The BGS Lexicon of Named Rock Units — Result Details

Ullapool Gneiss

Computer Code: ULLA Preferred Map Code: AFO
Status Code: Full
Age range: Archaean Eon (AR) — Palaeoproterozoic Era (AL)
Lithological Description: The Ullapool Gneiss is a dark pink to white weathering, variably foliated and poorly layered, medium- to coarse-grained orthogneiss, rich in alkali feldspar. The gneiss has a broadly syenitic to granitic composition. Samples from the type locality typically comprise K-feldspar (perthite and microcline) + plagioclase + guartz + bluish green amphibole, with minor epitode and accessory titanite. The mafic minerals occur in clusters several millimetres across, which commonly gives the rock a pink-green speckled appearance. The felsic gneiss contains some pods of more mafic, amphibole-rich gneiss. The gneissic layering is considerably less well developed than in the typical Scourian Gneiss. Amphibolitic mafic dykes cross-cut the unit.
Definition of Lower Boundary: The unit always occurs within the Moine Thrust Belt, and the lower structual boundary is always tectonic.
Definition of Upper Boundary: The upper structural boundary is formed by the Moine Thrust or by the unconformity with overlying Torridon Group sediments.
Thickness: 20m - 200m
Geographical Limits: The Ullpool Gneiss occurs in thrust slices within the Moine Thrust Belt. The most northerly thrust slice occurs in Strathkanaird, just east of Langwell Lodge. A major thrust slice extends from Ullapool River (just west from Loch Achall), via Corry Point across Loch Broom to Loch Lagaidh. Similar gneisses have been identified in the Kinlochewe area by earlier mappers (see references).
Parent Unit: Lewisian Complex (L)
Previous Name(s): Logan Rock (-3742)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  A 200m-thick thrust sheet comprised of orthogneiss on either side of the Ullapool River; quarried in part. 
Reference(s):
Geikie, A. Notebook A. Quoted in Oldroyd (1990). 
Heddle, M F. 1883. The geonosy and mineralogy of Scotland. Sutherland-Part V. Mineralogical Magazine 5, 217-263. 
Callaway, C. 1883. The age of the newer gneissic rocks of the northern Highlands. Quarterly Journal of the Geological Socety of London 39, 355-422 
Oldroyd, D R. 1990. The Highlands Controversy: Constructing Geological Knowledge through Fieldwork in Nineteenth Century Britain, University of Chicago Press. 438pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable