A spectacular gorge cut by glacial melt water and the 45-metre-high Falls of Measach tell the dramatic story of what water and ice can do to a landscape
Lochbroom sits in a wide valley deepened by glaciers during the Ice Age, with several narrow, deep gorges feeding small streams into it. The main gorge, at the head of Lochbroom, is Corrieshalloch, which was formed at the end of the last glaciation by large volumes of meltwater carrying glacial debris. At times, this meltwater may have been trapped at high pressures under the ice and this cut the gorge at a greater rate. The current river is much reduced in capacity, especially now there is a hydroelectric scheme on Dirrie More.
The bedrock in the Corrieshalloch area belongs to a group of metamorphosed mudstones and sandstones called the Moine. The original sedimentary rocks were deposited in shallow water, and then caught up in a mountain building event where the high pressures and temperatures caused new crystals to grow. This mountain building took place as the wide Iapetus Ocean closed, and England and Scotland collided and welded together. In the final stages of this continental collision, the Moine metamorphic rocks were lifted up and pushed westwards: at Knockan Crag, you can see the ‘thrust plane’ where the Moine rocks ended up on top of the sedimentary rocks of the North West Highlands.
At Corrieshalloch, the result of this tectonic movement was to form a particular kind of rock, called mylonite, that splits easily along flat planes. In mylonite, the grains – especially quartz crystals, are flattened into lens shapes. The significance of these shapes was recognised by Charles Lapworth (Professor of geology at Mason College of Science, a forerunner of the University of Birmingham) in the Loch Eriboll region. He found that the platey rocks were formed by shearing, likening the process to working hot metal in a steel rolling mill. It is this flattening and smearing action that gives the Moine rocks their flaggy appearance.
The rock also contains vertical faults and cracks called joints. These flaws in the rocks meant that the meltwater did not have to grind the rock away to form the gorge – rather large blocks were removed by the torrents of water, having possibly first been loosened by freeze/thaw action.
The gorge is not perfectly straight; it has sharp bends along its length. The directions of the small faults and joints that cross the gorge in the bedrock cause this. As rock has been removed from the gorge, the waterfalls have gradually worked their way upstream. The Falls of Measach, at 45 metres high, can be looked down on from the suspension bridge or viewed from a specially constructed platform further downstream.
The huge quantity of debris carried by the meltwater was deposited on the floor of the main valley, when the sea level was higher than it is today. Initially, the sea occupied the valley right up to the base of the gorge and the waves washed this debris around, making the valley bottom very flat. The area is often described as a raised beach, because the land has rebounded since the ice left, lifting this flat area clear of the sea. You can view the raised beach from the end of the trail at Corrieshalloch Gorge.
Text contributed by Pete Harrison, North West Highlands UNESCO Global Geopark
Find out more
The site is owned by The National Trust for Scotland, visit their website to download an information leaflet: www.nts.org.uk/Visit/Corrieshalloch-Gorge/
An Excursion Guide to the Moine Geology of The North West Highlands – Strachan, Alsop, Friend and Miller. ISBN 978-1-905267-33-0. Available from the Edinburgh Geological Society: http://www.edinburghgeolsoc.org/publications/geological-excursion-guides/
Corrieshalloch Gorge National Nature Reserve: https://www.nts.org.uk/visit/places/corrieshalloch-gorge
This web page is published by the Scottish Geodiversity Forum under a Creative Commons ‘Attribution Non-commercial’ (CC BY-NC) licence, which permits non-commercial reuse provided the original work is properly cited.