Rob Butler
On September 4th 1860, the 25 year old Archibald Geikie completed an audacious piece of rapid reconnaissance geology. He walked alone from Kinlochewe to Ullapool, tracing the Cambrian quartzites through what, even today, is some of the most remote countryside in the British Isles. His observations were used to reaffirm Murchison's view of stratigraphic continuity in the NW Highlands, and cast doubt on Nicol's competing hypothesis of tectonic disruption. This serious blunder was only recovered 25 years later. Is there a role for this type of reconnaissance geology? Just how much science could be done covering those 40 km (with a shade over 1000m of ascent and descent) during daylight hours?
I first heard of Geikie's remarkable solo expedition from David Oldroyd's equally remarkable, and highly entertaining book The Highlands Controversy. As an account of geological debate and the politics of Victorian science it is hard to beat. It tells the story of the discovery of what we now know as the Moine Thrust Belt and in many regards marks the start of research in thrust tectonics, at least within the English-speaking world. As a student in Leeds I cut my structural teeth on this ground, brought up on the century-old folk-law of NW Highlands geology: Peach and Horne mapping out miles of complex imbricate structures; the iconoclastic Charles Lapworth getting "over-excited" at the prospect of far-travelled thrust sheets of gneiss. But these successes had a less glorious past.
In
the middle of the nineteenth century, the scientific debate famously rested between that Victorian colossus, Sir
Roderick Impey Murchison and Aberdeen's James Nicol. Murchison contended that NW Highland geology represented
a simple stratigraphic progression while Nicol argued for tectonic disruption. Geikie was Murchison's protégé,
and was involved in most of the field work that was used to rubbish Nicol's interpretations. He did very nicely
out of the association. Through Murchison's patronage Geikie gained the Chair in Geology at Edinburgh and went
on to become Director of the Geological Survey and President of the Royal Society. As Oldroyd puts it, he was
always the winner. In contrast, those on the other side of the debate were hounded by Murchison.
Although the Highlands Controversy has long been told in terms of a battle between Murchison and Nicol, the key science was done by Geikie on one side and variously by Charles Lapworth and Charles Callaway (the two independently "discovered" thrust tectonics), followed by Peach and Horne. The key difference was that while Geikie did rapid tours and reconnaissance traverses, in the early 1880s Lapworth, followed by Peach and Horne, mapped out geometric relationships. In modern terms the difference in approach is like comparing the insight on basin structure offered by 3D seismic with that gained from a few wells and a poor 2D line. In the face of better mapping, the old Murchison-Geikie interpretations fell apart and the world has been left with a legacy of thrust tectonics, albeit rather different to Nicol's view of faulting, that broadly remains valid today.
Oldroyd's book was particularly poignant for me at the time because it was my bedtime reading during a rather rapid field study around the Himalayan peak of Nanga Parbat. Was I falling into the trap of looking at complex terrain too quickly? Geikie's early adventures certainly made me wonder whether reconnaissance merely reconfirms initial prejudices. Some kind of re-enactment seemed appropriate.
10 years passed by, the gap in the map beckoned. Rather than walk alone I was joined by Richard Morgan and Steve Matthews. Back in 1983 we had been contracted by Mike Coward to map part of Geikie's country. For a week that snowy April we nibbled away at the edges then got back to our own PhD researches. This time we opted for the better chances of fair weather offered by the Spring Bank Holiday weekend. Our plan was to meet in Ullapool on the Friday night, leave support cars at Corrie Hallie and above the traverse' end at Allt na Airbhe and get a good meal. Then first thing we'd drive around to Kinlochewe and start walking. Richard, Steve and I were joined by Tim Needham and John Wheeler, both fellow inmates from early 80s Leeds, together with Clare Bond from Scottish Natural Heritage to ensure fair play!
6.45 am saw Tim's over-full estate roll into the car park at Incheril. A misted up Sierra parked in the corner offered up a very bleary-eyed Wheeler. He had reached the car park at two, having been kept in Liverpool until five the previous evening by an examiners' meeting, and then by the convoys of caravans heading for the Lakes. A quick crunchie bar and we were off, out along the path worn into a trench by scores of Munro-baggers that leads to Loch Maree and Slioch.
Geikie's field note books are blank for the day of his traverse. Oldroyd suggests that the field descriptions were written up in Ullapool the following day - a proposal that introduced a healthy air of skepticism to our proceedings. Armed with all the paraphernalia, field slips, notebooks, cameras and even a GPS receiver, we followed beneath crags of Torridonian and Lewisian rocks that lie in the Kinlochewe thrust sheet. The weather was brilliant, hazy blue skies that show the Highlands at their best yet are the harbingers of rain. Perhaps low cloud obscured Geikie's view. The first rocks on the path are the Cambrian quartzites that run up to the spectacular rampart of Bonaidh Dhonn overlooking the head of Loch Maree. Ahead is the Torridonian mountain massif of Slioch with the basal unconformity roller-coastering across the Lewisian basement, probably the most dramatic Precambrian landscape in Europe.
