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Geology of the Glen Roy district. Memoir for 1:50 000 geological sheet 63W (Scotland)

R M Key, G C Clark, F May, E R Phillips, B C Chacksfield and J D Peacock

Bibliographic reference: Key, R M, Clark, G C, May, F, Phillips, E R, Chacksfield, B C, and Peacock, D. 1997. Geology of the Glen Roy district. Memoir of the British Geological Survey, Sheet 63W (Scotland).

Contributors: Stratigraphy B W Glover; Hydrogeology W M Edmunds; Geochemistry P J Henney

British Geological Survey

London: The Stationery Office 1997. NERC copyright 1997 First published 1997. ISBN 0 11 884519 5. Printed in the UK for The Stationery Office J27049 C6 11/97

The grid used on the figures is the National Grid taken from the Ordnance Survey map. (Figure 1) is based on material from Ordnance Survey 1:50 000 scale map numbers 34, 41 and 42 with the permission of The Controller of Her Majesty's Stationery Office. © Crown copyright. Ordnace Survey Licence No. GD272191/1997.

(Front cover) Cover photograph: Dissected alluvial fans of Allt Feith Bhrunachan on south-east side of Glen Roy at Brunachan [NN 318 896]; upper parallel roads visible above on shoulder of Leana Mho. (D4624) (Photographer: T S Bain)

(Rear cover)

(Frontispiece) Folded veins of the Toman Liath Vein Complex cutting pelitic schists of the Leven Schist Formation. An axial planar schistosity in the veins extends into the schists as a crenulation cleavage. River Roy [NN 3700 9225] (D4616).

Other publications of the Survey dealing with this and adjacent districts

Books

Maps and atlases

Acknowledgements

The primary survey of the Solid geology in the northern half of the Glen Roy district and the resurvey in the south were carried out by Drs G C Clark, F May and R M Key between 1986 and 1990. Small areas in and around the Great Glen were surveyed by Drs D I Smith and A J Highton. The survey of the Drift geology was undertaken in 1990 and 1991 by Dr J D Peacock. The work was accomplished under the supervision of Drs D J Fettes and D I J Mallick as Regional Geologists.

Most of the memoir was written by Drs Key, Clark and May, but Dr Peacock wrote the Quaternary chapter and Mr B C Chacksfield wrote the final chapter on geophysics. The results of ground geophysical traverses by Mr Chacksfield and Mr K E Rollin are described in this chapter. As well as writing the sections on regional metamorphism and on the thermal aureole around the Strath Ossian Granitic Complex, Dr E R Phillips contributed to the structural geology section of Chapter Six. He describes the results of his detailed petrographical studies and mineral chemical analyses from metasedimentary rocks south-east of the Great Glen. Dr B W Glover contributed to Chapter Four, particularly on the sedimentology. The account of the early basic intrusions includes a contribution by Dr P J Henney on the geo-chemistry of some of the Glen Roy appinites. Dr W M Edmunds contributed to the section on water in Chapter Fifteen. The memoir was edited by Dr Mallick and Dr S G Molyneux.

Grateful acknowledgement is made to the landowners and the people of Roybridge, Invergarry and Fort Augustus for their generous help during the survey.

Notes

Throughout this memoir the word 'district' means the area included in the 1:50 000 Geological Sheet 63W (Glen Roy).

Figures in square brackets are National Grid references within 100 km squares NN and NH.

Five-figure numbers preceded by S are references to thin sections held in the BGS Scottish Collection in Murchison House, Edinburgh.

Numbers preceeded by D and TS refer to photographs in the BGS collection in Murchison House, Edinburgh.

Preface

The Glen Roy district lies at the south-western end of the Monadhliath mountain range in the Grampian Highlands of Scotland. It is an area of outstanding natural beauty. The three main roads in the district are confined to the larger valleys so that the large mountainous tracts form a remote wilderness which attracts a growing number of hill-walkers and mountaineers. Two national nature reserves, in Glen Roy and around Creag Meagaidh, have been established to protect this unspoilt environment which contains geological features long known to the general public. These include the famous Parallel Roads of Glen Roy which represent the former shorelines of glacial lakes that existed during the Quaternary Period.

The memoir presents the first account of the geology of the district and is based mostly on the primary survey undertaken between 1986 and 1991. As well as providing descriptions of the well-known features and deposits related to Quaternary climatic changes, there are summaries of all the facets of a very varied bedrock geology. The district forms part of the root zone of the Caledonian orogenic belt. Much of the memoir is concerned with unravelling the stratigraphy and structure of the Precambrian metamorphosed sedimentary rocks, and elucidating their relations with the numerous igneous rocks intruded at different stages in the tectonic and thermal evolution of the Caledonian mountain belt during late Precambrian and early Palaeozoic times. Postorogenic uplift during the Devonian Period was accompanied by major faulting, with the Great Glen Fault Zone, one of the main British faults, cutting across the north-western part of the district. There is an ongoing debate about the correlation of rocks across this fault zone, and in this memoir the metasedimentary rocks of the Moine Super group found north-west of the Great Glen are described separately from the Dalradian metasedimentary succession on the south-east side of the fault zone.

The geological mapping formed one part of a multidisciplinary investigation of the Glen Roy district, and accounts of geophysical and geochemical studies are also given. Regional geophysical studies provide insight into the nature of the deep crust, and more local studies provided new data on the physical properties of the bedrock. A regional geochemical study of stream sediments was used as an exploration tool for metalliferous mineralisation, and several small anomalies from the district are described. A more localised study of stream waters was used to monitor the effects of bedrock chemistry on the purity of the stream waters. The main natural resource of this underpopulated area is its unpolluted surface water. The memoir will provide much information to help in preserving the unspoilt beauty of the Glen Roy district, and will add to the enjoyment of those who visit the area and wish to know more about how the area has evolved with geological time.

Peter J Cook, CBE, DSc, CGeol, FGS. Director, British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham. NG12 5GG

Geology of the Glen Roy district—summary

The district described in this memoir lies at the southwestern end of the Monadhliath mountain range and includes the major glaciated valleys of Glen Roy, Glen Spean and the Great Glen. The area is chiefly underlain by Neoproterozoic sedimentary rocks, intruded by large granitic bodies and subsequently metamorphosed in a progressive ductile deformation event during the late Proterozoic and early Palaeozoic Caledonian Orogeny. The metasedimentary rocks include altered shallow-water elastic deposits of the Moine Supergroup, within and to the north of the Great Glen Fault which traverses the north-western part of the district. South of this major structure, there is a thick sequence of Grampian Group psammitic and semipelitic lithologies with feldspathic quartzites, in which lateral and vertical facies changes reflect dynamic changes in the original depositional basin where deep- and shallow-water sedimentation occurred. This sequence is overlain by Appin Group pclites and quartzites, laid down during a marine transgression, with increasing amounts of carbonate rock higher in succession. A distinctive suite of metamorphic rocks immediately south-east of the Great Glen Fault Zone underlies an overthrust Grampian Group succession and is of uncertain stratigraphical affinity.

The metamorphic rocks were intruded by a wide variety of igneous rocks, with intrusive activity extending from Ordovician to early Permo-Carboniferous times, a period of late- to post-tectonic regional cooling and uplift. The igneous rocks include early granitic vein complexes, large masses and minor intrusions of calc-alkaline lamprophyric rocks (mostly appinites), large multiphase calc-alkaline granitic masses and contemporaneous vein complexes, and calc-alkaline dyke swarms. The final phase of igneous activity is represented by rare Permo-Carboniferous camptonitc-monchiquite dykes north-west of the Great Glen.

Major faulting during the Devonian Period locally controlled elastic sedimentation; there are Old Red Sandstone deposits within the Great Glen Fault Zone.

The effects upon the pre-Quaternary landscape of repeated glaciations during the Pleistocene are profound, and large areas are blanketed by extensive glacial deposits. The last glacial event, the Loch Lomond Readvance, took place some 11 000 to 10 000 years ago.

(Front cover) Dissected alluvial fans of Allt Feith Bhrunachan on south-east side of Glen Roy at Brunachan [NN 318 896]; upper parallel roads visible above on shoulder of Leana Mho. (D4624) (Photographer: T S Bain)

(Frontispiece) Folded veins of the Toman Liath Vein Complex cutting pelitic schists of the Leven Schist Formation. An axial planar schistosity in the veins extends into the schists as a crenulation cleavage. River Roy [NN 3700 9225] (D4616).

(Geological succession) Geological sequence in the Glen Roy district.

Chapter 1 Introduction

Area and physical features

The north-western corner of the district contains lower Glen Garry on the north-west side of the Great Glen. However, most of the Glen Roy district lies at the southwestern end of the Monadhliath mountain range, and includes the high ground between Glen Tarff in the north and middle Glen Spean in the south (Figure 1). In the central and eastern parts of the area, the high ground is dominated by the peaks of the Creag Meagaidh and Corrieyairack ranges, some of which exceed 900 m above sea level. South of Glen Spean and west of Loch Treig, the ground rises steeply towards the 'Grey Corries' north of Ben Nevis. Similarly steep ground on the east side of Loch Treig rises towards several 'Munros', including Aonach Beag (1114 m) immediately adjacent to the south-eastern part of the district.

A well-developed drainage system in the north flows north-eastwards into the North Sea, either through Loch Ness or along the Spey valley. The drainage system in the remainder of the district flows south-westwards, either through Loch Lochy or along Glen Spean. The main rivers and lochs occur in ice-carved valleys, and a major influence on the district's landscape has been the Quaternary glaciations. The interplay between glacial processes and landform development is discussed in Chapter Fourteen.

Much of the area is open land, used either for deer stalking or forming parts of nature reserves (Glen Roy and Creag Meagaidh), with agriculture and forestry confined chiefly to Glen Spean, the Great Glen and Glen Gloy. The district forms part of Ordnance Survey 1:50 000 Landranger sheets 34, 41 and 42.

Regional setting and summary of the geology

The Glen Roy district lies wholly within the Caledonian orogenic belt, and is mostly underlain by metasedimentary rocks assigned to the Neoproterozoic Moine and Dalradian supergroups which were deformed, metamorphosed and cut by a variety of igneous rocks during the late Proterozoic and early Palaeozoic. Metasedimentary rocks in the north-west part of the district comprise altered, shallow-water deposits of the Loch Eil Group, the youngest of the three Moine groups (see Harris and Johnson, 1991). These are separated by the Great Glen Fault Zone (Figure 2) from Dalradian rocks which comprise a wider variety of altered sediments (Johnson, 1991). Grampian Group sediments at the exposed base of the Dalradian sequence infilled an early Dalradian basin prior to regional subsidence which heralded deposition of the main Dalradian sequence (Glover and Winchester, 1989). In the district, the Grampian Group comprises a thick turbiditic clastic sequence overlain by shallow-water, tidal and deltaic deposits, now represented by metasandstones (psammites) and metamorphosed silty sandstones (semipelites). The basal part of the overlying Appin Group consists of tidal shelf deposits, laid down during a marine transgression caused by regional subsidence. They are overlain by deeper-water sediments, mainly sandstones and shales but including carbonates, which form the youngest part of the succession in the district (see Hickman, 1975; Anderton, 1985). There were primary sedimentary variations in thickness and lateral changes in facies; in addition ductile deformation resulted in tectonic thickening and thinning of strata (see Bailey, 1934; Anderton, 1988). Thus the Fort William Slide, which defines the western limit of an attenuated Appin Group stratigraphy in the general area of Fort William has been interpreted as an intra-Appin Group unconformity (Glover, 1992); as a listric fault which defined the western limit of the original sedimentary basin (Anderton, 1988); and as a major ductile shear (Bailey, 1934).

The Grampian Group metasedimentary rocks are separated by ductile shears from structurally underlying metasedimentary rocks of uncertain stratigraphical affinity. These include the fault-bounded, lower-grade meta-sedimentary rocks beneath the Eilrig Shear Zone immediately south-east of the Great Glen Fault Zone, as well as the dominantly psammitic sequence beneath the Gairbeinn Slide of Haselock et al. (1982). Subvertical ductile shears within the Ossian–Geal Charn Steep Belt separate off a different lithological sequence in the south-eastern corner of the district. This sequence includes a limestone-pelitic schist-amphibolite association, initially thought to be equivalent to the Ballachulish succession of the Appin Group (Anderson, 1956) but lithologically more similar to the basal Ord Ban Subgroup of the Grampian Group (Winchester and Glover, 1988).

The Moine and Dalradian sequences have both suffered polyphase ductile deformation with accompanying regional metamorphism. Early, north-eastward trending recumbent folds (nappes), first recognised by Bailey (1934), affect the Moine, which here forms part of the Sgurr Beag Nappe, and also the Appin and Grampian groups in the southern part of the district, south-west of the Strath Ossian and Corrieyairack granitic complexes (Figure 2). These folds are overprinted by upright, north-eastward trending folds which are more widely developed. Ductile shearing accompanied the folding, with thrusting towards the north-west along the Eilrig Shear Zone, contemporaneous with the peak of regional, amphibolite facies metamorphism. Folded Dalradian rocks may have been transported up to 60 km northwestward on the Eilrig Shear Zone. Neither the sense nor the amount of movement has been proved on the other major shears shown in (Figure 2).

Within the district, intrusive igneous rocks of varied ages and different types occur in the orogen. Early granitic rocks, which share the tectono-thermal history of their host metasedimentary rocks, are represented by granitic gneiss within the Moine succession. Pre-tectonic metagabbroic rocks (amphibolites and hornblende schists) are also confined to this north-western area. More widespread in the district are late- to post-tectonic (Ordovician to early Devonian) igneous rocks which include granite vein complexes and approximately contemporaneous appinite complexes, as well as major granitic masses that thermally altered their folded meta-sedimentary country rocks.

Major block faulting in the orogen was initiated in late Silurian times as deep crustal fracturing at the time of continental collision, following the closure of the Iapetus Ocean (Watson, 1984). This faulting continued into the early Devonian (Smith and Watson, 1983) to accompany regional uplift and clastic sedimentation. Red, coarse-grained deposits found in the district within the Great Glen Fault Zone were laid down in fault-controlled basins at this time. Contemporaneous (early Devonian) magmatism with high-level igneous intrusions and accompanying volcanicity took place in the Ben Nevis and Glen Coe district (Bailey and Maufe, 1960). Dykes from two centres, at Etive and Ben Nevis, extend into the district south-east of the Great Glen Fault Zone. In contrast, the youngest of the igneous rocks, Permo-Carboniferous camptonite-monchiquite dykes, are found only on the north-west side of this fault.

Previous research

Geological surveying of Sheet 63W commenced at the beginning of the century with mapping of the southwestern area of the Glen Roy district, the work overlapping from the primary survey of sheets 53 and 54 (Bailey and Maufe, 1960; Hinxman et al., 1923). The results of this early work were included in a regional synthesis describing the tectonic evolution of the south-western part of the Grampian Highlands (Bailey, 1934). A rapid geological survey of the south-eastern part of the district, subsequently extended northwards, was undertaken in the 1930s and the results were reported by Anderson (1956). The primary survey of Sheet 62 was undertaken intermittently between the years 1950 and 1963, and included the mapping of the western margin of Sheet 63W. Sheet 62E was published in 1975 without an accompanying memoir.

Thereafter, the main geological input into the Glen Roy district was through university research (Glover, 1989; Haselock, 1982; Hickman, 1972; Okonkwo, 1985; Parson, 1982; and papers based on these theses quoted in the relevent parts of the present memoir), although BGS surveyed a narrow strip of ground south-east of the Great Glen between 1980 and 1983 as part of the Great Glen Project. Review papers on the sedimentation and tectonics of the Dalradian metasedimentary rocks (Anderton, 1985; Thomas, 1979) provide alternative interpretations to that of Bailey (1934) but were based chiefly on studies undertaken outside the Glen Roy district. University research has been undertaken on the acid vein complexes and intrusions (Clayburn, 1981), and the Strath Ossian Granitic Complex was studied during investigations for radioactive waste disposal (Henderson, 1982).

The Parallel Roads of Glen Roy, regarded as glacial lake shorelines, were first described by Pennant (1776) and are a geomorphological feature that has aroused considerable interest since that time, giving rise to more than 90 scientific publications. Quaternary deposits of the southern part of the district were mapped during the early primary surveys, but, in contrast, very little has been published on the Quaternary in the north of the sheet. Recent research has concentrated on the deposits and landforms created during the late Devensian Loch Lomond Re-advance (see Chapter Fourteen for a fuller review of all previous work on the Quaternary).

Chapter 2 Moine

Metamorphosed sediments of the Moine succession crop out over a large area of the Northern Highlands and extend from the trace of the Moine Thrust eastwards and southwards as far as the Great Glen. Some of the metasedimentary rocks south-east of the Great Glen were previously assigned to the Moine (Johnstone, 1975; Harris and Pitcher, 1975) but recent interpretations place these rocks in the Grampian Group, a thick succession of psammitic and semipelitic rocks that are conformable with the overlying Appin Group of the Dalradian. Harris et al. (1978) and Haselock (1982) proposed that both groups should be regarded as part of the Dalradian Supergroup mainly because of the locally conformable boundary between them. However, the status of the Grampian Group with reference to either the Moine or Dalradian successions continues to be a subject for debate. Stratigraphical continuity exists between the Grampian Group and the Appin Group. Similarities of age and lithology between the Grampian Group and the Moine suggest the possibility of resurrecting some sort of correlation between the two in the future. For the following account, rocks that are unequivocally assigned to the Moine occur only north-west of the Great Glen. Rocks tentatively correlated with the Moine also occur within the melange of intensely brecciated rocks that make up the Great Glen Fault Zone.

The Moine rocks were originally laid down as elastic sediments, probably in a shallow-water marine environment, but subsequent metamorphism has converted the sandstones to quartzofeldspathic psammitic rocks, the shales to mica schists (pelites) and the sandy shales or siltstones to quartz-mica-schists (semipelites). Calcareous sediments, probably originating as concretions, are represented by thin beds and lenses of talc-silicate rock. The talc-silicate layers form a very minor constituent of the total Moine succession, but regionally they are of significance because of their value as indicators of metamorphic grade (Kennedy, 1958).

Distinctive marker horizons are rare within the Moine succession and, over large areas of the Moine outcrop, metamorphism and several phases of folding have obscured the sedimentary evidence that would enable the order of deposition to be determined. Only locally have stratigraphical successions been established. In western Inverness-shire, the Moine rocks were divided into three tectonostratigraphical units, termed the Morar, Glenfinnan and Loch Eil divisions (Johnstone et al., 1969). This broad three-fold division has been extended, with some local variations, to most of the Moine outcrop in the Northern Highlands. Recent work (Roberts et al., 1987; Harris and Johnson, 1991) has led to the proposal that the original three units should be regarded as formal lithostratigraphical groups, with only minor amendments to their descriptions for which the names Morar, Glenfinnan and Loch Eil should be retained. In the area of Sheet 63W to the north-west of the Great Glen, only representatives of the Loch Eil Group are exposed; the rocks have been assigned to that group because of the continuity of their outcrop with that of the type area to the south-west.

Loch Eil Group: Upper Garry Psammite Formation

Lithology

In the Glen Roy district, the Loch Eil Group is made up of a thick, monotonous sequence of grey psammites; other rock types are either not present as major constituents of the group, or do not have outcrops sufficiently wide to be mapped separately. The psammitic rocks vary from a pale grey, sparsely micaceous psammite to a dark blue-grey, highly micaceous psammite containing a relatively higher proportion of biotite. The highly micaceous psammite grades laterally or across strike into sparsely micaceous psammite, and every gradation between the two main rock types occurs. It was found to be impracticable to map out the different psammite rock types mapped separately.

Quartz and feldspar (plagioclase and/or potassium feldspar) are the main constituents of the psammitic rocks and are present in almost equal proportions. They are of uniform, equigranular grain size, rarely exceeding 2 mm in maximum diameter. Variations in grain size are not significant and pebbly horizons, formed by concentrations of larger fragments of quartz and feldspar, have not been recognised. Muscovite, when present, is always less abundant than biotite. The main accessory minerals include garnet, zoisite, epidote, sphene, apatite and iron oxide. The biotite content of the psammitic rocks, as well as defining gross lithological variations between sparsely micaceous and highly micaceous rocks, also varies within each rock type. Biotite-rich laminae, 1 to 3 mm thick, alternate with layers of similar thickness in which the biotite content is less, giving the rock a layered fabric and imparting a colour banding that defines the foliation. Such colour banding is interpreted as representing the original sedimentary bedding. Sedimentary cross-bedding structures are rare but when present are defined by the attitude of the biotite laminae. The biotite content of the laminae may increase to form thin micaceous layers, 2 to 5 mm thick, that form prominent planes of parting. Such micaceous layers may thicken and pass into thin beds of pelite, rarely more than 0.5 m thick, which are interbedded with the psammite but are impersistent along strike.

Thin talc-silicate layers occur throughout the psammite outcrop but form a very minor constituent of the total succession. The layers, 2 to 10 cm thick, give rise to light grey bands at outcrop and usually contain visible garnet and amphibole. They consist of quartz, varying amounts of plagioclase (andesine or bytownite), and at least some of the following minerals, in varying proportions: zoisite, epidote, garnet, biotite and hornblende. The bands are either lenticular or parallel-sided and some may be traced for several metres along strike. The talc-silicate layers are always concordant to the bedding of the surrounding psammite. The concordance and along-strike continuation of the layers suggests a sedimentary origin and they are interpreted as calcareous concretions formed during the process of diagenesis following sedimentation.

Narrow discontinuous pods of hornblende schist occur throughout the psammite outcrop but are more common north of the River Garry. These hornblendic bodies vary in thickness from a few centimetres to masses more than 1 m thick; all have sharp concordant contacts with the surrounding psammites. An igneous origin is favoured for the hornblende schists. They were intruded prior to the regional metamorphism and deformation, when all the original igneous texture was obliterated. Such meta-igneous bodies are a characteristic feature of both the Loch Eil and Glenfinnan groups.

Stratigraphy

Sedimentary structures are rarely preserved in the Moine rocks and an order of deposition has not been established. In the absence of evidence to the contrary, it is assumed that the psammitic rocks lie in their order of deposition.

The psammite is structurally underlain by the Fort Augustus granitic gneiss, with an interbanded but sharp contact between the two rock types. The contact is more or less parallel to the foliation in both the psammite and granitic gneiss. The granitic gneiss is interpreted as originating as a granite intruded into Moine sedimentary rocks that were undergoing metamorphism at the time of the intrusion (Barr et al., 1985). The psammite extends north and west beyond the limits of Sheet 63W and is continuous with thick psammitic sequences termed the Loch Eil Psammite (Johnstone et al., 1969; Strachan et al., 1988) and the Upper Garry Psammite (Roberts and Harris, 1983). This extensive, thick succession of monotonous psammite was probably laid down during a single continuous period of sedimentation, in a shallow marine or possibly fluvio-deltaic depositional environment (Strachan, 1985). The thin semipelitic and pelitic layers represent short periods of decreased elastic sedimentation but are of no stratigraphic significance.

Structure

The Moine psarnmitic rocks have variable but generally low to moderate dips. South of Loch Garry, the main foliation (parallel to bedding) of the psammite trends approximately north–south, with a moderate, locally steep dip to the west. North of Loch Garry, in the area of

Loch Lundie [NH 2910 0310], the foliation trends approximately north-eastwards with a low dip to the north-west, i.e. there is a broad swing in the foliation trend. This swing is approximately parallel to the boundary of the psammite with the structurally underlying Fort Augustus granitic gneiss.

Structural evidence from outside the area indicates that the Moine psammitic rocks have undergone a history of deformation involving early phases of recumbent folding and later phases of upright folding (Strachan, 1985). In the north of the Glen Roy district, minor folds of tight, recumbent style and later open folds are represented, but major folds of either early or late deformational phases have not been recognised. It thus appears that the Moine psammitic rocks form a thick, uninverted sequence, some 1.5 km in structural thickness, which youngs towards the north-west with the lowest part of the succession overlying the Fort Augustus granitic gneiss.

Early minor folds are tight to almost isoclinal in style and fold the primary lithological handing. Locally, in layers of very micaceous psammite, an axial-planar schistosity is developed in the fold hinges. The axial plunge of the minor folds is generally less than 30°, and the axes may plunge towards either the north or south.

Later open minor folds are less common than the early folds, but occur throughout the area. These folds are upright with vertical or steeply dipping axial planes trending approximately east–west, and low to moderate axial plunges, chiefly to the west. These late minor folds have wavelengths up to the order of several metres.

Metamorphism

The metamorphic rocks of the Glen Roy district have provided little information on the tectonothermal evolution of this part of the Sgurr Beag Nappe. Metamorphic index minerals, e.g. aluminosilicates, are notably absent from the semipelitic rocks of the Upper Gary Psammite Formation. Information from the adjacent Invermoriston district (May and Highton, in preparation), however, indicates that the rocks reached middle to upper amphibolite facies conditions contemporaneous with the earliest (late-Precambrian) phase of deformation in the nappe. This resulted in the development of a gneissose segregation, and localised migmatisation, in the semipelitic lithologies (and Fort Augustus Granitic Gneiss; see Chapter Seven). A relict early pyroxene + bytownite + hornblende + garnet assemblage has been recorded in calc-silicate rocks from the Invermoriston district. However, the hornblende + andesine + garnet + epidote assemblages recorded in the Glen Roy district reflect recrystallisation under lower amphibolite facies conditions during the period of upright folding associated with Ordovician reworking of the nappes.

Metamorphic rocks in the Great Glen Fault Zone

The metamorphic rocks within the Great Glen Fault Zone (i.e. between the Great Glen and Glen Buck faults) form an extensive, elongated outcrop with a maximum width of 2.0 km, which is bounded by subparallel faults of unknown but probably considerable displacement. Published descriptions of the metamorphic rocks are few. Eyles and MacGregor (1952), when describing the Great Glen Crush Belt, drew attention to rocks involved in the fault deformation 'that were possibly originally gneisses'. In the same paper, a description of the metamorphic rocks within the Great Glen Fault Zone north of Sheet 63W was given by J G C Anderson who recognised 'a wide belt of intensely crushed rocks which appear to have been, originally, granulites and semipelitic schists of Moinian type. The bedding or schistosity has been largely obscured and the characteristic flagginess of such rocks is absent. Chlorite has been developed on a widespread scale'. A more complete account of the rocks described by Anderson was given by Parson (1982), who descibed the along-strike equivalents in the area immediately north of Sheet 63W as a mixed assemblage of metasedimentary and meta-igneous rocks.

Lithology

The metamorphic rocks in the Great Glen Fault Zone consist of quartzofeldspathic psammites which are locally gneissose. Semipelitic rocks are very rare; Parson (1982) recorded one exposure of a semipelite, at locality [NH 351 030]. Pelitic rocks appear to be absent. The psammitic rocks are intruded by white and pink acid pegmatites. The metamorphic rocks have suffered widespread, complete, intense brittle deformation as a result of movement within the Great Glen Fault Zone and there are very few exposures which yield specimens sufficiently undeformed to provide evidence of the original features of the main rock types.

The dominant rock type is a mica-poor siliceous psammite that is gneissose in some outcrops, and which locally includes relatively thick layers of quartzite. In specimens where the effects of the brittle deformation are least, thin sections of the psammite show it to be composed of abundant quartz, less common sodic plagioclase and subordinate potassium feldspar. Muscovite is an uncommon constituent but is more abundant than biotite which may show complete alteration to chlorite and iron oxide. The quartz and plagioclase display a granoblastic texture, the quartz showing undulose extinction and a crude, dimensional shape orientation. The plagioclase is always sericitised to some degree with muscovite crystals developing along the cleavage planes. Some crystals show albite twinning. The potassium feldspars are unaltered and show irregular intergranular shape. The muscovite and less common biotite occur as discrete isolated crystals that are dimensionally oriented but are insufficiently abundant to form continuous mica-rich layers. They impart the lithological banding, in shades of grey, to the rock.

The quartzites recognised in the field differ from the psammites described above only in the increase in quartz with concomitant decrease in feldspar content, the quartz forming more than about 70 per cent of the rock by volume. They should be described more properly as quartzose psammites rather than quartzites. Micas are rare, muscovite being the more abundant, and well-defined mica-hearing foliae are absent.

The mineralogy of the gneissose psammites appears to be similar to that of the psammites, but the quartz and feldspars are more coarsely crystalline and muscovite is slightly more abundant, forming thin micaceous foliae in some thin sections.

