A dyke partially crosses the ridge of Cefn Gelligaer, on open moorland to the immediate east of Tyla-glas farm and to the west side of a modern road. In its visble extent it consists of a stony bank measuring about 95m long, aligned east-north-east by west-south-west, which can be followed from SO11180132 to SO11090128. There is a 3m wide gap at SO11140130. The bank is spread up to 5m wide and averages 0.25m high on its south side. Along the north side of the bank is a broad flanking ditch the base of which is now 0.8m below the top of the bank and 0.3m below its north edge.
No trace of the bank can be seen across moorland beyond the road. On the west, the bank stops short of a field wall alongside which runs a rough track. On the other side of the wall the bank has been mostly ploughed out but its course is visible on air photos for a further 56m. The total length of this feature is thus likely to have been at least 180m. A second dyke lies about 1.25km to the south-south-east (NPRN 305947).
Additional sources:
RCAHMW, Glamorgan Inventory vol.1 pt 3 (1876), p.8, no.804.
RCAHMW AP94-CS 0779.
RCAHMW AP945106/48-50.

David Leighton, RCAHMW, 9 November 2015