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Report on Geophysical Survey, November 1997
INTRODUCTION
A geophysical survey was carried out over the long barrow 600m WSW of Shepherds Shore (Bishops Cannings, Wiltshire, NGR SU 03 66), in response to a proposal by the MPP for rescheduling the monument. The burial mound forms one of three features already scheduled as WILTS 49 95, national monument number 21896.
Long barrows represent the traditional barrow construction form in the earlier Neolithic period (RCHME 1979), and were usually oriented roughly E-W with flanking ditches on either side. If the barrow was trapezoidal (as opposed to rectangular) then human burials were often located at the broader eastward end in a structure of turf or timber, which was sometimes burnt. Alternatively, the end structure was sometimes constructed of stone or soil (Ashbee 1970).
This long barrow is aligned ENE-WSW on a false crest approximately 450m south of the Wansdyke on a clayey topsoil developed over Upper Cretaceous chalk. The surviving earthwork appears roughly oval in plan (35m long and 16m wide), with its original height much reduced by cultivation. Recorded as 27m long and 17m wide, partial excavation of the mound in 1860 and 1927 revealed a cremation deposit and four adult inhumation burials; the mound was found to be constructed of chalk rubble covering a mortuary area at the eastern end with a paving of oolite and sarsen blocks (Kinnes 1992).
During the current fieldwork the height of the existing earthwork was measured with an electronic distance-measuring device and found to be 0.42m.
Available information suggested that the burial mound was constructed without the flanking ditches common to long barrows (Kinnes 1992). The existing schedule had therefore only allowed a boundary of 2m around the visible archaeological features. It was proposed that the monument be rescheduled to include the same area as previously determined. The aim of the geophysical survey was therefore to confirm the presence or absence of flanking ditches in order to verify whether the current scheduled area was sufficient.
METHOD
Magnetometry was chosen as the main method of survey over the monument, since it is a proven method of locating buried ditches and pits (Clark 1996), and has produced useful results over chalkland longbarrows elsewhere (Payne 1998 in prep). Resistivity survey was used to try to obtain further information about the internal structure of the barrow as stone structures or compacted turf can produce anomalous resistivity values if prevailing soil moisture conditions allow.
A grid of 30m x 30m squares was set out over the visible structure, aligned with Grid North (Figure 1b).
Geoscan FM36 gradiometers were used to record magnetic data at 0.25m intervals along survey lines separated by 1m. The instrument traverses were walked in a N-S direction to ensure that the weakest features present would give the strongest possible magnetic anomalies. Once the position and extent of the archaeological features had been located with the magnetometers a grid of four squares was surveyed with a Geoscan RM15 resistivity meter. Maintaining a N-S traverse direction and 1m separation, resistivity data was recorded at 1m intervals using a twin-probe array with 0.5m mobile probe separation.
The results of the survey are illustrated in Figures 2, 3a, 3b and 4, with interpretative diagrams in Figures 3c and d.
Magnetic Data
RESULTS
The magnetic data is presented in Figure 3a after application of a despiking filter of radius 1m. The linear greyscale has limits of +/-1.5nT.
The lines of two ditches are immediately obvious in the magnetic data as positive anomalies (max 5 nT) oriented roughly ENE-WSW along either side of the main body of the long barrow. The northernmost ditch has an average width of 4m, whilst the southern ditch is tapered, its eastern end being 6m wide, narrowing to a width of 2m at the west.
The barrow mound is shown by an area of magnetic disturbance approximately 10m wide and 30m long, and roughly oval in shape corresponding with the central earthwork area as shown on the OS map (Figure 4a).
A linear anomaly approximately 1m wide, representing a ditch, runs from the NE of the survey area along the inside edge of the northern flanking ditch, curving around the western end of the barrow where it joins another similar anomaly along the south side of the mound. A third ditch anomaly of the same width runs NNW from the westernmost point of the others, curving northwards out of the survey area.
Several sub-parallel lines of very weakly positive data can be seen with a general NE-SW trend, separated by 10-14m. These are not obviously related to the other anomalies and may be related to former cultivation of unknown date.
