Cut into the surface of the rock outcrop are rectangular troughs, each roughly six metres by four metres and up to half a metre deep, their curved negative impressions left behind where millstones were lifted clear of the stone. Nearby, a number of rough-outs remain in place, abandoned partway through the shaping process, giving the site the quality of a workshop caught mid-task and simply left.

Millstones were the essential grinding technology of medieval and early modern life, and sourcing good stone for them was a serious industrial concern. The cliffs here stand some thirty to forty metres above the estuary, and the medieval settlement on Great Island, visible across the water, is known to have produced millstones during the Middle Ages, quite possibly drawing on this very outcrop. The earliest written confirmation comes from Robert Leigh, who noted in 1684 that millstones were being quarried from rock over Ballyhack, a detail preserved in the historical record by Philip Herbert Hore in the late nineteenth century. Whether quarrying here began earlier, in the medieval period associated with Great Island, remains an open question, though the presence of the rough-outs suggests a sustained and organised operation rather than casual extraction.