Seeing beneath the Soil: Prospecting Methods in Archaeology

Seeing beneath the Soil: Prospecting Methods in Archaeology

Seeing beneath the Soil: Prospecting Methods in Archaeology

Seeing beneath the Soil: Prospecting Methods in Archaeology


Scientific soil prospecting methods can give dramatic pictures of buried archaeological sites, and sometimes information on what occurred within them, before any earth has ben removed. Dr Clark, who was one of the earliest to work in this field, has written the first general survey of an increasingly important area of practical archaeology.The emphasis is on the principles and practical application of the well established techniques of resistivity, magnetometry and magnetic susceptibility, with shorter sections on emerging and less common techniques such as ground-penetrating radar, electromagnetic methods and phosphate survey.This paperback edition updates and enhances the earlier book, adding new material such as the large-scale evaluation exercises now required as a precondition of planning consent for major developments.


The previous chapters may well have left a rather blurred impression of a number of techniques with different capabilities, and a feeling of uncertainty about which to actually use when confronted with a site to survey. This chapter provides some guidance, as well as a brief indication of methods for judging the archaeological potential of a landscape.

Choice of method

Fig. 93 is an attempt to summarize the kind of response to be expected from each of the standard techniques to typical features on a straightforward British lowland site on fairly level ground with even soil cover—the kind of site that might show as typical crop marks from the air. I must acknowledge the pioneering use of this type of presentation by Greene (1983).

A reasonable magnetic susceptibility contrast between natural and topsoil is assumed. At the top is a schematic diagram of the site as it originally was, with the subsurface features shown in cross-section. a and B are a medium-sized ditch and bank of the kind that would have surrounded a settlement containing the Iron Age round house, D, with a central clay surface hearth (shown black) and a dump of domestic waste, or midden, C, outside. the house has an outer ring of posts set in a circular foundation trench, and inner posts with individual post-holes. E is a bell-shaped storage pit and F a shallow pit or scoop. G is a fenced animal compound and H a small Roman building with substantial outer wall foundations and the lighter foundation of an inner wall. Finally, J is a pottery kiln of baked clay, together with its stoke-hole.

The second diagram shows these features as they appear now, after prolonged ploughing. the visible remains of the round house are reduced to the bottoms of the foundation trench and post-holes, while the remains of the hearth have been ploughed into the topsoil. the cattle pen is also only represented by post-holes. the Roman building has been robbed for its material but the lower parts of the foundations remain, with a scatter of smaller fragments in the soil. the superstructure of the kiln has collapsed into the interior. Relatively high concentrations of occupation soil affected by burning are shown by stippling.

The responses of the different techniques are shown as idealized graphs below the two cross-sections. the figure as a whole is designed as a quick reference in its own right, but the following comments provide some additional information.

Resistivity This is the most complicated technique to show, because of its dependence on climatic conditions. To start with, the time is assumed to be the latter part of the year when a net moisture deficit gives generally optimum conditions. the lines represent behaviour over extreme soil types, the continuous line chalk and the dotted line gravel. An approximation to the chalk response can be expected over clay, loam and other close-textured naturals, while welldrained sands will behave similarly to gravel.

The ditch a contains a relatively loose soil filling on chalk, and gives a positive anomaly. On gravel, this is likely to be negative because the gravel is well drained, and also because the soils that develop on gravel often contain alluvium and so are relatively silty and close-textured. the bank has been levelled but its effect may still be faintly detectable from its remnants ploughed into the topsoil, which may also be slightly shallower where the bank has protected the natural from erosion. This may give a slightly lower reading on chalk, but higher on gravel.

The midden heap, C, is not detected, nor are the post-holes of the house, D, because they have too small a volume but slight responses can be expected from the foundation trench. the pit, E,

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