Geophysical surveys of three pond barrows in the Lake Down Barrow Group, near Wilsford, Wiltshire

Geophysical Surveys of Three PondBarrows in the Lake Down BarrowGroup, near Wilsford, Wiltshire

MARK COLE {Archaeometry Branch, Ancient Monuments Laboratory, English Heritage,23 Savile Row, London W1X 1AB, UK

ABSTRACT Speculation by Ashbee et al (1989) that pond barrows conceal buried shafts, such as thatdiscovered beneath Wilsford 33A, had led Mr Edward Flaxman, with the help of Mr John Trust, toinvestigate a number of barrows in Wessex, including two on Lake Down, using ground penetratingradar (GPR), in September 1995. The radar profiles proved difficult to interpret and it was hopedthat additional magnetometer and resistance surveys would assist this process by elucidatinggreater detail of the morphology and contents of the barrows. Both surveys, undertaken by theAncient Monuments Laboratory, successfully mapped the barrows revealing a number ofsignificant features. Analysis of the resistivity data and GPR profiles suggests that the underlyingsolid Chalk is close to the surface at the centre of the barrows which in turn suggests that shaftsare unlikely to be present. *c 1997 John Wiley Sons, Ltd.

Archaeol. Prospect. 4: 113–121, 1997.No. of Figures: 5. No. of Tables: 0. No. of Refs: 11.

Key words: ground penetrating radar; magnetometry; pond barrow; resistivity; shafts.

Introduction

Grinsell (1953) defines pond barrows as consist-ing of a `slight depression, well and regularlyformed, the material from which has been placedaround the circumference to form an embankedrim'. Ashbee et al (1989) report that althoughWilliam Stukeley was the first to excavate a pondbarrow and publish the results in 1740, it was infact Colt Hoare who first introduced the term. Todate approximately 60 pond barrows have beenidentified in the Wessex region, separated intothree main groups near Avebury, near Stone-henge and on the south Dorset ridgeway(Grinsell, 1953). The barrows discussed in thisreport are part of the Lake Down barrow groupnear Wilsford, Wiltshire (see Figures 1 and 2).

Magnetometer and resistivity surveys wereundertaken over the locations of three of the fourpond barrows on Lake Down in March 1996. Thesurveys were instigated at the request ofMr Edward Flaxman, a retired civil engineercurrently investigating the possibility, originallymooted by Ashbee (Ashbee et al, 1989, pp. 7, 134,138), that pond barrows may conceal the loca-tions of buried shafts such as that discoveredbeneath the site of pond barrow Wilsford 33A.

In an attempt to test this theory, Mr Flaxmanarranged for ground penetrating radar (GPR)profiles to be recorded at several sites, includingthe site of the Wilsford Shaft and two of the fourpond barrows in the Lake Down Group. Thiswork was undertaken by Mr John Trust inSeptember 1995 (Flaxman, 1996). Pairs of ortho-gonal radar profiles were recorded, in each casewith a GSSI SIR2 kit using 500 MHz and100 MHz antennae. The 500 MHz profile overthe Wilsford Shaft appears to have successfully

{Correspondence to: M. Cole, Archaeometry Branch, AncientMonuments Laboratory, English Heritage, 23 Savile Row,London W1X 1AB, UK.

CCC 1075±2196/97/030113±09$17.50 Received 18 April 1997# 1997 John Wiley & Sons, Ltd. Accepted 9 May 1997

Archaeological Prospection, Vol. 4, 113±121 (1997)

located the sides of the upper part of theweathering cone (see Figure 3a). However, theshaft itself, the top of which is buried at a depthof approximately 6 m, proved to be beyond therange of the equipment used. As the other sitesare all located on the same Chalk geology, thissuggested that, if a shaft was located beneath anyof the barrows under investigation, the techniquestood a reasonable chance of locating at least partof the weathering cone, if not the main body ofthe shaft itself. At Lake Down, however, thetransects did not produce the same reflectionpattern: instead a shallow interface at the barrowcentres was indicated.

The surveys reported upon here were intendedto help provide more information regarding thenear-surface characteristics of these barrows andthereby aid the interpretation of the GPR profiles.They also provided an opportunity to assess howconducive the site conditions are for furtherinvestigation using more penetrative geophysicalconfigurations. Barrows WS 77, 77A and 78were each surveyed using both resistivity and

magnetometry, although to date only WS 77 and78 have been included in the GPR coverage.

