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In 2019, the Chilterns National Landscape team launched Chalk, Cherries and Chairs (CCC), funded primarily by the National Lottery Heritage Fund and the HS2 Community and Environment Fund as well as a range of other match funders. The project was a five-year landscape partnership scheme designed to celebrate, restore and improve the environmental and cultural heritage of the Central Chilterns. Working with landowners, local authorities and different communities the project team ran projects to restore habitats, monitor vulnerable species, enhance cultural collections, increase public access and increase knowledge of this area.

One of the projects within CCC explored ‘The Mystery of Grim’s Ditch’ in Buckinghamshire, where the history of this historic Scheduled Monument was brought to residents of and visitors to the Chilterns by partner Buckinghamshire Council, through a number of events and activities, including guided walks, school visits and new research on the monument.

Grim’s Ditch is a linear archaeological earthwork, comprising a ditch and bank, which runs from Bradenham across Buckinghamshire and out into Hertfordshire. It is the longest and largest archaeological asset within Buckinghamshire, but despite its size and significance, relatively little is known about it. It is commonly believed to date to the Iron Age, and to be a land boundary, but neither its date nor its purpose has ever been conclusively established. Therefore, in addition to promoting this incredible heritage asset, one element of the CCC project was to further our understanding of the monument.

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 and whether they are real or not. By that, we mean, was the monument constructed as a sequence of disconnected segments, or did the bank and ditch continue in these sections, which have been subsequently 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 at and close to this section of the monument as part of the HS2 works 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 westwards from the Hunts Green section and towards Road Farm, suggesting it may survive in these areas as both an upstanding earthwork and as a buried archaeological feature.

In 2021, CCC 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. The aim was to see if we could prove whether the ditch continued in this location.

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 the earth resistance survey measures 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 earth work to the north-east at Hunts Green. The earth resistance survey focused on this area and identified a corresponding linear anomaly which interestingly corresponded with a linear feature present on the LiDAR coverage.

The AOC survey raised interesting questions, and further surveys were clearly needed. The difference between the magnetic gradiometer results and the earth resistance results also 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 had been visited, where information about the ditch was shared, and discussions raised about 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 geophysics work themselves. On this basis, the CCC partnership invited the Aylesbury and Chiltern branches of the Young Archaeologists Club (YAC), along with the New Shoots members (another CCC initiative allowing practical conservation skills to young people aged 14-18), to undertake the geophysical survey.

The surveys were carried out over two days on the 16th & 17th of 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; and
• 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;

• one hectare (ha) Frequency Domain Electromagnetics (FDEM);
• four Electrical Resistivity Imaging (ERI) profiles;
• eleven Ground Penetrating Radar (GPR) profiles; and
• nine Transient Electromagnetics (TEM) profiles.

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

For this survey, the 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 overlie and thus 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 is a geophysical survey method that involves sending electrical currents into the ground and by measuring the resulting voltages, it is able to identify differences in the resistivity of the subsurface materials, and thus recognise different buried remains.

During the survey, this technique identified a resistivity anomaly of a substantial c.10-14m wide ditch structure which follows the projected path of Grim's Ditch and cuts into the chalk bedrock to a depth of approximately 2.5 to 3m. A second area of survey also identified this feature, suggesting it extends a further c.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 suggest it does indeed extend through Road Farm.

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

Of the eleven GPR profiles taken, only one presented a clear reflector indicating the presence of a 10m wide, 1.5m deep ditch supporting the EMI results. Why only one profile was presented a clear result is uncertain, but the successful profile presented a clear reflection inside the ditch which interestingly showed results which could either represent that the ditch was backfilled in two separate phases, or that the ditch was recut, representing maintenance of the boundary.

Transient Electromagnetics 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.

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 c.2m below ground level, confirming the results of the other surveys in regards the depth of the feature.

In conclusion, the aims of the survey weekend were met and we now know more about this section of Grim’s Ditch. The SW-NE 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. Whilst 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 and interestingly, its fills. In addition, the TEM survey was able to locate the base of the feature where it cuts into the chalk bedrock. In general, the ERI surveys were the most successful as an individual approach, but the results of all four surveys, when combined all added to the interpretation and understanding of the buried remains on site. The young participants were all able to experience at least two different geophysical survey techniques, and the final result was new information about the monument gained, and more knowledge of the monument shared.

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 www.chilterns.org.uk/GrimsDitch