Mathers, Hannah (2014) the Impact of the Minch Palaeo-Ice Stream in NW
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Mathers, Hannah (2014) The impact of the Minch palaeo-ice stream in NW Scotland: Constraining glacial erosion and landscape evolution through geomorphology and cosmogenic nuclide analysis. PhD thesis. http://theses.gla.ac.uk/5302/ Copyright and moral rights for this thesis are retained by the author A copy can be downloaded for personal non-commercial research or study, without prior permission or charge This thesis cannot be reproduced or quoted extensively from without first obtaining permission in writing from the Author The content must not be changed in any way or sold commercially in any format or medium without the formal permission of the Author When referring to this work, full bibliographic details including the author, title, awarding institution and date of the thesis must be given Glasgow Theses Service http://theses.gla.ac.uk/ [email protected] The impact of the Minch palaeo-ice stream in NW Scotland: Constraining glacial erosion and landscape evolution through geomorphology and cosmogenic nuclide analysis. Hannah Mathers Thesis submitted to the University of Glasgow For the degree of Doctor of Philosophy School of Geographical and Earth Sciences University of Glasgow June 2014 I, Hannah Mathers, hereby certify that this thesis, which is approximately 48,000 words in length, has been written by me, that it is the record of work carried out by me and that it has not been submitted in any previous application for a higher degree. Date 25.06.2014 Signature of candidate I was admitted as a research student in October 2006 and as a candidate for the degree of PhD in October 2007; the higher study for which this is a record was carried out in the University of Glasgow between 2006 and 2013. Date 25.06.2014 Signature of candidate I hereby certify that the candidate has fulfilled the conditions of the Resolution and Regulations for the degree of PhD in the University of Glasgow and that the candidate is qualified to submit this thesis in application for that degree. Date 25.06.2014 Signature of supervisor Unrestricted In submitting this thesis to the University of Glasgow I understand that I am giving permission for it to be made available for use in accordance with the regulations of the University Library for the time being in force, subject to any copyright vested in the work not being affected thereby. I also understand that the title and abstract will be published, and that a copy of the work may be made and supplied to any bona fide library or research worker. Date 25.06.2014 Signature of candidate ABSTRACT The British-Irish Ice Sheet (BIIS) is predicted to have deglaciated rapidly from ~ 18 ka, in response to rising sea level and temperature, similar forcings experienced by modern polar ice sheets. As the main conduits of ice mass loss, the reaction of ice streams to these forcings is thought to have been central in determining the mode and timing of this deglaciation. However, lack of understanding of ice stream influence on the glaciology and deglaciation of ice sheets limits confidence in ice sheet model predictions. NW Scotland is an area of the last BIIS predicted to have been dominated by ice stream onset conditions. This thesis presents results from a geomorphological and terrestrial cosmogenic nuclide (TCN) analysis study which resulted in the production of a composite ice-sheet thermal regime map and retreat chronology for the last BIIS in this region. Mapping and surface exposure dating suggest that the regional glaciology and landscape evolution was dominated by the presence of ice- stream onset zones during Greenland Stadial-2 (GS-2). Mountain top erratics were uplifted and transported to high elevation during GS-2, before 16.5 ka BP. By inference, mountain summits were covered by ice during maximal ice sheet conditions. The existence of sharp thermo-mechanical contrasts, developed in response to ice streaming, are proposed as the main controls on bedrock erosion and terrestrial sediment deposition. The interpretation of ‘trimlines’ in NW Scotland as englacial thermo-mechanical boundaries, is verified by the identification of ‘rip-offs’, a newly recognised geomorphic feature in the UK, and by quantitative demonstration of the increase in glacial erosion in the vicinity of these boundaries. Geomorphic and TCN data supports a conceptual model of thermal inversion following ice-stream cessation. The first description of ‘till tails’ in the UK provides insight into the