University of Bradford eThesis This thesis is hosted in Bradford Scholars – The University of Bradford Open Access repository. Visit the repository for full metadata or to contact the repository team © University of Bradford. This work is licenced for reuse under a Creative Commons Licence. DEVELOPING ARCHAEOMAGNETIC DATING IN THE BRITISH IRON AGE Sarah-Jane CLELLAND Submitted for the degree of Doctor of Philosophy Department of Archaeological Sciences University of Bradford 2011 Abstract Developing archaeomagnetic dating in the British Iron Age Sarah-Jane Clelland Archaeomagnetism is an area of research that utilises the magnetic properties of archaeological materials to date past human activity. This research aimed to use the evidence of past geomagnetism, as recorded by archaeological and geological materials, to identify and characterise short timescale changes in the Earth’s magnetic field. This contribution to the discipline focused on the first millennium BC, as there is evidence that during this time the Earth’s magnetic field experienced rapid changes in direction. This work focused on an established weakness in archaeomagnetic studies, i.e. the application of archaeological information to assign a date range to the magnetic directions. The date ranges for 232 magnetic directions from 98 Iron Age sites were reviewed and a programme of fieldwork produced 25 new magnetic directions from 11 Iron Age sites across Britain. The approach developed in this thesis has made significant improvements to the data examined, which represent the prehistoric section of the British secular variation curve (SVC). These data have been incorporated into the British archaeomagnetic dataset that now comprises over 1000 magnetic directions and will be used to generate future British SVCs. The potential of the near continuous records of geomagnetic secular variation from British lake sediment sequences to SVCs was explored. This showed that these sediments have recorded the relative changes in the Earth’s magnetic field but the dating and method of constructing the British master curve requires revision. As SVCs are predominately used as calibration curves for archaeomagnetic dating, this work provides a foundation for a revised and extended British SVC. This revision would be to the mutual benefit of studies in archaeology and archaeomagnetism, as the latter could potentially enable high- resolution dating of Iron Age material, providing a viable alternative to radiocarbon dating. Keywords: archaeomagnetism, dating, Iron Age, secular variation, calibration curve, Britain, first millennium BC, geomagnetic Contents Table of figures ..................................................................................................... v List of tables .......................................................................................................... x Acknowledgements ............................................................................................ xiii Glossary .............................................................................................................. xiv 1. Introduction ...................................................................................................... 1 1.1 Research outline ............................................................................................. 1 1.2 Scheme of research ........................................................................................ 2 1.3 Research rationale .......................................................................................... 5 1.4 Structure of the thesis .................................................................................... 8 2. Previous developments in the research of Iron Age archaeology and archaeomagnetic dating in Britain ........................................................................ 9 2.1 Introduction .................................................................................................... 9 2.2 The chronological framework of the British Iron Age .................................. 10 2.2.1 Early Iron Age ................................................................................ 10 2.2.2 Middle Iron Age ............................................................................. 12 2.2.3 Late Iron Age ................................................................................. 12 2.3 Overview of the archaeology of the British Iron Age ................................... 13 2.3.1 Settlement ..................................................................................... 13 2.3.2 Economy ........................................................................................ 14 2.3.3 Ritual ............................................................................................. 15 2.4 Dating the British Iron Age ........................................................................... 16 2.4.1 Pottery typology ............................................................................ 16 2.4.2 Continental metalwork ................................................................. 18 2.4.3 Radiocarbon dating ....................................................................... 19 2.5 A history of archaeomagnetic dating in Britain ............................................ 21 2.5.1 The early work ............................................................................... 21 2.5.2 The period of establishment of British archaeomagnetic studies 24 2.5.3 Further developments .................................................................. 27 2.5.4 The future ...................................................................................... 31 2.6 Summary ....................................................................................................... 31 3. Current understanding of the fundamental aspects of archaeomagnetism. ..... 33 3.1 Introduction .................................................................................................. 33 3.2 Developments in understanding of the geomagnetic field .......................... 33 3.3 The magnetic elements ................................................................................ 36 3.4 Principles of rock magnetism for archaeomagnetic dating .......................... 41 3.5 Application of rock magnetism to archaeomagnetism ................................ 46 3.5.1 Viscous remanent magnetisation ................................................. 47 3.5.2 Causes of secondary magnetisation ............................................. 47 3.5.3 Precision and accuracy in archaeomagnetic directions ................ 49 3.6 Thermoremanent mechanisms .................................................................... 53 3.6.1 Advantages and limitations of TRM .............................................. 53 3.7 Depositional remanence mechanisms ......................................................... 56 3.7.1 Differences between DDRM and PDRM ....................................... 59 i 3.7.2 Advantages and limitations of DRM ............................................. 61 3.8 Summary ....................................................................................................... 63 4. A procedure for identifying and evaluating the chronometric placement of magnetic data recovered from archaeological material ....................................... 65 4.1 Introduction .................................................................................................. 65 4.2 Collating the data ......................................................................................... 66 4.3 Assessing the data ........................................................................................ 67 4.3.1 Meta data ...................................................................................... 70 4.3.2 Magnetic data ............................................................................... 72 4.3.3 Chronological data ........................................................................ 76 4.4 Method for reassessing the archaeological age estimates .......................... 80 4.4.1 The problem .................................................................................. 81 4.4.2 The procedure ............................................................................... 81 4.5 Application of the method for reassessing the archaeological age estimates .................................................................................................................................... 91 4.5.1 Ethical considerations ................................................................... 92 4.6 Case studies .................................................................................................. 93 4.6.1 Case study 1: Old Scatness ............................................................ 94 4.6.2 Case Study 2: Ferrybridge ........................................................... 101 4.6.3 Case study 3: Vitrified hillforts .................................................... 102 4.7 Results from reassessing the archaeological age estimates ...................... 114 4.7.1 Summary statistics and reliability of the dataset ....................... 115 4.7.2 Impact of the reassessment on the precision of the dataset ..... 121 4.7.3 Effect on the chronological distribution of the dataset.............. 130 4.8 Summary ..................................................................................................... 136 5. Programme of archaeomagnetic dating