INTRODUCTION TO THE OUTER HEBRIDES
Modified from an article written in 2000 by J R Mendum, principle author of the BGS memoir on the area, a field guide by John Mendum and Doug Fettes and an unpublished Geologist’s Association guide by Rod Graham.  

The Outer Hebrides are a chain of islands in the Caledonian foreland of Britain some 200 km from end to end. We will be visiting only the central part of the chain (North Uist, Benbecula and South Uist) where Mike Coward did most of his work. 
The  Hebrides form an upstanding basement high that has a discrete faulted eastern margin against Mesozoic and earlier Torridonian sedimentary basins in the Minches. On their western side lies an extensive shallow shelf, termed the Outer Hebrides Platform with Lewisian Gneiss at or near the bedrock surface. It extends west of the Uists for some 60 to 70 kilometres. The solid geology of the Outer Hebrides is dominated by Archaean and Proterozoic gneisses – Lewisian, after the island of Lewis,  analogous in age with rocks forming the Caledonian foreland of mainland Scotland, but separated by from them by  a  sediment filled Mesozoic half graben beneath the Minch.  Part of this is exposed as the Stornoway beds (near the town of Stornoway in Lewis), the only non- Lewisian rocks in the Hebrides.  

History of Research

The Outer Hebrides were first visited geologically by J. MacCulloch (1819) who carried out a cursory survey. Unlike the Scottish mainland, the region was not mapped by the Geological Survey in the late 19th and early 20th centuries, and the first systematic study was undertaken by T.J. Jehu and R.M. Craig of the University of Edinburgh in the 1920’s. The results were described in a series of reports in the Transactions of the Royal Society of Edinburgh between 1923 and 1934. Jehu and Craig recognized the main elements of Hebridean geology including the South Harris Igneous Complex, the Outer Hebrides Fault Zone, mafic and ultramafic bodies, and the main outcrops of granites. A further gap in research followed, and it was only following the publication of J. Sutton and J.V. Watson’s (1951) seminal work on the mainland Lewisian that work resumed in the Outer Hebrides. R. Dearnley mapped parts of South Harris (Dearnley, 1963) and also attempted to correlate wider features of Hebridean geology with the mainland (Dearnley, 1962). Later, together with F.W. Dunning, he recognized the importance of the deformation state of a suite of Palaeoproterozoic mafic dykes, here termed the ‘Younger Basic’ suite, which are equivalent to the Scourie Dyke-swarm of mainland NW Scotland (Dearnley and Dunning, 1968). This work also included detailed studies of well-exposed coastal sections in Benbecula and South Uist and recognized the heterogeneity and rapid spatial variations in later, Laxfordian strain patterns. Also in the 1960’s, Watson, assisted by numerous PhD students at Imperial College, London (including, of course, Mike Coward), carried out a programme of work to map and understand the solid geology of the Outer Hebrides. A co-operative project with the Geological Survey to produce maps and reports followed. Areas not covered by Ph.D. theses were mapped at 1:10 000 scale, existing material was compiled, and petrographical studies and chemical analyses of the main lithologies were carried out. The end result was the compilation of four 1:100 000 maps (Institute of Geological Sciences, 1981) and an accompanying memoir together with separate structural maps at 1:100 000 (Fettes et al., 1992). 
In the 1970s differing views were put forward regarding the number of major basic dyke suites in the Outer Hebrides. Bowes and Hopgood (1975) and Taft (1978) argued for several suites and Coward (1973), Francis (1973) and Davies et al. (1975) suggested that there is effectively only a single suite. The problem was discussed by Fettes et al. (1992) and the latter viewpoint was upheld. More recent work has focussed on the Outer Hebrides Fault Zone (OHFZ), describing its long and complex structural history (Sibson, 1977a; Fettes and Mendum, 1987; Butler et al., 1995; Imber et al., 1997, 2001), and dating of the various elements in the Lewisian Gneiss Complex (Cliff et al., 1998; Whitehouse, 1990a; Whitehouse and Bridgwater, 2001; Friend and Kinny, 2001). In addition there has been much detailed mapping, and geochemical and metamorphic studies in the south-west part of South Harris, notably on the various elements of the South Harris Igneous Complex, but also on the Langavat and Leverburgh metasedimentary belts (e.g. Witty, 1975; Horsley, 1978; Dickinson and Watson, 1976). The most recent work here has described high-pressure metamorphic assemblages in the Leverburgh metasedimentary rocks (Baba, 1998,1999a,b). 

