Geological excursion guide to the North-west Highlands of Scotland

From Earthwise
Jump to navigation Jump to search
Buy the book EGS. Also available from Amazon
A simplified geological map showing the areas covered by the excursions detailed in this guide.

Goodenough, Kathryn M. and Krabbendam, Maartin (Editors) A geological excursion guide to the North-west Highlands of Scotland. Edinburgh : Edinburgh Geological Society in association with NMS Enterprises Limited, 2011.

Contents[edit]

Contributors

Acknowledgements

Foreword

Editorial introduction

Background information

Topographic maps

Geological maps

Background reading

Geological framework of the North-west Highlands[edit]

A. The structural units - The Foreland[edit]

Lewisian Gneiss Complex

Stoer Group

Torridon Group

The Cambro-Ordovician succession

The structural units - The Moine Supergroup[edit]

The Moine Thrust Zone

B. Igneous rocks[edit]

Excursion 1 Loch Assynt and the Achmore Duplex[edit]

Excursion 1A: The foreland succession of the Loch Assynt roadside[edit]

Locality 1.1 Lewisian outcrops, Loch Assynt. [NC 2125 2507]

Locality 1.2 Basal unconformity of the Torridon Group. [NC 2134 2516]

Locality 1.3 Relief on the basal Torridon Group unconformity. [NC 2175 2503]

Locality 1.4 Base of the Applecross Formation. [NC 2194 2497]

Locality 1.5 Glacial striae. [NC 2225 2489]

Locality 1.6 Typical outcrops of Applecross Formation. [NC 2248 2479]

Locality 1.7 The basal Cambrian unconformity. [NC 2273 2470]

Locality 1.8 The base of the Eriboll Formation. [NC 2291 2472]

Locality 1.9 Typical outcrops of the Basal Quartzite Member. [NC 2308 2453]

Locality 1.10 Classic outcrops of the Pipe Rock Member. [NC 2349 2440]

Locality 1.11 Outcrops of the Fucoid Beds Member. [NC 2359 2423]

Locality 1.12 [NC 2372 2408] Outcrops of the Salterella Grit Member and Ghrudaidh Formation.

Locality 1.13 Vogesite sills and Sole Thrust. [NC 2382 2403]

Excursion 1B: The stratigraphy and structure of the Achmore Duplex and adjacent foreland[edit]

Locality 1.14 Imbricates in the stream section west of Achmore Farm. [NC 2366 2509 to NC 2369 2528]

Locality 1.15 [NC 2381 2558]

Locality 1.16 [NC 2363 2570]

Locality 1.17 [NC 2310 2540]

Excursion 2 A Transect through the Canisp Shear Zone, Achmelvich[edit]

Locality 2.1 Surface expression of Lewisian basement fabrics – viewpoint looking inland towards Canisp. [NC 0753 2554]

Locality 2.2 Badcallian gneiss on Achmelvich beach. [NC 0565 2503]

Locality 2.3 Little-deformed Scourie Dyke and localised Laxfordian dextral shear. [NC 0570 2512]

Locality 2.4 Inverian deformation on north beach. [NC 0591 2525]

Locality 2.5 Faulted margin of the Laxfordian Canisp Shear Zone. [NC 0575 2551]

Locality 2.6 Ductile and brittle Laxfordian shear fabrics indicating multiple deformation events. [NC 0575 2556]

Locality 2.7 Unfaulted margin of the Laxfordian Canisp Shear Zone. [NC 0533 2574]

Locality 2.8 Refolded folds, sheath folds and reworked Scourie Dykes. [NC 0508 2611]

Excursion 3 Stoer Group at Stoer Peninsula[edit]

Locality 3.1 Bay of Clachtoll: Stoer Group – Lewisian Gneiss Complex unconformity (low tide only). [NC 0404 2702]

Locality 3.2A Stoer Group – Lewisian Gneiss Complex unconformity. [NC 0412 2672]

Locality 3.2B Basal Stoer Group breccio-conglomerate. [NC 0412 2666]

Locality 3.3 A’ Chlach Thuill (‘Split Rock’). [NC 0384 2673]

Locality 3.4 Bay of Clachtoll beach: Clachtoll Formation. [NC 0405 2715]