Our route now ran NNE, climbing the rugged stalkers' path up Glen Bianasdail. Being on the west side of the valley we weaved up and down across the sub-Torridonian unconformity. But the eastern side shows the structure. Geikie had been along here with Murchison a few days before his walk but never ventured onto the rocks. Upon a plinth of Torridonian sandstones lies the continuous cliff-line of bright white Cambrian quartzites. And there on top, sitting on a bench, is the Lewisian. For Geikie the thrust sheet was conformable stratigraphy and from a distance he could be forgiven. No chance of a quick diversion today - ground-truthing would have to wait until we reached the shores of Lochan Fada.
We reached the lochan, after a rather wobbly crossing of the stepping stones, by 10.45. Stunned by the scenery, renewing old debates on unconformities, thrusts and highland geology, and even collecting sporadic field observations, we had dawdled for these first 12 km. Chocolate and crunchie bars for refuelling, rough outcrops of gneiss for debate, the first drops of rain for added refreshment. We were now sat on metamorphic rocks of the Kinlochewe thrust sheet; banded gneisses, acid sheets and even an altered metabasic dyke. In part the argument between the Murchison-Geikie camp and Nicol centred on the difference between metamorphic rocks found west (the "fundamental gneiss" below) and east (the "eastern schists" above) of the tract of Torridonian and Cambrian strata. The outcrops of Lochan Fada, and elsewhere in the Kinlochewe sheet are clearly Lewisian - Geikie would surely have grouped them with his "fundamental gneiss" if they had cropped out further west. Just to the south, poking out of the peat, are exposures of Durness limestone and beyond lie the quartzites, both structurally beneath the gneisses. So the Kinlochewe thrust repeats stratigraphy, as all good thrusts should. But the situation is complicated by a late fault that drops the Kinlochewe sheet down against foreland Torridonian. If Geikie had been in cloud perhaps these relationships would only have become apparent after careful mapping. In reasonable visibility they stand clear, particularly relating them back to the views in Glen Bianasdail.
Looking onto Mullach Corie Mhic Fhearchair under spring snow. What is now known as the Kinlochewe Thrust is picked up, carrying Lewisian basement onto Cambrian sediments.
The most informative viewpoint lies about 1500m NE of Lochan Fada, at the foot of Coire Mhic Fhearchair. A panel of Cambrian quartzites with the gentle south-easterly dip of the foreland arches around the north side of the corrie to the summit of Mullach Coire Mhic Fhearchair. The stream in the floor of the corrie shows spectacular bedding surfaces in the Pipe Rock, studded with the Skolithos trace fossils. And above, forming the rugged peak of Sgurr Dubh, is the Kinlochewe sheet. Geikie will have crossed the line of the thrust heading for the pass of Bealach na Croise, just west of Beinn Bheag. We followed, the ground thankfully dry after a particularly arid May. This sector has no path and could have been slow going in the normal damp conditions. But there are big, lichen-covered outcrops of Lewisian rocks, much of it dominated by acid sheets. These exposures are not of the quality of those down by Lochan Fada and perhaps could be dismissed. The clue lies in the landscape, with those big views of Mullach Coire Mhic Fhearchair providing a natural cross-section.
A vague little path heads over Bealach na Croise, picking its way through boulders of Moine mylonites. These have fallen from higher on Beinn Bheag, the Moine thrust being visible on crags above the pass. The next tract of land is key. Rounding a corner and suddenly huge slabs of dipping quartzites sprang into view. Leading down from the north side of Mullach Coire Mhic Fhearchair these surfaces must be the largest single exposures of bedding planes in the British Isles. In the, now few, sunny intervals they looked like snow fields. And there ahead perched on top was a klippe of Lewisian rocks, forming the hill of Meallan an Laoigh. The klippe looks like some scaled up late 80s cuisine, perhaps a slice of goats cheese, served up on a vast white plate. Every first year student struggling with structure contours should see this - what a pity it is so far from the road. It certainly made an impression to Peach and Horne with the classic 1907 memoir describing these sections in some detail.
Looking to Meallan an Laoigh from Lochan Nid.
