SDO14490 - New Barn Farm, Knowlton, Dorset. Geophysical Survey Report.

‘Ground penetrating radar (GPR) survey was conducted over land at New Barn Farm, near Knowlton in Dorset. The project was commissioned by Smiths Gore on behalf of their client St. Giles Farms with the aim of establishing the presence, or otherwise, and nature of detectable archaeological anomalies on the site ahead of proposed redevelopment. The geophysical survey forms part of an ongoing programme of archaeological work at the site and follows a previous Desk-Based Assessment (DBA). The site comprises a disused farmyard and is positioned immediately south of the intersection of the B3078 and Lumber Lane, 6km west of Verwood and 10km north of Wimborne Minster. The central portion of the site is designated as a Scheduled Monument, and known as the South Circle forming part of the group collectively known as the Knowlton henge monuments. The site is largely flat, lying at c. 52m above Ordnance Datum, and is surrounded by arable farmland. English Heritage advised that a Ground Penetrating Radar (GPR) survey should be undertaken prior to any intrusive archaeological evaluation given the complex arrangement of the existing buildings, areas of hardstanding and possible presence of archaeological features relating to the henge monument. Data was collected at 0.02m intervals along traverses spaced 0.5m apart within seven separate survey areas using a Mala Geoscience shielded antenna of 250MHz central frequency; where possible, data was collected on similar orientations but the number of obstacles within the survey area forced several different orientations to be used. The data was then timesliced and gridded to provide plan views of the survey area, although individual radargrams were also inspected. The data is of generally high quality and the results show the presence of numerous anomalies of high amplitude, regions of increased response and several probable modern services. Whilst it is difficult to determine the origins of the majority of the anomalies identified, some are more clearly modern or historic and relate to existing or former buildings. It is interesting to note that few anomalies consistent with the henge ditch have been identified, and individual radargrams collected perpendicular to the alignment of the henge do not show responses typical of a sizeable ditch-like feature; the excavation of the eastern circuit of the henge ditch determined its dimensions to be approximately 5m wide and over 5m deep. Whilst the reason for this is somewhat unclear, it is possible that the monument has been significantly truncated by the construction of the farmyard and associated buildings, that it was less substantial around its western circuit compared with the east, or that the survey was conducted entirely over a former entrance to the henge. The GPR data has not allowed for a definitive interpretation of the relationship between the base of the concrete pads or gravel/hardcore areas and the underlying geology, and it is therefore unclear if these have been laid directly on the top of the bedrock geology or whether a soil horizon is still present underneath the concrete.’