Today we think of Oxford as lying at the heart of a
homogeneous geographical region formed by the Thames Valley, but for
much of the last few thousand years this part of England was an area
of intense political and economic rivalry and territorial dispute.
In the Iron Age, and perhaps earlier, the River Thames and its
northern tributary, the Cherwell, appear to have served as political,
tribal boundaries. The story was to be repeated in Anglo-Saxon times
when the Thames again divided kingdom from kingdom, Mercia to the
north and Wessex to the south.
The political map of the Iron Age is complex, but
distributions of coinage for this area suggest that the Catuvellauni
occupied the area to the east of the Thames / Cherwell, the
Atrebates to the south of the Thames, and the Dobunni to the west
(with a possible 'sub-Dobunni' group to the south-west). The section
of the Thames extending from the Cherwell downstream to Wallingford
represents the area where all three tribal powers met.
An overall sense of this historical geography has
been understood for years; but the picture became much clearer
recently when a massive, thirty-three hectare, late Iron Age defensive
enclosure, or oppidum, was discovered underlying the town centre of
Abingdon, south of Oxford, during excavations in 1990-1991 by Tim
Allen of the Oxford Archaeological Unit. The oppidum was enclosed by
a substantial multiple-ditched defensive circuit, and by projecting
the line of the defences it appeared they cut off the angle between
the Thames and its tributary, the Ock, which together acted as its
other defensive sides. The oppidum was established on a site already
occupied in the early to middle Iron Age, and after its main period
of use it continued in occupation throughout the Roman period.
Most importantly, however, the oppidum joins a
series of four or five major earthworks of about the same period -
roughly 100 BC to the time of the Roman conquest - in this short
stretch of the Thames Valley. Together they amount to one of the
most significant concentrations of late Iron Age and early Roman
defensive or territorial earthworks in Britain.
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 thirteen square kilometres and eventually
eighty square kilometres, 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.
Next in the sequence going south is Abingdon in the
territory of the sub-Dobunni or the Atrebates, and then the Dyke
Hills oppidum at Dorchester, another very large, forty hectare
defensive enclosure, very similar in form to Abingdon, in what might
have been Catuvellaunian territory. This had two substantial banks
either side of a large ditch, probably an artificial channel linking
the Thames to its tributary the Thame to form a defensive moat.