In themselves, limpet shells are modest things. But gathered in volume and deposited deliberately, they constitute a midden, the archaeological term for an accumulated rubbish deposit, typically of food waste, that can tell researchers a great deal about how people fed themselves and how long they occupied a place.

The ditch, or fosse, in which the shells were found was no ordinary pit. It formed part of the inner defences of a promontory fort, a type of coastal enclosure in which a headland or peninsula is cut off from the mainland by one or more banks and ditches, using the sea itself as a natural barrier on the remaining sides. The excavation was carried out under licence in 2006, with findings published by Lynch and Jones in 2009. The particular geography of this site, a neck of land so narrow that it barely connects Horse Island to the shore, would have made it naturally defensible, and the inner fosse suggests the occupants were not relying on the sea alone. That someone also deposited shellfish waste within that same ditch points to domestic life continuing inside the fortified area, meals eaten, refuse discarded, the ordinary rhythms of occupation layered into the same ground that once marked a boundary worth defending.