Scotland’s North-West Highlands Geopark doesn’t just offer some of the most stunning scenery in the country: it also contains some classic geological locations. Covering around 2000 square kilometres in the far north west of Scotland, the park's geology and scenery is based upon some of the world’s oldest rocks.
The Moine Thrust
The Moine Thrust is the name given to a huge fault zone which extends from the north coast of Scotland near Sango Bay, across to the south of Skye where it reaches the sea. The zone (made up of many faults with the same trend, rather than a single fault) separates the rocks of the Hebridean Terrane in the west from those of the Northern Highlands to the east.
The significance of this area for geologist lies in its complexity. Victorian geologists disagreed over what caused the particular layering of rocks. Some, led by Sir Roderick Murchison, thought that this was a normal sequence with younger rocks overlying older ones. Others disagreed. It wasn’t until 1912 that the conundrum was solved by the work of two geologists, Ben Peach and John Horne.
Peach and Horne succeeded in demonstrating that the structures of the Moine in fact represented a thrust zone – that is, an area in which massive earth movements have thrust old rock layers above younger ones. In other words, the huge forces of continental collision forced great thicknesses of rock as much as 20km westwards along the fault zone.
The Moine is a huge feature and there are several places where it is visible in the landscape. At a distance it is obvious in Glencoul (see picture) where quartzites of Cambrian age (younger than around 540 million years) are overlain by much older rocks, known as gneisses (Leeds University).
At Knockan Crag near Inchnadamph, the thrust can be can be seen close up. Four-hundred-million-year-old limestone lies below the layers of ancient and much-deformed gneiss. The area lies in the Inchnadamph Nature reserve, where a visitor centre and a trail explain the geology of the thrust zone and provide information both on it and on the many limestone caves exposed in the cliff.
The Mountains of Coigach
The scenery of Coigach is striking: steep sided hills such as Suilven and Stac Pollaidh may not be high (not much more that 600m) but they rise dramatically from a low-lying area of rock. Coigach is a very ancient landscape and the distinctive topography is a product of geology and the particular sequence of rocks which is in place.
The base rocks are Lewisian gneiss. Among the oldest rocks in the British Isles at around 3,000 million years. Above them, a layer of sandstone hundreds of metres thick was laid down over millions of years; and on top of that was laid down a layer of a hard rock called quartzite.
Over time the overlying rocks have been eroded and much of the sandstone, which is relatively soft compared to both the gneisses and the quartzite, has been stripped away by processes including ice, wind and water. Where the quartzite cap protected the underlying sandstone the erosive process was slower and this is how the hills of Coigach were formed.
Not all of the hills retain the quartzite capping. Some, like Stac Pollaidh, have lost it to erosion in the relatively recent past and so the top of the mountain is characterised by accelerated erosion of the sandstone layer, giving it a shattered and serrated appearance. The smoother lower slopes of the mountains are the product of glaciation in the last ice age (North West Highlands Geopark).
Ancient Rocks: The Lewisian Gneiss
The rocks which underlie the north-west Highlands and form Scotland’s Western Isles are not only the oldest in Britain; they are among the oldest in the world. Over a period of 3,000 million years they have been buried, exhumed, eroded and altered on many occasions. This antiquity alone makes them worthy of further investigation.
Gneiss is characteristically a banded rock, with dark and light minerals arranged in thin but distinct layers. It is by definition a rock which has been metamorphosed (or changed) and so it is difficult to identify its origins with any exactness. The gneisses of the north-west Highlands are thought to be mainly (though not exclusively) igneous in origin and may well have been caused during periods of continental collision, possibly buried up to 30km deep (McKirdy et al).
Over their history the various movements of the highly dynamic Earth have altered the uniform nature of the rocks. They have been cut across by intrusions of magma which shows up as dark fingers of finer-grained material across the bands (these are known as dykes) and they have been intruded by larger masses of younger igneous rocks (such as granite). These intrusions can be seen today.
Sources and Further Information
Alan McKirdy, John Gordon and Roger Crofts Land of Mountain and Flood: the Geology and Landforms of Scotland Birlinn 2009
North West Highlands Geopark website, northwest-highlands-geoparkorg.uk, accessed 5 July 2010
University of Leeds “Assynt Geology”, see.leeds.ac.uk, accessed 5 July 2010
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