Archaeomagnetism

This summary describes the archaeomagnetic investigation of two features encountered during excavations at Street House, North Yorkshire. This work was carried out as part of research into improving the archaeomagnetic reference curve for the UK. Archaeomagnetic dating is a derivative dating method, matching regionally specific patterns in the Earths magnetic field. The variation of Earths magnetic field is not predictable (Batt 1997) so the pattern of change has to be established by independent dating, typically: historical records, radiocarbon or dendrochronology. One of the aims of this research is to provide further dated magnetic directions from the Iron Age to improve archaeomagnetic dating in this period. The features sampled were: an area of clay that displayed colour change consistent with heating (context 889) along the western edge of a rectangular structure, butting as stone platform and a halo of reddened clay surrounding a discrete area of flat stones (context 845) interpreted as a stone lined hearth. A total of 20 orientated samples were collected from cleaned horizontal deposits within both features using the standard disc method (Clark et al. 1988). There was no evidence of any local magnetic anomalies, so the samples were orientated before removal with a magnetic compass. After sample preparation, the direction and strength of the remnant magnetisation of all samples was measured with a Molspin fluxgate spinner magnetometer (Evans & Heller 2003: 56). Just over half the samples from both features had a measureable remanence; indicating that not all of the material sampled contained sufficient magnetic minerals to record a stable magnetic direction. This raises some concerns regarding the ability of these samples to produce a reliable archaeomagnetic date. From a pilot study of the sample behaviour from both features, an alternating field of 7.5mT was chosen to provide the optimum removal of less stable components, leaving the magnetisation of archaeological interest. These demagnetisation data were also assessed using methods defined by Tarling and Symons (1967) to provide an indication of the magnetic stability of the measurements.