It is worth noting that Grinsell (1973) refers tothe excavation of the pond barrows on LakeDown by the Reverend Edward Duke (1790±1850) of Lake House, Wilsford. This account wasoriginally published in the Wiltshire Archaeo-logical Magazine for 1907±8 (WAM xxxv, 585),which records that all four pond barrows on LakeDown (see Figure 1) were excavated (namely WS76A, 77, 77A and 78). Pond barrows WS 76A islocated just to the north of the other three and canjust be made out at the lower centre right ofFigure 1. It appears that within each barrow anarea roughly in the centre was opened and inthree cases `nothing found'. In the fourth acircular cist containing burnt bone was discov-ered within 1.5 feet of the surface. Unfortunatelyit is not stated clearly to which of the fourbarrows this discovery refers.

The area surveyed (see Figure 2), centred onSU 1180 3926, lies over Upper Chalk (BritishGeological Survey, 1950).

Figure 1. The Lake Down Barrow group from the air looking southwest (#Crown Copyright/MOD). The three pond barrowssurveyed can be seen at the centre.

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# 1997 John Wiley & Sons, Ltd. Archaeological Prospection, Vol. 4, 113±121 (1997)

Method

A grid of 30-m squares was laid out at the site byMr Flaxman which encompassed three of thebarrows (i.e. WS 77, 77a and 78 Ð see Figure 1).Each of these grid squares was then surveyed

using a Geoscan FM36 fluxgate gradiometer.Measurements were recorded at 0.25 m intervalsalong traverses 0.5 m apart and the data wasdownloaded periodically to a microcomputer inthe field. Greyscale and graphical trace plots ofthis data appear on Figure 4.

Figure 2. Location plans of (a) the Lake Down barrow group and (b) the geophysical survey.

Geophysical Surveys of Pond Barrows, Wiltshire 115


Each square was then resurveyed with aGeoscan RM15 resistivity meter using the TwinElectrode configuration with a 0.5 m electrodespacing. Measurements were recorded at 1.0 mintervals along traverses spaced 1.0 m apart. Theresulting data are illustrated in this report in theform of greyscale images on Figure 4. The latterincludes a plot of the raw data as well as a plot ofthe data after statistical treatment using a high-pass Gaussian filter (Scollar et al, 1990).

During the earlier GPR survey Mr Flaxmanhad obtained a rudimentary measure of thesurface topography of barrows WS 77 and 78by stretching a tape horizontally across each

barrow. Measurements to the ground surfacewere then recorded at regular intervals along thistape. This procedure was carried out along thetraverses used for the radar profiling which,unfortunately, occupied a different alignment tothat used for the magnetometer and resistivitysurveys. However, the corresponding transectshave been extracted from the relevant data-sets toallow a visual comparison with the surfacetopography to be made. In addition, an approxi-mate depth-scale has been added to the GPRprofile over WS 78 (Figure 5b) by assuming aradar velocity of 0.05 m/ns. This assumption wasbased on values published in Davis and Annan

Figure 3. Ground penetrating radar (GPR) profiles (a) over the Wilsford Shaft (WS 33A) and (b) over pond barrowWS 78 (see also Figure 5). (These profiles have been reproduced with the kind permission of Mr Edward Flaxman andMr John Trust.)

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Figure 4. Plots of the survey data. (a) Greyscale of magnetometer data; (b) trace plot of magnetometer data; (c) greyscaleof raw resistivity data; and (d) greyscale of contrast-enhanced resistivity data.



(1989). Meats and Tite (1995) also assumed asimilar value when interpreting their radar data(also obtained using GSSI SIR equipment) at asite on Chalk near Andover, Hampshire. All ofthe results from the west±east transect over WS78 are presented for intercomparison in Figure 5b.

Results

Magnetometer survey

It is clear from the plots (Figure 4a and b) that themagnetometer survey has successfully detectedthe three barrows, although the response iscomplex. The process of interpretation is notmade easier by the very subdued magneticresponse Ð the frequency distribution histogramon Figure 4a shows that the great majority of thereadings recorded lie between +0:7 nT, which isclose to the maximum precision of the instru-ment. This implies that the intensity of many ofthe archaeological anomalies mapped lies nearthe level of background noise.

From the surface, all three barrows appear tobe broadly similar (see Figure 1), with WS 77Aapparently a smaller scale version of barrows77 and 78. The data, however, suggest that thismay indeed be a superficial view of the barrowsand that significant differences in their make-upexist.

WS 78

The magnetometer response to WS 78 reveals thisto be the most uniformly circular of the three, anda sequence of subtle concentric circular magneticanomalies, both positive and negative, has beenmapped. Comparison of this data with both thetopographic and resistivity data (see below andFigure 5b) suggests that the top of the barrowbank has been detected as a positive magneticanomaly. This appears to be flanked inside andout by a narrow negative anomaly. There is alsothe suggestion of further circular positive anoma-lies both to the inside and outside of the latter.The detection of a bank as a positive anomaly isunusual although a possible explanation is thatthe bank has a magnetic (turf) core. Alternatively,magnetically enhanced topsoil may, for somereason, be concentrated on the top of the bank, as

is the case in the detection of ridge and furrow(see for example Cole, 1992). The effects of thetopography also must not be ruled out as acontributing factor to the form of the magneticanomalies.

