BECKHAMPTON, Nr AVEBURY, WILTS.
Report on geophysical survey, May 1999
This report describes results of magnetometer and resistivity surveys undertaken in Longstones Field, Beckhampton, near Avebury, Wiltshire, during May 1999, as well as results of an earlier magnetometer survey undertaken in 1989. The aim of the more recent survey was to provide information on the character and location of possible archaeological features in advance of excavations due for August 1999.
In the south-western corner of Longstones Field are two standing stones, Adam and Eve, whose documented history only begins certainly with the field observations of William Stukeley in 1724 (Ucko et al., 190). Despite some limited excavation in 1912 (Cunnington 1914), their subsequent interpretation has been a matter of extended debate, following Stukeley's unconfirmed proposal that they were once part of a stone setting or `cove' associated with an `Avenue' leading westwards from Avebury henge (Burl 1979, 1988). A reassessment of the documentary evidence, as well as geophysical surveys in 1975 and 1989, was unable to resolve the nature of the settings and, in particular, whether or not the Beckhampton Avenue had once been a reality (Ucko et al 1990).
To compound the complexity of the issue, aerial photographs taken by the RCHME in 1997 indicated the presence of a large oval enclosure (approximately 154m x 108m) which includes Eve within its southern perimeter. Although not observed on previous aerial photographs of Longstones Field (eg Crawford and Keiller 1928), this cropmark has provoked comparison with the palisade enclosures at West Kennet (Whittle 1997).
Previous geophysical survey
Both magnetic and resistivity surveys were undertaken by the AML in the vicinity of the Longstones in 1989 and the resistivity results were reported in Ucko et al (1990, 196-199). At that time the magnetometer survey was thought not to have yielded significant results; however, following the identification of the oval cropmark, the original field data was re-plotted and examined afresh in 1997 - successfully revealing a very faint and intermittent linear anomaly believed to correspond with the cropmark. The opportunity is taken to report the 1989 magnetometer survey in more detail below.
The 1989 resistivity data (Figure 2) was dominated by the response to former ridge and furrow cultivation and a trackway at the south-western edge of the field (ibid. Plate 63). Evidence for previous sarsen settings was very tentative, limited to only four possible examples on a conjectured route of the avenue.
The 1999 geophysical survey
The need for further geophysical survey arose as a necessary component of a new fieldwork project initiated by Mark Gillings (Leicester University) and Josh Pollard (University of Wales College, Newport). The aims of this new work include the determination, through survey and excavation, of the character and date of the cropmark enclosure, and the final determination of the existence, or otherwise, of the Beckhampton Avenue (Pollard 1998).
Geophysical survey was requested as a means of re-defining the exact position of the cropmark on the ground and if possible to find any associated features within the enclosure. Such survey was also needed for the re-location and, if possible, for the refined definition of anomalies interpreted in 1990 as possible avenue stone settings: `three weak negative anomalies disposed at the corners of a suitably oriented right-angled triangle. If an avenue indeed exists, then these anomalies, albeit very faint, might be taken to be suggestive of two stone positions on the N side of the alignment and a single one on the opposite side' (Ucko et al, 198, Plate 63, grid square 8C). Re-definition of these anomalies was necessary in advance of the proposed area excavation.
The geophysical surveys reported below were intended specifically to examine the three resistivity anomalies described above and, if possible, to locate the adjacent part of the cropmark. More extensive survey may be necessary in subsequent phases of the project.
Ground conditions for the May surveys were good - moist but not sticky - and the field was planted with a cereal crop still only a few centimetres high. The soils are calcareous and silty, of the Andover 1 Association (Soil Survey of England and Wales, 1983, Sheet 6) developed over Lower Chalk (Institute of Geological Sciences 1974).
A survey area measuring 60m x 60m was centred over the known location of the three 1989 resistivity anomalies (SU 0897 6939: see Figure 1) with the intention of fully accommodating a proposed excavation area of 50m x 50m. The survey area was divided into quadrants each of which was surveyed by magnetometry and resistivity during 19th and 20th May. Previous experience at the site (see above), and elsewhere in the Avebury area, has demonstrated the value of these methods in detecting both former stone settings and palisade ditches; magnetometer survey might, in addition, locate other features such as pits, hearths and large post-holes.
Resistivity measurements were made with a Geoscan RM15 meter, MPX15 multiplexer and PA5 probe array, using the Twin Electrode probe configuration. Readings were taken at 0.5m intervals with mobile probe spacings of 0.5m and 1.0m. The data for both mobile probe spacings is presented in Figure 3a-d: 3a and 3c have been `de-spiked' (to remove single anomalous responses) and 3b and 3d have been de-corrugated using a directional cosine filter.
Measurements of the gradient of the local magnetic field were made with Geoscan FM36 fluxgate gradiometers using a sample interval of 0.25m along traverses spaced at 0.5m intervals apart. The instruments were deliberately held at a low level with the lowermost sensor within 0.1m of the ground surface to maximise the detection of weakly magnetic features. The resulting raw data is illustrated in trace format in Figure 4a; a linear greyscale of the data following low-pass filtering is shown in Figure 4b, and an interpretation in Figure 4c.
