A fosse is a defensive or boundary ditch, typically dug around a settlement or enclosure in the early medieval period and often the last surviving trace of structures long since eroded away. At roughly 50 to 60 metres in diameter, the enclosure is substantial, its sides largely straight though the western edge curves slightly and the north-west and south-west corners appear rounded rather than angular, giving it a character that sits somewhere between the geometric and the organic.

The enclosure came to light through an aerial photograph taken on 9 July 1964 as part of the Cambridge University Collection of Aerial Photography. Cropmarks of this kind appear when buried features such as ditches, which retain moisture differently from undisturbed subsoil, cause the vegetation above them to grow at a slightly different rate or colour from the surrounding field. On the right day, at the right angle of light and at the right stage of the growing season, these differences become legible from above in ways that are entirely undetectable on foot. What makes Outrath particularly interesting is that the enclosure does not stand alone. A second, very similar but slightly smaller enclosure lies approximately 10 metres to the north, also visible as a cropmark, suggesting that whatever activity once took place here was repeated or mirrored in close proximity. Whether the two were contemporary, or whether one succeeded the other, remains an open question.