GCR block - Lewisian (LEW)


Block Description

UK map showing distribution of GCR sites of GCR block LEW
Distribution of GCR sites of GCR block Lewisian

The geology represented by this GCR Block are metamorphosed rocks of Precambrian age in Scotland that are older than the Dalradian/Torridonian (see DAL, TOR). ‘Precambrian’ is a broadly used term for rocks that pre-date the Cambrian Period (540–510 Ma), i.e. were formed before the Phanerozoic Eon. It encompasses a vast span of time, extending back to at least 4000 Ma, and has been subdivided into two Eons, the Archaean and the younger Proterozoic Eon. The Precambrian rocks selected for this block belong to Archaean Eon.


The crust of north-west Scotland, together with parts of Greenland and North America that make up the ancient continent of Laurentia, was built up mainly from igneous rocks that crystallized during Precambrian time around 2900 to 2700 million years ago (Ma).

At that time, the rocks that crop out at the surface now were deep in the Earth's crust. They were deformed and metamorphosed at very high temperatures, producing gneisses with a folded layering – the Lewisian Complex. There were, in fact, two periods of deformation and metamorphism, separated by a stable period when the crust fractured and allowed in basic magma that crystallized as a set of dykes (Scourian dykes). The early metamorphic event was of high grade (where the typical mafic mineral in the rock is pyroxene), whereas the later ‘Laxfordian’ metamorphism was of medium grade (typical mafic mineral is hornblende).

The Lewisian gneisses are overlain unconformably by the unmetamorphosed cover of Torridonian or younger strata, see TOR, and can be thought of as the ‘basement rocks’ of the crust of Britain brought to the surface during tectonic events.

Several rock types are found in the Lewisian Complex. They can be broadly subdivided into felsic gneisses (rich in light-coloured minerals like feldspar and quartz), mafic gneisses (containing a large proportion of dark mafic minerals such as pyroxene, hornblende, or biotite mica), and ultramafic gneisses (containing exclusively mafic minerals). The felsic gneisses are the most abundant, and are the metamorphosed equivalents of granite or, more characteristically, tonalite (like granite but without potassium feldspar). Mafic and ultramafic gneisses are generally found together, and can be understood as metamorphosed varieties of gabbro and peridotite.

Site List

Your selection found 24 GCR sites. Sites are sorted alphabetically by country, local authority and then by site name -
CodeNameCountryLocal AuthorityGrid RefGCR Site Account
2435An ArdScotlandCaithness and Sutherland and Ross and CromartyNG805751Not available
2444An Fharaid Mor - ClachtollScotlandCaithness and Sutherland and Ross and CromartyNC038270Not available
2458BadcallScotlandCaithness and Sutherland and Ross and CromartyNC146461Not available
2436Creag Mhor ThollaidhScotlandCaithness and Sutherland and Ross and CromartyNG827756Not available
2460DiabaigScotlandCaithness and Sutherland and Ross and CromartyNG825590Not available
2453FlowerdaleScotlandCaithness and Sutherland and Ross and CromartyNG819747Not available
2434KerrysdaleScotlandCaithness and Sutherland and Ross and CromartyNG826738Not available
2433Loch Braigh Horrisdale to Sidhean MorScotlandCaithness and Sutherland and Ross and CromartyNG788770Not available
2439Loch DrumbegScotlandCaithness and Sutherland and Ross and CromartyNC132320Not available
2461Scourie BayScotlandCaithness and Sutherland and Ross and CromartyNC148448Not available
2459Scourie MorScotlandCaithness and Sutherland and Ross and CromartyNC144450Not available
2457Sithean MorScotlandCaithness and Sutherland and Ross and CromartyNC150460Not available
2438Tarbet to Rubha RuadhScotlandCaithness and Sutherland and Ross and CromartyNC174506Not available
2456Cnoc an FhithichScotlandEilean Siar (Western Isles)NF655047Not available
2455Cnoca BreacScotlandEilean Siar (Western Isles)NF733337Not available
2437Gearraidh Siar and Baile a' MhanaichScotlandEilean Siar (Western Isles)NF767555Not available
2462Gruinard RiverScotlandEilean Siar (Western Isles)NG980883Not available
2451LeinisScotlandEilean Siar (Western Isles)NL706998Not available
2454Loch SgioportScotlandEilean Siar (Western Isles)NF812384Not available
2440Na Buirgh (Borve)ScotlandEilean Siar (Western Isles)NG009937Not available
2441North PabbayScotlandEilean Siar (Western Isles)NF873885Not available
2443North Uist CoastScotlandEilean Siar (Western Isles)NF738767Not available
2452RhughasinishScotlandEilean Siar (Western Isles)NF821448Not available
2442RoineabhalScotlandEilean Siar (Western Isles)NG050855Not available
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