APPENDIX B: REPORT ON GEOPHYSICAL SURVEY, JULY 1998

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1 APPENDIX B: REPORT ON GEOPHYSICAL SURVEY, JULY 1998 by Andy Payne Introduction A geophysical survey was carried out at Le Yaudet, Ploulec h, Brittany as part of the ongoing investigation into the archaeology of the area. The project thus far has uncovered evidence of occupation at the site from the Late Iron Age period to post-medieval times. The local geology consists of granite interspersed with bands of granitic sand. The aims of the survey were to test for the potential presence of archaeological remains in areas of the site not yet excavated, and also to assess a range of geophysical techniques in the local geological conditions to discover if they have a useful archaeological application at the site and in the wider surrounding area. It was intended that the resulting information would assist in planning future excavation at Le Yaudet and on similar sites in the surrounding region. Methods Because of the untested nature of the site and the local geological conditions and the uncertainty this engendered, resistivity and magnetometer surveys were carried out at Le Yaudet in the hope that at least one of the methods would prove suitable for the detection of the archaeological features present. Hopes of obtaining good results at the site were still not high, because the granite bedrock lies very near to the surface in some areas and this would be expected to cause problems for both geophysical techniques. Not only is the granite highly resistive but it could possibly be non-uniformly magnetic as well and therefore the influence of geology on the resistivity and magnetometer data could quite easily override any response to the often quite small archaeological features previously recorded at Le Yaudet. Magnetometers respond to local modification of the geomagnetic field by magnetic iron oxides that have become concentrated in archaeological features. The fluxgate magnetometers used for the survey at Le Yaudet can in most cases detect features such as silted-up ditches and pits, walls constructed from materials with contrasting magnetism to the surrounding soil, fired clay structures and deposits of burnt material. Despite a slower speed of operation than magnetometer survey, resistivity survey is a similarly well established geophysical technique used in archaeology. Although resistivity is capable of identifying larger pits and ditches, it is more suited to the detection of sub-surface building foundations and other buried masonry features such as road surfaces. Because the electrical resistance of the ground varies according to soil moisture content, the presence of relatively impermeable masonry will cause an increase in resistance at that location. Conversely buried ditches and other silted-up features, cut into the natural subsoil or bedrock, generally retain more water than the surrounding deposits, giving rise to low resistance anomalies. Resistivity can also be used as a slower alternative to magnetometer survey for detecting buried ditches and similar features, in cases where magnetometer survey is ineffective (for example in areas where there is magnetic interference from buried modern pipes and services or igneous intrusions). The trial geophysical surveys were carried out in two separate areas of the site (Fig. 1). The first to be surveyed (Area 3, the Central Valley) was a stretch of grassland adjoining the car park,

2 hemmed in on its remaining three sides by dense vegetation. The land here slopes away from the car park towards the west in the direction of the Léguer estuary and was bracketed on each side by previous trial trenches (21 and 22) excavated in These contained a number of features (including in T21 a ditch (F453) on a N-S alignment) that were expected to continue into the survey area. It was anticipated that the existing knowledge of the buried deposits in the area, provided by the trial trenches, would assist the interpretation of the survey results, enable the geophysical response to known features to be assessed and possibly (depending on the outcome of the surveys) enable the results from the limited trial trenches to be put into a wider context. The second and larger of the two areas surveyed (Areas 1 and 2) included the small approximately triangular hay-field next to the chapel and the area immediately to the north enclosed by stone walls. The whole area slopes gradually up to the south-west and is bounded by dense vegetation cover to the north and west, the chapel and modern habitation to the northeast and east. Bare rock (belonging to the rock formations known as Les Rochers du Chateau and Rochers de Beaumanoir) outcrops in the south-westernmost part of the area and the remains of the Late Iron Age period rampart run alongside the south-east edge of the survey in the hayfield. The two areas chosen for examination were divided as far as possible into 30 m x 30 m squares (see Figure 1), and resistivity and magnetic surveys carried out over each. i) Magnetometer survey Geoscan FM36 Fluxgate Gradiometers were used to take measurements of the vertical component of the ambient magnetic field within each of the grid squares, along traverse lines separated by 1 m. Readings were recorded at intervals of 0.25 m along each traverse. ii) Resistivity survey Also using 1 m traverse separations, a Geoscan RM15 resistivity meter was used to collect resistance readings over the selected areas using a twin-probe array, with mobile electrode separations of both 0.5 m (sample interval 0.5 m) and 1.0 m (sample interval 1m) obtained by using an MPX15 multiplexer unit. This enabled the instrument to gather data from different depths and, since the effects of the local geology were expected to be strong, it was hoped that these could later be removed from the shallower data to reveal any nearer surface archaeological contribution. This technique was employed in Areas 1 and 3. A 1 m sample interval and 0.5 m mobile probe spacing were utilized in Area 2 (due to the requirement to return some of the equipment to the UK before the fieldwork was finished). Data gathered during the surveys are presented as greyscale plots in their relative positions on Figures 2 and 3. A summary interpretative diagram relating all the geophysical information to the plan of the site is presented in Figure 4. Results i) Magnetometer survey The results of the magnetometer surveys are disappointing. The magnetic response over both areas surveyed is noisy and erratic and contains few if any anomalies that can be reconciled with

