Report on geophysical survey, 1994

Summary

A geophysical survey was carried out at Trevigue near St. Genny's in northern Cornwall to detect any surviving remains of a shrunken medieval settlement thought to have existed on the site. Results were disappointing and no definite archaeological features were detected with either magnetometry or resistivity. Nevertheless, several linear anomalies were revealed in the resistivity survey that may indicate the presence of poorly preserved wall footings.

Introduction

The site of Trevigue near St. Genny's in northern Cornwall was identified during a rapid reconnaissance survey of the area by the Royal Commission for Historic Monuments as being the likely location of a shrunken medieval settlement. Evidence to support this conclusion includes both the shape and size of the enclosure surrounding the extant farm buildings and the presence of undulations, suggestive of earthworks, within the enclosure and to the north of the standing buildings. The Archaeometry Branch of the Ancient Monuments Laboratory carried out a geophysical survey at the site during June 1994 at the request of Peter Rose of the Cornish Archaeological Unit, with the intention of determining the existence and probable extent of any buried remains.

The site is located at national grid reference SX 136 952, on a small rise, in an area where the predominant geology is Upper Carboniferous Namurian Shale; outcrops of this substrate are visible at several places in the survey area. Geophysical survey of the site is complicated by the presence of a public road running diagonally from the north-east through its centre and by an area of hard standing for agricultural vehicles at its western end, adjacent to the road. Resistivity coverage was not possible over these features, owing to the need to insert electrodes into the ground, and magnetometer measurements were greatly disturbed in their vicinity.

Method

The area to the north of the farm buildings was divided up into a grid of 30m squares, located by triangulation to the enclosure boundaries. A resistivity survey was conducted over the entire area, subject to the caveats mentioned above, and using the standard method outlined in Annex 1, note 1. To test the effectiveness of magnetometry on such sites, two of the 30m squares were resurveyed with a fluxgate gradiometer using the method described in Annex 1, note 2.

Results

The Magnetometer Survey

The results of the magnetometer survey are depicted, superimposed onto a 1:500 scale map of Trevigue, in Annex 2 [45Kb GIF]. The data has been corrected for 'striping' between adjacent traverses and is plotted using equal area greyscale assignment. Measurements made in the western most square in the plot are extremely disturbed owing to the hardstanding described above and an old plough parked near the pond. This extraneous interference has prevented the detection of weaker anomalies typical of archaeological features.

The eastern square is less affected, except over the line of the road. An area of increased magnetic field gradient is visible but it coincides with an outcropping of shale visible on the ground, and is unlikely to be of interest. Several other, localised, high readings can be seen in this square but, given their high intensity and isolation these are most likely to be due to recent disturbance or responses to smaller outcroppings of shale. No anomalies suggestive of characteristic archaeological features are apparent.

The Resistivity Survey

The results of the resistivity survey are depicted, superimposed onto a 1:500 scale map of Trevigue in Annex 3 [60Kb GIF]. The data has been filtered with a high pass gaussian convolution mask to accentuate anomalies less than 3 metres in width and remove regional trends caused by variations in drainage across the site. Quite a number of anomalies can be seen in this plot, so a schematic interpretation plan that highlights the features to be discussed, is included as Annex 4 [13Kb GIF].

A group of strong, high resistance anomalies are visible at the eastern end of the survey in the area marked (1a) in Annex 4. These coincide with an area of outcropping shale, marked on the map, so in this case the resistance meter is responding to a geological target although it may have been utilised in the past as an integral part of a building wall. Two other linear high resistance anomalies, marked (1b), exhibit a similar response and despite the absence of outcropping shale in this area are likely to reflect the presence of this substrate just below the surface. It is also possible, given their strength and proximity to the existing hardstanding, that they represent the remains of a comparatively recent feature associated with the farm.

Nevertheless, not far from (1a), some less intense linear high resistance anomalies are marked. These are depicted in grey in Annex 4and marked as (2a), (2b) and (2c). Their width is consistent with them being the buried wall footings of previous buildings, those at (2a) describing an outline suggestive of a building plan. However, no complete building plan is evident and it is possible that these anomalies are due to variations in the underlying shale or, in the case of (2b), to relatively recent field drains. The linear feature running north from the farm buildings and marked (2c) aligns with the present entrance to the farm courtyard and is likely to represent the line of a previous path rather than being a wall footing.

Some intriguing low resistance anomalies are also visible: two are linear and are marked on Annex 4 as (3a) and (3b); the other, (4), is an approximately rectangular area some 8 metres in length. Given that both (3a) and (3b) run towards the pond, it is possible that they represent the course of filled in drainage ditches, although it is impossible to suggest from how long ago they date. The feature marked (4) may well represent a larger, backfilled excavation. However, a faint high resistance anomaly can just be discerned appearing to enclose it, so it is possible that it represents the interior of a sunken floored building. With no associated anomalies nearby it is not possible to expand upon this interpretation and a recent origin cannot be discounted, as this area has been used as a depot for agricultural machinery by both the previous and present owners of the farm.

Conclusion

The resistivity survey has revealed several linear anomalies that may, potentially, be the remains of buildings that used to stand on the site. However, these anomalies mostly contrast only faintly with the general background response and no clear building plan emerges. This suggests that, if these anomalies are indeed of archaeological origin, then their preservation is poor. This conclusion is unsurprising, given that the area appears to have been landscaped in the recent past.

The magnetometer results were disappointing, particularly since the prevailing geology of the region tends to produce soil that is highly responsive to anthropogenic activity. In the case of Trevigue the negative results are likely to be due to the poor state of preservation of any surviving archaeology and the amount of disturbance caused by recent activity on the site. This latter effect was readily apparent in the magnetometer plot shown in Annex 2. It leads to the conclusion that magnetometer survey of such settlements is only likely to be successful where there has not been extensive activity on the site since the original settlement was abandoned.

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