Report on geophysical survey, December 1999
Introduction
A geophysical survey of approximately 4.7ha was conducted over two sites in Marshchapel, Lincolnshire, in support of the ongoing Humber Wetlands archaeological survey. The first area of investigation was undertaken in a field locally known as 'Burnt Mound' (TF 35 98), from which a large amount of 12th - 13th-century AD pottery and building material has been recovered (Van de Noort, pers comm.). The second area (TF 35 97), some 500m to the south, was believed to be a Roman settlement, with surface finds of pottery, roof and hypocaust tiles (Van de Noort, pers comm.). Both sites were also believed to have been used for the salt making industry, possibly prior to more formal occupation.
The aim of the survey program was to assist the Humber Wetlands project in determining the extent of occupation and industry on these sites, and so to highlight areas for investigative excavation.
Both sites lie on stoneless clayey soils of the Newchurch 2 association (Soil Survey of England and Wales 1983) developed over a drift geology of marine and estuarine alluvium with an underlying solid geology of Burnham Chalk (Institute of Geological Sciences 1990). At the time of the survey the Burnt Mound area was in uncultivated 'set-aside' ground but the second site (Marshchapel) was ploughed and planted with a cereal crop.
Method
Magnetometer survey
Magnetic survey (see Annex 1, note 2) was chosen both because of its rapidity of coverage and because of the favourable magnetic response previously noted on similar sites in the Humber catchment area (Cole and Cottrell 1999). Both a traceplot and linear greyscale of the raw data for each site have been 'de-spiked' (using a median filter to remove single anomalous readings created by ferrous objects); 'un-bunched' (corrected to zero the median of each instrument traverse to remove heading errors); and 'de-staggered' (to remove offsets between adjacent traverses caused by heading error). Plan A depicts the data for Burnt Mound, at a scale of 1:750. Plan B represents the data for Marshchapel at a scale of 1:1250.
Magnetic Susceptibility
Soil samples were collected from each site at 30 metre intervals along a central traverse, orientated along grid square edges (Figure 1). The results are presented as bar charts showing the variation of topsoil mass specific magnetic susceptibility and superimposed upon the location plans in Figure 1.
Results
Burnt Mound
A graphical summary of all significant anomalies discussed in the following text is provided in Plan C.
Magnetometry
The direction of modern ploughing (visible in part on the surface of the field) can be seen over most of the plot but is most noticeable at [1]. Other remnants of agricultural regimes are a possible field system [2], where a series of linear anomalies orientated in a north-south direction can be seen.
The magnetic data is dominated by several linear highly positive magnetic anomalies and clusters of thermoremanent responses.
The thermoremanent features at [3]-[6] have very intense magnetic signatures (~ 75 nT). [4] and [5] are both adjacent to less intense pit-type anomalies. The group as a whole may be related to the adjacent enclosure [7], which possibly also incorporates the line of anomalies at [8]. West of features [3]-[6] at [9] are some less intense linear anomalies suggestive of an associated rectilinear structure. Other anomalies are apparent to the east at [10] and [11] and, to the north is a ditch [12], and at least two pits [13] and [14].
The positive magnetic response at [15] may form part of an enclosure, respecting the corner of the current field system and including the more intense response [16]. Adjacent to [15] are two intense thermoremanent responses [17] and [18], each with a neighbouring pit-type anomaly. Nearby, at [19], some less intense amorphous anomalies might represent associated activity. Just outside and to the north of [15] are two more pit like responses, [20].
Across the north-western edge of the survey grids is an area of more subdued positive magnetic anomalies [21], some of which form linear patterns. Overall, however, these responses are too indistinct to define clearly any specific features.
Magnetic Susceptibility.
Values for mass specific susceptibility (X) are high, falling within the range of 43-176 × 10-8 [m3kg-1] (Figure 1). As might be expected, the peak value falls close to [4], and there is a fall-off to the east where the gradiometer readings are more subdued.
