Magnetometry surveys virtually always reveal magnetic debris, ranging from just a few isolated dipolar anomalies to widespread high magnitude anomalies that mask weaker features. The magnetic response is often characterised by discrete, often circular zones of positive magnetism surrounded by a negative halo. Sometimes the response may be negative surrounded by positive or conjoined positive and negative zones. At high densities the morphology of individual dipolar responses may not be clear and highly complex and variable anomalies may be apparent in magnetometry datasets.
The potential value of these anomalies becomes more apparent with high resolution datasets. At Archaeological Surveys Ltd as standard we tend to collect data along traverses separated by 0.5m with along traverse sampling frequently less than 0.1m. Other factors to consider though are the height of the sensors above the ground surface and the characteristics of the fluxgates (or other types of sensor). Where sensors are held well above the surface and data collection is at comparatively coarse resolution e.g. 1m x 0.25m, the mapping of small ferrous objects as potential archaeological indicators becomes ineffective - perhaps this is one reason why the value of 'magnetic debris' has been overlooked.
What sort of archaeological information can we derive from topsoil magnetic debris? Our own work has proved useful for the location of former structures, areas of former industrial activity and areas of military activity. It's worth noting that we are considering the magnetic debris as a proxy where there are no surviving structures or cut features but of course where there is surviving archaeology, magnetic debris can also prove highly useful.
Research surveys with the Wiltshire Archaeology Field Group at the Mother Anthony's Well site demonstrated the presence of areas of magnetic debris in parts of the site where other cultural material from surface collections and nearby corn drying kilns suggested that there should be other Roman structures. Evaluation trenching revealed just one very truncated and disturbed chalk wall suggesting that modern cultivation had removed former structures but by mapping the zones of magnetic debris we were able to understand more fully the likely extent of the area of occupation. Subsequently, part of the area was tested using metal detection to identify the exact location of small ferrous objects allowing them to be mapped using RTK GNSS and to sample the range of objects forming the magnetic debris. The most frequent objects identified were nails with a large proportion of these being typical of those used to hold Roman roof tiles in place, a significant proportion were also Roman hobnails. Within the core area of magnetic debris, as identified from the magnetometer data, approximately 1 ferrous object per 1m x 1m was located but towards the periphery this became less dense at around 1 object per 2m x 2m area. The results of the magnetometry infer a significantly higher density of small ferrous objects than located by metal detection, and it is likely that this is due to the differences in the mode of operation between the magnetometer and the metal detector which is effectively an electromagnetic instrument. The magnetometer may well have been responding to objects such as pottery sherds close to the surface, fragments of burnt soil or daub and very small iron objects not located by the metal detector.
Commercial magnetometry surveys within and close to the Forest of Dean have defined spreads of ferrous material associated with early iron working. Many of these site were probably transitory and structural remains have been completely removed by ploughing. However, where zones of magnetic debris are located along with ferrous slag visible as surface finds, it is possible to map the extent of early iron working sites across the landscape. Our surveys are hinting at a significantly higher proportion of potential sites extending between Gloucester and the Forest than previously envisaged.
We advocate detailed analysis of magnetic debris when located by magnetometry. Rather than dismissing it as modern noise or rubbish, careful consideration to the origin of the material should always be carried out as part of the interpretation process. Appropriate highlighting of areas of debris and its archaeological potential, given supporting information from sources such as field surface collections, HER data, old mapping, regional research frameworks, etc., can assist in understanding former human activity on a site where archaeological features no longer survive. Magnetic debris may be the only indicator and evidence of certain types of historic event or activity.