A geological excursion guide to Rum: the Paleocene igneous rocks of the Isle of Rum, Inner Hebrides

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Emeleus, C. H. and Troll, V. R. A geological excursion guide to Rum: the Paleocene igneous rocks of the Isle of Rum, Inner Hebrides. Edinburgh : Edinburgh Geological Society in association with NMS Enterprises Limited, 2008.


Contents

Acknowledgements

Introduction, maps, travel and accomodation, Rum - an excursion|

Summary of the geology of Rum

Pre-Paleocene Geology

Lewisian Gneiss Complex

Torridon Group

Mesozoic strata

Palaeocene

Pre-Stage 1

The Rum Central Complex

Stage 1

The Main Ring Fault

Minor intrusions: the commencement of Stage 2

Stage 2: The Layered Centre (formerly the ‘Layered Suite’)

The Canna Lava Formation on Rum

Events post-dating the Rum Central Complex

The Pleistocene and later

Excursion 1 Kinloch and surroundings

Excursion 1 Kinloch and surroundings. A. West of Kinloch

Locality 1.1 Kinloch Glen – sandstone intruded by dolerite plug and sheet [NM 3927 9987]

Locality 1.2 Bridge over Allt Bealach Mhic Neill – peridotite plug in sandstone [NM 3803 9983]

Locality 1.3 Allt Bealach Mhic Neill – spectacular spherulithic structures in baked sandstone adjoining gabbro plug [NM 3803 9966]

Excursion 1 Kinloch and surroundings. B. South side of Loch Scresort

Locality 1.4 White House, Kinloch – picrite dyke cutting sandstone [NM 4044 9928]

Locality 1.5 Cro nan Laogh – Torridonian sandstone displaced along the low-angle Mullach Ard Fault [NM 4208 9893]

Excursion 2 The Northern Marginal Zone (NMZ)

Coire Dubh – Meall Breac – Am Màm– Priomh-lochs area

Locality 2.1 Allt Slugan a’Choilich – steeply dipping, indurated mudstone within strands of Main Ring Fault [NM 3935 9835]

Locality 2.2 Allt Slugan a’Choilich – Inner Main Ring Fault [NM 3932 9828]

Locality 2.3 Coire Dubh – basal Coire Dubh Breccia, with incipient fracturing in nearby sandstone [NM 3917 9807]

Locality 2.4 Coire Dubh – tuffisite dyke cutting Coire Dubh Breccia [NM 3927 9794]

Locality 2.5 Coire Dubh – ‘mixed-magma’: rhyodacite plug with mafic enclaves [NM 393 979]

Locality 2.6 Cnapan Breaca – bedded breccias, sandstone and rhyodacite ash flows [NM 396 976]

Locality 2.7 Coire Dubh – Epiclastic sandstone underlying tuffs at base of rhyodacite ash flows [NM 3933 9769]

Locality 2.8 Coire Dubh – rhyodacite fiamme deformed by ‘bombs’ – c. [NM 3915 9775]

Locality 2.9 South end, Meall Breac – intrusion breccia at rhyodacite/gabbro contact [NM 3845 9797]

Locality 2.10 South-west Meall Breac – deformed fiamme in rhyodacite ash flow [NM 3837 9810]

Locality 2.11 Loch Gainmich – Am Màm Breccia underlying gabbro megablock [NM 3825 9874]

Locality 2.12 East of Loch Gainmich – gabbro/gneiss contact, megablock in Am Màm Breccia [NM 3825 9873]

Locality 2.13 Am Màm – Am Màm Breccia with gabbro, gneiss and sandstone clasts [NM 3830 9885]

Locality 2.14 East of Am Mam – gabbro of megablock cut by Am Màm Breccia and rhyodacite [NM3856 9867]

Locality 2.15 North Meall Breac – Am Màm Breccia cut by, and grades into, rhyodacite [NM 3870 9860]

Excursion 2 The Northern Marginal Zone (NMZ). Optional: Traverse west, towards Priomh-lochs

Locality 2.16 Loch Gainmich – small intrusive plug of gabbro inside Main Ring Fault [NM 3805 9895]

