Magnetic survey has become increasingly important in locating and characterising sites of archaeological potential. Magnetometry and topsoil magnetic susceptibility are the principal methods employed. In recent years advances in the design of magnetometers have made their use almost universal, somewhat to the detriment of magnetic susceptibility survey. This is especially true when the efficacy of various geophysical methods in locating sites of archaeological potential is compared. Such direct comparisons are, however, open to serious question. When sealed below the soil archaeological features acquire a degree of permanence, and their relatively static nature allows them to be identified by magnetometer survey on morphological grounds. By contrast, soil is continuously exposed, and over a period of 'deep time' is subject to the vagaries of human activity. The soil itsc1fbecomcs an artefact and the information it contains at any given time need not be site - specific but extensive. The problem addressed here is that of the interpretation of such topsoil magnetic data. Through occupation, industry, and agriculture, communities have historically affected the nature and proportion of the iron minerals (mostly oxides) in the soil, which establishes its magnetic identity. The problems of identifying the cultural influences which impact on the magnetic signature are simplified here by introducing three separable dynamic agencies: viz. chemical, exotic (inclusion) and mechanical (intervention, or perturbation). The incorporation of exotic materials into the soil, and human mechanical perturbation, arc both capable of creating soil magnetic patterns without directly affecting the soil's natural minerals. A search of early agricultural literature has identified activity which aids the interpretation of otherwise enigmatic data. The results of over 200 surveys have been reviewed, and from them examples selected to illustrate the three dynamic models. The results confirm that magnetically enhances soils often indicate buries archaeology but also may have no underlying archaeological source, the historic information residing exclusively within the topsoil itself. Long-term dispersal of magnetic material within the topsoil is examined and seen to be related to both bioturbation and agricultural activity. Documentary evidence shows that historic enclosure, selective land-use, and ploughing have contributed to soil magnetic pattern formation. Far from being a poor relative of other prospecting methods, topsoil susceptibility has its own unique contribution to make. Information resides within, not only below the soil, and the full potential of the method has not yet been realised. Recently the idea of cultural soilscapes has emerged, an historical perspective gained from archaeological soil surveys increasingly aids an understanding of how soils have been utilised, exploited and, more recently, degraded.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:617598 |
| Date | January 2013 |
| Creators | Johnson, Anthony |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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