GIANT HILL, THORGANBY GRANGE, N. YORKSHIRE

Report on Geophysical Survey, December 1997

INTRODUCTION

A geophysical survey was carried out over Giant Hill, Thorganby Grange (N. Yorkshire, NGR SE 69 39), to form part of the Humber Wetlands Project (HWP: Centre for Wetlands Archaeology, University of Hull).

Giant Hill is an earthwork with a ditch and upcast bank, and is recorded as a castle ringwork in the North Yorkshire SMR (No. 556300000). It is situated on a promontory in the Derwent flood plain, bounded on one side by the course of the old river Derwent, and on another by the existing river Derwent. The promontory geology consists of red Bunter Sandstone overlain by silt and clay; alluvial deposits from the rivers bound the promontory to the south and east.

With a steep drop to the flood plain, the location is suitable for a defensive structure and is thought to have been utilised no later than the 11th-century AD (R. Van de Noort, pers comm).

The land including the earthwork is currently under pasture and, being situated close to the floodplain, has good potential for waterlogged deposits.

METHOD

An initial grid of 30m x 30m squares was set out by the HWP team for optimum coverage of the earthwork.

Magnetometry was chosen as the method of survey for this site for several reasons: firstly, the use of gradiometers has been proven to be a quick and efficient method of data collection, often producing informative and reliable data. Secondly, no modern-day interference which would obscure any archaeological features was known about. Finally, the frozen state of the ground meant that an initial survey using resistivity equipment would progress very slowly and not necessarily yield very reliable results.

Four Geoscan FM36 gradiometers were used to gather magnetic data from the site; traverses were separated by 1m and walked in an approximate N-S direction, with data being logged at 0.25m intervals.

Once the earthwork and its interior had been surveyed, the gridded area was extended westwards to investigate the region immediately outside the ditch.

RESULTS

The results of the magnetic survey are displayed in Figure 2 as raw data (greyscale plot and x-y trace) and after filtering in Figure 3a. To arrive at the latter, the dataset was first clipped at +/-0.5nT to remove the effects of the modern disturbance, then convolved using a Gaussian filter of window radius 1m. A Wallis filter was then applied (ratio 0.1) to sharpen the picture slightly.

Unfortunately, a modern system of drains installed in the area dominates the magnetic response. For clarity, these modern features are not included on the interpretation diagram in Figure 3b.

A ridge and furrow pattern exists to the north of the earthwork, oriented NW-SE, and with a consistent maximum ridge width of approximately 8m.

An area of rather amorphous magnetic disturbance adjoining the end of the ditch may indicate a former eastward extension of the earthwork. A very slight negative magnetic anomaly just to the north of the earthwork might perhaps have an archaeological origin although its pattern is unfortunately partially obscured by the response from one of the drains.

Just to the west of this anomaly there appears to be a very subtle boundary which separates ground where the ridge and furrow is relatively strongly detected from the main survey area to the east where it is not so clearly resolved. This may reflect a local variation in the drift geology.

Positions of anomalies which may represent pits are marked on the interpretation Figure 3b.

CONCLUSIONS

Despite being located within and around the prominent earthwork of Giant Hill, the survey has revealed only a few magnetic anomalies of possible archaeological significance. There is good evidence that the ditch extended at least 12m further to the east, and there is a hint of an isolated archaeological feature, perhaps a building, to the north of it. Elsewhere within the survey area, the magnetic response very subdued except where interrupted by the pattern of field drains.

E. Bray
P. Cottrell
N. Linford
A. Payne

E. Bray Date of Report: 13/2/1998

Archaeometry Branch
Ancient Monuments Laboratory
English Heritage

List of Figures

Figure 1 Location of survey grids
Figure 2 Raw magnetometer data: a) histogram equalised greyscale plot
b) trace plot
Figure 3 a) Linear greyscale plot of filtered data
b) Interpretation diagram

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