Report on the geophysical survey, November 1996.
Geophysical survey was undertaken at Fawler Roman Villa, Oxon, (SAM no. OX 73) in response to a request from the Monuments Protection Programme (MPP) of English Heritage. The scheduled status of this site is currently under revision and it was felt that insufficient information was available for this to be undertaken effectively. Geophysical survey was requested, therefore, in order to obtain a better understanding of the full extent of the villa site within the easily accessible areas to the south and west of Bury Close. It was also hoped that the survey would allow a broader understanding of the nature of any surviving buried remains within this area.
A wealth of evidence relating to the Roman activity at Fawler has been discovered over the last 100 years or so, although only the most recent findings have been fully documented (Allen 1988). The latter followed excavations undertaken in 1986 along the course of a water service pipe and includes a summary of the archaeological information currently available for Fawler from which the following has been elicited. In the mid 19th-century, the remains of a building containing a mosaic were identified during the laying of a land drain between Bury Close and the Evenlode. Similarly, during the building of the Great Western Railway (GWR), a building containing a mosaic was discovered. It is not certain, however, how these findings relate to each other. Whatever the case, the building revealed during the construction of the GWR was largely destroyed at that time by the diversion of the Evenlode. The original course of the river (as marked by the route of the parish boundary on Fig 1) ran at an oblique angle to the railway and as such would have made the construction of the bridge more difficult.
A limited excavation was undertaken in 1926 by the Oxford branch of The Classical Association which unearthed some silver coins and a 2nd-century Samian cup. In 1970 a watching brief for a Thames Water Authority pipe revealed 1st-century Roman gravel pits in the area between Bury Close and the railway bridge. Allen's 1986 excavations took place just to the east of this latter investigation and were once again prompted by the laying of a Thames Water Authority pipe. Much evidence of Roman activity was unearthed including the remains of a substantial building which incorporated a hypocaust. It is worth noting, however, that Late Saxon remains as well as widespread evidence of 12th - 13th Century occupation were also revealed by these excavations. Numerous coins have also been found in the village, most of which are of Late Roman date, suggesting that the Roman activity at Fawler was prolonged.
The underlying solid geology at the site is Jurassic Cornbrash which is overlain along the valley floor by a silty clay subsoil (Allen 1988).
In an attempt to maximise the information recovered from the site, both magnetometer and resistivity surveys were carried out. Magnetometer survey is a well established technique of archaeological prospection which provides a rapid and non-invasive means of investigating many types of rural occupation site (Clark 1990). The magnetometer is capable of detecting a wide range of buried archaeological features such as pits, ditches, gullies, kilns, ovens and hearths. Resistivity survey is a similarly well established technique also capable of identifying pits and ditches but is particularly well suited to the detection of sub-surface building foundations and other masonry features. Due to its more rapid speed of deployment, magnetometer survey was used throughout the surveyed area whilst resistivity was applied more selectively in the area thought to contain the buried remains of buildings.
To complement the magnetometer survey, samples of topsoil (of approximately 100g) were retrieved along a central east-west transect (see Fig 1) in order to measure their magnetic susceptibility (MS). MS is a natural attribute of soils which becomes artificially enhanced by anthropogenic activity, in particular by the use of fire (Thompson and Oldfield 1986). It is the enhanced MS of soils infilling archaeological features such as pits and ditches, that allows them to be detectable with a magnetometer. Relatively high MS values can thus, in their own right, be an indicator of former occupation and MS is also a valuable aid to the interpretation of detailed magnetometer survey.
Two separate grids of 30m squares were established either side of the railway embankment, both laid out roughly parallel to the latter (see Fig 1). Each of these squares was then surveyed using Geoscan FM36 fluxgate gradiometers. Measurements were recorded at 0.25m intervals along traverses spaced 1.0m apart. Greyscale and graphical traceplots of this data appear in Figures 2 and 5.
A number of these squares were then resurveyed with a Geoscan RM15 resistivity meter using the Twin Electrode configuration with a mobile probe separation of 0.5m. Measurements were taken at 1.0m intervals along traverses spaced 1.0m apart. The resulting data is illustrated in this report using greyscale plots (see Figs 3 and 4). Due to the proximity of the survey area to the Evenlode, a broad variation in background apparent resistivity was encountered across the site and to counter this the data has been statistically treated using a range of image enhancement algorithms (see Fig 4).
Magnetometer Survey (see Figs 2 & 3; text refers to interpretation diagram on Fig 6)
Unfortunately, the results of the magnetometer survey have been greatly disturbed by responses to modern ferrous material. The latter includes modern fencing and extraneous surface litter as well as the Thames Water Authority pipes described above (see Figure 6). The character and extent of this modern interference is perhaps best illustrated by Figure 3 (plot 2) where the sharp vertical deflections in the traces represent ferrous material. It is unfortunate that this disturbance is particularly conspicuous within the area immediately south of Bury Close, the core of the scheduled area.
