Glaciated pavements on the shore of a reservoir show spectacular examples of folded rock layers that have influenced and inspired geologists from around the world
When continents collide to make mountain ranges – as is happening today in the Himalayas – the rocks caught up in the collision can become dramatically crumpled. This folding can result in wonderful rock structures, and one of the most famous places in world to study them is on the shore of Loch Monar, in the northern Highlands. It was here in the 1950s that a young geologist called John Ramsay was studying for his PhD. He worked out that the folded shapes – which at first glance appear to form shapes as complex as the Gordian Knot – could be understood as the products of ‘superposed folding’. This means that the layers have been folded several times, first one way and then another, rather like a sheet of paper shaped by origami. Erosion has cut through the shapes revealing exquisite patterns – like egg-boxes, snake-like tongues and bat shapes. Ramsay was able to work out the sequence of folding and, by collecting lots of further measurements, he was able to relate this sequence to other structures in the adjacent area.
In fact, the methodical approach that Ramsay developed here at Loch Monar has become a standard method for geologists attempting to unravel the complex processes that happen deep beneath mountain ranges. This remote spot remains one of the most important (and photogenic) places in world geology.
The rocks of Loch Monar were once sedimentary and form part of what is known as the ‘Morar Group’ of the Moine metamorphic rocks. They were deposited in shallow seas over a billion years ago and became deeply buried to form part of the continental crust. This crust – ancestral northern Scotland – became involved in the so-called Caledonian mountain belt. The structures at Loch Monar formed at this time, around 450 million years ago, probably over 25 kilometres beneath the Earth’s surface. Erosion over geological time, most recently by the glaciations that have provided the remarkable slabs on the shores of the reservoir, has brought the rocks to the surface.
Actually, the rocks at Loch Monar tell a story that involves more than just crumpling. The sedimentary rocks of the Morar Group are sedimentary no more. Deep burial and the heating that accompanied it has altered the strata – the rocks are now classed as metamorphic. Many of the minerals from which they were originally made experienced chemical reactions under heat and pressure and have formed new minerals; the most obvious of these are the shiny mica crystals. The rocks have also melted – indicating that they probably reached temperatures greater than 650C. The white seams that cross the main banding in the rocks are small patches of granite, formed by small amounts of melting of the surrounding strata.
Text contributed by Rob Butler
Find out more
Access is via a private road up Glen Strathfarrar and permission is required for use by motor vehicles (April-October, ask at gatehouse at Struy). The road is also a great bicycle ride (20km each way). Cross the first dam, and continue to the far (southern) side of the next one and saunter out onto the rock slabs on the shore of the reservoir.
Mendum, J.R.. 2009. Loch Monar. 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, 494-498.
Ramsay, J.G.. 2010. Excursion 8: Glen Strathfarrar and Loch Monar. 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. 153-161.
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