A midden is essentially a prehistoric or early historic refuse deposit, the accumulated cast-offs of shellfish meals, and the one on Oileán na gCapall, or Horse Island, a small islet sitting close to the south-western end of Bartragh Island, is now visible in cross-section in that exposed bank face.

The deposit runs roughly four metres from north-north-west to south-south-east and is between fifteen and twenty centimetres deep, a modest but legible stripe in the soil. It is composed mainly of cockle and limpet shells, with smaller quantities of periwinkles, mussels, and oysters mixed through. Above it sits a layer of sod and dark brown topsoil; below it, a light brown stony subsoil. That stratigraphy, the layering of materials in sequence, is what gives the deposit its archaeological significance. The shells did not wash there naturally; they were brought, eaten, and discarded, and the ground simply closed over them in time. The precise date of the deposit is not recorded, but middens of this kind occur across Ireland from the Mesolithic period onward, and their contents can, when analysed, reveal a great deal about diet, season of occupation, and the species that once populated local waters.