Report on Geophysical Survey, 1994.
INTRODUCTION
Geophysical survey was undertaken at Merdon Castle, Hampshire (Scheduled Ancient Monument Hampshire No. 29) in response to a request from Steven Trow, the Inspector of Ancient Monuments responsible for the site. The construction of the well preserved and substantial ringwork in c.1138 by Henry de Blois, Bishop of Winchester, is recorded in the Annals of Winchester. These earthworks are located eccentrically within an outer enclosure, presumed to be of Iron Age origin (Williams-Freeman, 1915). The substantial ruins of a flint masonry building stand on the northern side of the ringwork, with the remains of an in-filled well some 30m to the south (see Fig. 1).
The site is centred on NGR SU 42 26 and lies over Upper Chalk (British Geological Survey, 1966).
METHOD
The earthworks themselves are steeply banked and covered with large trees and dense undergrowth in places and as such are not suited to geophysical survey. It was decided, therefore, that the most suitable areas to survey would be the open ground within the Norman ringwork itself as well the area immediately to the east in the space between the surrounding enclosure and the ringwork (see Fig. 1).
Two separate grids of 30m squares were established on a roughly north-south alignment within the two areas outlined above. Each of these squares was then surveyed with a Geoscan RM15 resistivity meter using the twin electrode configuration. Measurements were taken at 1.0m intervals along traverses 1.0m apart and the data was periodically down-loaded to a microcomputer in the field. The resulting data is illustrated in this report using greyscale plots (see Plans 1, 2 & 3). Buried stonework is indicated on these by alignments of high resistance (shown as white) as well as broader more amorphous areas of disturbance. In an attempt to clarify visual recognition of significant anomalies, the data has been statistically enhanced using a high-pass gaussian filter (Scollar et al, 1990). The resistivity data has also been plotted over a scanned-in version of the RCHME ground survey (see Plan 1) to facilitate a comparison of the results of the two surveys.
A number of the grid squares were subsequently resurveyed using a Geoscan FM36 fluxgate gradiometer (see Fig. 1). Measurements were recorded at 0.25m intervals along traverses 1.0m apart. The resulting data is represented in this report using both greyscale and graphical trace plots (see Plans 3 & 4).
RESULTS
Resistivity Survey (Plans 1, 2 & 3)
Inner Ward (grid squares 1-6)
The resistivity survey of this area has detected a number of linear high resistance anomalies strongly suggestive of former buildings. In particular, there are two parallel anomalies (in grid square 1) aligned with the bank, from which two further parallel anomalies appear to run, if somewhat intermittently, southward towards the bailey. Two lengths of a flint rubble revetment, built into the inner face of the earthwork to the west (see RCHME survey on Plan 1), run parallel to these latter resistance anomalies and, in combination, suggest that the buildings in this area may have been arranged in a polygonal formation around a central courtyard.
To the south of the inner ward (within grid squares 4 & 6) is a rectilinear arrangement of high resistance anomalies which, given its location, may well represent the remains of a gatehouse between the inner ward and the bailey. Significantly, these anomalies are situated in the vicinity of a "marked cleft" in the earthworks (Donachie, forthcoming). This arrangement of anomalies contains an area of distinctly low resistance, compared to the local background, indicative of a moisture retaining environment possibly created by buried foundations preventing natural drainage down slope into the bailey or containing a locally deeper accumulation of soil. Immediately to the west is a discrete area of low resistance of unknown origin, although this may be associated with the water pipe running NW-SE (see below).
The results from the eastern half of the inner ward are rather enigmatic and no distinct
features have been detected with the exception of a linear high resistance anomaly running
east-west immediately to the north of the well, which may represent a buried wall.
The Bailey (grid squares 7-15)
The most striking characteristic of the results from the bailey is an amorphous spread of high resistance which covers the majority of the area surveyed. This zone of disturbance generally lacks any internal patterning although there are some slight linear effects suggestive of structural remains (see grid squares 7 & 14 in particular). Some of the latter correlate well with the earthwork survey (see Plan 1). This type of anomalous response is often associated with buried rubble but the possibility remains that its origin is not of archaeological significance.
There is a broad band of very low resistance inside the SW edge of the bailey bank and this is most probably caused by the damp clayey topsoil present in this area. A standing water tap (in grid square 8) used by campers may well be the origin of this dampness.
The two low resistance linear anomalies detected in grid squares 7 & 8 may be a response to the in-filled remains of a ditch surrounding the inner ringwork. However, this interpretation is perhaps questionable due to their discontinuous nature. Further caution must be also be taken in the interpretation of the anomaly in square 7 as it is aligned with the trackway entering the bailey here, the surface of which was damp and clayey, and littered with pot-holes at the time survey was carried out.
