TOWER HILL, Ashbury, Oxfordshire
Report on geophysical survey, March 1993
Introduction
A limited geophysical survey was conducted over the find-spot of a significant late Bronze Age hoard discovered by a walker crossing Tower Hill, Oxfordshire, on an established footpath during the spring of 1993. The find consisted of a group of 21 socketed bronze axes and other copper alloy objects which were interpreted as a metal worker's hoard and dated to 8-7th century BC. The aim of the survey was to identify evidence for any archaeological activity associated with the find spot to assist the location of subsequent excavation conducted by the Oxford Archaeological Unit (OAU), funded by English Heritage. This report makes available full details of the geophysical survey results to complement the summary text account included in the OAU excavation monograph (Cromarty et al - forthcoming).
The site (SU 2846 8397) lies on a substrate Middle Cretaceous Chalk over which a shallow layer of well drained calcareous silty soil of the Andover 1 association has developed. Notable soil erosion accelerated by ploughing was reported by the land owner, Mr Erik Penser, which has led to quantities of chalk and flint being scattered across the site. At the time of the survey the field was sown with winter cereal.
Method
Magnetometer survey
Magnetic survey was considered the most appropriate technique to apply due to the favourable chalk geology, which has produced a wealth of successful results in the locality (eg Payne 1996) and the possible presence of highly magnetised thermoremanent features related to metal working. The survey was conducted over all the numbered squares in Figure 1 centred over the immediate location of the findspot using the standard method outlined in note 2 of Annex 1.
The results are presented at a scale of 1:1000 in Plan A.1 as a stacked trace plot of the raw data, the only correction to the measured values being to zero-mean each instrument traverse to remove heading errors. A greytone image of the data is presented in Plan A.2 which has been further enhanced to reduce the detrimental effects produced by surface iron objects through the application of a 2m by 2m thresholding median filter (Scollar et al 1990).
Topsoil magnetic susceptibility survey
In addition to the magnetometer survey topsoil magnetic susceptibility values were recorded at a 15m sample interval over 2.5ha of the surrounding area, as shaded on Figure 1, with a Bartington MS2 susceptibility meter and MS2D field coil. Results from this survey are presented as a grey scale plot superimposed over the Ordnance Survey base map at a scale of 1:2500.
Results
Magnetometer survey
The magnetic response of the site is extremely subdued with the majority of the readings falling in a range between ± 0.8nT, close to the practical noise limit of the instrument. The only magnetic anomalies of note are the scatter of intense responses caused, no doubt, by near-surface ferrous litter (visible in the traceplot of raw data A.1).
Topsoil magnetic susceptibility
Topsoil magnetic susceptibility falls within a range of 5 - 30 x10-5 (SI) over the site with no apparent concentration of enhanced readings either in the vicinity of the findspot or elsewhere within the wider area encompassed by the survey. Slight variations in the topsoil susceptibility would appear to reflect the topography of the site with lower values recorded over the crest of the hill where plough action has promoted topsoil erosion and increased the quantity of low susceptibility chalk mixed with the remaining soil.
Conclusion
The failure of the magnetometer survey to detect the more ephemeral features revealed by the subsequent excavation, such as the post-holes, is unsurprising given the latters' small physical size and weak magnetic contrast with the surrounding subsoil. However, the absence of any thermoremanent anomalies is more intriguing and suggests that any metal-working activity in the vicinity of the hoard was either of short duration or has been severely truncated by ploughing. Due to the limited extent of the magnetometer survey it is, of course, impossible to rule out the existence of a more remote metal-working site some distance from the hoard; however, it would be remarkable for such semi-industrial activity, even if it were highly truncated, not to be preserved as a discernable topsoil susceptibility anomaly.
References
British Geological Survey, 1971, Abingdon, England and Wales Sheet 253, Drift Geology, 1:50,000.
Cromarty, A., Miles, D., Palmer, S., Lock, G. and Gosden, C., The Evolution of the Ritual Landscape: Investigations at Whitehorse Hill, Uffington and Tower Hill, Ashbury, Oxford Archaeological Unit thames Valley Monograph, Oxford., forthcoming
Payne, A., 1996, The Use of Magnetic Prospection in the Exploration of Iron Age Hillfort Interiors in Southern England, Archaeological Prospection, v3, pp163-184.
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 and plans:
Figure 1 Location plan of survey grid squares showing area covered by the topsoil magnetic susceptibility survey (1:2500).
Figure 2Greytone plot of the topsoil magnetic susceptibility data superimposed upon OS base map (1:2500).
Plan A Traceplot of raw magnetometer data together with a linear greytone of magnetometer data after numerical processing to suppress near-surface ferrous responses (1:1000).
