwm (NPRN 302786).

Main sources:
Powell, T.G.E. (1973), Excavation of the Megalithic Chambered Cairn at Dyffryn Ardudwy, Merioneth, Wales, Archaeologia Volume CIV (104), 1-49.
Lynch, F. (1976), Towards a chronology of megalithic tombs in Wales, in: Boon, G.C. & Lewis, J.M. (eds.) Welsh Antiquity, National Museum of Wales. 63-79

T. Driver, RCAHMW, 31 July 2008.