4 February 2016

The Highlands Controversy, by David Oldroyd

Posted by Callan Bentley

highcont While I’ve spent quality time in Ireland on previous trips, I’ve never been to Scotland. To me, it is terra incognita, and I am eager to explore it this summer.  It was with delight then, that I delved into David Oldroyd’s The Highlands Controversy, which at once tickled many parts of my brain: the structural geology part, the history of geology part, and the part that gets giddy with anticipation of my upcoming travels in Europe.

The Northwest Highlands of Scotland are a neat place – one of the most remote and least populated portions of the British Isles, a site of classic structural geology (the Moine Thrust), and a new geopark. A great resource you may be interested in is this website from Rob Butler at the University of Leeds, UK, which explores the geology of the region in great detail.

In a nutshell, though, this is a place where thrust faults (low angle reverse faults, where the fault plane is almost identical in orientation to the bedding plane) were first used as a viable mechanism for explaining the geology. Rocks of various types along these faults were smeared out to make mylonites, and this was the spot where that lovely term was first applied.

This history explores the development of geologic thinking through about a hundred years of field work in the Northwest Highlands, starting with Roderick Murchison, king of the Silurian and his disciple Archibald Geike. Geike did a legendary hike traversing some of this geology, and here’s a great notion – recreating that hike in the modern day. Because I’ll have my three year old son in tow this summer, I doubt that’s in my future, but it’s a tantalizing notion. Oldroyd reveals Murchison as being obsessed with expanding the domain of the Silurian (you’ll recall he had a falling out with Adam Sedgewick on this topic, vs. the Cambrian). Hence, Murchison saw everything in the northwest highlands as Silurian in age. Because the rocks were either metamorphic or sparsely-fossiliferous sedimentary strata, he had little data to countermand his bias. Though he wasn’t able to explain how highly metamorphosed schist lay atop unmetamorphosed sandstones and limestones, Murchison wasn’t bothered. Superposition reigned supreme, so Murchison reasoned that some of these layers must simply be more susceptible to metamorphism than others. Geike towed the party line, and benefited greatly from Murchison’s patronage, eventually rising to direct the British Geological Survey.

Antagonizing Murchison, but eventually swept under the rug by Geike, was James Nicol, who advocated a role for faulting, but not thrust faulting – that wasn’t conceptually an option that was available to him at the time. Nicol’s faults were steeper, and feature prominently in the cross-sections he drew as interpretations of his field work. Many people caricature the Highlands Controversy as “Nicol got it right, but Murchison was more powerful, and eventually Peach and Horne vindicated Nicol.” But the reality Oldroyd reveals is more complicated than that: a story with multiple strands, and no single moment of scientific revelation. Nicol’s conclusions were flawed like Murchison’s, just in a different way.

Then along came Charles Lapworth. Not only did Lapworth steal away part of the Silurian and part of the Cambrian to make the Ordovician (through the clever use of graptolites as index fossils, a groundbreaking technique in its detail), but he also suggested that the Northwest Highlands were far more interesting than mere layer-cake geology with anomalously metamorphosed layers. He suggested serious deformation as the cause. Detailed mapping of the terrain disproved suppositions that stemmed from Murchison’s model, and Lapworth benefitted from reading about other examples of what today we would call fold and thrust belts, in the Alps (the work of Arnold Escher von der Linth and his protégée Albert Heim) and the Appalachians (the work of William Barton Rogers and his brother Henry Darwin Rogers). Lapworth didn’t solve the structure of the Northwest Highlands, but his work was more right than wrong, and critically served as catalyst to make the Geikefied Survey decide that they needed to take another look.

The detailed mapping was key to unraveling the enigma, and eventually John Horne and Benjamin Peach engaged in a concerted campaign of mapping on behalf of the survey that “solved” the problem once and for all. Thrust faulting was an ingredient they mixed into their recipe to great effect. This was when the Moine Thrust and its structurally lower counterparts were first decisively delineated.

Oldroyd’s treatment of the history of this particular issue is very well done, and methodical enough that I don’t feel I missed anything. At any rate, I could follow the arguments and the divergences in thinking, the social factors, etc. through time. His exposition is thorough, and readable. One thing he points out that I was astonished by is how shoddy the graphical images were that the earliest geologists to work in the area produced and published. The first map wasn’t published until thirty years into the controversy (Bonney, 1880), and it’s a sorry thing – no scale, no “north” arrow, no indication of topography, and no precise location of the site being mapped. Everything hitherto had been conceptual cross-sections, essentially the sorts of cartoons I like to draw, unconstrained by too many empirical facts.

In summary, The Highlands Controversy is worth your time to read if (a) you are a structural geologist, (b) you are interested in the history of science, or (c) if you plan to travel to northwest Scotland and want a solid background of the local geology soaked in the context of how that understanding developed.