Rare outcrops of a dark green schistose amphibolite within the psammite rock have not been identified in the Glen Roy district, but were recorded by Parson (1982) in the area to the north on Sheet 73W. He considered them to represent meta-igneous rocks, occurring as thin sheet-like intrusions that are impersistent along strike. The intrusions are foliated, the foliation being parallel to the margins that, in places, are intensely deformed and displaced by numerous slip planes. In thin section, Parson (1982) reported the typical mineral assemblage to be hornblende, biotite, chlorite, epidote (zoisite), labradorite (An65), calcite and sphene with subordinate quartz.

The psammitic rocks are intruded by numerous pegmatitic and granitic veins, usually not more than 20 to 30 cm wide, which occur throughout the outcrop and, locally, are sufficiently abundant to form a vein network (see Chapter Ten).

Stratigraphy

The metamorphic rocks within the Great Glen Fault Zone are separated from other mapped lithological units by faults of unknown, but considerable, displacement. Direct stratigraphic correlation is therefore not possible. The local development of a gneissose fabric is a feature that cannot be observed in other psammitic rocks of the Glen Roy district. The overall lithological composition, however, of mica-poor psammite, does bear comparison with major lithological units identified on both sides of the Great Glen. The presence of gneissose psammitic rocks, the abundant granitic veins, and the presence north of the Glen Roy district of rare hornblende schist bodies, provides the evidence for a tentative correlation of the psammitic rocks with the Moine rocks north-west of the Great Glen.

Structure

At outcrop, the psammite occurs as an apparently homogeneous, grey-weathering rock in which there is rarely evidence for the presence of a bedding or foliation planar structure. On the weathered surface, numerous thin fractures and slip surfaces can be recognised, separating the rock into irregularly shaped fragments of varying size. The fractures are infilled with chlorite, or less commonly, carbonate, and may in places include dark grey-black, finely comminuted rock material. Freshly broken outcrop surfaces are dark green in colour, due to the rock breaking along such chlorite-filled fractures. Fresh surfaces of the psammitic rock are difficult to obtain as breakage always occurs along fracture planes. Despite the intense brecciation, the rock is lithified and forms positive physical features, the unit responding to erosion and weathering in the same manner as less fractured rock units.

The original bedding/foliation is only rarely observed, and when seen shows a variable orientation so that no persistent trend can be traced from one exposure to another. Where it occurs, the foliation is recognised as a lithological layering, appearing at outcrop as grey colour banding, formed from mica-rich foliae alternating with quartz-feldspar foliae. In general, the majority of the foliation strike measurements lie in the north-east quad- rant but changes in foliation trends from north–south to east–west can occur in an abrupt and haphazard manner. Such disorientation of foliation is considered to be the result of disruptive movement of rock masses of exposure size. It is concluded that the whole rock outcrop has been disrupted into blocks and fragments of varying size, from the scale of exposure to small 2 to 3 cm fragments, that have rotated relative to each other as a result of movement in the crush zone, so that their original attitude cannot be determined.

Chapter 3 Dalradian: Grampian Group

Much of the Monadhliath mountains is underlain by metamorphosed psammitic and semipelitic rocks assigned to the Gra mpian Group (formerly Grampian Division, Piasecki, 1980). In the Glen Roy district, these rocks occur south-east of the Great Glen Fault Zone where they consist mainly of psammitic rocks, including thick feldspathic quartzites, together with thin pods and layers of calcsilicate rock. The base of the group is defined by ductile tectonic breaks which separate it from underlying metamorphic rocks of uncertain stratigraphical affinity. In the northern part of the Glen Roy district, this basal tectonic contact is defined by the Eilrig Shear Zone and Gairbeinn Slide (Figure 2). The top of the Grampian Group is exposed in central and southern parts of the Glen Roy district where it is overlain by pelitic and quartzitic metamorphic rocks of the Appin Group. Locally a conformable boundary between the two groups is modified tectonically, and there is also a local lateral facies change between the highest formation in the Grampian Group (Dog Falls Psammite Formation) and the basal formation of the Appin Group (Loch Treig Schist and Quartzite Formation). In the extreme south-east, rocks forming the upper part of the Grampian Group are in tectonic contact with rocks of uncertain stratigraphical affinity within the Ossian–Geal Charn Steep Belt. The total thickness of the Grampian Group successions in the district varies up to about 5250 m; thickness variations are both a primary feature and also due to subsequent deformation.

The lack of geographical continuity caused by the emplacement of the large Corrieyairack Granitic Complex, coupled with lateral facies changes within the Grampian Group, have necessitated the adoption of local formation names to the south-east and north-west of the granitic intrusion. These two successions are described separately, with the north-western area corresponding to Area 2 and the south-eastern area corresponding to Areas 3 and 4 (Figure 3). Three of the uppermost formations can be traced with confidence across the outcrop of the Corrieyairack Granitic Complex.

North-west of the Corrieyairack Granitic Complex (Area 2)

North-west of the Corrieyairack Complex, the Grampian Group is made up of a conformable succession of meta-sedimentary rocks at least 3000 m thick. The succession is folded about the Stob Ban and Creag a' Chail synforms (Figure 10), and there are marked differences in litho-logical character between the eastern and western limbs of these two folds. The eastern succession is dominated by a psammitic sequence, at least 2500 m thick, with intercalated units of interbanded semipelite and psammite. The succession identified on the western limb is lithologically more heterogeneous, and is made up of an interbanded succession of semipelite, psammite and quartzite.

The base of the Grampian Group to the north of the Corrieyairack Complex is defined by a tectonic break which is known as the Eilrig Shear Zone on the western fold limb (see Chapter Six) and the Gairbeinn Slide on the eastern limb (Haselock et al., 1982). The sequences of metasedimentary rocks below these tectonic breaks, dealt with in Chapter Five, have no proven stratigraphical link with the Grampian Group and their lithostratigraphical status is unknown, although Haselock et al. (1982) regarded the Glenshirra Succession, described from outside this district and below the Gairbeinn Slide, as representing a lower succession within the Grampian Group. The boundary of the Grampian Group with the overlying metasedimentary rocks of the Appin Group is a sharp lithological contact, partly conformable and partly defined by the Fort William Slide.

Lithology

The Grampian Group is made up of a conformable succession of psammites, feldspathic quartzites and semipelites. Each rock type forms mappable units that can be traced for several kilometres along strike, but may also occur interbanded with other rock types. All rock types show rapid lateral facies changes, and boundaries between rock types are nearly always gradational; rapid transitions can be recognised, from quartzite, to psammite, to micaceous psammite, to semipelite.

The psammites are quartz-feldspar-biotite rocks with an obvious well-developed planar foliation. They can be flaggy or massive at outcrop, with parting planes developed along thin micaceous laminae, 1 to 2 mm thick, which may be spaced at 20 cm intervals. Biotite is disseminated throughout the rock but the content is variable and imparts a colour banding. The colour banding, and the micaceous laminae, are interpreted as representing original bedding, virtually undisturbed by the development of the metamorphic foliation. Cross-bedding and other laminated sedimentary structures are locally common.

Sequences have been recognised, several hundred metres thick, in which semipelite is the dominant rock type. It is always well foliated, the foliation being defined by thin parallel lenses of micaceous foliae. Such banding is almost entirely of secondary tectonic origin, and original bedding structures are preserved only within thin intercalated psammites.

Rock units occur in which semipelite and psammite are present in almost equal proportions, with the two lithologies forming thin interbeds, 5 to 20 cm thick. As such, individual rock types cannot be mapped separately and formations of mixed lithologies have therefore been recognised.

Calc-silicate layers occur in both the psammitic and semipelitic rocks. They vary greatly in abundance but occur throughout the Grampian Group succession. The calc-silicates are mostly lenticular; they are typically up to 50 mm thick but taper out along strike within 1 to 2 m. Some calc-silicate layers include large green hornblende porphyroblasts showing poorly defined dimensional orientation. The calc-silicate layers are interpreted as representing metamorphosed calcareous concretions.

At three levels within the Grampian Group, quartzite forms thick mappable units in the west and south-west of the area that can be traced for several kilometres along-strike; they thin rapidly and taper out when traced northwards to Creag a' Chail [NN 403 959]. Nearly all the quartzites are banded, with prominent planes of parting parallel to bedding, defined by thin mica-rich laminae up to 3 mm thick. The quartzites may be intimately inter-banded with beds of psammite and semipelite. On freshly broken surfaces, the quartzites are white in colour, but weathered surfaces of some lenses show a pinkish tinge due to the presence of a significant amount of pink potassium feldspar.

Lithostratigraphy

A lithostratigraphy has been established for the Grampian Group north-west of the Corrieyairack Complex (Figure 4). Although there is along-strike continuity of lithological units, there is rapid lateral facies change and the proportions of different rock types that make up the total succession in any one area vary greatly. In the west of the area, around Glen Gloy and Glen Roy, sedimentary structures show that the succession youngs consistently to the south-east, and the Grampian Group has been divided into seven formations (Table 1). Some thin and taper out along strike; the thick quartzites that define three of the formations thin when traced northeastwards and pass laterally into an interbedded sequence of psammite and semipelite.

Coire Nan Laogh Semipelite Formation

The rocks of this formation were first described by Haselock et al. (1982). The formation only occurs east of the axial trace of the Creag a' Chail fold (Figure 10) and is exposed on the south-western slopes of Gairbeinn [NN 459 980]; the base of the formation is marked by the Gairbeinn Slide (Haselock et al., 1982). The actual contact with the slide is not well exposed in the sheet area but its trace can be located to within approximately 100 m on the south-eastern slopes of Gairbeinn [NN 459 983].

The formation consists of a lower gneissose semipelite, up to 150 m thick, made up of coarse quartz-feldspar segregations separated by highly micaceous (chiefly biotite) foliae. Garnet is abundant and thin talc-silicate lenses are present. The basal units locally have a blastomylonitic fabric, formed during movement along the Gairbeinn Slide. This lower migmatised semipelite unit is overlain by an interbanded sequence, at least 350 m thick, of thin micaceous psammite beds, up to 20 cm thick, alternating with semipelite layers, both rock types being more or less equally abundant.

Sedimentary structures were not recorded and the stratigraphical position of the formation is based upon younging directions in the overlying Glen Doe Formation. The interbanded assemblage forming the upper part of the formation marks an upward lithological transition from semipelite rocks to the thick massive psammites of the overlying formation.

Glen Doe Psammite Formation

This formation comprises of a monotonous sequence of psammitic rocks, at least 2500 m thick and makes up at least half of the Grampian Group succession east of the Creag a' Chail Synform. The outcrop is disrupted by the Sronlairig Fault, but the psammite has been traced beyond the northern edge of the sheet and it underlies large areas of Sheet 73E (Foyers) and 73W (Invermoriston). To the south, the formation is interrupted by the Corrieyairack Granitic Complex, but a massive psammite exposed south-east of the complex and termed the Loch Laggan Psammite Formation (see below) is probably its lateral equivalent.

The lower boundary of the formation is poorly exposed, but is located south-east of Geal Charn [NN 444 989], where semipelite layers of the underlying Coire nan Laogh Semipelite decrease in abundance and the succession passes into a dominantly psammite assemblage. The boundary is defined at the base of the first massive psammite bed more than 1 m thick. The upper boundary is exposed in a stream section [NH 431 010] and is defined by the rapid increase of semipelite beds, up to 50 cm thick, which mark the rapid transition to the overlying Tarff Banded Formation.

The formation is made up of massively bedded psammite, with parting planes up to 1 m apart and defined by the presence of thin micaceous laminae. Thin semipelite beds are uncommon but do increase in abundance to impart a flaggy character to the psammite, well displayed within about 200 m of the upper and lower boundaries. Cale-silicate lenses are very uncommon and the rarity of this rock type is a distinctive feature. Sedimentary structures, particularly cross-laminations, are present in the outcrop south of the Sronlairig Fault and demonstrate a consistent younging direction to the north-west.

Allt Goibhre Semipelite Formation

South of Coire an t-Sidhein, this formation forms the lowest part of the succession in Glen Gloy and is separated from the Eilrig Shear Zone (defining the lower boundary of the Grampian Group) by a major fault exposed in the Allt Goibhre [NN 284 932]. Outcrops of garnetiferous semipelitic rock north-west of the fault are probably faulted elements of this formation.

The Allt Goibhre Semipelite Formation, up to 400 m thick, is chiefly made up of semipelite, with numerous talc-silicate lenses that contain conspicuous hornblende porphyroblasts. The talc-silicate lenses, generally less than 50 mm thick, occur within layers of grey micaceous psammite averaging about 10 cm in thickness. The semipelite of the formation is a garnet-biotite-schist in which quartzofeldspathic microlithons are separated by films of mica. A bed of flaggy feldspathic quartzite, more than 10 m thick, occurs interbedded with psammite in which well-preserved trough cross-lamination has been identified, confirming that this formation lies stratigraphically below the Auchivarie Psammite Formation.

The Allt Goibhre Semipelite can be traced for some 5 km to the north-east where it passes laterally into a sequence of grey psammites that are indistinguishable from the psammitic rocks of the overlying Auchivarie Psammite Formation.

Auchivarie Psammite Formation

The Allt Goibhre [NN 289 930] provides an almost continuous section through this formation. In a small tributary, its basal contact with the Allt Goibhre Semipelite, defined by an abrupt change from semipelite to psammite, can be located to within 5 cm. In the Allt Goibhre, the formation is at least 300 m thick and consists almost entirely of grey psammite with a regular parallel-bedded structure. The rock is fine grained, and micaceous partings lying parallel to the bedding and spaced at intervals of 2 to 10 cm give the formation a flaggy appearance. The grey colour is due to the presence of disseminated biotite, reflecting the mud content of the original sediment. Beds, generally less than 0.5 m thick, are picked out by colour variation and some appear to be slightly graded. Cross-bedding has not been detected and talc-silicate rock is absent from the Allt Goibhre section.

Thin sections ((S92462), (S92484)) show that the psammite contains almost as much muscovite and biotite, in well-aligned flakes, as rocks classified as semipelite. However, the psammites are finer grained and the mica is evenly disseminated, a texture which contrasts with its segregation into films separating quartzofeldspathic microlithons in semipelites, giving the latter their characteristic laminated lenticular structure. The psammite contains approximately equal quantities of quartz and plagioclase, and small (less than 1 mm) garnets are common.

Flaggy, fine-grained grey psammite can be followed to the north-east from the type area at Auchivarie. At Beinn Bhan [NN 332 965], where the semipelitic rocks of the Allt Goibhre Formation taper out, the Auchivarie Psammite Formation has a thickness of more than 500 m and contains thin lenses of talc-silicate rock throughout the succession. Interbedded units of semipelite occur, and to the north-east these increase in abundance. To the north of the Allt Dubh Appinite [NN 370 960], the lower part of the formation passes laterally into the sequence of interbanded semipelites and psammites of the Tarff Banded Formation. The upper part of the Auchivarie Formation can be traced to Cam Leac [NN 467 977], where it is equivalent to the Cam Leac Psammite described by Haselock et al. (1982).

Glen Gloy Quartzite Formation

The base of the Glen Gloy Quartzite Formation is well exposed at several localities south-west of Auchivarie in the River Gloy, for example at [NN 291 928] where it is marked by a transition, generally less than 20 m thick, in which quartzite is interbedded with grey psammite. Individual beds of quartzite have sharp planar boundaries with adjoining beds of grey psammite.

The Glen Gloy Quartzite is up to 270 m thick and is an important marker which has been traced from the lower part of Glen Gloy to the margin of the Allt Dubh Appinite, a distance of 14 km. The quartzite is feldspathic, typical of the Grampian Group, and the beds, generally less than 1 m thick, are colour laminated parallel to the bed contacts. Trough cross-bedding, in sets 3 to 10 cm thick, is commonly displayed on the clean, scoured surfaces in the River Gloy and its tributaries. Synsedimentary folding is rare but has been recorded, for example at [NN 2851 9200] where laminations in a few of the beds are isoclinally folded. Intervals of muddy sedimentation are represented by lenses of psammite and semipelite. The former shows flaser bedding, grey cross-laminated ripples being draped by fine-grained, dark grey psammite a few millimetres thick. Channels have not been recognised but evidence of penecontemporaneous erosion occurs at one locality [NN 2776 9119] where the lower part of a bed of grey psammite, 3 m thick, contains abundant clasts of semipelite. The clasts, up to 30 mm in length, have a 'spiky' shape and are commonly folded, probably as a result of compaction before lithification. At many localities, the micaceous psammite and semipelite beds have a well-developed layered structure, some of the layers containing porphyroblasts of hornblende. Pale-coloured talc-silicate rock, also with abundant large hornblende porphyroblasts, cross-cuts the layering and indicates that diagenetic concretions developed within the calcareous beds.

The Glen Gloy Quartzite Formation thins out towards the north-east, its lateral equivalents being grey psammites that merge into a thickened Auchivarie Psammite Formation.

Tarff Banded Formation

North of Coire Odhar Beag [NN 372 970], the Auchivarie Psammite Formation passes laterally into the interbedded semipelites and psammites of the Tarff Banded Formation. The latter can be traced northwards as far as Glen Tarff where it is displaced sinistrally about 6 km by the Sronlairig Fault. The Tarff Banded Formation consists of an interbedded sequence of thin micaceous psammites and semipelites, with subordinate beds of psammite and quartzite. Only locally are the psammitic and quartzite beds sufficiently thick and continuous along strike to be mapped separately. The formation has an apparent thickness of at least 2700 m, its base being defined by a tectonic break, the Eilrig Shear Zone.

The formation is chiefly made up of thin beds of psammite, 5 to 30 cm thick, alternating with only slightly thinner layers of semipelite. Locally, either rock type may form units several metres thick, but only rarely can individual beds be traced for any distance along strike. The psammitic and semipelitic fractions are approximately equal in abundance but, in general, the psammitic lithologies become more abundant as the formation is traced to the south-west. Semipelite is the common rock type in the area of Meall an Odhar [NN 396 003]. Southeast of Eilrig, in the Allt Lagan a' Bhainne [NN 377 994], the lowest exposed unit of the formation, directly overlying the Eilrig Shear Zone, is a micaceous psammite in which talc-silicate lenses with hornblende porphyroblasts occur. The psammite includes thin semipelitic beds which increase in abundance, and there is an upwards transition into a mixed assemblage of interbanded psammite and semipelite.

Calc-silicate lenses occur throughout the formation. They are interbedded with both the psammitic and semipelitic units. Such talc-silicate lenses are rarely more than 5 cm thick. A distinctive feature of them, particularly common in the lower part of the formation, is the presence of large hornblende porphyroblasts which lie within, or near to, the plane of foliation but lack strong linear dimensional orientation.

Quartzite beds occur within the formation but few can be traced for any distance along strike. On the northwestern slopes of Beinn Shan [NN 324 972], a quartzite with a maximum thickness of some 150 m has been traced for at least 3 km. Other impersistent horizons of quartzite occur in upper Glen Tarff.

In the lithostratigraphical succession proposed by Haselock et al. (1982), a succession of psammitic and semipelitic rocks that forms extensive but isolated outcrops in the Invermoriston (73W) and Foyers (73E) districts was named the Monadhliath Semipelite Formation. Although there is no continuity of outcrop, the Tarff Banded Formation is regarded as the lateral equivalent of that formation. The Tarff Banded Formation is overlain by a psammitic unit termed the Cam Leac Psammite by Haselock et al. (1982). The present mapping indicates that the Cam Leac Psammite is laterally equivalent to the upper part of the Auchivarie Psammite Formation, the latter being the lithostratigraphical term used in this account.

Glen Fintaig Semipelite Formation

The passage between the Glen Gloy Quartzite and Glen Fintaig Semipelite formations is exposed in the River Gloy, [NN 2716 9044] and in several streams on the north-western side of the Glen. The passage, generally about 10 to 20 m thick, is marked by the interbanding of flaggy quartzite and semipelite. The beds of semipelite are 1 to 3 m thick and have sharp contacts (top and bottom) with quartzite.

The Glen Fintaig Semipelite Formation is more than 1000 m thick, and is well exposed in the River Gloy and the Fintaig Water. Continuous sections also occur in many of the side streams, for example in the Allt nan Reigh [NN 280 912]. In all these sections, the semipelite has a lenticular laminated structure due to the segregation of the quartzofeldspathic and micaceous components. It contains garnets with quartz-filled pressure shadows which, together with the pervasive laminated structure, are aligned parallel to an intense mica fabric. These secondary tectonic structures are, in most exposures, parallel to the primary bedding.

Although predominantly semipelitic, beds of garnetiferous micaceous psammite are abundant, and parts of the succession in the Fintaig Water consist mainly of thin-bedded psammite. Cross-bedding is very rarely seen, and talc-silicate rock is absent in the type area though it has been recorded to the north-east. Lenses of thin-bedded (less than 0.5 m) quartzite, lithologically identical to the underlying Glen Gloy Quartzite Formation, and up to 5 m thick, are common.

As the Glen Fintaig Semipelite Formation is traced north-eastwards, its central part becomes increasingly psammitic. The formation thins towards the north-cast, and at the hinge of the Creag a' Chail Synform it is no more than 300 m thick. East of the fold hinge, there is rapid lateral transition to psammitic rocks indistinguishable from those of the underlying Auchivarie Formation. Psammite and semipelite are well exposed on Meall a' Chomhlain and continuously in the Allt Teanga Bige [NN 320 946]. The psammite has micaceous laminae, ± 1 mm thick, spaced at 1 to 20 mm intervals. The amount of disseminated biotite is variable, giving a colour banding in shades of grey. All gradations occur, from psammite through micaceous psammite to semipelite. The layering (0.1 m to 1.0 m) clearly represents original bedding. The internal lamination is mainly parallel to the bedding, but cross-lamination in sets 10 to 25 cm thick has been found at several localities. An example of trough cross-bedding, forming a coset 1 m thick and sets 20 cm thick, occurs on Meall a' Chomhlain [NN 3176 9419]. Flagginess is a typical feature, dependent on the schistosity lying parallel to the bedding. In the Allt Teanga Bige, the regional flaggy appearance gives way to a more massive appearance in the section between the axial traces of late folds, where schistosity is strongly crenulated. This contrast in appearance is clearly a secondary structural feature of no sedimentological significance. The proportion of rock which can be described as semipelite is very variable. Some parts of the formation, several hundred metres thick, are predominantly of semipelite with bedding picked out by thin lenses of micaceous psammite. An intense, bedding-parallel schistosity (biotite fabric) is deflected around garnets. Calc-silicate stripes in psammite and semipelite have been found, but appear to be uncommon or absent in most places. They are typically up to 50 mm thick and usually taper out along strike within 1 m.

In most places lenses of feldspathic quartzite are a minor constituent of the succession, but along its entire strike length the uppermost 500 m of the Glen Fintaig Semipelite Formation is composed of interbanded quartzite and semipelite in approximately equal amounts. On freshly broken surfaces the quartzite is white, but on weathered surfaces it is pinkish and obviously feldspathic. A premetamorphic clastic texture is preserved and original grain-size variation from bed to bed can be detected. The pale pink colour of the quartzite contrasts with the grey of the psammite. Jointing in the quartzite gives sharp angular blocks which are resistant to weathering. The psammite, in contrast, weathers to give more rounded slabs and blocks.

Beinn Iaruinn Quartzite Formation

The boundary between the Glen Fintaig Semipelite and Beinn Iaruinn Quartzite formations is drawn where quartzite reaches approximately 80 per cent of the succession. The Beinn Iaruinn Quartzite Formation has a maximum thickness of 1260 m, which diminishes to only 120 m on Creag a' Chail [NN 403 959].

Micaceous (muscovite and minor biotite) partings separate the quartzite into beds mainly less than 0.5 m thick. Most beds are parallel-laminated but cross-lamination is quite common and good examples occur in the Allt Dearg [NN 3323 9142]. Here the sets are 10 to 20 cm thick and an apparent unidirectional palaeocurrent flow towards the north-east is evident from horizontal surfaces. One bed, 0.5 m thick, is cross-laminated in its lower part and convoluted in the top 15 cm. Most of the quartzite originated as mud-free arkosic sandstone and is white in colour with greyish laminae, although some thin beds are of darker colour due to the presence of a small amount of biotite. Much disseminated feldspar, mainly microcline, is invariably present and a few of the thicker beds contain streaks, up to several centimetres thick, of coarse-grained pink feldspar. The feldspar crystals are augen-shaped, 1 to 3 mm long, and are deformed detrital grains which, together with parallel-oriented muscovite, define a tectonite fabric lying parallel to the bedding. Beds of semipelite with sharp planar contacts against quartzite are very common.

Brunachan Psammite Formation

The upper part of the Beinn Iaruinn Quartzite Formation includes more psammitic beds towards the southwest, so that in the Coire nan Eun area [NN 29 89] it is difficult to draw a boundary with the psammitic rocks of the Brunachan Psammite Formation. Rocks included within the latter formation lack the beds containing coarse-grained pink feldspar which are typical of the Beinn Iaruinn Quartzite. Along strike to the north-east, the base of the Brunachan Psammite is well defined by this change of lithology.

Disruption of bedding into fold mullions is very common, and it is therefore difficult to estimate the original thickness of this formation. In the south-west, the width of the outcrop is controlled by the presence of large-scale folds (Appin Synform, Bohuntine Antiform).

Good examples of sedimentary structures are preserved and well displayed in the River Roy e.g. [NN 303 887] where three subfacies occurring in cycles, each up to several tens of metres thick, can be recognised. Subfacies (a) consists of lenticular-bedded grey psammite and quartzite. The beds, 20 to 50 cm in thickness, show trough cross-bedding and channels can be recognised. A few of the beds show synsedimentary folding. Subfacies (b) is more thinly bedded with ripple cross-lamination. Subfacies (c) is also thinly bedded, but in contrast to the above it is parallel-bedded. Each cycle (a–b–c) represents deposition under conditions of decreasing energy, with subfacies (c) deposited from sus pension. The psammite in the Allt na Reinich [NN 331 904] is almost entirely of subfacies (c). The general environment was probably marine in a zone transitional between coastal sand spread and open shelf. Shallow conditions below wave base are indicated.

The bedding of the psammite is marked by colour variation from mid-grey to pale-grey. The colour contrasts are due mainly to variations in the content of disseminated biotite, reflecting the mud content of the original sediment. Calc-silicate nodules are absent, and instead a characteristic feature is the widespread presence of hornblende porphyroblasts. They are up to several centimetres long, almost completely altered to biotite and chlorite, and commonly display a bow-tie texture on bedding surfaces. Their distribution is very patchy, even within a single bed, and is probably related to the composition of the cement present in the sandstone before metamorphism.

March Burn Quartzite Formation

Beds of quartzite at the top of the Brunachan Psammite Formation mark the transition into the March Burn Quartzite Formation, which forms the uppermost part of the Grampian Group in Glen Roy. The March Burn [NN 338 909] provides a section through the formation, which is about 100 m thick. It consists of feldspathic quartzite which is thinly bedded, breaking into slabs 10 to 20 cm thick along muscovitic parting planes. Cross-bedding has not been found. Some of the beds contain a small amount of biotite and a few can be described as grey psammite rather than quartzite. Locally, for example in the Allt na Reinich [NN 332 903], the March Burn Quartzite consists mainly of flaggy grey psammite. However, the absence of the quartzite in some sections is the result of displacement on a strike-parallel fault.

In the River Spean, 10 km to the south-west (Sheet 62E) there is a passage upwards from the lateral equivalent of the March Burn Quartzite Formation into the Spean Viaduct Quartzite Formation (Appin Group). The latter is lithologically distinct, being a recrystallised quartz-arenite in contrast to the feldspathic quartzite (white psammite) typical of the March Burn Quartzite and other Grampian Group quartzites (Glover and Winchester, 1989). The Spean Viaduct Quartzite is absent in Glen Roy, the March Burn Quartzite being separated from the Leven Schist Formation (Appin Group) by a very sharp contact which is exposed in the March Burn and several other stream sections on the south-east side of Glen Roy. In each case, it can be located to within 1 cm and has been interpreted as the Fort William Slide (Anderton, 1988). However, although the quartzite close to the contact is extremely schistose, the texture cannot be described as mylonitic and the contact is probably a strained sedimentary boundary.