Locality 2.17 Ridge east of Priomh-lochs – ‘matrix banding’ in peridotite tongue [NM 3735 9900]

Locality 2.18 Priomh-lochs – faulted and unconformable relationships of gritty sandstone to gneiss [NM 3715 9876]

Locality 2.19 North of Priomh-lochs – explosion breccia injected along line of Main Ring Fault [NM 367 994]


Excursion 3 Hallival and Askival

Locality 3.1 Coire Dubh dam – intrusion breccia on edge of Eastern Layered Intrusion [NM 3892 9782]

Locality 3.2 Coire Dubh – chromite seams and anorthositic troctolite at Unit 7/8 boundary [NM 3896 9742]

Locality 3.3 North of Hallival – ‘wavy contact’ within Unit 9, at troctolite/gabbro boundary [NM 3950 9694]

Locality 3.5 Shelf north-west of Hallival – classic Unit 11/12 boundary with chromite seam [NM 3895 9679]

Locality 3.7 Hallival summit – panoramic views of Eigg, north-west Highlands, Skye and Outer Isles [NM 3953 9626]

Locality 3.8 Coire nan Grunnd – details of troctolite layering displayed in fallen blocks c. [NM 400 961]

Locality 3.9 Allt Mòr na h-Uamha – rafts of hornfelsed basalt in layered troctolite [NM 397 972]

Excursion 4A The Central Intrusion

Locality 4.1 Harris road near Kilmory fork – quarry shows sandstone baked by dolerite plug [NG 3696 0016]

Locality 4.2 Bridge south of Long Loch – evidence for Long Loch Fault in riverside exposures [NM 3639 9944]

Locality 4.3 North of Harris road – layering in troctolite deformed by ‘dropstones’ [NM 3615 9929]

Locality 4.4 ‘Whaleback’ west of Long Loch – depositional and erosional sedimentary structures in troctolite [NM 3620 9908]

Locality 4.5 ‘Whaleback’ – layering in troctolite, transgressed by underlying peridotite [NM 3623 9893]

Locality 4.6 South of Long Loch – layered troctolite/peridotite blocks in deformed layered peridotite [NM 3635 9792]

Locality 4.7 Loch an Dornabac – poikilo-macro-spherulitic structures in peridotite of Central Intrusion [NM 3569 9769]

Locality 4.8 North of Loch an Dornabac – small-scale layering in peridotite of Western Layered Intrusion [NM 3576 9800]

Locality 4.9 Road east of Ard Nev – dense, black picrite dyke intruding peridotite [NM 353 984]

Locality 4.10 Roadside north-east of Ard Nev – baked microgranite and (nearby) altered dolerite, near peridotite [NM 3540 9870]

Locality 4.11 East of Ard Mheall – harrisitic structures in peridotite, some showing erosional features [NM 3502 9772]

Excursion 4B The Central Intrusion

Locality 4.12 North of Harris Bay – roadside exposures of harrisitic structures [NM 341 963]

Locality 4.13 Harris – spectacular layering in gabbros north of bridge [NM 3377 9602]

Locality 4.14 West end, Harris Bay – contact of layered gabbro (with harrisite) and microgranite [NM 3350 9560]

Locality 4.15 Cove west of Harris Bay – contact between gabbro and hybridised microgranite [NM 3348 9565]

Locality 4.16 Harris Bay – shelves eroded in well-layered gabbro [NM 373 958]

Locality 4.16 Harris Bay – shelves eroded in well-layered gabbro [NM 373 958]Locality 4.17 East end Harris Bay – intrusion breccia at gabbro/microgranite contact [NM 3405 9505]

Locality 4.18 Abhainn Rangail – peridotite breccia in Central Intrusion, with chromite seams [NM 3450 9557]

Excursion 5 The Canna Lava Formation in north-west Rum

Locality 5.1 North-west of Malcolm’s Bridge – contact between Central Intrusion and Western Granite [NM 354 998]

Locality 5.2 ‘West Minishal’ – conglomerates containing clasts derived from central complex [NG 3475 0064]

