The Highlands controversy was a scientific controversy which started between British geologists in the middle of the nineteenth century concerning the nature of the rock strata in the Northwest Highlands of Scotland. The disagreement stemmed from the apparent ages of the strata, particularly the existence, now confirmed, of older rock above younger rock as well as duplicated and inverted strata, which could not be satisfactorily explained by contemporary geology. This rock formation and surrounding controversy were the impetus for Albert Heim's theory of thrust faulting, which, in conjunction with anticlines and imbrication, are now commonly accepted as the primary geological mechanisms that created the Northwest Highlands rock strata.
At the time, the debate became contentious, even acrimonious, because of some of the personalities involved and because it pitted professional geologists of the Geological Survey against academic and amateur geologists. An initial resolution was achieved by about 1886 but the great complexity and scientific importance of the discovery of the Moine Thrust Belt and the geological processes involved in its creation led to field work continuing for a further twenty years culminating in the 1907 publication by the Geological Survey of a book of fundamental geological significance: The Geological Structure of the North-West Highlands of Scotland.
The acrimony was an important factor in the political decision to set up the Wharton Committee of 1899 to review the state-funded Geological Survey. The committee's report probably precipitated the retiral of Archibald Geikie, the Survey's director-general, who had been slow to accept the new geological paradigm. However, in retirement Geikie's status flourished as he went on to become president both of the Geological Society and the Royal Society and to receive the Order of Merit.
The northwest highlands region of Scotland is now known to be where part of the Iapetus Ocean closed with the collision of the continents of Laurentia and Baltica about 400 million years ago. The consequent Caledonian Orogeny produced intense folding and compression of rocks – at thrust faults older rock strata slid for miles over younger rocks and, at nappes, the sequences of rock strata became inverted and duplicated at overturned anticlines.
Background Geological science in the mid nineteenth century
From around 1830 geologists were beginning to date rocks according to the embedded fossils. The law of superposition whereby younger rocks lie above older ones was very well established and it was recognised that some layers may be missing because they had been eroded away. Folding and faulting of strata were recognised and in 1841 Arnold Escher von der Linth discovered that sometimes older rocks lay above younger ones. However his explanation involved such large horizontal movements of rock and folding on such a massive scale that he was afraid to publish his results because his theory would seem ridiculous. It was not until after his death that his pupil Albert Heim published the findings in 1878. Thrust faulting involving a considerable horizontal movement of older strata over younger had not yet been identified.
Roderick Murchison's expedition to Wales in 1831 led to his identification of the Silurian period and he came to regard the Silurian geological system as being his own territory. He went on to decide that Silurian rocks extended into parts of England and southern Scotland and this caused bitter arguments with his friend Adam Sedgwick who had previously identified the rocks as Cambrian − the intervening Ordovician period was yet to be characterised. Murchison identified the Silurian on the basis of the types of fossils the rocks contained whereas earlier geologists had studied the type of rock. The strength of Murchison's views became buttressed by his knighthood in 1846 and when he was appointed director-general of the Geological Survey in 1855 he decided to turn his researches to the little-known and even less understood Northwest Highlands of Scotland expecting to extend his Silurian domain up there.
Northwest Highlands of Scotland
The Northwest Highlands were, and still are, remote and difficult to access. Along a coastal strip some long and wide the terrain is austere with isolated mountains rising above barren lower ground where knolls of bare rock lie among lochans and peat bogs. This geological region runs from the Sleat peninsula of Skye northward through Kyle of Lochalsh, Ullapool and Assynt to Cape Wrath and Loch Eriboll.
Assynt offers diverse geological formations and varied landscapes. The low-lying terrain to the west consists of Lewisian gneiss, a hard metamorphic rock that represents some of the oldest bedrock in Britain. Overlying this basement are relatively undisturbed sequences of Torridonian sandstone, quartzite, and Durness limestone. Historically, dating these strata presented challenges: the sandstone yielded no fossils, the fossilized trace fossils known as 'Pipe Rock' within the quartzite were not initially recognized as organic in origin, and the fossils within the limestone could not be definitively dated at the time. Further east, the landscape is characterized by strongly metamorphosed Moine rocks, which in some areas overlie non-metamorphosed strata.
For nineteenth-century geologists, this was a major puzzle because younger rocks are expected to lie above older ones and so non-metamorphosed should lie above metamorphosed – what were the ages of the rocks and why did they seem to be in the wrong order? Why had seemingly upper and lower layers of quartzite apparently been discovered both above and below basement gneiss?
