Armstrong, Kayt

Cheetham, Paul

English

Bournemouth University Research Online

Archaeological geophysical prospection in peatland environments: Locating the SweetTrack at Canada Farm, Shapwick Heath (Somerset)Kayt Armstrong & Paul CheethamSchool of Conservation Sciences, Bournemouth University, Talbot Campus, Fern Barrow, Poole,Dorset BH12 5BB(Contact: karmstrong@boournemouth.ac.uk)“Geophysical techniques can, as yet, have little part to play in wetland evaluation.”(English Heritage, 2008, 17)Peatland environments preserve organic material, including anthropogenic objects, because theyare waterlogged, and therefore anaerobic. These objects are not preserved on most other sites, sopeatland sites have contributed enormously to our understanding of the past. Peat also preservespaleoenvironmental evidence and is the primary resource for understanding the past climate andecology of the UK. However, these sites often lie within or at the base of wet, deep, homogenouspeat which makes them invisible to surface observers. As a result, they most often come to lightwhilst being destroyed. Once located, they are difficult to excavate, leading to a bias towardssmall area, detailed excavations, rather than open area work which allows monuments to beplaced within a landscape context. Furthermore, these sites and the environments they inhabit arevery sensitive, so invasive evaluation techniques are ruled out.  There is a need for non-invasivetechniques to detect, monitor and contextualize peatland sites. A research project to evaluate theuse of geophysical prospection for this purpose is being undertaken at Bournemouth University.In the past it has been suggested that peat deposits are too wet, too deep, too homogenous orsimilar to the target for traditional dry land geophysical techniques to be of any use. However, asthis research demonstrates, there are detectable physical and chemical differences between thearchaeological deposits and structures and the surrounding matrix. The overall research projectsets out to document these and find survey techniques that can recognize them.The project has defined four peatland environment types: upland and lowland, each divided intosites with sub peat and intra peat archaeological deposits. Eight case study sites have beenselected that represent these. This presentation focuses on work in lowland peat deposits, and acase study of these at Shapwick Heath in the Somerset Levels, which was prospecting for theSweet Track. Dated at 3200 BC (approx) this is the oldest known trackway in Britain and is of aunique single plank construction. The surveys over a preserved section of the track all located ananomaly in the known location of the trackway. The resistivity and conductivity (EM) surveysshowed a linear anomaly running north/south, 2m wide and 16m in from the western edge of thegrid. They also show a gradient of increasing resistivity values from west to east across the grid.The resistivity inversion modeling showed that this gradient appears to show a more resistivelayer that starts at about 5-8m in from the western edge, growing more intense to the east andextending down about 0.75m. To the east and underneath this is a generally much less resistivebody. The magnetic survey was very quiet but showed an area of enhancement in the far southeast corner of the survey and a faint linear negative anomaly about 1m wide running north-southabout 14m in from the western grid edge. The GPR showed a large dendritic reflector in the southwest quadrant of the grid between 10-28ns/ 0.4m deep. There is a faint linear reflector at roughlythe same depth running north/south, 2m wide and about 16m in from the western grid edge. Fromthis depth the northern edge of the grid and in particular the north-east corner shows highamplitude responses that seem to be ‘cut’ at the 16m mark, in line with the previously notedanomaly. The EM38B magnetic susceptibility survey revealed no anomalies of any significance.The linear north/south anomaly is interpreted as being, or being caused by the Sweet Track. Thestronger dendritic anomaly in the GPR data is interpreted as being a bog oak.[pic]Figure 1: Top of image is northern grid edge (see figure 2). GPR Timeslice at 14-17 ns. The squared amplitude of theGPR signal, split into forward and reverse directions then recombined to reduce antenna noise. Low pass filtered.Line of trackway (right of image at 16m east) together with the much stronger response from a bog-oak (bottom left).Darker colours represent higher amplitudes. Created with GPR-SLICE.However, it seems that only the GPR was directly responding to the trackway. The electrical datashows an anomaly in the location of the track, but not the bog oak, whereas the radar shows both.If they were responding to the same physical properties of the wood, the bog oak should alsoshow in the EM and RM15 data. There is also a gradient in the resistivity of the peat, and acommon depth to the changes. This common depth is around 0.75m from the surface; the samedepth as the trackway in that location. The track was built on a semi-stable surface in the peat andwas later subsumed so it is possible this reflects a change in the type or physical and chemicalproperties of the peat itself. The gradient could also be as a result of this, or of different groundwater bodies.Figure 2: Location of survey and major anomalies with reference to known archaeology and previous GPR survey byUsti Electronics (Utsi Electronics, 2001).The reasons for the trackway showing like this, especially in the magnetometer data are not clear.The current working hypothesis is that the track itself is influencing the hydrology within the peatcausing the loss or collection of minerals within the pore water and peat matrix, and it is thesevariations that are being detected. Further work is planned on the site to sample the water and peatand conduct chemical analyses to determine if there is a variation in the chemical composition thatreflects the changes shown in the geophysical data.References:English Heritage, 2008. Geophysical Survey in Archaeological Field Evaluation (2nd Edition). 2nd ed. Swindon: English Heritage.Utsi Electronics, 2001, Trial GPR surveys of three sites in the Somerset Levels: Part 1- A sectionof the Sweet Track 2nd July 2001:  UNPUBLISHED Held By: English HeritageAcknowledgements: This work is being undertaken under a Bournemouth University funded PhD Studentship underthe supervision of Prof. T Darvill and P. Cheetham. The research was conducted under a Section 42 license fromEnglish Heritage, and with the permission and support of Phil Holms, the Natural England reserve manager atShapwick Heath.

Archaeological geophysical prospection in peatland environments: Locating the Sweet Track at Canada Farm, Shapwick Heath (Somerset)

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