We were now back on well-beaten ground as a stalkers' track led north towards An Teallach. The rain poured, we delayed lunch and headed on to the mouth of Loch an Nid. This little lake, tucked under the 600m western slopes of Creag Rainich, is held back by a natural dam as the Cambrian quartzites sweep across the valley. The next 4 km effectively follow the sub-Cambrian unconformity with Torridonian rocks. The Moine and a now very thin panel of Torridonian and Lewisian rocks representing the last northern part of the Kinlochewe sheet all lie a steep climb above the valley. We pushed on as the showers became more frequent.
Time was now pressing. Although making good progress as the rain curtailed all but the briefest examination of the geology, our schedule had been delayed by the slower deliberations in the sunshine. At the head of Strath na Sealga a jeep track climbed out, over another pass and down to the road at Corrie Hallie. We still had over 6 km to the road and another 7 to reach Allt na Aibhe. It was 2.45. The last ferry over Loch Broom was at 5.30. No chance. We reasoned that in Geikie's day the boats ran longer or indeed could be summoned on request, but not on a busy holiday weekend. We pressed on.
The track out of Strath na Sealga climbs up through the stratigraphy, crossing the quartzites, the fucoid beds and then up into the Moine mylonites. This presumably will have appeared completely normal to Geikie. In the 1860s the mylonitic fabric was interpreted as relict bedding planes. It was some 25 years later that Lapworth and then Peach and Horne were to recognise these rocks as being strongly deformed. Tell-tale stretching lineations show the WNW shearing direction. But we only spent a few minutes on these rocks as the rain poured. Oldroyd correctly points out that the track gives stunning views of eastern side of An Teallach, the Cambrian quartzites preserved as outliers on its ridges and the basal unconformity sweeping through the valley. We saw the insides of the hoods on our waterproofs. In bad weather this is indeed a desolate place, even with those spectacular mylonites.
Looking down onto the pass down to Corrie Hallie from high on An Teallach.
By 4.15 we'd reached the road at Corrie Hallie. For John and Tim, who had done so much driving the day before, the lure of the bar in the Dundonnell hotel proved strong. Oldroyd speculated that Geikie had called in at nearby Dundonnell House for a cup of tea. A quick munch on a chocolate bar and a dump of rucksacs in my car left the remaining four of us replenished for the final foot-slog. Geologically there is little to see. The action happens above the road and is largely obscured by modern plantations. The same basic rock sequence is maintained, Torridonian, Cambrian and Moine. Hidden in the bog over the back lies the "Dundonnell structure", a culmination of imbricates in the thrust belt that has achieved the unlikely distinction of being Boyer and Elliott's type example of an antiformal stack duplex. Geikie won't have nipped up the escarpment to have a look and he won't have missed much. It's probably the worst exposed classic structure in the NW Highlands.
The four of us reached the top of the road at 5.45, too late for the ferry. Now well soaked it seemed time to stop. Steve's car sat there. It would have been simple to hack down to the shores of Loch Broom but the thought of walking back up to the car wasn't appealing. And we were hardly dressed for an overnight stop at Allt na Airbhe - one of the more exclusive hotel-restaurants in Britain. So it was off to the Dundonnell hotel to pick up John and Tim, both of whom had fallen asleep over their first pint, and hence back to Ullapool. Phew!
So what of the traverse? Certainly the route from Kinlochewe to Dundonnell is spectacular. Moving quickly it all looks pretty obvious. The key outcrops lie back at Lochan Fada. Only here can rocks within the thrust belt that are clearly Lewisian, and therefore correlatable with the foreland, be positively identified without peeling off lichen. Of course critical outcrops only achieve this status in retrospect. For Geikie the logistics must have been more pressing, covering in effect the entire distance across a modern 1:50,000 OS Landranger map. This type of reconnaissance work is useful - to be built on later. But don't let's kid ourselves it's objective, all fieldwork relies on some level of preconception or hypothesis. Geikie as an observer, and we in his footsteps, were strongly limited by the weather, our mood and training. It's no wonder that we emphasise to our students that all these factors must appear in field notebooks so that the quality of observations can be judged long after the event. As for the team, we're off to walk another stretch of the Moine Thrust Belt next year. And my reconnaissance work at Nanga Parbat - I couldn't possibly comment.
If you want to re-enact the traverse, you'll need the 1:50k OS sheet 19 (Gairloch-Ullapool). The walk is covered by a single 1:25,000 OS topographic map (Explorer series) - sheet 435 (An Teallach & Slioch). If you want geological assistance take sheet 92 (the 1962 Inverbroom sheet, with the geology mapped in the 1880s under Geikie's supervision, largely by Peach and Horne). Read Oldroyd's The Highlands Controversy (published by Chicago University Press in 1990: ISBN 0-226-62635-0).
Rob Butler
Earth Sciences
School of Earth and Environment
The University of Leeds
Leeds LS2 9JT
This article first appeared in Geoscientist magazine, October 2000.
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