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   The Great Plume Debate

AGU Chapman Conference: The Great Plume Debate


Askival (left peak) and Hallival (right peak) viewed from ferry approaching Kinloch from East.

Rum Excursion

Sunday 21 st – Friday 26th August, 2005

Leaders: Dr C. Henry Emeleus1 and Dr Valentin R. Troll2

  1. Dept. Earth Sciences, Univ. Durham, South Road, Durham, DH1 3LE, United Kingdom. (c.h.emeleus@durham.ac.uk)
  2. Dept. Geology, Univ. Dublin, Trinity College, Dublin 2, Ireland. (trollv@gemini.tcd.ie)

Summary Itinerary

Sunday 21st Arrive Mallaig late afternoon/evening, stay overnight at the Marine Hotel.

Monday 22nd 09.45 (latest): assemble at the Caledonian Macbrayne pier next to the Small Isles ferry “Loch Nevis”, for a 10.15 departure.

Arrive Rum 12.45 and walk to Kinloch Castle (c 1.5 km; your baggage will be transported by Land Rover). Local geology in afternoon.

Tuesday 23rd – Thursday 25th Day long excursions designed to illustrate most aspects of the Paleocene igneous geology of Rum (see provisional summary below).

Friday 26th Features (geological and otherwise) around Kinloch will be examined in the morning.

Departure from Rum: If you wish, you may board the ferry at 14:15. If you do this you should be at the slipway by 13:45. The ferry will go to the Isle of Canna and then return to Rum at 16:35. If you want to spend a little time on Canna, you can take this option for a small additional ferry charge. People who would prefer to spend more time on Rum may board the ferry when it returns to Rum at 16:35, and then sails straight to Mallaig. Be sure to be there well in advance of 16:35 to allow time for boarding.

Luggage must be ready well in advance, to be loaded onto the Land Rover going to the slipway. You must be sure that you check your own luggage onto the ship.

Arrive Mallaig 17:55. Transport will be arranged to take the group from there to Ft. William.

If you are planning to stay a further day on Rum privately, a Saturday morning boat from Rum will get you to Mallaig at about 13:00 hours, with ample time to get the afternoon train to Ft. William. Please note that the charge for the main excursion does not cover the additional costs arising from a further day's accommodation or independent travel from Rum to Ft. William.

Costs

The inclusive cost of the excursion (Mallaig –> Rum –> Mallaig –> Ft. William, including B&B in Mallaig 21st August) is 270.

General

The Isle of Rum is the largest of the Small Isles in the Inner Hebrides, NW Scotland. It a National Nature Reserve, owned and managed by Scottish Natural Heritage (SNH) (www.snh.org.uk). In addition to spectacular geology, the island is noted for its herd of red deer (the subject of a long-term study), feral goats, plant life, birds (Rum was used as the base for the reintroduction of the Sea Eagle to the Hebrides), and insects (including the midge).

Rum has a population of about 20, the majority living at Kinloch where the SNH has its office and staff quarters, and where there is a shop/post office and public telephone (NB: mobile/cell phone reception is poor except on the extreme east coast of the island). The party will stay at Kinloch Castle, an Edwardian country house/shooting lodge.

The island is served by a ferry service (foot passengers only) from the fishing port of Mallaig, which is connected by road and railway to Fort William (45 miles) and Glasgow (150 miles), and by road to Inverness (110 miles). 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. Ferry services are operated by Caledonian MacBrayne (www.calmac.co.uk) and train services by Scotrail (http://www.firstgroup.com/scotrail/).


Schematic view of the Isle of Rum viewed from East. Click on image to enlarge.
Field work on Rum commonly involves walking in the rain and  wind, over rough, damp ground, so please come prepared! It is essential to have suitable walking boots for day use and adequate waterproof clothing. Temperatures in August are generally in the range 15 – 18oC. All excursions will be on foot (the SNH Land Rovers are exclusively for the use of their staff), with distances up to about 12 km. There are no paved roads on Rum, only rough tracks and a few paths. Most excursions will involve cross-country walking over wet ground, climbing up to about 700 m (c. 2200 feet) elevation on one day.

