Knockan Crag

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Name: Knockan Crag
Grid reference: NC 188 090 - NC 196 099
Theme: Tectonic structure
Subject: Moine Thrust
Main feature: thrust fault
Key reference: BUTLER, R W H, 2009. Knockan Crag. In: MENDUM, J R, BARBER, A J, BUTLER, R W H, FLINN, D, GOODENOUGH, K M, KRABBENDAM, M, PARK, R G, STEWART, A D. (Editors), Lewisian, Torridonian and Moine rocks of Scotland. Geological Conservation Review Series, 34. Joint Nature Conservation Committee, Peterborough, pp. 292–296.
DIAGRAM/LOCATION MAP HERE
Schematic geological map of the Northern Highlands, showing extent of the Moine Thrust.


Author(s): M Krabbendam (BGS), KM Goodenough (BGS)

Summary[edit]

Knockan Crag is a key locality for the Moine Thrust, the main tectonic structure in the North West Highlands. The Moine Thrust is a gently dipping tectonic fault, along which old and high-grade metamorphic rocks (the Precambrian Moine Supergroup rocks) where translated over low-grade and younger Cambrian rocks, such as the Durness Limestone. The fault developed during the Caledonian Orogeny, 460–430 myrs ago. Knockan Crag is one of the best exposed and accessible places to view the Moine Thrust. Knockan Crag was important site during the ‘Highlands Controversy’, an at times acrimonous scientific debate in the late 19th century that centred on the question whether or not large volumes of rocks could be translated over large distances along low angle faults.

Knockan Crag is located c. 15 km north of Ullapool. The site is an SSSI, a Natural Nature Reserve and a GCR site. There is a Visitor Centre in which the geological features are interpreted, managed by Scottish Natural Heritage. No sampling is permitted at the site.

The ‘Knockan Puzzle’: a geological model of Knockan Crag, showing the different strata in the footwall, the Moine Thrust, and the hangingwall. Knockan Crag Visitor Centre. P527531.

Location - access[edit]

Knockan Crag is an escarpment along the A835 Ullapool – Lochinver road, some 15 km north of Ullapool. The site is public access, with good parking facilities (also for coaches) and toilet facilities. The Visitor Centre is open 24/365. A well maintained track leads past most critical exposures. Part of the site allows disabled access.

Geological description[edit]

The Moine Thrust is the main feature at Knockan Crag. Metamorphosed metasediments of the Neoproterozoic (c. 1000 ma) Moine Supergroup were emplaced by the the thrust on top of low-grade Cambro-Orodovician and older rocks below. The Moine Thrust lies directly on rocks of the undeformed Foreland; the complicated imbricate structures that elsewhere in the NW Highlands form Moine Thrust Zone are absent.

DIAGRAM TO GO HERE
Stratigraphy of strata present at Knockan Crag (coloured) in the footwall of the Moine Thrust.

Footwall[edit]

The footwall of the Moine Thrust at Knockan Crag is the undeformed Foreland of the Moine thrust Zone, and comprises Cambro-Ordovician strata in stratigraphical order. At Knockan Crag, rocks of the Pipe Rock Member (Eriboll Formation), Fucoid Beds Member, Salterella Grit Member (An ‘t-Sron Formation) and Ghrudaidh Formation are present. The rocks of the Ghrudaidh Formation are highly altered: normally these are dark-gray due to the presence of organic material. At Knockan Crag, the Ghrudaidh Formation carbonate is ochre-coloured due to secondary alteration and are highly fractured.

Hangingwall[edit]

The hangingwall of the Moine Thrust comprises siliclastic mylonite, mainly of psammitic composition. These are fine-grained, finely layered rocks formed by intense ductile shearing. Some 30-50 m structurally above the Moine Thrust the intensity of mylonisation gradually decreases, and the psammitic rocks become flaggy, with a typical layer thickness of 10-20 cm. The biotite fabric is still sub-parallel to the lithological layering. A good outcrop of these mylonites is at ‘Eagle Rock’ [NC 188 086].

DIAGRAM TO GO HERE
Geological map of Knockan Crag and surrounding area. From: Goodenough & Krabbendam (2011).

The protoliths of the mylonite at Knockan are psammite (metasandstone) of the Morar Group, the basal group of the Moine Supergroup. In areas of low strain, the metasandstone contains recognizable sedimentary structures such as cross-bedding. In the mylonites at Knockan such features are completely obliterated by the ductile shearing.

Thrust[edit]

The Moine Thrust at Knockan is a very sharp and planar fault plane, dipping very gently to the east (<5°). The thrust plane comprises a reddish fault gouge up to about 2 cm thick. This gouge is easily weathered and causes the sharp incut below the main escarpment at Knockan Crag. Above this, the rock is highly fractured and brecciated for a few metres. The clasts within the fault breccia are themselves highly foliated, showing a brittle overprint following ductile shearing.

Tectonic interpretation[edit]

The exposure of the Moine Thrust at Knockan Crag is a very clear fault plane, but this represents only the latest brittle movement. In effect, the Moine Thrust at Knockan crag ‘contains’ the entire Moine Thrust Zone. The thrust clearly emplaces highly metamorphosed and older rocks (Moine Supergroup) over younger, unmetamoprhosed rocks of Cambrian age. The nature of the brittle faulting, however, is less clear. The thrust plane is very gentle dipping with a lower dip than the strata in the Foreland to the west which dip c. 10-12° to the east (see cross-section). Coward (1985) argues that the brittle movement was partly caused by extensional movement as the thrust pile collapsed. Alternatively, the brittle movement represent the latest component of a cooler and shallower thrust movement as the thrust pile evolved under progressively cooler conditions as seen elsewhere in the thrust belt, for instance in the Assynt Culmination.

DIAGRAM TO GO HERE
Geological cross-section of Knockan Crag and surrounding area. From: Assynt (bedrock) map.

Gallery

Painting of Knockan Crag by Elizabeth Pickett, NW Highlands (oblique aerial view). Picture is based on an aerial photo by P and A Macdonald. P552065. Painting of Knockan Crag by Elizabeth Pickett, NW Highlands (oblique aerial view). Picture is based on an aerial photo by P and A Macdonald. P614985. The Moine Thrust at Knockan Crag, north of Ullapool. Mylonitic Moine rocks above Durness Limestone). P531955.
Knockan Crag. Moine Thrust. Blue-grey Moine mylonite is thrust over ochreous weathered Durness Limestone.

P219710.

Moine Mylonite, northern end of Knockan Crag. C. 30 m above the Moine Thrust. P512960. Detail of Durness Limestone, just below Moine Thrust, Knockan Crag. P512957.

See also[edit]

North-west Highlands

  • Knockan Crag (key locality for Moine Thrust)

External Links[edit]

Knockan Crag Visitor Centre (site from Scottish Natural heritage)