Anraboll thrust: © Rob Butler
Large-scale faults are important for stacking up the continental crust to form mountain ranges in convergent plate boundaries – such as the Himalayas and the Alps. These are called thrusts – and active thrusts generate the largest earthquakes we experience.
Research on thrusts was given great impetus in the late nineteenth century by fieldwork in North West Scotland. It was here that the Moine Thrust Belt was discovered – a major zone of ancient thrusting that formed the edge of the old Caledonian mountain belt. This research began on the north coast of Scotland, and even today provides information on faulting processes and shows the complex structures that can result from thrusting.
Ben Arnaboll, on the eastern shores of Loch Eriboll, is therefore one of the most influential sites in world structural geology. It was here that the term “thrust” was coined – published by Sir Archibald Geikie (1835-1924) in 1884 in Nature, and used to describe a type of break in the Earth’s crust where older rocks from lower stratigraphic positions are pushed over higher, younger ones, thus disrupting the sequence. It was also here that mylonites – rocks which have been deformed by the high temperatures and pressures of shear zones – were first identified.
These advances came about thanks to the pioneering fieldwork of Charles Lapworth, then Professor of Geology at Mason Science College – a forerunner of the University of Birmingham. Although subsequent research in the 1880s by the British Geological Survey, headed up by Ben Peach and John Horne, established the scale and continuity of what we now call the Moine Thrust Belt, the story begins at Arnaboll.
Visiting the site
Arnaboll is located at the northern edge of the NW Highlands Geopark and lies on the North Coast 500 route (the scenic journey promoted as Scotland’s “Route 66”). To access it, park up at the lay-by on the A 838 overlooking the northern end of Loch Eriboll. Arnaboll lies a km stroll across moorland to the east.
Before leaving the parking site gaze down and across Eriboll. It provides a stunning example of the edge of the structures of the Caledonian mountain belt – formed around 430 million years ago. The slopes of western Eriboll are a gently tiled panel of Cambrian age quartz sandstones. These lie just outside the Caledonian belt – forming the “foreland”. At the head of Loch Eriboll, leading on to the mountain massif of Foinaven to the SSW, are disrupted piles of the quartz sandstones – pushed up like waves by thrusting. We are looking side-on to what Lapworth termed a “great crushing sledge” – rocks pushed up like snow ahead of the advancing plough of the Moine Thrust Sheet. Arnaboll allows us to examine these deformed rocks directly.
Walk over the moorland, and gaze ahead to the ridge (looking east). The upper part of the ridge is made of Lewisian gneiss – part of the fundamental continental crust of NW Scotland. The lower part of the slope is made of Cambrian quartz sandstones, forming prominent cliffs. The contact is the Arnaboll Thrust, where much older metamorphic rocks (Lewisian gneisses) have been thrust onto sedimentary rocks (Cambrian quartz sandstones).
The most photogenic outcrops lie on the northern slopes of the ridge. With care, drop onto the NW corner of the Arnaboll plateau. These are the type sites for mylonite – where the original gneisses are altered to highly platey green rocks through high temperature and pressure. It was into this narrow zone (about a metre wide) that somewhere over 10 km of movement has been focused. It is the formation of mylonite, much weaker than the original rock, that allows these types of fault to move. The rocks below are the quartz sandstones, which contain fossil burrows (this is the so-called “Pipe Rock’) – which have been deflected over in the direction of shear – like the trunks of saplings in a gale.
As Geikie noted: “When a geologist finds…gneiss overlying gently inclined sheets of fossiliferous quartzite, shale and limestone, he may be excused if he
begins to wonder whether he himself is not really standing on his head.”
Literature
Butler, R.W.H. 2009. Eriboll. In: Lewisian, Torridonian and Moine rocks of Scotland. (edited by Mendum, J. R, Barber, A. J., Butler, R. W. H., Flinn, D., Goodenough, K. M., Krabbendam, M., Park, R. G. and Stewart, A. D.) Geological Conservation Review Series, volume 34, Joint Nature Conservation Committee, Peterborough, pp. 721.
Butler R.W.H. 2010. The Geological Structure of the North-West Highlands of Scotland – revisited: Peach et al. one hundred years on. In: Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. (eds R.D. Law, R.W.H. Butler, R.E. Holdsworth, M. Krabbendam & R. Strachan), Special Publication of the Geological Society, 335,. 7-27.
Butler, R.W.H. 2010. Excursion 11: The Moine Thrust at Loch Eriboll. In: A Geological Excursion Guide to Moine Geology of the Northern Highlands of Scotland (edited by R A Strachan, G I Alsop, C R L Friend & S Miller). National Museums of Scotland. 191-216.
Butler, R.W.H., Raine, R. & Smith, M.P. 2011. Excursion 15: The Moine Thrust Zone at Loch Eriboll. In: A Geological Excursion Guide to the North-West Highlands of Scotland (edited by K.M. Goodenough & M. Krabbndam) National Museums of Scotland, 181-197.
Wibberley C.A.J. & Butler, R.W.H. 2010. Structure and internal deformation of the Arnaboll Thrust Sheet, north-west Scotland: implications for structural styles and strain localization. In: Continental Tectonics and Mountain Building: The Legacy of Peach and Horne. (eds R.D. Law, R.W.H. Butler, R.E. Holdsworth, M. Krabbendam & R. Strachan), Special Publication of the Geological Society, 335, 321-333.
Butler, R.W.H., Matthews, S.J. and Morgan, R.K. (in review). Geikie's field researches and their geological controversies. In: Aspects of the Life and Works of Archibald Geikie. Geological Society Special Publication
Twinned with: Glarus, Switzerland
The Glarus Thrust, in the Alps of eastern Switzerland, like the thrust seen at Ben Arnaboll in Scotland, is among the world’s most famous thrust faults – low angle reverse faults in which older rocks can be pushed up and over higher strata. It formed during the Alpine Orogeny throughout the Miocene and Oligocene, as a result ...continue reading