The Edinburgh Geologist
Issue no 42
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The Edinburgh Geologist | | ||||||||
by Caroline Paterson
We were based in the Glebe Barn Field Centre, set up by Simon and Karen Helliwell in a stately old building to provide a hostel for field groups and individuals visiting the island. Arriving on the CalMac ferry from Mallaig on a damp and windy day, we step through the ferry side on to the 'flit-boat' which shuttles in through rocks and shallows to the island's small jetty. "Jump when it comes level" says the kind ferryman, helping old ladies across the gap between the ferry and the boat. Then a brisk walk up the hill to the Glebe Barn while our baggage trundles up in Simon's trailer. First impressions of the place are enhanced by Karen's welcoming tea and cake. Only time before dinner for a quick stroll out to get a view of the trap basalts at this southeast corner of the island, and to be impressed by the commanding rampart of the Sgurr of Eigg towering to nearly 1300 feet above the bay. Sunday we spend along the south coast where cliffs of basalt back a grassy shoreline with views to Muck and the Ardnamurchan peninsula. These basalts are products of the Tertiary igneous activity known so well from nearby Rum, Skye, Mull and Ardnamurchan. The post-glacial sea has eroded a series of caves out of the highly vesicular lava flows along the cliff base, two being sites of events in the island's history. Cathedral Cave, high-roofed as its name suggests, is said to have been used for church services at the time of the nineteenth century Disruption in the Presbyterian Church, when the parish minister left his own church and manse and with his congregation joined the newly formed Free Church of Scotland. Nearby Massacre Cave has a grim history. Sometime in the 1520s the whole population of the island, some 395 people, crowded into it to escape pillage by a party of MacLeods from Skye in what seems to have been a revenge attack between the traditionally feuding MacLeod and MacDonald clans. The MacLeods lit a huge fire across the narrow mouth of the cave which asphyxiated all inside, either by smoke inhalation or by heat and oxygen deprivation. Hugh Miller, visiting this notorious cave three centuries later in 1844, describes in The cruise of the Betsy seeing the bones of adults and children in family groups with the charred remains of their straw mattresses and small household objects. All were finally removed for burial in hallowed ground. A grisly lunch-spot in 21st century sunshine. Only a few of us venture through the very narrow entrance into the large cave behind. The afternoon
is occupied with a gentle potter back along the coast to see several pitchstone
dykes, presumed to be related to the pitchstone of the nearby Sgurr of Eigg. The
Eigg pitchstone is the youngest rock known in the Scottish Tertiary, produced
at a late stage of the whole Tertiary igneous event. Then on past the pier, café
and shop (centres of the island economy) and out along the coast towards Kildonnan
to look at more dykes and the basic 'Kildonnan sheets' which evidently belong
to a late cross-cutting intrusive event. Handsome basalt columns of an immensely
thick lava flow form an impressive cliff face above our homeward path back to
showers, drinks and an excellent dinner. On Tuesday the decision is made to tackle the longest itinerary on the island, walking the eastern coastline below the basalt cliffs of Beinn Bhuidhe to the island's most northerly point, and returning by the road through the centre of the island. This traces the steps of Hugh Miller, whose discovery of fish remains and dinosaur bones in a red limestone of the Jurassic on Eigg he graphically describes in The cruise of the Betsy. There is great anticipation in the feeling that we are treading in Miller's footsteps of nearly 160 years ago, and we are not disappointed. The route north along the coast from Kildonnan follows the base of the Tertiary lavas and the top of the Jurassic, though the latter is extensively landslipped along much of this shore. The most recent fall in the winter of 1999/2000 brought down a huge slab of the Valtos Sandstone from the cliff above. On the foreshore about a mile and a half north of Kildonnan are exposures in the Lealt Shale Formation, the lowest Jurassic found on Eigg, where a series of shales, siltstones and limestones with their fossil fauna again record salinity changes in a coastal lagoon. The lowest bed exposed above the low tide mark is the only known outcrop of Miller's Reptile Bed, where he discovered plesiosaur vertebrae and rib and pelvic bones. No hammering here! An hour or so looking at the fossiliferous beds (lots of the mussel Praemytilus strathairdensis) and we take lunch on the landslip and continue northwards up this charmingly wild bit of coast. With celandines and bluebells decorating the slopes and buzzards wheeling overhead, this is a perfect island day. Finally at the north end of the island
we reach a bay with a stony beach opposite the skerry of Eilean Thuilm, the Seal
Island. The beach has sea-worn boulders of a densely shelly red limestone, packed
with with gastropod and bivalve fragments and with fish scales, fin bones, teeth
and bone fragments. These loose blocks are the source of the plesiosaur bones
and teeth found by Hugh Miller in 1844, then 'a thing new to Scotch geology'.
Though not here seen in the bedrock, they have a lithology identical to that of
the Reptile Bed we saw in the morning. We are less lucky than Miller in
discovering reptile remains, but fortunately John Hudson and Tony Irving have
some of their own specimens on display back at the Glebe Barn for us to see. Turning
for home, we are once more welcomed at the Helliwell's house in Cleadale for tea
and cake before beginning the weary tramp back along the island road.