The Beinn Iaruinn Quartzite, Brunachan Psammite and March Burn Quartzite formations merge northeastwards into a single unit of interbanded psammite and quartzite, some 260 m thick, that can be traced round the fold closure at Creag a' Chail. The continuation of this unit to the south-east of the Corrieyairack Granite Complex, at Meall Ptarmigan [NN 425 904], provides the basis for correlation with the Inverlair Formation of the Grampian Group in that area (Figure 4).

South-east of the Corrieyairack Granitic Complex and the southern part of the district (Area 3 and Area 4)

Within this area, the Grampian Group comprises interbedded psammitic and semipelitic lithologies, with more siliceous rocks occurring less frequently. A lithostratigraphical terminology different from that above is used (Figure 4) and (Table 2), based upon the lithostratigraphy of Winchester and Glover (1988). It should be emphasised that all formation boundaries are transitional. Lateral as well as vertical facies changes become increasingly common up the succession. Concomitant with this primary lithological variation are lateral thickness changes; the maximum thickness for the Grampian Group in this area is about 5250 m.

Lithological units in the extreme south-east have not, as yet, been assigned lithostratigraphical names. This is primarily because their main development lies farther east in adjacent parts of Sheet 63E, where mapping is largely incomplete, so that their type areas have not yet been defined. Tectonic complications also mean that their position in the regional lithostratigraphy is, as yet, uncertain. These rocks are described in a later section (p.24).

Loch Laggan Psammite Formation

This lowest exposed formation underlies an area of about 25 km2 around Loch Laggan, where good crag exposures and incised stream sections abound. The most accessible exposures occur along the northern shore of Loch Laggan and include artificial cuttings along the A86 between Moy and the car park at Rubha na Magach [NN 456 849], immediately east of the sheet (see Hambrey et al., 1991).

That part of the formation within the Glen Roy district has a total thickness of about 2500 m, which compares with a thickness of about 3200 m for the whole formation (Okonkwo, 1985). The formation is dominated by flaggy micaceous psammites.

The base of the Loch Laggan Psammite Formation is not seen in the district, although Okonkwo (1988) shows a conformable contact with the underlying Coire nan Laogh Semipelite Formation immediately to the east. There is a gradual increase in the proportion of semipelites towards the top of the Loch Laggan Formation and the boundary with the overlying Ardair Semipelite Formation is taken where semipelites become the dominant lithology. This transition is also well exposed south of Loch Laggan, e.g. near Lochan An Tuirc [NN 427 806], and north of Loch Laggan on the steep south and east slopes of Creag Meagaidh, including Coire Ardair [NN 432 880].

The main lithology of the Loch Laggan Formation is a medium- to thickly bedded micaceous psammite; each bed contains pelitic schist laminations and commonly shows grading upwards into semipelite or pelite. The micaceous psammites are medium-grained (quartz and feldspar grains from 0.2 to 0.5 mm in diameter and micas up to 1 mm in length), speckled (' salt and pepper' texture) rocks, mostly grey but with pink-weathered tints to the south of Loch Laggan. There is a gradual decrease in the average bed thickness up the succession. Beds in the lower half of the formation, to within 1000 m of its top boundary, are mostly between 30 and 150 cm thick. The overlying 600 m consists mostly of more fissile beds, about 20 cm thick on average, whereas in the topmost 400 m the beds average about 10 cm in thickness. Concomitant with the decrease in bed thickness is an increase in the proportion of semipelitic and pelitic laminations and thin beds. These micaceous beds are up to 15 cm thick. Consequently the topmost part of the formation is characterised by a striped unit (Moy Member of Glover and Winchester, 1989) of finely interbedded micaceous psammites and semipelites/pelites. Good examples of this striped unit are exposed at [NN 4119 8389] and alongside the A86 west of Moy (see Glover and Winchester, 1989, for logged sections).

White talc-silicate pods are common throughout the formation and are up to 10 cm thick and 100 cm long (Plate 1). Their oblate shapes are, at least in part, due to post-diagenetic flattening. They are coarser grained than their host psammites, with stellate hornblende blades in a white, coarse, feldspathic groundmass which locally contains red garnet porphyroblasts. The pods are aligned parallel to bedding and occur mainly within more psammitic beds.

The dominant sedimentary structure is the fine micaceous planar lamination, accentuated by alignment of the metamorphic mica flakes. Individual beds are invariably tabular; minor cross-cutting relationships, including channelling, are confined to basal parts of the formation to the east of Sheet 63W. Low-angle (i.e. cross-beds dip at angles less than 15° to the bedding plane, in part due to tectonic flattening) tabular cross-laminations are exposed at [NN 4155 8237], [NN 4100 8438], [NN 4315 8208], with trough cross-laminations at [NN 4374 8167] and [NN 4514 8326]. Ripple cross-laminations, convolute bedding, rip-up clasts and scouring can be seen on the water-washed surfaces on the north shore of Loch Laggan (e.g. (Plate 1)).

The normal, graded beds of this formation commonly exhibit well-developed Bouma sequences (Bouma, 1962) indicative of turbidite sedimentation. Beds of intermediate thickness exhibit more or less complete Bouma sequences, Tabcde, while thin beds commonly comprise Tae Tbe, Tce and thicker beds Ta, Taband Tabe (see Okonkwo, 1985; Glover, 1989; Glover and Winchester, 1989; Hambrey et al., 1991).

The psammites are composed of quartz, sericitised plagioclase and less common potassium feldspar, biotite with less common muscovite, microcline and accessory apatite, opaque minerals, garnet, epidote, chlorite, sphene, zircon and calcite (e.g. in (S76883), (S76888), (S8000)). Brown biotite flakes define the foliation and are variably altered (commonly along cleavage planes) to chlorite. Parallel muscovite flakes may accompany the biotite in foliae, with late muscovite porphyroblasts overprinting this fabric. Quartz is the dominant mineral phase, occurring as embayed strained grains of various sizes but mostly slightly larger than accompanying feldspar grains. Garnets occur as clusters of small (about 0.3 mm diameter) anhedral grains, locally including quartz.

Both the Loch Laggan and Glen Doe formations consist of massive psammites at least 2500 m thick. However, those of the Loch Laggan Formation are chiefly very micaceous and include a significant proportion of thin semipelitic and pelitic layers, white talc-silicate lenses are common throughout. In contrast, the psammites of the Glen Doe Formation are thickly bedded and are only markedly micaceous, with thin semipelite intercalations, near the upper and lower boundaries of the formation. Calc-silicate lenses are not common. Despite these appa rent lithological differences the two formations are regarded as lateral equivalents, separated by the Corrieyairack Granitic Complex.

Ardair Semipelite Formation

This formation consists of finely interbedded semipelitic schists and micaceous psammites which crop out as a continuous belt, helping to define the Loch Laggan Antiform, between Lubvan [NN 446 790] in the southeast and Coire Ardair [NN 435 880]. From Coire Ardair, the formation continues north-eastwards across the poorly exposed Allt Coire Ardair valley to the crags at Coire a' Chriochairein [NN 445 894]. It then strikes north-eastwards through Sròn Garbh Choire for about 2 km as far as the Corrieyairack Granitic Complex. It is correlated with the Tarff Banded Formation north of the granite. North of the A86, the Ardair Formation forms the middle slopes of Creag Meagaidh which is capped by an incomplete development of the Creag Meagaidh Psammite Formation. South of the main road, the formation underlies less well-exposed lower ground.

The upper boundary of the formation is with thinly bedded psammitic flagstones of the Creag Meagaidh Formation. On the eastern slopes of Creag Meagaidh and south of the A86, this upper contact is sharply defined by a change from semipelite to psammite. However, immediately north of the A86 it is more difficult to define. In this area the Ardair Formation comprises almost equal amounts of micaceous psammite and semipelite in discrete beds (Figure 5), and the overlying Creag Meagaidh Formation is made up predominantly of graded micaceous psammite beds with semipelitic tops. Bed thicknesses are similar in both cases.

In the type area at Coire Ardair, the formation is 770 ± 70 m thick (based on outcrop width and dip variations). This thickness is constant along the whole length of the northern limb of the Loch Laggan Antiform. In the south-east near Lubvan it is thinner, about 400 m.

The formation comprises finely interbedded semipelitic schists and micaceous psammites, as discrete beds (couplets) and as normal graded units with psammitic bases passing upwards into more pelitic tops. Individual beds are less than 30 cm thick, except for rare massive psammitic beds up to 150 cm thick. The micaceous psammites and semipelites are identical in terms of grain size, texture and structure to those of the Loch Laggan Formation. Again, the micaceous psammites have pink tints in the south-east. Good examples of graded beds are exposed at [NN 4272 8600] and [NN 4431 8707], and finely interlayered (ribbed on weathered surface) sequences are well exposed alongside the A86 in the Craigbeg area [NN 407 816]. Calc-silicate pods, identical to those described from the Loch Laggan Formation, are common. Non-micaceous psammites are rare and confined to thin (less than 50 cm thick in most cases) beds at [NN 4024 8210], [NN 4504 8967], [NN 4102 8422] and [NN 4190 8575]. In the north, the upper part of the formation is predominantly semipelitic.

Graded bedding and interlayering of micaceous psammite and semipelitic schist are the principal sedimentary structures. Planar, low-angle cross-laminations were seen only at [NN 4165 8489], [NN 4153 8179] and [NN 4300 7958]. Beds are tabular and less commonly lenticular (e.g. at [NN 3978 8442] and [NN 4200 8488]). At Craigbeg [NN 4136 8180], a local unconformity with a low angular discordance is exposed.

Thin sections e.g. (S76901), (S76886), (S80617) show equigranular assemblages of strained quartz, sericitised plagioclase (An.57 in one rock), brown biotite and muscovite (highly variable modal content) with clusters of pink, anhedral garnets and accessory sphene, apatite, opaque minerals, zircon and rare carbonate. Chlorite (penninite) commonly replaces biotite. The calcsilicates have plagioclase rather than quartz as the principal modal component, with garnet and clinozoisite. Okonkwo (1988) noted that these rocks are hornfelsed within 400 m of their contact with the Corrieyairack Granitic Complex (see Chapter Ten). Granitic streaks and lenses have formed as a result of partial melting on Creag a' Bhanain [NN 433 911]. Rare andalusite is present within the aureole of the Strath Ossian Granitic Complex (see Chapter Ten).

Creag Meagaidh Psammite Formation

This formation of flaggy micaceous psammites underlies the summit plateau of Creag Meagaidh and can be traced north-eastwards through

Coire Ardair and the Sron Garbh Choire area [NN 440 900] to strike westwards into the Corrieyairack Granitic Complex at Meall Ptarmigan [NN 426 904]. Southwards, it forms a continuous narrow outcrop around the Loch Laggan Antiform through Craigbeg to Lubvan. The upper part of the Creag Meagaidh Formation is very well exposed north of the A86, all along the eastern cliff's of Beinn a' Chaorainn [NN 390 850]. Consequently, the boundary with the overlying fissile semipelitic schists of the Clachaig Formation is easily defined immediately below the summit spine of this mountain. Immediately south of the A86, the upper part of the Creag Meagaidh Formation is intruded by the Strath Ossian Granitic Complex. However, in the south-east there is a poorly exposed boundary with pelitic and semipelitic gneisses of the Meall Cos Charnan Formation.

The thickest development of the Creag Meagaidh Formation is on the eastern side of Beinn a' Chaorainn where it has a total thickness of about 765 m. Further south, at Lubvan, it is about 640 m thick. There is a northwards reduction in thickness and at Meall Ptarmigan [NN 428 909] the formation is only about 150 m thick.

The dominant lithology is a grey to dark grey, micaceous psammite with a strong fissility that breaks the rocks into flags, mostly less than 10 cm thick (Plate 2). It is laminated on a millimetre scale, with white feldspathic and parallel muscovite and biotite laminae and lenses. Grading on a scale of several centimetres is common, from fine- to medium-grained micaceous psammite upwards to dark grey semipelitic schist (less than 20 per cent of the graded unit). This lithology is well exposed in the quarry [NN 3977 8133] alongside the A86.

However, there are mappable lateral lithofacies changes. South of Lubvan, the succession comprises a basal 170 m of the dominant flags, overlain by about 200 m of more massively bedded micaceous psammites containing semipelitic schist laminations, with an uppermost 270 in of assorted micaceous psammites, semipelites, psammites and rare feldspathic quartzite (at locality [NN 4449 7841]). Farther north, on the eastern cliffs of Beinn a' Chaorainn, the main flaggy psammites are associated with thicker, graded beds (Tab or Tad of the Bouma sequence) with a mean thickness of about 16 cm, and massive psammites with bed thicknesses of mostly between 30 and 60 cm but locally up to 200 cm. These thick beds are commonly graded and are most common to the northeast of the summit of Beinn a Chaorainn (Figure 6). For the most part, beds have planar contacts, although lensing of individual beds, e.g. at [NN 3942 8418], and irregular, eroded contacts, e.g. at [NN 3800 8449], have been observed.

Garnetiferous, white talc-silicate lenses, common south of the A86, decrease in importance northward and are rare on Creag Meagaidh. In the aureole of the Strath Ossian pluton, all lithologies become gneissose e.g. (S76919).

Thin-section examination of the micaceous psammites show the same mineral assemblages and textures as other Grampian Group micaceous psammites (e.g. in (S76910), (S76923), (S80012), (S80094), (S82236). Dominant quartz grains are strained and may show a weak shape-alignment parallel to a mica foliation. Plagioclase grains are strongly sericitised. Biotite to muscovite ratios vary, with late muscovite plates overprinting the shape-orientated flakes. Ilmenite, apatite and zircon are accessories with local trails of apatite e.g. in (S76923).

Clachaig Semipelite and Psammite Formation

The north-trending ridge west of the Allt na h-Uamha [NN 403 845], north of the Strath Ossian Complex, is capped by semipelitic schists of the Clachaig Semipelite and Psammite Formation. The formation wedges out northwards and is laterally equivalent to semipelites and pelites of the Meall Cos Charnan Formation to the south-east.

The basal contact between micaceous psammites of the Creag Meagaidh Formation and semipelitic schists of the Clachaig Formation is well exposed near the top of the eastern cliffs, from Meall Bhaideanach [NN 39 83] to Coire Buidhe [NN 38 86]. However, inter-beds and lenses of micaceous psammite and massive, cross-bedded psammite persist within the lower part of the Clachaig Formation, e.g. at [NN 3848 8625], [NN 3844 8644] and [NN 3833 8549]. The top of the formation is poorly exposed on the gentler western slopes of the mountain. A transitional upper contact is well exposed only in the area of the cairn [NN 3804 8741] where pink, fissile psammites and feldspathic quartzites form the basal part of the overlying Inverlair Formation.

The thickness of the Clachaig Formation increases southwards from zero to about 350 m near the cairn [NN 3804 8741], to about 580 m at Coire Buidhe, and to about 820 m south of Beinn a' Chaorainn where some tectonic thickening has taken place.

North of Coire Clachaig [NN 38 83], the principal lithologies are pale to dark grey semipelitic schists and thinly laminated micaceous psammites which are commonly interbedded on a centimetre scale. More massive interbeds (less than 10 cm thick) of psammite and quartzite are confined to basal and upper parts of the Clachaig Formation in the north. White calcareous pods, less than 10 cm thick, are confined to upper parts, in the stream draining west of Coire Odhar [NN 3721 8480].

The Clachaig Formation is lithologically more diverse south of Coire Clachaig. Basal semipelitic schists are overlain by a thick sequence of interbedded, laminated semipelites and micaceous psammites. The latter sequence containing a pink and white feldspathic quartzite, up to several metres thick, which forms a useful marker bed. Laminated micaceous psammites are dominant in the upper part of the formation, with thin (less than 1 m thick) pink psammitic beds appearing towards the top of the formation. White calcareous lenses are confined to basal beds. Red garnets are present in the semipelites, mostly above the middle metaquartzite marker. Quartz-leaf fabrics and quartzofeldspathic lenses within the mica fabric are common close to the contact with the Strath Ossian Complex, as well as farther north e.g. at [NN 3785 8610]. The various lithologies are identical in thin sections to their counterparts in older formations (e.g. in(S76929), (S80606), (S80616), (S80625). Winchester and Glover (1988) reported apatite bands 2 and 3 mm thick in the semipelites, which they interpreted as altered phosphatic bands. Alternatively, they could originally have been heavy mineral bands.

Meall Cos Charnan Semipelite Formation

Anderson (1956) showed a wedge-shaped extension of his Monadhliath Schists (equivalent to the Monadhliath Semipelite of Haselock et al., 1982) protruding from the east side of the Strath Ossian Granitic Complex at Meall Cos Charnan [NN 433 775]. These rocks are here referred to as the Meall Cos Charnan Semipelite Formation and are regarded as lying stratigraphically above the Monadhliath Semipelite. They are laterally equivalent to the Clachaig Formation; the Strath Ossian Complex separates the outcrop of the two formations.

There are numerous good crag exposures of semipelitic and pelitic schists and gneisses around Meall Cos Charnan and on Meall Luidh Mor, some 3 km to the north-west. Elsewhere the formation is poorly exposed.

A sharp boundary between semipelitic gneisses and micaceous psammites of the Creag Meagaidh Formation is exposed in the Allt Cam at [NN 4448 7791], and on an adjacent crag [NN 4406 7837]. The contact appears to be conformable, with pegmatite sheets locally emplaced along this plane. A similar, bedding-parallel, sharp upper contact with micaceous psammites, lateral equivalents of the Inverlair Formation, is exposed at locality [NN 4333 7662].

The outcrop area is severely affected by the intrusion of the Strath Ossian Granitic Complex and by regional folding, making estimation of thickness difficult. Locally, the succession is about 750 m thick.

The dominant lithology is a coarse-grained, laminated gneissose semipelite, comprising white quartzofeldspathic and black biotite-rich seams and pods up to sev eral millimetres thick. Quartz and mica grains have strong shape orientations which define planar and linear fabrics. Within the semipelites, there is a 160 m thick, silver-coloured, muscovitic gneiss (metapelite) which is well exposed on Sron Allt Fearna [NN 4408 7792]. Muscovite flakes occur with biotite in micaceous seams. Pale grey calcareous seams are common in the Meall Ludh Mor area.

The outcrop of the Meall Cos Charnan Formation is wholly within the thermal aureole of the Strath Ossian Granitic Complex; the coarser gneissose fabrics are a direct result of the contact metamorphism. Andalusite and garnet porphyroblasts are readily visible in the field and thin sections reveal the presence of cordierite and sillimanite which are also thermal mineral growths (see Chapter Ten). Induration related to the contact metamorphism has produced ribbed outcrop surfaces, with protruding quartzofeldspathic seams. The calcareous rocks of Meall Luidh Mor have common epidote associated with hornblende, carbonate, garnet, quartz and plagioclase. There is widespread alteration of plagioclase to sericite and biotite to chlorite. Zircon, apatite and opaque minerals are ubiquitous accessories in all lithologies (e.g. in (S76921), (S76925), (S80015), (S80026)).

Inverlair Psammite Formation

This formation consists of psammites and feldspathic quartzites with interbedded layers of semipelite. The formation is a useful marker in the southern half of the district; its component white or reddish metaquartzites can be traced south-south-eastwards from the eastern side of the Corrieyairack Granitic Complex near locality [NN 400 900], to Inverlair and Meall Laire [NN 333 787]. Throughout its strike length, the Inverlair Formation is well exposed in hillside crags and in numerous stream sections. The most impressive exposures are in the Inverlair gorge of the River Spean, either side of the Fersit road bridge [NN 3408 8055]. Exposures on Meall Laire are largely hidden by a conifer forest, although new track cuttings through the forest compensate for the lost exposures.

In the north, the thickness of the formation is generally between about 410 and 700 m. However, it is reduced to about 170 m on the north-east side of a small synform near Meall Ptarmigan, where the formation is directly overlain by the Leven Schist Formation. South of the Corrieyairack Granite Complex, the formation is tectonically thickened in the core of the Inverlair Antiform.

Laminated micaceous psammites, similar to those of the Inverlair Psammite Formation, occur north-west of the Corrieyarick Granite Complex where they are about 500 m thick. Folding has resulted in a much wider outcrop there than to the south-east of the intrusion. A weak colour banding is visible in exposures of the psammites, but the dominant structure is a schistosity marked not only by parallel orientation of biotite flakes but also by the segregation of biotite into discontinuous laminae about 1 m thick. The layering and schistosity are generally subparallel, but in places the layering, presumably bedding, is tightly folded and the schistosity is axial planar to the folds. The tectonic fabric again becomes more dominant towards the contact with the Corrieyairack Granitic Complex, which suggests that this intrusion may occupy the site of a major tectonic break.

Augen-shaped grains of quartz seen in the psammites north-west of the Corrieyairack Granitic Complex may be relict detrital granules. No cross-bedding has been found, and any present in the original sediment has almost certainly been destroyed during deformation. In most places, talc-silicate lenses are rare or absent but a few have been found, for example at locality [NN 3775 9116] where they have reddish brown centres, greenish epidotic margins and are commonly boudinaged.

In the north, the Inverlair Formation is generally overlain by fissile micaceous psammites of the Dog Falls Psammite Formation. South of the Corrieyairack Granitic Complex, the Inverlair Formation forms the core of the upright Inverlair Antiform. On its north-west limb, the psammites are overlain by interbedded semipelites and micaceous psammites comprising the Loch Treig Schist and Quartzite Formation of the Appin Group. Unfortunately, the boundary between the two formations is not well exposed and could, at least locally, be tectonic. At localities [NN 3232 8134] and [NN 3243 8127], there are ductile shears with quartz-chlorite veins and pods along the boundary. On the south-east side of the antiform, the Inverlair Formation passes south-eastwards into garnet schists of the Loch Treig Formation. The boundary is not exposed. In the core of the fold, micaceous psammites of the Eilde Flags conformably overlie psammites of the Inverlair Formation, although there is a lateral transition between these two formations, both of which in places directly underlie the Loch Treig Schist and Quartzite Formation.

North of the A86, the formation is composed predominantly of thinly bedded, grey and pink psammites and feldspathic metaquartzites, both with semipelitic interlayers. Close to both the lower and upper boundaries of the formation, the rocks are notably fissile due to fine inter-layering of the three lithologies. Sedimentary structures within beds are rare; possible low-amplitude ripples are exposed at locality [NN 3695 8606]. Individual psammite and quartzite beds up to 2 m thick, are fine to medium grained, with pink tints caused by weathered interstitial feldspar grains. Internal lamination is diffuse on a centimetre scale, with minor biotite flakes disseminated throughout the beds.

Southern exposures, in the Inverlair and Meall Laire areas, are dominated by grey to off-white, massive psammites and feldspathic quartzites, superbly exposed in the Inverlair gorge section along the River Spean (see (Plate 3)). Individual beds are mostly up to 1 m thick, with inter-beds of dark grey, semipelitic or pelitic schists which are up to 8 m thick (in the Inverlair gorge section). Garnetiferous calc-silicate pods are rare and were only seen at locality [NN 3422 7902]. Sedimentary structures are ubiquitous in good exposures of the massive psammites and impure quartzites. These commonly consist of either planar cross-bedding e.g. at [NN 3310 7850] and [NN 3402 8069], or trough cross-bedding, e.g. in the Inverlair gorge. Small-scale ripples are preserved on the underside of a psammite bed at [NN 3469 8144] and indicate currents from the north-west. Scouring is exposed at [NN 3492 8135] with soft sediment slumping at [NN 3356 7956]. The psammites of the Inverlair gorge are frequently arranged in upwards-fining packages, up to 10 m thick, with erosive bases.

Thin-sections (e.g. samples (S80086), (S80104), (S80131), (S80137), (S80607), (S82223)) reveal quartz-rich ground-masses with up to about 10 per cent feldspar in the quartzites and over 10 per cent feldspar in the psammites. The quartz grains may be recrystallised with triple junction contacts, or strained and sutured, as well as shape-oriented in a mica foliation. The feldspars are strongly sericitised; staining shows altered plagioclase and potassium feldspar. The main mica phase is biotite with variable chlorite alteration. Muscovite flakes are shape-oriented in the biotite fabric or form poikiloblastic overgrowths. Common accessories include zoned zircon, apatite, sphene, epidote and opaque minerals (dust and anhedral grains) which may define heavy mineral bands. Rare metamorphic garnets are replaced by biotite and, more commonly, chlorite. Garnets in heavy mineral hands may he detrital. The semipelitic interlayers consist of aligned biotite and muscovite flakes with local parallel shape-orientation of quartz, opaque minerals and plagioclase grains. Secondary chlorite is again patchily developed. Andalusite is present in very small amounts in semipelitic schists in the Invelair gorge, within the aureole of the Strath Ossian Granitic Complex.

Eilde Flags Formation

Metamorphosed psammitic rocks of this formation are confined to three separate outcrop areas west of Loch Treig. Between the Allt nam Bruach and Sgurr Innse [NN 290 748], the formation is well exposed in the main streams and on the mountains, where it overlies the Inverlair Psammite Formation in the core of the Inverlair Antiform (see Chapter 6). Farther east, there is a smaller but still well-exposed outcrop strip south-west from Meall Cian Dearg [NN 332 759] where the flags form the core of the complementary Blackwater Synform. Isolated exposures of similar psammitic flags are also found immediately west of the Strath Ossian Granitic Complex. These exposures can be traced southwards where they were informally referred to as the Corrour granulites (Hinxman et al., 1923). The Eilde Flags are stratigraphically overlain by the Loch Treig Schist and Quartzite Formation, and within the Inverlair Antiform this superposition is maintained. However, within the Blackwater Synform, due to early recumbent folding, the stratigraphical order is reversed so that the flags actually overlie schists, i.e. the synform is downward facing (see Bailey, 1934; Hickman, 1978).

On the south-eastern limb of the Inverlair Antiform, a sharp boundary between psammitic flagstones and quartz-mica-schists of the Loch Treig Formation is exposed along the Allt Laire, and locally may be tectonic, e.g. at locality [NN 3376 7859] where the psammites have a strong platy fabric. A similarly abrupt contact is exposed on the south-eastern side of Cruach Innse [NN 2825 7554], where the schists form a cliff face rising above a plateau underlain by psammites. However, the schist-flagstone contact is less well defined and gradational on Creag an Fhireoin [NN 2716 7508]. In part, this is because the schists have been indurated by the intrusion of an appinitic complex. There is also a transitional passage from micaceous psammites with common quartzite seams into quartz-mica-feldspar-schists which also have numerous quartzite seams. Farther north, the top of the formation is poorly exposed. South-east of Beinn Chlianaig [NN 2960 7736], the uppermost psammitic beds are garnetiferous and have a strong lenticular spaced foliation with common retrogressive chlorite. The overlying schists are also garnetiferous.

In contrast, there is a well-exposed gradation between the Eilde Flags and the Loch Treig Formation on Meall Cian Derg [NN 332 760]. The upper schists are feldspathic and grade into fissile micaceous psammites.

The dominant lithology in the main western outcrop area is a fine- to medium-grained, variably flaggy, banded pinkish white and grey, micaceous psammite. The lamination, which is on a millimetre scale, is cut by a spaced cleavage, defined by mica flakes separated by quartz-rich microlithons, 1 to 2 mm thick. Muscovite and biotite are present in the foliation planes; biotite also occurs in lath-shaped aggregates which appear to replace hornblende. Garnet porphyroblasts up to several millimetres in diameter may be present, with secondary chlorite variably replacing both garnet and biotite flakes. Pyrite cubes occur at [NN 2861 7552]. Interbedded with the micaceous psammites are white quartzites which vary in thickness from several centimetres up to several metres. There are also persistent quartz-mica-schist interbeds which commonly contain pebbles, up to 2 cm in diameter, of quartz with potassium feldspar overgrowths, e.g. at locality [NN 2999 7689]. The pebbles comprise up to about 5 per cent of the modes. Individual pebble beds are up to several centimetres in thickness, whereas individual micaceous psammite beds are up to about 150 cm thick, but mostly within the range 5 to 30 cm. Generally, beds are parallel sided, although low-angle cross-beds are preserved at locality [NN 2912 7509].

