THE LATE QUATERNARY HISTORY OF THE RIVER ERME, SOUTH DEVON SUSAN CREAK A thesis submitted in partial fulfilment of the requirements of the Council for National Academic Awards for the degree of Doctor of Philosophy July 1991 Polytechnic South West In CoUabora-hoo lOiVk ^f^iv^s'iKj College, Sioou\5ftc^ "*"HE LATE (OoftTE^rvi^iey M/STO^X OF TMG iZive^ ^EooG-^ SOUTH l>eKcJr\J ABSTRACT This thesis examines a typical 'drowned* river valley of south-west England. The River Erme drains southern Dartmoor and, in common, with other rivers of the region, has a buried channel graded to c. -50m OD. The channel has been infilled with a variety of sediments, of which the top 6-7m have been analysed for this study. Augered cores have been collected from the modern floodplain and salt marsh areas in the lower part of the valley and have been studied using particle size, diatom, molluscan and radiocarbon analyses. The main sediment types recovered include a fine silt unit representing the most recent phase of fluvial deposition, v^ich overlies variable horizons of fluvial granitic sands and gravels and brackish organic sands and silts. These are, in places, replaced at depth by shelly silts, sands and gravels deposited under more estuarine and marine conditions. The organic layer has been radiocarbon dated to between 1000 and 2000 years BP and has been correlated with a former area of marshland recorded in the tithe maps of the region. It is suggested that the Erme valley was previously more estuarine than today and that alluviation and infilling of the Erme's channel in the last 1000-2000 years has been aided by forest clearance and tin mining on Dartmoor. All of the sediments recovered from the buried channel are derived from local sources of bedrock and have probably been deposited in the last 4000-6000 years. Evidence from the tithe maps suggests that the channel'has been stable in the past 200 years. This work seeks to contribute to the Quaternary knowledge of the south-west of England, and because of the paucity of data concerning these burled rock channels, makes this study of the River Erme a preliminary model against which other rivers in the South West may be compared. I PC5.YTECHN1C SOUTH WE^vT | I iSBRARY SERVICES ACKNOWLEDGEMENTS I would like to thank a number of people for their help in the completion of this thesis. Mr. Peter Sims for supervising the project and for his support and help in finishing the thesis. Professor Nick Stephens for supervision, providing the funds for radiocarbon dating of the sediments and his generosity during trips to Swansea. Dr. David Croot for supervision in the initial stages and the setting up of the project. I should like to thank Mr. Mildmay-White, the owner of the Flete Estate, for giving us unlimited access to the Erme Valley, and allowing us to work in such a beautiful area of South Devon. There are a number of people from the Department of Geographical Sciences who contributed to the field investigation and provided technical support in the laboratory. I should primarily like to thank Dr. Martin Coard for his ingenuity and patience during all aspects of the field work and for help with diatom analysis of the sediments. Joff Atkinson for help with the drilling program and surveying. Adrian Holmes for printing out draft copies of chapters and converting my documents to Word Perfect format. Anne Kelly, Kevin Solman, Sheila Ternan, Jenny Wyatt and Brian Rogers also provided technical and cartographic support. I would also like to thank the Department of Geology for the loan of the drilling equipment and technician time; John Bowers. Hugh Balhatchet and Graham Mott assisted in the field. I would like to thank Dr. David Smith of Coventry Polytechnic for the loan of drilling equipment used in the early stages of the project and Dr. David Gilbertson of the University of Sheffield for providing valuable help in the identification of mollusca from the sediments. I should also like to thank Dr. Brian Whalley of Belfast University for supplying plates of quartz grains and help in the SEM analysis of my sediments. I would like to thank the staff of Devon Record Office who allowed me access to tithe maps and records for the South Hams area. The Learning Resources Centre (of the Polytechnic) for their inter-library loan service. Media services in the LRC for making the plates of diatoms, SEM studies and field work and Steve Johnston, of the LRC, for help with photographs of the molluscs and for supplying the aerial view of the River Enme. Devon County Council provided funding for the project and I would also like to thank the Research Degrees Committee at Polytechnic South West for their continuing support: in particular Professor Les Ebdon for his encouragement and diplomacy. I am indebted to the department of Physical Geography at the University of Reading for providing such a stimulating environment for my undergraduate studies and in particular Professor Peter Worsley for his enthusiastic approach to the teaching of Quaternary Studies. I'd like to thank the friends who helped with this research project: Dr. Anne Gellatly for