Murchison and Nicol in the Northwest Highlands
In 1827 Murchison had made a cursory survey of the area from the sea but in 1855, hearing news of the discovery of fossils in the limestone at Durness, he visited again with James Nicol, professor of geology at Aberdeen University. The fossils were thought to be Devonian and the limestone was clearly more recent than the underlying sandstone. This was a problem because the sandstone, later to be called Torridonian sandstone, was thought to be equivalent to the Old Red Sandstone on the east coast of Scotland which certainly contained Devonian fossils. Because of the conditions on the ground they were unable to conduct a thorough geological survey but Murchison considered there was an ascending series (becoming younger) of strata exposed on the surface as one moved west to east. He concluded that this exemplified the stratigraphic column of Britain. The strata must dip down from west to east, he thought, so at any particular elevation the rocks towards the east were younger than those to the west and so, he assumed, the schist and gneiss of the north of Scotland were Silurian sediments above a basement.
James Nicol, professor of geology at Aberdeen University disagreed. Following a separate visit in 1856 he claimed that a geological fault ran right down along the northwest coast and that the seemingly younger rocks were in reality far older having been shifted upwards compared with those to the west. A matter that was to become especially difficult was that Murchison considered that there were two quartzite layers of different age at two stratigraphic levels whereas Nicol claimed there was one layer but vertically displaced along the fault. However both geologists had to gloss over the difficulty that the quartzite was greatly folded and in some places seemed to fold over itself.
In 1859, following another tour of the highlands when he was accompanied by his second-in-command at the Geological Survey, Andrew Ramsay, Murchison addressed a British Association in Aberdeen where he explained what he considered was the essential simplicity of the geology of the north of Scotland. It seems his lecture was regarded as a triumph by the attendees and by The Times and the Scottish press. John Phillips spoke of the high estimation in which Murchison was held as "master of the Silurian". Nicol, meanwhile, had been preoccupied organising the geology section of the meeting – his paper hardly got a mention in the press and seems to have received little support at the meeting.
Murchison involves Geikie
In 1860 Nicol returned to the Northwest Highlands to investigate further and Murchison, after hearing of Nicol's travels, made a quite separate journey taking along with him a junior but very ambitious member of the Survey, Archibald Geikie. Murchison confirmed in his own mind what he had already observed and Geikie, anxious to please, became a strong supporter of Murchison's views. However, in his posthumous biography of Murchison he was to say of the expedition that Murchison "stuck to his leading principle, from which no amount of contradictory detail would make him swerve".
When Nicol announced the results of his latest investigations to the Geological Society in December 1860 he refuted Murchison's findings and rejected the presence of Silurian sediments. He considered igneous rock was to be found along a "zone of complication" from Durness to Skye and faulting was responsible for the perplexing strata. He utterly rejected Murchison's idea that metamorphic rock could lie with no unconformity (with no intervening period of erosion) over unaltered sedimentary layers. There was no evidence that the highlands were of Silurian age. Murchison was enraged and wrote to his protegé Archibald Geikie "we have a fight in which our reputation and veracity are at stake". In February 1861 he and Geikie delivered their paper to the Geological Society and this and Nicol's papers were published in the same volume of the Quarterly Journal of the Geological Society.
At that time the geological community in Britain had not formed a definite opinion on these matters but Murchison's view was the one that was to prevail at least for a time. Murchison held great prestige and Geikie was a persuasive writer and speaker who was well able to place the best possible gloss on Murchison's views, sometimes by writing favourable anonymous reviews of the two geologists' own publications. For example, Geike wrote a seemingly independent review of his and Murchison's 1861 map. So, the Murchison–Geikie view became orthodox.
Nicol and Murchison never resolved their differences and Nicol, after making little progress persuading the geological community of his theory, ceased publishing on the matter after 1866 although he continued to make geological field studies in the north of Scotland throughout his career. He died in 1879.
Geikie was appointed director of the Scottish Geological Survey in 1867 and, in 1871, the year of Murchison's death, he was appointed to the newly created chair of Murchison professor of geology at Edinburgh University, endowed by Murchison himself. He continued to promulgate Murchison's Silurian theories and carried the professional geological establishment with him.
Rising dissent in the 1880s
At this time the Geological Society's members were academic geologists and amateurs, some of them becoming very knowledgeable and well respected. There were often differences of approach and opinion between, on the one hand, members of the Society and, on the other, senior spokesmen and managers of the "professionals" employed in the Geological Survey who carried out most of the actual fieldwork. The Survey's directors considered themselves better informed than amateurs and academics (who they regarded also as amateurs). From around 1878 papers by members of the Society started to appear, sometimes supporting Nicol and at other times just drawing attention to the inconsistencies in both contemporary theories and the incompatibility of the various field observations.