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Archived News ~ Week ending Friday 12th March 2004.
Ladybridge Farm Evaluation - Geophysical Pilot Study
As part of the ongoing evaluation of Ladybridge Farm, a geophysical survey has been undertaken involving a magnetometer survey of 20% of the area (see archived news, 5th March 2004) followed by a soil resistance pilot study of 1 hectare. The soil resistance pilot study was completed last month and the results and interpretation are summarised here. The results of the pilot study indicate that resistivity is a more effective method of mapping buried archaeological remains at Nosterfield than magnetometry.
The soil resistance pilot study was carried out using an RM15 Advanced soil resistance meter. The soil resistance meter passes a small electrical current into the ground via metal probes and measures the grounds resistance to the current. Variations in the grounds resistance are normally caused by changes in the underlying geology, or the presence of archaeological features. In this case, the RM15 was used with a multiplexer connected to a multiple probe array fitted with three probes which allows a series of different readings to be taken at the same point. The survey then produces two sets of data, a high resolution image of shallow soil resistance features, and a lower resolution image of deeper features.
Two pilot study survey areas totalling 1 hectare were selected. Area A was located over a group of possible archaeological features defined by the magnetometer survey, while Area B was sited to investigate a group of ephemeral cropmarks identified by aerial photography.
The pilot study successfully defined a large number of soil resistance features, many of which may be archaeological. The two data sets produced similar results suggesting that most of the features are relatively deep.
Low resolution 1m probe results
High resolution 0.5m probe results
Several linear features were detected as low resistance anomalies (see interpretation plot, below). Some are very straight or right angled and may be modern field drains, others are curvilinear, such as the interrupted ditch feature in Area A, and may be of prehistoric date. Other low resistance features appear as circular or sub-circular anomalies. These may be deep swallow holes, although some may be pits. Three features in Area B appear to be deliberately aligned, and could be interpreted as a possible pit alignment, although swallow holes, due to their geological nature also appear in apparently ordered rows. Swallow holes are produced by seams of marl being eroded deep below ground which gradually encourages subsidence of gravel and sand in a restricted area often appearing at ground level as a circular almost well-like features. Seams of erosion below ground produce rows of swallow holes and it is possible that this is what we are seeing in Area B.
High resistance features also include a possible curvilinear enclosure in Area A situated within, but not necessarily associated with, the low resistance interrupted ditch feature. A sub-ovate enclosure has been excavated elsewhere in the Nosterfield landscape by Jan Harding and this may be another example. The soil resistance survey also detected changes in the underlying geology in both Area A and B.
The soil resistance pilot study has demonstrated that resistivity is a more effective geophysical technique than standard magnetometry in defining buried archaeological features at Ladybridge Farm. As a consequence, it was decided that the southwest field of the evaluation area, which until recently had been inaccessible due to the presence of a beet crop, would be the subject of a soil resistance survey. The results of this survey will be used to inform the location of future evaluation trenches in that area. The effectiveness of the geophysical surveys at Ladybridge can only be assessed fully once the evaluation trenches have been excavated and the archaeological and geological features have been mapped and compared with the results of the geophysics.
Interpretation of probe results