At Grange in County Wexford, a subcircular enclosure roughly 25 metres across shows up as a cropmark on aerial photography, the faint signature of an ancient fosse, or defensive ditch, pressed into the soil beneath what is now ordinary pasture. Walk the field and you would see nothing at all.

A rath, the Irish term for a roughly circular earthen enclosure typically dating from the early medieval period, was the standard settlement unit of rural Ireland for centuries, a farmstead defined by an earthen bank and ditch that offered both status and a degree of security. Most were home to a single farming family. The Grange example belongs to a cluster of such sites: three other rath monuments lie within 150 to 200 metres to the north-west and north-east, which suggests this was once a reasonably well-settled patch of landscape rather than an isolated farmstead. The cropmark itself is defined by a single fosse, the enclosing ditch whose differential effect on crop growth, wetter or drier soil retaining or repelling moisture differently from the surrounding field, allows aerial survey to detect what centuries of ploughing and pasture have otherwise erased.