3 archaeological features. The only obvious anomalies in the data are caused by objects of ferrous metal which are most likely to be of recent origin. If any archaeological features are present in the areas surveyed there is presumably insufficient magnetic contrast between them and the underlying geology or a variable magnetic signal from the granite is obscuring any response to them. ii) Resistivity survey In comparison to the magnetometer survey, the resistivity data from Le Yaudet contains a great deal more that is of interest. The field next to the chapel (Area 1) contains a wide linear low resistance anomaly (approximately 10 m across) running diagonally across it, on a roughly east-west alignment [A on Figure 4]. It appears to continue to the west in the next field on a heading that would take it just to the right (or north) of Les Rochers du Chateau but skirting the summit of the site. The low resistance anomaly appears to have parallel deeper troughs of low resistance along each of its sides and it is possible that it may represent a trackway with side ditches. Immediately to the north of this feature, and aligned along it, are several oblong-shaped blocks of high resistance [B and C] which may represent the remains of stone buildings fronting the possible road. These features are on a similar axis to the chapel and if they do represent buildings may therefore be elements of a medieval settlement. A further area of high resistance [D], visible to the south of the low resistance linear anomaly, may also represent part of the same series of structures. A note of caution should be introduced at this point, because while it is very tempting to see the plans of buildings in the shapes of high resistance mapped in Area 1 by the resistivity survey, the anomalies lack any internal structural detail and it is possible therefore that the anomalies represent something more mundane perhaps areas of bedrock nearer the surface or heaps of some dumped material. The more certain geological features mapped by resistivity in Area 3 also suspiciously share a very similar alignment to the chapel and anomalies A D in Area 1. In the same way, the low resistance linear anomaly could be caused by natural bands of sand or soil in the granite. In Area 2 (to the west of Area 1) a series of regularly aligned fairly narrow (c.2.5 m wide) linear low resistance anomalies [E/F] are visible, sharing a similar orientation to the wider linear anomaly [A] to the north. It is possible that these represent former boundaries (perhaps of small fields) or features linked to former cultivation of the land. They appear to be linked by other less distinct linear anomalies. Low resistance features such as these could also be caused by linear variations in the underlying rock. Near the eastern edge of Area 2 there appears to be a group of three or four localized low resistance anomalies which could represent large irregular elongated pits or hollows. These are arranged in a wavy line running approximately north-south towards the major linear anomaly [A]. A further possible pit-type feature is present near the south-east corner of the field. The significance of these features is uncertain, but it is possible that they might be archaeological. The wide linear variations in resistance in Area 3 are probably an effect of the geology, based on what is already known from trial trenching. A series of faint linear low resistance anomalies forming two sides of a rectangle in the extreme south-west of the area may be a response to a buried structure terraced in to the hill-slope (suggested by the presence of occupation deposits in the trial trench immediately to the south).

4 Conclusions The surveys have shown that resistivity survey has the most potential to reveal the presence of buried archaeological features at Le Yaudet. Magnetometer survey proved to less successful, and the further application of this method at the site is unlikely to prove rewarding. The resistivity has indicated the presence of several possible previously unknown groups of archaeological features (particularly a pattern of linear anomalies in Areas 1 and 2) which are likely to be worth further investigation. The precise character of the features responsible for these anomalies will have to await confirmation by excavation as nearly all the anomalies could also be attributable to geological causes including near surface bedrock and bands of sand within the underlying granite.

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