SEEING BENEATH THE SOIL Seeing Beneath the Soil Prospecting methods in archaeology Anthony Clark To Richard Atkinson First published 1990 by B.T.Batsford Ltd Revised paperback edition 1996 Simultaneously published in the USA and Canada by Routledge 29 West 35th Street, New York, NY 10001 Routledge is an imprint of the Taylor & Francis Group This edition published in the Taylor & Francis e-Library, 2004. © 1990 Anthony Clark All rights reserved. No part of this book may be reprinted or reproduced or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data ISBN 0-203-16498-9 Master e-book ISBN ISBN 0-203-25936-X (Adobe eReader Format) ISBN 0-415-21440-8 (Print Edition) Contents Preface 7 Preface to the second edition 8 Acknowledgments 9 Acknowledgments to the second edition 9 1 The development of archaeological prospecting 11 2 Resistivity 27 3 Magnetometry 64 4 Magnetic susceptibility 99 5 Other methods 118 6 Choice of method: choice of site 124 7 Interpretation and presentation 132 8 Survey logistics 158 References 165 Glossary 168 Supplement 171 Index 188 Preface When I began this book, I thought it would be tions, which have already been well exposed in fairly easy to produce a general worldwide review the archaeological literature. I can only excuse of geophysical prospecting in archaeology; but as myself by saying that some of the old examples my allowance of pages and illustrations was con- simply do remain the best and clearest, and it is sumed, I realized how big the subject had grown, perhaps useful to gather them together in a defini- and how impossible it was to encompass fully in tive way. Others have long lurked in my mind as one reasonably sized book. potential examples, and it is good also finally to Therefore the book has become largely a mani- make use of these. festo for the methods we have adopted and devel- Writing about his book A Brief History of Time, oped in Britain. Perhaps this is no bad thing, because Stephen Hawking said that he was told by his I know of no other country where archaeological publisher that every equation he included would prospecting is so routinely and widely applied. halve his readership. Even so, he felt that he had Clients have come to expect extensive and economi- to run the risk of including E=mc2, but that its cal surveys and this has led, by a process of quite familiarity might make it scarcely detectable. I natural selection, to the development and survival hope that the same will apply to I=V/R. Other of the most practical techniques, and the experi- equations do creep in, but perhaps mentioning ence is worth sharing. This is not to say that we metal detectors and dowsing will help to redress owe no debt to colleagues abroad, as I hope I shall the balance. make clear. My debt to others for kindness and help over I suppose all authors of this type of book hope the years is enormous; I am really only reporting to strike just the balance that will inform the pro- on their achievements. Many are acknowledged fessional and appeal to the general inter ested reader. in Chapter 1, and I will reserve this paragraph for I have certainly borne in mind the needs of the special thanks to my colleagues and successors in smaller-scale and amateur practitioner as well as the Geophysics (now Archaeometry) section of the the professional, and have gone to war on mystifi- Ancient Monuments Laboratory, notably Alister cation and jargon—which I don’t understand any- Bartlett (still a colleague in the world outside), way. Of course, technicalities have to be included Andrew David and David Haddon-Reece, who when dealing with such a subject, but I do not think have brought great intelligence and ingenuity to that much of the sense of the story will be lost by the subject and have endured dreadful hardships glossing over them, at least on a first reading. at my behest. They have been extra-ordinarily Whatever the technicalities, most of us ultimately helpful and generous in providing illustrations admit that the appeal of archaeological prospect- from their own material. ing, which can be uncomfortable and tedious, is This book has proved impossible to the romance and enduring excitement of this pe- compartmentalize neatly. It was essential to have culiarly civilized detective work—restoring to vis- chapters on survey logistics and the interpretation ibility the settlements of long-forgotten people who and presentation of results, but material that would also knew our lands, by means of technology be- have fitted into them had to be introduced in ear- yond their dreams. lier chapters to illustrate and explain the survey I suspect that a groan will go up from many of techniques. Magnetic susceptibility as a survey my colleagues when they see some of the illustra- method seemed to follow naturally after 8 Preface magnetometer survey, yet had to be introduced in tral, essential themes of survey and presentation, the earlier chapter. Phosphate