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Resistivity survey of two settlement sites at Tofts Ness, Sanday, Orkney

Resistivity as a method of geophysical survey is potentially ideally suited to the location of archaeological sites and component features of the important archaeological landscape surviving at Tofts Ness, Sanday, Orkney. The presence of stone archaeological anomalies should be seen in strong contrast against the surrounding surface geology of a machair-type of sand subsoil in conjunction with a relatively high water table. This is supported by the positive results of a preliminary resistivity survey in 1983 over known archaeological features (Dockrill 1984, 586-8). In 1984 two suspected settlement sites (mounds A and B, illus 1) were surveyed using the resistivity survey method. Both of these sites had been identified as broad mounds and planned by conventional field-survey techniques by J B Stevenson (RCAMS). Evidence of a partially exposed sub-circular structure upon the north-west edge of mound A, together with midden deposits exposed in rabbit burrows in both sites, are suggestive of an occupational function. A detailed conventional plan of the sub-circular structure was made, using a planning frame upon the resistivity grid (illus 4). It was hoped that such a detailed plan of a known feature would enable direct comparison with the resistivity results. The aim of the survey was to supplement the conventional earthwork survey of the above-ground remains to show the extent of archaeological deposits and to reveal any structural features within the two mounds. The geophysical survey was performed with the use of a Geoscan RM4 resistivity meter using the twin probe configuration. This particular resistance meter was used because of its digital display and tolerance of resistivity extremes present upon this site. Resistivity readings were taken at one metre intervals over the survey area. An area of 5200 sq m was examined over mound A and one of 600 sq m was surveyed over mound B. The data were processed upon an Epson HX20 microcomputer to allow on-site analysis. Final analysis and presentation as dot-density plots (illus 2 & 3) were made with the use of a Hewlett Packard 2100 mini-computer. High resistance features are generally associated with areas containing concentrations of stone such as rubble or walling, where there is less moisture in the soil thus providing a high resistance. Such an anomaly upon a dot-density plot is seen as a strong concentration of dots. Low resistance anomalies are associated with features containing a greater moisture content such as fills of ditches and would appear as areas of few or no dots upon the plots. A number of anomalies can be identified upon the plots and these are described below.