Carn y Defaid is a large stone cairn located in heather moorland on Waun Carn-y-defaid on the summit of Mynydd y Garn-fawr at 503m OD. It consists of a pile of loose rubble, mixed grade stones and small boulders, with its centre hollowed out. Overall it measures 17.5m (east by west) by 14.5m and 1.2m high on the west. Its height on the east is nearer 2m and probably reflects the likely presence of outcrop upon which the cairn was heaped. The central hollow is 4m across and 1m deep. Despite its disturbance, both centrally and around its edges, the cairn displays no clear evidence for structural detail. The cairn is a major administrative boundary marker.
Nearby is a small cairn (see NPRN 306238).

David Leighton, RCAHMW, 25 November 2015