A re-investigation of the scientific dating evidence from Guiting Power, Gloucestershire

Archaeomagnetic dating is a derivative dating method, matching regionally specific patterns of secular changes in geomagnetism, therefore needs a calibration curve. The calibration curve is a record of the secular variation in the geomagnetic field so is referred to as a secular variation curve (SVC). The current SVC for the UK is the accumulation of 60 years of research by geophysists (Aitken & Weaver 1962; Aitken & Hawley 1967; Clark et al. 1988; Batt 1997; Zananiri et al. 2007) and currently the section of the curve that relates to the first millennium BC is described by just under 100 magnetic directions. The pattern of change for each magnetic phenomenon has to be established by other chronometric methods, typically: historical records, radiocarbon or dendrochronology, as the secular variation pattern of geomagnetism is not predictable (Batt 1997). It is therefore necessary to assign a date to every magnetic direction used to construct the SVC. The main problem with the current British SVC relates to the chronological placement afforded to the majority of magnetic direction determinations that comprise it. At the moment these date ranges are based on the date range estimated whilst the excavation was still ongoing. Therefore it does not include any of evidence that came to light during post-excavation; in addition it is heavily dependent on typological sequencing. Unfortunately this has meant that many points have just been described as Iron Age so ascribed the date range 700BC-AD43. Consequently, the temporal assignments of many points are ill defined with little confidence in their reliability (Clark et al. 1988). In order to address this issue it was decided to review the archaeological evidence associated with each archaeomagnetic direction in the reference curve as it should be possible to improve on the current situation and may enable the age range associated with each data point to be reduced from 750 years. The magnetic directions from Guiting Power are included in the current SVC, making it one of the sites that could benefit from re-evaulation.