Archaeological Geophysical Prospection in Peatland Environments
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Archaeological geophysical prospection in peatland environments Kayt Armstrong Dissertation submitted in partial fulfilment of the requirements for the degree ‘Doctor of Philosophy’, awarded by Bournemouth University 2010 Volume 1 of 2 This copy of this thesis has been supplied on the condition that anyone who consults it is understood to recognise that copyright rests with its author and due acknowledgement must always be made of the use of any material contained in, or derived from, this thesis. Abstract Waterlogged sites in peat often preserve organic material, both in the form of artefacts and palaeoenvironmental evidence as a result of the prevailing anaerobic environment. After three decades of excavation and large scale study projects in the UK, the sub- discipline of wetland archaeology is rethinking theoretical approaches to these environments. Wetland sites are generally discovered while they are being damaged or destroyed by human activity. The survival in situ of these important sites is also threatened by drainage, agriculture, erosion and climate change as the deposits cease to be anaerobic. Sites are lost without ever being discovered as the nature of the substrate changes. A prospection tool is badly needed to address these wetland areas as conventional prospection methods such as aerial photography, field walking and remote sensing are not able to detect sites under the protective over burden. This thesis presents research undertaken between 2007 and 2010 at Bournemouth University. It aimed to examine the potential for conventional geophysical survey methods (resistivity, gradiometry, ground penetrating radar and frequency domain electromagnetic) as site prospection and landscape investigation tools in peatland environments. It examines previous attempts to prospect peatland sites, both in archaeology and environmental science. These attempts show that under the right circumstances, archaeological and landscape features could be detected by these methods, but that the reasons why techniques often fail are not well understood. Eight case-study sites were surveyed using a combination of conventional techniques. At three of the sites ground truthing work in the form of excavations, bulk sampling and coring was undertaken to validate the survey interpretations. This was followed up by laboratory analysis of the physical and chemical properties of the peat and mineral soils encountered. The key conclusion of the case study work undertaken is that conventional geophysical prospection tools are capable of detecting archaeological features in peatland environments, but that the nature of the deposits encountered creates challenges in interpretation. Too few previous surveys have been adequately ground truthed to allow inferences and cross comparisons. The upland case studies demonstrated that geophysical survey on shallow types of upland peat using conventional techniques yields useful information about prehistoric landscapes. The situation in the lowlands is more complex. In shallow peat without minerogenic layers, timber detection is possible. There are indications that in saturated peat the chemistry of the peat and pore water causes responses in the geophysical surveys, which could be developed as a proxy means to detect or monitor archaeological remains. On sites where the sediments are more complex or affected by desiccation, timbers were not detected with the methods attempted. However, important landscape features were and there are indications that geophysical surveys could be used as part of management and conservation strategies. This thesis concludes that geophysical prospection contributes to theoretically informed wetland archaeology as a tool for site detection, landscape interpretation, and conservation. Future research should aim to further our understanding of the relationship between geophysical response and peatland geochemistry, alongside a more extensive programme of surveys and ground-truthing work to improve survey methodologies and archaeological interpretations. 