Marshchapel
A graphical summary of all significant anomalies discussed in the following text is provided in Plan D.
Magnetometry
In contrast to the Burnt Mound field, the magnetic response over Marshchapel is generally more subdued although a wealth of anomalies have been detected. The weaker of these, which underlie most of the survey area are probably geomorphological in origin, but superimposed on this background are localised areas of archaeological activity. Along the north-west part of the survey area a linear zone of relative magnetic inactivity [1] may correspond with the edge of a palaeo-channel.
The archaeologically significant anomalies on this site appear to be grouped into four main areas spanning the field from the south-west to the north-east. There are, however, some more isolated anomalies such as the three pit-type responses at [2] and the more intense anomaly [3].
In the south-west of the survey grids lies an area of increased magnetic response, with the majority of intense readings around [4]. These anomalies mainly consist of pit-type responses and linear features. South of these, at [5] is a more subdued, but nevertheless positive area of readings.
A distinct negative linear anomaly [6] extends from [4] towards the next concentration of significant readings at [7]. The latter includes some suggestion of a rectilinear patterning but no particular structural shapes are apparent.
To the north-east of [7] is another group of intense positive readings [8]. There seem to be less pit-type anomalies at [8] than at [4]. An apparent reduction in magnetic activity within [8], identifying an approximately rectangular central area, may be significant.
To the north of [8] is a further concentration of positive magnetic readings [9] which are noticeably less intense than those recorded over [4], [7] and [8]. It is possible that [9] represents activity taking place on the bank of the palaeo-channel.
Magnetic Susceptibility.
The recorded values for mass specific susceptibility (X) fall within a range of 11-23 × 10-8 [m3kg-1], and are very markedly lower than those at Burnt Mound (see above). As at the latter site, however, the peak values coincide with the more intense concentrations of magnetic activity detected by the magnetometer.
Conclusions
The soils in the Marshchapel area have proved suitable for magnetic survey, with archaeological remains being detected clearly at both sites.
The high magnetic susceptibility readings at Burnt Mound suggest that activity involving burning was taking place, as indicated by the field name, and the intense magnetic anomalies here may well be the remains of industrial structures. There is evidence for additional features and enclosures but it is not possible to ascertain whether or not these are indicative of permanent occupation of the site.
On the main Marshchapel field the palaeo-channel has been detected as an absence of the natural patterning of anomalies visible over much of the field. To one side of the channel is evidence for a series of four distinct clusters of activity arranged, in part, along what is possibly a north-south trackway [6]. Magnetic susceptibility readings were lower in this field than at Burnt Mound, possibly suggesting settlement rather than industrial activity on this site. In support of this the gradiometer readings were also less intense than those recorded in the Burnt Mound field. Another explanation, however, may be the possibility of a deeper overburden in this field, reducing the peak anomaly strength. The response here closely resembles that encountered at Adlingfleet during surveys in 1996 and 1998 (Cole and Cottrell 1999).
Acknowledgements
The author wishes to express the thanks of English Heritage to the Humber Wetlands Project team, in particular to William Fletcher and Gavin Thomas.
References
British Geological Survey, 1990 Grimsby and Saltfleet, England and Wales, Sheet 90-91, Solid and Drift Geology, 1:50,000.
Soil Survey of England and Wales, 1983, Soils of England and Wales, Sheet 4, Eastern England.
List of enclosed figures and plans:
Figure 1 Location plan of survey grid squares over base OS map with results from the topsoil magnetic susceptibility transects (1:2500).
Plan A Traceplot and linear greyscale of magnetometer data from Burnt Mound (1:750).
Plan B Traceplot and linear greyscale of magnetometer data from Marshchapel (1:1250).
Plan C Graphical summary of significant anomalies from Burnt Mound (1:750).
Plan D Graphical summary of significant anomalies from Marshchapel (1:1250).
