1. SULLY ISLAND. The E. end of the island, rising to about 45m 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
3. Royal Commission survey visit, 30th April 2025
The earthwork defences are much as described in the 1976 Glamorgan Inventory. Sully Island measures nearly half a kilometre long, east-west, with the eastern end rising to 21m above O.D, a locally prominent summit and commanding position for the fort. There is a saltmarsh or pond on the north side of the island, potentially representing a source of freshwater and wildfowling opportunities in prehistory. Coastal erosion along the southern side of the island has created a series of spectacular cliffs and undercut rock ledges, which are leading to the loss of the promontory earthworks on this side. Dense vegetation comprising elder woodland and brambles has taken hold of much of the central and north-eastern parts of the island, leaving a very narrow path along the south side to access the eastern part of the island and the promontory fort.
The promontory fort is enclosed by three ramparts; the outermost rampart measures approximately 51m north-south. It has a shallow outer ditch to the west which does not appear to be exposed in the cliff section, while the rampart stands around 1m high. This rampart is more of a scarp.bank, with a steeper west face showing rubble exposed in path, but merging with the contours on the east side.
The main second rampart measures approximately 47m north-south and has an outer west ditch which has been cut through the bedrock, evident in the eroding cliff face on the south side. The ditch measures 5.30m wide; within it, upper contexts of tumbled walling blocks are visible on eastern side at foot of rampart.
The inner eastern rampart is rather more of a scarp bank, with a steeper front (west) face with. This rampart may have been developed on natural step in bedrock. Deep path erosion through this inner rampart reveals lower, internal wall 1m in from front face, perhaps an earlier phase or internal construction wall. Today this bank stands more clear of vegetation than the other banks. Cliff erosion on the south side suggests a 2.5m wide outer ditch, shallowly excavated through upper levels of bedrock, to a depth of 0.5m.
The innermost ‘citadel’ or liminal point of the promontory fort is also the highest point on the island, a prominent rocky dome on which stands the footings of an Early Bronze Age round barrow. It is entirely possible that the barrow has been reused as a beacon-mound or similar in later centuries.
During the visit, the entire innermost enclosure was found to have been metal detected, and covered with holes ranging from 10-40cm in size.
Dr Toby Driver and Louise Barker, RCAHMW, May 2024