surveying seems to a structural problem that was not unique. pop up all over the place. I am grateful to my The book includes some results of original re- brother, Perry Clark, who guided me through these search, especially in resistivity, which have not difficulties, explaining that they arose from the fact been previously published. that the different components of the subject were related among themselves as well as to the cen- Guildford, June 1989 Preface to the second edition tures that can be produced by archaeological geo- physics, have led to an increasing demand for The need for a second English edition of this book instrumental prospecting as part of the standard has arisen fairly quickly. It therefore seemed ap- routine of archaeology, and there are now a propriate to leave the original layout largely un- number of small companies in Britain and abroad touched apart from corrections, and to put most specialising in providing this service. The role of new material into a supplement at the end. This the Ancient Monuments Laboratory has shifted revised version also forms the basis of the first from that of main practitioner increasingly to- Japanese edition. wards development, setting standards and moni- The first publication of the book in 1990 coin- toring the work of others. cided with the appearance in Britain of the enor- Dealing with large evaluation exercises has mously influential government planning policy mainly fallen to magnetometry and magnetic sus- guidance note, PPG16, which advises regional ceptibility, because of their speed and strength of planning authorities to require developers to fund response to former human activity. The use of archaeological investigation as a precondition of magnetic susceptibility has especially developed the granting of planning permission. This has as a result, and Chapter 4 has had to be consid- changed the emphasis of much archaeological erably complemented. prospecting from a mainly site-specific role to a The book has become adopted as a text for search for archaeological sites on a landscape college courses. Therefore I have included a little scale. Road and rail corridors many k ilometres in more mathematical substance on resistivity in the length, and areas for mineral extraction, often 50 supplement. hectares or more in area, must be searched eco- nomically and quickly for the presence of remains. Guildford, January 1996 This need, and steady improvement of the pic- Acknowledgments Material for this book has been gathered from 78, 85, 86, 88, 94, 96–104, 106, 107(B), 110; many sources. Quotations have been acknowl- Tsuneo Imai: Fig. 92; John Mead: Fig. 3; Megger edged in the text, and I am grateful to the follow- Instruments Ltd: Fig. 1; National Monuments ing for permission to use illustrative material, Record, Royal Commission on the Historical much of it copyright. Monuments of England: Figs. 42, 84; Mark Noel: Academic Press Ltd: Figs. 34, 39, 54, 55, 78; Fig. 48; Northampton Development Corporation: Martin Aitken: Figs. 7, 79; Edward Arnold Ltd: Fig. 54(A); Jeffery Orchard: Figs. 54(C), 65, 66; Fig. 49; Richard Atkinson: Fig. 2; Bartington In- OYO Corporation: Fig. 92; Frank Philpot: Figs. struments Ltd: Fig. 81(A); Alister Bartlett: Figs. 13, 14; Plessey Company: Fig. 13; Research Labo- 105, 107(B, C); B.T.Batsford Ltd: Fig. 87; Martin ratory for Archaeology, Oxford: Figs. 9, 51, 64; Bott: Fig. 49; British Gas PLC, Fig. 98; Cambridge Julian Richards: Fig. 86; Graham Ritchie: Figs. Committee for Aerial Photography: Figs. 4, 73, 74; Royal Commission on the Ancient and 107(A); Clarendon Press, Fig. 79; Fondazione Historical Monuments of Scotland: Figs. 73, 74; Lerici: Figs. 14, 25(A), 37, 104; Geonics Ltd: Fig. Irwin Scollar: Figs. 8, 9; Sheffield University: Fig. 25(B); Geoscan Research: Fig. 53; P. Goodhugh: 48; Society of Exploration Geophysicists: Fig. 92; Fig. 43; David Graham: Fig. 108; Edward Hall: Surrey University: Fig. 20; Vivien Swan: Figs. 14, Fig. 51; Colin Heathcote: Fig. 63; Historic Build- 15; Thames & Hudson: Fig. 21; Thamesdown ings and Monuments Commission (English Her- Archaeological Unit: Fig. 105; Michael Tite: Fig. itage), Ancient Monuments Laboratory and 64; Trust for Wessex Archaeology: Fig. 86; David Publications Section: Figs. 55, 57, 58, 72, 75, 77, R.Wilson: Fig. 87. Acknowledgments for the second edition 125; Historic Buildings and Monuments Com- mission (English Heritage), Ancient Monuments I must add my gratitude to the following for Laboratory and Publications Section, and Neil material they have allowed me to use in the sec- Linford: Fig. 120; Yasushi Tanaka and STG Cor- ond edition of this book. poration, Shanghai: Fig. 127; Stratascan, RPS Buckinghamshire County Museum and Depart- Clouston and Severn-Trent Water plc: Fig. 123. ment of Transport: Fig. 124; Cotswold Archaeo- I am also grateful to J.C.E.Jennings for helpful logical Trust and Wales Built Homes Ltd: Fig. suggestions.