On Meall Cian Dearg, the lithologies are less variable and comprise fine- to medium-grained, micaceous psammitic flags with semipelitic laminae, and a have a penetrative biotite foliation oblique to well-defined bedding. Bedding is on a scale of several centimetres. At locality [NN 3173 7459], there is scree of fissile micaceous psammite containing white quartzite beds on a centimetre scale. All the micaceous psammites consist of quartz, plagioclase, muscovite and biotite assemblages, with or without minor amounts of opaque minerals, potassium feldspar (untwinned), garnet, apatite, sphene and zircon, and with minor secondary chlorite, carbonate and sericite (e.g. in (S80113), (S80634), (S58488)). The potassium feldspar (microcline and microperthite) overgrowths in the pebble beds are strongly deformed (in (S80635), (S80640)). The psammites are gneissose within the thermal aureole of the Strath Ossian Granatic Complex (see Chapter 10).

Dog Falls Psammite Formation

The Dog Falls Psammite Formation consists of flaggy micaceous psammites and is the uppermost of the Grampian Group formations north of the A86, where it overlies the Inverlair Psammite Formation. Its outcrop area is bisected by the Corrieyairack Granitic Complex, with hornfelsing of psammitic metasediments on both margins of the intrusion. The formation has a diachronous relationship with the Leven Schist and Loch Treig Schist and Quartzite formations, which accounts for its absence along strike to the south-west of the Corrieyairack Granitic Complex. To the north-west of the Corrieyairack Granitic Complex, the psammites of the Dog Falls Formation strike south-westwards into pelitic schist of the Leven Schist Formation. South-west of the granitic complex, the Dog Falls Formation is replaced along strike by interbedded pelitic schists and psammites of the Loch Treig Schist and Quartzite Formation. In addition to this lateral transition, there is also a vertical transition from the Dog Falls Formation into the overlying Leven Schist Formation. A continuous section in the Burn of Agie [NN 370 905] exposes this transitional boundary. The top of the Dog Falls Psammite Formation is marked by a gradual increase in the amount of muscovite, and a decrease in the feldspar to quartz ratios in the psammites.

Exposure of the Dog Falls Formation is very variable with the best exposures on remote, high ground. To the north-west of the Corrieyairack Granitic Complex, there are good exposures of psammite in the Burn of Agie (including the Dog Falls section) and on Creag an Breac, near [NN 380 900]. Crags on the steep, eastern side of Beinn Teallach [NN 362 860] provide an almost complete section across the south-eastern outcrop of the formation.

East of the Corrieyairack Granitic Complex, the Inverlair Psammite Formation is overlain by a sequence of flaggy micaceous psammites with semipelites, which can be divided into two main units. At the base are about 250 m of flaggy, fissile, fine- to medium-grained, graded and laminated micaceous psammites with semipelitic tops. The more psammitic parts have pink-weathered tints due to alteration of their feldspar grains. Micaceous laminations are up to 1 mm thick at about 5 mm spacing. The beds, mostly less than about 15 cm thick, consistently young to the west, i.e. away from the Inverlair Formation.

A further 250 m of fissile, laminated micaceous psammites and semipelites overlies the graded units. A thin-bedded grey quartzite at locality [NN 3608 8539] and garnetiferous schists appear near the top of the laminated rocks, close to the contact with the Corrieyairack Granitic Complex. The laminations in the psammites are, in part, tectonic in origin, and are defined by biotite lithons within a spaced schistosity or cleavage (in (S80689), (S80610)). This tectonic fabric is increasingly dominant towards the contact with the Corrieyairack Granitic Complex. Graded beds are rare in this upper unit; at locality [NN 3620 8566], there is a 50 m section containing six graded units younging westwards. Quartz lenses are common within the semipelitic laminae.

South-east part of the district

Overlying the Meall Cos Charnan Semipelite Formation is a sequence of fissile micaceous psammites with quartzite interbeds which extend eastwards onto Sheet 63E (Table 3). Graded psammitic beds indicate that this, as yet unnamed unit, youngs south-westwards and may be equivalent to the Inverlair Psammite Formation. Tight, regional folding repeats the psammitic unit on both sides of Meall Nathrach [NN 448 759], where semipelitic schists (e.g. (S90039)) form the core of the Beinn a' Chlachair Synform (Figure 12).

Psammites and the less common quartzites are well exposed on the high slopes of the major mountains, on the north-west side of Aonach Beag in the vicinity of Meall Nathrach, and on the south-west slopes of Beinn a' Chlachair [NN 471 782]. There are also isolated exposures farther west, close to the Strath Ossian Granitic Complex. Sharp contacts with the overlying semipelite are well exposed at Meall Nathrach. The south-eastern limit of the psammite/quartzite unit is defined by a sharp vertical contact with mylonitic semipelites which is well exposed at [NN 4432 7433].

The psammite/quartzite unit has a thickness of about 800 m on each limb of the Beinn a' Chlachair Synform. The dominant micaceous psammites are thinly laminated. Preferential weathering of the semipelites produces distinctive ribbed outcrops. Thin quartzite inter-beds are widespread. There is a 50 m-thick quartzite and semipelitic schist band which defines an isoclinal reclined fold at the south-eastern limit of the unit. Trough cross-beds are preserved in quartzite in a stream exposure [NN 4445 7459].

The semipelites which form the core of the Beinn a' Chlachair Synform are completely exposed on the steep northern slope of Meall Nathrach, where they have a tectonic thickness of about 200 m. Lithologically and petrographically, they are identical to the stratigraphically underlying semipelitic gneisses of the Meall Cos Charnan Formation. The Meall Nathrach exposures are within the thermal aureole of the Strath Ossian Granitic Complex.

The semipelite unit defining the south-eastern limit of the psammite/quartzite unit is characterised by rocks with a strong quartz-leaf fabric parallel to biotite foliation, and the ubiquitous presence of tightly folded quartz knots. These impart a linear element to the strong tectonic fabric of these mylonitic schists and gneisses. The vertically dipping semipelites have a maximum thickness of about 250 m. On the western spur of Aonach Beag, near [NN 4440 7423], there is a well-exposed tectonic lens within the semipelites containing tightly interfolded talc-silicate rock ((S80052), (S80053)), amphibolite, quartzite and semipelitic to pelitic schist and gneiss. Individual beds are laterally discontinuous. The amphibolites are about 1 m thick, occurring as fine-grained, massive, dark grey, hornblende-rich rocks speckled by plagioclase. They constitute conformable beds next to the calc-silicate rocks which are medium to coarse grained and handed with green epidotic seams and pinkish white feldspathic seams. Sphene forms brown grains visible to the naked eye. The stratigraphical position of this unit is not known.

In the extreme south-east corner of the sheet there is a mixed sequence of semipelitic gneisses (S80047) and micaceous psammitic flags (S80048) with minor quartzite interbeds (S80046), and concordant amphibolite sheets up to about 1 m thick e.g. at [NN 4472 7418]. These rocks are in tectonic contact with mylonitic semipelites; their stratigraphical position is not known (Table 3).

Summary of Grampian Group lithofacies

Glover and Winchester (1989) provided a comprehensive account of the Grampian Group which includes an analysis of its sedimentology and development. They noted that the main part of the succession (their Corrieyairack Subgroup), from the Coire nan Laogh Semi-pelite Formation up to and including the Creag Meagaidh Psammite Formation, represents turbiditic deposition in a deepening basin in which the rate of subsidence outpaced sediment supply. The overlying formations (Glen Spean Subgroup) represent near-shore coastal sediments. Fining-upwards sequences in the Loch Laggan Formation are compatible with submarine channel sedimentation. Unidirectional cross-bedding in the Loch Laggan Formation suggests that its sediments were supplied from the south-west. Okonkwo (1985) also suggested a southerly source for the sediments of the Creag Meagaidh Formation, which agrees with the lateral lithofacies variations described in the present account. Chemical analysis of the metasedimentary rocks show increasing maturity up the succession.

Table 4 lists mean whole rock analyses of semipelites and psammites from Sheet 63W, taken from Winchester and Glover (1988) and Okonkwo (1989). The Grampian Group semipelites are chemically very similar to one another, and collectively are chemically different to the semipelites or pelites of the overlying Appin Group. These whole rock analyses indicate a granitoid source area (Zr, Y enriched sediments) lacking mafic lithologies (depleted Ni and Cr sediments), with deposition on a passive continental margin or in an intracratonic basin (see Winchester and Glover, 1988).

Chapter 4 Dalradian: Appin Group

The predominantly psammitic and semipelitic lithologies of the Grampian Group are overlain in the central and south-western parts of the Glen Roy district by rocks of the Appin Group. The lowest part of the Appin Group is dominated by pelitic schists and quartzites, but increasing amounts of calcareous schists and limestone appear higher in the succession. This younger sequence can be traced into the south-west of the district from its type area south of Fort William. However, a northerly attenuation of Appin Group stratigraphy in the Glen Roy district is regarded as a primary feature. The present mapping has highlighted deficiencies in the established stratigraphical nomenclature and a new lithostratigraphy is proposed for much of the Lochaber Subgroup (Figure 7). Several schist and quartzite units, previously assigned separate formation names (see Hickman, 1975), are now interpreted as parts of one formation, here referred to as the Loch Treig Schist and Quartzite Formation. The quartzites form discontinuous lenses and are reduced in rank to members, except for the basal Spean Viaduct Quartzite Formation. The new terminology avoids duplication of geographical terms within the lithostratigraphy of the Lochaber Subgroup (Figure 7). The rocks described in this chapter are found in the southern part of Area 2, that part of Area 3 west of the Corrieyairack Granitic Complex, and throughout Area 4 of (Figure 3).

Lithostratigraphy

Spean Viaduct Quartzite Formation

On the western side of the main Leven Schist Formation outcrop, an underlying quartzite unit with considerable along-strike thickness variations can he traced northwestwards from Sheet 62E around the Appin Synform and Bohuntine Antiform (see Chapter 6 and (Figure 10)). On the published geological map of Sheet 62E, this quartzite is referred to as the Eilde Quartzite. However, there is no proven correlation with the Eilde Quartzite in its type-area, and the unit is referred to here as the Spean Viaduct Quartzite Formation, after Glover (1993), on the basis of excellent exposures along the River Spean on Sheet 62E. This quartzite is the lateral equivalent of the whole of the Loch Treig Schist and Quartzite Formation, (Figure 4). On Sheet 63W, the quartzites of the Spean Viaduct Quartzite Formation are best exposed in variably spaced stream sections; they are poorly exposed on the badly drained eastern slopes of Bohuntine Hill and west of the Allt Coire Ionndrainn, between the Coire Ionndrainn and the Allt Coire Ceirsle. The unit comprises white to light grey, fissile flaggy and massive, fine- to medium-grained, fractured quartzites (altered quartz arenites).

In outcrop, the Spean Viaduct Quartzite has conformable boundaries with various Grampian Group lithologies. Due to facies changes in the older rocks, this basal boundary is both sharp and transitional. The upper boundary is invariably sharp against schists of the Leven Schist or Ballachulish Limestone formations. Locally, this contact is tectonically modified by the Fort William Slide (Chapter 6).

The formation has a maximum thickness of about 170 m. Its local absence may reflect a primary uneven distribution, local erosion prior to deposition of the overlying pelites, and/or tectonic attenuation.

Most of the quartzites are poorly bedded; their flaggy character is due to a penetrative foliation. Locally, the foliation is at an acute angle to preserved bedding, e.g. at locality [NN 2818 8644]. A pebbly quartzite is exposed adjacent to the Fort William Slide, in the nose of the Appin Synform at [NN 2843 8694]. Angular quartzite clasts, up to about 1 cm long, are embedded in a quartzose matrix, with lesser amounts of white feldspar and shape-aligned muscovite and biotite flakes.

Loch Treig Schist and Quartzite Formation

This formation consists of dark grey pelitic schists with prominent quartzite members, and has a total maximum thickness of about 1550 m in the Glen Roy district. It includes pelitic schists, previously assigned to the Eilde Schists, Binnein Schists, Loch Treig Schists, part of the Lairig Schists and the basal part of the Leven Schists Mark Leven Schists'). Two quartzites, the Binnein and Glencoe quartzites, can be traced continuously from their type areas to the south-west of the district, whereas other quartzites are only found in the Glen Roy district and its immediately adjacent ground. The latter are the Innse Quartzite (Hickman, 1975) and the Stob and Reservoir quartzites (Hinxman et al., 1923). A common feature of these quartzites is that they are metamorphosed quartz-arenites and are less feldspathic than the older Grampian Group quartzites. The outcrop pattern of the Loch Treig Schist and Quartzite Formation is controlled by the regional folding (Chapter 6) with two areas of excellent exposure: on the western limb of the Inverlair Antiform and farther east, on both limbs of the Blackwater Synform (Figure 10). The western exposures around Cruach Innse [NN 280 764] afford a complete section through the formation, selected as a reference section. The eastern area is structurally more complex and it is more difficult to interpret the stratigraphy. Here the best exposures are found on the flanks of Meall Cian Dearg [NN 332 769] and Sròn na Garbh-bheinne [NN 357 753].

The nature of the generally conformable boundary with underlying Grampian Group psammitic lithologies is described in Chapter 3. In addition to the vertical superposition of the Appin Group on the Bnmachan Psammite, Inverlair Psammite and Eilde Flags formations of the Grampian Group, there is also a lateral facies change north-eastwards from the Loch Treig Formation into the basal part of the Dog Falls Psammite Formation. This is due to a progressive increase in psammite interbeds in the Lochaber Subgroup schists, with psammites eventually becoming dominant north-east of [NN 370 869] to form the Dog Falls Formation. The top of the Loch Treig Formation is marked by a transition into pale grey pelitic strata of the Leven Schist Formation. This coincides with a change in the magnetic character of the rocks, the pelitic schists of the Leven Schist Formation being more magnetic.

In its reference area around Cruach Innse, the formation's total thickness of about 1500 m, is made up as follows:

The thickness of the formation decreases northeastwards from the reference area, due to thinning out of the major quartzites to leave a succession composed predominantly of schists. South of Creag Uilleim [NN 345 836], the formation is about 1000 m thick. In the Loch Treig area, it has been tectonically attenuated (see Chapter 6) to between about 600 and 900 m. In this eastern area, the Stob and Reservoir quartzites are only 20 and 140 m thick respectively.

The basal schists of the reference area are dark grey, laminated, and fine to medium grained, with more biotite than the overlying pelites. Garnet porphyroblasts are unevenly distributed; for example they are common east of Beinn Chlianaig near [NN 297 780]. Clean quartzite beds, up to several centimetres thick, are common through out these schists, e.g. at [NN 2770 7470], [NN 2942 7721] and [NN 2785 7520]. The schists are strongly disrupted adjacent to the Innse Appinite near Clach Cartaidh [NN 276 752]. Details of mineral assemblages of the various Appin Group lithologies are provided in the section on metamorphism in Chapter 6.

The Stob Quartzite Member, which is white, flaggy and micaceous, can be traced north-eastwards from its type area, Stob Coire Easain on Sheet 54W (Hinxman et al., 1923), as far as locality [NN 3232 7476]. This quartzite has a conformable lower boundary with micaceous psammites of the Eilde Flags on the south-eastern limb of the Blackwater Synform. The Stob Quartzite is about 20 m thick, and passes along strike to the north-east into more feldspathic rocks which show a vertical transition down into the Eilde Flags. There are thin quartzite beds in the transition zone.

The Binnein Quartzite Member, which forms the core of a reclined fold, is exposed around locality [NN 2742 7472] at the southern margin of Sheet 63W. Here, the quartzite is a white, fissile, flaggy rock with transitional boundaries into adjacent semipelitic schists. Traced south-westwards onto Sheet 54W, the member thickens to about 300 m (Hickman, 1975).

The thick quartzite which can be traced from the southern edge of the map, through Cruach Innse to about 1 km north-east of Beinn Chlianaig, is referred to as the Glencoe Quartzite Member. It is regarded as the northerly continuation of the Glencoe Quartzite on published BGS maps (sheets 53 and 54W), following Bailey (1934) and Hickman (1975), a correlation which differs from that of Treagus (1974) who equated it with the Binnein Quartzite, based on mapping around Loch Leven. On Sheet 63W, despite gradually tapering to the north-east from a maximum thickness of about 660 m at the district's southern margin, the Glencoe Quartzite is separated from the Grampian Group psammites by a more or less constant thickness of semipelitic and pelitic schists. Excellent hill exposures show that the Glencoe Quartzite comprises interbedded white, massive, cross-bedded and flaggy quartzites, grey, quartz-rich schists and quartz-mica-schists, with or without garnet. (Figure 8) shows details of a well-exposed section, through about 350 m of steeply dipping rocks of the Glencoe Quartzite, from the summit spine of Cruach Innse [NN 280 763].

Both the lower and upper boundaries of the Glencoe Quartzite are transitional, with thin quartzite beds persisting into adjacent schists over thicknesses of about 10 m. The quartzites are typically white to light grey, medium grained and slightly micaceous. Feldspar grains are locally visible in hand specimens from the north-eastern extremity of the outcrop. At [NN 2930 7752], basal coarse-grained beds contain disseminated mafic grains which weather to produce a brown spotting. A similar quartzite is exposed at [NN 2671 7602], near the top of the member. Bed thicknesses vary, up to a maximum of 2 m, at [NN 2720 7554]. Trough and tabular cross-bedding ('herring-bone'), especially common in the south-west, shows that the Glencoe Quartzite youngs consistently to the north-west. Good examples of cross-bedding occur at [NN 2741 7571], [NN 2759 7548], [NN 2715 7538] and [NN 2738 7508] where pelitic laminae drape the foresets.

Bed thicknesses are slightly variable over several metres of strike. This is, at least in part, a function of deformation.

Adjacent to the Innse Appinite the quartzites are strongly brecciated and recrystallised following local melting. They are also invaded by appinitic veins.

The schists within the Glencoe Quartzite are fine grained. In the northern part of the outcrop, these schists have a well-developed continuous cleavage, for example at [NN 2928 7827]. Quartzite lenses vary from thin seams at [NN 2660 7543] to the units 30 m thick at [NN 2931 7820]. Disseminated pyrite grains are present in schists adjacent to the thick quartzite lens, as at [NN 2671 7551].

The Innse Quartzite Member comprises medium-grained, white to light grey quartzites which form beds up to 50 cm thick. There are minor interbeds of quartz-mica-schist and pink-tinted psammites (in the folded southern exposures). The thickness of the Innse Quartzite decreases very gradually north-eastwards over about 5 km of strike, from a maximum of 33 m at the south-western margin of the district. The northernmost exposure of the Innse Quartzite, in a stream at [NN 2936 7886], comprises cream-coloured quartzites with a penetrative muscovitic foliation. Feldspar grains are seen locally in these northerly exposures.

In the south, the base of the Innse Quartzite is sharp, with local quartz veining along the contact with underlying schists. The top of the member is transitional over a thickness about 10 m. Good exposures of the Innse Quartzite occur on Stob Coire Gaibhre [NN 261 757], the north-western end of Cruach Innse, the summit of Beinn Chlianaig, and in a stream section on the southern slopes of this hill [NN 2893 7800].

North of the River Spean, there are no thick quartzite members, and the Loch Treig Schist and Quartzite Formation is dominated by dark grey, fine-grained, semipelitic schists, which are variably garnetiferous, with thin psammite beds. A thin quartzite is exposed on Creag Uilleim where all the rocks are cut by several generations of felsic veins. In this northern area, there appears to be a random distribution of coarse-grained garnet-schists, e.g. south-east of Creag Dhubh [NN 322 824].

The main planar fabric in the schists is defined by shape-oriented biotite and subsidiary muscovite flakes. Quartz grains define seams and lenses, up to several centimetres thick, which parallel the mica fabric. Parallel, rare, pale green calc-silicate seams are of similar thickness.

The Reservoir Quartzite Member occurs at the top of the Loch Treig Schist and Quartzite Formation, and consists of up to 140 m of persistent beds of massive, pale brown-weathering, medium-grained quartzites with iron-staining. It is well exposed on both sides of Loch Treig, with the most accessible exposures being on the western shore of the loch [NN 3392 7465]. Muscovite flakes coat exposed bedding surfaces. The quartzite beds, up to about 1 m in thick, are separated by silver-grey mica-schists, identical to the overlying schists of the Leven Schist Formation.

Leven Schist Formation

This formation consists of laminated, pale grey, pelitic schists which are locally flaggy and variably garnetiferous. Thin carbonate-bearing beds appear towards the top of the formation, in association with more calcareous schists and phyllites. In contrast to the underlying Loch Treig Formation, very few quartzites are present in the Leven Schist Formation. Its main outcrop occurs on the eastern side of Glen Roy and the hilly ground south of Roybridge, and lies within the core of the Stob Ban Synform (Figure 10). This belt of schists continues to the south-west of the Glen Roy district into the formation's type-area around Loch Leven (see Bailey, 1934; Treagus, 1974; Hickman, 1975; Litherland, 1980). A second, parallel belt of pelitic schists assigned to the Leven Schist Formation can be traced from east of Loch Leven into the Loch Treig area. These schists were originally referred to as the 'Ermine-rock' within the Reservoir Schists by Hinxman et al. (1923). A small northern outlier of the Leven Schist Formation occurs on Meall Ptarmigan [NN 426 904], separated from the main outcrop area by the Corrieyairack Granitic Complex.

In the southern part of the main outcrop area, as well as around Loch Treig, Leven Schist Formation pelites conformably overlie rocks of the Loch Treig Schist and Quartzite Formation. Elsewhere the pelites directly overlie Grampian Group lithologies (Figure 4). In the latter case, the base varies from transitional (with the Dog Falls Psammite Formation in the upper reaches of Glen Roy), to sharply conformable (e.g. with the Inverlair Formation at Meall Ptarmigan), to sharp with evidence for tectonic reworking (north of Roybridge). In the last case, an extremely thin (and locally completely excised) Leven Schist Formation is sandwiched between the Ballachulish Limestone Formation and older psammites and quartzites (of the Brunachan Psammite, March Burn Quartzite and Spean Viaduct Quartzite formations).

A conformable, locally transitional upper boundary with the Ballachulish Limestone Formation is well exposed in the Allt Ionndrainn (Plate 5) and its tributaries near Bohuntine [NN 2749 8349], in the south-west corner of the district in the Allt Leachdach [NN 26 77], and in the Coire na Reinich south of Brae Roy Lodge [NN 34 89].

The formation is well exposed in river sections and on hillside crags. Easily accessible east–west sections across the complete outcrop occur in Glen Roy: in the Allt Ionndrainn, and the Gleann Glas Dhoire and its tributaries. There are also superb sections in the River Roy, and in the Monessie Gorge of the River Spean between [NN 2831 8055] and [NN 2994 8100].

North-eastwards thinning of the formation in its main outcrop area is regarded as a primary feature, although present thicknesses have also been tectonically modified, notably on the eastern limb of the Stob Ban Synform (see Chapter 6). In the Allt Leachdach area, there are about 2200 m of tectonically thickened section between the Innse Quartzite and Ballachulish Limestone Formation, on the eastern limb of the Stob Ban Synform (Table 5). North of Roybridge the total thickness is about 1250 m on the western limb, as measured in the Allt Ionndrainn (Table 5), and about 900 m in the Coire na Reinich section between the top of the Grampian Group and the base of the Ballachulish Limestone Formation. At Meall Ptarmigan, the total thickness of the Leven Schist Formation is probably less than 200 m, overlying an attenuated uppermost Grampian Group succession. Here the Leven Schist lithologies are very similar to those found immediately below the Ballachulish Limestone Formation farther south.

The main lithologies in the formation are laminated, silver-grey, muscovitic schists and laminated, flaggy, coarse-grained quartz-mica-schists which are variably garnetiferous. In both lithologies, the laminations are due to quartz-rich microlithons parallel to micaceous foliation planes. There is commonly more than one generation of these tectonic fabrics (Plate 4). A coarser, primary fabric is defined by quartzose beds, up to about 10 cm thick and commonly packed with garnets. There are also calcareous beds which may form the cores of the quartzose beds and which contain randomly oriented hornblende blades as well as garnet porphyroblasts. Individual beds can be followed across entire exposures, resulting in a striped or banded appearance to the outcrop. The quartz-mica-schists have a green tint and become noticeably fissile where they contain tremolite. Elongate biotite flakes define a mineral lineation on early schistosity planes. Disseminated pyrite and chalcopyrite are locally visible (e.g. at [NN 3012 8550], [NN 2724 8316] and [NN 3082 8556]). Common quartz lenses within the schistosity locally contain pink-weathering feldspar as well as occasionally having chloritic sheaths. On the western map edge, immediately south of the River Spean, the uppermost rocks of the formation are fine grained and phyllitic.

On the west side of Loch Treig, the pelites are distinctive silver-grey, muscovite-rich schists, speckled by black amphibole porphyroblasts or by biotite aggregates after the amphiboles. They are medium- to coarse-grained rocks with common thin psammite or quartzite seams, but less common calc-silicate lenses. East of the loch, these rocks are hornfelsed in the aureole of the Strath Ossian Granitic Complex (see Chapter 10).

Dolomitic marble beds and lenses, up to about 3 m thick, occur in association with carbonate-bearing pelites towards the top of the formation (Plate 5). North of the River Spean, in the vicinity of Meall a' Chail [NN 3360 8455], there is a black and pale green, laminated, calcsilicate rock which can be traced along strike for several hundred metres. It has, a thickness of about 40 m (see Anderson, 1956).

Quartzite beds are rare. A cross-bedded quartzite is poorly exposed north of Creag Dhubh, at [NN 3167 8358], and thin quartzite beds were noted at [NN 2729 7655].

At Meall Ptarmigan, the main lithology is a fresh and very tough, biotite-rich hornfels within the aureole of the Corrieyairack Granitic Complex (Chapter 10). Randomly oriented porphyroblasts of sillimanite, up to 15 mm long and 3 mm across, are prominent on weathered surfaces. Light-coloured layers, about 10 cm thick, alternate with darker layers up to 2 cm thick, and are tightly folded. Quartzose lenses, up to 2 mm thick, represent a prehornfelsing schistosity lying parallel to the axial planes of the minor folds. Vein quartz forms lenses up to 20 cm thick.

Ballachulish Limestone Formation

In the Glen Roy district, the Ballachulish Limestone Formation comprises three main lithologies. These are:

  1. Grey and white, variably flaggy, laminated calcareous (usually tremolitic) schists.
  2. Fissile, greenish calcareous phyllites.
  3. Pale brown-weathering, impure limestones which are mostly dolomitic.

These lithologies are unevenly distributed, the phyllites and massive limestones being more common in the west. The formation has a north-eastward trending outcrop strip of more or less constant width from its type area at Loch Leven as far as the Ben Nevis Granite (Sheet 53, Ben Nevis). The outcrop then continues along strike northeast of the granite and can be traced continuously across Sheet 62E into the south-west of the Glen Roy district. Here, the outcrop is terminated north of Roybridge in the core of the south-westward plunging Appin Synform (Figure 10). In this fold, the Ballachulish Limestone Formation passes by upward transition from the Leven Schist Formation and is conformably overlain, with a sharp boundary, by the Ballachulish Slate Formation. This upper contact is well exposed in the Allt Bo-loin and Allt Coire Ceirsle. Complete sections through the Ballachulish Limestone Formation are exposed in these two rivers, as well as in the Allt Ionndrainn and Allt Coire Ionndrainn (Figure 9). Away from the watercourses, exposure is very poor.

The maximum thickness of the formation, as measured in the Allt Bo-loin section, is about 800 m. Elsewhere it has been tectonically attenuated, either by folding or by ductile shearing.

Individual limestones vary from massive beds, up to about 60 m thick, to centimetre-thick seams intercalated with the other two main lithologies. During deformation, the limestones were disrupted into pods and it is generally not possible to trace one bed between adjacent stream sections with certainty. Preserved bedding in the various lithologies is accompanied by tectonic laminations, with more than one generation of microlithons. Disharmonic folds are defined both by primary laminae and planar tectonic fabrics. These folds are offset by brittle and ductile microfractures, locally infilled by vein calcite. All three main lithologies may be spotted by tremolite rosettes up to several centimetres in diameter, and by reddish brown dolomite. Silver-coloured muscovite is commonly concentrated along microfractures. The reference section for the northern part of the Ballachulish Limestone Formation is in the Allt Bo-loin where the following sequence, summarised in (Figure 9), is exposed, going upstream onto sheet 62E:

40 m Calcareous schist
60 m Calcareous phyllite
20 m Calcareous quartz-schists
10 m Calcareous schists
1 m Schistose limestone
80 m Infold of Leven Schists (core of antiform)
250 m Tremolite-schists with marble pods
130 m Sericite-tremolite-schists
30 m Impure dolomitic limestone
40 m Sericite-tremolite-schists with marble pods
16 m Impure dolomitic marble with schist bands
125 m Tremolite-schists with tremolitic limestones
110 m Striped phyllites
50 m Tremolite-schists with tremolitic limestones
70 m Ballachulish Slate Formation (core of Appin Synform)
40 m Tremolite-biotite-schists
30 m Calcitic limestone
16 m Tremolite-phyllites
8 m Calcitic limestone
80 m Calcareous phyllites
(Tectonic contact with the Grampian Group)

The lack of apparent symmetry about the fold axes indicates small-scale lateral variability within the Ballachulish Limestone Formation.