The 1989 magnetometer survey data was captured with a Geoscan FM36; readings were logged at 0.25m intervals along traverses 1.0m apart. The data is illustrated here as both greyscale and trace plots (Figure 5a-b).
The plots of the resistivity data (Figure 3a and 3b) are characterised by narrowly spaced linear anomalies which cross the entire survey area and correspond exactly with similar NNW-SSE alignments detected in 1989. These are clearly the effects of cultivation and are probably recent; the field is presently cultivated on this alignment. The effects are visible in both the 0.5m and the 1.0m datasets. The 1989 data, which covers a much wider area (approx 3.2 ha), shows broader linear anomalies on the same alignment, and elsewhere at right angles to it, suggestive of earlier cultivation regimes, perhaps ridge and furrow.
In addition to such traces of cultivation in the 1999 data, but at an angle to them, are two or three weakly defined and broad positive linear anomalies (approx 3m across: see Figure 3e). These might represent the vestiges of former banks, but their explanation as either anthropogenic or natural features is uncertain. Similar anomalies, equally difficult to understand (and of perhaps limited significance) are apparent elsewhere in the field (Figure 2; Ucko et al 1990, Plate 63).
Otherwise the most prominent features of the recent dataset are the three central negative anomalies, anticipated by the previous survey, and in addition, a single very distinct positive anomaly situated at the fourth corner of the rectangle so formed. All four anomalies have similar shapes and dimensions, being roughly circular and with diameters of about 2.5m-3.0m. The anomalies are at their most pronounced in the data generated from the 0.5m mobile probe spacing; all three of the negative anomalies diminish in the 1.0m dataset, one vanishing altogether. These three anomalies may therefore be caused by relatively shallow features - pits, for example, at a depth of approximately 0.5, not far below ploughing levels. The positive resistivity anomaly is prominent in both datasets and, if a buried sarsen, could be deeper than the other features; however, the effect of a highly resistive body at a shallow depth might be expected to persist in data from the 1.0m spacing as well. It is possible, therefore that all four anomalies represent features at approximately the same depth, not very far below the ploughed horizon.
Magnetically, the response within the 1999 survey area is mostly undistinguished, except by many reactions to scattered iron objects amidst a soil background `noise' of +0.5nT. Four anomalies stand out as larger and stronger than any others (Figure 4c); three are characteristic of buried iron debris and, as they have no corresponding resistivity anomalies, are unlikely to be of archaeological significance. The fourth anomaly is composed of a distinct group of more subdued and predominantly positive responses extending over an oval area of approximately 6.0m x 3.5m overlying one of the three negative resistivity anomalies referred to above. Whilst modern ferrous debris may be present here, it is also a possibility that this cluster of anomalies is a response to fragments of burnt sarsen - possibly associated with a large pit. A fifth anomaly, although much weaker and therefore even less certainly of archaeological origin, overlies the most northerly of the four resistivity anomalies.
Apart from the anomalies referred to above there is very little secure evidence for linear features, such as the line of the cropmark enclosure, within the survey area. The dashed lines marked on the interpretation plan (Figure 4c) are tentative and indicate anomalies that barely exceed background magnetic variation. Further discussion of the magnetic evidence is best taken up in the light of the 1989 survey data.
The 1989 magnetometer survey
The plots of the data collected in 1989 are shown in Figure 5a-b which includes a tentative interpretation (5c) and the relative location of the smaller 1999 survey area (5a and 5c).
The general character of the magnetic response over the wider area covered in 1989 is the same as that of the 1999 survey with one significant exception: over half of the cropmark circuit has been intermittently detected. The anomaly is rendered visible more on account of its continuity than on its magnetic strength alone (less than 0.5nT) which can be matched widely, but less systematically, across the entire survey area. There are indeed many other instances in the data where anomalies of similar strength are present, but these lack extended continuity or patterning; some of the more credible of these have been cautiously indicated on Figure 5c. Such interpretations as the latter, based on data at or below the threshold of natural background variation, result as much from an individual's wishful and partial pattern recognition as from any comprehensive and objective image analysis.
The cropmark anomaly has only been detected over a combined distance of about 75m; elsewhere it fades from the image entirely. Most notably it is almost completely absent within the area surveyed in 1999 where the more sensitive and highly resolved survey was not able to detect it with certainty. A very weak linear anomaly was detected in 1999 extending about 5m into the 1999 survey area from the south (Figure 4c) in about the right position to correlate with the projected 1989 anomaly; however, the two anomalies (1989 and 1999) are displaced from one another by about 3m - either a mis-match between the two survey grids or an example of the fickleness of `tentative' interpretation in these soil conditions. Whatever the case, it seems that the cropmark feature is either too slight to be detected over a length of about 30m, or is entirely absent over this distance. One possible scenario is that ploughing and erosion have removed the upper and more magnetic levels of the feature, although allowing its less magnetic base to survive. However, the cropmark is also unclear or absent in this area.
