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North Park Farm, Bletchingley, Surrey, England

The quarry site was already known to contain archaeological deposits of various eras, due to excavation work carried out by Surrey County Archaeology Unit. Artefacts on site included mesolithic and neolithic flints - there was also evidence for activity during the Bronze Age, Saxon and Early Medieval periods.

This is interesting from an archaeological point of view but the issue of how to detect features associated with such deposits is a difficult one. As with any quarry, the whole set of deposits could be expected to be removed. In this case, the quarry is for sand so pure it is used for lens glass. The stratigraphy consists mainly of sand-derived soils and is not very magnetically active.

Our brief was complex - before the next area of excavation commenced an attempt was made to discover as much as possible about it. Two methods were used to make an overview of the 3D shape of the deposits - ground probing radar (GPR) and electrical resistance tomography (ERT). In addition a caesium magnetometer survey provided information about potential archaeological remains as a planar view. After the excavation had been cleaned down to the neolithic and mesolithic layers, the caesium magnetometer survey was repeated and some magnetic susceptibility readings made.

Overall the geophysical exercise proved useful together with our initial topographic survey. The likely form of the strata was shown, with correlation between the GPR and ERT. Although the magnetic contrast was low, both the "before" and "after" survey results showed anomalies probably due to archaeological features, including pits.

Client

Surrey County Archaeological Unit