Grim’s Ditch Geophysical Survey

The Chalk, Cherries and Chairs Landscape Partnership, along with the Chilterns and Aylesbury branches of the Young Archaeologists Club, recently conducted a geophysical survey of Grim’s Ditch.

The line of Grim’s Ditch through Buckinghamshire is not a continuous feature, instead it comprises sections of bank and ditch with gaps in between. A key research question relates to these gaps – was the monument constructed as a sequence of disconnected segments? Or did the bank and ditch continue in these sections that have been removed? Understanding this will help to better understand the purpose of the ditch.

One of the shortest sections of the upstanding earthworks of Grim’s Ditch is at Hunts Green, near to Great Missenden.

Works close to this section of the monument, as part of the HS2 construction, had shown that the ditch and bank in this area originally extended northwards but had been infilled and taken out of use as early as the Roman period. Recently flown LiDAR imagery also suggested that the ditch and bank may have continued towards the south-west, with linear features visible extending westward from the Hunts Green section, towards Road Farm, suggesting it may survive in these areas as both an upstanding earthwork and as a buried archaeological feature.

Known route of Grim’s Ditch as it crosses through Buckinghamshire © Buckinghamshire Council

In 2021, Chalk, Cherries and Chairs commissioned AOC Archaeology Group to undertake two forms of geophysical survey in the fields to the west of Road Farm, Great Missenden, to see whether they could identify a continuation of the ditch in this location.

Geophysical survey is an archaeological technique which aims to create plans of buried archaeological features in a non-invasive manner. There are different types of geophysical survey, but all work on the general principle that they are detecting buried archaeological remains by recording when and where their physical properties contrast with their surroundings.

Results of AOC Earth Resistance Survey showing the processed data and the interpretation showing the linear area of high resistance © AOC Archaeology

The AOC survey carried out a magnetic gradiometer survey and an earth resistance survey. Magnetic gradiometer surveys measure the changes in the Earth’s magnetic field as a result of the presence of buried remains, and earth resistance surveys measure variations in soil moisture as a result of the presence of buried remains.

The gradiometer survey did not identify any definite archaeological anomalies but it did identify a number of linear magnetic responses, of which two responses align with the prominent section of the Grim’s Ditch earthwork to the north-east at Hunts Green. The earth resistance survey focused on this area and identified a corresponding linear anomaly which corresponded with a linear feature present on the LiDAR coverage.

The AOC survey raised interesting questions and further surveys were needed. The difference between the magnetic gradiometer results and the earth resistance results raised the question of which geophysical technique would be best placed for searching for a large historically infilled ditch.

Earlier in the project, both branches of the Buckinghamshire Young Archaeologists Club discussed the potential purpose and function of the monument. Based on the success of these sessions, it was suggested to undertake an open survey weekend, offering young people the opportunity to carry out the geophysical work themselves. Chalk, Cherries and Chairs invited the Aylesbury and Chilterns branches of the Young Archaeologists Club (YAC), along with the New Shoots members (another initiative offering practical conservation skills to young people aged 14-18), to undertake the geophysical survey.

The surveys were carried out over two days in September 2023 with instruction and supervision by competent operators and archaeologists, with the following aims:

Gather further evidence for the presence of Grim’s Ditch in this location,
Evaluate the effectiveness of four different geophysical methods,
Introduce the different geophysical techniques to a new audience.

To compliment the work previously undertaken, the survey was focussed on the area of the AOC surveys where the anomaly had been detected and the geophysical methods utilised included:

Members of Aylesbury YAC carrying out ERI survey © Aylesbury YAC

1 hectare Frequency Domain Electromagnetics (FDEM),
4 Electrical Resistivity Imaging (ERI) profiles,
11 Ground Penetrating Radar (GPR) profiles,
9 Transient Electromagnetics (TEM) profiles.

Frequency Domain Electromagnetics work by simultaneously targeting electrical and magnetic variations, which allows for both the presence, and depth, of variations to be recorded.

For this survey, the Frequency Domain Electromagnetics (FDEM) results showed alternating bands of high and low resistivity, which have been interpreted as medieval ridge and furrow – a more recent archaeological earthwork which would hide any earlier underlying archaeology.

One anomaly on the geophysics, which was on a similar alignment as the medieval ridge and furrow, did not follow the regular pattern and was thought a potential candidate for the line of Grim’s Ditch. However, this anomaly was only present in the north-east half of the survey area and could not confidently be considered to be part of the monument.

Electrical Resistivity Imaging (ERI) is a geophysical survey method that involves sending electrical currents into the ground and measuring the resulting voltages. It can identify differences in the resistivity of the subsurface materials and recognise different buried remains.

Members of Chilterns YAC carrying out FDE survey © Chilterns YAC

During the survey, this technique identified a resistivity anomaly of a substantial 10m to 14m wide ditch structure which follows the projected path of Grim’s Ditch and cuts into the chalk bedrock to a depth of approximately 2.5m to 3m. A second area of survey also identified this feature, suggesting it extends a further 110m to the south-west into the neighbouring field. The size and depth of the linear feature, and the fact the alignment matches a known section of the monument, suggests this is likely to be the infilled ditch of Grim’s Ditch and that it does extend through Road Farm.

Ground Penetrating Radar (GPR) sends radar pulses into the ground and measures the reflected signals from the various subsurface interfaces.

Of the 11 GPR profiles taken, only one presented a clear reflector indicating the presence of a 10m wide, 1.5m deep ditch. This could either mean that the ditch was backfilled in two separate phases, or that the ditch was recut, representing maintenance of the boundary.

Transient Electromagnetics (TEM) induce electric and magnetic fields and then measure the subsequent decay of the electric currents. It is normally used for measuring variations at a greater depth than traditional archaeological geophysics, such as during mineral exploration and groundwater mapping. Due to the potential depth of Grim’s Ditch, it was considered worthwhile including this method within the survey weekend.

New Shoots members conducting the Ground Penetrating Radar survey © Chiltern Rangers

The deeper coverage offered by the TEM method meant that the upper fills of the ditch feature were not as clear as the other techniques, however the results clearly depicted the bedrock interface at 2m below ground level, confirming the results of the other surveys in regards the depth of the feature.

Results from the survey weekend show that the southwest/northeast bank seen on the LiDAR coverage appears to be the extant remains of a disused field boundary, which overlies the buried remains of Grim’s Ditch.

While the FDEM results failed to present conclusive evidence of the presence of Grim’s ditch, the GPR and ERI surveys were able to identify the linear feature. In addition, the TEM survey was able to locate the base of the feature where it cuts into the chalk bedrock.

The young participants were all able to experience at least two different geophysical survey techniques, and the final result provided new information about the monument.

New Shoots members conducting the Transient Electromagnetic survey © Chiltern Rangers

To find out more about what the project achieved, and to explore a virtual tour of the Buckinghamshire Grim’s Ditch area and guided walk, see The Mystery of Grim’s Ditch.

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