The application of environmental magnetism to archaeological prospection : a semi-quantitative approach
Magnetic survey is the most widely used shallow geophysical techniquefor the location of archaeological activity in the United Kingdom but is often discountedftom use within alluviated landscapes. Results presented in this thesis from fluxgate gradiometer surveys conducted over an alluviated flood plain near the village of Yarnton, Oxfordshire, UK, demonstrate a wide variation of magnetic response between adjacent sites. This suggests a more complicated relationship between the rock magnetic properties of underlying archaeological sediments and the resultant magnetic anomalies recorded during surface magnetometer surveys. This study aims to investigate this relationship further and determine the influence of post-depositional mineral dissolution in water logged soils; a factor that together with increased alluvial overburden, has often been cited as an explanation for disappointing magnetic results over similar sites. A wide ranging study has been conducted including geophysical surveys, environmental magnetic analysis of archaeological sediments recovered during excavation and experimental work to investigate the influence of burning. Initial, rapid determination of isothermal magnetic parameters, such as susceptibility, has been complemented, for selected samples, by more detailed hysteresis measurements and thermomagnetic variation over a temperature range from 20 to 973K. Interpretation of the resulting data has been assisted through the development of semi-quantitative numerical models to describe the complex magnetic mixtures present. The results, including over 20ha (20, OOOOm2)of geophysical survey and the analysis of more than 500 samples, demonstrate the important role of fire for magnetic enhancement and also provides evidence, under suitable conditions, for more esoteric biogenic mechanisms. The main conclusions reached suggest archaeological features magnetised before the onset of floodplain conditions may still be detected through geophysical survey, particularly if more sensitive, caesium vapour magnetometers are applied In addition, semi-quantitative unmixing models allow both the thermal history of burnt sediments to be estimated and provide a means for identifying biogenically enhanced samples.