Knockan

Knockan Crag is one of the most accessible and popular sites in Assynt. This popularity is ensured these days by the development of visitor facilities including interpretation resources by Scottish Natural Heritage. The site is now a Site of Special Scientific Interest, designated for it's scientific and educational value to the nation's heritage. As such no geological (or other) specimens may be collected, even from loose rock. The site was important in the early debates in the Highland Controversy, notably between Murchison and Nicol. So what's there?

Knockan Crag is basically a single cliff section up through the Cambrian strata and into the Moine. The contact between them - the Moine thrust - is well-exposed at a series of places in the hillside. It is roughly parallel to bedding in the Cambrian strata - a feature that misled Murchison into thinking that the contact itself was a simple bedding plane.

Working up from the main road we find the upper few metres of Pipe Rock, followed by the succession though Fucoid Beds, Salterella and into the Durness carbonates. Many of these units are undeformed. The Pipe Rock preserves dramatic worm burrows that remain perpendicular to bedding. There are depositional structures (e.g. ripples) to be found in the Fucoid Beds. However, within a few metres of the thrust plane the Durness carbonates are heavily deformed, with many fracture surfaces, making it difficult to recognise bedding. The thrust surface itself is knife-sharp.

Coming into the Moine Thrust from its upper side, the higher parts of the Moine rocks at Knockan are strongly banded. The banding is picked out by different concentrations of the dominant constituents, feldspar, mica, chlorite and quartz. Murchison thought that this banding represented bedding. However, following the deductions made by Lapworth in the Loch Eriboll district in the 1880s, geologists now consider the banding to have been produced by intense shearing. This type of rock is called a mylonite (a word coined by Lapworth to mean a rock that has been intensely sheared so that its constituent minerals are strongly flattened and streaked out). The intensity of banding increases towards the thrust. In places it is strongly folded (best seen in road cuttings about 1-2 km south of Knockan). Within a metre or so of the thrust the mylonite has been broken up and is fragmentary, although this is difficult to spot (use a hand-lens) as the whole lot has been cemented together. Geologists call this type of shattered rock produced by fault motion a "cataclasite". The transition from mylonite to cataclasite could imply that either the rate of straining increased near the fault (probable) and/or that the temperature was gradually decreasing as the Moine Thrust moved (also likely as the rocks got closer to the Earth's surface). However, along the thrust itself the cataclasite is streaked out again (perhaps representing the final slowing down of deformation along the Thrust).

The cliff section at Knockan is deceptively simple. But in fact the Durness carbonates have been stacked up into a pile of minor thrust sheets (called imbricates) which were then over-run by the Moine Thrust. You can get a flavour of this complexity from the cross-sections of Peach and co-workers. On the ground you can see this effect in a stream section gained by walking along the thrust behind the crag.

Knockan is important to geologists then because of its place in the Highland Controversy, for providing a dramatic transect into the Moine Thrust and for showing the relationship between the Moine Thrust and other thrust structures below. It is an important conservation area but, rather controversially, also now hosts a "visitor attraction".

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