Exposed in the crumbling land face, roughly two metres above the shoreline and just below the modern sod line, lies a thin but distinct layer of cockle shells, the remnant of a midden. A midden is essentially a refuse deposit, the accumulated kitchen waste of people who lived, fished, and ate at a particular spot over time. This one stretches approximately four to five metres east to west and sits only ten to twenty centimetres thick, a compressed record of repeated meals that has waited, sealed in the ground, until the slow work of coastal erosion began to peel back its cover.

The shells are predominantly cockle, which suggests a community making regular use of the estuary's shellfish, a pattern well documented along the Irish coastline from prehistoric times through to the early modern period. Without excavation it is impossible to say precisely when this deposit formed, and the site's position makes close examination difficult. What can be said is that it does not stand alone: a second midden lies roughly forty metres to the west, raising the possibility that this part of Bartragh Island was a place of sustained, repeated human activity rather than a single episode of occupation. The River Moy estuary, which empties into Killala Bay, has long been one of the more productive stretches of water in the west of Ireland, and the presence of two shell middens within metres of each other on a low island in its mouth fits neatly into that longer story of people and shellfish and tidal margins.