Coflein says;
Remains of a probable long cairn, dating from the Neolithic period (c. 4,400 BC – 2,900 BC), situated upon a wide terrace in the centre of a sheltered valley. The stone built cairn is wedge shaped and measures 17m from NW to SE by about 10.5m transversely (at the SE end) and about 5 m transversely (at the NW end) The cairn has been disturbed in the past, presumably the result of antiquarian investigation. The cairn measures about 0.8m in height, raising slightly towards the SE end.
Source: Cadw scheduling description of August 2006.