In thin section, the lithologies are seen to comprise assemblages of carbonate and quartz, with variable amounts of tremolite and phlogopitic biotite and less common muscovite, chlorite, talc and opaque minerals. Zoned, strongly poikiloblastic plagioclase laths overprint a schistosity adjacent to the Fort William Slide.

Ballachulish Slate Formation

Dark grey, fine-grained, chloritic schists with a remarkably constant outcrop width can be traced north-eastwards from the River Spean on Sheet 62E as far as the Allt Coire an t-Seilich on Sheet 63W. These rocks have been assigned to the Ballachulish Slate Formation by all previous workers, forming the uppermost unit in the core of the Appin Synform. This correlation is retained because the schists overlie the Ballachulish Limestone, although there are lithological differences from the type area, around Ballachulish, where the formation comprises pyritic, graphitic slates. On Sheet 63W, the formation is best exposed in the Allt Coire an t-Seilich and Allt Coire Ceirsle where it has a maximum thickness of about 160 m. Here, it comprises dark grey, fissile, finely laminated and chloritic schists. A continuous cleavage is cut at acute angles by several spaced fabrics. The schists also contain common brittle microfractures. A single 5 cm thick limestone band, exposed in the Allt Coire Ceirsle [NN 269 849] provides the only lithological variety within the formation. The schists are not conspicuously pyritic and small garnets are uncommon. A sample (S82433) from [NN 2691 8493] comprises intergrown chlorite and sericite which define a continuous cleavage. Quartz occurs in seams parallel to the cleavage. Relatively large biotite flakes are shape-aligned in the cleavage, but anhedral garnets, partly replaced by chlorite, predate the fabric.

Chemistry of the Appin Group lithologies

Table 6 lists mean whole-rock analyses of the main Appin Group lithologies, excluding quartzite. The pelites are depleted in CaO, MnO, Ni, P2O5  and Sr, but enriched in Nb, Rb, Y and Zr relative to Grampian Group semipelites. They are also relatively potassic. The low CaO and Sr values arise from the low modal amounts of plagioclase in the pelites. According to Lambert et al. (1981), the chemistry of the pelites reflects the illite-poor and feldspar-poor nature of the original argillites, which in turn suggests that the source-area rocks were extensively degraded. Chemical analyses confirm the dolomitic character of the limestones. The uppermost schists of the Ballachulish Slate Formation are peraluminous and chemically resemble typical black, potassic shales.

Hickman and Wright (1983) presented a detailed study of the whole-rock chemistry of the various Appin Group quartzites from their entire outcrop area between Lismore and Glen Roy. There is considerable overlap in the chemical make-up of the main quartzites because they all show considerable spatial facies variation. However, the work does show that all quartzites from the northern part of their study area (the south-western part of Sheet 63W) were originally highly mature quartz sands (over 95 per cent Si02), deficient in feldspar and clay minerals.

Lambert et al. (1982) reported the results of Rb-Sr whole-rock isotopic studies on metapelites of the Leven Schist Formation from the Glen Roy–River Spean area. Sixteen analyses produced an isochron recording an age of 655±25 Ma with an initial 87Sr/86Sr ratio of 0.7232 ± 10. However, the significance of this age is not clear.

Sedimentation

The progressive shallowing associated with the final infilling of the Grampian Group basin was followed by a phase of slow subsidence, resulting in a marine transgression across a hinterland of low relief. This led to the deposition of the Lochaber Subgroup pelites and mature quartz sands (see Chapter 3 and also Hickman, 1975; Anderton, 1985). The lithologies of the Loch Treig Schist and Quartzite Formation represent altered tidal shelf deposits. A significant increase in water depth, during which subsidence may have periodically outstripped sedimentation, led to the deposition of the fining-upwards sequence of the Leven Schist Formation. In part, the pelites of the Leven Schist Formation represent a lateral facies change from the Loch Treig lithologies so that in the north-east they directly overlie Grampian Group strata. An analysis of cross-bedding attitudes within the quartzites led Hickman (1975) to conclude that they were originally laid down by northerly flowing currents. The Glen Spean area was regarded as distal to a southerly source area. The present study suggests that there was also a northerly source area for the Loch Treig Formation.

The succession from the Leven Schist Formation to the Ballachulish Slate Formation records a period of progressive diminution of clastic supply to the basin, culminating in the formation of limestones in an open marine setting and finally in the deposition of organic-rich muds (Ballachulish Slate Formation) in an anoxic basin (Hickman, 1975). It is considered that the present Appin Group outcrop area in the Glen Roy district essentially reflects the outline of the original sedimentary basin. The northerly attenuation of the succession is related to decreasing sedimentation as the northern basin margin in the Meall Ptarmigan area is approached.

Chapter 5 Rocks of uncertain stratigraphical affinity south-east of the Great Glen

The Grampian Group succession north-west of the Corrieyairack Granitic Complex is underlain by a sequence of metasedimentary rocks of uncertain lithostratigraphical affinity. These are bounded by tectonic contacts and thus have no continuity with lithostratigraphical units established elsewhere on the sheet. However, units of this sequence do show some lithological similarities with rocks assigned to the Grampian and other groups.

The rocks considered here occur in two areas:

  1. Along the southern flank of the Great Glen. The rocks extend in a narrow zone across Sheet 63W and onto Sheet 73W (Invermoriston) where they are faulted out. They may reappear farther north, near the boundary of Sheet 73W with 73E (Foyers). This area is bounded to the north-west by the subvertical Glen Buck Fault, and to the south-east by the Eilrig Shear Zone which dips steeply south-eastwards. Within Sheet 63W, the rocks are mainly pebbly psammites of the Glen Buck Formation, but there are also unassigned thin developments of black phyllite, quartzite, limestone and rare metabasite. The Glen Buck Formation and associated rocks dip to the south-east or east-south-east, and structurally underlie the Grampian Group rocks which appear to be at a significantly higher metamorphic grade; amphibolite facies overlying green-schist facies (Phillips, 1992).
  2. On the south side of Gairbeinn below the Gairbeinn Slide. The rocks of this area, which is mainly on Sheet 63E (Dalwhinnie), were attributed by Haselock et al. (1982) to the Glenshirra Succession, interpreted as lying stratigraphically below the Corrieyairack Subgroup of the Grampian Group. Only the topmost unit of the Glenshirra Succession, the Gairbeinn Pebbly Psammite Formation, extends onto the eastern edge of Sheet 63W.

Glen Buck Psammite Formation

Psammitic rocks, with or without pebbly beds, form a unit termed the Glen Buck Psammite Formation, which can be traced for some 20 km from the western boundary of the district at Corriegour Burn [NN 2650 9250] to the northern boundary. The outcrop is displaced by the late brittle Sronlairig Fault, along which there is sinistral displacement of about 6 km. North of the Sronlairig Fault, the Glen Buck Formation trends in an approximate northeast to south-west direction, with a moderately steep dip to the south-east. Its western boundary is exposed in the Calder Burn [NH 348 018] and is defined by the Glen Buck Fault. The eastern boundary is exposed in stream sections [NH 354 017] and [NH 348 008] and is defined by the mylonites of the Eilrig Shear Zone. Immediately south of the Sronlairig Fault, the formation has an approximate east-north-east strike, but the trend changes southwards to an approximate north-easterly direction. The rocks dip steeply to the south-south-east. The contact with the Sronlairig Fault is exposed in the Allt Innis Shim [NN 329 984], Allt na Larach [NN 339 988] and Allt Lagan a' Bhainne [NH 391 011]. The upper, south-eastern boundary of the formation is exposed in the Allt Innis Shim [NN 329 979] and Allt na Larach [NN 341 984] and is again defined by the mylonitic rocks of the Eilrig Shear Zone. Thus, north and south of the Sronlairig Fault, the upper and lower boundaries of the formation are tectonic and its true thickness is not represented in the area. Major folds have not been recognised; the formation appears to consist of a simple dipping sequence, and in Glen Buck a thickness of 800 m is exposed in the streams south-east of the Aberchalder Burn.

The formation consists of a well-foliated quartz-feldspar-muscovite rock, made up of alternations, some 4 to 20 cm thick, of mica-rich and quartz-feldspar-rich layers. Where the mica-rich layers predominate, the rock is semipelitic; where the quartz-feldspar bands thicken and become dominant, the rock is psammitic in character. Both rock types contain angular clasts forming pebble beds. The margins of the pebble beds are conformable to the overall lithological banding of the rock and such banding is considered to be an original sedimentary bedding structure. Few other sedimentary structures have been recorded; equivocal cross-bedding occurs in the Allt a' Ghlinne [NH 3455 0055] and possible graded bedding in a pebbly layer at [NH 3477 0187], both structures implying younging to the south-east. The only reasonably common sedimentary structures are thin and impersistent ]entitles of heavy mineral grains at the base of some quartzose layers.

The pebble layers are commonly between 2 and 10 cm thick. At localities in Glen Buck [NH 347 015], pebble layers are common and occur within the succession at intervals of 40 cm or less. They are laterally impersistent, being traceable along strike for only a few metres. The boundaries of the pebble beds are usually sharp, and gradational contacts with the adjacent non-pebbly layers have not been identified.

Within the pebble beds, the individual clasts are usually discrete and matrix supported, and are remarkably uniform in size at about 5 mm in diameter, near the defined boundary between granule and pebble size. Exceptionally, clasts up to 10 mm in size occur. Clasts are also distributed throughout the rock as isolated fragments, and may be present when well-defined pebbly bands are absent.

The clasts are always formed of fragmental quartz and pink feldspar; other minerals or rock fragments have not been identified, although granite pebbles have been recorded from correlatives of the Glen Buck Formation in the south of the Invermoriston sheet (Sheet 73W) (Parson, 1982).

The presence of pebble beds and isolated clasts is variable, and their overall abundance and size decreases up the structural succession, that is, towards the southeast or south-south-east. In the upper part of the formation, as the Eilrig Shear Zone is approached, pebble beds become rare and may be absent.

The evidence also indicates a gradual decrease in clast size and abundance of pebble beds when the formation is traced along strike from north to south. In outcrop, that part of the formation in which pebbly layers are most common and clasts are of greatest size occurs in Glen Buck, in the lowest part of the succession immediately south-east of the Glen Buck Fault. South of the Sronlairig Fault, these lower, more pebbly beds have been cut out by faulting so that as the formation is traced towards Loch Lochy, only the upper elements of the formation are exposed, consisting of psammites in which pebbly beds are less common.

To the north, in the Invermoriston district (Sheet 73W), Parson (1982) described pebbly psammitic rocks that are the along-strike equivalents of the Glen Buck Formation in which clasts up to 150 mm in diameter (conglomerate size) occur. The regional decrease in clast size from the north-east towards the south-west probably reflects variation in the depositional environment of the original sediment. The decrease in clast size towards the top of the formation may also reflect a primary sedimentary feature but can be shown to be in part tectonic, granulation and even obliteration of clasts occurring as the Eilrig Shear Zone is approached.

Throughout most of the outcrop, the pebbly psammitic rocks are muscovite-chlorite-rich, and appear to be mineralogically distinct from and at a lower metamorphic grade than the garnetiferous biotite-bearing psammitic and semipelitic rocks of the overlying Grampian Group (see (Figure 16)). Muscovite is the most common mica in the Glen Buck Formation rocks, and most exposed surfaces have a silvery lustre due to the presence of this mineral. Many exposures have a greenish tinge due to the presence of chlorite. Biotite is present only in the upper part of the succession where the rocks have been modified by the effects of the Eilrig Shear Zone, and may be accompanied by small porphyroblasts of garnet. Gale-silicate lenses, locally common in some formations of the Grampian Group, have not been recorded in the Glen Buck Formation. In some exposures, thin stringers, about 2 to 3 mm thick, or small clusters of heavy minerals have been seen. These are regarded as very thin, discontinuous placers.

In thin section, the psammitic rocks are characteristically inequigranular, fine- to coarse-grained, grain- to matrix-supported, texturally and chemically immature arkosic quartz-arenites. They contain angular to weakly rounded detrital clasts of mono- and polycrystalline quartz, and potassium feldspar. Potassium feldspar clasts are the most common and include perthite, microcline and mesoperthite. The matrix is primarily composed of fine-grained quartz and feldspar. The muscovite-chlorite rich layers usually contain abundant, irregularly shaped clusters of epidote with lesser amounts of iron oxide and sphene. The foliation defined by the muscovite-chloriterich layers wraps around the feldspar clasts.

The pebbly psammitic rocks show a progressive increase in intensity of deformation towards the Eilrig Shear Zone, resulting in the development of protomylonites in the upper levels of the succession. In thin section, this increase in intensity of deformation is shown by an overall diminution of grain size, especially within the matrix. The quartz clasts are progressively deformed and flattened into 1 to 2 mm-thick lens-shaped quartz foliae, with the development of an undulose to sweeping extinction, deformation bands, and discrete subgrains along locally serrated margins. In contrast, the feldspar clasts remain angular or subrounded; the feldspars are more resistant to the effects of deformation than the quartz. An incipient mylonitic foliation is developed within the matrix, defined by thin lenticular mica foliae and dimensionally orientated quartz.

Other lithologies

Black phyllitic schists

Black phyllitic schists have been recorded at several localities, occurring as lensoid exposures elongated parallel to the regional foliation. They form impersistent layers within the pebbly psammitic rocks. The rock is dark grey-black, thinly foliated phyllitic schist, and readily breaks into slickensided, fracture-bounded, lens-shaped fragments. North of the Sronlairig Fault, a unit of black phyllitic schist, some 100 m thick, is exposed in the Allt na Larach [NN 3375 9905] and in a subsidiary stream to the south [NN 336 988]. It is subvertical, possibly graphitic in places, and includes small porphyroblasts of garnet. Thin quartz veins are locally abundant. The same unit of black schist also occurs in a stream section on the northern slopes of Meall an Odhar [NH 341 007], where it is only 50 m thick; it is assumed to taper out to the north. The most northerly outcrop of black phyllitic schist occurs in the Allt Lagan a' Bhainne [NH 392 011] in a fault-bounded outcrop adjacent to the Sronlairig Fault.

South of the Sronlairig Fault, a layer of black phyllitic schist with common small garnets, about 50 m thick and exposed only in stream sections, can be traced for some 2.5 km from the Allt Innis Shim [NN 327 982] to a stream section [NN 320 976], beyond which it appears to thin and taper out.

In the Allt an t-Sidhein [NN 201 959], several separate layers of black schist have been recognised. One of them contains numerous euhedral crystals of pyrite and is locally interbanded with pale ochreous weathering marble (see below). In a subsidiary stream to the Allt an t-Sidhein [NN 2997 9570], there are exposures of black schist in which thin, 2 to 4mm-thick, brown-weathering concordant carbonate lenses are a distinctive feature. A bed of black phyllitic schist, structurally interbanded with south-eastwards dipping mylonitic rocks, occurs in the Corriegour Burn [NN 2660 9230] and can he traced from 600 m to the north-east, to locality [NN 2690 9300]. Thin units of black schist are also present in the steep gullies in the upper stretches of the Allt a' Choilich [NN 2900 9475] and Allt na h-Atha [NN 2860 9450].

Limestone

Pale cream coloured, coarsely crystalline marble crops out at several localities. The most northerly outcrop is in a subsidiary stream to the Allt na Larach [NN3365 9870], where a small exposure of marble occurs a short distance north of the trace of the Sronlairig Fault. Thin marbles present in the Allt an t-Sidhein [NN 3003 9580] can be traced in a subsidiary stream to the south [NN 302 957]. These marble layers, up to 3 m thick locally, are complexly folded within the enclosing partly mylonitised psammitic rocks. The marble–psammite contact is sharp and abrupt, but has a very irregular outcrop pattern. The marble units include irregularly shaped, discontinuous pods of black phyllitic schist, and are interlayered with white quartzite horizons. Further isolated exposures occur to the south-west, in the Allt na h-Atha [NN 2895 9426], and in a stream section [NN 2721 9274] where a thin layer of marble is exposed and continues for some 600 m to the south.

Rock (1989) reported chemical analyses of the marbles in the Allt an t-Sidhein and Corriegour Burn. He pointed out that the marbles at both localities are notably Mg-rich, and that the rocks should be regarded as dolostones. Rock's (1989) opinion was that they could not be equated with the geographically nearest limestones of the Dalradian Appin Group and, on chemical evidence, he considered the calcareous rocks within the Great Glen (other outcrops occur outside the area of Sheet 63W) to form a separate assemblage whose relationship to the Lewisian, Moine and Dalradian has not been established.

Quartzite

Thin pale ochreous quartzites crop out in the Allt an t-Sidhein and in hillside exposures along the steep south-eastern slope above Loch Lochy, between the Allt a' Choilich [NN 291 948] and Coire Tarsiunn-eas [NN 271 922]. The quartzites occur as discontinuous layers, generally not more than 2 m thick, and lie structurally above the limestones and black phyllites. The quartzites show no obvious bedding but a strong platy fabric, formed by regular parallel parting planes, may be developed locally. Where such platiness is absent, the quartzites are massive but in most exposures show the results of some brittle deformation with the development of thin, closed, irregular fractures.

Metabasite

Two exposures of metabasite occur in the Allt an t-Sidhein, near the outcrops of marble and quartzite. The exposures, up to 2 or 3 m thick, are made up of green chloritic schist in which occur numerous euhedral crystals of pyrite and less common magnetite. The foliation, defined by the parallel alignment of chlorite and other micaceous minerals, is parallel to the lithological boundary of the metabasite. Thin sections ((S94966), (S94967)) of the metabasite reveal a well-developed mylonitic fabric, defined by very fine-grained ribbons of cryptocrystalline quartz, granular epidote chains and length-aligned, fine-grained chlorite and actinolitic amphibole. Rounded hornblende porphyroclasts are locally pseudomorphed by actinolite, chlorite and green biotite, and are enclosed by thin, locally well-developed asymmetrical pressure shadows. The bulk of the rock is composed of fine-grained chlorite, actinolite and epidote with quartz forming thin wispy ribbons. The amphibole porphyroclasts are deformed, have a well-developed sweeping extinction, kinks and dislocations, and are enveloped by the mylonitic fabric. The actinolite porphyroclasts are locally sigmoidal in shape, and are enclosed within reaction rims and pressure shadows of slightly coarser grained green chlorite. Post-kinematic pyrite porphyroblasts are common. The abundance of chlorite and amphibole, and the relative sparsity of quartz and feldspar, points to a probable igneous origin for the metabasites but metamorphism and deformation have destroyed evidence of their original nature.

Gairbeinn Pebbly Psammite Formation

The rocks of this formation were first described by Haselock et al. (1982). In the Glen Roy district, the formation has a limited outcrop and can be traced from the southern slopes of Gairbeinn [NN 459 980] south-westwards to the intrusive contact with the Corrieyairack Granitic Complex, an along-strike distance of less than 1 km. Rocks of the formation form extensive outcrops in the areas of Sheet 63E (Dalwhinnie) and Sheet 73E (Foyers), and the formation has a considerable strike-length within the Monadhliath Mountains.

On Sheet 63W, the Gairbeinn Pebbly Psammite is made up of psammitic rocks in which pebbly layers are a common and distinctive feature. The pebble clasts rarely exceed 5 mm in diameter and are formed of subrounded to subangular fragments of quartz and pink feldspar. The clasts are discrete, matrix-supported and occur in pebble beds up to 20 cm thick which are laterally impersistent. Haselock et al. (9182) recorded grading within the pebbly beds, defined by upwards decrease in clast size and increase in biotite content towards the top of each graded unit. Semipelite bands, not more than 10 cm thick, occur within the psammitic rocks but are a subordinate rock type within the formation.

The stratigraphical contacts of the formation are not exposed within the area of Sheet 63W, but the upper boundary is defined by the abrupt contact with the structurally overlying migmatitic pelitic rocks of the Coire nan Laogh Semipelite Formation, interpreted as the trace of the Gairbeinn Slide. The psammitic rocks develop a platy fabric and the pebble clasts become flattened and elongated as the slide is approached (Haselock et al., 1982).

Lithostratigraphical correlation

There are obvious general similarities between the Glen Buck and Gairbeinn formations; both are pebbly in parts and both occur immediately beneath tectonic breaks the Eilrig Shear Zone and Gairbeinn Slide respectively. However, there are differences between the two situations in the Glen Roy district. Firstly, the stratigraphical level in the Grampian Group immediately above the Eilrig Shear Zone is different to that above the Gairbeinn Slide. Furthermore, there is no evidence to show whether or not the Eilrig Shear Zone and Gairbeinn Slide are separate outcrops of a single folded tectonic dislocation. There are some differences of lithology between the Glen Buck and Gairbeinn formations, the former being more micaceous and the latter, north of Sheet 63W, being chiefly psammitic with conglomerate beds. Also, the formations are at different metamorphic grades. The Glen Buck Formation is greenschist facies (Phillips, 1992), quite distinct from the amphibolite facies of the overlying Grampian Group. In contrast, Haselock (1982) described kyanite from rocks below the Gairbeinn Formation near Gairbeinn, suggesting little difference in grade from the migmatitic Coire nan Laogh Semipelite Formation immediately above the Gairbeinn Slide. The lack of clear stratigraphical relationships between the successions above and below the tectonic breaks in the Glen Roy area has led to the exclusion of the Glen Buck and Gairbein formations from the Grampian Group (see also Glover and Winchester, 1989). Geochemical and sedimentological differences between the successions above and below the Gairbeinn Slide have been described by Haselock (1984) and Okonkwo (1985).

The areally subordinate black phyllitic schist, marble, quartzite and metabasite occur as narrow, thin units within the pebbly psammitic rocks of the Glen Buck Formation and, locally, within the Eilrig Shear Zone. They are lithologically similar to units of the Appin Group, although Rock (1989) considered that the geochemical characteristics of the marbles did not support correlation with that group. Moreover, possible correlatives of the metabasites have not been recorded in the Appin Group. The association of black phyllite, marble, quartzite and metabasite is similar to, but at a lower metamorphic grade than, the kyanite schist-marble-quartziteamphilobite association of the Ord Ban Subgroup (Glover and Winchester, 1989). The tectonic contacts between the different lithologies, and their occurrence adjacent to or within a thick shear zone, prevents any lithostratigraphical correlations, and thus the status and affinites of the metasedimentary rocks within and below the Eilrig Shear Zone remain unknown.

The metamorphic rocks of uncertain stratigraphical affinity in the south-eastern corner of the district are tectonically interbanded with Grampian Group lithologies and have been described with these rocks in Chapter 3.

Chapter 6 Structure and metamorphism south-east of the Great Glen

Over most of their outcrop area, the distribution of the various Grampian Group and Appin Group stratigraphical units is controlled by upright, open to tight D2 folds with north-east to south-west trending axial traces. The D2 folds deform bedding (So) and a bedding-parallel foliation (S1) formed during an earlier (D1) deformation episode. It is only possible to identify two regional D1 folds, the Treig or Appin Syncline and the Kinlochleven Anticline (Figure 10), in the south-west of the district, around Loch Treig and in Glen Spean. The D1 nappefolds were recognised by Bailey (1934) during the survey of the Ben Nevis district (Sheet 53), and later by Hickman (1978) in the Loch Treig area. They are north-west facing and were originally recumbent structures. The amplitude of these early folds decreases northwards and westwards, and they die out in the Appin and Grampian Group rocks north of Meallan Odhar, near [NN 330 840], and west of Cnoc nan Ceann Mora [NN 272 774].

The regional D2 folds found in the district are, the Appin Synform, Bohuntine Antiform, Stob Ban Synform, Inverlair Antiform, Blackwater Synform, Loch Laggan Antiform, Corrieyairack Synform, Tarff Synform, Tarff Anticline, and the tight folds within the Ossian–Geal Charn Steep Belt ((Figure 10) and (Figure 12)). Some of these structures have been described by previous workers (Bailey, 1934; Anderson, 1956; Treagus, 1974; Hickman, 1978; Thomas, 1979; Haselock et al., 1982). On a large scale, these folds plunge south-westwards so that the youngest stratigraphical units appear in that direction.

Petrographical work indicates that the nappe-folds and later upright folds were developed progressively, concomitant with an increase in regional PT conditions which peaked in the amphibolite-facies grade of metamorphism during D2.

Ductile shears have been recognised in the metasedimentary succession, including the Eilrig Shear Zone along which Grampian Group rocks are thrust over an underlying, lithologically distinct sequence of metasedimentary rocks of lower metamorphic grade. Other ductile shears occur, and the Gairbeinn Slide and Fort William Slide locally define the base and top respectively of the Grampian Group. However, recent papers have suggested that the Fort William Slide, while being a high strain zone, may represent a primary stratigraphical break rather than a purely tectonic discontinuity (Anderton, 1988; Glover, 1992). The following account provides general descriptions of the main regional structures and the various tectonic fabrics.

D1 nappe structures

Evidence for the presence of nappe-folds is found on the west side of Loch Treig where an inverted stratigraphical sequence is exposed on the limb of the D2 Blackwater Synform. Elsewhere, the only manifestation of D1 is a layer-parallel foliation and the local occurrence of minor D1 isoclines. West of Loch Treig, gently dipping Eilde Flags Formation rocks capping Meall Cian Dearg [NN 332 759] are known to be inverted from cross-bedding found along strike to the south-south-west (Bailey and Maufe, 1960; Treagus, 1974; Hickman, 1978; Glover, 1989). The rocks young downwards into Appin Group pelitic schists to form the inverted limb of the Kinlochleven Anticline and underlying syncline, referred to as the Treig Syncline by Hickman (1978) (Figure 11). The axial trace of the syncline passes through the small outcrop of the Leven Schist Formation on both sides of Loch Treig and can be recognised by the change in vergence of minor folds, as well as from changes in the trend of mapped lithostratigraphical boundaries. The axial surface of the Kinlochleven Anticline in the Loch Treig area lies above the present land surface. Westwards, the axial surfaces are folded around the upright D2 Blackwater Synform and Inverlair Antiform, and can be identified in the main outcrop of the Leven Schist Formation in the Glen Spean area. Here the axial traces are located on the eastern limb of the D2 Stob Ban Synform by changes in the angular relationships between bedding and an early schistosity, as well as by changes in the vergence of minor D1 folds. Cross-bedding in psammitic rocks at locality [NN 3167 8358] indicates that these rocks young to the east, in contrast to the westwards younging recognised elsewhere on the eastern limb of the Stob Ban Synform. The reversal of younging is the result of inversion on the common limb of the refolded early anticline/syncline fold pair (Figure 11)a.

The axial traces of the early folds have not been identified on the north-west limb of the Stob Ban Synform by the present survey, although Bailey (1934) suggested that the axial trace of an early syncline, which he referred to as the Appin Syncline, was located in the Ballachulish Limestone Formation. In the Glen Roy district, these rocks young upwards within an upright synformal fold which Bailey (1934) referred to as the Appin Core (here referred to as the Appin Synform), a D2 structure which he thought folded the early syncline. Certainly the Appin Synform defined in the outcrop area of the Ballachulish Limestone in the Glen Roy district is not a simple stnicture, and more than one generation of penetrative structures are found in the various lithologies. The attitude of the various fabrics indicates that an early syncline has been tightened during the D2 deformation. Bailey (1934) does in fact report a change in the attitude of the Appin Syncline from a recumbent nappe in the east (the Treig Syncline of Hickman, 1978) to a more upright western fold. Evidence for the nappe-folds is restricted in the Glen Roy district to the area south of the Corrieyairack Granitic Complex and it is probable that these early structures did not extend throughout the sheet area.