glaciological organisation and thermal evolution of the BIIS. A dated (17.6 ka BP) terrestrial glacial limit on the north Sutherland coast indicates early ice retreat from the shelf and provides a minimum i constraint on ice-stream cessation. This indicates rapid loss of ice extent and volume following shutdown of the Minch palaeo-ice stream. Major ice sheet reorganisation c. 15-16 ka BP is suggested by the correlation of some lateral margin ages with high elevation erratic deposition ages implying significant ice thinning and margin retreat prior to this time. Additionally, thinning of ~300 m is predicted for some areas prior to 14 ka BP. ii TABLE OF CONTENTS ABSTRACT i LIST OF FIGURES vii LIST OF TABLES xi LIST OF ACCOMPANYING MATERIAL xii ACKNOWLEDGEMENTS xiii PREFACE: A GOOD DAY BY NORMAN MACCAIG xiv CHAPTER 1: INTRODUCTION 1 1.1. Thesis Conceptual Framework and Research Objectives 6 1.2. Study Area 9 1.3. Geology of NW Scotland 10 1.4. Palaeo-ice streams and glaciological organisation 12 1.5. Constraining the basal thermal regime of the last British-Irish Ice Sheet 16 1.5.1. Spatial and chronological retreat of the last BIIS 18 1.6. Thesis scope 26 1.7. Wider Implications 28 CHAPTER 2: METHODOLOGY 29 2.1. Introduction 29 2.2. Geomorphic mapping 30 2.2.1. Mapping, observation and sampling 31 2.3. Terrestrial Cosmogenic Nuclide (TCN) Analysis 32 2.3.1. TCN theory 33 2.3.1.1. TCN Production 33 2.3.1.2. Utilisation of TCN data in glacial studies 35 2.3.2. Processing from rock to TCN concentration 37 iii 2.3.2.1. Sampling 37 2.3.2.2. Extracting Be and Al from a whole rock sample 40 2.3.2.3. Calculating a surface exposure age 43 2.3.2.4. Use of a local production rate 44 CHAPTER 3: RESULTS 46 3.1. Introduction 46 3.2. N coast, Sutherland 48 3.2.1. Regional geomorphology 48 3.2.1.1. Kyle of Durness 51 3.2.1.2. Rispond 52 3.2.1.3. Eriboll Valley 56 3.2.1.4. Eriboll: W flank 56 3.2.1.5. Srath Beag 61 3.2.1.6. Cranstackie 69 3.2.1.7. Eriboll: Eastern flank 75 3.2.1.8. Loch Hope 80 3.2.1.9. Ben Loyal 82 3.2.1.10. Cape Wrath Area 83 3.3. W Sutherland: Oldshoremore & Sheigra 85 3.4. Assynt Mountains: Glas Bheinn, Beinn Uidhe, Conival, Breabeg 92 3.5. Coigach 100 3.5.1. Cul Beag 100 3.5.2. Beinn an Eòin 103 3.5.3. Stac Pollaidh 105 3.5.4. Ben More Coigach & Spidean Còinnich 106 3.5.5. Tanera Mòr 108 3.6. Wester Ross 112 iv 3.6.1. Glen Achall 112 3.6.2. Sail Liath 116 3.6.3. Little Loch Broom 119 3.6.4. Gruinard Bay 120 3.7. Newly identified geomorphic features 122 3.7.1. ‘Rip-off zones’ 122 3.7.2. Till Tails 128 CHAPTER 4: DISCUSSION: HORIZONTAL & VERTICAL ICE MARGIN RETREAT CHRONOLOGY 133 4.1. Introduction 133 4.2. Lateral ice margins 133 4.2.1. North Sutherland coast 133 4.2.2. Sheigra – Oldshoremore 140 4.2.3. Summer Isles – Glen Achall 147 4.2.4. Glen Achall 150 4.2.5. Little Loch Broom 150 4.2.6. Gruinard Bay 150 4.3. Ice sheet thinning 151 4.4. Ice sheet stability 154 4.5 Ice sheet sector deglaciation: Major findings regarding chronology of retreat and glaciological organisation 158 CHAPTER 5: DISCUSSION: THERMAL STRUCTURE AND FLOW REGIME OF THE LAST BIIS IN NW SCOTLAND 159 5.1. Introduction 159 5.2. Relict surfaces & Glacial modification 159 5.2.1. High and intermediate level surfaces: Tors, Blockfields & Rip- offs 163 5.2.2. Low level surfaces 187 v 5.3. Thermal transitions & Flow Organisation 192 5.3.1. Vertical thermal zonation 192 5.3.2. Flow Disruption & Organisation 197 5.3.3. Vertical transportation of erratics 206 5.3.3.1. Glaciological significance of erratic uplift 209 5.3.3.2. Timing of erratic deposition 213 5.4. Ice sheet thermal and land surface evolution 215 5.4.1. Towards an understanding of thermo-mechanical organisation of the last BIIS in NW Scotland 215 5.4.2. Glaciological evolution of the BIIS in NW during GS-2 219 5.4.3. Evidence of active periglacial processes during the Holocene 221 5.5. Key findings regarding the thermal and landscape evolution history of NW Scotland 223 CHAPTER 6: CONCLUSIONS 224 REFERENCES 227 vi LIST OF FIGURES Figure 1.1: North west Scotland field area. 3 Figure 1.2: Schematic of the western calving margins of the last BIIS 4 Figure 1.3: Relative sea-level (RSL) observations and model predictions. 5 Figure 1.4: Schematic of thesis knowledge structure 7 Figure 1.5: ‘Typical’ landscapes of the field study area in NW Scotland 11 Figure 1.6: Ice sheet models predicting ice streaming in The Minch. 14 Figure 1.7: The Minch palaeo-ice stream at maximum extent. 15 Figure 1.8: Schematic W-E transect across Scotland 17 Figure 1.9: A poorly mapped BGS Quaternary Geology sheet.