Sigwells, Somerset, England: Regional Application and Interpretation of Geophysical Survey

Article excerpt

The South Cadbury Environs Project is distinguishable from conventional regional surveys by the use of geophysical survey as its principal technique. Our large-scale, continuous survey has generated images derived from raw numerical data which are interpretable precisely because of the extent of coverage. Varying orientations of linear anomalies, linked to the morphology of enclosures and building foundations, have allowed us to identify episodes of planned structuring in the landscape as well as adaptation within existing frameworks. For the purposes of this paper interpretation on the large scale has been emphasized at the expense of some detail detectable in the graphically plotted data.

The project area encompasses 64 sq. km centred on Cadbury Castle, a defended hill at the neck of Britain's southwest peninsula (FIGURE 1), at the interface between a Jurassic limestone ridge and low-lying lias clays. There has been no previous systematic archaeological research in this distinctive geological niche, sandwiched by the chalk Downs of central southern Britain and the peats and clays of the Somerset Levels, excepting excavation on Cadbury Castle itself (Alcock 1972; 1995; Barrett et al. forthcoming). The objective of the survey is to discover how and when landscapes were modified and defined by successive boundary systems over several millennia.

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Prioritizing geophysics in regional survey

In recent decades regional or landscape archaeological survey has shifted from being a means for locating `sites' to a multidisciplinary process for describing the human place in changing landscapes. Whilst these more sophisticated analyses of ploughzone artefacts and their distributions allow increasingly resolved diagnoses of function and approximate contemporaneity, they provide little direct evidence for systemic relationships between concentrations.

Previously regional programmes have employed geophysical prospection in narrow transects analogous to those used for surface artefact collection, or to discover the extent of structures where surface artefact concentrations occur (Carrete et al. 1995: 219-24; Barker 1995: 51-4). Survey at Wroxeter in the English Midlands has mapped and phased a near-complete 70-ha Romano-British city plan (White & van Leusen forthcoming). Even on the latter grand scale geophysical techniques were applied in a site-based rather than regional manner.

Air photography provides an apposite model. In a review of photographs from central southern Britain Palmer pioneered the subjective interpretation of feature type (i.e. ditch, bank etc.). He then classified these features morphologically and looked for signs of their organization in the landscape, often on the basis of alignment (Palmer 1984: 5). By adding extrinsic data from other archaeological work in a heavily researched and repeatedly photographed area, he was able to propose outline maps of monuments and boundaries for four broad-span periods over four millennia (Palmer 1984:124-31). Geophysical data offer the potential for the discovery of a wider range of features and increased resolution with fuller characterization of features by their strength as anomalies. It has been claimed that geophysical survey can inform the stratigraphic analysis of excavated sites (Lyall & Powlesland 1996: 2).

In an area of poor air-photographic coverage, our work shows that large-scale geophysical survey can reveal coherent, roughly synchronous layouts or systems which can be treated as chronologically discrete. Contingent development within systems can be distinguished in adjustments of enclosures and the subdividing of large bounded areas. Phasing the systems has been achieved by judging whether features within them are superimposed upon or respect features from other systems. Known morphological characteristics are particular to certain systems, and have made it possible to construct a chronologically anchored sequence of bounded landscapes over three millennia. …