Bedding-parallel ductile shear zones, thought to be contemporaneous with the early folds, are found in rocks on the lower limb of the Treig Syncline. The extremely deformed rocks may be up to several metres thick, within psammites of the Eilde Flags Formation in the core of the Inverlair Antiform. Other shear zones are present at a higher stratigraphical level, in the Dog Falls Psammite Formation in the centre of the district [NN 368 863], where they occur as 20 m thick zones of attenuated and tightly folded vein quartz within schists with quartz-leaf fabrics and are traceable for up to several hundred metres. A strong penetrative early foliation is present in the psammitic rocks adjacent to these shear zones. The Fort William Slide was recognised by Bailey (1934) as a lag structure on the right-way-up western limb of the Appin Synform where it separates the Leven Schist Formation and younger rocks from underlying Appin Group (Spean Viaduct Quartzite Formation) and Grampian Group lithologies. In the Glen Roy district, the slide is exposed in stream sections north of Roybridge: in the Coire Ionndrainn [NN 2784 8660] and [NN 2817 8644], the Allt Coire Ceirsle [NN 2668 8491], and the Allt Ionndrainn [NN 2754 8384], and does not appear to be an important structure (see also Anderson, 1956). Ductile fabrics close to the slide plane are similar to the penetrative fabrics in adjacent schists. The main evidence for the presence of the slide is the attenuation of Appin Group stratigraphy across its trace, although most of the attenuation could be the result of primary lithological thinning (Anderton, 1988; Glover, 1992). The slide is folded around the Appin Synform and bedding in the psammites has been dragged out so that it is parallel with the slide on the eastern limb of the fold, for example in the Caol Lairig [NN 274 850]. Bailey (1934) traced the Fort William Slide to the south of Roybridge as a continuous structure on the western limb of his Appin Syncline. However north-eastwards, away from the Appin Synform, along the western limb of the Stob Ban Synform, the trace of the Fort William Slide has not been recognised. The basal boundary of the Leven Schist Formation, which is generally well exposed, is sharply defined but conformable with underlying Grampian Group rocks. The lag movement along the Fort William Slide in the south-west may be more widely dispersed in the Glen Roy district, as slip parallel to the bedding and early schistosity within Grampian Group psammitic rocks lying below the early nappes.

The Grampian Group rocks east of the Corrieyairack and Strath Ossian complexes have a well-developed early foliation which is subparallel to bedding. The bedding/early-foliation intersection lineations plunge gently south-westwards. Minor D1 folds are rare, although in the Moy Burn [NN 4175 8431] a calcareous pod within a psammitic bed is asymmetrically folded, the fold limbs being sheared out along the bedding surfaces of the psammite and indicating bedding-plane slip. Farther north, Haselock et al., (1982) have suggested that the Gairbeinn Slide is also a D1 structure, although in the present account it is described with the D2 shear zones.

Early isoclinal folds with subvertical axes occur within the Ossian–Geal Charn Steep Belt (Thomas, 1979), a zone of subvertical rocks of mixed lithologies found in the extreme south-east of the district. These rocks form part of the regional north-eastwards trending, 4 km-wide steep belt which is interpreted as a composite D1/D2 structure. Interference structures between D1 and D2 folds are exposed in the outcrop of the Meall Cos Charnan Formation, just north of the steep belt around [NN 44 78]. In the same unit is an early recumbent fold with pelitic schists in its core, as shown on the map cross-section.

D2 folds and related shear zones

Appin Synform and Bohuntine Antiform

The Appin Synform and complementary Bohuntine Antiform are upright folds located to the north-west of Bohuntine [NN 288 832] (Figure 10), and have axial planes that dip steeply towards the east-south-east. The folds have an axial-trace separation of about 800 m, and form a parasitic fold couplet on the western limb of the much larger Stob Ban Synform (Figure 11)a. The Bohuntine Antiform dies out along the axial trace and is confined to the Glen Roy district, although the western limb of the Appin Synform does extend beyond the district's western boundary for an unknown distance. Both folds have curvilinear axes with variable plunges, mostly to the south-south-west but steepening to plunge down the dip of the axial planes where the folds are reclined. The style of folding also varies along the axial traces. The Appin Synform is a tight fold on Creagan a' Mhuilinn [NN 265 842] but becomes more open in style when traced northwards to Brunachan [NN 318 896]. The Bohuntine Antiform has a tight fold style at Beinn a' Mhonicag [NN 288 854] and displays a double fold crest at Maol Ruadh [NN 266 828], the narrow strip of Ballachulish Limestone marking the position of the crestal synform.

Both folds are upwards-facing, indicated by cross-bedding preserved in psammitic rocks on all fold limbs. Some ductile attenuation of the stratigraphy has occurred on both limbs of the Appin Synform along the Fort William Slide (see above and (Figure 11)b. Minor folds and fold mullions are commonly associated with both the Appin Synform and Bohuntine Antiform, and are defined by folding of bedding and early microlithons (Plate 5). The minor folds show a steeply dipping axial planar schistosity. (Plate 6) shows tight folds of bedding in a Grampian Group psammite on the western side of the Stob Ban Synform, and illustrates the control of folding by the ductility contrast between beds of varying lithology.

Stob Ban Synform

The Stob Ban Synform is a major structure profoundly influencing the distribution and attitude of Grampian Group and Appin Group rocks. The synform has a northeastwards trending axial trace that can be identified throughout the Glen Roy district and which continues to the south-west, probably as far as Loch Leven, a distance of at least 50 km (Hickman, 1978). The Stob Ban Synform dies out towards the north-east and the northern limit of the Leven Schist Formation is controlled by the Creag a' Chail Synform which is a later structure (Figure 10).

The Stob Ban Synform is an upright fold with an axial plane that dips steeply to the south-east and a variable axial plunge. It has an asymmetrical profile; north of the Corrieyairack Granitic Complex, the fold has a subvertical eastern limb and a western limb dipping steeply to the south-east. Sedimentary structures on both limbs of the fold show a consistent sense of younging towards the axial trace, and thus it is upward-facing with the youngest unit of the succession, the Ballachulish Limestone Formation, disposed in elongated outcrops along the axial trace of the fold. North and north-west of the Corrieyairack Complex, there is no evidence for duplication of the sedimentary succession by isoclinal or tight, reclined folding. East of the Stob Ban Synform is the complementary Inverlair Antiform, described below.

The minor folds associated with the Stob Ban Synform have variable plunges and are tight. In the Leven Schist Formation, an associated penetrative fabric has axial planar and linear components. The latter is defined by lens-shaped porphyroblasts of biotite on muscovitic schistosity planes. It plunges consistently to the east-north-east and generally diverges at a high angle from the north-easterly trending axes of the D2 folds. It is not influenced by variations in the attitude of the fold axes and sheath folds have been recorded where they coincide [NN 2703 8092]. The relationships indicate that the lineation was the extension direction during D2 strain.

Inverlair Antiform

The Inverlair Antiform, first named by Hickman (1978), has a north-eastwards trending axial trace that can be recognised for about 9 km, from Sgurr Innse through Creag nam Meann [NN 310 770] as far as the Inverlair gorge [NN 341 805]. North of the Inverlair gorge, the antiform is truncated by the Strath Ossian Granitic Complex. The Loch Laggan Antiform, found to the northeast of the intrusion (Figure 12), may represent the north-eastern continuation of the Inverlair Antiform. South of the Glen Roy district, the fold is continuous with the Mamore Antiform on Sheet 54W, which can be traced for at least 10 km, as far as Loch Leven (Hickman, 1978; Thomas, 1979).

The Inverlair Antiform is an upright, asymmetrical, tight fold with a subvertical south-eastern limb and a more moderately inclined north-western limb. Thinning of the easterly limb occurred due to shearing during D2 folding (see section on tectonic fabrics), resulting in an attenuated Appin Group stratigraphy. The fold has a core of Grampian Group psammitic rocks which are shown by cross-bedding to be the right-way-up along the axial trace. Local reversals in younging in psammitic rocks on the south-east limb are attributed to minor folding, associated with DI shear zones in these rocks. At Cnap Cruinn [NN 303 776], vertically plunging, minor D1 isoclinal folds, within a tectonic slice of Appin Group pelitic schists, are folded around the Inverlair Antiform. Bedding and an early foliation in Grampian Group rocks are folded around the Inverlair Antiform, with the local development of an axial planar foliation in the more micaceous beds. D2 minor folds are common, and M-zones developed within the core of the antiform are well exposed in the Inverlair gorge and at Creag nam Meann [NN 31 77]. The minor folds are tight to isoclinal in style with moderate plunges to the south-west. Ductile shears are common along the minor fold limbs as well as on both limbs of the Inverlair Antiform (see (Figure 15)).

Blackwater Synform

The north-eastward trending axial trace of this upright fold, complementary to the Inverlair Antiform, can be traced northwards along the summit ridge of Meall Cian Dearg [NN 330 758] to the aureole of the Strath Ossian Complex near Fersit [NN 352 780] (Figure 10). Southwest of the district, the axial trace has been recognised for about 20 km towards the River Leven within the outcrop of the inverted Eilde Flags Formation (Treagus, 1974; Hickman, 1978). In the Glen Roy district, the rocks in its core are therefore inferred to be inverted. The fold is asymmetrical with a sub-vertical north-western limb and a south-eastern limb that is gently inclined to the north-west (Figure 11). The fold axis has a very low south-westerly plunge.

Parasitic folds are common on both limbs, and are recognised either as well-exposed fold closures in the pelitic schists on both sides of Meall Cian Dearg, or indirectly by changes in the angular relation of early planar fabrics with a later subvertical foliation which formed during the D2 folding. Minor folds are common and most plunge to the south-west at acute angles to D1 fold axes. Interference folds (Figure 13), formed by D2 folds superimposed upon early D1 folds, are well exposed on flat exposures of pelitic schists at the Loch Treig valve station [NN 343 760].

The second foliation is axial planar to minor D2 folds, and is widespread in both pelitic and psammitic lithologies folded by the Blackwater Synform. However, at [NN 3349 7508] this foliation is arcuate and is at an angle to the axial trace of a D2 fold.

Loch Laggan Antiform

The Loch Laggan Antiform and the subparallel Ossian–Geal Charn Steep Belt are structures of regional significance which control the distribution and attitude of metamorphic rocks east of the Corrieyairack and Strath Ossian granitic complexes. The Loch Laggan Antiform has a broad hinge zone trending north-eastwards and is an open structure which becomes tighter in style to the north-east of the district, around Aberarder [NN 479 876]. Bedding and a subparallel early foliation are folded by the antiform. In general, no axial planar penetrative cleavage is associated with the folding, although a locally developed crenulation cleavage, seen in semipelitic rocks and parallel to the axial plane of the antiform, may be contemporaneous with the D2 folding. At locality [NN 4525 8332], there is an isolated example of an axial-planar fracture cleavage in micaceous psammites. The Loch Laggan Antiform plunges gently south-westwards except in the thermal aureole of the Strath Ossian Granitic Complex where the plunge steepens (see Chapter 10).

North-west of Creag Meagaidh [NN 41 87], the northwestern limb of the Loch Laggan Antiform steepens and is subvertical adjacent to the Corrieyairack Granitic Complex Parasitic monoforms, with an axial-planar crenulation cleavage dipping steeply to the north-west, are seen on the north side of Creag Meagaidh. The north-western limb of the antiform is deformed by later folds, the effects of which are less obvious on the southeastern limb. The mean dip of bedding and the early foliation on the south-eastern limb steepens progressively to the south-east towards the complementary Beinn a' Chlachair Synform (Figure 12). Minor D2 folds on the south-eastern limb of the Loch Laggan Antiform were identified only near Meall Cos Charnan [NN 432 774].

Corrieyairack Synform

The Corrieyairack Synform has been described by Haselock et al. (1982) as a major D2 syncline that can be traced north of the Sronlairig Fault as far as Loch Killin on Sheet 73W (Invermoriston). In the Glen Roy district, the axial trace of the fold trends north-east to south-west and has been identified in the psammitic rocks of the Glen Doe Formation north of the Sronlairig Fault (Figure 10). Minor folds associated with the synform plunge at low angles to the north-east. The complementary D2 antiform to the north-west of the Corrieyairack Synform was recognised by Haselock et al. (1982) in the River Tarff by vergence of associated minor folds. The antiform, named the Tarff Anticline by Haselock et al. (1982), has been traced for some 8 km to the north-east as far as Cam Clach na Fearna [NH 432 089]. In the Glen Roy district, the axial trace of the antiform is located approximately by vergence of the minor folds and is recognised as far south as the Sronlairig Fault. The fold trace has not been located south of the fault.

Tarff Synform

The Tarff Synform has a north-north-eastwards trending axial trace and can be recognised for a distance of some 5 km southwards from the Sronlairig Fault to Cam Leac [NN 407 978] where the fold either dies out or is overprinted by the late Creag a' Chail Synform. The Tarff Synform (Figure 10) has a subvertical axial plane and a steep axial plunge to the south-south-west. Vergence of the associated D2 minor folds locates the axial trace within a micaceous psammite of the Auchivarie Formation, well exposed at the summit of Cam Leac (Figure 14). Because of the axial plunge of the fold, the psammite, named the Cam Leac Psammite by Haselock et al. (1982), tapers out when traced northwards to the Sronlairig Fault.

The Tarff Synform is displaced sinistrally by the Sronlairig Fault. North of the fault, the axial trace of the fold has been located by vergence of the minor folds and trends in an approximate north-north-easterly direction through Cam Bad na Circe [NH 383 014]. It is continuous with a D2 major synform recognised in the Tarff gorge by Haselock et al. (1982); the associated minor structures plunge at very low angles to the north-northeast.

Ossian–Geal Charn Steep Belt

The Ossian–Geal Charn Steep Belt is a composite structure made up of early D1 isoclinal folds with subvertical axes (such as that defined by prominent quartzites south-east of Meall Nathrach [NN 450 756] ) and later upright, tight D2 folds with axes showing variable plunge. The wavelength of the major D2 folds in the eastern part of the Glen Roy district decreases southeastwards from about 8 km to about 0.7 km in the steep belt. Tight D2 folds include the Beinn a' Chlachair Synform, which effectively defines the north-western limit of the steep belt, and may represent the eastward continuation of the Blackwater Synform; and the complementary Meall Nathrach Antiform with an axial trace passing through the summit of Meall Nathrach (Thomas, 1979) (Figure 12). The axial trace of the Beinn a' Chlachair Synform is located in the semipelitic schists on the steep northern slope of Meall Nathrach, and can be traced north-eastwards out of the district to Beinn a' Chlachair [NN 471 783]. Here the fold is tight in style with a well-developed axial-planar foliation in the fold core.

Small-scale folds related to both main fold phases are common in the steep belt and around Meall Cos Charnan. These minor structures vary in style from isoclines to asymmetrical tight folds with variable vergence. Their axes plunge moderately or steeply south-westwards, or, less commonly, to the north-east. They are mostly coaxial, although on the northern slopes of Meall Nathrach an early lineation is folded around a tight D2 fold closure. Axial-planar fabrics in the steep belt vary from a crenulation cleavage to a penetrative schistosity, with quartz-leaf fabrics in narrow, ductile shear zones. The amount of attenuation, the relative direction of movement, and the displacement within and across the shears are all unknown. Interference folds in the semipelitic rocks around Meall Cos Charnan create an 'egg-tray' effect on weathered rock surfaces.

Eilrig Shear Zone

The Eilrig Shear Zone consists of a thick sequence of mylonites and phyllonites, interbanded with less-deformed arkosic psammites. It defines the base of the Grampian Group succession on the western limb of the Stob Ban Synform and has been traced north-eastwards for at least 20 km, from Corriegour on the eastern slopes of the Great Glen above Loch Lochy to the Allt Lagan a' Bhainne [NH 392 011] where it is displaced sinistrally by the late, brittle Sronlairig Fault. North of the Sronlairig Fault, the shear zone can be traced to the northern boundary of the Glen Roy sheet where it occurs at the same structural level as a slide identified previously by Parson (1982).

The shear zone has a maximum thickness of approximately 1.0 to 1.2 km. In the Corriegour Burn [NN 267 921], it dips at varying angles to the south-east and is made up of bands of mylonite, several metres thick, separated by units of relatively undeformed Glen Buck Formation psammite in which pebbly layers can be recognised locally. Interbanded within the shear zone, in this stream section only, are discontinuous lenses of black phyllitic schist, quartzite, dolomitic marble and rare lenses of metabasite. At Eilrig [NN 371 997], the shear zone dips steeply to the south-east, and also consists of thick bands of mylonite separated by less-deformed psammite in which pebbly layers can be recognised. North of the Sronlairig Fault, the shear zone narrows and becomes more diffuse to include thin mylonitic layers and garnet-bearing phyllonites.

The Eilrig Shear Zone is structurally overlain by the 2 Group rocks that make up the western limb of the Stob Ban Synform (Figure 10). The base of the Grampian Group succession is obliterated and overprinted by ductile shear movements. Parts of the lower units of the

Grampian Group have been incorporated in the shear zone and are now represented by garnetiferous phyllonites. Beneath the Eilrig Shear Zone, the psammites and subordinate semipelitic bands of the Glen Buck Formation are muscovite-chlorite-rich, and are mineralogically distinct from and of lower metamorphic grade than the biotite-garnet-bearing rocks of the overlying Grampian Group (Figure 16). The Eilrig Shear Zone evidently separates distinctive me tasedimentary successions, and the stratigraphical relationships between the two successions are unknown. Kinematic indicators, represented by deformed pebble clasts in the mylonites, and rotated garnet porphyroblasts in the phyllonites, yield a consistent north-westward directed sense of shear that corresponds to a north-westward directed sense of overthrusting of the Grampian Group over the underlying metasedimentary rocks (Phillips et al., 1993).

Gairbeinn Slide

The Gairbeinn Slide was first described by Haselock et al. (1982) and was interpreted as a slide separating metasedimentary successions that are petrologically and geochemically distinct (Haselock, 1984). In the northeastern part of the Glen Roy district, the Gairbeinn Slide can be traced for a short distance, from the southwestern slopes of Gairbeinn [NN 458 981] to the contact of the Corrieyairack Granitic Complex. The Gairbeinn Slide has been recognised in the Dalwhinnie district (Sheet 63E) and has an extensive strike length in the Foyers district (Sheet 73E).

The Gairbeinn Slide separates a thick succession of psammites with upper units that contain distinctive pebbly bands — the Glenshirra Succession of Haselock et al. (1982) — from the semipelite and psammite succession of the overlying Grampian Group. The slide is marked by a zone up to 400 m thick north-east of the Glen Roy district, in which a foliation is progressively developed within the pebbly psammitic rocks as the slide contact is approached (Haselock et al., 1982). The pebble clasts become noticeably flattened and are elongate locally, defining a lineation which plunges down-dip to the south-east. Haselock (1981) reported that the maximum elongation, calculated from the deformed clasts, was in the north-west-south-east direction. The position of the slide is located at the sharp lithological contact between the pebbly psammite beds and the structurally overlying semipelites. The Gairbeinn Slide, like the Eilrig Shear Zone, separates metasedimentary sequences that are lithologically distinct. Both the Gairbeinn Slide and Eilrig Shear Zone are significant tectonic breaks, although the relationship between them is not fully understood. The metasedimentary rocks that structurally overlie the Gairbeinn Slide are at a lower stratigraphical level within the Grampian Group succession than those that overlie the Eilrig Shear Zone. Extensive development of mylonites, a distinctive feature of the Eilrig Shear Zone, is absent from the trace of the Gairbeinn Slide in the Glen Roy district.

Tectonic fabrics

Tectonic fabrics developed during both the DI and D2 fold phases. The early (S1) mica foliation is generally parallel to bedding, and varies from a penetrative schistosity to a domainal schistosity in which shape-oriented muscovite, biotite and rare chlorite are separated by quartzose laminae on a millimetre scale. Aligned biotite flakes define a linear fabric on the early schistosity planes within pelitic schists of the Leven Schist Formation. The early schistosity is variably overprinted by a later (S2) crenulation to transposition cleavage, and progressive stages of the development of the late cleavage have been recognised within Appin Group pelitic schists, largely based on observation of the relationships between different planar fabrics and synchronous garnet porphyroblasts (see Bell and Rubenach, 1983). The stages progress from undeformed to crenulated schistosity, to differentiated crenulation cleavage, to the complete development of a second transposition fabric (Phillips and Key, 1992).

Micaceous rocks in the cores of the main second folds have an axial-planar second foliation. On the fold limbs, reactivation of the early schistosity can be seen in thin sections (Phillips and Key, 1992), and is probably due to bedding-plane slip. For example, a detailed examination of the south-eastern limb of the Inverlair Antiform indicates that shearing continued along the developing second cleavage mica-lithons after reactivation, due to the common sense of shear between flexure slip and the bulk shear which induced folding (Figure 15). With continued deformation, the quartz domain fabrics were progres sively destroyed as the fabric homogenised. In contrast, on the north-western limb of the Inverlair Antiform, reactivation of the boundary between the Grampian and Appin groups resulted in a localised shift in the pattern of deformation partitioning (as described by Bell, 1985), so that most of the progressive shear was distributed along the bedding-parallel fabric (Figure 15). The local shear sense on this limb is antithetic to the bulk sense of displacement on the fold. As deformation continued, the second fabric's quartz domains were dissected, leading to the development of an anastomosing foliation which encloses lenticular, disrupted quartz domains. This new fabric is composed of segments of locally reactivated early schistosities as well as recently formed second cleavages. A more detailed description of these porphryroblast-fabric relations is given in Phillips and Key (1992).

Late structures

There is evidence for local ductile deformation after the generation of the upright D2 folds. Crenulation of fabrics generated during the early folding is widespread, although variable in its intensity. Locally, crenulations developed into a penetrative S3 cleavage which trends approximately east-west with steep northerly dips, and which has been recognised at Toman Liath [NN 355 903], near the valve station on Loch Treig [NN 3433 7600], at Meall a' Mheanbh-chruidh [NN 395 895], and at [NN 3259 8512]. Contemporaneous open, asymmetrical folds include monoforms that are coaxial to the east-west crenulations. Northward trending monoforms near Creag Tharsuinn [NN 4440 8540] have subvertical, westward dipping limbs. An open monoform with rounded hinges occurs at Meall a' Chomhlain [NN 32 94], where the flaggy character of the psammitic and semipelitic lithologies (due to S1 being parallel to So) becomes less evident as an oblique, north-eastward trending crenulation cleavage is developed. This cleavage dips steeply towards the north-west and is axial planar to the monoform.

The outcrop of the Leven Schist Formation is terminated to the north by the closure of the Creag a' Chail Synform (Figure 10). The dominant planar structure, the D2 schistosity, is folded showing that the synform is a late structure. The limbs are straight, i.e. without associated minor folds, and the interlimb angle is 70°. The hinge zone is exposed at [NN 397 954] where it is only about 50 m wide. Minor chevron-style folds with limbs 10 to 20 cm long occur in the hinge zone, but crenulations are very weakly developed and completely absent outside the hinge zone, which is surprising considering the highly micaceous lithology. The synform plunges steeply towards the south and the axial plane, which is taken as the bisector of the limbs, dips steeply to the east-south-east. Dips of bedding in the stream section north-west of Creag a' Chail, in quartzites and psammites of the Grampian Group, indicate that the hinge zone there is more rounded. However, at Carn Leac [NN 40 97] the synform is tight and indistinguishable in style from D2 folds.

Open folds, with interlimb angles exceeding 90°, are common at Toman Liath and the Allt Chonnal [NN 388 947] where they fold trondhjemitic veins (see Chapter 10). The crenulation cleavage at Toman Liath postdates the emplacement of appinites and the veins. A penetrative schistosity in both of these intrusive rocks can be traced into a crenulation cleavage in the host schists (see Frontispiece). However, the thermal effects associated with the emplacement of the main granodiorite of the Corrieyairack Granitic Complex are superposed on the crenulation cleavage. This evidence provides a chronometric framework for the growth of the crenulation cleavage.

Late chevron folds at Creag an Breac [NN 38 90] have a weak axial planar cleavage. Numerous reclined chevron folds, mostly plunging at low angles to the west-north-west with an associated crenulation cleavage, also occur in semipelitic schists north of Lochan a' Choire [NN 437 882].

A synform is developed in the Leven Schist Formation outcrop at Meall Ptarmigan [NN 426 904] (Figure 12). This open fold has a subhorizontal, north-westward trending axis, and folds the S1 early foliation and small isoclinal folds of bedding. Minor, probably unrelated folds in adjacent Grampian Group rocks vary from reclined folds with axial planes dipping gently south-eastwards to more open, asymmetrical warps with no preferred orientation.

Late conjugate kink bands and parallel microfractures are present in the pelitic schists of the Leven Schist Formation. These are particularly well exposed along the River Roy, upstream from Roybridge. Two main trends of the microfractures are recognised: between about 080° and 120° with sinistral offset, and between about 150° and 160° with dextral offset.

Regional metamorphism

Metasedimentary rocks of the Grampian and Appin groups have a relatively uniform regional metamorphic grade across the Glen Roy district, with garnet and biotite forming the peak metamorphic index minerals in pelitic and semipelitic lithologies. In marked contrast, the Eilrig Shear Zone represents a major metamorphic as well as structural boundary, separating Grampian Group rocks from the structurally underlying and lower grade Glen Buck Psammite Formation. However, there does not appear to be a change in metamorphic grade across the Gairbeinn Slide, as Haselock et al. (1982) recorded the presence of kyanite in the metasedimentary rocks below this structure.

The Grampian and Appin groups share the same thermal history of regional amphibolite facies metamorphism, resulting in the development of the following mineral assemblages:

Grampian Group

Appin Group

(Mineral abbreviations: Bio = biotite; Cc = calcium carbonate; Chi = chlorite; Ep = epidote; Gt. = garnet; Hb = hornblende; Mg-Chl = magnesium chlorite; Mu = muscovite; Ph = phlogopite; Pl = plagioclase; Ksp = potassium feldspar; Qz = quartz; Sp = sphene; St = staurolite; Ta = talc; Tr = tremolite)

Prograde metamorphism accompanied the early deformation history of these rocks, the thermal peak roughly coinciding with the imposition of S2. Three successive generations of fabrics have been identified, deforming rocks of both the Grampian and Appin groups. These are an early, bedding-parallel S1 fabric (Bio + Mu ± Chl); a variably developed S2 crenulation to transposition fabric (Bio + Mu + Gt); and a weak, locally developed S3 crenulation cleavage (Mu). A lower-grade metamorphic event accompanied the imposition of S3, and is represented by new muscovite growth parallel to this fabric as well as the formation of late muscovite porphyroblasts, oriented at random, where S3 is absent. A regional retrogressive event postdated the main tectono-thermal history of the Grampian and Appin groups, and resulted in the localised alteration of garnet and biotite to late chlorite (± opaque oxides).

The thermal history of the Grampian and Appin groups is recognised from the prograde mineral changes in the pelitic, semipelitic and calc-silicate rocks. The imposition of S1 was accompanied by regional biotite grade (?) metamorphism, resulting in the development of the assemblage Qz + Mu + Bio + Chl + Pl within the pelitic and semipelitic lithologies, and the assemblage Pl + Qz + Ep + Chl ± Bio, Mu, Cc, and opaque minerals within the spatially related calc-silicate rocks. Porphyroblasts of poikiloblastic, anhedral to subhedral biotite (annite fraction 0.40–0.52, phlogopite fraction 0.32–0.45) contain elongate inclusions of quartz, opaque oxide (ilmenite) and zircon which define a weakly developed straight inclusion fabric passing through the porphyroblast and concordant with the S1 fabric, in the matrix. Possible early chlorite is rarely pre- served as a metastable phase within S1 developed within the pelitic schists. No P-T estimates have been determined for Di.