Geological History
The Lewisian Gneiss Complex is a product of polyphase depositional, intrusive, deformational and metamorphic history spanning the time period between 3100 million years and 400 million years. The main lithologies result from a combination of different age and type of protolith, and different amounts and degree of structural and metamorphic reworking. 
Classically two major periods of tectonic and metamorphic activity have been recognized - Scourian and Laxfordian. The main elements of the gneiss complex were formed during the Scourian. Originally granodioritic, tonalitic and basaltic intrusions emplaced at mid or lower crustal levels they were subjected to deformation and metamorphism (gneiss formation) between 3100 Ma and 2500 Ma. Granite bodies and microdiorite sheets were intruded near the end of the Archaean in what is termed the late Scourian at around 2600 Ma (Fettes et al., 1992). There followed the widespread intrusion of the ‘Younger Basic’ suite of dolerite and basalt dykes and lenticular sheets in the Palaeoproterozoic at around 2400 Ma. Significant later accretion of possibly arc-related metasedimentary and metavolcanic rocks and intrusion of the South Harris Igneous Complex occurred at around 1880 Ma. The gneisses and earlier igneous bodies were reworked significantly between 1850 Ma and 1600 Ma, during the Laxfordian event, and it is the resultant Laxfordian folds, fabrics and metamorphic mineralogy that dominate the Lewisian rocks of the Outer Hebrides. 
The most obvious major structure of the Outer Isles is  Outer Hebrides Fault Zone, a gently ESE-dipping, structure with evidence of polyphase compressional, strike slip and extensional displacement, a major eastward dipping thrust zone, marked by pseudotachylite, cataclasite and brecciated 'mashed gneiss'. Later mainly extensional mylonites (in part phyllonites) of Caledonian age are also found in South Lewis and the Uists. A zone of low grade, hydrous, greenschist facies retrogression, broadly associated with the thrust zone, is well developed from North Uist through Harris and Lewis. The trace of the Outer Isles Fault Zone runs down the eastern side of the Outer Hebrides, and reaches up to 18 kilometres wide in the Park district of Lewis, but lies offshore from parts of South Harris. It is not clear whether the thrust zone exposed on Barra is only a forethrust with the main trace of the Outer Hebrides Fault Zone lying offshore to the southeast.
The steep Minch Fault, which now spatially corresponds closely with the surface trace of the earlier low angle thrust/extensional Outer Hebrides Fault Zone, generally lies immediately offshore to the southeast of theOuter Hebrides. 
and may have formed originally as a Grenvillean (c. 1100 Ma) thrust belt. It was re-activated as an extensional structure at the time of deposition of the later parts of the Torridonian succession (c. 1000 Ma). Subsequent re-activation and further development occurred near the end of the Caledonian Orogeny during the main WNW-directed thrusting event at c. 435-425 Ma. This was rapidly followed by further extension, which gave rise to dip-slip movements. Development of the roughly coincident Minch Fault occurred in the Mesozoic, and this structure later controlled the nature of the Palaeogene basin there Mike Coward recognized the ‘Outer Isles Thrust’ as a fundamentally important structure. Some recent interpretations suggest that the fault zone may coincide with a suture resulting from Lewisian terrane amalgamation between c.1500 Ma and 1100 Ma (Friend et al. 2001),
The Corodale Gneiss (sometimes termed the Eastern Gneiss), a massive partly banded granulite facies metadiorite that lies above the main thrust of the Outer Hebrides Fault Zone in South Uist, is also of Scourian age. These older metasedimentary and basic/intermediate units are intruded by abundant Archaean tonalitic, granodioritic or granitic intrusions, which now constitute the bulk of the typical grey, banded quartzofeldspathic gneisses. The intrusions were emplaced at deep crustal levels around 2900 to 2800 Ma, and subject to recrystallisation, migmatisation and gneiss formation soon after emplacement. No major fold structures are recognised to result from this event (dS2) although a pervasive gneissose foliation was formed. Numerous mafic and minor ultramafic intrusions also form part of this igneous suite. A further period of Late Scourian (ca 2600 Ma) igneous activity is also recognised and locally significant dioritic, monzonitic and granitic bodies are common in parts of the Outer Hebrides. The diorites and monzonites are commmon on the southeastern side of Barra, and the lenticular granitic intrusions occur notably in the northern and western parts of North Uist, on Berneray, and in Northwest Lewis. These elements were significantly affected by a Late Scourian deformation and metamorphic episode, which resulted in the formation of asymmmetrical folds and the regional NNE trend to the overall gneiss foliation (dS3). Shear zones that post-date these structures are also recognised (dS4). Late Scourian pegmatites are common throughout the Outer Hebrides.