Within WS 78 a discrete positive anomaly hasbeen detected, just to the east of centre andbetween two dipole responses, which mayrepresent a pit. Further weight to this interpreta-tion is provided by the resistance survey, whichhas detected a low resistance anomaly at thesame location (see below).

WS 77The sequence of anomalies detected over WS 77appears to be the same as that over 78. Theinnermost positive anomaly, however, is not onlymore prominent than at WS 78 but is shapedmore in the form of a horseshoe than a circle, witha possible opening to the north. The presence ofsuch a feature is not evident on the surfacealthough some supporting evidence is providedon close examination of the aerial photograph(Figure 1). Further support for this interpretationis provided by Atkinson et al (1951) who point outthat `in some pond barrows there is a single well-marked entrance-gap in the surrounding bank,recalling the single entrances of Class I hengemonuments'. It may be significant that thepositive response over the bank is distinctlyelevated at two discrete positions in the samelocality.

The only obvious anomaly located within thisbarrow is a dipole response, just to the east ofits centre, due to a ferrous object. Although thismay simply be extraneous surface litter, it ispossible that it is associated with Duke's excava-tions (see above).

WS 77AThe response to this barrow is perhaps the leastcomplete of the three. An outermost negativeanomaly is only partially visible as an arcrunning around the southern half of the barrow.The most clearly detected anomaly is thesubcircular positive over the centre of the barrow(cf. WS 77). The circuit of this anomaly isincomplete and varies in magnetic intensitywith apparent interruptions to both the east andthe west. Comparison with the results of the

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Figure 5. (a) Greyscale of the magnetometer data showing the location of the GPR profiles and (b) the topographic dataplotted alongside the undulating GPR interface, and the extracted magnetic and resistivity data from the W–E traverseacross WS 78.



resistivity survey reveals that this feature neatlycircumscribes a discrete zone of high resistance(see below).

In the area between the barrows, the magnet-ometer survey has located a positive linearanomaly running approximately north±southfor about 20 m just to the east of WS 77. Thecourse of the former field boundary visible on the1975 Ordnance Survey 1 : 2500 map (see Figure 2)has not been detected.

On the southern edge of grid square 4 themagnetometer has recorded a characteristicallystrong response due to a modern ferrous pipe. Adeep plough furrow at the edge of the arable fieldto the west has also been detected as a negativeanomaly running north±south through gridsquares 1 and 4.

Resistivity survey

Unfortunately, owing to the limited amount oftime available in the field and problems with theequipment, it was only possible to conduct thissurvey at a lower resolution (1:0 m� 1:0 m) thanhad been planned (0:5 m� 0:5 m). An informa-tive plan of the barrows has nevertheless beenobtained.

As with the magnetometer survey, theresponse to the barrows is broadly similar ineach case. The barrow banks have been identifiedas broad rings of high resistance surrounding anarea of predominantly lower resistance (Figure 4cand d). Interestingly, within the latter, all of thebarrows show a central core of higher readings.This appears to correspond well with the GPRdata, which indicate a shallow interface at thecentre of the barrows (see west±east profile overWS 78 on Figure 5b). Given the conditions at thesite, the clearest interface evident in the GPRprofile, a strong undulating reflection, is likely tobe that corresponding with the underlying solidChalk (John Trust, pers. comm.). It is also feasiblethat this interface is a response to a distinctchange in the moisture balance down the soilprofile created by an earlier downpour. How-ever, as Figure 5b so clearly illustrates just howclosely the GPR profile reflects the surfacetopography, it would seem likely that the formerinterpretation is correct. This would imply thatthe radar impulses have not been affected by

the internal structure of the barrow bank and thattopographic correction would result in a more orless horizontal interface.

Taken together, however, the GPR and resist-ivity results do indicate that the surface of thesolid Chalk is closest to the ground surface atthe centre of the barrow. Furthermore, the GPRprofile suggests that immediately to either side ofcentre the depth to the Chalk interface increasesslightly. (It is worth pointing out that the verticalscale of this profile is not in units of distance butin reflection time (nanoseconds). As a result theimage gives an exaggerated impression of thevariation in depth across the barrow.) This mayindicate an accumulation of topsoil against theinside barrow bank, or conceivably an innerditch. This would in turn offer an explanationboth for the reduced resistance and also thepositive magnetometer readings in this area(see above). The central area of high resistanceis particularly pronounced within WS 77A Ðunfortunately no GPR profiles have, as yet, beenrecorded over this barrow.

