INTRODUCTION

Geophysical survey was undertaken in January 1994 at Hagnaby Lock, Stickford, Lincolnshire, in response to a request from the Fenland Management Project (FMP). Bronze Age pottery, struck flints, burnt cobbles and burnt flint had been collected over an area of approximately 120m x 150m during the Fenland Survey (Hall and Coles 1994). As such, the site at Hagnaby Lock (FMP site code SKD1) represented one of a number of Bronze Age sites situated adjacent to the contemporary fen edge. Further investigation by the FMP, including excavation, was recommended due its rarity, group value and potential for providing well preserved environmental and economic information (Trimble 1994).

The geophysical survey (centred on TF 34 60) was undertaken in advance of the excavations in an attempt to locate any buried archaeological features and thus aid the location of the trenches.

The site is located on hummocky sands deposited on the western flank of a spur of glacially deposited clay jutting in to the fen just south of West Keal. These sands are sealed by marine deposits but (as is the case here) appear in places as islands above the surrounding marine clay.

METHOD

A grid of 30m squares was laid out, on a roughly north-south alignment (see Fig 1), over the approximate location of the surface scatter of finds. Unfortunately, the survey was undertaken following a sustained period of wet weather and much of the area of interest was under standing water. As a result it was only possible to survey a limited area using the magnetometer (see Fig 1) although a more extensive (but less intensive) investigation was possible with the magnetic susceptibility field loop (see below).

Geoscan FM36 fluxgate magnetometers were employed with measurements being recorded at 0.25m intervals along north-south traverses 1.0m apart. The data was periodically down-loaded to a microcomputer in the field. The resulting data is illustrated in this report using both greyscale and graphical trace plots (see Figs 2-4).

In addition, a magnetic susceptibility (MS) survey was undertaken in order to complement the magnetometer survey. Measurements of topsoil MS were recorded at 10m intervals using a Bartington MS2 meter and MS2D field loop. Presentation of this data has been enhanced by using a local median filter to reduce the distracting effect of random measurement noise and is illustrated here in the form of a greyscale image (see Fig 4).

RESULTS

Magnetometer Survey (Figures 2-4)

The magnetic response at the site was generally subdued and only a limited number of anomalies of potential archaeological significance were detected. The most obvious of these is the group of fairly strong positive anomalies (up to 10nT) visible to the south of the surveyed area, in grid square 24. However, during the subsequent excavations (ie of trench 2 on Figure 1) no obviously archaeological features were revealed at this location. Instead, the anomalies appeared to be a response to deposits of iron-rich sand within large tree-throw hollows (Trimble 1994).

Immediately to the east of the above, an isolated positive anomaly was detected (in the corner of grid square 25) which may well represent a pit. Unfortunately this area was not excavated and so this interpretation must remain speculative.

The survey also detected a number of zones of amorphous positive magnetic disturbance (of up to 2-3 nT in strength) which are unlikely to be archaeological in origin. Similar responses have been recorded by other magnetometer surveys within the Lincolnshire Fens, most notably at Hoe Hills Dowsby (Cole 1995). At the latter site, comparable anomalies were recorded over the course of a former river channel and these were interpreted as accumulations of relatively high MS sediment alongside and within deposits of much lower MS. A similar explanation is perhaps reasonable, therefore, for the anomalies mapped at Hagnaby Lock. Additionally, it is possible that the strong anomalies (in grid square 24) described above might reflect concentrations of such sediment within the tree-throw hollows.

The plots of the data are characterised, particularly in grid squares 15 and 20, by a slightly noisy response, an effect which is most apparent on the traceplot on Figure 3. An explanation for this may well be the distribution of burnt cobbles located in this general area during field walking (Tom Lane pers comm) - these cobbles possibly having acquired a remanent magnetisation when they were heated.

The survey has also detected, at least in part, the course of a former field boundary, visible on the OS map (used for Figure 1) but no longer evident on the ground.

Magnetic Susceptibility Survey

The results of the magnetic susceptibility survey show an average value of approximately 30 SI x10-5. The former field boundary located by the magnetometer survey appears broadly to divide the survey into an area of low readings to its north and an area of generally higher readings the south. Within the latter there are subtle areas of relatively enhanced MS (in grid squares 11 and 12, and grid square 26) which may reflect the presence of former settlement activity. Unfortunately, these areas could not be investigated with the magnetometer due to the poor weather conditions.

CONCLUSION

Poor weather conditions, which limited the area available for survey, and a generally weak magnetic response conspired to make the geophysical data of only limited use in identifying suitable areas for excavation. Indeed the most promising features located by the survey, a cluster of strong positive magnetic anomalies, were shown to be a response to an apparently natural deposit of iron-rich sediment within large tree-throw hollows. Neither of the excavation trenches produced clear evidence of settlement activity, with the exception of two pits and a number of postholes in Trench 2 (Trimble 1994), and this perhaps goes some way to explaining the generally poor response recorded by the magnetometer.

References

Cole, M A 1995 Hoe Hills, Dowsby, Lincolnshire. Report on geophysical surveys, October 1994 and March 1995, Ancient Monuments Laboratory Report Series, 17/95.

Hall, D, and Coles, J, 1994 The Fenland Survey: An essay in landscape and persistence. London: English Heritage.

Trimble, D, 1994 Fenland Management Project, Heritage Lincoln: Hagnaby Lock, Stickford, Fenland Research, 9 .

List of figures

Figure 1 Location plan of survey (1:2500)

Figure 2 Location plan with magnetometer data overlain (1:2500)

Figure 3 Plots of magnetometer data (1:1000)

Figure 4 Plots of magnetic susceptibility and magnetometer data (1:1500)

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