Where the circuit of the cropmark has been detected the anomaly is quite distinctly intermittent in places, with apparent gaps of various widths (up to about 3m across), most obviously in the northern arc. Such gaps may represent the original pattern of construction or may result either from differential preservation or the contrasting magnetic susceptibility of different fillings along the course of the feature.
The 1989 and 1999 magnetic datasets reveal different distributions of responses to iron objects in the soil, as would be expected following an interlude of 10 years of cultivation. It is important to note that anomalies detected in 1999 and attributed to iron debris, and perhaps burnt sarsen associated with a pit, are not replicated in the earlier dataset. Any archaeological significance placed on these anomalies must therefore be highly circumspect.
Apart from the cropmark circuit, the identification of other features in the 1989 survey area is difficult - as explained above with respect to other possible linear features. There is no apparent contrast in magnetic activity between the interior and exterior of the cropmark enclosure, for instance. There are no anomalies which can be certainly ascribed to pits; three possible candidates have been indicated on Figure 5c but the same caveats as above apply; the northernmost example, for instance, was not detected in 1999 and may therefore have been a subdued response to a mobile iron object.
Both the magnetic and the resistivity results outlined above have, respectively, recovered very significant information. The magnetic data reveals the perimeter of most of the cropmark enclosure and shows the circuit to be a discontinuous feature about 1m wide. Whilst wider gaps may be a result of truncation it seems that narrower gaps (0.5m-3m) could perhaps be part of the original design. The data is otherwise not distinguished by anomalies that are unequivocally of archaeological significance. The general magnetic response is very subdued and the more tentative patterning indicated may have arisen through the random coincidence of natural background variations. Other anomalies appear to be ephemeral.
The resistivity data collected in 1999 confirms the presence of isolated negative anomalies detected in 1989 and then conjectured to be former stone positions. A fourth (positive) anomaly was also detected in both datasets, but much more emphatically in 1999. Together, these four anomalies fall at the corners of an approximate rectangle of sides (approx 15m x 23m) roughly similar to the equivalent distances between stone positions on the West Kennet Avenue (approx 17m x 24m). Extending this patterning, there is certainly a temptation to include a pair of isolated high resistivity anomalies present in the 1989 data (but excluded from the earlier interpretation: Ucko et al. 1990, Plate 63), which might perhaps represent a further pair of stone positions (indicated by question marks on Figure 2b).
Taken with the fact that the north-western pair of 1999 resistivity anomalies align with Adam and Eve, it is difficult to resist the speculation that Stukeley's avenue may be a reality (Figure 2; Ucko et al 1990, 198). A problem with this interpretation (ibid. 198) is that three of the critical anomalies are of low rather than high resistance; buried stone positions on the West Kennet Avenue have been shown conclusively to present high resistance anomalies only (ibid. 189). Perhaps local soil conditions at Beckhampton inhibit the loss of soil moisture from stone burial or burning pits, or perhaps buried stones lying flat are retaining moisture on their upper surfaces. The reversed anomalies do raise suspicions, however, and clearly declare a need for further investigation by probing and/or ground penetrating radar; the planned excavation should then settle the matter.
Burl, A., 1979, Prehistoric Avebury, Newhaven, London
Burl, A., 1988, Coves: structural enigmas of the Neolithic, Wiltshire Archaeological and Natural History Magazine 82, 1-18.
Crawford, O. G. S. and Keiller, A., 1928, Wessex from the Air, Oxford.
Cunnington, M. E., 1914, The re-erection of two fallen stones and discovery of an interment with drinking cup, at Avebury, Wiltshire Archaeological Magazine 38, 1-11.
Institute of Geological Sciences 1974, Geological Survey of Great Britain (England and Wales), Marlborough, Sheet 266.
Pollard, J., 1998, Fieldwork and excavation at Beckhampton, near Avebury, Wiltshire: an application for funding. Unpublished.
Whittle, A., 1997, Sacred Mound Holy Rings, Oxbow Monograph 74, Oxford.
Ucko, P. J, Hunter, M., Clark, A. J. and David, A., 1990, Avebury Reconsidered, Unwin Hyman, London.
List of enclosed figures
Figure 1 Location plan of 1989 and 1999 survey grids superimposed upon base map (1:2500).
Figure 2 Greyscale plot of 1989 resistivity data and interpretation (1:1250)
Figure 3 Greyscale plots of 1999 resistivity data for both mobile probe spacings, before and after de-corrugation and a summary of significant anomalies (1:750).
Figure 4 Traceplot and greyscale of 1999 magnetometer data and a summary of signifcant anomalies (1:500).
Figure 5 Traceplot and greyscale of 1989 magnetometer data and a summary of signifcant anomalies (1:1000).