commenting on an early draft, for her enthusiasm and encouragement to finish the thesis and for her generosity towards the Plymouth postgraduates. I'd like to thank Cris Russell for being so patient and for printing the thesis and photocopying the diagrams. And finally thank you to my fellow postgraduates and friends for all the happy times in Plymouth, in particular: Niki Botton, Greg Croxford, Maggie Bochel, David and Jan Pickering, Michele Webb and Bonita Searle^Bames. CONTENTS Page CHAPTER 1 INTRODUCTION 1 CHAPTER 2 A CONSIDERATION OF THE FACTORS CONTROLLING THE FORMATION AND DEVELOPMENT OF RIVER VALLEYS IN SOUTH-WEST ENGLAND 9 2.1 Introduction 9 2.2 The Effect of Quaternary Glaclations on the Area of Study 9 2.2.1 North Devon and the Isles of Scilly 11 2.2.2 The English Channel and South Devon 12 2.3 Periglacial Deposits 13 2.4 Quaternary Sea Level Changes 17 2.4.1 Long Term Tectonic Changes in the Earth's Crust.. 17 2.4.2 Glaciallsostasy 18 2.4.3 Hydro-lsostasy 19 2.4.4 Gepid Changes 19 2.4.5 Eustatic Sea Level Changes 20 .2.4.6 Miscellaneous Causes of Changes in Sea Level 20 2.5 High Quaternary Sea Levels 21 2.5.1 Planation Surfaces 21 2.5.2 Evidence for Former High Sea Levels in South• west England 23 2.5.3 Depositionai Evidence of Higher Sea Levels in South-West England 24 i) St. Erth 25 ii) St. Agnes Beacon 25 2.5.4 The Problems of Identifying and Mapping Erosion Surfaces 26 2.5.5 Raised Beaches 27 2.6 Low Quaternary Sea Levels.. 29 2.6.1 Buried River Channels 30 2.6.2 Drowned Shorelines 38 2.6.3 Deposits Associated with Low Sea Levels 41 2.7 The Flandrian Transgression 43 2.7.1 The Nature of the Sea level Rise 44 2.7.2 Important Sites in Flandrian Studies 47 i) The Somerset Levels 47 ii) The Bristol Channel 48 iii) Devon 48 iv) The Isles of Scilly 49 v) Northem France 49 2.7.3 Inaccuracies in the Collection and Interpretation of Data 50 2.8 Fluvial Activity during the Late Fiandrian 52 2.9 Conclusions 54 CHAPTER 3 THE STUDY AREA 56 3.1 Introduction - Ttie River Erme Valley 56 3.2 Solid Geology 60 3.3 Geomorphological Relationships 63 3.4 The Age of the Erme Valley and it's Deposits 70 3.5 Climate and Current River Discharge..! 73 3.6 Present Sea Level and Tides 74 3.7 Vegetation and Soil Development During the Flandrian 77 3.8 Archaeological Data 80 3.9 The Area to be Studied 83 CHAPTER 4 LITHOLOGY OF THE VALLEY SEDIMENTS: FIELD METHODS AND LABORATORY ANALYSIS 84 4.1 Field Methods 84 4.1.1 Drilling into Floodplain Deposits 84 4.1.2 Location of Drilling Sites 87 4.1.3 Collection of Samples 94 4.1.4 Surveying to Ordnance Datum 99 11 4.2 Laboratory Methods 104 4.2.1 Logging of Sediment 104 4.2.2 Particle Size Analysis of Samples 104 4.2.2.1 Sieving 104 4.2.2.2 Pippette Analysis 105 4.2.3 Mineral Composition of Sediments 106 4.2.4 Scanning Electron Microscope (S.E.M.) Analysis of Sediment 107 4.2.4.1 The Method 107 4.2.4.2 Selection of Material for Analysis 108 4.3 Results 109 4.3.1 Description of the Cores 109 4.3.2 Size Analysis 134 4.3.3 Mineral composition 155 4.3.4 S.E.M. Analysis of the Cores 158 4.4 A Brief Discussion 162 CHAPTER 5 A DETAILED STUDY OF DIATOMS PRESERVED IN THE BURIED CHANNEL DEPOSITS 164 5.1 Introduction 164 5.2 Classification and Identification 165 5.3 Ecological Groupings of Diatoms 167 5.4 Methodology 179 5.4.1 Choice of Samples 179 5.4.2 Preparation of Samples 180 5.4.3 Slide Preparation 180 5.4.4 Examination and Identification 180 5.4.5 Counting 181 5.4.6 Presentation of results 181 5.5 Results of Diatom Analysis 182 5.5.1 Combined Salinity Diagrams 195 5.5.2 Individual Species 198 5.6 Discussion 204 5.7 Conclusions 207 111 CHAPTER 6 MOLLUSCAN FAUNA OF THE BURIED CHANNEL DEPOSITS 208 6.1 Introduction 208 6.2 Methodology 208 6.2.1 Extraction of Molluscan Shell Fragments From the Samples 208 6.2.2 Counting of Species, Identification and Presentation of Results 209 6.3 Results 210 6.4 Discussion 230 6.5 Conclusions 232 CHAPTER 7 EVIDENCE OF CHANGES IN HISTORICAL TIME IN THE ERME VALLEY 234 7.1 Introduction 234 7.2 Mining Activities on the Moor 235 7.2.1 Introduction 235 7.2.2 Evidence of Prehistoric Tin Mining 236 7.2.3 Later Tin Mining 237 7.2.4 Methods of Tin Extraction and their Resulting Effects on the Landscape 239 7.2.5 Sites of Tin Mining in the Erme Catchment 241 7.2.6 China Clay Production 245 7.3 Documentary Evidence of Changes in the Lower Valley 246 7.4 Radiocarbon Dating 252 7.4.1 Choice of Samples for Radiocarbon Dating 252 i) Samples from SB1 253 ii) Samples from A4 254 ill) Samples from BH6 254 7.4.2 Results of Radiocarbon Analysis 255 7.5 Discussion 259 IV CHAPTERS DISCUSSION 261 8.1 Introduction 261 8.2 A Summary of Results From Earlier Chapters 261 8.3 Correlation of Sediments 262 8.4 The Main Sediment Types Recorded in the Lower Erme Valley : 284 8.5 Age of Sediment 290 8.6 A History of the Ernie Valley 292 8.6.1 Ttie Devensian 293 8.6.2 The End of the Devensian (10.000) to 6.000 Years BP 296 8.6.3 6000-2000 years BP 296 8.6.4 2000 years BP to the Present 298 8.6.5 The Present Day 302 8.7 Correlations with Other Sites in the South-West of England 302 CHAPTER 9 CONCLUSIONS 305 REFERENCES 310 APPENDICES 334 2 .