Molluscan Biostratigraphy of Flandrian Slope Deposits
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MOLLUSCAN BIOSTRATIGRAPHY OF FLANDRIAN SLOPE DEPOSITS IN EAST SUSSEX. By CAROLINE SARAH ELLIS B.Sc. A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of membership of the Imperial College. 1985 Department of Geology, Royal School of Mines, Imperial College of Science and Technology, LONDON SW7 2BP. ABSTRACT Until recently Quaternary Scientists have concentrated almost exclusively upon the botanical evidence, especially pollen, for both palaeoenvironmental and chronological interpretation. However, highly oxidized calcareous sediments such as chalk slope deposits do not preserve pollen well, but do contain abundant fossil Mollusca. Since the pioneering work of Sparks (1961, 1964), Kerney (1963) and Kerney et al (1964) molluscs have been increasingly used as valuable environmental indicators. More recently, a series of molluscan biozones spanning the Late-glacial and Postglacial (Flandrian) periods has been established from work in Kent (Kerney 1977; Kerney et al 1980), and these have allowed sites to be correlated biostratigraphically. The molluscan biostratigraphy of sixteen sequences of mainly Flandrian slope deposits from six sites in East Sussex has been studied, and these are described in detail. The results are presented in the form of percentage frequency histograms allowing easy comparison between sequences and sites. The slope deposits consist of a tripartite sequence of sediments. Poorly stratified Late-glacial chalk silts and rubbles are overlain by a forest soil which in turn has been buried by several metres of colluvium. The Late-glacial deposits are interpreted as having formed by a combination of fluvial and mass movement processes, and between them and the earliest Postglacial deposits there is a hiatus of several thousand years. The in situ soil formed under stable woodland which covered the valley sides and floors and represents the earliest Postglacial sediment found in the valleys. Lastly woodland clearance by man for agriculture and settlement caused an acceleration of soil erosion, creating colluvium. Analysis of these slope deposits has shown that the molluscan biozones can be recognised on the South Downs and so can be used to correlate sites both within Sussex and elsewhere in southern England. These studies have elucidated the anthropogenic biozones e and f, especially the order of arrival into the area of the accidentally introduced molluscan species (Monacha cartusiana, Helix aspersa, Cernuella virgata, Candidula gigaxii, Candidula intersecta and Monacha cantiana). A general molluscan succession has also been established for the eastern end of the South Downs, relating the changes in the molluscan assemblages to alterations in vegetation and land use. Radiocarbon dates from the buried soils and basal colluvium have been obtained from four sites, indicating that forest clearance began in some areas in the Neolithic period and in others as late as the Iron Age. These dates have added significantly to existing information about the onset and duration of the molluscan biozones. Ii ACKNOWLEDGEMENTS Firstly I should like to thank my supervisor Dr. Michael Kerney for his support, encouragement and helpful discussion throughout this project for without him this work could never have been undertaken. I am also grateful to all the many friends and colleagues who have helped me with fieldwork and by making helpful comments during writing up. These include my mother, Mr. M. Wingman, Mr. S. Salmon and Dr. M. Bell. I would especially like to thank Mr. D. Gordon for his helpful advice and to also acknowledge the joint work described in Chapter 16. Also I would like to thank Dr. R. B. G. Williams for arranging and helping with the excavations at the Devil's Dyke and for organising the transportation of the samples. Specialist opinions were sought from the following sources:— Dr. J. Boardman, Earth Sciences, Brighton Polytechnic (Palaeosols); Mr. A. Currant, British Museum Natural History (Bones); Miss R. Gale, Royal Botanical Gardens, Kew (Charcoal); Mr. D. Gordon, Geography Department, Bristol (S.E.M.); Dr. D. Holyoak, Geography Department, Nottingham (Seeds); Mr. I. Kinnes, British Museum (Pottery); Dr. H. Rendell, Geography Department, Sussex (Thermoluminescence); Dr. C. Turner, Botany School, Cambridge (Pollen). Advice and technical assistance was kindly given by Mr. N. Morton, Photography Department, Imperial College and from Mr. I. i v Watts, Cartography Department, Sussex. The radiocarbon assays were kindly provided by Mr. R. Burleigh at the British Museum Research Laboratory. Thanks must also be given to the people and Institutions that gave permission for access to the sites. To Brighton Corporation for allowing excavations in the floor of the Devil's Dyke and to Mr. Lee for giving access across