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Geophysics at Mere End

One effective use of geophysical survey is in quickly and accurately locating and defining archaeological sites, so that effective management strategies can be established. A case in point is a disc barrow (Scheduled Ancient Monument OX199) on Mere End Down, Oxfordshire, initially mapped by the National Mapping Programme (NMP) of the Lambourn Downs.

More recently, questions have been raised over the state of preservation of the barrow and therefore a new management solution is being sought by the Inspector of Ancient Monuments, Chris Welch.

Magnetometer aerial view of disc barrow on Mere End Down

Magnetometer Plot and Location of Grass Island

The first part of the investigation strategy was to undertake a detailed study of the aerial photographic record. This was conducted by Helen Winton from the Aerial Survey Team based in Swindon. Her report showed that the barrow was still extant as a topographic feature in the 1950s, but has since suffered from significant plough damage.

Aware of this, the farmer has been leaving an unploughed area of grass over the site of barrow to prevent further damage. However, owing to the lack of nearby landmarks, it has not been possible to accurately maintain the correct size and position for this grass island from year to year.

In August 2004 our team undertook a caesium magnetometer survey of the barrow, using our cart system. The aim of this survey was to accurately locate the round barrow on the ground and to identify if there were any related external features so far not recorded on aerial photographs. A topographical survey was also conducted to collect data to create a Digital Terrain Model (DTM – a 3D model of the site) to help with the interpretation of the magnetometer data. This data was collected with our Global Positioning System (GPS), which also gave us accurate co-ordinates for our survey grid and therefore the important anomalies revealed in the magnetometer survey.

Despite the difficult site conditions – due to the uncultivated island (overgrown and bumpy) surrounded by a freshly ploughed arable surface (very soft) – the survey has successfully located the true position of the barrow ditch. This is shown in the data to the right by a white circular anomaly in the centre of the plot. There is also an indication of an inner mound platform and possibly the outer bank. The course of an unrecorded pipeline passing close to the monument shows up as a strong linear anomaly. In this instance, no significant features outside the barrow bank can be interpreted as being directly related to the barrow itself.

Whilst examining the data in the field we calculated the position of the centre of the barrow and, with the help of the farmer, marked it for future reference. Back at the office we then had a chance to fully process the data and calculate how large the protective area should be. This information was passed on to Chris Welch and the farmer, and the area to be left unploughed can now be correctly identified. In the future it is hoped to mark it permanently, either with fixed markers or with trees.

This figure is a false colour plot of data draped over the DTM.  Owing to the pronounced slope of the hillside a first order surface of best fit was subtracted from the DTM to enhance local variations in surface height and the vertical scale has been exaggerated by a factor of 15.

References

Martin L 'Mere End Down Barrow, Oxfordshire. Report on Geophysical Survey, August 2004' Centre for Archaeology Report No. 68/2004, 2004

Winton, H  'Mere End Down Barrow (Oxfordshire 199) Oxfordshire' Aerial Survey Report Series AER/2/2004, 2004