THE FORT DITCH: INTERIM REPORT

Anthony E. Collins (December 1979)

The 1948/49 excavations directed by Major Brinson for the Ministry of Public Buildings and Works located an east-west defensive ditch just outside the north gate of the later walled town. At the time this was thought to be the south ditch of a fort lying to the north, on the highest ground available.It was not until plans were being prepared for the M11 motorway that the subject received further attention.Aerial photographs taken by Dr. L. K. St.Joseph were studied by the M11 Archaeological committee, and on their behalf trial trenching along the line of the motorway was undertaken by Mr. M. G. Spratling in 1971 and Miss S. Lunt in 1972.They showed that the fort lay not to the north but to the south of Brinson's ditch. The results were summarised by Dr. W. Rodwell (5), who suggested that a cropmark continuing in an easterly direction on the same line as the section found by Brinson marked the north-east corner of the fort some 170m east of the Bl383 (formerly A11), thus giving a fort of 35-37 acres (14-15 ha).

In 1972 a watching brief was carried out by the writer within the presumed eastern sector of the fort at Grid Ref: TL 50434331. The work involved a mechanically excavated trench, 1m wide and 130m long, to connect the recreation ground pavilion to the main sewer.For a distance of 25m from the B1383 a dark occupation soil with much pottery and bone was found overlying natural gravel.At a point 21m from the road a large pit or ditch at least 2.10m wide and in excess of 0.68m deep was recorded.Since this coincided with a manhole it may well have been wider.The occupation layers continued for a further 4m before the gravel subsoil gave way to a fluviatile loam deposit sterile of any archaeological evidence for the next 105m.The greater part of this presumed section of the fort has clearly always been waterlogged and is still subject to seasonal flooding.This watching brief therefore cast serious doubt on the whole presumed eastern half of the fort.

In 1979 the opportunity presented itself for a complete reappraisal.

Study of more recent aerial photographs taken by the Ordnance Survey showed clearly the north-east corner spotted by Rodwell, but there was no sign of the eastern crop-mark continuing for more than about 80m. An auger survey was carried out by the Great Chesterford Archaeology Group, followed by a short trench, which proved beyond doubt that there was no eastern ditch continuing beyond the 80m point for more than a few metres.The ditch was therefore presumed to turn back in a westerly direction.Permission was obtained from the farmer, Mr. C. E. Dyball, to investigate a strip adjoining the sewerage sub-station alongside the B1383 (formerly A11) main road, since this seemed a likely place to pick up the ditch again.

A trench 110m long was excavated by hand. This revealed various features of a later date which included evidence for metalworking, a cobbled surface probably belonging to a building and 3rd/4th century plough marks.Then a major ditch running north-south was found at a point 10m east of the B1383.This was a double ditch 4.5m wide (15 ft) and 1.30m (4ft 3in) deep, clearly a military ditch modified to meet local conditions, namely a high water table and a sandy subsoil.The defensive system had to be made effective but a deep steep-sided ditch here was not feasible. So it would seem that the Roman engineers heeded Caesar's notes from Alesia (De Bello Gallico VII 73-80) and provided a small counterscarp bank and a shallow outer ditch, in the bottom of which were holes to receive either slender pointed stakes (cippi) or thorns having much the same effect as a modern barbed-wire entanglement (cervi).The rampart was constructed on a levelled gravel base 6 m wide.The front face was revetted with vertical timbers, the rear was battered with stacked turves in the usual way.Rainwater was dealt with by the provision of a drainage gulley. This north-south ditch and rampart was certainly the east side of the main fort.

It is now possible to re-assess the size of the fort. As pointed out by Rodwell, even though the south ditch is not visible on aerial photographs, the position of the two roads heading for a point almost central to the later walled town plainly indicates the south gate of the fort.The north side was located by Brinson in 1948/49, the east side by the Great Chesterford Archaeology Group in 1979, leaving only the west line which is limited by the river Cam.The north-south dimensions of the fort are therefore 300m, and the east-west 360m. This gives a fort of around 24.5 acres (9.9 ha) which compares closely with Newton-on-Trent (9.7 ha) and Longthorpe (10.3 ha).As suggested recently by Frere (6), with the army tending to operate in half-legionary units during the early years of the Roman occupation, forts of this size would have been suitable for half a legion and a few auxiliary units.

To complete the 1979 investigation a trench of 25m was excavated by machine to cut across the main crop-mark near the north-east corner noted by Rodwell.This trench disclosed a ditch l.8m wide by O.75m deep with a slot at the bottom. There w as no sign of an accompanying rampart.In all probability, therefore, this was an extension or annexe less heavily fortified than the main fort, measuring 140m east to west and 78m north to south, with an internal area of 2.5 acres (I hal).Neither the excavation nor several field inspections have produced anything to indicate the particular purpose of this annexe, but the general scarcity of pottery suggests it was not occupied for very long.

diagram of the fort ditch

References

Rodwell Britannia III 1972, 290.

W. Frere S. Britannia 1967, 71.

Acknowledgement

'The Group gratefull acknowledges the cooperation of Mr. C. E. Dyball.