The layer, roughly a metre long and no more than twenty centimetres deep, is composed almost entirely of cockle shells, compacted into the earth about half a metre above the base of the cliff face. It is a midden, the archaeological term for a refuse deposit, usually of food waste, that accumulates over time wherever people repeatedly process and consume what the sea provides. This one sits half-buried, partly eroded out of the scarp face and further concealed by vegetation and the collapsing overhangs that coastal erosion produces. Its full original extent remains unknown.

What makes the find quietly compelling is not its size but its company. Approximately 120 metres to the east, at the very tip of the island, there is a second midden. Two separate deposits of shell waste on the same short stretch of shoreline suggest a pattern of use rather than a single casual event, though the notes do not extend to dating either deposit or naming the people responsible. Bartragh is a low, largely uninhabited island in Killala Bay, exposed to the Atlantic approaches of north Mayo, and the presence of repeated shellfish processing here hints at a sustained relationship between people and this particular stretch of coast, one that the eroding cliff face is slowly, irreversibly dismantling.