his fields. Also to Blue Circle Cement Company who allowed excavations at Asham and to Mr. Newby and Mr. Gascoigne for access at South Heighton and Exceat respectively. Finally I should like to thank the Geology Department at Imperial College for the use of facilities and to N.E.R.C. for providing a research studentship. I would also like to acknowledge the money provided by the Fourth International Flint Symposium for the excavations at the Devil's Dyke. CONTENTS Page No. ABSTRACT ACKNOWLEDGEMENTS Contents List of Figures ix List of Tables CHAPTER 1: INTRODUCTION 1 CHAPTER 2: ENVIRONMENTAL HISTORY OF THE SOUTH DOWNS 7 i) Geology and Geomorphology 7 ii) Soils 11 iii) Palaeobotany 15 iv) Fossil Molluscan Work 22 v) Archaeology of Sussex 30 vi) Present day Conditions 33 CHAPTER 3: GEOMORPHOLOGICAL PROCESSES RESPONSIBLE FOR SLOPE PROCESSES 36 i) Introduction 36 ii) Late—glacial Deposits 37 iii) Postglacial Deposits 40 v i Page. No. CHAPTER 4: METHODS 48 i) Fieldwork 48 ii) Laboratory Work 50 iii) Data Presentation 57 CHAPTER 5: INTERPRETATION OF THE FOSSIL ASSEMBLAGES 61 i) Introduction 61 ii) History of the Technique 61 iii) Recent Advances 66 iv) Biozones 68 v) Taphonomy 72 CHAPTER 6: NOTES ON CERTAIN MOLLUSCAN SPECIES 78 i) Biozones y and z 78 ii) Early Postglacial Biozones a-c 78 iii) Biozone d 79 iv) Biozones e and f 81 CHAPTER 7: DEVIL'S DYKE 85 i) Introduction 85 ii) Pit 1 89 iii) Pit 2 94 iv) Pit 3 95 v) Pit 4 97 vi) Pit 5 100 vii) Other Remains 104 viii) Dating 104 vi i Page No. ix) Discussion and Conclusions 106 CHAPTER 8: ASHAM QUARRY 109 i) Introduction 109 ii) South Section 111 iii) North Section 120 iv) Discussion and Conclusions 131 CHAPTER 9: SOUTH HEIGHTON 135 i) Introduction 135 ii) Stratigraphy 135 iii) Mollusca 137 iv) Interpretation 140 v) Other Remains 140 vi) Dating 140 vii) Discussion and Conclusions 141 CHAPTER 10: HOPE GAP 143 i) Introduction 143 ii) Stratigraphy 143 . iii) Mollusca 144 iv) Discussion and Conclusions 147 CHAPTER 11: EXCEAT 149 i) Introduction 149 ii) Formation of Strip Lynchets 149 iii) Exceat 1 150 viii Page No. iv) Exceat 2 156 v) Discussion and Conclusions 159 CHAPTER 12: COW GAP 162 i) Introduction 162 ii) Central Infill Section 165 iii) Interfluve Deposits. 175 iv) Discussion and Conclusions 178 CHAPTER 13: ARION GRANULES 182 CHAPTER 14: DATING OF THE DEPOSITS 186 CHAPTER 15: NOTES ON POMATIAS ELEGANS 194 CHAPTER 16: OPTIMUM SAMPLE SIZE 202 CHAPTER 17: DISCUSSION AND GENERAL CONCLUSIONS 213 REFERENCES 224 APPENDIX 259 ix LIST OF FIGURES Fig. No. Page No. 1. Dry Valley System in Sussex. 8 2. Fossil Mollusc and Pollen Sites in Sussex. 17 3. S.E.M. Photographs of the Helicellids. 55 4. Idealised Diagram of the Molluscan Biozones. 70 5. Taphonomic Processes. 74 6. Long Profile of the Devil's Dyke. 7. Sketch Diagram of Pit 1. 90 8. Molluscan Histogram of Pit 1. 92 9. Sketch Diagram and Molluscan Histogram of Pit 3. 96 10. Sketch Diagram and Molluscan Histogram of Pit 4. 99 11. Sketch Diagram of Pit 5. 101 12. Molluscan Histogram of Pit 5. 103 Fig. No Page No. 13. Molluscan Histogram Redrawn from Williams 1971. 110 14. Sketch Diagram of Asham, South Section. 112 15. Molluscan Histogram South Section, Column A. 116 16. Molluscan Histogram South Section, Column B. 119 17. Sketch Diagram of Asham, North Section. 121 18. Molluscan Histogram North Section, Column C. 124 19. Molluscan Histogram North Section, Column D. 128 20. Sketch Diagram of South Heighton. 136 21. Molluscan Histogram of South Heighton. 138 22. Sketch Diagram and Molluscan Histogram of Hope Gap. 145 23. Section Through a Lynchet. 151 24. Sketch Diagram of Exceat 1. 153 25. Molluscan Histogram of Exceat 1. 155 xi Fig. No. Page No. 26. Sketch Diagram of Exceat 2. 157 27. Molluscan Histogram of Exceat 2. 160 28. Molluscan Histogram of Cow Gap taken from Kerney. 163 29. Sketch Diagram of Cow Gap, Central Infill Section. 169 30. Location of Pottery from Bell 1981a. 170 31. Molluscan Histogram of Cow Gap, Central Inf ill Section. 171 32. Stratigraphy and Molluscan Histogram of Cow Gap, Interfluve Section. 177 33. Histogram of Shells and Anon Granules from Cow Gap, Central Infill Section. 184 34. Distribution of Pomatias elegans in the British Isles taken from Kerney 1976a. 196 35. Distribution of the Sizes of Pomatias elegans at Asham. 199 36. Distribution of the Sizes of Pomatias elegans at Cow Gap. 200 xii Fig. No. Page No. 37. Number of shells vs. sample weight 204. 38. Number of species vs. sample weight. 205 39. Cumulative number of species vs. sample weight. 207 40. Sample weight vs. percent error. 209 41. Molluscan histogram of the different sample weights. 212 42. Succession on the Chalk on the South Downs. 221 LIST OF TABLES A. Characteristics of valley inf ill deposits. APPENDIX I 1. Devil's Dyke, Pit 1 2. Devil's Dyke, Pit 2 3. Devil's Dyke, Pit 3 4. Devil's Dyke, Pit 4 5. Devil's Dyke, Pit 5 6. Asham, Column A 7.