Geophysical Survey at Mount Pleasant Henge
Fifty years after geophysics was first used at Mount Pleasant new results reveal a more complex monument, demonstrating the value of continued research to monitor its condition.
Mount Pleasant is a large, Neolithic, ditch and bank monument comparable in size to other ‘'mega-henges', such as Avebury and Durrington Walls in Wiltshire, but remains almost hidden from view under arable fields to the east of Dorchester, Dorset.
Much is known about the henge owing to Geoffrey Wainwright’s excavations between 1970 and 1971, informed by Tony Clark’s contemporaneous geophysical surveys. Martyn Barber’s more recent analysis of aerial photography has further enhanced the picture. This research has established a broad understanding of the phases of its development, and recognition of the relationship between the henge and the surrounding archaeological landscape. The Conquer Barrow, a substantial earthwork probably dating to the late-Neolithic and currently covered with trees, is situated adjacent to the west entrance of the henge and a number of prehistoric barrows lie to the south and east.
Recent aerial photographs show extensive areas of chalk exposed on the raised banks to the south of the henge where the thin, overlying soils have been eroded. Geophysical survey was suggested to assess the survival of the prehistoric remains and variation in the depth of soil cover protecting the archaeology from the plough.
As there were only a few days available to conduct the fieldwork in March and then later in August 2019 after the harvest, a vehicle- towed magnetometer array was used to cover the whole site. Ground Penetrating Radar (GPR) coverage concentrated on the main henge and obvious areas of soil erosion.
The magnetic survey has mapped the main ditch and internal palisade of the henge with exceptional detail including sections of apparent’gang-digging’ where the ditch appears to have been constructed in an irregular, segmented form, perhaps representing episodic building of each section. When viewed in detail it appears each segment may have been enlarged from an original circle of pits, shown on the image below, marking out the course of the ditch.
There is also evidence to suggest additional entrances where breaks are found in the henge ditch and bank. One of these entrances to the south west was previously identified from aerial photography, but the apparent entrance revealed to the north east has not previously been detected and is most evident in the magnetometer data, which also indicate a possible outer work formed by a second ditch and bank. The later construction of an internal palisade inside the entire circuit of the henge ditch may have created a deliberate, more complete, enclosing structure to conceal views of the monument from those outside
Avenue to the river
Other larger henge monuments tend to be associated with linear avenues and there are crop mark indications that Mount Pleasant was linked in some way to the River Frome. Both geophysical techniques support this suggestion and show a broad, linear anomaly heading north east from the henge down towards the river. Further investigation of the approach beyond the current survey area over the railway line to the north would be useful to better understand the potential relationship between the monument and the river valley.
Survival under the plough
Comparison of the geophysical survey with the excavation evidence confirms that the majority of features still survive
A good example from the excavation is the penannular (ie a broken circle-shaped) ditched feature with internal post-holes known as Site IV, located on a raised plateau within the centre of the main henge.
This circular structure, initially of five broadly-concentric rings of timber posts within a pennanular ditch appears to have been replaced by a structure of sarsen stones in around 1700 BC.
The GPR data suggests, however, that the topsoil protecting the archaeology is quite shallow in places, perhaps as little as 0.2 metres in depth, and there is a pattern of damage from deeper plough scours visible in the underlying chalk immediately to the south. Historic mapping shows the scour pattern is limited to one of the former fields before all the hedge boundaries were removed, suggesting this might be due not to the modern agricultural regime but, perhaps, to an episode of nineteenth to early twentieth century steam ploughing.
A partial inner palisade, hitherto identified on a single aerial photograph only in the north- west quadrant of the henge inside the main palisade, has been replicated in both the magnetic and GPR data sets. It does not appear to be any more extensive than suggested by the aerial photography and, perhaps, either never continued inside the full circuit of the main palisade or has simply not survived.
The GPR has also revealed more detail over the raised banks to the south of the henge. Profiles through the bank suggest an outer weathering layer on top of chalk packing, sealing an apparent buried land surface beneath. It is possible that the area of surface topsoil erosion is due to a later phase of enlargement, represented bythe chalk packing to form a raised bank, south of the main ditch. Material to construct the raised bank may have come from deepening the main ditch or from the ditch found immediately to the south of the bank. Due to the depth of the henge ditch its full profile has not been resolved by the GPR survey beyond a depth of approximately 2 metres.