Locality 5.4 East of Fionchra – varied clasts in fluviatile conglomerates; palaeovalley in lavas [NG 342 005

Locality 5.5 North-east side, Fionchra – hyaloclastites (including pillows) exposed in fallen blocks [NG 341 005]

Locality 5.6 North side, Fionchra – stream section through plant-bearing silty sandstones [NG 3365 0068]

Locality 5.7 North side, Fionchra – coarse fluviatile conglomerate underlying feldspar-phyric lava [NG 3346 0088]

Locality 5.8 Coire na Loigh – conglomerates and lavas faulted against Torridonian sandstone [NG 3322 0098]

Locality 5.9 Coire na Loigh – lava with thin, glassy basal selvedge overlying conglomerate [NG 3314 0092]

Locality 5.10 South side, Fionchra – lava and conglomerate ponded against wall of hyaloclastite [NG 3363 0038]

Locality 5.11 Bealach a’Bhràigh Bhig – lava resting on rubbly microgranite surface [NG 3395 0010]

Locality 5.12 Orval – classic locality where lavas were shown to rest on weathered microgranite [NM 335 992]

Locality 5.13 Bloodstone Hill – lavas lying on weathered sandstone, overlain by valley-fill lavas [NG 317 006]

Locality 5.14 North end of Bloodstone Hill – lava on weathered sandstone [NG 3149 0082]

Locality 5.15 West side of Bloodstone Hill – rhyodacite boulder in conglomerate [NG 3107 0035]

Excursion 6 Minishal and north-west Rum

Locality 6.1 Kilmory River – alkaline segregations in peridotite of Central Intrusion [NM 3639 9962]

Locality 6.2 West of Salisbury’s Dam – replacement ‘finger structures’ in peridotite [NG 3583 0018]

Locality 6.3 Minishal – Main Ring Fault, sandstone and microgranite cut by peridotite [NG 3550 0022]

Locality 6.4 Path north of Minishal – peridotite plug with some layering, intruding sandstone [NG 356 002]

Locality 6.5 Sgaorishal – ‘fissure-breccia’ in sandstone, which is baked and bleached [NG 3523 0175]

Locality 6.6 ‘West Sgaorishal’ – contact of sandstone and peridotite exposed in plug [NG 3446 0198]

Locality 6.7 ‘West Sgaorishal’ – matrix banding and strong jointing in peridotite plug [NG 3430 0224]

Locality 6.8 ‘West Sgaorishal’ – Triassic cornstones at peridotite contact containing chalcopyrite and malachite [NG 3427 0229]

Locality 6.9 Monadh Dubh – plant and ostracod remains in Triassic silty sandstones [NG 3404 0300]

Locality 6.10 North of Glen Shellesder – cornstones permeating brecciated Torridonian sandstone [NG 3301 0262]

Locality 6.11 Coast at Glen Shellesder – heavy mineral bands in Torridonian sandstone [NG 3300 0248]

Locality 6.12 Guirdil Bothy – bloodstone and agate pebbles on the foreshore [NG 3196 0134]

Excursions 7, 8 and 9 The Southern Mountains and Dibidil

Excursion 7 Kinloch – Allt nam Bà – Beinn nan Stac – Lower Glen Dibidil

Locality 7.1 Allt na h-Uamha – magmatic sediments [NM 4095 9670]

Locality 7.2 Welshman’s Rock – a doming-related ?slide[NM 417 963]

Locality 7.3 Allt nam Bà – margins of a ultrabasic magma chamber and evidence for subsidence on the Main Ring Fault [NM 4093 9424]

Locality 7.4 Lower south-east flank of Beinn nan Stac – Paleocene basalt lavas and Lewisian gneiss slivers in the Main Ring Fault zone [NM 4033 9343]

Locality 7.5 Beinn nan Stac – folded and tilted Torridonian rocks [NM 4004 9403]

Locality 7.6 Peak of Beinn nan Stac – a volcano-sedimentary succession? [MN 3977 9405]

Locality 7.7 Views from Beinn nan Stac [NM 3693 9410]

