In a pasture field in Prusselstown, County Kildare, there is an ancient enclosure that cannot be seen at all from the ground. No earthwork rises from the grass, no ditch interrupts the slope, no stone or timber survives to mark the spot. The only evidence that anything was ever here came from the air, and from a single photograph taken in 1967.

Cropmarks are among archaeology's quieter revelations. Where buried features such as ditches or walls lie beneath a field, the soil above them retains moisture differently, and the crops or grass growing overhead respond in kind, producing faint variations in colour and vigour that become legible from altitude even when they are invisible at eye level. The 1967 aerial photograph, taken as part of the Cambridge University Committee for Aerial Photography survey, captured exactly this kind of ghostly outline at Prusselstown: the cropmark of a fosse, or ditch, tracing a rectangular area roughly 20 metres along its northeast to southwest axis and around 10 metres across. Rectangular enclosures of this kind are less common in the Irish record than circular ones, and the shape alone makes this site worth noting. The same photograph also revealed cropmarks in the adjacent field to the northeast suggesting a possible rath, the circular earthen enclosure that was a commonplace feature of early medieval Irish settlement, along with what appears to be an associated field system. Whether the rectangular enclosure and the possible rath were contemporary with one another, or represent activity from entirely different periods, is not known.