This is a midden, a term for the accumulated domestic refuse of past communities, most often shellfish remains, that archaeologists read as evidence of diet, settlement, and the rhythms of coastal life. At Roeillaun, that layer runs between 0.2 and 0.3 metres deep, concentrated around a knoll at the island's north-western end and across to a small islet opposite it.

The island itself adds an extra layer of strangeness to the site. Roeillaun is now abandoned and tidal, reachable only at low water via a narrow stone causeway from the mainland to the south-east. The small islet facing the knoll was not always separate: Ordnance Survey six-inch maps show it as a promontory still attached to the island, its connection to the main landmass intact. Winter storms have since eroded that neck of land away entirely, leaving what was once a continuous feature divided by open water. The oyster shells that make up the midden are visible along the eroding peat shelf above the shoreline, and loose shells are scattered across the coral beach below, a beach of roughly fifty metres in length. The site was brought to wider attention by local archaeologist Michael Gibbons.

Access depends entirely on the tide. The stone causeway from the mainland is only passable at low water, so anyone visiting needs to time their approach carefully. Once on the island, the midden is most clearly visible where the peat is actively eroding above the shoreline, exposing the compacted shell layer in cross-section. The islet opposite the north-western knoll, now separated from Roeillaun by storm erosion, holds further concentrations of material at its south-eastern end.