Locality 7.8 Lower south-west flank of Beinn nan Stac – subsided slivers of Mesozoic and Paleocene rocks [NM 3988 9341]

Locality 7.9 Dibidil River – ‘intrusive tuffs’ [NM 3934 9307]

Locality 7.10 Dibidil foreshore – a ring fault exposed [NM 3947 9374]

Excursion 8 Lower Glen Dibidil – Nameless and Forgotten corries – Upper Glen Dibidil – Sandy Corrie – Sgurr nan Gillean

Locality 8.1 Lower east side of Sgurr nan Gillean – contact between intrusive breccia and mesobreccia [NM 3899 9294]

Locality 8.2 Lower east side of Sgurr nan Gillean – enigmatic Torridonian rocks above rhyodacite (extrusive?) and mesobreccia [NM 3876 9293]

Locality 8.3 Nameless Corrie – an ignimbrite vent system [NM 3828 9346]

Locality 8.4 Nameless Corrie – a hybrid contact rock (‘needle-rock’) [NM 3863 9349]

Locality 8.5 Forgotten Corrie – tilted Torridonian basement, bedded mesobreccia, and extrusive vs. intrusive rhyodacite [NM 3851 9427]

Locality 8.6 Upper Glen Dibidil – Hughes’ back-veining contact between the ultrabasic and felsic rocks (rhyodacites) [NM 3821 9453]

Locality 8.7 Bealach an Fhuarain – views of Glen Dibidil, Trollaval, Fiachanis and Harris [NM 3790 9487]

Locality 8.8 Sandy Corrie – a rare caldera floor through volcanosedimentary infill succession [NM 3743 9404]

Locality 8.9 Headwall of Nameless Corrie – intrusive rhyodacite meets the extrusive rhyodacite sheet of Sgurr nan Gillean peak [NM 3783 9330]

Locality 8.10 Peak of Sgurr nan Gillean Peak of Sgurr nan Gillean – views of the Hebrides and Western Scottish Highlands [NM 3802 9303]

Excursion 9 Lower Glen Dibidil – shoulder of Sgurr nan Gillean – Papadil

Locality 9.1 Along the Papadil path – evidence for multiple movements on the Main Ring Fault [NM 3916 9263]

Locality 9.2 Southern lower slope of Sgurr nan Gillean – depositional contact between mesobreccia and uplifted gneiss [NM 3845 9248]

Locality 9.3 Southern lower slope of Sgurr nan Gillean – contacts between mesobreccia, rhyodacite, and intrusive Am Màm-type breccia [NM 3826 9245]

Locality 9.4 Shoulder of Sgurr nan Gillean – alternating bedding in the mesobreccia [NM 3810 9246]

Locality 9.5 Shoulder of Sgurr nan Gillean – the Papadil Microgranite [NM 378 923]

Locality 9.6 Sandstone melted by gabbro at Inbhir Gil [NM 3579 9247]

Locality 9.7 Peridotites around Loch Papadil [NM 3620 9228]

Locality 9.8 Melted sandstone around the bay south of Loch Papadil [NM 3660 9173]

Locality 9.9 South-east of Papadil – minor intrusions and heavy-mineral bands in Torridonian sandstone [NM 3671 9164]

Acknowledgements

Our understanding of the geology of Rum has profited greatly by many discussions with J. Barraud, J. Bedard, B. R. Bell, G. P. Black, M. H. P. Bott, D. Brown, G. M. Brown, M. Cheadle, C. H. Donaldson, A. C. Dunham, M. Errington, J. Faithfull, M. Forster, A. Fowler, K. Goodenough, J. R. Graham, R. Greenwood, C. J. Hughes, R. H. Hunter, M. Holness, D. Jerram, D. Kitchen, J. McClurg, I. Meighan, G. Nicoll, P. J. Nicholson, B. O’Driscoll, R. Renner, R. Sides, M. Smith, B. G. J. Upton, J. Volker, W. J. Wadsworth and I. M. Young. We are especially grateful to G. Nicoll for help with drafting many of the maps and for providing a number of photographs.

