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Ham Hill Geophysics

Geophysical survey was undertaken in response to the managed expansion of stone quarrying at the Iron Age hillfort on Ham Hill, 6km west of Yeovil in Somerset. Ham Hill is one of the largest Iron Age hillforts in England, enclosing an area of about 88 hectares, defined by two ramparts and ditches characteristic of developed hillforts spanning the Early to Middle Iron Age periods.

The site occupies a plateau with a north projecting spur formed of Upper Lias Shelly Limestone (Ham Hill Stone). This distinctive stone is in demand locally as it is the primary building material used in the historic building fabric of South Somerset.

Colour aerial photograph showing a hill with large ramparts. It has been heavily quarried in the middle and is also very scrubby
Ham Hill looking south, photographed on 31-AUG-2004, with the north spur and former quarrying disturbance visible in the foreground and the main plateau area investigated by the geophysical survey at the top of the image (NMR 23702/11) © Historic England

Geophysical survey

The conservation of Ham Hill is a delicate balance between the competing demands of preserving an archaeological site of significant importance and the need for a sustainable supply of Ham Stone for the repair of the historic built environment in South Somerset.

Parts of the site have been quarried for Ham Hill Stone since the Roman period and two quarries are still in operation, with the remainder managed as a country park by South Somerset District Council.

Caesium magnetometer survey was undertaken by the Geophysics Team of the Remote Sensing Group of English Heritage (now Historic England) in the unthreatened plateau area of the hillfort. This was to complete previous geophysical coverage of the monument and augment intrusive investigations carried out during 2011-2013 in advance of renewed quarry expansion to the west.

The geophysical survey employed an array of between four and six high sensitivity caesium magnetometer sensors mounted on a non-magnetic cart or vehicle-towed sledge system in conjunction with a mobile GPS receiver for simultaneous recording of positional data.

This rapid and high-resolution magnetic survey technique provided exceptionally clear definition of buried occupation remains within the hillfort, such as pits, hearths, ovens, circular gullies and foundation slots for timber round-houses.

Colour photograph showing two men in a field with a large wooden wheeled contraption
Non-magnetic cart with array of 4 specially modified Scintrex SM4 Caesium vapour magnetometer sensors and GPS for simultaneous positional recording in operation at Ham Hill, May 2011 © Historic England

Geophysical interpretation

The surveys have provided detailed evidence for activity within the hillfort interior from the Bronze Age to Roman periods.

Grayscale image with numerous features visible as lighter lines against a mainly darker background
Caesium magnetometer data from Ham Hill in relation to the plan of the eastern area of the hillfort © Historic England: Geophysics Team

Evidence highlighted by the geophysical survey includes:

  • An early phase of probable Bronze Age field systems and linear boundaries.
  • Iron Age activity represented by a major arterial roadway running between the main entrances to the hillfort and numerous ditched enclosures with associated pit clusters and round-houses.
  • Subsequent Roman activity in the form of a small corridor villa with further associated enclosures and quarries.
  • Several early phases of quarrying activity possibly dating as far back as the Roman period. These appear to have disturbed earlier traces of occupation where they occur, though fortunately their extent is limited.

Colour image showing the different types of anomalies in red, yellow and other colours against a largely cyan background
Graphical summary of significant magnetic anomalies detected at Ham Hill © Historic England: Geophysics Team

In conclusion Ham Hill appears to have been an occupied hill-top settlement with a long and complex sequence of use. Combined with earlier geophysical surveys undertaken in the 1990s a near complete archaeological map of the internal character of the hillfort has been obtained, providing an enhanced understanding of an important later prehistoric monument under continued pressure from mineral resource exploitation.

The full results of this survey can be downloaded as reports RRS 22-2012 & RRS 67-2014 from the Historic England research reports database.

Additional information on further geophysical surveys of hillforts in southern England can be found in the Wessex Hillforts volume:

The Wessex Hillforts Project

The Wessex Hillforts Project

Published 15 September 2006

An archaeological survey of the Iron Age prehistoric earthwork forts that crown many hills in Southern England. The survey distinguishes several classes of hillfort, and reveals the great variation in complexity among sites.


The images used on this page are copyright Historic England unless specified otherwise. For further details of any photographs or other images and for copies of these, or the plans and reports related to the project, please contact the Historic England Archive.

For further information on a project or any other aspect of the work of the Remote Sensing Team please contact us via email using the link below.

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