April 2020 Issue Table of Contents
How Far to Glencoe?
In their doomed attempt to keep me near the cutting edge of technology, one of my sons recently bought me a grey cylinder that, when connected to an electricity supply, answers (literally) to the name of Alexa. She speaks BBC English with a mellow, alto voice and can, sometimes, answer my questions. Early in our relationship, to see how smart she really is, I asked her a question with three answers, ‘How far is it to Glencoe?’
Glencoe is a village in the West Highlands of Scotland. ‘Glen’ is Scottish Gaelic for a steep-sided valley. Glen Coe is a splendid example of a U-shaped glaciated valley (Fig. 1), now occupied by the River Coe, ‘Coe’ being a word of uncertain, and possibly ancient, origin. When a village is named after a feature, the words are usually combined, so we find Glencoe village on the seashore at the western foot of Glen Coe.
My question for Alexa has three answers because northwest Scotland has an intensely indented fjord coastline. As the crow flies, Glencoe is 19.34 km SSE of my house. If I drive round the head of the fjord, which my house overlooks, it is 54.8 km away, and if I take a ro-ro (‘roll on–roll off’) ferry across the lower part of the fjord it is 37.4 km and I am £8 poorer.
Alexa’s confident reply was surprising. I did not record the exact value, but it was somewhat more than 7,000 km. In 2009, when I was working in New Mexico Tech, in Socorro (USA), with Matt Heizler, I visited another Glencoe, in southern New Mexico (Fig. 2). On a great circle, it is 7,525 km from my home. It is curious that Alexa chose this Glencoe rather than the one only 19 km away. Did she look up the history of my travels in some monster Big Brother computer? According to Wikipedia, there are three Glencoes in Canada, three in Australia, one in New Zealand, one in South Africa, and an astonishing 17 in the USA. The majority of those in the US and Canada are built on mid-western agricultural land that is perfectly flat, and, therefore, devoid of anything remotely resembling a glen.
There are few points of similarity between the villages in Scotland and New Mexico. The archetype has a population of 374 and the new-world version 210. The Scottish version is at sea level, the New Mexico version at 1,749 m. Both have a dark side. The Scottish Glencoe is infamous for the massacre of local people by largely English soldiers in 1692; the New Mexico version was settled by the Coe family, two of whom were outlaws, responsible for shootings and hangings.
Scotland’s Glen Coe is a grand place of brooding dark cliffs (Figs. 1 and 4). Its mountains are not high (the highest is 1,150 m), but they start from near sea-level and in places are very steep. It is rightly famous in the history of geology, because it cuts through the first ancient caldera to be recognized by geological mapping anywhere. A paper by Clough, Maufe and Bailey (1909) introduced the term ‘cauldron subsidence’ to the world. Greater detail and a description of the regional setting were provided by Bailey and Maufe (1916). A superb modern geological guide and map has been produced by Kokelaar and Moore (2006).
Understanding of the geology of the Scottish Highlands gradually developed during the latter part of the 19th century. The logistics were not easy. Immediately to the south of the Highland Boundary Fault is the great industrial city of Glasgow, which in the first years of the 20th century built a fifth of the world’s steamships, but of the largely trackless mountains a hundred miles to the North, James Nicol, Professor of Natural History at Aberdeen University, could write, as a footnote to a paper in the Quarterly Journal of the Geological Society for 1857:
In justice to my predecessors I may mention, that at the time they examined the West Highlands there were even less facilities for visiting these remote parts of the country than at the present day, when steamboats have done so much to render them accessible. The uncertain climate, too, places a great bar to geological investigations; and Sedgwick and Murchison specially state that the stormy and wet weather which they encountered in these districts prevented their full exploration of the mountains. Sir R. Murchison and myself, in 1855, were again impeded in our researches by the rains and mist that so frequently obscure the best and most instructive sections on this humid coast.
