A Neolithic chambered tomb set on a crag at the highest point of the Mynydd Cnwc headland overlooking Trecastle Bay. The tomb was excavated in 1952-3 and has since been consolidated and otherwise presented for public access within a modern glass-topped mound.

The tomb was covered by a great turf based mound some 27m in diameter. On the north side was an inturned forecourt from which a 7.0m long passage led into the irregular central chamber. Three smaller chambers branched off this central space producing a cruciform plan. These smaller chambers at least were roofed by great stone slabs, only one of which survives. The west chamber had a smaller annex within which the cremated remains of two men were found. The end stones of the east and west chamber and three of the stones in the inner end of the passage were decorated with abstract pecked patterns of zig-zags, lozenges and spirals, in a tradition of megalithic rock art common among the tombs of the Boyne Valley in Ireland, including the famous Newgrange, and chambered tombs in Britanny and Spain.

Traces of a long lasting fire were found in the central chamber. This had been quenched by a 'stew' or magical potion, followed by a quantity of limpet shells and pebbles. However, alternative explanations offered for this unusual deposit of largely inedible mammals and fish is that it is faeces from an otter who may have been resident in the derelict tomb.

Research in 2005 and 2006 by a team from Bristol University discovered further pecked rock art on Stone 7, in the left hand or eastern recess of the chamber, described in Nash et al 2005, Archaeology in Wales 45, 11-16.

Sources: RCAHM Anglesey Inventory (1937), 97-8
Powell and Daniel 'Barclodiad y Gawres: excavation of a megalithic chamber tomb' (1956)
Lynch 'Prehistoric Anglesey' (1970), 34-40
Yates and Longley CADW Guide to Anglesey 3rd edition (2001), 36-9

John Wiles, RCAHMW, 13 August 2007, with additional notes by T. Driver, RCAHMW, 20 September 2010.