An increase in metamorphic grade during deformation, which resulted in the imposition of S2, is recorded by the development of the assemblage: Qz + Mu + Bio + Gt + Pl (An13 to An22) ± rare St (in the Leven Schists only) within the pelitic schists. Where S2 is well developed within the pelitic and semipelitic schists, earlier developed biotite porphyroblasts and clusters of small biotite flakes were deformed during D2 to form lenticular to sigmoidal 'mica-fish'. Localised extension within the plane of S2 resulted in the opening of biotite cleavage planes which are infilled with coarse recrystallised quartz. It is possible that biotite porphyroblastesis within these pelitic lithologies occurred during the early stages of S2 development, the porphyroblasts being progressively deformed as deformation continued. Anhedral to euhedral, syn-kinematic garnet porphyroblasts (3 to 5 mm in diameter) developed within the pelitic schists, and contain inclusion trails which record the progressive development of S2 (Phillips and Key, 1992). These porphyroblasts locally comprise an inclusion-rich core enclosed by a relatively inclusion-free rim which cross-cuts the enveloping S2 mica-lithons, indicating that garnet growth continued after the imposition of this fabric. Analysed garnet porphyroblasts are almandine-rich (e.g. core: grossular 17–20%, pyrope 4–6%, almandine 74–77%; rim: grossular 12–15%, pyrope 6–8%, almandine 77–82%). Zoning profiles obtained for these porphyroblasts are characterised by typical bell-shaped Xspessartine profiles, with an antipathetic variation in Xalmandine. The garnet porphyroblasts which preserve the various stages of S2 development show only slight variation in composition. This is consistent with the growth of garnet during a single metamorphic event. Rare anhedral to rounded staurolite porphyroblasts in the pelitic Leven Schists overgrow S2 mica-lithons (Figure 15), indicating that peak metamorphism continued after D2.

The increase in metamorphic grade during the imposition of S2 resulted in the development of the assemblage PI (An15 to An36) + Qz + Hb + Gt + Ep + Bio, Mu, Chl (ripidolite), carbonate and opaque minerals within the calc-silicate rocks. Early chlorite flakes and small porphyroblasts are preserved as inclusions within later garnet and hornblende porphyroblasts. Anhedral to subhedral garnet porphyroblasts (grossular 16–24%, pyrope 6–12%, almandine 67–76%) contain fine-grained inclusions of quartz, epidote, opaque oxides and chlorite which preserve a locally intense planar fabric. Locally, zoned hornblende porphyroblasts (ferro-edenitic hornblende in the core to ferro-edenite at the rim) partially enclose garnet, and contain coarser-grained inclusion fabrics similar to the grain size of the matrix. Hornblende growth, therefore postdated that of garnet, at least in part, and possibly accompanied the recrystallisation of the matrix during peak (post-S2 ?) metamorphism. The absence of any reaction textures between these two porphyroblast phases suggests that garnet was not involved in the hornblende growth reactions.

Previously published P-T estimates for the garnet-micaschists and calc-silicate rocks from the Leven Schist Formation in the Spean Bridge to Roy Bridge area range from T = 535°C at a depth of 18.5 km (Richardson and Powell 1976), to T = 540° +/ −30°C with P = 6.5 ± 0.5 kbars (Powell and Evans 1983), to T = 525°C with P = 5.0 kbars (Wells 1979). Further P-T estimates of final matrix equilibration within pelite schists of the Appin Group have been made using the calibrations and appropriate mixing models of Hodges and Spear (1982) for the garnet-biotite geothermometer (Ferry and Spear 1978), and the garnet-muscovite-plagioclase recalibration of Ghent and Stout (1981) (Table 7). Other pressure estimates were obtained using the thermobarometer of Hodges and Crowley (1985) (Table 7) and (Figure 18). Estimates for peak metamorphism from the new data for samples containing the assemblage Gt + Bio + Mu + Pl + Qz ( ± Chl), lie in the range T = 500° to 600°C and P = 7.1 to 8.5 kbars. P-T estimates for the spatially related calc-gilicate rocks have been made using the garnet-biotite geothermometers of Hodges and Spear (1982) and Ferry and Spear (1978), and garnet-hornblende Fe-Mg exchange geothermometer of Graham and Powell (1984). In general, temperature estimates obtained for the calc-silicates are comparable to those determined for the garnet-micaschists (Table 7). Quantitative P-T paths have been determined for zoned garnet porphyroblasts from pelitic schists of the Appin Group, pelitic schists using the procedures and thermodynamic data reported by Spear and Silverstone (1983) and employing the programme P-T PATH of Spear (1986). Start P-T conditions were established using the above standard geothermometers and geobarometers for the assemblage: garnet + biotite + plagioclase + muscovite + chlorite + quartz + H2O. The resultant P-T paths all describe increasing P and T during garnet growth, indicative of a burial P-T path (AT = 10° to 25°C and AP = 0.5 to 2.1 kbars) (Figure 18). A number of paths constructed for the Appin Group pelitic schists commence with an initial increase in P and to a lesser extent T, followed by a period of more rapidly increasing T with respect to P (Figure 18). Consequently, the overall P-T path followed by the Appin Group pelitic schists is interpreted as having an initial period of rapid burial, possibly associated with nappe emplacement, which was then followed by a period of increasing T and to a lesser extent P, possibly due to heating and thermal equilibration of the schists.

Below the Eilrig Shear Zone, psammitic and semipelitic schists containing the assemblage: Qz + Mu + Chl + Pl + opaque oxides represent the peak metamorphic conditions reached by the Glen Buck Formation, in greenschist facies. Within the Eilrig Shear Zone the pebbly arkosic psammites of the Glen Buck Formation are intensely deformed, resulting in the development of a sequence of protomylonites, quartz-feldspar-mylonites, ultramylonites and phyllonites, interbanded with relatively less-deformed psammites.

At the base of the shear zone these quartzofeldspathic mylonites comprise the assemblage Qz + Pl + Ksp + Mu + Chl. In the more semipelitic lithologies, the mylonitic fabric is composed of alternating quartz and mica-rich (muscovite and chlorite) domains. Towards the top of this major shear zone, biotite locally appears within the mineral assemblage of both the quartz-feldspar-mylonites and the semipelitic mylonitic rocks of the Glen Buck Formation (Figure 16). An overall increase in the intensity of late recrystallisation of these mylonitic rocks is also observed towards the top of the Eilrig Shear Zone and in the base of the overlying Grampian Group. Detailed petrographic work has identified three discrete mineral assemblage zones (based upon key index minerals) which occur parallel to the strike of the Eilrig Shear Zone (Figure 16), with an increase in metamorphic grade towards the top of this major shear zone and the boundary with the structurally overlying, higher-grade Grampian Group.

At Eilrig [NN 371 997], the quartz-feldspar-mylonites of the Glen Buck Formation are interlayered with subordinate, highly foliated, locally garnet-bearing, micaceous quartz-mylonites and phyllonitic schists of uncertain origin (they may be altered Grampian Group rocks). In thin section, these highly foliated rocks comprise the assemblage Qz + Bio + Mu + Pl + Chl ± rare Gt. Garnet (almandine = 69–75%; grossular = 18–25%; pyrope = 5–8%) forms small, anhedral poikiloblasts which are partially enclosed by weakly to moderately well-developed pressure shadows. The poikiloblasts contain rounded to elongate inclusions of quartz and minor opaque oxide which define straight inclusion trails passing through the porphyroblast concordant with the external matrix foliation, and indicate post-kinematic garnet growth.

Near to the contact with the Eilrig Shear Zone, the regional S1 and S2 fabrics in the Grampian Group are locally transposed by a well-developed mylonitic foliation, resulting in the development of garnet-bearing phyllonites and phyllonitic schists in the Grampian Group. The mylonitic fabric in these phyllonites is defined by fine-grained muscovite with subordinate biotite and opaque oxides. Muscovite also forms small, late (post-mylonitisation), porphyroblasts, oriented at random and cross-cutting this fabric. Anhedral to subhedral (1 to 2 mm), retrogressed, syn-kinematic garnet porphyroblasts (almandine = 67–77%; grossular = 13–29%; pyrope < 10%) preserve fine-scale snowball, spiral and 'S' shaped inclusion trails which yield a north-westerly directed sense of shear (Phillips et al., 1993). Biotite (Xannite = 0.42–0.53; Xphlogopite = 0.32–0.41) is locally retrogressed to pale green chlorite. Initial pressure and temperature estimates for the various garnet-bearing phyllonites and rare garnet-bearing quartz-mylonites incorporated within the Eilrig Shear Zone (P = 6.5 to 8.0 kbars and T = 450° to 575°C, Phillips et al., 1993) suggest that mylonitisation, resulting from ductile movement along the shear zone, occurred during prograde amphibolite facies metamorphism. These initial P-T estimates are comparable with estimates for peak regional metamorphism which accompanied regional deformation of the Grampian and Appin groups.

Summary of tectono-thermal events

The metasedimentary rocks of the Grampian and Appin groups in the Glen Roy district were initially folded during a polyphase (D1/D2) ductile deformation event which was accompanied by regional metamorphism. The north-western limit of early nappes, first described in detail by Bailey (1934) from the Ben Nevis district, occurs in the southern part of the Glen Roy district, south of Glen Spean. Here, the amplitude of these earliest folds, which affect uppermost Grampian Group and Appin Group rocks, decreases between Loch Treig and Roybridge, concomitant with a change in their attitude from recumbent to more upright (Figure 17)a. Elsewhere in the district, the only manifestations of the early D1 deformation are bedding-parallel foliation surfaces and ductile shears, mostly found in Grampian Group lithologies. Some local folding also took place adjacent to the larger ductile shears, notably in the south-east in the Ossian-Geal Charn Steep Belt. The nappe structures resulted in bulk transport to the north-west; the sense of movement along the underlying Eilrig Shear Zone also indicates north-westwards directed thrusting. Presently exposed rocks were at depths of about 10 to 12 km with temperatures raised to about 450°C at the end of D1 (Figure 18).

Continuing deformation resulted in the development of north-eastward trending upright D2 folds throughout the district, together with ductile movement along major shear zones, notably the Eilrig Shear Zone and probably within the Ossian–Geal Charn Steep Belt. Northwestward directed thrusting has been proved along the Eilrig Shear Zone (Phillips et al., 1993), and represents transport at deeper structural levels relative to DI. The rocks overlying the Eilrig Shear Zone were raised to temperatures of about 550°C during D2, at a depth of 20 ± 5 km. The increase in depth relative to D1 may be due to the propagation of the early nappes at higher structural levels in rocks now eroded away. The hot mass of folded Grampian Group and younger rocks was thrust over a cooler suite of rocks which now form the footwall of the Eilrig Shear Zone. P-T calculations based on the metamorphic mineral assemblages in these lower-grade rocks indicate that they were buried to depths of about 5 km prior to thrusting. It is difficult to quantify accurately the amount of lateral tectonic transport directed north-westwards along the Eilrig Shear Zone, as its regional dip at depth is not known. Assuming regional dips of 10° and 30° would give figures of 60 and 20 km respectively, using the depth figures quoted above for the juxtaposed footwall and hanging-wall successions.

The polyphase deformation preceded the emplacement of all the igneous rocks cutting rocks of the Grampian and Appin groups in the district. Consequently, an isotopic age of about 450 Ma for the oldest of these intrusions provides a minimum age for D1 and D2. There is no maximum age on deformation, other than an assumed Neoproterozoic age for the original sediments. Neither is there any constraint on the duration of the early deformation. The post-D2 folding occurred during the period of Ordovician to Silurian igneous activity.

Small arrows represent calculated P-T paths from zoned synkinematic garnet porphyroblasts. Large coloured arrow represents P-T path followed by the Grampian and Appin groups during prograde regional metamorphism which accompanied D2. Reaction [1]: garnet in reaction taken from Yardley (1989); Reaction [2]: staurolite in reaction taken from Symmes and Ferry (1991). Open squares = average P-T estimates for Appin Group pelitic schists; open diamonds = average P-T estimates obtained for the Appin Group semipelitic schists; filled diamonds = average P-T estimates obtained for the Grampian Group calcsilicate rocks.

Chapter 7 Pre-Caledonian igneous rocks

Granitic gneiss

A large, elongate body of granitic gneiss crops out immediately north-west of the Great Glen Fault Zone, and forms a continuous outcrop extending from North Laggan at the southern end of Loch Oich, to Fort Augustus and the south-western shores of Loch Ness (Figure 3). The body is approximately 2 to 3 km wide and some 14 to 15 km long. It is truncated to the south-east by faults of the Great Glen Fault Zone, and is overlain structurally by psammitic rocks of the Moine succession to the west, north and east. Only the south-western part of the granitic gneiss body occurs in the Glen Roy district, where it underlies an area of some 15 km2 extending north-east and south-west of the River Garry. The contact of the granitic gneiss with the Great Glen Fault Zone can be traced from North Laggan to near the ruins of Invergarry Castle [NH 316 008]. The granitic gneiss, called the Fort Augustus granitic gneiss by Parson (1982), Barr et al. (1985) and Brown (1991), is the most easterly of several large discrete masses of granitic gneiss that can be traced along a sinuous outcrop pattern, from Fort Augustus in the east through Loch Quoich and Loch Arkaig to the most westerly body, the Ardgour gneiss, at Glenfinnan (Barr et al., 1985)

The granitic gneiss is homogeneous in appearance throughout its outcrop in the Glen Roy district, and is a pale grey, coarsely crystalline, well-foliated rock of granitic composition, in which the foliation is defined by thin layers of oriented biotite that are separated by coarser segregations of feldspar and quartz. In many exposures it is cut by acid granodioritic veins and includes schistose metabasite intrusions.

Thin sections show the gneiss to be made up predominantly of microcline and quartz. Oligoclase is commonly present, and the mineralogy of the gneiss can be defined in terms of a two-feldspar-and-quartz assemblage throughout its outcrop. Myrmekitic intergrowths, formed by the replacement of potassium feldspar by plagioclase and quartz, are commonly present. Biotite, with less common muscovite, is a subordinate constituent but is always present, making up no more than 5 per cent of the rock volume (Parson, 1982). Garnet is a widespread accessory mineral, and small, disseminated crystals of hornblende occur locally. Other accessory minerals include epidote, sphene, zircon, rutile and iron oxide.

The contact of the granitic gneiss with the structurally overlying psammitic rocks is sharp and distinct. There is no evidence of 'soaking' of the psammitic rocks by granitic gneiss or the development of diffuse margins. Near to the contact with the granitic gneiss, the psammitic rocks are intruded by subconcordant sheets of granitic gneiss, some more than 1 m thick, and the evidence indicates a close interleaving of the two rock types at the boundary. Such interleaving is nearly always concordant with the foliation of the psammitic rocks and with that developed in the granitic gneiss; only rarely are transgressive contacts seen, formed by thin sheets of granitic gneiss cross-cutting the foliation in the psammitic rocks at a low angle.

North of the River Garry, the contact between the psammitic rocks and the granitic gneiss, which is typically parallel to the foliation in both rock types, has a gentle to moderate dip to the north or north-west, and it is clear that the granitic gneiss structurally underlies the psammitic rocks. South of the River Garry, the inclination of the contact and foliation steepens, and shows a moderate to high dip to the west.

The granitic gneiss is folded, the foliation showing rapid abrupt changes in attitude which define the existence of broad open folds with north-north-eastward trending axial planes and subhorizontal axial hinges. These late open structures fold earlier, tight, recumbent minor folds that, although uncommon, are present throughout the granitic gneiss outcrop. Such early folds occur as folded quartz-feldspar foliae. Locally, the gneissose foliation appears to be axial planar to the early folds.

Barr et al. (1985) concluded that the major granitic gneiss bodies between Fort Augustus and Glenfinnan represent a suite of deformed and metamorphosed granite intrusions, emplaced in the Moine country rocks during the earliest recognised tectonic event. The significance of a Rb87/Sr86 isochron of 1028 ± 43 Ma from the Ardgour granitic gneiss at Glenfinnan (Brook et al., 1976) is not fully understood. Radiometric dates from other granitic gneiss bodies have not been determined. If the present interpretation that all the granitic gneiss bodies were emplaced during the same phase of igneous activity is correct, then the age determination from the Ardgour granitic gneiss should be applicable to the Fort Augustus granitic gneiss exposed in the Glen Roy district, and may point to a Grenvillian age of intrusion.

Metabasite

Both the Moine psammitic rocks and the granitic gneiss frequently contain inclusions of metabasite as layers, pods and lenses, varying, from thin discrete layers only millimetres thick, to pods up to 2 m thick. They occur most commonly as concordant, well-foliated sheets of hornblende schist, which may vary from 0.1 to 1.0 m in thickness and which taper out along strike. Less commonly, massive pods or bands of amphibolite also occur. (Also present is a body of variable hornblendic rocks interpreted as a metamorphosed Caledonian intrusion and described in Chapter 9).

The hornblende schist and amphibolite are dark coloured, fine- to medium-grained rocks, composed predominantly of greenish brown hornblende and plagioclase with subordinate amounts of biotite. Sphene, epidote and iron oxide form the most common accessory minerals. The plagioclase nearly always shows variable but widespread alteration, and the hornblende usually shows some degree of alteration to biotite and chlorite, with chlorite locally forming pseudomorphs after biotite.

The foliation of the hornblende schists is defined by thin laminae, rich in hornblende, alternating with thin, discontinuous, feldspar segregations. The hornblende crystals may also be dimensionally oriented, and a strong linear fabric may be developed.

The contacts of the hornblende schists and amphibolites with the surrounding country rocks are always sharp and there is no evidence for the presence of gradational contacts. The foliation developed within the hornblende schists remains strictly concordant with that in the adjacent country rock, and is regarded as having formed during the same tectonic event.

The bodies of hornblende schist and amphibolite are interpreted as metamorphosed basic intrusions. Smith (1979) described similar rock types that make up his amphibolite suite and are distributed throughout the Northern Highlands. They are considered to have been emplaced as minor doleritic intrusions early in the deformation history. The emplacement of the intrusions postdates the formation of the granitic gneiss, and the metabasites are invaded by 428 ± 6 Ma pegmatites in Glen Urquhart (Brook and Rock, 1983). Rock et al. (1985) suggested that the metabasites were possibly of Grenvillian age.

Chapter 8 Early vein complexes

A considerable proportion of the Glen Roy district is made up of late- to post-tectonic granitic rocks, of which the Loch Laggan and Dog Falls vein complexes are important components (Figure 19). Anderson (1956) described the Loch Laggan Pegmatite Complex as a 3 to 6 mile-wide, 12 mile-long belt of sharply defined dykes and sheets, extending north-eastwards from the Strath Ossian Granitic Complex towards Garva Bridge (Sheet 63E). Clayburn (1981) recognised a whole suite of intrusive rocks within the complex, varying from aplite, micro-granite and granite to pegmatite, and confirmed its areal extent. Clayburn (1981) also regarded veins centred on the leucogranite of Beinn Teallach [NN 360 850] as an early complex. However, there is evidence that these were emplaced at a later stage, and they are now considered to be a part of the Corrieyairack Granitic Complex (Chapter 10). The Dog Falls Complex is probably a satellite of the much larger Loch Laggan Complex. Both have been affected by the final stage of regional (D1/D2) ductile deformation, and both largely predate intrusion of the Appinite Suite, although exposures to the south of Loch Laggan suggest some overlap with a prolonged period of mafic plutonism which continued after emplacement of the acidic veins. Members of the Loch Laggan and Dog Falls complexes are cut by the Ben Nevis and Etive dyke swarms.

Loch Laggan Vein Complex

The Loch Laggan Complex (Figure 19) has a restricted range of compositions but texturally it is extremely varied. The rock types can be grouped into grey microgranite, pink leucogranite, pegmatite and aplite. Granite and pegmatite are by far the most abundant, and the greatest concentrations of intrusions are at Binnein Shuas ([NN 46 82], Sheet 63E) and Tom Ban [NN 42 83]. Both Anderson (1956) and Clayburn (1981) observed a decrease in the intensity of veining with altitude, and Clayburn claimed that there is a roof zone to the complex above 850 m OD in the Creag Meagaidh area. However, recent mapping by BGS does not support this conclusion as there are equally low proportions of veins on the high and low ground in this northern peripheral area of the complex. The abundance of the intrusions decreases away from the core areas of Tom Ban and Binnein Shuas. An accurate estimate of the relative proportions of country rock and intrusions is precluded over much of the complex because of poor exposure. Clayburn (1981) suggested that at least 80 per cent of the core area is composed of pegmatite and related rocks. The peripheral area contains a much lower proportion and the limit of the complex (Figure 19) is, very approximately, where the figure falls below 1 per cent.

Most of the veins are 1 m or less thick and only 16 per cent exceed 5 m (Clayburn, 1981). Many of the thickest are in the core area but some occur more marginally; for example, a vein in the cliffs west of Loch Roy [NN 413 893] is up to 100 m thick. At Tom Ban [NN 42 83], pegmatite forms a vertical-sided stock with an outcrop of about 1 km2, and veins in the core area to the south-east of Loch Laggan are commonly up to 60 m wide. Apart from the thicker dykes, it is not usually possible to trace a particular intrusion beyond a single exposure, and most are inferred to be no more than 100 m long. They are generally subvertical and trend north-eastwards, but some dip less steeply.

In many exposures, several intersecting veins show no regular order to the cross-cutting relationships, with pegmatite cutting pegmatite irrespective of orientation, pegmatite cutting granite and vice versa (Plate 7). In the few localities where grey microgranite occurs, it is cut by granite and pegmatite, for example at [NN 4131 8773]. Dykes may splay, with apophyses at various angles to the main dyke. Opposite margins of individual dykes are not necessarily parallel; wedge-shaped intrusions and elongate lensoid veins are common.

The thicker intrusions commonly contain screens of country rock which have not been rotated during detachment from the adjoining vein wall. They probably lie at the boundary between successive phases of injection, and indicate that a vein that is otherwise homogeneous is, in fact, multiple. Small xenoliths tend to be orientated at random. Intrusive contacts are mostly sharp with no sign of shearing. Diffuse contact zones of partly assimilated country rock and partly digested xenoliths have also been recorded.

Large multitextured veins in the core of the complex show gradational contacts between granite and pegmatite. In the peripheral areas, many of the veins are composite with a sharp, but extremely irregular contact separating leucogranite from pegmatite. Crystals of feldspar, up to 20 cm long, project into the leucogranite, and it is possible to infer the nucleation of crystals on the vein walls followed by inward growth, with magma continuing to flow in the central region. During this process, an abrupt change of conditions, probably a decrease in volatile content, led to the crystallisation of granite in the spaces between previously crystallised pegmatite.

Many of the otherwise uniform pegmatite veins contain pockets in which quartz makes up much of the rock. Mica, mainly muscovite in books up to 3 cm thick and 10 cm across, tends to be concentrated within these pockets. Narrow (less than 5 cm) zones lying parallel to the strike of a few of the veins contain crystals of garnet up to 10 mm in diameter. Pegmatite is commonly cut by quartz veinlets, each lying approximately normal to the trend of the host vein and tapering out before reaching its margin, relationships which indicate that they are infilled tension gashes. Folded pegmatite veins are rare, although an example has been found at [NN 4457 8526].

Clayburn (1981) described the mineralogy of the various textural types within the Laggan complex. All are composed of potassium feldspar (microcline), plagioclase (albite-oligoclase), quartz, biotite and muscovite in varying amounts, with or without traces of garnet, sulphides and zircon. No valuable or rare minerals have been found. Commonly, microcline is graphically intergrown with quartz, and plagioclase laths tend to be smaller than adjacent crystals of microcline. Quartz grains are commonly strained and microshears occur locally.

Clayburn (1981) concluded that pegmatites are best dated by a feldspar isochron plot, giving an age of 439 ± 7 Ma with an initial 87Sr/86Sr ratio of 0.70964. This age is slightly older than the regional cooling age of the country rock below 300°C at 405 ± 9 Ma, which implies that the pegmatites were emplaced into hot rocks. The country rocks reached a maximum temperature of about 560°C during the regional metamorphism; its climax preceded the emplacement of the pegmatites (Clayburn, 1981; Wells, 1979).

Clayburn (1981) also undertook analyses of the isotopic lead content of 16 samples of the pegmatites. The range of 206, 207, 208pb to 204Pb ratios within the Loch Laggan complex is similar to other pegmatite complexes in the Grampian Highlands. The data imply an old lower crust depleted in U and Th, with non-radiogenic Pb and a low Th/U ratio, and indicate that the pegmatite complex resulted, at least in part, from crustal reworking. These data complement the strontium isotope results whose low initial 87Sr/86Sr ratios also imply derivation by melting of continental lower crust.

Dog Falls Vein Complex

The veins consist of pink leucocratic granite and pegmatite, and are well exposed on clean scoured surfaces at Dog Falls [NN 373 896]. Some of the granite veins have pegmatitic margins and, although all the veins belong to a single complex, cross-cutting relationships show that there was multiple injection. Pegmatite most commonly cuts granite but the reverse relationship is also found. Vein thickness ranges from a few centimetres to about 4 m. The veins make up no more than 2 per cent of the total rock at any locality, and the limit of the complex can only be placed very approximately (Figure 19). The main planar structure (schistosity) of the country rocks strikes north-eastwards, and the granite veins tend to be concordant whereas the pegmatite veins are commonly discordant with a north–south strike and steep dip. However there is much variation; veins branch and have tapering offshoots. Concordant veins are mostly straight but close examination often reveals slight necking with the schistosity of the country rock psammite curving into the necks. Completely separated boudins occur in a few of the exposures, and quartz-filled tension fractures are common. Veins lying at a large angle to the schistosity are slightly curved or strongly folded and examples of folded discordant pegmatite veins cutting straight concordant veins of granite have been recorded. All the veins are very much less deformed than the pre- to syntec tonic quartz veins in the adjoining country rock, and they have only been affected by the final stage of strain associated with the development of the regional schistosity.

Chapter 9 Early basic intrusions

Many of the minor intrusions and a few of the larger igneous masses in the Glen Roy area are composed of the distinctive rocks making up the appinite suite (Figure 19). These calc-alkaline lamprophyric rocks consist mainly of appinitic diorite and its finer-grained equivalent, spessartite, though rock types ranging from ultrabasic to acidic composition occur. Emplacement of the appinite suite was associated with the formation of pipes which are infilled with breccia composed entirely of meta-sedimentary clasts. Volatile-rich basic magma reached high levels in the crust and volcanicity probably took place at the surface. The emplacement of the appinite suite closely followed the injection of the acid pegmatites discussed in Chapter 8, and there is evidence in the southern part of the area of some overlap in the time of emplacement. Examples of lamprophyre and appinite cut by members of the Ben Nevis and Etive dyke swarms have been found at numerous localities. In the Toman Liath area [NN 37 91], minor intrusions of the appinite suite are cut by veins of trondhjemite, and both possess a penetrative fabric which developed during a period of north–south compression. The latter postdates the trondhjemite but predates the Ben Nevis Dyke Swarm.

Achavady Appinite

Field relationships and emplacement history

An excellent, easily accessible, north to south section (Figure 20) through the appinite is exposed in the River Roy below the ruins of Achavady [NN 296 866]. To the west of the river, exposure is confined to small streams and a few small crags on the steep hillside. Better exposure to the east can be found in the larger streams and on several large crags. The exposures and a distinctive vermiculite-bearing soil allow the margin of the intrusion to be traced, and show that it is elongated in a northwesterly direction across the boundary between the Grampian and Appin groups.

The major lithologies in this multiple intrusion are a micaceous mafic appinite which may be foliated (notably near the northern margin of the intrusive complex) and a xenolithic appinite (chaotic breccia) which forms discrete sheet-like bodies. There are several generations of felsic veins including the late pink-red linear granitic veins shown in (Figure 20). A small breccia pipe cuts the northern xenolithic appinite sheet, which is also cut by lamprophyric dykes.

The mafic micaceous appinites are mostly isotropic, fresh coarse-grained rocks comprising black hornblende, commonly euhedral, which may be enclosed by poikilitic tremolite, with silvery phlogopitic biotite flakes and talc. A mottled texture is caused by segregation of black horn- blendes. The feldspar content is variable, with discrete white crystals disseminated through the groundmass as well as diffuse pink-white segregations and late thin vein-lets. Poikilitic feldspars (microperthite) include both biotite and hornblende crystals. In some of the exposures, feldspar forms augen, wrapped by a mica fabric, and macroscopically the rock is foliated. The foliation is also deflected around hornblende augen. Small, rounded, pale green ultramafic xenoliths are distributed at random in the massive appinite which also has rare angular country rock rafts near the margins of the complex.

The xenolithic appinites (chaotic breccias) comprise a felsic appinite matrix which supports a variety of poorly sorted appinitic and metasedimentary xenoliths. The xenoliths of appinite are always more melanocratic than the enclosing matrix. This is also true of xenoliths within xenoliths, e.g. rounded ultramafic clasts are present in massive appinite xenoliths in the xenolithic sheets. The appinite xenoliths locally contain their own feldspathic veinlets. All the igneous xenoliths tend to be rounded and smaller than the various metasedimentary clasts, which are angular and may be up to several metres long. Among the latter, the most common are psammite and metaquartzite from the adjacent country rock, although schist and marble occur in the largest sheet in the northern part of the section. Partial melting of the psammitic xenoliths has produced plastically deformed veins which penetrate the supporting appinite. Stringers of appinite also penetrate the xenoliths. The more feldspathic appinitic matrix may be foliated parallel to the strike of the xenolithic sheets.