These Scourian elements were all intruded by a suite of basic to ultrabasic dykes, sheets and lenticular intrusions at mid-crustal depths mainly at around 2400 Ma, but with a later phase at around 2000 Ma. This suite, known on the mainland as the Scourie Dyke Suite, and in the Outer Hebrides as the Younger Basic Suite, are an important marker which separate two distinct periods of crustal accretion, deformation and metamorphism. The bulk of the dykes are metadolerites of continental tholeiite affinity, but locally olivine metadolerites, metanorites and sepentinised picrites are found. There is some evidence that intrusion occurred synchronous with localised shearing and amphibolite grade metamorphism but recent Nd-Sm dating of minerals and dykes around the Laxford Front (M.T. George – Geoscience 2000) suggest that the peak conditions of the Inverian metamorphic event post-dated dyke emplacement by 100 to 150 Ma. The dykes provide an element whose state of deformation can readily be used to distinguish between the earlier Scourian migmatisation, deformation and metamorphism and the later Laxfordian effects that reworked most parts of the Lewisian Complex in the Outer Hebrides. The thicker basic bodies and weakly deformed dykes preserve igneous fabrics and have distinctive pyroxene- and garnet-bearing, granulite facies mineralogies. These mineralogies reflect the conditions of dyke emplacement, and even the least recrystallised bodies show metamorphic pyroxene and feldspar compositions. The nature of the dykes and interpretation of their role during Laxfordian reworking caused several disputes in the 1960s and 1970s. However, it is now largely accepted that the Younger Basic Suite can be correlated with the Scourie Dyke Suite, and that they represent a period of widespread basic intrusion that separate Scourian and Laxfordian tectonometamorphic events.
Overlapping in time with Scourie Dyke intrusion was the emplacement of large layered mafic to felsic intrusions ranging from anorthosite to peridotite, diorite and tonalite. The most prominent of these intrusions form the South Harris Igneous Complex, but the  Corodale Gneiss of South Uist was thought formerly also to be of this age. The South Harris Igneous Complex comprises four major igneous intrusions and numerous smaller bodies. Field relationships show that the order of intrusion was metagabbro, meta-anorthosite, metanorite, and metadiorite. The complex was intruded at lower crustal levels and crystallised initially under granulite facies metamorphic conditions. A later history of Laxfordian uplift and metamorphism at progressively shallower levels can be documented from the meta-igneous bodies and their adjacent metasedimentary belts. Geochemically the igneous complex is characterised by overall calc-alkaline trends.
Laxfordian reworking took place at amphibolite grade and resulted in folding, locally pervasive planar and linear fabrics, migmatisation and granite and pegmatite intrusion. It occurred between 1800 Ma and 1550 Ma. As it post-dated Scourie Dyke intrusion the deformational and metamorphic state of the dykes are a good guide to the intensity of the Laxfordian effects. The folding and metamorphism occurred early in the Laxfordian event and two main phases can be distinguished. The earlier (dL2) resulted in folds with flat-lying or gently dipping axial planes and a strong axial planar fabric. The later phase (dL3) resulted in regional asymmetrical folding with broad hinge zones and attenuated steep limbs. Locally the Laxfordian recrystallisation and migmatisation resulted in the development of a coarse pegmatitic or agmatitic fabric, with the original Scourian elements being masked or even erased. In the more water-deficient granulite facies gneisses, Scourie Dyke metabasic bodies and other large intrusive bodies, Laxfordian reworking is mainly limited to small and large-scale shear zone development and some formation of pseudotachylite. However, the South Harris Igneous Complex shows regional inversion and large scale folding, generally attributed to Laxfordian deformation. between 1700 and 1550 Ma