Locality 3.5 Aird na Mòine: Bay of Stoer Formation. [NC 0361 2726]

Locality 3.6 Sgèir na Tràghad: Bay of Stoer Formation. [NC 0358 2751]

Locality 3.7 Bay of Stoer: Clachtoll – Bay of Stoer formation boundary. [NC 0379 2834]

Locality 3.8 [NC 0355 2853]

Locality 3.9 Stac Fada: Stac Fada Member. [NC 0333 2854]

Locality 3.10 Stoer cemetery: basal Stoer Group conglomerate. [NC 0411 2842]

Locality 3.11 Culkein – Port Feadaig: Stoer Group /Torridon Group unconformity. [NC 042 329]

Excursion 4 Stoer Group at Enard Bay[edit]

Locality 4.1 Achnahaird Bay: Meall Dearg Formation. [NC 0221 1312]

Locality 4.2 Achnahaird Bay: Meall Dearg Formation. [NC 0215 1335] to [NC 0217 1400]

Locality 4.3 [NC 0222 1419]

Locality 4.4 Camas a’ Bhothain: Meall Dearg Formation – Poll a’ Mhuilt Member boundary. [NC 0271 1464]

Locality 4.5 ‘Salmon Bothy’ Bay, west side: ‘drop stones’ in the Clachtoll Formation. [NC 0278 1463]

Locality 4.6 ‘Salmon Bothy’ Bay, east side: basal conglomerate and algal limestone, Poll a’ Mhuilt Member. [NC 0286 1462]

Locality 4.7 Camas a’ Bhothain: Diabaig Formation. [NC 030 146]

Locality 4.8 Camas a’ Bhothain: Poll a’ Mhuilt Member. [NC 0301 1456]

Locality 4.9 Camas a’ Bhothain: Stac Fada Member. [NC 0308 1456]

Locality 4.10 Rubh’ a’ Choin: Torridon Group. [NC 0331 1469] to [NC 0339 1462]

Locality 4.11 Headland of Creag a’ Choin Mhóir: conglomerate of the Stoer and Torridon groups. [NC 0360 1467]

Locality 4.12 Rubha Dunain, Achiltibuie: unconformity between the Torridon and Stoer groups. [NC 0242 0678]

Locality 4.13 Reiff: Applecross Formation. [NB 962 147]

Excursion 5 Ullapool River, Creag nam Broc and Glen Achall[edit]

Locality 5.1 Torr an Eas quarries. [NH 1490 9535]

Locality 5.2 Lithologies in the Ullapool Thrust Sheet. [NH 1550 9560]

Locality 5.3 Ullapool gneiss in the Eas Dubh Waterfall. [NH 1510 9550]

Locality 5.4 Creag nam Broc. [NH 1480 9580]

Locality 5.5 The Moine Thrust at Meall Mòr. [NH 1430 9470]

Excursion 6 Knockan Crag and the Knockan Klippen[edit]

Excursion 6a: Knockan Crag[edit]

Locality 6.1 Knockan Crag visitor centre and Moine Thrust trail. [NC 188 092]

Locality 6.2 Viewpoint at top of Knockan Crag. [NC 194 094]

Locality 6.3 Eagle Rock. [NC 188 086]

Excursion 6b: The klippen at Knockan[edit]

Locality 6.4 Cnoc a’Choilich Mhor Klippe. [NC 2056 0962]

Locality 6.5 Moine Thrust exposures. [NC 200 093 to NC 204 094]

Locality 6.6 Uamh an Tartair Klippe. [NC 213 092 to 217 092]

Excursion 7 Traligill and Bealach Traligill[edit]

Locality 7.1 Durness Group dolostone outcrops. [NC 2561 2189 to NC 2671 2140]

Locality 7.2 View of the Traligill Thrust. [NC 2693 2112]

Locality 7.3 Lower Traligill Cave. [NC 2706 2089]

Locality 7.4 Cnoc nan Uamh Klippe. [NC 2765 2047 to NC 2767 2046]

Locality 7.5 The summit of Cnoc nan Uamh. [NC 2778 2048]

Locality 7.6 Pipe Rock Member in the Breabag Dome. [NC 2895 1995]

