In the summer of 1970, an aerial photograph taken over farmland at Grange in County Kilkenny captured something invisible at ground level: a circular cropmark roughly twenty metres in diameter, tracing the outline of a long-buried fosse. A fosse is a defensive or boundary ditch, and when one is cut into the subsoil and later fills in, the disturbed earth retains moisture differently from the surrounding ground. In a dry summer, growing crops directly above it stay greener and taller for longer, drawing the buried line up into visibility only from altitude and only for a matter of days.

The cropmark at Grange indicates a circular enclosure, a form with a very long history in the Irish landscape, ranging from prehistoric ring-ditches to early medieval ringforts used as enclosed farmsteads. At around twenty metres across, this is a modest example, though size alone tells us little about date or function without excavation. The field was in tillage when the photograph was taken, which is precisely the condition that makes cropmarks legible; grassland tends to mask them. The image was recorded on the 16th of July 1970 as part of the Cambridge University Collection of Aerial Photography, a systematic programme that documented enormous amounts of Irish and British archaeology from the air during the mid-twentieth century, capturing features that subsequent development or land-use change has since obscured or destroyed entirely.