MORNINGTON HOUSE FARM, GOSBERTON, LINCOLNSHIRE.

Report on Geophysical Survey, September 1992.

INTRODUCTION

Geophysical survey was undertaken at Mornington House Farm in response to a request from the Fenland Management Project (FMP). Fieldwalking had discovered a surface scatter of Saxon material, including 8 lava querns, and aerial photography indicated the presence of an enclosure near these finds. The latter suggested that the site might be wealthier than some contemporary sites in the locality. The aim of the survey was to attempt to map any buried archaeological features in advance of a programme of trial trenching.

The site is located on a major roddon at a point where it divides into three courses. This silt soil is generally at a higher elevation than the surrounding marine clay. The survey was centred on NGR TF 17 31.

Subsequent to the geophysical survey, two trenches were excavated by FMP (see Fig. 1) which confirmed the Saxon origins of the site and revealed numerous pits and ditches as well as the remains of a rectangular post-built structure, presumably a dwelling.

METHOD

A grid of 30m squares was established over the roddon running approximately N-S (see Fig. 1). Each of these squares was then surveyed using a Geoscan FM36 fluxgate gradiometer. Measurements were taken at 0.25m intervals along N-S traverses 1m apart within each 30m square. The resulting data is illustrated in this report using both greyscale and graphical trace plots (see Fig. 2).

A topsoil magnetic susceptibility (MS) survey was also carried out, using a Bartington MS2 meter and field probe. Measurements were made at 10m intervals along the N-S grid lines. During the subsequent FMP excavation, soil samples were retrieved from excavated archaeological features and from the 'natural' soil surrounding them. A number of topsoil samples were also collected from grid square 5 (see Fig. 1). The mass specific MS of these samples were then measured in the laboratory using a Bartington MS1 meter and MS2B bench sensor. Additionally, soil samples from one of the pits were heated in a furnace (following Cole et al 1995) in an attempt to achieve that maximum magnetic enhancement. Their MS was measured before and after heating in order to calculate their fractional conversion. The results of all of this work are presented in Table 1).

RESULTS

Magnetometer Survey

Disappointingly, only a very weak magnetic response was detectable. Indeed, analysis of the frequency distribution of the data (see Fig. 2) shows that approximately 90% of the readings recorded lie within 1nT. The majority of the readings, therefore, lie close to the sensitivity of the instrument. Despite this, the survey has detected some very weak magnetic variations which may have an archaeological relevance. Of these the most significant are shown in red on figure 2.

Magnetic Susceptibility

The field loop MS survey conducted over the survey area produced consistently low values (< 10 SI x10-8 m3 Kg-1). Indeed the dynamic range of the data collected was so small (all values falling in the range 5 - 10 SI x10-8 m3 Kg-1) that a plot of the data has not been included in this report.

In addition, all of the soil samples retrieved during the excavation produced very low values of MS (8 - 14 SI x10-8 m3 Kg-1 - see Table 1)). None of the features sampled demonstrated a discernable difference in MS between the soil removed from the archaeological contexts (pits, ditches etc) and the natural substrate into which they were cut. The topsoil samples also displayed similarly low values. Significantly, the results of the heating experiment indicate that the samples had only achieved a small fraction of their potential enhancement in situ (<6%) and emphasise the lack of contrast between topsoil, feature-fill and subsoil. The latter certainly helps to explain the poor response to the magnetometer.

CONCLUSION

The site has responded only very weakly to the magnetometer and the survey has failed to provide any substantial evidence of the Saxon settlement activity subsequently revealed by excavation. This poor response is demonstrably due to a lack of magnetic contrast between the buried archaeological features and the substrate that they are cut into. This in turn is presumably due to a paucity of iron oxides in the silt soils of the roddon available for magnetic enhancement by anthropogenic activity.

Archaeometry Branch
Ancient Monuments Laboratory

Reference

Cole, M A, Linford, N T, Payne, A W & Linford, P K 1995 Soil magnetic susceptibility measurements and their application to archaeological site investigation, in Beavis, J (ed), Science and Site: Evaluation and Conservation, Archaeological Sciences Conference 1993, London (in press).

List of Figures

Figure 1 Location plan of survey showing FMP trenches (1:2500)

Figure 2 Magnetometer data (1:1000)

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