Report on geophysical survey, December 1999
Introduction
A geophysical survey of approximately 2.6 ha was conducted over the site of Butterbump Round Barrows, Lincolnshire (SAM No. LI296) in support of the ongoing Humber Wetlands archaeological survey. Previous fieldwork at the site by Mr Barry Beeby included excavations of the most south-easterly of the mapped barrows which produced, among other finds, a cremation burial. Surrounding the barrow was an "irregular circle of turves" and nearby a perforated whetstone and bronze dagger with wooden sheath were found (Greig 1982, 11).
The aim of the survey was to investigate the geophysical response to the barrows recorded by the Ordnance Survey and determine whether it would be feasible to identify other barrows. The results of the survey were due to be corroborated through limited excavation by the Humber Wetlands Project team in January 2000.
The site (TF 49 72) lies on fine loamy soils of the Holderness association (Soil Survey of England and Wales 1983) developed over glaciofluvial deposits of sand and gravel with some peat (Institute of Geological Sciences 1996). At the time of the survey the field containing grid squares 1-4 (see Figure 1) was ploughed and the field containing the remaining grid squares was under grass.
Method
Magnetometer survey
Magnetometry was considered the most appropriate geophysical technique to use, taking into account the relatively large size of the area to evaluate within the limited time available. The survey was conducted over all the numbered grid squares (Figure 1) using the standard method outlined in note 2 of Annex 1. To compare the geophysical anomalies with the mapped features, a greyscale plot of the data (after processing as described below) has been superimposed over the base OS map in Figure 2, at a scale of 1:2500. Plots of the dataset are additionally presented as both an XY traceplot and a linear greyscale, at a scale of 1:1250 in Plan A. The only corrections made to the measured values displayed in the plots were to zero-mean each instrument traverse to remove heading errors and to ‘despike' the data through the application of a 2m by 2m thresholding median filter (Scollar et al 1990) to reduce the detrimental effects produced by surface iron objects. In addition the traceplot has had the more extreme upper and lower values trimmed for presentation purposes.
Topsoil magnetic susceptibility survey
Topsoil samples were collected at 15m intervals along two traverses, one in each field, orientated along grid square edges approximately central to the survey areas, in an east-west direction. The first traverse ran between the squares in the ploughed field (see Figure 1; W1 E1) and the other between the grid squares in the main field (see Figure 1; W2 E2). Measurements of mass specific magnetic susceptibility were made in the laboratory at two frequencies (420Hz and 4200Hz) with a Bartington MS2 AC susceptibility meter and MS2B sensor. The results are presented as bar charts on Figure 1.
Results
The magnetic response from the site is in general rather subdued except where modern ferrous material occurs. Background variation rarely exceeds + 1 nanotesla (nT), but several archaeological features have been detected within this range. A graphical summary of all significant anomalies discussed in the following text is provided in Plan B.
There is a large self-contained area of extreme disturbance [1] in the north-eastern corner of the larger field (grid square 15) central to one of the known barrow locations. The identity of this barrow seems to be confirmed by the negative curvilinear anomaly [2] which is assumed to represent part of its encircling ditch. The central disturbance [1] is composed predominantly of positive magnetic anomalies (max 200 nT; average ~35 nT) with a halo of negative deflections. This unusual pattern suggests either burnt features and/or some ferrous materials. The cause is likely to be relatively modern, such as the remains of an excavation, or a structure, but a prehistoric origin cannot be altogether ruled out. Other anomalies that may represent modern disturbance of the site can also be seen at [3], [4], [5] and [6].
Another known barrow is indicated at grid square 19, this time by a broader and more diffuse negative anomaly (~ -1nT) corresponding with its ditch [7]. Within this, but slightly off centre is a roughly circular area of subdued but distinct positive and negative readings [8], some 25 sq m in extent, which is again difficult to interpret definitively as of ancient or modern derivation. The scatter of smaller anomalies at [9] over the south-western boundary of the barrow is more probably attributable to recent ferrous material. To the south of the barrow is a negative, slightly angled, linear anomaly [10] that may represent an infilled ditch.