Despite this widespread disturbance the survey has nevertheless succeeded in detecting clear evidence of buried archaeological features. The most impressive of these is the pattern of broad, linear positive anomalies ( A ) which is apparently interrupted by the present course of Evenlode. Good correlation is evident here with a pattern of linear high resistance anomalies located by the resistivity survey (see below).
Immediately to the west of the above, an area of anomalous magnetic disturbance ( B ) has been detected. Whilst there appear to be some linear and discrete anomalies within this, visualisation of these is greatly restricted by the disturbance associated with modern ferrous fencing and litter.
Further to the west (ie grid squares 1-12, 18-21), a pattern of narrow linear magnetic anomalies ( C ) has been detected which appears to emerge from the gardens along the western side of Bury Close. At D there is an approximately rectilinear arrangement of negative magnetic anomalies which correlates well with a similarly shaped zone of slightly elevated resistance readings. A possible interpretation of these is that they constitute a response to the remains of a former building or building platform. Almost all of the features of archaeological potential detected in this section of the survey are confined to its eastern half but some can undoubtedly be seen to extend beyond the limits of the scheduled area.
To the south of the railway embankment, two broad but indistinct parallel anomalies have been detected running roughly west-east across the surveyed area. Their linearity indicates an anthropogenic origin - perhaps some form of drainage or enclosure feature. It may be significant that this area is often flooded during the winter months - perhaps lending weight to an interpretation as a drainage feature.
The courses of both of the Thames Water Authority pipelines have been detected as narrow negative anomalies. A number of discrete positive anomalies have also been detected along the course of the northernmost pipeline and are, therefore, most probably associated with this feature. There is, however, an isolated positive anomaly at the southwestern corner of the survey which may be archaeologically significant.
Resistivity Survey (see Fig 3 & 4 and also the interpretation diagram on Fig 6)
Due to the proximity of the Evenlode, a broad variation in the background apparent resistivity was encountered. Within this, the presence of buried features is nevertheless discernable, the most distinct of which are marked on Figure 6. The image enhancement algorithms that have been applied to the data aid the definition of archaeologically relevant responses from natural variation.
As noted above, there is good correlation between the resistivity and magnetometer surveys both at A and further to the west at E . Once again, however, whilst giving the distinct impression of the presence of buried structures throughout, there does not appear to be any obviously recognizable pattern to the anomalies. Some of the broad amorphous areas of high resistance visible in the data may represent spreads of buried rubble but more probably reflect changes in the background resistivity. The latter are due to variations in underlying geology, topsoil thickness, soil moisture content and the slope of the land.
The trench dug for a service pipeline has been detected as a low resistance anomaly running northwest-southeast at the western end of the survey area. The low resistance anomaly linear anomaly ( F ) is likely to represent the eastward continuation of this pipeline.
Topsoil Magnetic Susceptibility (see Fig 7)
The values of topsoil magnetic susceptibility (MS) vary markedly across the site from a minimum of 16 x10-8m3Kg-1 at the western and eastern extremities to a maximum of 64 x10-8m3Kg-1 towards the middle of the surveyed area. The distribution of values clearly reflects the pattern of anomalies as located by the magnetometer survey, and supports the assertion that the main focus of activity lies within the scheduled area.
Despite of the severe impact of 19th- and 20th-century interference (including the disturbance associated with the railway, ferrous fencing, service pipes, surface litter etc) the geophysical surveys have successfully managed to detect evidence of buried archaeological features. In general, the latter appear to radiate outwards from the gardens of Bury Close suggesting that it in all probability the latter represent the focus of the archaeological activity at Fawler. It is clear from the geophysical evidence that the currently scheduled area broadly reflects the distribution of surviving remains although some features certainly extend beyond the protected area to the west. In addition, the two broad parallel features detected to the south of the railway embankment are of interest and, although their association with the villa site is unclear, potentially highly significant.
Unfortunately it is not possible to confidently attribute any of the features detected to a particular period based on the geophysical data alone. However, the limited excavation evidence available demonstrates a sustained period of high status Roman occupation which in turn suggests that at least some of the geophysical anomalies are likely to be Roman in origin. Any such interpretation must, however, remain tentative without the support of further corroborative information.
Allen, T G, 1988 Excavations at Bury Close, Fawler, Oxon., Oxoniensia, 53 , 293-315.
Clark, A J, 1990 Seeing Beneath the Soil: prospecting methods in archaeology, B T Batsford, London.
Scollar, I, et al, 1990 Topics in Remote Sensing 2: Archaeological Prospecting and Remote Sensing, Cambridge.
Thompson, R, and Oldfield, F, 1986 Environmental Magnetism, Allen & Unwin, London.
List of figures
Figure 1 Location plan of geophysical survey (1:2500)
Figure 2 Magnetometer survey overlain on location plan (1:2500)
Figure 3 Plots of magnetometer survey (1:1250)
Figure 4 Resistivity survey overlain on location plan (1:2500)
Figure 5 Plots of resistivity data (1:1250)
Figure 6 Interpretation of geophysical anomalies
Figure 7 Results of topsoil magnetic susceptibility measurements