Link

Grim’s Ditch
Dyke
From the journal of British Archaeology

Paragraph five says: At the northern end of this political and economic frontier zone, near the modern towns of Woodstock and Charlbury, in what was probably the Iron Age territory of the Dobunni, there is the massive North Oxfordshire Grim’s Ditch, an earthwork enclosing an area of initially about 13km2 and eventually 80km2, within which a notable cluster of early Roman villas later appeared. No-one knows exactly what this enclosure was for; but it may have been intended to demarcate a special-status area for settlement. To the south, a much smaller, possibly unfinished defensive enclosure of about ten hectares, known as Cassington Big Ring, lay on the banks of the Thames near Eynsham.