We carried out our last planned phase of our Druitt Gardens geophysical survey during the first week of March 2020. This involved primarily a magnetometer survey of the survey lines we had previously established in August and December last year, and which we had surveyed using GPR and ERT. In view of the success we had in confirming the GPR results with ERT, we also established to further ERT lines. The survey was carried out by our contractor Dr. Martin Bates using the Bartington 601 fluxgate gradiometer (see photo) for the magnetometry and ABEM equipment for the resistivity imaging (ERT). It was decided that highly-sensitive magnetometers were probably inappropriate on site due to the presence of metal seats, as can be seen in the photograph, lamp standards, fences and underground services.
While we were initially fortunate in being able to start the survey in fine weather, heavy rain at the end of our period on site meant that we had some difficulty in completing the work schedule!
At the beginning of December 2019, we carried out the second phase of our geophysical investigation of the scheduled site in Druitt Gardens, Christchurch. In August, we carried out a survey with ground-penetrating radar (GPR) in our search for geophysical evidence of the remains of the Saxon defences of the burh of Christchurch-Twynham. Archaeological evidence dating from the excavations of the 1970s predicted the continuation of the remains of a bank and ditch into and through the Gardens, but likely to be at some 2m below the present ground surface.
Previous geophysical surveys had used techniques which were unlikely to provide penetration of the soil to such depths and in this survey we had chosen techniques such as GPR and, in the present phase, resistivity imaging which are capable of penetration to 2m. The picture shows the resisitivity array laid out during the survey one of our longest lines of some 80m. The survey equipment automatically selects and combines various groups of 4 electrodes in a Wenner array of varying dimensions, and allows a resistivity profile through the soil to be imaged.
The August GPR results had identified a bank/ditch-like feature some 40m from the eastern end of our line. Initial results from the resistvity imaging seem to pick up this same feature, though we lack any sort of ground-truth to confirm the nature of this geophysical anomaly.
During November, we joined with AVAS to investigate the site of the long barrow near Sopley which had been flattened during the construction in 1944 of an airfield for the support of the D-Day landings. The barrow had been well-documented in early records but never recorded as such on early OS maps. Last year, a geophysical survey led by AVAS and TCA member Mike Gill located a feature matching exactly the position and extent of the barrow mound and in November this year we obtained permission from the land owner to try to confirm the former barrow by locating evidence of the side ditches. Now, with a trench 1m wide by some 11m long we have found clear evidence in the stratigraphy of a former ditch extending alongside the barrow, of which no surface feature now remains.
Some members will be aware that for some time we have been negotiating with Historic England and Christchurch Town Council for permission to conduct a geophysical survey within Druitt Gardens and on the site the Scheduled Ancient Monument protecting the supposed known course of the Saxon defences of our ancient town. We are grateful to the Town Council for providing funding for the survey and also to Historic England for granting us a Section 42 licence in response to our request to survey the SAM. Although geophysical surveys have been carried out on site in the past, they did not reveal evidence of the defences and it is generally acknowledged that the techniques used were probably insufficient to reveal archaeological remains shown by excavations outside the Gardens during the 1970/1980s to be buried at depths approaching 2 metres.
The first part of our survey took place on 20/21 August 2019 under licence from the Town Council, and was carried out by Dr Richard Bates of St. Andrew’s University who was able to give us the benefit of his expertise and specialised equipment while in the area prior to taking part in an extensive survey at Stonehenge. Dr. Bates used a MALA Ground Penetrating Radar (GPR) fitted with a 200 MHz antenna, a frequency which was aimed at providing depth penetration. Initial results indicated that penetration to some 3m was being obtained. We were very encouraged to see that on some lines interesting anomalies indicative of bank/trench -like features were apparent on the basic data plots which we hope to study with further processing and enhancement. A view of the GPR in action is shown operating in the garden of Little Millhams in an attempt to see if any evidence of an eastern ditch to the town’s defences could be found.
In the time available, Dr. Bates was also able to deploy another deep-sensing piece of equipment. generally known as an EM31, or electromagnetic conductivity meter. This equipment is shown in action on the left. Also in the picture is the DGPS position-fixing equipment used by Dr. Bates, one component carried in a rucksack and the other component a fixed station on a tripod. Use of this equipment provides simultaneous collection of geophysical and location data.
We hope and expect this to be the first part of a series of surveys with a variety of techniques over the currency of our six-month Section 42 licence.
An exciting event was happening this week at nearby Hengistbury Head in which this site of archaeological importance celebrated prehistoric crafts with a hands-on experience of pottery-making, flint knapping and the preparation of pigments for decoration. Our chairman took along some the clay-like layers which augering has revealed at Millhams and produced a very handsome pot. While many of the replica pots made with modern processed clay shattered in the firing, the sand-rich clay from Millhams remained intact and fired to a fine terracotta colour.
Since we started the augering of the site in 2018, as reported in a previous blog post, we have completed some 8 lines across the site in an E-W direction, each containing boreholes spaced at 2m apart. The length of each line varies according to the site boundary but containing usually about 15 boreholes. The various strata have been examined as they are logged with a view to determining the ancient ground surface, which could be river gravels or an orange clay. Above these levels are generally river sediments (over the gravel) or layers containing silt due to subsequent flooding and/or organic remains such as twigs or small riverine snails.
In order to visualise this ancient land surface underneath the modern, generally flat, appearance of the gardens, we have constructed a digital elevation model (DEM) of the surface within the geographic information system QGIS which we use to plot the Millhams project. An interim result is shown in this blog, which comprises a snapshot from a 3D model of the DEM with shading and colour contouring. Although not complete, the impression we have gained of an ancient channel running through the site can be clearly seen.
TCA held its 2019 AGM on 26 June in the Harbour View Room of Stanpit Village Hall. The Treasurer presented a financial report for the 12 months up to 31 May 2019 which was duly accepted by the meeting. The Chairman presented a report on the activities of the society over the last 12 months, mainly comprising the archaeological project at MIllhams but also including involvement in other projects concerned with the history and heritage of Christchurch. In the absence of any other nominees or volunteers our current committee members and officers agreed to serve for a further year.