OUTCROP INFORMATION

Ardivachar Point (South Uist), Garry a  Siar and Balivanich  (Benbecula) 
The classic sections on the west coast of Benbecula around Balivanich and Garry-a-siar. The overall structure of South Uist and Benbecula were described by Coward (1973a). These sections were originally described in detail by Dearnley and Dunning (1968) and by Mike Coward (1973b). Similar relationships are also seen around Ardivachar Point at the northwest extremity of South Uist. Although the areas generally lie within restricted zones of low Laxfordian strain all the authors stress the heterogeneity and varied scale of the strain effects. They differ as to the number of Laxfordian deformation phases present and in the interpretation of the deformation and metamorphic history of the zones. 
Mike (Coward, 1973a,b) recognised four Laxfordian deformation phases, dL1 to dL4, and his notation is used here. dL1 planar fabrics trend northwest, approximately parallel to the general trend of the 'Scourie Dykes'. Fabric intensity is very variable over short distances. dL1 folds of the dykes are uncommon except where apophyses at right angles are present. In contrast during dL2 deformation, which in most areas is the principal strain event, the metabasic dykes lay at a high angle to the compression. dL2 fabrics and fold axial planes trend northeast; hence dL2 folding is more common and variations in dL2 strain are broader with less sharp boundaries. dL3 strain and deformation again results in northwest- and NNW-trending fabrics and sharp variations in the intensity of deformation. Large scale dL3 folds resulted with lenticular zones of low strain in or near their hinge zones. All related linear features, e.g. fold axes, lineations, are coaxial and now plunge gently to moderately to the NNW. 
Mike (Coward, 1973b) also used the relationships of the 'Scourie Dyke'metabasic intrusions to their adjacent gneisses to estimate the amount and mode of strain and constrain mechanisms of deformation. He noted that the ductility contrast between metabasic dyke and adjacent quartzofeldspathic gneiss during Laxfordian deformation differed markedly, dependent on whether the metabasic dyke had granulite or amphibolite facies mineralogy. Granulite facies dykes show cuspate structures that V into the dyke, and are commonly folded and boudinaged. Amphibolite facies dykes and apophyses to granulite facies dykes show cuspate structures that V towards the gneisses, and commonly are not folded, even in zones of strong Laxfordian deformation.
At Balivanich  a large Scourian banded basic body at the northern end of the section, move southwest through a dL3 synformal zone and into a zone of higher dL2 deformation. Towards the southern end of the section discdorant metabasic dykes are seen and apart from a narrow zone on the southern promontory, Laxfordian deformation is low.
At Garry-a-siar Laxfordian deformation is generally lower overall and excellent but complex, metabasic dyke-quartzofeldspathic gneiss relationships can be seen. The relationship to both Late Scourian and Late Laxfordian quartz-feldspar pegmatites is also well seen. Mike Coward (1973b) has documented these features and shown how the gneiss foliation-dyke angular difference varies around  folded ‘Scourie’dykes. 

Other possible outcrops in South Uist 
A small low quarry opposite the Bornish turn at [NF 752 295] where the characteristic Lewisian quartzofelspathic banded gneisses with lenticular amphibolitic metadolerite are well exposed. They range from leucocratic to biotitic and locally hornblendic. Localised Laxfordian agmatitic and migmatitic effects are seen, and discordant Laxfordian pegmatite veins are present. The metabasic rocks are probably mainly ‘Scourie dykes’.

Howmore Quarry [NF 766 366], the original faces have been partly modified by building of the new road, but although access is slightly more difficult, the previously reported features are still eminently visible. The section shows deformed Scourie dykes in their various manifestations within the grey gneisses. Open to tight Laxfordian folds (dL2 and dL3) are present, and variations in the degree of agmatisation adjacent to the dykes is also well-seen.

