INTRODUCTION

Geophysical survey was undertaken at Westwoodside, Humberside, in response to a request from the Humber Wetlands Project. In recent years three bronze socketed axes, a flint tranchet axe, a dolerite axe and many flints (including arrowheads) have been discovered on agricultural land immediately to the west of the village. The survey was intended to locate any buried archaeological features that might be associated with these finds and thereby inform the consideration of the site for scheduling by the Monuments Protection Programme (MPP) of English Heritage.

The site at Westwoodside is located on the south-western limits of the Isle of Axholme (SE 74 00) on an outcrop of blown sand (aeolian reworking of either Humber Lake clay or Mercian mudstone: Robert Van de Noort pers comm) which is surrounded by peat.

METHOD

Given the limited amount of time available for the survey and the type of features likely to be encountered (ie ditches and pits) magnetometry was selected as the most suitable technique to employ.

Staff from the Humber Wetlands Project laid out a grid of 30m squares oriented to include the main archaeological find spots and extending across three separate arable fields (see Fig 1). Each of these grid squares was then surveyed using Geoscan FM36 fluxgate gradiometers. Measurements were recorded at 0.25m intervals along traverses 1.0m apart. The data thus collected was periodically down-loaded to a microcomputer in the field. In this report the presentation of this data has been enhanced by the application of a local median filter to reduce the intense response to ferrous material (Scollar et al 1990). Greyscales and a graphical traceplot of this data appear in Figures 2, 3 and 4 and an interpretation diagram is provided in Figure 5.

To complement the magnetometer survey, samples of topsoil were collected at 15m intervals along a central transect (see Fig 1) in order to measure their magnetic susceptibility (MS). MS is a natural attribute of soils which becomes artificially enhanced when these soils have been in contact with fire, as would be the case on an ancient occupation site. It is the enhanced MS of soils infilling archaeological features such as pits and ditches, that allows them to be detectable with a magnetometer. Relatively high MS values can thus, in their own right, sometimes be an indicator of former occupation and thereby act as a valuable aid to the interpretation of detailed magnetometer survey. In this instance, samples of approximately 100g in weight were collected. Having been dried in air for 48 hours, they were measured for MS in the laboratory using a Bartington MS1 meter and MS2B bench sensor.

RESULTS

Magnetometer Survey (see Figures 2, 3, 4 & 5)

In general, the magnetic response at the site can be seen to be very subdued. The majority of the readings recorded lie well within 1nT (see the frequency distribution histogram on Fig 3) which is close to the maximum sensitivity of the instrument. Nevertheless, the survey has succeeded in detecting some significant magnetic variations.

In contrast to the generally quiet response, one feature has been detected particularly clearly. This is the distinct linear anomaly running northwest-southeast through the western half of the survey area. Interestingly, the response to this feature, probably a ditch, varies markedly along its course and is particularly strong (up to 10nT) in grid square 13. The strength of this response is perhaps suggestive of the presence of some fairly intensive activity in this vicinity in the past, a proposal that is further supported by the results of the MS measurements (see below). Just to the northwest of this area of enhanced response, one or two further anomalies are evident in the data which do not form any distinct pattern (see Fig 5). The most obvious of these is a short linear anomaly (approximately 3m in length) which lies almost at right angles to the linear ditch, which it appears to intersect.

At the easternmost end of the survey area, an arrangement of subtle and amorphous anomalies has been detected which is of a roughly circular form (approximately 30m in diameter). Visualisation of these anomalies is impeded by the presence of a telegraph pole in the southern half of grid square 2 making an adequate interpretation difficult. Although an archaeological source cannot be excluded, these anomalies are perhaps more suggestive of natural features.

Throughout the surveyed area, the magnetometer has been affected by the presence of modern ferrous material, the majority of which is probably buried litter. This disturbance is at its strongest in grid squares 1 and 26 where strong magnetic interference has been created by a corrugated iron shed and supportive cabling for a telegraph pole respectively.

The magnetometer has also detected the effects of modern cultivation which is evident as a narrow parallelism in the data, mostly on a northwest-southeast alignment, throughout the westernmost field. Ploughing headlands can also be seen running along the western and eastern edges of this field. That these cultivation effects have been detected so clearly is slightly surprising given the generally low level of magnetic enhancement of the topsoil (see below).

Magnetic Susceptibility (see Figure 5)

The of results of the MS measurements yield an average value of 23 x10-8m3Kg-1. Whilst this is unremarkable, the readings do show a conspicuous peak towards the southeast of the westernmost field. This may well be significant, particularly as it overlaps with the area of increased magnetometer response to the linear feature (see above).

The MS readings generally fall below the average value towards the southeastern end of the traverse. Indeed, the lowest value was recorded over the amorphous feature mapped by the magnetometer (see above) perhaps lending weight to its interpretation as a geomorphological feature.

CONCLUSION

Despite the weak magnetic response over the site, some significant features have been detected. Of these, the most striking is a linear ditch bisecting the survey area. No obvious evidence of occupation was visible in the data, although a possible focus of former activity is suggested by a zone of increased magnetic response corresponding with an elevated level of topsoil MS.

Whilst magnetometry was clearly the most suitable technique to employ at this site, it is worth stressing that certain categories of archaeological features likely to be encountered here (such as post-holes, stake-holes and inhumations) will be too small or too poorly contrasted with their physical surroundings to be detectable. On sites such as this, where the background level of topsoil MS is generally low, more substantial features may also prove to be indistinguishable.

Archaeometry Branch
Ancient Monuments Laboratory

Reference

Scollar, I, Tabbagh, A, Hesse, A, and Herzog, I, 1990 Topics in Remote Sensing 2: Archaeological Prospecting and Remote Sensing, Cambridge.

List of figures

Figure 1 Location plan of survey (1:2500)

Figure 2 Greyscale of magnetometer survey overlain on location plan (1:2500)

Figure 3 Greyscale of magnetometer data (1:1000)

Figure 4 Traceplot of magnetometer data (1:1000)

Figure 5 Interpretation of magnetometer survey and results of MS measurements

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