Introduction

Prior to the construction of the Sevenmilestone to North Country section of the Cornwall Spine Water Main a geophysical survey was conducted along the pipeline corridor to the E of the medieval farming settlement at Stencoose Farm, Cornwall (Geophysical Surveys of Bradford 1996). The site is situated in an area identified as ancient enclosed farmland where the current field boundaries reflect the original pattern of medieval strip field systems (Countryside Commission 1996). This initial survey successfully identified a number of significant magnetic anomalies that were later confirmed by excavation as the remains of a well preserved Iron Age or Romano-Cornish "U" shaped structure together with a series of pits and ditches (Jones 1996).

Further geophysical survey was requested through the English Heritage Inspector of Ancient Monuments by the Cornwall Archaeological Unit (CAU) to place the discovery in a wider context. It was hoped that the high degree of archaeological preservation revealed by the excavation would augur well for the location of further geophysical anomalies to compliment the initial survey data.

The site (centred on SW 71 45), set on a gentle south facing slope, is located on soils of the Denbigh 2 association (Soil Survey of England and Wales 1983) developed over a substrate of Devonian Mylor Series slate and sandstone. At the time of the survey all areas were down to pasture.

Method

Due to the success of the initial magnetometer survey (Geophysical Surveys of Bradford 1996) it was this technique that was adopted to cover the extended area beyond the pipeline corridor. Figure 1 details the 30m grid established over the site and also shows the location of the initial magnetometer survey. Data was collected from each 30m grid square using a Geoscan FM36 fluxgate gradiometer along N-S traverses following the standard method outlined in note 2 of Annex 1.

Plan A shows a greytone image and X-Y traceplot of the magnetometer data after statistical processing of each survey line to provide a zero-centred mean. This process eliminates offsets between adjacent survey lines that may occur due to the directional sensitivity of fluxgate gradiometers when data is collected from alternate "zig-zag" traverses and considerably improves the presentation and interpretation of the resulting data.

Results

Modern interference

The presence of the ferrous water main has completely saturated the response of the fluxgates through squares 3, 8, 12 and 16. Figure 2 shows the complete survey superimposed over the base OS map but as the data from the former squares contains no significant archaeological anomalies it has been removed from subsequent plots. In addition, after analysis of these four pilot squares no further data collection was attempted along the course of the pipeline corridor. Further ferrous disturbance has occurred along the W edge of the survey (squares 15, 19 and 20) due to the proximity of modern agricultural buildings.

Faint patterns of closely spaced (~2m) parallel linear anomalies [1] are visible throughout the survey area. These occur on differing orientations in the two fields covered by the survey and most probably represent a recent ploughing pattern. Few of the more significant magnetic anomalies share either of these orientations and thus their identification has not been significantly impaired. An additional linear anomaly [2] of alternating positive/negative response is evident running through squares 5 and 10 and is reminiscent of a modern services pipe or cable. However, given the comparatively remote location of the site it is not immediately clear which properties this service connects. Alternatively, [2] may represent a ?ceramic field drain, however, this seems unlikely given the slope of the field and the apparent absence of any similar anomalies throughout the rest of the survey area.

Archaeological anomalies

The most striking anomalies visible in the data are a number of strong (>5nT) linear responses that intersect over much of the site . Unfortunately, due to the absence of data from the pipeline corridor and the limited extent of the initial survey more precise interpretation of these anomalies is difficult. However, they appear to represent a multi-phase palimpsest of enclosure and boundary ditches whose various orientations and alignments fail to respect either each other or the current field boundaries.

It is of interest to note that a number of ditch-type anomalies are concentrated in the vicinity of the excavated structure (Plan B) and one of these [3] appears to form part of a rectilinear enclosure. To the N of this, anomaly [4] correlates with the position of the ploughed-out bank revealed during the excavation (Jones 1996; Layer 7) although the response suggests it represents an associated ditch rather than the bank itself. Given the diffuse nature of [4] , it seems plausible to suggest the continuity of this with the anomaly to the W of the pipeline. Anomaly [5] running through squares 1 and 2 exhibits a similar diffuse response and may also represent a ploughed out ditch and bank.

Further examination of the excavation data also suggests a correlation between anomaly [6] and a ditch (Jones 1996; cut 54) revealed immediately S of the excavated structure. This anomaly was also replicated in the initial geophysical survey where the presence of a possible rectilinear enclosure was suggested (Geophysical Surveys of Bradford 1996).

A series of discrete positive pit-type anomalies [7] are also visible in the data. But no unequivocal association between these anomalies and the ditch-type responses can be made. The magnitude of response exhibited by the majority of these pit-type anomalies may well be caused by either a concentration of burnt material within the feature or, perhaps, an in situ burnt feature such as a hearth or kiln. The limited examples of pits revealed along the pipeline corridor excavation provide evidence to support both potential modes of magnetic enhancement (Jones 1996).

Square 27 contains a highly tentative oval anomaly [8] defined by a concentric halo of positive/negative response. The nature of this anomaly is similar to that recorded at two other Cornish sites; Boden Vean, St. Anthony in Meneage, (Linford 1998) and Reawla, Gwinear (Bartlett 1978). At the latter site the anomaly was confirmed by excavation (Appleton-Fox 1992) to be a an Iron-Age dwelling and it is possible that [8] represents the response from a similar feature, although its oval nature may imply a Romano-British date (Quinnell 1986).

Conclusion

The survey has successfully revealed a complex palimpsest of enclosure and field boundary ditches together with a scatter of discrete pit-type anomalies surrounding the site of the initial survey and subsequent excavation. Unfortunately, the absence of data along the course of the pipeline corridor has curtailed a more complete interpretation. In the vicinity of the excavated "U" shaped feature a combination of anomalies recorded by the initial survey and the work reported here confirms the presence of a rectilinear pattern of ditches enclosing the structure.

References

Appleton-Fox, N., 1992, Excavations at a Romano-British round: Reawla, Cornwall, Cornish Archaeol., 31 , pp69-123.

Bartlett, A., 1978, Report on the magnetometer survey at Reawla, Gwinear, Cornwall, Ancient Monuments Laboratory Rep. ser., 2565 (Geophysics 9/78), English Heritage, London.

Countryside Commission, 1996, Cornwall Landscape Assessment 1994, Report prepared by CAU and Landscape Design Associates, Cornwall County Council.

Geophysical Surveys of Bradford, 1996, CORNWAL North Country, geophysical survey report, 96/40 . - unpublished

Institute of Geological Sciences, 1974, Geological Survey of Great Britain, Sheet 346, Newquay - Drift.

Jones, A., 1996, A report to South West Water Services Ltd., An Archaeological Investigation at Stencoose, Cornwall 1996, Cornwall Archaeological Unit Archive Report.

Linford, N. T., 1998, Geophysical survey at Boden Vean, Cornwall, including an assessment of the microgravity technique for the location of suspected archaeological void features, Archaeometry, 40 (1) , pp187-216.

Soil Survey of England and Wales, 1983, Soils of England and Wales, Sheet 5, South West England.

Quinnell, H., 1986, Cornwall during the Iron Age and Roman period, Cornish Archaeol., 25 pp111-34.

Enclosed Figures and plans

Figure 1 Location of the geophysical surveys March 1996 and July 1997. (1:2500)

Figure 2 Greytone image of raw magnetometer data (July 1997) superimposed on the base OS map. (1:2500)

Plan A (1) Greytone of raw magnetometer data, (2) X-Y traceplot of magnetometer data. (1:1250)

Plan B Summary of significant anomalies from the March 1996 and July 1997 geophysical surveys together with features revealed by the CAU excavation. (1:1250)

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