(C) Historic Buildings & Monuments Commission for England.
Report on geophysical survey, July 1998.
The headland around Scarborough castle (NGR TA 04 89) has been occupied since the Late Bronze Age (Linzey 1997), and the castle itself dates from before 1135. The castle complex is of several phases and the fabric has suffered a history of sieges and bombardments up until 1914 (Clark 1997).
The inner bailey, built in the 1150s, contains the keep and the vestiges of a former hall and service buildings around its inner perimeter. In the northern corner the presence of a further hall is suggested by joist-holes in the curtain wall (Clark 1997, fig 6). Today the area outside the keep is largely turfed over except for the preserved foundations of the ancillary buildings and a medieval well.
A geophysical survey of the headland was requested by the Inspector for Historic Properties (North) with the eventual aim of improving the understanding and presentation of its historical development. The survey of the inner bailey, reported here, represents an initial phase of this project.
The inner bailey area was first divided as far as possible into 30m x 30m squares (see Figure 1). Geophysical targets anticipated within the inner bailey were masonry foundations, and therefore resistivity was chosen as the main method of survey. A Geoscan RM15 resistivity meter and MPX15 multiplexer unit were used, with a Twin Electrode probe configuration and a probe spacing of 0.5m. Readings were taken at 0.5m intervals along traverses 0.5m apart.
In addition, magnetic measurements were collected with a Geoscan FM36 fluxgate gradiometer along the same 0.5m-spaced traverses, at a sample interval of 0.25m.
The results of the surveys are shown in Figures 2 and 3 as greyscale and trace plots, and an interpretative diagram is presented in Figure 4. Both sets of data reveal significant anomalies although the interpretation of these is difficult.
The magnetic data (Figure 2) do not show any specific details of structures but instead indicate generalised areas of disturbance where anomalous magnetic activity is concentrated. The most prominent of these areas (labelled A on Figure 4) covers almost 400 sq m near the centre of the inner bailey south of the keep. Another similarly disturbed area (B) has been detected adjacent to the inside of the northern curtain wall, apparently next to the position of the hall implied by the presence of joist holes in the curtain wall. Anomalies within these disturbed areas may be caused by ferrous materials, burnt stone, hearths, or other magnetically enhanced materials within refuse, occupation and destruction deposits. Unfortunately, although these concentrations of anomalies are suggestive (as are the spaces of relative inactivity amongst them), it is not possible to distinguish particular structures within what must be a palimpsest of former events focused within the confines of the bailey enclosure. Some tentative alignments have been indicated in Figure 4.
Certain more extreme magnetic anomalies, also indicated on Figure 4, are responses to modern services and other relatively recent interference.
The plots of the resistivity data (Figure 3) reveal a different and rather more coherent picture. Both high and low resistance anomalies combine within a complex image which succeeds in suggesting at least some useful structural information - even if particular detail is again elusive.
Perhaps most obvious is the partial rectangular outline of low resistance values, seen off-centre of the survey area (labelled C in Figure 4), within and around which are amorphous areas of higher resistance values. Such an outline may indicate either a ditch, or perhaps a robber trench, with a more moisture-retaining backfill than its surroundings; a further possibility is that the anomaly is caused by a moisture-retentive structural material such as brick. The higher readings in the vicinity are likely to be the responses to coarser deposits such as bedrock, collapsed masonry, cobbling, paving, or other accumulations of building debris. Such high readings are widespread over much of the inner bailey and other partial low resistance alignments occur in places amongst them (see Figure 4); but, apart from a general rectilinearity with the keep and the eastern curtain wall, little coherent patterning is discernible. Rectilinear features are suggested by high resistances at positions H and J (Figure 4).
Two linear anomalies stand out and deserve separate comment. One is a high resistance anomaly (D) extending some 12m NNE from the extant building foundations in the southern corner of the inner bailey. This feature, detected faintly in the magnetometer survey as well, may be a wall or conduit. Between this and the well is an area of marked low resistance, suggestive of deeper soil and/or a lack of building foundations in this sector of the enclosure. The second anomaly (E), of pronounced low resistance, can be seen running southwards from the area of the NE corner of the keep towards the curtain wall. It either intersects the curtain wall or, as hinted at in the plot, follows around its inner perimeter. There is no magnetic evidence for this feature, as might be expected were it a ditch, and it remains unexplained.
It is difficult to correlate the separate evidence from the magnetic and resistance data. The
central area of magnetic disturbance (A) is broadly coincident with resistivity evidence which
could indicate the presence of buildings (see above). However, high resistivity readings north
and west of the well, suggestive either of building debris or bedrock, are not complemented
by significant magnetic activity. Conversely, the magnetic activity (B) adjacent to the opposite
curtain wall is not complemented by obvious patterning in the resistivity plot here. The
position of a building in the northern corner of the enclosure (evidenced by the joist holes in
the wall) coincides with a roughly rectangular area of reduced magnetic activity and markedly
low resistance, neither dataset being particularly indicative of stone foundations.
Although both the magnetic and the resistivity surveys have undoubtedly added information to previous knowledge about the inner bailey it is disappointing that the resulting images cannot be interpreted more confidently. As anticipated, the resistivity data is the more informative of the two methods, and several linear anomalies appear to be significant. Evidence for specific buildings is elusive although there seems, at least, to be some evidence for a building (or buildings) near the centre of the survey area and in places around its periphery. The various amorphous areas of high resistance are difficult to explain either as the influence of building debris and/or bedrock.
Resolving some of the questions raised by these surveys, including the relative age of the features detected, can only be satisfactorily achieved by excavation. Without digging, further resistivity survey repeated at times of differing moisture contrast might clarify some of the patterning. Ground penetrating radar would complement such surveys and might assist with their interpretation by providing estimates of depth. Mapping of any parch marks during periods of drought could obviously also be very informative.
E. Bray Date of Report: 14/10/98
Clark, J. 1997. Scarborough Castle, The Archaeological Journal, 154 . Royal Archaeological Institute, London.
Linzey, R., 1997. Scarborough Castle and Headland - brief for conservation plan, 10/6/97. English Heritage, Unpublished.
List of Figures
Figure 1 Location of the survey grids
Figure 2 Magnetic survey results
Figure 3 Resistivity survey results
Figure 4 Interpretation of data