We would also like to thank numerous research and undergraduate students, particularly P. K. Byrne, E. Donoghue, L. McCourt, C. Flanagan and F. Sheehan.

D. Stephenson and K. M. Goodenough are thanked for editorial handling of the various versions of this manuscript.

Our work on Rum has been made possible through the help and encouragement of the past and present scientific and estate staff of the Nature Conservancy Council and Scottish Natural Heritage, on and off the island.

Introduction

The Isle of Rum is the largest of the Small Isles in the Inner Hebrides, north-west Scotland (See image). It is a National Nature Reserve, owned and managed by Scottish Natural Heritage (SNH)* and there are several geological Sites of Special Scientific Interest (Emeleus and Gyopari, 1992). In addition to the spectacular geology, the island is noted for its herd of red deer (the subject of a long-term study initated in the 1950s), feral goats, plant life, birds (Rum was used as the base for the reintroduction of the Sea Eagle to the Hebrides), and insects. Rum has a population of about 20, the majority of whom live at Kinloch.

Visitors to Rum can usually freely explore the immediate surroundings of Kinloch, where there are several well-marked nature trails. At certain times during the year there are restrictions on access to parts of the island, especially the northern area around Kilmory which is the centre for ongoing deer studies. Notification of these activities is usually given on the information boards outside the White House. Those walking or working outwith Kinloch should always fill in daily route cards (available outside the White House) and make sure that these are completed on return. Leaders of parties visiting the island must contact the Reserve Manager well in advance of their intended visit; geologists should note that rock sampling can only be carried out with the Reserve Manager’s permission. Collecting from loose material is usually not a problem but hammering exposures is not generally permitted.

Rum and the other Small Isles (Eigg, Canna, and Muck) are served by ferry (foot passengers only) from Mallaig, which is connected by road and railway to Fort William (70 km) and Glasgow (240 km), and by road to Inverness (180 km). The nearest airports are at Glasgow and Inverness. There is also a regular vehicle ferry connection (c. 40 minutes) between Mallaig and Armadale on the Isle of Skye.

*SNH, The White House, Kinloch, Isle of Rum, PH43 4RR; Tel. 01687-46-2026; www.snh.org.uk

Fieldwork on Rum generally involves cross-country walking over rough, damp ground and climbing to between 500–800m elevation. Rain and strong winds are common. It is therefore essential to have good walking boots and adequate waterproof clothing. There is no public transport and all vehicles on the island are for the use of SNH employees. There are no paved roads, only rough tracks and paths.

The island is well known for the ferocity of its midges, which can be very trying on still, humid days. Visitors should bring their preferred repellant and midge nets, for these may not be available on the island. Ticks occur in the areas frequented by deer and goats. The best protection against both of these is provided by long trousers and long-sleeved shirts. There are no snakes on Rum.

A selection of maps and books relating to Rum is listed below. Topographic maps can be obtained through most booksellers or from Edward Stanford Ltd, 12-14 Long Acre, London, WC2E 9LP (www.stanfords.co.uk). SNH publications are available from the Publications Section, Scottish Natural Heritage, Battleby, Redgorton, Perth, PH1 3EW (www.snh.org.uk) and some may be bought from the SNH office on Rum. The SNH 1:20,000 geological map is only obtainable from SNH on Rum. Publications (maps, memoirs, etc.) of the British Geological Survey (www.bgs.ac.uk) can be purchased from: BGS, Murchison House, West Mains Road, Edinburgh, EH9 3LE; BGS Keyworth, Nottingham, NG12 5GG; The Natural History Museum, Earth Sciences Galleries, South Kensington, London; or through approved stockists. (NB: BGS publications required for educational purposes and ordered through an educational establishment may attract a discount.)

Maps

Visitors to Rum should bring a copy of the Ordnance Survey 1:25,000 map and use this at all times when in the field.

Ordnance Survey topographic maps:

1:50,000 Landranger series: Sheet 33, Rum and Eigg

1:25,000 Explorer series: Sheet 397,

Rum, Eigg, Muck, Canna and Sanday

Geological maps

1:50,000 British Geological Survey Scotland Sheet 60, Rum (Solid & Drift) (1994)

1:20,000 Scottish Natural Heritage, Rum – Solid Geology

(Second Edition, 1992) (obtainable only from SNH on Rum)

Selected books, etc.

