Figure 1; Copy of plot 2a marking anomalies of interest.
Report on geophysical survey, 1993
Introduction
This report describes the results of the geophysical survey carried out at Ashampstead Common near the town of Goring in Berkshire. During landscaping work, a medieval pottery kiln was discovered in a bank at the edge of the field adjoining the bungalow named "Uncommon". This kiln was excavated by the Trust for Wessex Archaeology and the geophysical survey was conducted subsequently, in an attempt to locate any further kilns or associated archaeological features.
The site (SU 57 74) lies in an area where the Upper Cretaceous Chalk is overlain by clay-with- flints and loam, although the clay layer thins out towards the south-western corner of the surveyed area.
Method
The field was divided into a grid of forty-two 30 metre squares, as depicted in the survey location plan included as Annex 1. Unfortunately, this area was last mapped by the Ordnance Survey in 1981 and it appears that since then some alteration has occurred to the northern and eastern field boundaries. Thus, the survey grid actually aligns more closely with the present boundaries than is shown in the location plan. Each 30 metre square was then surveyed using a Geoscan FM36 fluxgate gradiometer along successive north-south traverses separated by 1.0m intervals. Readings were logged every 0.25m and the data was transferred to a portable microcomputer in the field. In addition to the magnetometer coverage, the same area was surveyed with a Bartington MS1 Magnetic Susceptibility Meter and field loop, on a 15 metre grid, so that any changes in topsoil magnetic susceptibility caused by past anthropogenic activity might be detected.
Annex 2 depicts the results of the gradiometer survey of square 37, the area around the excavated kiln, at 1:200 scale in both greyscale (2a) and trace plot (2b) format. Annex 3 depicts the results of the entire gradiometer survey at 1:1250, again in both greyscale and trace plot formats. Annex 4 depicts the results from squares 40, 42 and 42 at larger (1:500) scale and Annex 5 depicts the results of the magnetic susceptibility survey, in greyscale format, at 1:2500 scale, superimposed on the location plan.
Results
The area around the kiln (Annex 2)
Unfortunately, the gradiometer response from the area immediately surrounding the remains of the kiln, in square 37, was greatly distorted by the presence of iron structures in the vicinity. This is particularly apparent in the bottom six traverses of the trace plot (2b) and in a rectangular area just below the south-western corner in both plots. Furthermore, recent landscaping of the bank has also caused disturbance in the survey results. Nevertheless, the characteristic double peaked, response over the kiln is clearly visible in the trace plot, seven metres east of the south-west corner of the square. The peak amplitude of this response is about 30nT above the background level; any other kilns detected by the gradiometer might be expected to produce an anomaly similar in both magnitude and shape.
A copy of the greytone plot (2a) is reproduced as figure 1 and marked on this are the kiln, and a faint anomaly that can be discerned about 24 metres to its north. This latter anomaly is about 10 metres in length but has a maximum amplitude of only 2nT. It is possible that it represents a linear feature turning through a right angle, perhaps part of a structure associated with the kiln. However, the anomaly lies in the area that has been landscaped recently and could quite possibly be an artifact of the disturbance so caused.
The gradiometer survey as a whole (Annex 3)
Plots 3a and 3b show respectively greyscale and trace plot representations of the entire gradiometer survey. A number of the most obvious anomalies in these plots are due to modern activity. The circular area of random disturbance covering most of square 38, is caused by the infilling of a depression with spoil from the landscaping mentioned above. A similar, smaller area of disturbance occurs about 90 metres to the north at the edge joining squares 16 and 23. This anomaly coincides with evidence of a recent bonfire and the magnetometer has detected the rubbish associated with this. The two large circular low magnetic field anomalies in squares 7 and 24 are also modern, being due to telegraph poles.