The Market Stance Quarry (formerly roadstone) in the centre of the island. Contains a moderate to steep WNW-dipping sequence of metasedimentary rocks and thick metabasic units, in part banded and locally with abundant garnet. These are interpreted as mainly banded basics but there may also be thick ‘Scourie dyke’ age metabasic sheets in the quarry. The metasedimentary rocks range from garnetiferous semipelitic gneisses to quartzose psammitic varieties. The units lie on the western limb of a dL3 anticline, and a complimentary syncline is mapped to the east on Rueval (124m). 
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OUTER ISLES FAULT ZONE
The OHFZ is a complex zone as noted in the geological introduction. For example, in parts of southeast Lewis a widespead cataclastic to ductile fabric predates pseudotachylite formation over a wide area. Later cataclasite veins and localised development of thick mylonites are also found in this area. In South Uist the OHFZ marks the base of the Corodale Gneiss which is thrust westwards over the Laxfordianised, mainly quartzo-feldspathic gneisses of the lower western part of South Uist. The thrust zone is marked by pseudotachylite breccia with subsidiary cataclasite. Cataclasite is an aphanitic, structureless, cohesive cataclastic rock in which most of the fragments are smaller than 0.2mm diameter. The breccia varies in thickness and reaches up to 50m thickness south of Lochboisdale [NF 840 145]. It passes upwards into 'mashed gneiss' composed here of broken metadiorite (Corodale Gneiss) with veins of pseudotachylite and cataclasite. This lithology reaches up to 160m thick on the Lees and Eaval forming the marked 'dip slope' feature. On these hills the ‘mashed gneiss’ is developed in the typical quartzofeldspathic gneisses and metabasic sheets and pods which are strongly retrograded.
 	The Corodale Gneiss metadiorite normally consists of orthopyroxene-clinopyroxene-garnet-hornblende-plagioclase with accessory biotite, quartz, ilmenite, apatite and scapolite. The original assemblage was opx-cpx-(gt)-pl, with plagioclase ranging up to 50%. Plagioclase is generally andesine/labradorite. The rock is crudely banded on a centimetre to metre scale from ultramafic to felsic, and Coward (1972) suggests that there is an overall transition upwwards into more felsic diorite. The banding is cut in several places by metabasic dykes but their age is uncertain. The gneiss shows granulite facies mineralogies and textures.

NORTH UIST 
North Uist lies in a major F3 antiform, the largest of the F 3 folds which together define the major structure of the southern Islands of the Hebrides. The hinge zone of this structure contains a smaller scale (but still very large) synformal zone which crops out on the north-west coast of North Uist (North Uist synform).
Along the north coast of the island gently northerly dipping gneisses form the common 1imb between the antiform and a synform which closes in the island of Berneray in the Sound of Harris.F2 Laxfordian deformation is mostly strong (and the amphibolites derived from Scourie dykes are mostly concordant) but there are two small areas of low finite Laxfordian strain where cross -cutting relationships remain between amphiboltized Scourie dykes and
ancient gneisses, areas of low finite Laxfordian deformation almost identical with those described by Dearnley and Dunning and Mike Coward at Ardivachar and Garry a Siar. 
The Berneray Synform is asymmetrical, its short steep 1imb striking NW-SE through the islands of Berneray and Pabbay. Minor folds which are F 3 Laxforfdian in the sequence here show opposed senses on the islands of Berneray and Pabbay, on the one hand, and Ensay and Killegray on the other, and therefore a major closure -The Sound of Harris Antiform- is assumed to exist between them.
On the islands of Ensay and K1llegray the gneisses dip gently northeastwards in the flat limb of the antiform and beneath a belt of gneissic norite, one of the large masses of deformed igneous rock which occur in S. Harris This is effectively the southern limb of a synformal zone which exists in South Harris.