A third barrow is indicated by the negative anomaly [11]. An isolated negative anomaly at [12] may be significant on account of it position central to the barrow, although this may simply be a fortuitous natural variation. The two short alignments of negative anomalies indicated at [13], if responses to genuine features, are also not possible to interpret.
Rather more distinct are the two pit-type (positive) anomalies at [14] in grid square 12, which can be seen to fall within the mapped boundary of another barrow (Figure 2) which is not otherwise defined by an encircling ditch anomaly. Other pit-type responses can be seen at [15] and [16], but these also seem to be remote from any other associated anomalies or recorded barrows. A clear negative anomaly, perhaps also a pit, is visible at [17].
The position or existence of the barrow mapped near the western edge of the larger field (centred in grid square 6) cannot be confirmed by the magnetometer survey. This area is characterised by a generalised scatter of ferrous responses amongst which it is not possible to distinguish archaeologically significant anomalies; however, there is no sign of any curvilinear negative anomaly to compare with the ring ditches detected elsewhere. A possible but very tentative alignment of weak positive readings is indicated at [18].
The south-western edge of the larger field is marked by a broad band of positive magnetic readings [19], with an apparently segmented negative linear anomaly [20]. The latter is of unknown origin, whilst the broader positive anomaly is comparable to those noted elsewhere to be responses to palaeochannels (Cole, 1995; Linford 1994, 40-41).
In grid squares 1-4 in the arable field to the north, a subtle curvilinear negative anomaly [21] has been detected and a negative linear anomaly at [22] may also be significant. A more intense positive magnetic linear anomaly [23] (~ 2.5nT) passes close to the mature tree here (indicated on the plot by blank readings) and may well be linked to the modern building debris noted in this area.
As might be anticipated from the magnetometer data, the values for topsoil mass specific MS (X) are rather low. Variation between the values for the northern field (10-27 × 10-8 [m3kg-1]) and those from the grassed field (7-14 × 10-8 [m3kg-1]), are likely to reflect the differing agricultural regime between the two areas.
Conclusion
Despite a generally weak magnetic response the survey has successfully identified the position of three of the recorded barrows at Butterbump. Very unusually, these each have ring ditches defined by negative rather than positive magnetic anomalies, a phenomenon which suggests that the ditches are infilled with a material of exceptionally low magnetic susceptibility, perhaps peat. Less credibly the anomalies may be caused by walls, rather than ditches, or might somehow be responses to circles of turves like the one reported by Greig (1982). At least two other linear magnetic anomalies, [10] and [21], also seem best interpreted as ditches and, given that negative anomalies are thus evidently significant at this site, it may well be that more isolated examples, such as [12] and [17] are archaeological features too.
At least two of the detected barrows have significant anomalies near their centres, although the nature of the features responsible, whether ancient or modern (such as former excavation trenches), is not clear.
Geophysical evidence for the remaining four mapped barrows is lacking, although two pit-like anomalies [14] coincide with the location of one of them. Unfortunately this negative evidence does not prove that barrows are absent, particularly if they are not confined within ring ditches. Although the survey was not as extensive as desirable to prospect more widely for additional barrows it seems unlikely, given the partial response recorded above, that additional survey would have been as definitive as required.
References
British Geological Survey, 1996 (provisional series), Skegness, England and Wales, Sheet 116, Solid and Drift Geology, 1:50,000.
Greig, J.R.A. 1982, Forest Clearance and the Barrow Builders at Butterbump, Lincolnshire. Lincolnshire History and Archaeology Vol. 17 , 11-14.
Linford, N. 1994, Mineral Magnetic Profiling of Archaeological Sediments. Archaeological Prospection 1 , 37-52.
Scollar, I. Tabbagh, A. Hesse, A. and Herzog, I. (eds.), 1990, Archaeological Prospecting and Remote Sensing. Cambridge.
Soil Survey of England and Wales, 1983, Soils of England and Wales, Sheet 4, Eastern England.
List of enclosed figures.
Figure 1 Location plan of survey grid squares over base OS map with results from the topsoil magnetic susceptibility transects (1:2500).
Figure 2 Greyscale plot of processed magnetometer data over base OS map (1:2500).
Plan A Traceplot and linear greyscale of magnetometer data (1:1250).
Plan B Graphical summary of significant geophysical anomalies.