Locality 7.7 Outcrops on the north-east side of the Bealach Traligill Fault. [NC 2992 1937]

Locality 7.8 Bealach Traligill. [NC 3018 1921]

Locality 7.9 Breabag Tarsuinn. [NC 3020 1904]

Excursion 8 Conival and Ben More Assynt[edit]

Locality 8.1 Traligill River. [NC 2734 2093]

Locality 8.2 Traligill River. [NC 2794 2064] to [NC 2800 2061]

Locality 8.3 Traligill River. [NC 2867 2039]

Locality 8.4 Path west of Allt a’Choinne Mhill. [NC 2894 2049]

Locality 8.5 Path west of Allt a’Choinne Mhill. [NC 293 206] to [NC 299 212]

Locality 8.6 Pipe Rock steps and the upper corrie. [NC 297 208 to NC 300 210]

Locality 8.7 Conival–Beinn an Fhurain col. [NC 3005 2080]

Locality 8.8 Coire a’ Mhadaidh. [NC 302 210] to [NC 306 205]

Locality 8.9 Ben More Assynt–Conival col. [NC 308 202]

Locality 8.10 Ben More Assynt summit. [NC 318 203]

Locality 8.11 Conival. [NC 304 199]

Excursion 9 Glen Oykel and the Loch Ailsh Pluton[edit]

Locality 9.1 Track to Ben More Lodge.

Locality 9.2 Loch Ailsh syenites in the River Oykel. [NC 326 127]

Locality 9.3 Syenite outcrops at stream confluence. [NC 328 129]

Locality 9.4 Allt Sail an Ruathair. [NC 332 134]

Locality 9.5 Metamorphic Burn. [NC 334 154]

Locality 9.6 Sgonnan Mòr Syncline and basal Torridon Group. [NC318 158]

Locality 9.7 Peralkaline rhyolite dykes. [NC 327 136]

Excursion 10 Cam Loch, Ledmore and the Loch Borralan Pluton[edit]

Locality 10.1 Cam Loch Klippe. [NC 230 121] to [NC 224 140]

Locality 10.2 Loch Borralan syenites at Ledmore. [NC 247 120]

Locality 10.3 Ledmore Marble Quarry. [NC 252 137]

Locality 10.4 Bad na h’Achlaise. [NC 245 115]

Locality 10.5 The Loch Urigill carbonatite. [NC 247 104]

Locality 10.6 Allt a’Mhuillin Quarry. [NC 287 096]

Excursion 11 Glencoul[edit]

Locality 11.1 Glencoul Thrust Plane at Tom na Toine. [NC 2570 3000 to 2600 3010]

Locality 11.2 Imbricates below the Stack of Glencoul. [NC 283 288 to 288 289]

Locality 11.3 Stack of Glencoul. [NC 2888 2876]

Excursion 12 Scourie Mòr[edit]

Locality 12.1 Scourie Graveyard. [NC 148 448]

Locality 12.2 Scourie Mòr [NC 144 446] to Geodh’ Eanruig. [NC 143 443]

Locality 12.3 First Inlet. [NC 152 453]

Locality 12.4 Poll Eòrna. [NC 150 456]

Locality 12.5 Sìthean Mòr. [NC 150 461]

Excursion 13 Tarbet[edit]

Locality 13.1 North side of Port of Tarbet. [NC 1615 4902]

Locality 13.2 Cnoc Gorm. [NC 1675 4985]

Locality 13.3 Rubha Ruadh. [NC 1650 5115]

Excursion 14: Durness, Balnakeil Bay and Faraid Head[edit]

Excursion 14A: Balnakeil Bay and Faraid Head[edit]

Locality 14.1 Type sequence of the Eilean Dubh Formation. [NC 3762 6878]

Locality 14.2 Stromatolites in the Eilean Dubh Formation. [NC 3765 6879]

Locality 14.3 [NC 3769 6880]

Locality 14.4 Eilean Dubh–Sailmhor Formation boundary. [NC 3782 6878]

Locality 14.5 Face in Sailmhor Formation. [NC 3788 6881]

Locality 14.6 Sailmhor–Sangomore Formation boundary. [NC 3836 6885]

Locality 14.7 Sangomore Formation. [NC 3860 6886]

Locality 14.8 [NC 3864 6885]

Locality 14.9 Sangomore–Balnakeil formation boundary. [NC 3887 6876]

Locality 14.10 [NC 3903 6872] Balnakeil Formation.