1 Acknowledgements This thesis would not have been possible without the enthusiasm, guidance and support of my supervisors, Tim Darvill and Paul Cheetham. I have benefited greatly from their advice and intelligence over the course of my research, as well as their help in the field and laboratory. Bournemouth University provided me with a generous studentship, without which I could not have carried out the work. The School of Conservation Sciences has also provided vital equipment, laboratories, software and training to allow me to carry out the practical elements of this project. All of the staff have been very supportive, but special thanks are owed to Drs Mike Allen, Iain Green, Martin Smith, and Kate Welham. Louise, Laura, Marie, and Rebecca from the admin team provided vital logistical support. My examiners, Dr Charly French and John Gale have vastly improved this thesis with their comments and corrections and I am very grateful to them for their ongoing enthusiasm and support. Any errors that remain are entirely my own. Discussions with Neil and Paul Linford, Louise Martin, Chris Gaffney, Armin Schmidt, Alette Kattenberg, Lawrence Donnelly and David Jordan proved inspiring and helpful. Matt Canti from English Heritage let me move into his lab for almost a week and taught me how to use a Sedigraph. Nigel Cassidy at Keele University is assisting with laboratory tests on peat and wood samples. Professor Geoffrey Wainwright encouraged my involvement with the Preseli sites, helped with the surveys and put me up in magnificent Pembrokeshire. Bob Johnson and Helen Wickstead provided me with insights into their work on Dartmoor, and Erica Utsi gave helpful advice about radar survey. The case studies required close co-operation with a number of agencies and landowners, who variously gave permission, assisted in applications, provided access to sites, and gave their specialist knowledge of the area. Phil McMahon and Robert Iles, the monuments inspectors in my regions deserve specific mention, as do Jane Marchand and her colleague Andy Crabb; without them the work on Dartmoor would not have been possible. English Heritage and the Dartmoor National Park Authority funded the excavations at Yellowmead, which greatly enhanced this research. Mike Webber, the education officer at Flag Fen was a joy to work with, and I am very grateful to the Fenland Archaeological Trust for approving and supporting my work there, and to Marcus Brittain, for suggesting I consider it. Richard Brunning championed the important excavations over the Sweet Track, and helped with the fieldwork. His insights on the ground were invaluable. Phil Holms from Natural England also played a vital role here, granting permissions and providing expert knowledge. I have never worked with more interested and enthusiastic landowners than Dennis and Jenny Wright, who own Yellowmead Stone Circles. The surveys could not have happened without the help of my volunteers, so huge thanks to Sarah Clark (who braved Dartmoor in January and October), Rebecca Bennett, James Fenn, Eanna O’Flaitharta, Sarah Pelling, Lynsey Wills and Cat Morgan for giving up their time for free, and keeping me sane. My parents have always supported my ambition to be an archaeologist. I need to thank my Dad for taking me tumulus hunting all over the Scottish Highlands and North Yorkshire Moors in the school holidays, and my Mum for having faith in us and never (quite) calling out mountain rescue. My friends, fellow students and family have been unstinting in their support and encouragement, but I want to especially thank Nicky, Stu, Emma, Ralf, Cat, Adam and my stripy army of cheerleaders. Matt, I don’t know how you have survived my PhD, thank you most of all. 2 Declaration of publications The following article has been published related to this thesis: Darvill, T, Wainwright, G, Armstrong, K, & Ixer, R, 2009, Strumble-Preseli ancient communities and environment study (SPACES): Sixth report 2007–08. Archaeology in Wales, 48, 47–56. Case-study reports were produced during the course of the project to maintain reporting obligations to English Heritage, local HERs and landowners: Armstrong, K & Cheetham, P, 2008a, Archaeological Geophysical Prospection in Peatland Environments: Case Study Survey at Flag Fen. Area One pilot project, September 2007, Radar and Resistance Tomography Supplemental Report: Unpublished limited circulation report. Copy accessed at: Bournemouth University & Fenland Archaeological Trust Armstrong, K & Cheetham, P, 2008c, Archaeological Geophysical Survey in Peatland Environments: Case Study Survey of the Sweet Track in the Somerset Levels. Surveys Near Canada Farm and The Old Peat Works, November to December 2007, Preliminary Report: Unpublished limited circulation report. Copy accessed at: Bournemouth University, English Heritage and Somerset HER Armstrong, K & Cheetham, P, 2008e. Archaeological Geophysical Prospection in Peatland environments: Case study survey at Flag Fen Area One pilot project, September 2007. Unpublished limited circulation report. Copy accessed at Bournemouth University and Fenland Archaeological Trust Armstrong, K, 2009, Interim Reporting on Ground Truthing Excavations at Yellowmead Down, Sheepstor (Devon), October 2008: Unpublished limited circulation report. Copy accessed at: Bournemouth University, Dartmoor National Park HER & English Heritage Armstrong, K & Cheetham, P, 2009a, Archaeological Geophysical Prospection in Peatland Environments: Case Study Surveys