Klippen at Knockan, North-west Highlands - an excursion

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Simplified geological map of the Knockan area, after British Geological Survey (2007), showing the localities described in Excursion 6.
The Moine Thrust in the area of Locality 6.5, with Moine mylonites resting on dolostones of the Eilean Dubh Formation. (BGS photograph P537568, © NERC)

By Kathryn Goodenough and Maarten Krabbendam

Excursion 6B from: Goodenough, Kathryn M. and Krabbendam, Maartin (Editors) A geological excursion guide to the North-west Highlands of Scotland. Edinburgh : Edinburgh Geological Society in association with NMS Enterprises Limited, 2011.

For key information about this excursion See: Knockan Crag and the Knockan Klippen, North-west Highlands - an excursion


Introduction

Drive some two kilometres north-east from Knockan Crag to the hamlet of Knockan, and park in a large lay-by on the left of the road [NC 212 106]. Walk about a hundred metres south-east along the road, and then follow a narrow tarred road towards the east, passing some crofts. At a sharp bend, go through a gate and continue for about 500m along a track that runs south-east across grazing lands. Where the track crosses a small stream, leave the track and follow the stream in a south-easterly direction to a low hill, Cnoc a’Choilich Mhor [NC 2056 0962].

Locality 6.4 Cnoc a’Choilich Mhor Klippe. [NC 2056 0962]

The low hill of Cnoc a’Choilich Mhor is composed of quartz arenites of the Eriboll Formation (mainly Pipe Rock Member, although pipes can be difficult to find). These outcrops represent a small klippe of a thrust, which carries the Eriboll Formation over the imbricated Durness Group carbonate that underlies all the surrounding low ground to the north and east. The carbonates in this area are characterised by the presence of sink holes.

From here, walk in a WSW direction towards a stream valley south of Druim Poll Eòghainn, passing outcrops of Eilean Dubh Formation dolo-stones. Variations in the dip of the dolostones indicate that they are part of an imbricate stack, known as the Elphin imbricates.

Locality 6.5 Moine Thrust exposures. [NC 200 093 to NC 204 094]

The east-west stream section here follows the Moine Thrust, and the thrust can be studied at several places in the stream valley. At [NC 201 094], easterly dipping beds of Eilean Dubh Formation dolostone form the bed of the burn, and are overlain by mylonites which are well exposed in a low cliff (P537568). The thrust plane itself is not actually exposed, but the ground immediately above the stream contains some excellent outcrops in which small-scale structures associated with the Moine Thrust can be seen. The mylonites commonly contain strung-out quartz ribbons, and close to the thrust the mylonitic fabric is locally overprinted by a fracture cleavage or dense fracturing (e.g. at [NC 2069 0927]). The dolostones close to the thrust are fractured and locally show incipient brecciation. The fabric in the mylonitic Moine psammite dips south here and strikes parallel to the outcrop of the (brittle) Moine Thrust itself. This shows that the later, brittle Moine Thrust does not cross-cut the ductile, mylonitic Moine Thrust (cf. Coward, 1985).

Locality 6.6 Uamh an Tartair Klippe. [NC 213 092 to 217 092]

From the thrust exposures, walk eastwards to the prominent right-angle bend in the Abhainn a’Chnocain (the Knockan Burn) at [NC 213 092]. Here the outcrops change from carbonate into quartz arenite as the unexposed Uamh an Tartair thrust is crossed. At [NC 2153 0911] Basal Quartzite Member can be recognised within the Uamh an Tartair Klippe. The Moine Thrust occurs only just to the south and must decapitate this klippe.

Continue along the stream to the east to [NC 2164 0912], taking great care as you approach an abrupt small gorge and a large sinkhole, which betray the presence of Durness Group carbonate, lying beneath the thrust.

However, the small hillock 50m to the east is again composed of quartz arenite. Several explanations, including normal faulting, could explain this apparent repetition of the thrust. However, on the south side of the stream it can be demonstrated that the carbonate overlies the Basal Quartzite Member to the west (i.e. young over old). This arrangement is best explained by breaching, in that a thrust within the underlying carbonate breached the Uamh an Tartair Thrust and emplaced carbonate over quartz arenite.

The overall sequence of events that can be established from Localities 6.4–6.6 is as follows:

(1) Ductile movement along the Moine Thrust, forming mylonitic fabric;

(2) Movement along the Uamh an Tartair Thrust, emplacing quartz arenite over Durness Group carbonate;

(3) Imbrication within the Durness Group carbonate, forming the Elphin imbricates, with some thrusts breaching the overlying Uamh an Tartair Thrust;

(4) Decapitation of the Uamh an Tartair Thrust Sheet and the breaching thrusts by the (reactivated) brittle Moine Thrust.

From Uamh an Tartair it is best to go north, passing two large sinkholes along the way, to pick up the track back to Knockan and the main road.

References

At all times follow: The Scottish Access Codeand Code of conduct for geological field work