Knockan Crag

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Best Places to see Scotland’s Geology: Knockan Crag

The Assynt area in the Northern Highlands is an area that has posed controversy since the early days of James Nicol (1810 – 1879) and Sir Roderick Murchison. Geologically, the area has undergone complex structural deformation and it was these complexities that were the cause of the problems. It was not until Benjamin Peach and John Horne toured the area that the area’s structural evolution was first interpreted properly. Knockan Crag was one of the sites where Peach and Horne first understood the nature of thrust tectonics. Here rocks from the Moine Supergroup have been thrust on top of younger Cambrian rocks. Their understanding of this led to crucial, early developments in the field of structural geology.

Find out more – www.nnr-scotland.org.uk/knockan-crag/

cs_knockan_cragKnockan Visitor Centre – a visitor attraction that interprets the ‘big picture’ of the landscape at Knockan Crag National Nature Reserve and surrounding area. © Colin MacFadyen/Scottish Natural Heritage.

The Moine Thrust Zone consists of a series of easterly dipping thrusts that separate the Moine rocks of the Northwest Highlands from the predominantly Lewisian rocks of the Hebridean Craton. The Moine Thrust itself is the oldest and highest of these thrusts and runs from Loch Eriboll on the north coast, SSE along the west coast of the Northern Highlands, before continuing along the southeast coast of the Isle of Skye and the west coast of the Isle of Mull.

Ascending Knockan Crag, Cambrian rocks are found overlain by the older Moine rocks. The Cambrian rocks consist of Pipe Rock, Fucoid Beds, Salterella Grits and Durness limestone – all in stratigraphic sequence. Overlying the Durness limestone are the dark schists of the Moine. These rocks are Precambrian in age. At the thrust plane, the Durness limestone shows evidence of crushing, and the Moine rocks have been altered to mylonites. This is perhaps the simplest exposure of the Moine Thrust to be found in the region.

Further reading:

MacGregor, M. & Phemister, J. 1958. Geological Excursion Guide to the Assynt District of Sutherland. 2nd edn. Edinburgh Geological Society (Oliver and Boyd).

Peach, B.N., Horne, J., Gunn, W., Clough, C.T. & Hinxman, L.W. 1907. The Geological Structure of the North-west Highlands of Scotland (Memoirs of the Geological Survey, Scotland). His Majesty’s Stationery Office, Glasgow