In a field at Gorteenvacan in County Kildare, a circle invisible to anyone walking past becomes briefly legible from the air. An aerial photograph captures what is known as a cropmark, a phenomenon in which buried features affect the growth of vegetation above them. Where a fosse, or defensive ditch, was once cut into the ground, differences in soil moisture and depth cause crops or grass to grow in subtly different shades, tracing the outline of structures that have long since vanished from the surface. In this case, the cropmark reveals a circular enclosure, its ditch still describing a ring beneath the topsoil, with an entrance oriented towards the south-east.

Circular enclosures of this kind are relatively common throughout Ireland and are generally associated with the early medieval period, though without excavation it is difficult to assign a precise date or function to any individual example. They are sometimes the remains of ringforts, which served as enclosed farmsteads for families of some local standing, typically consisting of a raised interior platform surrounded by one or more earthen banks and ditches. The south-east facing entrance is a detail worth noting; this orientation recurs often enough in Irish enclosures to suggest it was a deliberate choice, perhaps related to the direction of prevailing weather or to the position of the rising sun at particular times of year. At Gorteenvacan, the enclosure survives not as an earthwork but as a memory held in the soil itself.