Harrow and Hillingdon Geological Society


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Assynt and the Moine thrust

Glyn Hatherall

Some highlights of a visit to Assynt include Knocken Crag, which explains the MoineThrust in a digestible way, Suilven, Scotland’s most iconic mountain and the Ledmore Marble Quarry (the marble is now crushed for aggregates), the David Cameron painting The wilds of Assynt and the classic photograph of Peach and Horne of the Geological Survey sat outside the Inchnadamph Hotel. The latter are famed because of their contretemps with Roderick Impey Murchison, who insisted that the stratification in Assynt was normal and he died before he was proved wrong and Peach and Horne sorted it out.

Assynt has a limited number of mountains but they all rise up from a relatively flat area of Lewisian Gneiss, above which is Torridonian Sandstone. All are within the North-west Highlands Geopark was designated as a European geopark in 2004. It is committed to provide public education of geology etc.

The Glen Coul thrust at Kylescue has Archaean gneiss on Cambrian quartzite then Archaean gneiss again.

The Moine thrust has Lewisian Gneiss, Torridonian Sandstone, Moine Schist, Basal Quartzite, Pipe Rock, Fucoid Beds, Salterella Grits, and Durness Limestone with Moine Schist at the top. The MOine Schist is twice the age of the Cambrian quartzite. Lewisian includes Scourian Gneiss and Scourie Dykes then Laxfordian gneiss and granite (when this part of Scotland was part of Laurentia, the proto-North America). Scourie dykes are dated at about 2.7 billion years, Laxfordian at 1.75B, the Stoer Group at 1.2B and the torridonian at 1.0B years.

The Lewisian Gneiss has cnoc and lochan topography from which rise the mountains, several of which remained as nunataks during the last ice age and consequently retain a quartzite capping.

In the Durness Limestone, there are bone caves in the valley behind the Inchnadamph Hotel, which included bones of lynx, cave bears and brown bears.

Rockstops are a feature of the Geopark, where parking is available and there are clear explanations of the geology. One fine example is at Laxford Bridge, where there are Scourie Dykes, pegmatites, granite and dolerite. Arkle has lots of imbrications in the Moine Thrust zone. Stac Fada has suevites and shocked quartz thought to be indicative of meteorite impact.

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