Report on geophysical survey, December 1998
Introduction
A geophysical survey of ~5.5ha was conducted over a series of crop marks (Fig 1) identified to the S of Copper Hall Farm in support of the ongoing Humber Wetlands archaeological survey. The site lies on a slight rise of improved pasture ~250m to W of the river Hull where previous excavations on the eastern bank revealed a well preserved Viking age structure believed to be the remains of a bridging point (Dent 1984). Given the location of the site above the immediate flood plain of the river it was thought that the crop marks may represent settlement activity, possibly associated with the river crossing. The intermediate field situated on the flood plain between the current site and the river Hull was not considered suitable for geophysical techniques but has been the subject of a detailed micro-topographic survey.
The aim of the survey was to investigate the geophysical response of the crop mark anomalies and determine, if possible, the extent of any settlement activity over the site.
The site (TA 062 546) lies on a raised terrace of glacial till deposits (British Geological Survey
1993) where soils of the Holderness association have developed as a thin layer upon the gravel
substrate (Soil Association of England and Wales 1993). At the time of the survey the field was
in pasture with an area of approximately 30m square in the centre of the grid unavailable for
survey due to the presence of a large manure heap.
Method
Magnetometer survey
Magnetic survey was considered the most appropriate technique to apply due to the area of land to be covered and the reasonable degree of success that has been achieved with the method over similar geological substrates in the region (eg Stephens 1995, Shiel 1995 and Noel 1996). The survey was conducted over all the numbered squares in Figure 1 using the standard method outlined in note 2 of Annex 1. The results are presented at a scale of 1:1250 in Plans A and B with a greytone image of the data superimposed over the base Ordnance Survey 1:2500 map in Figure 2. Plan A.1 shows a stacked trace plot of the raw data, the only correction to the measured values being to zero-mean each instrument traverse to remove heading errors. The greytone image of the data presented in Plan B.1 has been further enhanced to reduce the detrimental effects produced by surface iron objects through the application of a 2m by 2m thresholding median filter (Scollar et al 1990).
A graphical summary of significant anomalies discussed in the following text is provided in Plan B.2.
Topsoil magnetic susceptibility survey
Soil samples were recovered every 15m from the two orthogonal transects across the site as depicted on Figure 1. Measurements of magnetic susceptibility were made in the laboratory at two frequencies (460Hz and 4600Hz) with a Bartington MS2 AC susceptibility meter and MS2B sensor. The dry mass of each sample was determined after air-drying at room temperature. Figure 3 shows the variation of both mass specific and frequency dependence of magnetic susceptibility over the site (Thompson and Oldfield 1986).
Results
General response and modern interference
Some modern interference is evident at the edges of the survey due to the presence of ferrous stock fencing in the field boundaries (eg square 1). Additional intense responses throughout the survey area are related to agricultural implements, such as cattle feeders, but the detrimental effect of this disturbance is localised and has not hampered the identification of the otherwise quite subtle magnetic anomalies on the site. Due to inclement weather prior to the survey a number of badly rutted vehicle tracks had developed over the site and these have produced spurious magnetic anomalies [1] and [2].
Significant anomalies
Throughout the survey area a pattern of subtle linear anomalies [3] is evident running on an approximately EW alignment. These anomalies apparently represent a combination of a former plough pattern and a series of ceramic field drains sharing the same orientation. Certainly, to the N of the survey area a distinction can be made between a pattern of subtle negative responses due to ploughing and a series of slightly more intense anomalies of alternating polarity, such as those at [4]. This latter response suggests the presence of a thermoremanent magnetisation associated with the individual pipe sections of a ceramic field drain.
More significant ditch-type anomalies [5], [6] and [7] are evident to the S and apparently continue W beyond the area covered by the survey. The magnitude of response exhibited by these anomalies varies quite considerably along their course and hampers the identification of relationships between the individual ditch segments. A further series of more tentative linear anomalies [8] and [9] are visible to the E and are apparently bisected by two further ceramic field drains. Due to the subtle nature of [8] and [9] it is difficult to suggest any relationship between these responses and the other linear anomalies [5], [6] and [7].