Locality 14.11 Moine psammites in the Faraid Head outlier. [NC 3925 6965]

Locality 14.12 Gneissic mylonites in the Faraid Head outlier. [NC 3855 7070]

Locality 14.13 [NC 3785 7135] Moine Thrust viewpoint.

Excursion 14B: additional localities - Loch Borralie, Smoo Cave and Sango Bay[edit]

Locality 14.14 The Croisaphuill Formation at Loch Borralie. [NC 384 670]

Locality 14.15 Smoo Cave. [NC 418 671]

Locality 14.16 [NC 4100 6740] Sangobeg Fault.

Locality 14.17 Sango Bay. [NC 4080 6770 to 4070 6800]

Locality 14.18 Creag Thairbhe. [NC 4040 6850]

Excursion 15 The Moine Thrust Zone at Loch Eriboll[edit]

Locality 15.1 An t-Sròn peninsula: a stratigraphic overview. [NC 4440 5798]

Locality 15.2 The viewpoint at Ard Neackie: Eriboll overview. [NC 4520 9990]

Locality 15.3 Port an Altain: imbricated Cambrian strata. [NC 4568 6133]

Locality 15.4 [Lighthouse. NC 4586 1779]

Locality 15.5 Loch na Cathrach Duibhe. [NC 4604 6160]

Locality 15.6 [NC 4639 6180]

Locality 15.7 Folded Pipe Rock Member. [NC 4685 6124]

Locality 15.8 Ben Arnaboll: major thrusts and associated structures. [NC 4615 5958]

Locality 15.9 Unnamed lochan. [NC 4620 5910

Locality 15.10 Upper Kempie: the mylonites. [NC 4487 5726]

Excursion 16 Roadside stops around the North-west Highlands[edit]

Locality 16.2 Moine mylonites. [NC 169 056]

Locality 16.3 Elphin: extensional faults above the Sole Thrust. [NC 208 104]

Locality 16.4 Allt a’Mhuillin Quarry. [NC 287 096]

Locality 16.5 Loch Awe Quarry. [NC 250 157]

Locality 16.6 Stronchrubie cliffs. [NC 248 200]

Locality 16.7 Peach and Horne Monument at Inchnadamph. [NC 248 222]

Locality 16.8 Loch Assynt to Skiag Bridge section. [NC 210 250] to [NC 240 240]

Locality 16.9 Cnoc Breac: viewpoint for the double unconformity. [NC 234 248]

Locality 16.10 Loch Glencoul. [NC 236 321]

Locality 16.11 Loch na Fiacaill – the ‘multicoloured rock stop’. [NC 232 486]


Acknowledgements[edit]

Work in the North-west Highlands, carried out by British Geological Survey (BGS) staff and collaborators, has been greatly helped by the support of the local people. Particular thanks are due to the staff and volunteers of the North West Highlands Geopark; to the many local landowners who assisted with access for fieldwork; to Scottish Natural Heritage staff in Ullapool; and to Chris Rix and all at Inchnadamph Lodge. The contributors have also benefited greatly from fruitful discussions with many people, too numerous to list here, during field excursions and conferences in the area. The majority of the figures in this excursion guide were drafted by Craig Woodward, BGS Edinburgh. This edition of the excursion guide owes much to Suzanne Miller, who gave us the impetus to start work on a new version of the Assynt guide that had been published in 1979.

Foreword[edit]

Michael Johnson and Ian Parsons

In the early 1880s, Charles Lapworth’s discovery of the Moine Thrust in North-west Scotland and Marcel Bertrand’s discovery of the Glarus Thrust in the Swiss Alps established thrust tectonics as a fundamental mechanism of crustal shortening in orogenic belts. The subsequent heroic phase of map-ping of the North-west Highlands by the team of British Geological Survey geologists led by Ben Peach and John Horne (the ‘investigator twins’) estab-lished the region as classic, recognised as such around the world, not only for the clear display of the nature of thrust systems but also for the remark-able igneous, metamorphic and sedimentary geology. The Assynt area is famous because it encapsulates many characteristic features of North-west Highland geology and accordingly the Survey team devoted much effort in preparing a special map of it.