Two small anomalies, probably representing infilled pits, are present in the westernmost survey grid (Figure 3a).
Resistivity Data
The raw resistivity data was convolved using a Gaussian filter of window radius 1m, then a Wallis filter was applied (radius 8m). The display parameters in Figure 3b are 18-26.5 Ohms.
The main flanking ditches of the barrow are not revealed by the resistivity data as effectively as by the magnetic data. The northern ditch is most clear as a line of low resistance, but the southern one is much less apparent. The smaller ditch around the west end of the barrow detected by the magnetometer is shown clearly as a narrow line of low resistance.
The main body of the barrow is shown as a sub-rectangular area of high resistance approximately 12m wide and at least 55m long, considerably exceeding the area of magnetic disturbance. An adjacent strip of slightly lower resistance to the south of this high resistance area may be representative of a spread of material from the barrow centre.
The dark line of the narrow ditch cuts across the extended high resistance area of the barrow, suggesting a later construction.
Discussion
The magnetic disturbance seen in the eastern end of the barrow interior may be partially due to the excavations of 1860 and 1927 but may also represent the remnants of burning associated with funerary process.
The lowest resistance seen over the northern flanking ditch may be associated with a shallow depression on that side of the earthwork retaining more groundwater than the immediate surrounds.
The 1m wide ditches probably represent a later phase of land use than that of the long barrow - this is indicated by both sets of data. The resistivity data shows the small ditches cutting across the main body of the barrow, and both methods reveal that these ditches respect the edge of the northern flanking ditch and the western edge of the visible earthwork. Thus these narrow ditches may have been dug in a position influenced by the remaining earthwork of the barrow, but after the flanking ditches had been silted up. Linear ditches dating from the Late Bronze Age are a familiar feature of the southern English chalkland and their positioning seems to have been influenced by local topography and upstanding monuments such as barrows (Bradley et al 1994). It is therefore possible that the narrow ditches located with the magnetometer are part of such a linear ditch system, and the barrow was used as a landmark on which to sight them (Bradley et al 1994; RCHME 1979).
It appears that the outer edges of the flanking ditches shown by the magnetometer survey occur about 5m (northern ditch) and up to 13m (southern ditch) outside the outer edge of the barrow marked on the OS map. The high resistance area seen in the resistivity data is at least 55m long, and therefore at least 20m longer than the current recorded barrow length.
CONCLUSIONS
The aim of this geophysical survey was to determine whether flanking ditches were present in association with long barrow 495c in order to assess the validity of the currently scheduled area. The magnetic survey has most effectively demonstrated the presence of two wide linear ditches running either side of the mound. The resistivity data has subsequently shown that the total area of the long barrow seems to be larger than previously believed. It is therefore probable that the extent of scheduling for this monument should be reviewed.
E. Bray
A. Payne
E. Bray Date of Report: 30/1/1998
Archaeometry Branch
Ancient Monuments Laboratory
English Heritage
References
Ashbee, P., 1970. The Earthen Long Barrow in Britain, J.M. Dent & Sons, London.
Bradley, R., Entwistle, R., Raymond, F., 1994. Prehistoric Land Divisions on Salisbury Plain. The Work of the Linear Ditches Project, English Heritage, London.
Clark, A. J., 1996. Seeing Beneath The Soil, Batsford, London.
Kinnes, I., 1992. Non-Megalithic Long Barrows and Allied Structures in the British Neolithic, British Museum Occasional Paper No. 52, British Museum, London.
Payne, A., 1998 in prep. White Barrow, Tilshead, Wiltshire. Report on Geophysical Survey May 1997. Ancient Monuments Laboratory Report Series.
RCHME, 1979. Long Barrows in Hampshire and the Isle of Wight, Crown Copyright.
List of Figures
Figure 1 a) Location of long barrow
b) Location of survey grids over long barrow
Figure 2 a) Raw magnetometer data
b) Raw resistivity data
Figure 3 a) Processed magnetometer data
b) Processed resistivity data
c) Interpretation of magnetometer data
d) Interpretation of resistivity data
Figure 4 Location of data over long barrow earthwork
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