In a flat tilled field in the Goul river valley, County Kilkenny, a circular prehistoric feature lies completely invisible to anyone standing on the ground. No mound, no stone, no depression marks the spot; the field looks exactly like any other. Yet from above, the outline of a ring-ditch roughly fourteen metres in internal diameter becomes legible, traced in the differential growth of crops over soil disturbed long ago.

A ring-ditch is essentially the filled-in remnant of a circular trench, often all that survives of a burial mound after centuries of ploughing have levelled the raised earthwork above it. The monument at Rathlogan was first identified as a cropmark on an aerial photograph taken in 1970. Cropmarks appear when buried features cause the vegetation above them to grow differently, whether lusher or more stressed, depending on whether the underlying soil retains more or less moisture. From ground level there is nothing to see, but that 1970 photograph preserved the shape of something that had otherwise been erased entirely from the landscape, sitting quietly above the flood plain with open views across the valley in every direction.