It is not a dramatic ruin but a quiet arrangement of wooden stakes, most of them barely protruding above the estuary floor, forming an L-shape that would have funnelled fish into a trap as the tide retreated. Fish weirs of this kind were simple but effective: two fences of upright timber, woven together with wattle panels to create a enclosure from which fish could not easily escape when the water pulled back. The structure known as Boarland Rock 9 is one of several such weirs in this stretch of the estuary, though its uprights stand slightly higher out of the mud than most of its neighbours.

Radiocarbon dating places the weir's construction somewhere between 1268 and 1379, putting it firmly in the later medieval period. The long fence runs to roughly 16 metres, with a shorter return of about 3 metres, and what remains visible today amounts to fewer than 40 uprights. In 2009, a wattle panel, the woven brushwork that would originally have lined the fence, was newly exposed close to the main structure, measuring approximately 1 metre by 2 metres. It is a rare glimpse of the organic material that once filled the gaps between the stakes and made the whole thing watertight enough to hold fish. Stranger still is a cluster of uprights located just to the north of the weir, arranged in a rough circle. Researchers have noted their presence but their purpose remains unexplained.