The Quaternary deposits and glacial history of the area around Inchnadamph, Sutherland Integrated Geoscience Surveys (Northern Britain) Internal Report IR/03/120 BRITISH GEOLOGICAL SURVEY INTERNAL REPORT IR/03/120 The Quaternary deposits and glacial history of the area around Inchnadamph, Sutherland Tom Bradwell The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Ordnance Survey licence number GD 272191/2003 Key words NC22; Quaternary; glaciation Front cover Quinag, Sutherland [P513692] Bibliographical reference BRADWELL, T. 2003. The Quaternary deposits and glacial history of the area around Inchnadamph, Sutherland. British Geological Survey Internal Report, IR/03/1200. 25pp. © NERC 2003 Keyworth, Nottingham British Geological Survey 2003 BRITISH GEOLOGICAL SURVEY The full range of Survey publications is available from the BGS Keyworth, Nottingham NG12 5GG Sales Desks at Nottingham and Edinburgh; see contact details 0115-936 3241 Fax 0115-936 3488 below or shop online at www.thebgs.co.uk e-mail: [email protected] The London Information Office maintains a reference collection www.bgs.ac.uk of BGS publications including maps for consultation. 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Forde House, Park Five Business Centre, Harrier Way, Sowton, Exeter, Devon EX2 7HU 01392-445271 Fax 01392-445371 Geological Survey of Northern Ireland, 20 College Gardens, Belfast BT9 6BS 028-9066 6595 Fax 028-9066 2835 Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB 01491-838800 Fax 01491-692345 Parent Body Natural Environment Research Council, Polaris House, North Star Avenue, Swindon, Wiltshire SN2 1EU 01793-411500 Fax 01793-411501 www.nerc.ac.uk ii IR/03/120;Version 1.0 Last modified: 2003/11/12 15:59 Foreword This report is the published product of a study by the British Geological Survey (BGS) as part of the Moine Thrust Project, encompassed within the IGS (N) programme. Acknowledgements The author would like to acknowledged the support of fellow Moine Thrust Project members Dr M Krabbendam, Dr R Key, Dr K Goodenough, Dr E Pickett and Dr A G Leslie. Dr Tim Lawson (Merchiston Castle School) is thanked for making his unpublished fieldmaps available and also for providing valuable additional information on the glacial history of Assynt. The Assynt Estate are thanked for allowing unrestricted land access. iii IR/03/120;Version 1.0 Last modified: 2003/11/12 15:59 Contents Foreword .......................................................................................................................................iii Acknowledgements.......................................................................................................................iii Contents.........................................................................................................................................iv Summary ........................................................................................................................................v 1 Introduction ............................................................................................................................1 2 Quaternary Deposits ..............................................................................................................1 2.1 Till ..................................................................................................................................1 2.1.1 Subglacial till.............................................................................................................. 2 2.1.2 Supraglacial till........................................................................................................... 3 2.2 Moraines ........................................................................................................................5 2.2.1 The southern ‘corrie’ on Quinag [221 928]................................................................ 5 2.2.2 Loch na Gainmhich .................................................................................................... 7 2.2.3 N of Glas Bheinn and Beinn Uidhe............................................................................ 7 2.2.4 Around Loch Beag and Glencoul............................................................................... 8 2.2.5 Traligill and Poll an Droighinn valleys ...................................................................... 8 2.2.6 Inchnadamph to Achmore .......................................................................................... 9 2.2.7 Loanan Valley .......................................................................................................... 10 2.2.8 N shore of Loch Assynt............................................................................................ 11 2.2.9 W of Beinn Garbh .................................................................................................... 11 2.2.10 Beinn an Fhurain................................................................................................... 12 2.3 Glaciofluvial sand and gravel ....................................................................................12 2.4 Blockfield .....................................................................................................................13 2.5 Head .............................................................................................................................14 2.6 Talus.............................................................................................................................15 2.6.1 Talus cones ............................................................................................................... 16 2.7 Alluvium ......................................................................................................................18 2.7.1 Alluvial fans ............................................................................................................. 19 2.8 Peat...............................................................................................................................20 2.9 Other deposits .............................................................................................................20 2.9.1 Lacustrine deposits ................................................................................................... 20 2.9.2 Artificial deposits ..................................................................................................... 20 3 Quaternary history...............................................................................................................21 3.1 Previous Research.......................................................................................................21 3.2 The implications of new mapping : a discussion......................................................25 iv IR/03/120;Version 1.0 Last modified: 2003/11/12 15:59 References ....................................................................................................................................27 Summary This report describes the Quaternary geology of the 1:25,000 sheet NC22, covering the area around Inchnadamph, Sutherland. The extent and distribution of glacial deposits shed light on the glacial evolution of the Assynt district. Consequently, a new model of Late Devensian deglaciation is proposed. v IR/03/120;Version 1.0 Last modified: 2003/11/12 15:59 1 Introduction Assynt possesses some of the most dramatic glacial scenery in Scotland. The high ground around Ben More Assynt and Beinn Uidhe would have been ice accumulation areas during the last glaciation; whilst the Lewisian Gneiss landscape is classic ‘cnoc and lochan’ terrain, scoured by an ice sheet flowing from east to west. NC22 includes the ground from near the summit of Conival in the east to the Quinag ridge in the west. Inchnadamph is the only settlement. The map sheet includes the eastern half of Loch Assynt, an overdeepened lake basin. The sea protrudes into the fjord Loch Beag on the northern edge of the sheet. The area is covered by the six-inch county series maps Sutherland 60 and 71 and was originally mapped by Officers of the Geological Survey between 1886-1900 (BGS, 1904, 1907). Since 1923 the area has appeared on the Assynt Special Sheet (solid and drift). However, the Quaternary geology of the area has never been comprehensively mapped or described in a geological memoir. The glacial history of the area has been studied, in whole or in part, by Charlesworth (1955), Sissons (1977), Lawson