Others exist only as shadows, readable from the air but entirely invisible to anyone standing in the field. At Gorteencrin in County Wexford, an oval enclosure measuring roughly thirty metres north to south and twenty-five metres east to west falls firmly into the second category. When aerial survey work was carried out in 1973, the outline of the feature appeared clearly on vertical photographs, defined by what seems to be a fosse, a rock-cut or dug ditch that typically marks the boundary of an enclosure. On the ground, in a cereal crop, there was nothing to see at all.

The enclosure was identified from photographs taken as part of the Geological Survey of Ireland's aerial photographic programme in 1973. Crop marks of this kind form when buried features alter the soil's moisture retention or depth, causing the plants above a filled ditch to grow slightly differently from those in undisturbed ground. From altitude, that difference in growth rate or colour can be striking enough to reveal the shape of something buried for centuries. Whether this particular enclosure is prehistoric, early medieval, or of some other period is not recorded; without excavation, the aerial outline alone cannot say. What it does suggest is that the landscape around Gorteencrin, flat and unremarkable to a casual eye, has been organised and bounded by people at some point in the distant past, leaving a mark that only becomes legible when you are high enough above it to see the whole shape at once.