Knockan Crag: The Moine Thrust

Knockan Crag is the easiest place to see the Moine Thrust. It lies in the Inverpolly Nature Reserve, and a nature trail leads up and along the crag, through the succession of Cambrian sediments. Above the grey Durness Limestone seen at Skiag Bridge there is a fractured and sheared white limestone. Then, immediately overlying this with a sharp contact, are mylonites, fine-grained platy metamorphic rocks made mostly of mica and quartz. The mylonites were formed by intense shearing of older metamorphic rocks, the Moine Schists. These were originally sandstones and siltstones deposited far away to the south and east, and they were metamorphosed into schists in the earlier stages of development of the Caledonian mountain belt. Then, about 420 million years ago, horizontal forces pushed them many tens of kilometres up and over the Cambrian sedimentary rocks and their basement, into their present position.

View of the outcrop of the Moine Thrust at Knockan Crag, looking south. The dark layered rock in the upper part of the photograph is mylonite formed from Moine Schists. The Moine Thrust itself lies between these dark rocks and the white Durness limestone in the centre of the photograph. Below the white limestone can be seen grey Durness limestone and, behind the small trees, light grey Salterella Grit.

The view in the opposite direction (northwards) gives a clearer picture of the white sheared limestone beneath the Moine Thrust plane. Below the path (bottom left), the limestone becomes darker in colour and more like the undeformed lower part of the Durness limestone seen at Skiag Bridge.

Outcrop of Moine mylonites above the thrust, showing the well developed platy texture of the strongly-sheared rocks.

Rock types at Knockan Crag

D.J. Waters, Department of Earth Sciences, May 2003