The island is well known for the ferocity of its midges. If there is the slightest breeze these are generally not a problem, but they can be very trying on still, humid days. So, bring your preferred repellant for none may be available on the island. Ticks occur in the areas frequented by deer and goats. Your best protection against both is to wear long trousers and long sleeved shirts, and at the end of the day make sure that no ticks are attached to you. Unlike on the other large islands and the Scottish mainland, there are no snakes on Rum.

Travel and Accommodation

Unless you inform us to the contrary it will be assumed that everyone will arrive in Mallaig on the afternoon or evening of Sunday 21st (the last train is due in to Mallaig at about 23.30, having departed Glasgow ca. 18.00). The party will stay overnight at the Marine Hotel which adjoins the railway station. Cars may be left in the long-term car park on the west side of the road from Ft. William (A830), adjacent to the railway station (the car park was free in 2004). Be sure to lock your car securely and don't leave any coats or bags, etc visible.  Last minute shopping can be done on Monday morning, the shops, banks (with cash machines) and the post office are close to the Marine Hotel.

The Caledonian MacBrayne ferry “Loch Nevis” departs from a berth close to the Skye vehicle ferry, a few minutes walk from the hotel. We have already reserved return tickets for everyone. You must get to the ferry about 30 minutes before departure (which is scheduled for 10:15 ) in order to give time for baggage to be assembled and loaded onto the ferry. The ship is due to arrive at the Rum slipway at 12:45. There is a cafeteria onboard which will give a chance for you to buy a mid-day meal or snack (the first meal on Rum will be supper at about 19:00).

The party will stay in the Kinloch Castle hostel. You will share accommodation in rooms for 2, 3 or 4 persons. Please note that this is not a hotel and some facilities (e.g. for washing) may be limited, especially if the hostel is full. We shall be provided with breakfast, a packed lunch and dinner (soft drinks, wine, beer will be available but must be paid for individually). Please give the organizers advanced notice of any special dietary requirements. It should also be possible to make tea or coffee in the self-catering kitchen. There is a strictly enforced ban on smoking in the hostel (except in one designated common room) or castle courtyard. Please observe this.

There is a small shop/post office which is usually quite well stocked with snacks and drinks (alcoholic and otherwise). The shop is generally open between about 17:30 – 19:30. 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.

On Friday 26th the ferry departs for Mallaig at 14:15. The journey is normally via the island of Canna, west of Rum, and usually provides good views of NW Rum and the Skye Cuillin. You may also board it at 16:35 when the boat returns to Rum. Arrival at Mallaig is scheduled for 17:55. As this gives a very tight connection with  the last train to Ft. William (scheduled to leave at 18:15), we will arrange vehicular transport to Ft. William for the group as necessary. The official end to the excursion will be at Mallaig. Individuals should have made their own arrangements for accommodation for the night of 26th August.

NB: Ferry sailings can be delayed by 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. Prior to 2004 passengers were transported between the  ship and shore by a small motor boat.

Summary of the Geology and a provisional list of excursions.

The Paleocene Rum Central Igneous Complex (c. 60 Ma) is situated on a  ridge of Archean Lewisian gneiss and sandstones of the Mid-Proterozoic Torridon Group, bounded to the east and west by basins filled with Mesozoic sedimentary rocks and Paleocene basalt lavas. On Rum, the coarse- to medium-grained red Torridon sandstones form the country rocks to the Paleocene Igneous Complex. The sandstones cut by numerous NW- to N- trending (predominantly) basaltic dykes of Paleocene age and, in NW Rum are overlain by Triassic sandstones and lavas of Paleocene age. Relic masses of Jurassic sedimentary rocks and Lewisian gneiss occur within the central complex, together with Torridonian sandstones and Paleocene basaltic lavas.

The Rum Central Igneous Complex was formed in two distinct stages:

During Stage 1 a major arcuate fault system (the Main Ring Fault) developed, which encompasses most of the intrusive and extrusive igneous activity. Central uplift on the ring faults brought masses of Lewisian gneiss and the basal members of the Torridon Group, to the present erosional levels, while later subsidence brought down Jurassic sedimentary rocks and Paleocene basaltic lavas. Magmas of mixed basaltic and rhyodacitic compositions intruded along the ring faults, and also erupted as ash flows which accumulated in a caldera bounded by the ring faults. The flows buried contemporaneous coarse breccias (predominantly of Torridon Group rocks) resulting from the collapse of the caldera walls. Somewhat later, several granites were intruded, including the Western Granite which forms much of western Rum. During Stage 1, the country rocks were strongly domed over the central complex and there is evidence that large masses of sandstone slid off the dome.