Obviously, a guide to the Assynt region was an imperative. The first edition (1937) of the Assynt guide was the work of two members of the British Geological Survey: Murray Macgregor, whose work was mainly in the Midland Valley, and James Phemister, who mapped parts of Sutherland. It contained excellent coloured maps.

The second edition (1979) was revised and extended by us and retained most of the locality descriptions and the coloured maps but added sketches of the structural phenomena, which are marvellously displayed on moun-tain sides. The timing was appropriate, coinciding as it did with a renewed attack on the problems of the Assynt region in the light of ‘thin-skinned’ thrust tectonics imported from the Appalachians and the Canadian Rockies. New maps of the major igneous intrusions at Borralan and Loch Ailsh were provided.

In the preparation of the present edition a large team has been assem-bled to provide up-to-date interpretations of the structural relationships, sedimentology, and metamorphic history. On the igneous front, the rela-tionship between magmatism and thrusting is explored by expanding the discussion to include the minor intrusive rocks and the radiometric ages obtained since 1979. The area covered by the guide now includes the thrust zone between Assynt and Eriboll (Lapworth’s area) and more on the rocks of the foreland, which at the time of writing are the subject of intense study using methods such as zircon chronology.

The first two editions provided several generations of professional geologists, students and amateurs with access to the treasures of Assynt. The new expanded edition shows that this classic region continues to yield up its secrets!

Editorial introduction[edit]

Kathryn Goodenough and Maarten Krabbendam

This guide describes the bedrock geology of the North-west Highlands from Ullapool northwards, including many classic localities in the Moine Thrust Zone and its foreland. The area described here also largely corre-sponds to the North West Highlands Geopark. The geology of the Moine Supergroup, lying east of the Moine Thrust, is discussed in a companion guide (A Geological Excursion Guide to the Moine Geology of the Northern Highlands of Scotland, published by the Edinburgh and Glasgow Geolog-ical Societies). The North-west Highlands are also of great interest for their Quaternary geomorphology, which is not discussed in this guide but is described in several other publications. These include Classic Landforms of the Assynt and Coigach Area, published by the Geographical Association, and two excursion guides published by the Quaternary Research Association.

This guide has been written for those with some knowledge of geology, whether students, interested amateurs, or professional geologists. Less technical introductions to the geology and landscape of the North-west Highlands are provided by The Northwest Highlands: a Landscape Fashioned by Geology, published by Scottish Natural Heritage; and Exploring the Landscape of Assynt: A Walker’s Guide and Map, published by the British Geological Survey.

Access to the North-west Highlands has improved greatly since the earlier editions of this guide were written, with the construction of fast new roads and bridges such as that at Kylesku. Nonetheless, this remains a remote and mountainous area, with some of the most dramatic scenery in the British Isles. The excursions described here vary from roadside stops to challenging, full day mountain walks over extremely rough terrain. The weather in this area can change very rapidly, with sunny mornings giving way to pouring rain by lunchtime – and vice versa. Therefore, all parties following these excursions should be well equipped with warm and waterproof clothing, sturdy walking boots, and topographic maps of the area (a list of relevant maps is given below). From May to September, some form of midge repellent will also be essential. All localities are given with accurate grid references, and so the use of a GPS is recommended. At the time of writing, mobile phone coverage is reasonably good along the main roads and on the mountaintops, but largely non-existent in the more remote glens.

Under the Land Reform (Scotland) Act 2003, access to the Scottish countryside is open to all, but these rights bring with them responsibilities. You should always keep dogs under control; leave gates as you find them; never leave litter; avoid damage to the local environment, whether animal, vegetable or mineral; and follow reasonable advice from land managers if management operations are going on. This last point is particularly important in the Northwest Highlands, where deer-stalking can take place from July until February. Stalking is a crucial part of the sustainable management of deer populations and of the economy of this area, and should be respected by all visitors. During the stalking season, if you wish to follow excursions away from the road, please follow all requests made by estate staff, either in person or through signage. If at all possible, the relevant estate should be contacted in advance of the excursion. Estate contact details are not given here, because of the likelihood of these changing during the lifetime of this guide; useful information can be obtained from www.whoownsscotland.org.uk.

