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Archived News ~ Week ending Friday 5th March 2004.
Ladybridge Farm Evaluation - Geophysical Survey Results
Non-invasive archaeological evaluation at Ladybridge Farm, Nosterfield has been ongoing since last Autumn. An extensive field-walking survey has been largely completed apart from a small area still under crop. The results of the fieldwalking (see archived news, 19th December 2003) included background noise of relatively modern material from manuring, as well as prehistoric lithic concentrations. Aerial photographs have also been used to plot cropmarks within the area. An integral part of the evaluation has been an extensive geophysical survey. Due to the nature of land use and the underlying geology at Nosterfield, it is very difficult to gain information about underlying archaeology from non-invasive survey. In order to explore further the context of the lithic concentrations identified from fieldwalking and the features identified in aerial photographs a geophysical survey pilot study using a magnetometer was undertaken in late 2003.
The magnetometer survey was undertaken with a fluxgate gradiometer which is an instrument that measures subtle, localised variations in the earth’s magnetic field. If buried archaeological remains are more or less magnetic than the surrounding subsoil, they can be detected as either positive or negative magnetic anomalies. Archaeological features which contain burnt material and/or decayed organic matter can normally be located using a magnetometer. This makes the technique useful for plotting the positions of possible archaeological features such as hearths, ditches and rubbish pits. The identification of features defined by a magnetometer survey rests on interpretation of the resulting data. Non-archaeological features, i.e. natural or geological features cannot always be distinguished from archaeological features by the technique, for example, the swallow holes found all over Nosterfield are full of organic matter and would also produce positive magnetic anomalies.
Unfortunately, due to the sand, clay and gravel subsoil in the Nosterfield area, there is little magnetic variation between the fills of many archaeological features and the surrounding subsoil. The initial pilot study was deliberately positioned over the lithic concentrations and the features identified from aerial photographs on the assumption that these areas and features would be associated with underlying archaeological remains.
The pilot study survey area was divided into 20m x 20m grids and readings were taken at 0.5m intervals along 1.0m traverses. From the data a grayscale plot of results was produced. An area totalling 1 hectare was represented by the pilot study and survey was undertaken in areas B1 and C (see survey results, below). The results of the survey were disappointing as very few anomalies, which might have represented underlying archaeology, were found. Despite disappointing results from the pilot study, it was felt that a survey representing 20% of the Ladybridge Farm area should be undertaken in an attempt to locate any major monuments that may be present. However, this survey produced similarly few features.
Geophysical Survey Results
The interpretation of the anomalies identified in the survey has been undertaken and includes linear anomalies which may be archaeological but could simply represent old fence lines, hedgerows and land drains (see interpretative results, below). Large anomalies which are roughly circular could be pits, but are more likely to be swallow holes, tree pits or natural concentrations of magnetic minerals in the subsoil.
Interpretation of Geophysical Survey, with crop mark data superimposed.
The modern ploughing at Nosterfield has also affected the success of the geophysical survey. The plough furrows in the survey areas create an uneven surface which the surveyor has to walk over with the instrument. These undulations in the ground surface are detected by the magnetometer and the lines of modern furrows are visible in all the areas of survey.
Although disappointing, the magnetometer survey has proved important in demonstrating that underlying archaeology is not easily detected by this geophysical technique. A survey using a soil resistance meter has recently been undertaken and the data is currently being processed and will be summarised at a later date. The most valuable information gained about the area in recent years has come from invasive techniques. A context for the lithic 'hotspots’ and the aerial reconnaissance will be gained by the excavation of test sieving squares to test lithic concentrations over the area. Evaluation trenches are planned as part of the evaluation programme.