Stage 2 commenced with the intrusion of many basaltic dykes and a suite of basaltic cone-sheets. These were followed by the emplacement of the Rum Layered Suite (gabbros, troctolites and feldspathic peridotites) that now forms the high ground of central Rum. In eastern Rum, these mafic and ultramafic rocks form prominent, gently-dipping layers (generally termed “Units”) and comprise the Eastern Layered Intrusion (or “Series”). A number of processes operated during the formation of the layered structures, including crystal fractionation and the intrusion of sheet-like bodies. Layered rocks also occur in SW Rum (the Western Layered Intrusion). The two areas of layered rocks are separated by the Central Intrusion, a N-S belt of breccias composed of blocks and megablocks of bytownite troctolite and feldspathic peridotite enclosed in matrices of similar compositions. The Central Intrusion is regarded as the feeder zone for the layered rocks. It is located along a major N-S fracture, the Long Loch Fault. Numerous sheets and plugs of gabbro and feldspathic peridotite intrude the layered rocks and plugs are common throughout the country rocks. A few dykes, including rare picrites, intrude the Rum Layered Suite.

A major volcanic edifice likely built up during stages 1 and 2, but vigorous erosion soon reduced the central complex to a hilly landscape, at much the level of the present land surface. The evidence for this is found in NW Rum where the Western Granite and sandstones of the Torridon Group are overlain by predominantly basaltic lava flows (c. 60 Ma) with intercalated fluviatile conglomerates. The lava flows and conglomerates, which belong to the Canna Lava Formation, have buried and preserved a hilly landscape dissected by steep-sided valleys that drained central Rum. The conglomerates contain abundant clasts of red sandstone and gneiss, together with rhyodacite, granite, troctolite and gabbro, all clearly derived from the central complex. Flows and conglomerates belonging to the Canna Lava Formation form the nearby islands of Canna and Sanday and are part of the Skye Main Lava Group of SW Skye where they are cut by gabbros of the Skye Cuillin Centre. The Rum central complex (60.5 Ma) was clearly dead and thoroughly dissected before the presently-exposed member of the Skye central complex were intruded (at c. 59 Ma).

Rum was almost completely covered by the major Pleistocene ice sheet sourced in mainland Scotland and, at a later stage, it supported a local ice cap with several valley glaciers. The ice had gone by about 10,000 BP and there is evidence that Man arrived fairly soon thereafter: at Kinloch a site excavated a few years ago has yielded implements made from the bloodstone found in the lavas of NW Rum. Remains from this site have been dated at about 8,500 BP.

Provisional excursions (N.B. order and content subject to weather!):

(i) A half day will be spent around Kinloch, on the sandstones of the Torridon Group and the numerous minor intrusion. The latter include basaltic sheets and dykes, and several plugs or feldspathic peridotite and gabbro.

(ii) This will be concerned with the rocks of Stage 1 exposed in Coire Dubh and on the low hills south of Kinloch (Cnapan Breaca, Meall Breac and Am Mam). The traverse, from Kinloch into Coire Dubh to the south, will take us across in situ Torridon Group rocks to the Main Ring Fault, then onto caldera breccias, and ash flows and small intrusions of rhyodacite (with evidence of mixing of basaltic and rhyodacitic magmas). On Am Mam, a distinctive intrusive breccia is seen to pre-date intrusive rhyodacite.


Fiamme at the base of rhyodacite ash-flow sheets, Coire Dubh and Meall Breac.

Layered ultrabasic rocks with sedimentary structures, East slope of Hallival.
(iii) The aim will be to examine members of the Eastern Layered Intrusion. Using the same route as at the start of excursion (i), we shall continue south to the layered rocks (“Units 7, 8”) in the back of Coire Dubh where thin anorthosite and chrome-spinel-rich layers occur, then over ledges formed by resistant bytownite troctolite (“allivalite”) layers to a shelf (“Unit 9”) about 1 km N of Hallival. Here layered structures have been variously ascribed to sedimentary loading in the magma chamber or to metasomatic replacement. Continuing uphill to the ridge NNW of Hallival there are good examples of chromite seams developed between major layers (“Units 11/12”) and, continuing to Hallival summit (722 m), of extensive slumping and deformation in the “allivalite” layers.