In addition, the resistivity survey has locatedother anomalies worthy of comment. Within WS78, a distinct low resistance anomaly has beendetected, just to the east of its centre, whichcorrelates well with an anomaly detected by themagnetometer survey (see above), and which islikely, therefore, to represent a pit. Just to thenorth of this is a further, although less obvious,low resistance anomaly, which correspondsdirectly with a dipole response in the magnet-ometer survey. Again, this is perhaps indicativeof a former excavation. The unusual horseshoe-shape of the magnetic anomaly encounteredwithin WS 77 is clearly replicated as a low-resistivity anomaly.

There is little evidence in the data of a flintpavement similar to that found by Atkinson et al,(1951) at Winterbourne Steepleton, although it ispossible that the presence of such a feature mightexplain the central core of elevated resistanceobserved above.

Conclusions

The conditions on Lake Down proved conduciveto geophysical prospection and both resistivity

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and magnetometry surveys have succeeded inrevealing more information relating to the struc-ture of the three barrows than is visible from thesurface. Significantly, the results indicate thatdespite their outwardly similar topographicalappearance, differences in their subsurfacecharacter exist.

The magnetometer and resistivity data showgood correlation, especially for the barrow inter-iors. Here comparison of the resistivity data andthe GPR profiles suggests that within eachbarrow there is a central core area where theunderlying Chalk is apparently closest to thesurface (i.e. resulting in a high resistance and ashallow interface). Surrounding this area, thedepth of overburden then apparently increasesslightly towards the barrow bank. On thisevidence, therefore, it would seem unlikely thata shaft exists beneath any of the barrows. Somedisturbance in the magnetic data was detected inthe interiors of barrows WS 77 and 78, whichmay be associated with nineteenth centuryexcavations but there is no firm evidence thatthe find spot of the cist discovered at that timehas been detected.

The survey results are positive and suggestthat any further work investigating pondbarrows for shafts would usefully include amore deeply penetrating resistivity configura-tion. The GPR profiling could also usefully bebroadened by carrying out a series of parallel,closely spaced traverses thereby allowing theconstruction of horizontal time slices (cf. Good-man et al, 1995). Clearly, this would be combinedmost profitably with a detailed microtopographysurvey so that the radar data can be corrected fortopography. Microgravity measurement mayalso offer some potential for shaft locationalthough the contrast in density between therubble-fill of a shaft and the surrounding Chalkmay not be sufficient to create a detectablegravity anomaly.

Acknowledgements

I would like to thank Paul Linford for his help inconducting the field work and for his commentson the text, for which thanks are also due toAndrew David. I am also greatly indebted toEdward Flaxman and John Trust for their kindpermission to reproduce the results of their GPRinvestigation. O. Flaxman, D. J. Trust, I. J. Trustand Miss J. Trust must also be thanked for theirhelp with the GPR survey.

References

Ashbee, P., Bell, M. and Proudfoot, E. (1989). WilsfordShaft: Excavations 1960±62. London: Historic Build-ings and Monuments Commission for England,English Heritage Archaeological Report No. 11.

Atkinson, R. J. C., Brailsford, J. W. and Wakefield, H. G.(1951). A pond barrow at Winterbourne Steepleton,Dorset. Archaeological Journal 108: 1±24.

British Geological Survey (1950). Salisbury. England andWales Sheet 298. Drift Geology (Keyworth, Notting-ham: British Geological Survey).

Cole, M. A. (1992). Nuneham Courtenay, Oxon. Report ongeophysical survey, April 1992. London: AncientMonuments Laboratory, Report Series, 57/92.

Davis, J. L. and Annan, A. P. (1989). Ground-penetratingradar for high-resolution mapping of soil and rockstratigraphy. Geophysical Prospecting 37(5): 531±551.

Flaxman, E. W. 1996. A radar survey of pond barrows.Unpublished report.

Goodman, D., Nishimura, Y. and Rogers, J. D. (1995).GPR time slices in archaeological prospection.Archaeological Prospection 2: 85±89.

Grinsell, L. V. (1953). The Ancient Burial Mounds ofEngland. London: Methuen & Co Ltd.

Grinsell, L. V. (1957). In `Victoria History of theCounties of England: Wiltshire', Vol. 1, part 1.Oxford: Oxford University Press.

Meats, C. and Tite, M. S. (1995). A ground penetratingradar survey at Rowbury Copse Banjo Enclosure,Wiltshire. Archaeological Prospection 2: 229±236.

Scollar, I., Tabbagh, A., Hesse, A. and Herzog, I. (1990).Topics in Remote Sensing 2: Archaeological Prospectingand Remote Sensing. Cambridge: Cambridge Univer-sity Press.



Geophysical surveys of three pond barrows in the Lake Down Barrow Group, near Wilsford, Wiltshire

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