Beyond the main henge
Several barrow groups are known from aerial photography surrounding the main henge and the geophysical survey has been able to provide some significant additional information here. For example, evidence for an additional ring ditch of a new barrow has been found to the south, and three of the known barrows now appear to have been enclosed by segmented ditches or pits rather than continuous ditches.
These features may possibly represent small henge-type monuments or mortuary enclosures, although extending the geophysical survey further to the south might help determine whether they form part of a more extensive complex.
One curious discovery is a parallel double ditch found in the results of both magnetic and GPR techniques that crosses the site from east to west, and partially follows the circuit of the henge bank to the south. The two ditches are separated by almost 4 metres, too wide to be modern vehicle tracks, and appear to be over 2 metres in depth. They also appear to be overlain by the raised sections of bank to the south of the henge, perhaps suggesting a more significant prehistoric boundary contemporary with, or even marking out, the earlier phases of the main monument.
Fifty years on
The new geophysical field work was conducted almost exactly 50 years after Tony Clark made his initial magnetometer survey. All of the original measurements were made with an analogue instrument with individual readings written on paper and plotted by hand. While an impressive level of detail was drawn from this original survey, it has now been possible to digitise the measurements to present the data in the same format as the modern survey.
Both magnetic surveys revealed a number of large, circular pits up to 8 metres in diameter. When excavated one of these revealed a shallow oval pit with Iron Age and Roman pottery sherds, and another two were thought to be natural clay pockets. The new geophysical survey data suggests that similar, large, deep pits are found across the site, perhaps associated with relict deposits of sarsen stone or chalk fragments.
Comparison between the 1969 and 2019 magnetic datasets also provides an additional means of assessing the survival of the archaeological remains over time. There is even an intense response to a buried iron object that has not moved over the 50 years!
The new geophysical survey provides a comprehensive plan of the monument to enhance the excavation and aerial photographic record. Despite evidence for plough levelling of the earth works in places, much of the monument survives for now beneath the immediate reach of the plough. Covering the whole of the henge with both magnetic and ground penetrating radar techniques provides an important record of the location and depth of the surviving archaeological features. Perhaps in another fifty years this data set will provide a useful comparison to future geophysical survey campaigns.
The results of the geophysical survey are available as a full research report and have been used to help with the on-going management to ensure adequate protection of the site.
About the authors
Senior Geophysicist at Historic England
Neil has experience across a wide range of applied geophysical techniques. Whilst his PhD research focused on the magnetic properties of archaeological sediments, he also has expertise in all aspects of the use of GPR. He is an editor of the journal Archaeological Prospection, has served as the chair of the NERC Geophysical Equipment Facility, and recently co-edited Innovation in Near-Surface Geophysics featuring contributions on many aspects of archaeological geophysics.
Paul Linford, MSc, has worked as an archaeological scientist for English Heritage and Historic England since the mid-1980s and is head of the latter’s Geophysics Team. He has particular interests in archaeomagnetic dating and in developing the team’s caesium magnetometer array. Paul is also Treasurer of the International Society for Archaeological Prospection and a member of the Geological Society’s Near Surface Geophysics Group committee.
Geophysicist at Historic England
Andy has specialised in the practice of archaeological geophysics since the early 1990s, working widely across England and occasionally in France, Spain and the Channel Islands. He has contributed to numerous reports and publications. His archaeological career has also included working on excavations in Orkney and on the site of the Roman amphitheatre in London.
You can find more general information on geophysical survey in our research methods pages and technical advice pages.
Barber, M 2014: Mount Pleasant, Dorset. A Survey of the Neolithic ‘Henge Enclosure’ and Associated Features. Historic England Research Report, Series 70/2014.
Linford, NT, Linford, PK, Payne AW; 2019: Mount Pleasant, West Stafford, Dorset: Report on Geophysical Survey, March and August 2019. Historic England Research Report Series 92/2019
Wainwright, G 1979 Mount Pleasant, Dorset: Excavations 1970-71. Report of the Research Committee of the Society of Antiquaries of London.
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