A number of discrete pit-type responses are also distinguishable within the survey data including several quite intense-anomalies, [10]-[14], which are possibly accumulations of magnetically enhanced occupation related material.
Topsoil susceptibility survey
Results of the magnetic measurements from the two transects of topsoil samples crossing the site are presented as barcharts in Figure 3. Initial mass specific susceptibility (X) falls within a range between 25-80 10-8 [m3/kg] and displays no obvious correlation with the gradiometer data beyond a slight rise in X in the vicinity of anomalies [5], [6] and [7]. The frequency dependence of susceptibility (XFD%) is <4% for all samples which suggests the topsoil at this site contains only a low concentration of very fine grained magnetic material (<0.03µm) that is often associated with occupation activity (eg Thompson and Oldfield 1986, Cole et al 1995).
Conclusion
Magnetometer survey at this site has successfully identified a number of linear and pit-type anomalies which apparently extend to the W of the investigated area but do not, in themselves, suggest a focus to any settlement activity. Furthermore, correlation between the magnetometer data and the cropmark evidence is poor due to both an inaccessible area of the survey grid and the generally weak magnetic response. However, it is of interest to note the variation in magnitude of response demonstrated by a number of the linear anomalies which suggests that highly localised magnetic enhancement of the soil has occurred due to occupation activity (?burning) in the immediate vicinity of these ditches. The anomalies are not sufficiently well defined to suggest a tentative date based on their morphology or to support any conjecture relating this activity to the excavated bridging point.
Acknowledgments
The author wishes to express the thanks of English Heritage to the Smith family of Copper Hall farm for allowing access to their land. William Fletcher, Nick Duggan, Gavin Thomas and Henry Chapman of the Humber Wetlands Project are also thanked for their valuable field assistance during the survey and for providing location details for the survey grid.
References
British Geological Survey, 1993, Great Driffield, England and Wales Sheet 64, Solid and Drift Geology, 1:50,000, Provisional Series.
Cole, M. A., Linford, N. T., Payne, A. W. and Linford, P. K., 1995, Soil Magnetic Susceptibility Measurements and their Application to Archaeological Site Investigation, in Science and Site: Evaluation and Conservation (eds. J. Beavis and K. Barker), Proceedings of the Archaeological Sciences Conference 1993, Bournemouth University, September 1993.
Dent, J, 1984, Skerne, Current Archaeology, 91 , pp251-3.
Noel, M. J., 1996, Geophysical survey of site adjoining Annie Reed Road, Beverley, Geoquest Associates survey report.
Scollar, I., Tabbagh, A., Hesse, A. and Herzog, I. (eds.), 1990, Archaeological
Prospecting and Remote Sensing, Cambridge.
Shiel, D., 1995, Barton-upon-Humber, Geophysical Surveys of Bradford survey report, 95/118 .
Soil Survey of England and Wales, 1983, Soils of England and Wales, Sheet 1,
Northern England.
Stephens, C., 1995, Salthouse Road II Hull, Geophysical Surveys of Bradford survey report, 95/94.
Thompson, R. and Oldfield, F. (1986). Environmental magnetism. London: Allen and Unwin.
List of enclosed figures and plans:
Figure 1 Location plan of survey grid squares showing the transects of topsoil magnetic susceptibility samples (1:2500).
Figure 2 Greytone plot of raw data superimposed upon OS base map (1:2500).
Figure 3 Results from the topsoil magnetic susceptibility transect.
Plan A Traceplot of raw magnetometer data (1:1250).
Plan B Linear greytone of magnetometer data (B.1) after numerical processing to suppress near-surface ferrous responses together with (B.2) a graphical summary of significant anomalies discussed in the text (1:1250).