GOODENOUGH, K. and BRADWELL, T. (2004): Rum and the Small Isles: A Landscape fashioned by Geology (Redgorton, Perth: Scottish Natural Heritage).

BELL, B. R. and WILLIAMSON, I. T. (2002): ‘Chapter 14: Tertiary igneous activity’, in TREWIN, N. H. (editor): The Geology of Scotland (London: The Geological Society).

EMELEUS, C. H. (1997): ‘Geology of Rum and the adjacent islands’, Memoir of the British Geological Survey, sheet 60 (Scotland) (Nottingham: British Geological Survey).

EMELEUS, C. H. and BELL, B. R. (2005): British regional geology: The Palaeogene volcanic districts of Scotland(fourth edition) (Nottingham: British Geological Survey).

UPTON, B. G. J. (2004): Volcanoes and the Making of Scotland (Edinburgh: Dunedin Academic Press).

Travel

Caledonian MacBrayne operates a service from Mallaig to Rum and the other Small Isles. Details about the current timetable and fares should be obtained from Caledonian MacBrayne (Mallaig 01687-46-2403; or see the company’s website, www.calmac.co.uk). For train connections between Mallaig, Fort William and Glasgow, consult the National Rail Timetable or First Scotrail (www.firstgroup.com/scotrail). Buses operate between Mallaig and Fort William (Shiel Buses, Acharacle, Argyll, PH36 4JY; shiel.buses@virgin.net). As ferry departures from Mallaig are generally earlier than the arrival of trains (except on Saturdays during the summer), it is necessary to stay in Mallaig overnight. Hotel, B&B and other accommodation is available but should be booked in advance. A summer ferry service also operates between Arisaig and Rum on certain days; for details contact Arisaig Marine (tel. 01687-465224; www.greentourism.org.uk/ ArisaigMarine). Private vehicles (cars, motorcycles, etc.) are not permitted on the island and the SNH office on Rum should be consulted about the use of mountain bikes prior to arrival.

NB: Ferry sailings can be delayed or cancelled when there are adverse weather conditions. This rarely happens during the summer and delays are less frequent since the completion of the new slipways on Rum and the other islands.

Accommodation

Accommodation (self-catering/individual meals/full board) is currently available at Kinloch Castle hostel (contact: The Manager, Kinloch Castle, Isle of Rum, PH43 4RR, Tel. 01687-46-2037). Camping is allowed at Kinloch; elsewhere camping is strictly controlled and may only be arranged with the permission of the Reserve Manager. Bothies maintained by the Scottish Mountain Bothies Association are located at Dibidil [NM 393 927] and Guirdil [NG 320 013]; the Reserve Manager should be consulted if it is intended to use these.

A small licensed shop and post office next to the community hall [NM 403 997] at Kinloch sells a selection of groceries and beverages. Opening hours are from about 17.00–19.00. A limited selection of postcards may be available and postcards may also be on sale in the castle, along with Scottish Natural Heritage literature about Rum. A comprehensive selection of SNH publications about Rum is obtainable from the Reserve Office which is open on mornings, Monday–Friday. There is a public telephone near the old post office [NM 403 996] and in the castle courtyard. At present, mobile phone reception is possible on parts of the east side of the island, but is poor to non-existent elsewhere.

Items needed for the bothies

The bothy accommodation is spartan. There are two rooms, each of which has a fireplace, a table, and some benches. There is no plumbing and at Dibidil there is no nearby source of fuel (driftwood or otherwise). If you wish to light a fire in this bothy, bring a supply of fuel (kindling, firewood, coal, firelighters, matches, etc.). Sleeping bags, cooking utensils, a lightweight stove, all food, and candles/torches will be required. Mobile phone signal is generally good in the Dibidil bothy.

References

At all times follow: The Scottish Access Codeand Code of Conduct for Fieldwork .