A Geophysical Survey at Glastonbury Abbey, by Claire Stephens
Senior Archaeological Geophysicist & CAD manager at GSB Prospection
In 2009 GSB Prospection undertook a programme of geophysical survey in the grounds of Glastonbury Abbey. The work formed part of a wider research project and had two main aims: to identify previously known Abbey remains, and if, possible, any responses that might represent excavation trenches, thereby assisting in their accurate mapping; and to provide new information on the surviving buried deposits within the whole of the abbey precinct. Three techniques were employed: magnetic gradiometry, earth resistance and ground penetrating radar.
The various phases of building, rebuilding, demolition, destruction and post dissolution activity have all left their mark on the geophysical results, giving rise to complex data sets that have been challenging to interpret. Nonetheless all three techniques have contributed useful information to the project.
The magnetic survey has been the least successful. The data are dominated by large zones of severe magnetic disturbance produced by modern features (e.g. adjacent buildings, buried services, signposts and information boards, the use of iron/steel reinforcement in the reconstruction of the monastic building layout). This disturbance has effectively masked any weaker responses that might be of interest. Despite this, some anomalies of interest have been identified: in particular, the vallum monasterii ditch has been detected and it has been traced further south than previously mapped by excavation.
The resistance survey has revealed numerous anomalies relating to monastic remains. The variations in response suggest a combination of substantial intact wall lines, robbed out or excavated foundations, drains and areas of building rubble or compacted surfaces. Although, many of these seem likely to reflect post-conquest features, some responses have been located that are likely to represent remains associated with the earlier Anglo-Saxon church, the pre-conquest cemetery and St Dunstan's cloister. Anomalies at the western extent of the precinct suggest a possible range of buildings and/or a substantial bank or wall. The vallum ditch is not apparent in the resistance results; it is suggested that the various phases of infilling of the ditch has produced insufficient soil moisture contrast to leave a discernible trace amongst so many other strong responses from other nearby features.
The GPR survey has provided a wealth of three dimensional information. It has helped to clarify the nature (and in some cases, levels of preservation) of features detected by the other two techniques and identified a number of additional features of possible archaeological/historic interest. In particular, an increased number of anomalies relating to the pre-conquest activity have been identified (cemetery and St Dunstan's cloister), and several building phases of the Abbot's House are represented. The vallum ditch has not produced any substantial GPR reflectors, but there are hints of it's position in places where overlying masonry responses appear to slump into the ditch.