Today, we know that the Scottish mountains came into being when Scotland was a part, albeit a tiny part, of Laurentia. Our oldest rocks, the Lewisian Gneiss, go back to 3 Ga. We remained on the margin of Laurentia for 98% of our known history until, at around 60 Ma, the North Atlantic began to open and we were marooned on the margin of Europe. The UK has recently voted to leave even the margin of Europe!
The envelope rocks of the caldera are part of a thick pile of varied sedimentary rocks called the Dalradian Supergroup, which make up all of Highland Scotland south of the transcurrent Great Glen fault, which strikes SW–NE across the Highlands. The rocks were deposited in marine basins on the edge of Laurentia over a very long period, from ~750 Ma to 500 Ma, during the breakup of the Rodinia supercontinent, and were folded and metamorphosed when Laurentia encountered the Taconic arc at around 470 Ma. In Scotland, this is the Grampian Orogeny, corresponding with the Taconic Orogeny in the Appalachians.
The western edge of the Glen Coe caldera is 12 km SE of the Great Glen. Metamorphic rocks to the north of the fault experienced a second orogeny, the Scandian, at ~430 Ma, caused by the collision of Baltica with Laurentia. The Dalradian sequence was not deformed during the Scandian, implying at least 500 km of subsequent sinistral transcurrent movement on the Great Glen fault, perhaps driven by the oblique collision of Avalonia (carrying what we now call ‘England’) with the margin of Laurentia.
The Highlands, both north and south of the Great Glen, were subject to intensive granite magmatism over a long period, from 447 Ma to 380 Ma, of which the Glen Coe caldera, emplaced at 420 Ma (the time of final closure of the Iapetus Ocean), was part. The sources of this long-lived magmatism are unclear. The caldera is defined by a ring fault that is also the locus of a discontinuous fault intrusion (strong red on Fig. 3) composed of many types of igneous rock from gabbro via tonalite to granite. In the SW, the caldera is overlain by a younger granite, and this and the caldera rocks are cut by an intense dyke swarm. The interior of the caldera is extremely complex (Kokelaar and Moore 2006). On much-simplified Figure 3, the purple rocks within the caldera are predominantly andesites, the light yellow rocks are rhyolites. In the field, the identity of the caldera rocks is very clearly brought out by the way they weather and provide nutrients for plant life (Fig. 4). The rhyolites and ignimbrites often have wonderfully convoluted flow lamination (Fig. 5). There is a great thickness of these rocks in Glen Coe. The caldera must have been a lively place when it was active.
Glencoe is still a lively place. A few years ago, I took part in a geological field excursion with the local U3A (University of the Third Age). The day ended with me giving a lift to four ladies from the village. At risk of using up my entire 2020 allowance of gender stereotyping, it is fair to say that there was a lot of chatter and laughter coming from the back seat. The most exciting part was when one of them announced: ‘My husband built an aeroplane. We tested the engine by tying it to a tree. He got it certified and we flew it from a field near Glencoe. He was teaching me to fly. I could take off OK, but I could never get the hang of landing’. I often pass through Glencoe and think of that couple, white-knuckled, making their approach over an arm of the North Atlantic to a small field beneath the brooding cliffs of the great, ancient caldera.
University of Edinburgh, UK
and Lochaber Geopark
Bailey EB, Maufe HB (1916) The Geology of Ben Nevis and Glen Coe and the Surrounding Country. Memoirs of the Geological Survey of Great Britain. His Majesty’s Stationary Office, London, 307+x pp [The second impression, 1976, is still on sale]
Clough CT, Maufe HB, Bailey EB (1909) The cauldron-subsidence of Glen Coe, and the associated igneous phenomena. Quarterly Journal of the Geological Society of London 65: 611-678
Kokelaar BP, Moore ID (2006) Classical areas of British geology: Glencoe caldera volcano, Scotland. British Geological Survey, 127 pp
Nicol J (1857) On the red sandstone and conglomerate, and the superposed quartz-rocks, limestones, and gneiss of the northwest coast of Scotland. Quarterly Journal of the Geological Society of London 13: 17-39