The Eastern Enclosure (grid square 16-22)
The results in this area are dominated by the response to a relatively modern trackway/field boundary (see below). There is, however, a broad low resistance linear anomaly, presumably a ditch, running roughly N-S and parallel to the earthwork to the east for which their is clear correlation with a positive linear anomaly of similar dimensions in the magnetometer survey (see below) and also with a subtle feature in the earthwork survey. The archaeological significance of this feature is uncertain: it may well be of fairly modern origin, perhaps associated with the military activity at the site.
Magnetometer Survey (Plans 3 & 4)
The magnetometer survey was greatly hampered by the presence of modern ferrous material, particularly within the ringwork itself (see Plans 3 & 4). The amount of ferrous disturbance is a result of the considerable activity that has occurred at the site since it was abandoned as a residence during the 14/15th century. This includes agricultural activity, the use of the site by the army during both World Wars, and more recently for recreational camping during the summer months.
The magnetometer results are useful, however, as they provide complementary evidence to the resistance survey in interpreting the network of ferrous water pipes (see below), characterised by their fluctuating positive and negative magnetic signature, and the former field boundary/trackway in the eastern enclosure. The magnetometer survey has also located the broad ditch-type anomaly in the eastern enclosure (see above).
Recent Features
Whilst the survey has detected buried archaeological remains (see above) widespread modern disturbance has also been detected. A network of low resistance anomalies has been located (running through grid squares 3, 6-9 & 13) which can confidently be interpreted as the response to the ditches dug for modern water pipes. This is confirmed both by the presence of a standing water tap in grid square 8 and by the strong ferrous sources (ie iron pipes) detected by the magnetometer survey (see above). Further weight to this argument is provided by the fact that these low resistance anomalies connect the three breaches of the bailey earthworks, the eastern and western of which appear to be the result of deliberate mutilation (Donachie, forthcoming) and as such can be presumed to be modern.
To the east, a broad high resistance anomaly curves through grid squares 18-21 which clearly
corresponds to the slight bank located by the earthwork survey (see Plan 1). This feature is
visible as a trackway on aerial photographs and as a fence line on the 1967 OS map (see Fig. 1) and clearly connects the road to the NE with the war-time buildings to the south (see Plan
1). The presumably modern track between the road and the bailey has also been detected as
a broad high resistance anomaly which stands up well to comparison with the earthwork
survey.
CONCLUSION
The resistivity survey has succeeded in locating a variety of features, a number of which are probably wall foundations. The majority of these are restricted to the inner ward of the castle. The complementary nature of geophysical and earthwork surveys in assessing and interpreting such sites has been demonstrated, as illustrated by the identification of a possible gatehouse between the inner ward and the bailey. The lack of structural evidence for buildings within the bailey is disappointing, although the anomalous response in this area is at least suggestive of rubble. It must be recognised, however, that any former buildings surviving within the bailey may well have been of wooden construction and as such would be difficult, if not impossible, to detect by resistance survey.
The magnetometer survey within the earthworks was severely impaired by the ferrous contamination created since the site was deserted as a residence in the 14/15th century, particularly during the two World Wars.
The survey was unable to shed any light on the origins of the outer enclosure surrounding the ringwork. The form of the earthwork is clearly suggestive of an IA hillfort especially on consideration of the ploughed out remains of the rampart to the SE located for the first time by the RCHME survey. Further magnetometer survey in the arable field to the south, west and east of the monument (and over this rampart) may well yield more information and help determine the layout of the park pales to the east of the monument.
Archaeometry Branch
Ancient Monuments Laboratory
Science and Conservation Services, RPS.
References
Donachie, J D, (forthcoming) Merdon Castle, Hursley, Hampshire: An earthwork survey by the Royal Commission on the Historical Monuments of England.
Institute of Geological Sciences, 1966 1" map Geological Survey of Great Britain, Sheet 299, Winchester - Drift.
Scollar, I, et al, 1990 Archaeological Prospecting and Remote Sensing, Cambridge.
Williams-Freeman, J P, 1915 An Introduction to Field Archaeology as Illustrated by Hampshire, London 255-8.
Plans Enclosed
Figure 1 Location plan of survey (1:2500).
Plan 1 Plot of resistivity survey overlain on RCHME ground survey (1:1000).
Plan 2 Plots of resistivity data (1:1000).
Plan 3 Plots of resistivity and magnetometer data (1:1000).
Plan 4 Plots of magnetometer data (1:1000).