From the summit we have a panoramic view of all the nearby Tertiary volcanic centres of the islands and the mainland, but it is not a place to linger in today's chilly wind and we are soon on the steep descent back round the foot of the Nose, following the contact of the pitchstone with the palaeovalley side as it cuts down through the underlying lavas. This is steep, scree-strewn and heathery terrain where walking sticks are a distinct asset for the less agile among us, but finally we reach easier ground along the south side and head for a point where the base of the pitchstone is undercut to form a series of overhangs much valued by the local sheep who use them for shelter. These notches are key sites for understanding the form of the pitchstone as a valley fill. Standing under the actual base of the pitchstone one looks up at the ends of its hexagonal columns. Beneath our feet (among the sheep dung) is a conglomerate, originally covering the palaeovalley bottom, while at the back of the recess, between conglomerate and solid pitchstone, is a much altered breccia of black pitchstone pieces in a crumbly yellow matrix, the cooled and altered rubbly bottom of the pitchstone body. Digging with a pick in the conglomerate, Hugh Miller found and carried away large pieces of fossil wood, named Pinites eiggensis, evidence of vegetation originally growing in the Tertiary valley. We have to be content with photographs of a few putative wood fragments in the conglomerate. Thursday sees the
party divided. Six walk the length of the south side of the Sgurr to get a view
of the exposed base of the pitchstone, and the conglomerate valley fill beneath,
where it emerges in a high sea cliff on the west coast of the island displaying
a vertical section through the whole structure. The rest go north again to explore
the Laig Gorge where the Jurassic Duntulm and Kilmaluag Formations, stratigraphically
above the Valtos Sandstone seen on the shore of Laig Bay, are exposed along the
bed of a burn where it cuts into the post-glacial sea cliff backing Laig Bay.
In the upper part of the gorge is a small remnant of the Cretaceous, a conglomerate,
sandstone and limestone, overlain in turn by basalt flows. A wonderful sunny island
day this, with a clear blue sky and for the Sgurr party a view of the Outer Isles
- the outline of the hills of South Uist, the island of Barra and the chain of
little islands running south to Mingulay are clearly distinguishable, and sharp
eyes pick out the Oigh-sgeir lighthouse, sited on the only other known outcrop
of pitchstone like that of the Sgurr. Warned not to disturb a pair of nesting
eagles on the Sgurr, we are rewarded by good views of both in flight. They seem
more disturbed by a troublesome raven which perpetually harries them. So we leave Eigg to get back to 'normal' life on the mainland, full of happy impressions of this charming island. With the community buy-out of the island from its former owner by the Isle of Eigg Heritage Trust in 1997, island residents are working hard to create a new island economy to rescue the place from its former moribund state of decaying houses and empty crofts. The island now seems a busy place. New houses have gone up, though many others still await renovation. A new pier is under construction at which the CalMac ferry will be able to dock rather than having to transfer people and freight via the flit-boat. I for one will be back. (I am indebted to David Blythe for his field notes which I have relied on for detail of a few parts of the geology which I did not attempt due to an injured ankle.) Members and friends taking part in the week were: Ann Allwright, Tony Benfield, David Blythe, Anna Bostock, Mike Cotterill, Henry Emeleus, Rosalind Garton, Angus Harkness, John Hudson, Nora Hudson, Tony Irving, Dennis Jeffery, Alison Kerr, Tom Kerr, Rhoda MacKenzie, David Moseley, Julian Overnell, Caroline Paterson, Sinclair Ross, Margaret Rusbridge, Myra Smith, Christine Thompson and Brian Upton. The Isle of Eigg has its own website with details of accommodation, transport and island facilities at www.isleofeigg.org The Glebe Barn's website, giving information on their facilities and various courses, is www.isleofeigg/glebebarn/ Further Reading An account of the human history of Eigg from prehistoric times, through the clan system of the 16th to 18th centuries, 19th century destitution and troubles with absentee landowners of the 20th century, to the final community buy-out of the island: Dressler, C. 1998. Eigg, the story of an island. Polygon, Edinburgh. A compelling read both for Miller's account of Scottish geology as he understood it in his time, and for his wonderful descriptions of places, scenery and weather just as we know them today: Miller, H. 1858. The cruise of the Betsey, with Rambles of a geologist. Thomas Constable and Company, Edinburgh. Facsimile edition published by NMS Publishing, National Museums of Scotland, Edinburgh, 2003. Geological references Emeleus, C.H. 1997. Rum and the adjacent islands. Memoir for 1:50 000 geological sheet 60. The Stationery Office, London. Emeleus, C.H. & Gyopari, M.C. 1992. in British Tertiary Volcanic Province, pp 101-107. Joint Nature Conservation Committee. Chapman & Hall, London. Hudson, J.D. 1966. Hugh Miller's Reptile Bed and the Mytilus Shales, Middle Jurassic, Isle of Eigg, Scotland. Scottish Journal of Geology, Vol. 2, pp 265-281. Hudson, J.D. 1983. Mesozoic sedimentation and sedimentary rocks in the Inner Hebrides. Proceedings of the Royal Society of Edinburgh, Vol. 83B, pp 47-63. Hudson, J.D. 2000. That man is little to be envied ... Scottish Journal of Geology, Vol. 36, pp 1-3. A geological guide to Eigg, used by us on our week's excursion: Hudson, J.D. & Allwright, E.A. 2003. The Geology of Eigg. Isle of Eigg Heritage Trust, Isle of Eigg [see page 36 of this issue for details of how to get hold of a copy of this book].
Caroline
Paterson is retired from a career in biomedical science as a Lecturer in Biochemistry
in the University of Aberdeen. In mid-career she enrolled as a student with the
Open University and fulfilled a long-held ambition to get informed about geology.
Ten years ago she rashly volunteered to take charge of the organisation of the
Society's annual Long Excursion. Any errors in the description of the geology
of Eigg and Muck are hers alone, and due entirely to her lack of proper attention
in the field.
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