1. SULLY ISLAND. The E. end of the island, rising to about 45 m above O.D. is defended by a succession of ramparts. There has been much erosion, particularly on the S., and the N. half is thickly overgrown; the present area of the main enclosure is about 0.4 ha, but it may originally have been twice as much.

The outermost rampart, on the W., now appears as a scarp about 1.5 m high with a slight external ditch; it runs N. to S. across the island which is here about 45 m wide. The cliff which forms the N. boundary of the enclosure ends just outside this defence. Fifteen metres further E. is the second bank, about 2.5 m high externally and nearly 1 m internally. It was accompanied by a ditch, now completely silted up but visible in the cliff face as 3-5 m wide and nearly 1 m deep, with rubble against its inner scarp from a fallen revetment; the overall width of this defence seems to have been about 7-5 or 9 m.

The promontory continues at about the same width for another 80 m, and then contracts suddenly to about 25 m. This narrower projection is protected by a third rampart, appearing as a stony scarp nearly 2 m high with a slight ditch in front; there is a gap 3 m wide, perhaps an entrance, between its N. end and the cliff edge. The promontory continues for another 25 m, narrowing slightly and with a south-easterly inclination. At its end is a low mound, perhaps a barrow, about 4.5 m in diameter and nearly 1 m high. It shows signs of disturbance at the centre.

Source: RCAHMW 1976: An Inventory of the Ancient Monuments in Glamorgan. Volume I: Pre-Norman. Part II. The Iron Age and The Roman Occupation. No 704, Sully. p. 71

2. The east end of Sully Island is defended by a succession of three ramparts. There has been erosion, particularly on the south, and the north half is thickly overgrown; the present area of the main enclosure is about 0.4 ha, but it may originally have been twice as much. The inner rampart, cutting off the subsidiary promontory at the point of contraction, was fronted by a ditch and is thought to have been revetted, or walled in stone.

D.K.Leighton, RCAHMW, August 1987

Updated. T. Driver, RCAHMW, 2024