The Structure of north-western North Uist
Mike Coward was probably the first person to appreciate the spectacular outcrops of the west coast of North Uist, and although he did not work there in detail, he visited it many times.
The structure revealed on the west coast of North Uist is a major asymmetric synform of F3 Laxfordian age with numerous parasitic folds (named on fig. 3) which are themselves of considerable scale (up to a kilometer in wavelength). In common with the major structure of Uist, the synforms tend to be tighter and show more deformation than the broad box-like anti forms .
Smaller scale structures are ubiquitous, parasitic on the intermediate scale folds. In the centre of the major synformal structure there is a kilometer wide north westerly trending belt of upright, regu1arly planar rocks in which boudinage structures are abundant, and the rare minor folds extremely tight and strongly flattened. The north western and south eastern borders of the belt contain large amphibolites which, though containing much interbanded acid gneiss, are uniquely thick as composite units. The planar belt lies between two smaller scale synforms whose axial surfaces are inclined downwards towards each other (fig. 3). Minor folds parasitic on these synforms also show a downward converging fan, and, more significantly, show opposed senses on either side of the belt of planar rocks, suggesting that it is an isoclinal antiform of the same age as the minor folds (F 3), the large amphibolites are parts of a single unit, repeated by the isocline.
The Tigharry isocline is an odd-looking structure, the Geological survey did not believe in it – it will be interesting to look for ourselves. Certainly its curious geometry is mimicked by smaller scale structures  on the coast which seem to be associated with re-folded boudins. The F3 fold structures of the North Uist Coast are mostly buckle folds modified by later homogeneous strain. Although the overall structure of the coast is given by these structures, the pile of gneisses folded around them contains second folds, often of considerable scale (tens of meters wavelength).
The refolding is co-axial and the second folds are most easily distinguished in the broad antiformal F3 hinges. Particularly good examples of co-axially refolded F 2 folds are visible at Raikinish Fabric variations associated with the F 2 deformation occur within amphibolites along the coast, and a fabric with increased linearity in fold hinges by the superimposition of F 1 and F 2 strains.
Near Griminish Point at Sloc Roe impressive natural arches and a blowhole are visible On the point are a series of deformed and boudinaged  ‘Scourie Dyke’ age metabasic sheets in grey quartzofeldspathic gneisses. Further northeast is the hinge zone of a major antiform, (Griminish Antiform) The metabasic bodies are still strongly podded and deformed showing that the antiform is a dL3 structure. A little farther northeast at Caisteal Odhair is a partially amphibolitised, granulite facies, 'Scourie Dyke' metabasic pod. Within this body are small Laxfordian shear zones, which were cited by Ramsay and Graham (1970) in a paper defining the geometry and significance of shear zones.

SELECTED REFERENCES (see all MPC listed at rear of guide)
Coward, M P. 1969. The structural and metamorphic geology of South Uist, Outer Hebrides. Unpublished PhD thesis, University of London.
Dearnley, R. 1963. The Lewisian complex of South Harris, with some observations on the metamorphosed basic intrusions of the Outer Hebrides, Scotland. Quarterly Journal of the Geological Society of London, Vol. 119, 243-312. 
Dearnley, R, and Dunning, F W. 1968. Metamorphosed and deformed pegmatites and basic dykes in the Lewisian complex of the Outer Hebrides and their geological significance. Quarterly Journal of the Geological Society of London, Vol. 123, 353-378. 
Fettes, D.J., Mendum, J.R., Smith, D.I. and Watson, J.V. 1992. Geology of the Outer Hebrides. Memoir of the British Geological Survey, 1:100 000 Sheets (solid edition) Lewis and Harris, Uist and Barra (Scotland) HMSO:London. 197pp.
Graham, R H. 1969. A structural analysis of Lewisian rocks in parts of North Uist and the South of Harris, Outer Hebrides. Unpublished PhD thesis, University of London.
Ramsay, J G, and Graham, R H. 1970. Strain variation in shear belts. Canadian Journal of Earth Sciences, Vol. 7, 786-813.
Sibson, R.H. 1977a Fault Rocks and Fault Mechanisms  J’l Geol Soc Lond,133 191-213
Sibson,  R.H. The Outer Hebrides 1977b Thrust : its structure, mechanism and deformationenvironment , Unpublished PhD thesis, University of London