On a clear day the view from Hallival summit is magnificent: it extends from Ardnamurchan in the SE, east to the isles of Eigg and Muck with the Western Highlands beyond (and with Ben Nevis in the far distance), and north to the Paleocene Skye Central Igneous Complex and the Torridon Group sandstone hills of Applecross. The Outer Hebrides occupy the far western horizon. After a steep descent to the col between Hallival and Askival, return over allivalite shelves, some of which expose large xenoliths of basic granulites, interpreted to be thermally metamorphosed lavas from the pre-Rum Central Complex Eigg Lava Formation.

(iv) Continued examination of Stage 2 rocks, now belonging to the Central Intrusion: from Kinloch, take the rough road west and SW for about 4 km to the low ridge immediately west of the Long Loch. The ridge exposes superb layered structures attributable to magmatic sedimentation proceses during formation of the “allivalites” and feldspathic peridotites. Continuing beyond the S end of the Long Loch, spectacular breccias contain large blocks of layered ultramafic rocks in feldspathic peridotite with wildly disturbed layering.

Outcrop in Central Layered Series showing harrisitic olivine (in-situ growth).

Ultrabasic Breccias, of central and SW Rum

In the latter part of this day there are the options of (a) continuing to the Western Layered Intrusion at Harris Bay (a longish walk), where the complicated contact with the Western Granite is well exposed, or (b) of examining the post-central complex lavas and conglomerates around Fionchra (another longish walk).

(v) There might be an opportunity for a quick reappraisal of rocks from Excursion 1 in Coire Dubh  (returning to Kinloch by 12.30 at the latest), or of taking it easy and possibly having a guided tour of the Castle (charge c. 3) 

Literature

Below are listed a number of maps and books relating to Rum and its geology. 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 (including the 1:20,000 geological map) are obtainable on Rum. Publications (maps, memoirs etc) of the British Geological Survey (www.bgs.ac.uk) can be purchased from BGS Keyworth, Nottingham NG12 5GG, BGS, Murchison House, West Mains Road, Edinburgh EH9 3JW, 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 reasonable discount.)

Maps

Ordnance Survey topographic maps

1:50,000 Landranger series, Sheet 33 Rum and Eigg, 6.49

1:25,000 Explorer series: Sheet 397, Rum, Eigg, Muck, Canna and Sanday, 7.49

Geological maps

1:50,000 British Geological Survey Scotland Sheet 60, Rum (Solid & Drift) (1994) 10.

1:20,000 Scottish Natural Heritage RUM-Solid Geology. (SNH, Rum; 5 – available on Rum)

Geological publications

(i) Rum (A guide to the excursions will be provided on arrival)

Emeleus, C.H. 1997. Geology of Rum and the adjacent islands. Memoir of the British Geological Survey, Sheet 60 (Scotland) 35.

Emeleus, C.H. and Forster, R.M. 1979. Guide to the Tertiary Igneous Rocks of Rhum. Nature Conservancy Council, Peterborough. (Obtainable from SNH office on Rum, 3)

 (ii) Palaeogene igneous rocks of NW Scotland, including Rum

Bell, B R and Williamson, I T. 2003. Tertiary igneous activity. In Trewin, N H (editor)    The Geology of Scotland (4th Edition). The Geological Society: London

Emeleus, C H and Bell, B R. 2005. British Regional Geology: the Palaeogene volcanic districts of Scotland (4th Edition) London: HMSO for the British Geological Survey. (Price to be announced.) This book is now hot off the printing press and copies will be made available for purchase at the Ben Nevis Hotel during the meeting.

CHE 28th February, 2005
details of logistics updated by GRF 12th August, 2005

Further Information:

For further information, please email one of the fieldtrip leaders:

c.h.emeleus@durham.ac.uk or trollv@gemini.tcd.ie

or conference conveners:

Ian Campbell
Gillian R. Foulger
James H. Natland
W. Jason Morgan

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