TWO LONG BARROWS, Nr AVEBURY, Wiltshire.
Report on geophysical survey, December 2000.
Magnetometer surveys were conducted over two long barrows near Avebury, Wiltshire. The first, known as South Street long barrow (National Monument Number: 21735), is situated near the Longstones at Beckhampton. The second, the Horslip long barrow (National Monument Number: 21716), is located on the flank of Windmill Hill. The surveys were requested by the Avebury World Heritage Site Management Officer with the objective of correctly locating the monuments prior to their removal from cultivation. The surveys might also be expected to provide information on the character and physical survival of the monuments, and their immediate surroundings.
The South Street long barrow (SU 0902 6928) was mostly excavated during 1964-5 and 1966-67 (Ashbee et al 1979) and the Horslip long barrow (SU 0860 7052) in 1959 (ibid). Both lie on well drained calcareous clayey and silty soils of the Blewbury and Andover 1 associations (Soil Survey of England and Wales 1983). South Street is underlain by Middle Chalk (ibid; 250) whilst Horslip is over Lower Chalk (Institute of Geological Sciences 1974). Their respective scheduled areas had been left in 'set aside' to allow for the survey. Due to the orientation of the survey grids and indications from initial results it was necessary at certain points to extend the survey onto the adjacent winter wheat crop.
Magnetometer survey has been shown to be effective in the Avebury region in the detection of buried ditches and features associated with long barrows (e.g. Bray 1998), and was therefore the method of first choice on this occasion. The survey was conducted over all the numbered grid squares (Figure 1 and 3) using the standard method outlined in note 2 of Annex 1, but with a traverse interval of 0.5m. Plots of the South Street data-set are presented as both an X-Y traceplot and a linear greyscale, at a scale of 1:650 on Plan A. Plots of the Horslip data-set are presented as both an X-Y traceplot and a linear greyscale, at a scale of 1:750 on Plan B. 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; 492) to reduce the detrimental effects produced by surface iron objects. In addition the lower and upper values of the South Street data have been trimmed for presentation as a traceplot on Plan A. Linear greyscales of both data sets are supserimposed over the base OS maps in Figures 2 and 4.
South Street long barrow
A graphical summary of the anomalies discussed in the following text is provided on Plan C1.
The magnetic response has been severely affected by modern disturbance  along the boundary with Nash Road (formerly South Street). Other ferrous interference can be seen at , and at [3-4], the latter probably responses to debris in the excavation backfill of the long barrow ditch (see below). The linear positive magnetic anomaly  is almost certainly an artefact of the excavation, perhaps the infilling of a section (across ditch cuttings II and VII, section l-n'; Ashbee et al 1979; figs 23-4) deepened at the time to expose periglacial involutions. The direction of recent ploughing  has been recorded in the southern field.
The two barrow ditches have been located at  and . The anomaly  arising from the northern ditch (~ 2.5nT) is interrupted which may either indicate the presence of a causeway (but see Ashbee et al 1979; 257), or be an effect of the continuation of the excavated section . The magnetic response to both barrow ditches is not significantly higher than the background level (see traceplot - Plan A1).
Anomalies [9-11] have been located between the ditches, and within the excavated area but no corresponding features are apparent (ibid fig 23). Just to the north of , and outside the area excavated in 1966-67, is a positive magnetic anomaly , perhaps a pit. Other anomalies nearby have a more ferrous character and may therefore be more recent.
Also in this field are two separate negative linear anomalies  and  although it is unclear what relationship, if any, they have with the long barrow. The remaining anomalies recorded in the southern field are isolated linear responses similarly difficult to interpret.
Horslip long barrow
A graphical summary of the anomalies discussed in the following text is provided on Plan C2.
The background response here is more subdued (< ± 1nT) than at South Street, and, as the survey area did not coincide with field boundaries, there is significantly less magnetic disturbance. The notable exceptions are at  and , which may well relate to huts used in the 1959 excavations (Ashbee et al 1979; plate 28a); and at  and  which probably represent detritus in backfilling (in particular, in the ditch cuts across C and D; ibid; fig 2).
The broad bands of slightly raised magnetic readings at  and  relate to the long barrow ditches. To the north are about four discrete positive anomalies , arranged in an approximate semi-circle across the ends of the two ditches, outside the previously excavated area.
A well-defined linear positive magnetic anomaly , probably a ditch, running approximately NE-SW to the east of , was only partially captured by the survey.
Both surveys are characterised by a very weak and partial response to known features, notably the flanking ditches. The debris from the previous excavations is very evident in places.
At South Street the position of the western end of the outer ditches has been identified. However, the response from the eastern end of  has not been completely recorded as Nash Road has bisected the survey area. At the time of the excavations in the 1960's, South Street was recorded as being approximately 2.8m wide (Ashbee et al 1979; fig 23). At the time of this survey Nash Road and its verges span an area of approximately 9m. Additionally, the disturbance from the ferrous fences affects a strip up to 8m wide, thus impeding the location of the south-eastern ditch terminal.
Unsurprisingly there has been no response to the greatly reduced barrow mound at South Street, and no obviously significant features have been detected outside the barrow ditches (apart, possibly, from at ).
The barrow ditches have also been located at Horslip, with an equal if not more subdued response. The ditches are mainly defined by the disturbed magnetic readings at their terminals, presumably resulting from the intrusive excavations here. One explanation for the lack of significant response to the substantial ditches (between 2.4m and 3.6m below the 1959 ground surface; ibid; 214) is that their magnetically enhanced soils are too deeply buried for the magnetometer to record a stronger response. Around the northern ends of the two ditches, and outside the excavated area, are a number of positive magnetic anomalies that may be speculated to represent features contemporary with the barrow.
Overall, the survey has successfully provided confirmation of the exact location of the barrows, although additional information on character and preservation is very limited. Further survey using resistivity might provide some corroborative information, for instance where magnetic interference has been a problem.
Ashbee, P., Smith, I. F., Evans, J. G., 1979, Excavation of Three Long Barrows Near Avebury, Wiltshire. Proceedings of the Prehistoric Society 45 207-300.
Bray, E., 1998, Long Barrow 495c, Shepherds Shore, Wiltshire. Report on a Geophysical Survey, November 1997. Ancient Monuments Laboratory Report Series 3/98.
British Geological Survey, 1974, Marlborough, England and Wales, Sheet 266, 1:50,000.
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 5, South West England.
List of enclosed figures.
Figure 1 Location plan of survey grid squares for South Street long barrow over base OS map (1:2500).Figure 2 Linear greyscale of magnetometer data for South Street long barrow superimposed over base OS map (1:2500).
Figure 3 Location plan of survey grid squares for Horslip long barrow over base OS map (1:2500).Figure 4 Linear greyscale of magnetometer data for Horslip long barrow superimposed over base OS map (1:2500).
Plan A Traceplot and linear greyscale of magnetometer data for South Street long barrow (1:650).
Plan B Traceplot and linear greyscale of magnetometer data for Horslip long barrow (1:750).
Plan C Graphical summary of significant geophysical anomalies.