Many of the localities listed in this guide are Sites of Special Scientific Interest (SSSIs). Where this is the case, it is normally stated in the text. There should be no hammering or in situ sampling of any kind at these localities. If you have a genuine research reason for sample collection, you should first request permission from the local office of Scottish Natural Heritage [www.snh.org.uk]. At sites that are not SSSIs, hammering is allowed, but is rarely needed, since the field relationships are generally very clear. If you really want to collect samples, please consider the other visitors that will come after you; repeated hammering by generations of geologists has already destroyed geological and geomorphological evidence at some localities in this area.

Travel to the North-west Highlands is easiest by car; the journey to Ullapool from Inverness takes around one hour, with the total journey from Ullapool northwards to Durness taking a further two hours. The nearest train stations are at Dingwall and Lairg, which are both reached by a small number of trains a day from Inverness. Scheduled buses run from Dingwall and Inverness to Ullapool, and connect with buses to Lochinver. At the time of writing, a postbus from Lairg serves Lochinver and Durness. Access to the majority of the excursions listed here will be difficult without some form of private transport. For larger parties, hired coaches may be useful and will have no problems with the main roads, but some of the localities described here are accessed by single-track roads that are not suitable for large vehicles.

The North-west Highlands are quite sparsely inhabited. Limited accommodation is available in a number of the towns and villages within the area of this guide, but booking in advance is always recommended, and indeed is essential in the summer. A choice of hotels, bed & breakfasts, self-catering accommodation and camping can be found in the larger centres of Ullapool, Lochinver and Durness. Individual hotels and/or hostels are situated in the villages of Achiltibuie, Achmelvich, Inchnadamph, Kylesku, Scourie, Rhiconich and Kinlochbervie.

Background information[edit]

Topographic maps[edit]

The best maps to use for these excursions are the Ordnance Survey 1:50,000 scale Landranger or 1:25,000 scale Explorer maps. The excursions described in this guide lie on Landranger sheets 9 (Cape Wrath); 15 (Loch Assynt); 19 (Gairloch and Ullapool); and 20 (Beinn Dearg and Loch Broom). Ordnance Survey maps are available in a range of outlets across the North-west Highlands.

Geological maps[edit]

Geological maps are published by the British Geological Survey (BGS), and can be purchased online via www.bgs.ac.uk, at the BGS offices in Edinburgh and Keyworth (Nottingham), or at the Natural History Museum, London. Maps are also available for free viewing from the BGS Maps Portal, this includes the recent Assynt special sheet. An overview of the geology is given by the 1:625,000 scale Bedrock Geology: UK North map. More detailed information is provided by the 1:50,000 sheets, and Excursions 1 and 6–11 lie within the Assynt Special Sheet. The remainder of the excursions are spread across sheets 101E (Ullapool); 101W (Summer Isles); 107E (Loch Glencoul); 107W (Point of Stoer); 113 (Cape Wrath) and 114W (Loch Eriboll).

Background reading[edit]

A vast number of papers have been written about the geology of the North-west Highlands over the last 100 years; many of these are referenced in the text, and details are given in the reference list at the back of the guide. A few books are recommended as general background reading. The first two in the list below offer information about the geology of the area and the history of research; both are written from a personal standpoint and do not require extensive geological knowledge to read. The second pair of books provide up-to-date, detailed reviews of the current state of scientific knowledge about the geology of Scotland.

OLDROYD, D. R. (1990): The Highlands Controversy: Constructing Geological Knowledge through Fieldwork in Nineteenth-Century Britain (Chicago: University of Chicago Press).

RIDER, M. H. (2005): Hutton’s Arse (Rogart: Rider-French Consulting). TREWIN, N. H. (ed.) (2002): The Geology of Scotland (London: The Geological Society). WOODCOCK, N. and STRACHAN, R. A. (eds) (2000): Geological History of Britain and Ireland (Oxford: Blackwell Science).

References[edit]