Apart from the response to these features, a number of discrete amorphous areas of increased magnetic field strength are apparent, measuring about 6 metres in diameter and scattered randomly across the survey area, particularly the eastern half. The strongest of these, in square 31, have a peak amplitude of around 8nT. Furthermore, square 23, immediately to the north, contains a large, almost linear feature of the same type, about 30 metres long and describing a reverse 'S' shape. These anomalies could quite possibly be caused by pits related to the pottery production process. However, unequivocal interpretation is frustrated by the lack of any associated responses to archaeological structures and a geological origin cannot be ruled out. The result of trial augering in square 38 was also ambiguous, revealing a change in the clay consistency over the larger of the two anomalies in the vicinity but no clear evidence of archaeological material.
Scrutiny of the trace plot (3b) shows no evidence for any further double peaked anomalies similar to that caused by the known kiln in square 37. However, two discrete anomalies are apparent near the south-western corner of the survey and these are discussed below.
The south-western corner of the field (Annex 4)
Squares 40, 41 and 42 have been plotted at 1:500 scale in Annex 4 to highlight two discrete features, each about 2.5 metres in diameter. In the greyscale plot (4a) the predominantly low magnetic field response at the south-western corner of square 40 and the similar circular response in square 42 are caused by modern agricultural debris. However, two circular high field strength anomalies about 2.5 metres in diameter are also visible, one in square 41 with a peak amplitude of about 16nT, the other in square 40 with a peak amplitude of about 25nT. The anomaly in square 41 also appears to have an amorphous area of increased magnetic field strength immediately to its west. These anomalies have been marked on the copy of the plot reproduced as figure 2.
Both anomalies lack the characteristic double peaked response registered over the known kiln but are otherwise similar in size and peak magnitude. Such anomalies could be of non-archaeological origin and, in the absence of any other evidence, cannot be interpreted with certainty. Nevertheless, it is possible that they represent pits or perhaps the remains of kilns similar to that excavated, with the central cavity filled with debris collapsed from their walls.
The topsoil magnetic susceptibility survey (Annex 5)
In addition to the magnetometer survey, topsoil magnetic susceptibility measurements were made over the survey area. Past industrial activity, such as the firing of kilns, often produces local enhancement to the soil magnetic susceptibility and it was hoped areas of such enhancement might help locate archaeological remains associated with the pottery production.
Annex 5 indicates that in general the soil susceptibility on the site is low, with a mean value around 15 x 10-5 SI and only two of the readings were above 40 x 10-5 SI. The most notable areas of enhancement occur over the modern bonfire and infilled depression mentioned above. It is notable that a high measurement of 69 x 10-5 SI occurs in the vicinity of the excavated kiln. Also, to its north, is an area of slight enhancement about 45 metres across; this is likely to be soil from near the kiln redistributed during recent landscaping.
In square 16 a smaller area of enhancement occurs with values around 20 x 10-5 SI, though this does not appear to correspond with any anomalies on the magnetometer survey. Furthermore, the proximity of this area to the bonfire cautions against an archaeological interpretation. An area of very slightly higher susceptibility also occurs in squares 19, 26 and 27, although again the magnetometer detected no significant anomalies in that area and interpretation is unclear. One final feature of note in Annex 5 is the generally low magnetic susceptibility in squares 40, 41 and 42, perhaps throwing doubt on the interpretation of the magnetometer anomalies detected in these squares as kilns.
Conclusion
The magnetometer detected a characteristic anomaly over the excavated kiln but no comparable responses were found elsewhere in the survey area. Two anomalies of similar size and amplitude are present, in the south-western corner of the survey, and perhaps represent pits or the remains of damaged kilns. However, magnetic susceptibility in this area was low, reducing the reliability of this interpretation.
The most striking anomalies in the magnetometer survey are the amorphous areas of increased field strength distributed apparently at random over the survey area. These may well be pits related to the pottery production but the lack of associated, recognisably archaeological anomalies, allows no firm conclusion to be drawn.
Archaeometry Branch,
Ancient Monuments Laboratory,
English Heritage.
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