Work at the Scheduled Ancient Monument at Owmby began in 1995 as a case study to examine how the significance of archaeological sites may be degraded by arable farming and illegal metal-detecting, to develop techniques and methodologies to identify and assess the impact of these threats, and to inform management strategies for mitigation or deterrence. The project is deploying a wide variety of appropriate recording techniques to evaluate their usefulness in comparative terms, and to develop and enhance the effectiveness of their individual application.
To study the impact of arable farming, the depths of current agricultural disturbance and topsoil cover have been established and compared, demonstrating the presence of differential zones of vulnerability (areas where the depth of topsoil is less or greater than the depth of disturbance). This information is being assimilated with other data (topography, geology, soil composition, and past land-use), to determine the agencies and mechanisms responsible for the creation of the differential soil cover zones. The assessment of the impact of agricultural operations on archaeological deposits within the subsoil has been assisted by a large-scale magnetometer survey carried out by the Ancient Monuments Laboratory with the assistance of Geophysical Surveys of Bradford, who also undertook a topsoil magnetic susceptibility survey. The results of the magnetometer survey in particular, have been spectacularly successful, greatly enhancing the information previously available from cropmark evidence, and detailing a variety of differing components relating to settlement development and land-use (see also section 4.14).
A successful methodology has been established to demonstrate the depth of agricultural disturbance through surface inspection. Monitoring stations had been set up within the scheduled area in 1995-96, with each station comprising a layer of small, brightly coloured glass chips placed at different depths within the plough-soil. Revisiting these stations showed that the readily recognisable material had been brought to the surface from depths of up to 0.35m. The cause of this disturbance was not ploughing, however, but ridging associated with potato production, an extensive linear spread of material was unexpectedly observed at each monitoring station. A band of glass chips was placed across two potato ridges in the path of a mechanical potato harvester to establish the reason for this, and once the machine had passed, the chips were plotted to determine the degree of dislocation that had occurred. The furthest chips were found at a distance of 11.5m from their place of origin in the direction of machine-travel, replicating the results observed at the monitoring stations. The accepted view of the effects of ploughing, with material being moved around its point of origin, may not apply therefore apply to potato cultivation, where continued linear dispersal is as likely as material being moved back towards its previous location. The results of this work have important implications for the interpretation of archaeological material retrieved from the plough-soil surface and sub-surface visually, or using a metal-detector.
Illegal metal-detecting continues at Owmby despite the deployment of signage, private security for limited periods, and our own highly visible metal-detecting survey, although this must have mitigated the impact of illegal activity by the removal of some material close to the surface. As part of the mitigation study, one 50m square and five 30m squares, intensively metal-detected in 1995, were re-detected in 1996 to help quantify the potential resources required to remove metalwork from the plough-soil completely. All classes of metal were retrieved and the results show that there was a 25% increase in the overall quantity of metalwork recovered in 1996. Only copper alloy finds showed a (very slight) decrease in numbers and even this is not a consistent trend over all of the areas detected. This rather surprising outcome probably results from the condition of the field surface during collection; in 1995 the field had only been ploughed and was quite uneven, but in 1996 it had been ploughed and rolled, clearly demonstrating the impact of slight variations in agricultural technique.
The copper alloy coins are being used to study the condition of metalwork in the plough-soil. Marked variations in corrosion levels have been noted and these have been categorised to investigate whether this is a result of context or coin composition. The study of the context of metalwork in the plough-soil continues and has been greatly facilitated by the information on subsoil features revealed in the magnetometer plot, and by the completion of the field-walking programme which will allow for a comparison between metalwork and other artefacts in the plough-soil. The entire surface of the scheduled area (c2ha) has now been field-walked for total retrieval of artefacts. Each find can be located to within 5m of its point of collection and this will enable comparisons of effectiveness of differing sub-sampling strategies (ie use of transects).
Future fieldwork will include the continuation of studies into the movement of artefacts within, and the retrieval of metalwork from, the plough-soil, together with geo-archaeological work to examine the causes of differential soil cover, and further extensive metal-detecting to address the questions of the context and condition of metalwork in the plough-soil. A resistivity survey will complete the suite of remote sensing techniques to be deployed and will complement the small-scale sampling of magnetic anomalies and sediments to inform a review of the suitability of current geophysical strategies.
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