The late breccia pipe which cuts the northern xenolithic appinite sheet (Figure 20) is several metres in diameter and has a steep easterly plunge. The breccia is clast-supported, and both the metasedimentary and igneous clasts generally range from about 2 to 8 cm in length, although some of the igneous clasts are up to 1.5 m long. The clasts of psammite and appinite have been deformed plastically to mould around each other at contact points. Clasts of vein quartz are angular and have been fractured. Evidence for the presence of a small amount of liquid magma during the formation of the breccia is provided by the presence of interstitial felsic appinite.

The thin pink-red granitic veins are fine grained although the 20 m-thick sheet emplaced along the southern margin of the large central psammitic raft is a medium-grained, two feldspar, hornblende granite (Figure 20). Two dark grey, fine-grained lamprophyre dykes which cut the northern xenolithic appinite sheet also cut the granite veins in this sheet. The thicker of the two dykes has offshoots which are chilled against the xenolithic appinite. The exact relationships of the granitic veins and lamprophyre dykes to the appinitic magma are unclear. It is possible that the granite veins emanated from the adjacent, roughly contemporaneous, red granite of the Corrieyairack Granitic Complex. The dykes may be late differentiates of the appinite; they are not part of the Ben Nevis Dyke Swarm.

The complicated intrusive history of the Achavady Appinite is not fully understood. However, the following is a tentative sequence of events.

  1. Intrusion of an appinitic magma into the crust with some crystal fractionation to produce a solid ultramafic phase, followed by intrusion to the present level and the incorporation of ultramafic xenoliths in mafic appinite.
  2. Violent brecciation of this magma (fluid/solid mix) and its country rock to produce the xenolithic appinites. Initial fracturing was followed by plastic movement (a process repeated in the late breccia pipes) which produced the rounded clasts.
  3. Emplacement of another pulse of primary magma into the xenolithic appinite. Internal stresses produced the marginal foliation in the fluid/solid magma.
  4. Intrusion of the late breccia pipe into the still hot appinite.
  5. A cooled appinitic complex is finally cut by the pink granitic veins and by the lamprophyre dykes.

Mineralogy and mineral chemistry

The appinitic rocks as a group are characterised by abundant amphibole and/or mica, co-existing with clinopyroxene and, less commonly, olivine. Feldspar is generally restricted to the groundmass. Amphibole occurs as euhedral phenocrysts and as later rims around early-formed clinopyroxene. The dark mica is generally later than the clinopyroxene and amphibole, and occurs as oriented blades at random in the groundmass and crosscutting early-formed phases, and as oriented aggregates defining a foliation around clinopyroxene and amphibole. Olivine is rare and usually highly altered. The amphibole has a range of compositions, including magnesian hornblende, actinolitic hornblende, edenitic hornblende and edenite. The clinopyroxene is predominantly magnesian augite with up to about 1 per cent Cr2O3, while the olivine grains have Fo contents up to 88 per cent and high Ni contents (up to about 0.4 per cent NiO). The dark mica is dominantly phlogopite with subordinate titanian biotite. The feldspars are predominantly orthoclase and albite, commonly almost pure end members in the groundmass, with some orthoclase containing up to 3.8 per cent BaO. Some less acidic plagioclase (An content up to 50 per cent) is also found in some of the more mafic samples. The co-existence of acid plagioclase and K-feldspar with magnesian clinopyroxene and olivine is a characteristic feature of appinitic magmatism (Hamidullah and Bowes, 1987).

Geochemistry

Representative analyses of three intrusive types are listed in (Table 8). The felsic appinite represents the matrix to the xenolithic appinite of (Figure 20). In common with lithologies described from the appinite type locality near Ballachulish (Wright and Bowes, 1979; Hamidullah and Bowes, 1987), the samples from Achavady are characterised by high Ba, Sr, Rb and K2O values coupled with high Ni, Cr and MgO. The mafic appinites show significantly greater Ni and Cr contents as well as higher K2O/Na2O ratios than the other intrusive lithologies, but lower Sr abundances (Figure 21), 1–3. The high modal mica content of many of the mafic appinites explains their high K2O and K2O/Na2O, but whether this reflects late-stage autometasomatism rather than original magma composition is not clear. The increase in Sr with SiO2 suggests that plagioclase was not an early separating phase in these magmas, probably because of the high H2O content of the appinitic melt. The high Ni and Cr content of many of the more mafic appinites confirms that these are cumulate in nature.

Primordial mantle-normalised 'spidergrams' (Figure 21), 4–5 reveal that the lithologies have prominent negative Nb anomalies with high LIL/HFS element ratios characteristic of subduction-related magmas. The appinites also have distinct negative Th anomalies, the origins of which are not yet clear. In (Figure 21), 6–8, the Achavady Appinite is compared with contemporaneous basic magmas from the south-west Highlands, namely the appinites of the Balluchulish district and the basaltic lavas of the Lorn Plateau. The majority of the Achavady samples have significantly higher K2O/Na2O ratios than either the Ballachulish appinites or Lorn lavas, but have a similar range of La/Nb values. The majority of both groups of appinites are generally less evolved than the Lorn lavas and have lower La/Y values.

Glas Bheinn Appinitic Complex

This complex, is 1.8 km by 0.9 km in plan (Figure 19) and (Figure 22) and comprises three components which are intimately associated with each other: (1) hornfelsed schist and schist breccia; (2) quartzite; and (3) ultramafic rocks and varieties of appinite. The boundary of the complex can be mapped fairly accurately but its interior is marked by smooth grassy slopes with scattered rocky knolls, the latter being mainly of breccia and quartzite while the intervening ground is underlain by deeply weathered igneous rocks (Figure 22). In contrast, the Allt na Glas Bheinne provides an almost continuous section which includes appinite, some of it fresh. The surrounding country rock is composed entirely of the Leven Schist Formation.

Breccia and large metasedimentary masses

The Leven Schist close to the margin of the complex shows very little contact metamorphism, although porphyroblastic andalusite occurs at one locality [NN 3840 9359]. Much thermally altered schist is present within the complex. It consists of hornfels in which quartz, plagioclase, biotite and muscovite have completely recrystallised with a general increase in grain size, the development of a decussate texture and the total destruction of the original tectonite fabric. Locally, plagioclase porphyroblasts (An35) make up a large proportion of the rock (S76730), giving it an igneous appearance in hand specimen.

Regional garnet is commonly preserved, and in some places it is unaltered or only partly replaced by biotite. Elsewhere, it appears to have been granulated and recrystallised. Extensive late-stage chloritisation of both garnet and biotite occurs at some localities. Patches of shimmer aggregate, probably after porphyroblastic andalusite, have been recorded, and fibrolite in muscovite is common.

Original bedding has been disrupted, and clasts up to many metres across are present but their boundaries have been obscured. However, pale, garnetiferous, calc-silicate rocks form elongated, slightly rounded, clasts that are easily seen. Much vein quartz in the form of angular clasts, less than 5 cm across, provides evidence of intense fragmentation and a lack of sorting during brecciation. In addition to the metasedimentary material, rounded and lens-shaped ultramafic igneous clasts make up about 1 per cent of the total (Plate 8). Pits marking the position of the igneous clasts, usually, 1 to 30 cm and exceptionally up to 1 m across, are very conspicuous in most exposures. Much of the breccia is isotropic, but elongated clasts in the north-east define a linear fabric which is approximately parallel to the lineation developed in the appinite exposed in the Allt Glas na Bheinne (Figure 22). In some of the exposures at the eastern end of the complex, the planar component of the fabric is dominant and strikes, on average, at 40° to the equivalent structure in the appinite exposed in the Allt na Glas Bheinne. The lack of similar structures in the surrounding country rocks indicates that the fabric is an internal feature of the complex and not the result of regional deformation.

At locality A (Figure 22), recognisable clasts lie in a garnet-bearing micaceous matrix with a lenticular planar structure (Plate 8). It appears that the rock has been homogenised by a combination of thermal recrystallisation and plastic strain, leaving only vein quartz and ultramafic igneous material as obvious clasts.

Evidence of partial melting of the schist breccia is rarely seen, although a pink heterogeneous rock at locality B (Figure 22), comprising potassium feldspar and quartz (S76720) and containing metasedimentary xenoliths, could be interpreted as a result of this process.

Surprisingly, clasts of quartzite in the schist breccia are uncommon at most localities. Instead, quartzite occurs separately in masses 50 m or more across throughout the complex (Figure 22). In some masses, bedding is recognisable and the quartzite is not brecciated. In others, it appears to be almost structureless and clasts can be recognised with difficulty. Thermal metamorphism has increased the grain size of the quartzite, obscuring the texture of the original breccia.

The quartzite at locality C (Figure 22) is 5 m thick, and bedding is marked by alternating feldspathic and pure quartz layers, 2 to 20 mm thick. It is extremely coarse grained and contains a few quartzofeldspathic lenses up to about 30 mm thick. These lie parallel to the bedding, have a granular texture, and are probably the product of partial melting of the quartzite.

The nearest quartzite outcrop is 150 m across strike from the edge of the complex, but this band is less than 50 m thick and the Beinn Iaruinn Quartzite Formation, 0.6 km across strike, is a more likely source of most of the quartzite in the complex. Its average dip is 68°, which unless the angle decreases downwards, implies that upward transport of more than 1.5 km has occurred in the appinite. The Brunachan Psammite Formation provided only a small proportion of the metasedimentary rocks within the complex.

At some localities, for example D (Figure 22), dark grey hornblendic material is present in addition to quartzite. It is obviously of igneous origin but does not occur as crosscutting veins and appears to be a component of the breccia. Its colour index is variable and it forms very irregular wisps and patches. These probably represent pockets of appinite which became incorporated as semi-molten magma during the breccia-forming process. Where the breccia comprises schist, the igneous material occurs as clearly defined clasts which were probably in a more completely crystallised condition during brecciation.

Intrusive igneous rocks

The clasts and disrupted pockets of appinite in the breccias constitute only a small proportion of the igneous material present. At least 50 per cent of the complex consists of appinitic rocks which appear to support large rafts of breccia. An example of appinite intruded into a large mass of non-brecciated quartzite occurs at locality E (Figure 22), where quartzite beds 2 to 20 cm thick and sheets of appinite, also only a few centimetres thick, are interleaved.

An unusual ultramafic rock (scyelite) exposed in the Allt na Glas Bheinne at locality F (Figure 22), is made up of olivine (Fo 87 per cent), phlogopite and a colourless amphibole (S76719). The olivine is euhedral, partially serpentinised and forms about 10 per cent of the rock. The phlogopite is more abundant than the amphibole, and both are anhedral. This rock type has not been found elsewhere in the complex and its field relations are not seen. However, it may be the parent rock of some of the vermiculite-bearing weathered material seen in other parts of the stream section.

Most of the intrusive rock is appinite which is very variable in texture and colour index. It is made up of hornblende, biotite, plagioclase (An30) and minor quartz (S76728). The hornblende is pale green, euhedral and has inclusions of biotite. Some of the appinite is very coarse grained, with hornblende up to 10 mm long. Plagioclase is anhedral against biotite and hornblende, with a tendency to be poikilitic. The biotite is pale brown and only slightly less abundant than hornblende. Felsic segregations, up to a few centimetres across and with diffuse margins, occur at many localities. They contain conspicuous euhedral crystals of hornblende and, in addition to being more feldspathic than the host rock, they are coarser grained.

A mesocratic rock, which can be described as felsic appinite, forms irregularly shaped masses at many localities. In places these have gradational boundaries with appinite, and in others cut it. In mineral composition, it is more feldspathic (greater than 50 per cent) than appinite ((S76716), (S76724)). Plagioclase forms zoned (An28 to An20) subhedral tablets, up to 5 mm long, showing minor turbid alteration. Some of the hornblende occurs in euhedral crystals, similar in size to the plagioclase, but most of it is finer grained, anhedral and interstitial to the plagioclase. All of it is pleochroic in shades of green. Biotite, in interstitial ragged flakes, is a major consituent. A small amount of quartz, showing strained extinction, is also present. A few large poikilitic crystals of potassium feldspar contain inclusions of plagioclase, hornblende and biotite, and at the edge of the crystal, where it is in contact with plagioclase, there is a small amount of myrmekite. A few small crystals of epidote with allanite cores have been recorded.

The felsic appinite contains abundant, small, rounded, ultramafic xenoliths. In the banks of the stream at locality G (Figure 22), completely weathered rocks remain undisturbed and igneous contacts are well displayed. Felsic appinite is decomposed to sandy material in which white feldspar is conspicuous. The latter makes it easy to distinguish from decomposed ultramafic rock with its abundant golden-brown vermiculite.

At some localities, for example F (Figure 22), the appinite is foliated and lineated. At locality B, discontinuous felsic layers up to 20 mm thick alternate with mafic layers of similar thickness. The felsic rock is isotropic, whereas the mafic rock has a biotite fabric which forms a schistosity lying parallel to the layering (S76718). In the mafic rock, pale green amphibole occurs as augen, wrapped by felts of coarse biotite. In the felsic rock, the biotite shows only a slight preferred orientation, and the feldspar, mainly microcline, forms oikocrysts with euhedral inclusions of amphibole. It is clear from the texture that hornblende and biotite crystallised early, forming a primocryst framework which locally collapsed with the expulsion of the residual melt and the development of the fabric. Crystallisation of the residual melt in those layers which escaped collapse gave rise to the poikilitic texture and lower colour index.

Emplacement history

The development of the Glas Bheinn Complex can be divided into four stages which postdate the regional metamorphism and predate the intrusion of the Ben Nevis Dyke Swarm.

  1. Development of a breccia pipe in the Beinn Iaruinn Quartzite and overlying formations. Small-scale intrusion of ultrabasic magma, followed by its disruption and incorporation into the breccia. At the level of the Beinn Iaruinn Quartzite, disruption occurred while the magma was still in a largely liquid condition, while at higher levels crystallisation was far enough advanced for rounded clasts to form.
  2. Compaction and consolidation of the breccia.
  3. Intrusion of appinite magma. Upward transport of large (greater than 10 m in diameter) masses of cohesive breccia as rafts. Hornfelsing and local partial melting of quartzite and schist breccia.
  4. Partial crystallisation of appinite and local collapse of its primocryst framework, probably as a result of continued forceful intrusion of magma into the complex. The residual melt expelled during this process crystallised as felsic appinite while the collapsed framework finally stabilised as foliated mafic appinite. The schist breccia underwent plastic strain during this stage.

Allt Dubh Appinite

Very variable appinitic rocks form a major intrusion, with a very wide aureole on its northern side but almost none on the southern side (Figure 23). The aureole is a remarkable feature, not only because of its asymmetry, but also because of disruption which has given rise to rocks that could be described as marginal breccia (Platten, 1983) or anatectic migmatite (Pattison and Harte, 1988). The appinitic rocks are almost only exposed in the Allt Dubh and its tributaries. However, the boundary of the intrusion can be located in most places to within a 100 m or so by a break of slope. The rocks of the aureole on the northern side form the high ground and are fairly well exposed. A few gullies and scrapings containing brown sandy material with flakes of vermiculite show that the hillside below is underlain by appinite.

Ultramafic rocks

Many of the exposures in the Allt Dubh are of ultramafic rocks which appear to be homogeneous, and it is probable that much of the main intrusion and the whole of the igneous mass north of Meallan Odhar is ultramafic (Figure 23). In hand specimen, these rocks are dark greenish grey, and it is usually possible to recognise mica and the sites of serpentinised olivine crystals. Thin sections show that they consist of phlogopite, tremolite, clinopyroxene and olivine in variable proportions. In some thin sections (e.g. (S82595)), olivine forms more than 40 per cent of the rock, whereas in others it is completely absent, the main constituents being phlogopite and tremolite (S82602). Clinopyroxene is locally abundant and forms irregularly shaped grains, partly enclosed by tremolite (S82204). Late-stage alteration is usually confined to the serpentinisation of olivine, but in one example (S92812) olivine has been completely replaced by fine-grained aggregates of talc, fringed by radiating growths of chlorite (S92812). A few of the phlogopite crystals in the same rock contain lenses of secondary talc. All the ultramafic rocks are non-foliated and lack xenoliths. Locally occurring, leucocratic segregations contain euhedral prisms of hornblende, up to 10 mm long, in a matrix of coarse-grained, poikilitic andesine (S92813). Most of the exposures have weathered into a brown, vermiculite-bearing sand, containing sharply bounded blocks of very fresh rock. The amount of weathering accounts for the absence of exposures away from the streams.

Appinite and felsic appinite

These rocks show great variations in texture and colour index, even within a single exposure (Plate 9). The variants occur as irregular streaks and lenticles, forming a foliation that strikes north-eastwards and dips steeply (Figure 23). The boundaries are mainly gradational over a few centimetres, but cross-cutting leucocratic veins also occur.

The appinite consists of biotite, hornblende, oligoclase, microcline and quartz. Biotite occurs as ragged flakes, commonly enclosed by hornblende. The hornblende is euhedral against the feldspars which are poikilitic (S81678). At some localities the biotite and hornblende have a preferred orientation, defining a planar fabric that lies parallel to the macroscopic lenticular structure noted above. The hornblende-biotite fabric is preserved within poikilitic feldspar (S81675) and is a structure that predates full crystallisation, similar to that in the Glas Bheinn Appinitic Complex.

The felsic appinite contains abundant large tablets of antiperthitic oligoclase rimmed by albite. Microcline and quartz are poikilitic, with abundant inclusions of oligoclase as well as hornblende and biotite (S82201). The felsic appinite also contains small (1 to 10 cm) rounded xenoliths, mainly of ultramafic rock although some are of mesocratic appinite. The xenoliths invariably have a greater colour index than the rock in which they occur, a fact which reflects the evolution of the appinitic magma, the final stages being represented by feldspathic segregations and cross-cutting leucocratic veins. Xenoliths of psammite and coarse-grained recrystallised quartzite, some more than 10 m across, occur in all the varieties of appinite.

Several granitic veins up to 1 m thick have been recorded. They occupy brittle fractures, contain xenoliths of felsic appinite.

Aureole

At locality A (Figure 23), the rock adjacent to the appinite is laminated grey psammite with a few micaceous bands. Up to 3 m from the contact, it is coarse grained but the biotite fabric is preserved. At 20 m it is a normal, fine-grained psammite. At localities B, C and D, psammite and semipelite are exposed within 200 m of the contact but there is no evidence of thermal metamorphism.

On the northern side of the intrusion, the margin of the appinite is nowhere exposed. However, its position is probably only a short distance south of the exposures on Leac nan Uan (Figure 23) where the rocks are predominantly psammitic and disrupted. Minor disruption is indicated by necking and local angular discordance between bedding planes. A few impersistent granitic veins, up to a few centimetres thick, run parallel to the bedding which is locally disrupted into lenticles.

At many localities the disruption is more profound (Plate 10). For example, at locality E in the north-east a chaotic mass of psammite and micaceous psammite fragments is exposed. Banding within the fragments is commonly bent, and irregularly shaped blobs of felsic appinite, up to 30 cm across, form between 1 and 5 per cent of the mass. The psammite has obviously been softened and infiltrated by appinitic magma. In some exposures near locality E, felsic appinite is absent. Instead, pink microgranite, cut by veinlets of pegmatite, occurs between the clasts.

At nearby locality F, the psammite is completely disrupted into lenses which have a parallel orientation, giving rise to a planar fabric that dips south at 60°. This arrangement contrasts with the isotropic fabric at locality E. The bedding within the lenses lies subparallel to their length, which ranges from a few centimetres to more than a metre. The bedding is commonly curved and truncated at the lens edge. Felsic appinite and granitic material form irregular, discontinuous veins lying parallel to the planar fabric.

Localities E and F are typical of areas of major disruption but excellent exposures also occur on Meallan Odhar. Here the clasts, which commonly include talc-silicate rock, vary in shape from equant to elongated parallel to bedding, and some, particularly those composed of micaceous psammite, are irregularly folded. The fabric is generally isotropic with the psammite fragments in contact with each other. However, a matrix with an igneous texture is locally present, and some of it is microgranitic with metasedimentary xenoliths represented by small micaceous flakes. A coarser-grained dioritic rock, forming abundant, irregularly shaped masses up to several metres across as well as narrow, discontinuous veins, also occurs. It contains subhedral feldspars up to 5 mm long, rounded ultramafic xenoliths and leucocratic segregations, all features typical of the felsic appinite in the main intrusion.

Origin of the aureole brecciation

The bedding of the psammite has been disrupted in a wide zone defining an aureole on the northern side of the intrusion. The planar fabric of the resulting breccia and the folding of some of the clasts show that the psammite was in a ductile condition during this process. The disruption was probably caused by deformation as the intrusion dilated in a northerly direction. This conclusion is supported by the general concordance of the planar fabric with the northern margin of the intrusion. Veining by felsic appinite would have raised the temperature of the aureole, probably resulting in local melting of the psammite. The presence of a high concentration of water in the appinite magma may have been a contributory factor. Mobilisation of pockets of melt probably gave rise to the microgranitic veins containing micaceous flakes (refractory fragments). The fact that both appinitic and microgranitic veins can be recognised suggests that the melts co-existed with only a minor amount of mixing. The country rocks to the south of the intrusion have not been invaded by large numbers of appinitic veins, and therefore did not receive sufficient thermal input for ductile deformation and disruption to take place. The veining to the north, and the proposed dilation of the intrusion in that direction, may have been the result of a lower confining pressure there.

Bohenie Appinite

A large intrusion of appinite occurs several kilometres north-east of Bohenie [NN 294 829] (Figure 19). Exposures are confined to the AIlt Glas Dhoire and its tributaries. Away from the watercourses, the appinite is covered by partly afforested vermiculite-bearing soils and the exact shape of the outcrop is not known. It is intruded into the Leven Schist Formation and, at its eastern end, is truncated by the leucogranite of the Corrieyairack Granitic Complex. The appinite is cut by numerous pink granite veins and also by microdioritic dykes of the Ben Nevis Dyke Swarm. Faulted contacts between the appinite and schists, as well as granite, are exposed at [NN 3280 8460] and [NN 3350 8495] respectively. The main lithology is a massive, medium- to coarse-grained, micaceous ultramafic rock in which a local foliation is defined by shape-aligned biotite flakes (S80646). The colour of this rock varies from black-green to speckled black and bright green, depending on the nature of the amphibole components. Angular xenoliths of its schistose country rocks are locally present, e.g. at [NN 3290 8406]. Here, a weak mica fabric in the ultramafic host rock is deflected around the angular xenoliths which are up to about 30 cm long. In thin section ((S80647) and (S80648)), the ultramafic rock is seen to comprise variable amounts of phlogopitic biotite, amphibole (hornblende and tremolite), talc and clinopyroxene.

Massive feldspar-bearing rocks (e.g. (S80591)), in which white plagioclase occurs interstitially to the mafic minerals as well as forming irregular clots up to several centimetres long, are also exposed. A primary, centimetre-scale colour banding at [NN 3268 8436] dips at 65° towards NO55°E.

No breccias were found associated with this intrusion.

Appinite raft in the Corrieyairack Granitic Complex

The leucogranite forming the southern part of the Corrieyairack Complex contains numerous xenoliths of its metasedimentary country rocks, but only one, albeit large (1.5 km X 0.25 km), appinite raft which is very well exposed in the Allt Glas Dhoire between [NN 3297 8536] and [NN 3272 8655] (Figure 19). The only other exposures occur for short distances up the few tributaries of the Allt Glas Dhoire and, as there are no exposures for several hundred metres on either side of this stream, the exact shape of the appinite is poorly constrained. Stream exposures of the contact between the raft and its host granite reveal a hybrid rock (S80604), comprising a bright green ultramafic host permeated by granitic material and invaded by diffuse and sharply defined pink granite veins. At [NN 325 865], this hybrid is about 70 m wide.

Four main lithologies are exposed in the raft. These include the two main lithologies found in the Bohenie Appinite, which comprise much of the southern and northern parts of the raft. In addition, a mica-lamprophyre exposed in the north is a massive rock which contains disseminated phlogopitic mica flakes as well as veins of this mica, set in a medium-grained groundmass of clinopyroxene, plagioclase and talc, with secondary calcite as well as chlorite aggregates (S80642). Rare xenoliths of a strongly altered, dark grey rock contain fine-grained calcite replacing both tremolite and strained plagioclase, with tiny biotite flakes overprinting the amphibole grains. Large plagioclase grains poikilitically enclose relict amphibole and clinopyroxene grains. In the centre of the main stream section is the fourth component, a dense, massive ultramafic rock comprising olivine (partly serpentinised), phlogopitic mica, clinopyroxene (with olivine inclusions) and disseminated opaque oxides (S80644). Secondary calcite is variably present in the four main rock types; it is most common in the bright green ultramafic rocks. Early mineral phases have been severely altered which makes it difficult to assess the nature of the primary lithologies.

Angular xenoliths of talc-silicate rock, up to several tens of centimetres in length, were seen only at [NN 3248 8552], in an ultramafic host. Immediately upstream from here, breccia identical to that infilling the late pipe in the Achavady Appinite is partly exposed on one side of the stream. Granite veins are found in all phases of the appinite except the central ultramafic part. The raft is also cut by several dykes of the Ben Nevis Dyke Swarm.

Innse Appinite and Breccia

The small appinitic plug south of Cruach Innse [NN 275 752] (Figure 19) is relatively felsic, compared to the other appinites exposed to the north. It is poorly exposed although a biotite-bearing monzonitic phase is seen in the stream section at the south end of the intrusion. White, pink-flecked, feldspar phenocrysts, up to about 8 mm in diameter, occur in a fine-grained, dull grey groundmass. Pink granite veins cut the appinite and the adjacent country rocks.

To the north-west of the appinite, a zone of breccia about 150 m wide comprises angular clasts of recrystallised quartzite. At the contact, detached fragments of quartzite are supported by appinite. Away from the contact, the quartzite is less brecciated and is cut by linear veins of appinite, indicating that brecciation preceded appinite emplacement. In contrast, the pelites with quartzite inter-layers which form the eastern wall of the appinite were deformed plastically, prior to the injection of appinite veins. However, the deformation appears to be a regional phenomenon, unrelated to the local appinite intrusion.

Breccia pipes

One large and several small pipes, infilled with breccia derived from the wall rock, cut the metasedimentary rocks in the central part of the area. A dyke-like body of breccia has also been found [NN 386 953]. All the occurrences are cut by, or closely associated with, mafic and ultramafic minor intrusions of appinitic affinity but igneous clasts are completely absent.

By far the largest pipe (600 m in an east–west direction) occurs at Cam Dearg Beag (Figure 19). It cuts the Beinn Iaruinn Quartzite Formation 2 km north-west of the Glas Bheinn Appinitic Complex. The breccia has been resistant to glacial erosion and forms the summit area of a hill rising more than 670 m above OD. The margin of the pipe can be located to within 1 or 2 m in a few places but along most of its length its position is only known approximately. The breccia consists almost entirely of quartzite fragments, mostly between 5 and 30 cm across although much larger (greater than 3 m) masses occur, particularly in the east. Close to the edge of the pipe [NN 3659 9419], semipelite in masses up to 3 m long has clearly been derived very locally from a semipelitic band in the adjoining quartzite. The quartzite fragments are angular, range from equant to elongated parallel to the schistosity, and are tightly packed. The breccia is isotropic except at [NN 3625 9391], where the alignment of elongated clasts gives a planar fabric dipping south at 60°. The breccia in this exposure is also unusual in that the clasts lie in a sandy matrix.

The pipe and the surrounding quartzite are cut by a few minor intrusions which are discontinuous and irregular in form. Some consist of appinite, others are more leucocratic, and an extremely xenolithic (80–90 per cent) microgranitic rock occurs at [NN 3650 9378]. The xenoliths comprise a biotite-rich ultramafic rock and a muscovitic metasedimentary rock in about equal proportions.

Minor intrusions

Minor intrusions of the appinite suite are very common over a large proportion of the area (Figure 19). They also occur in the area to the south (Sheet 53), where they were described by Bailey and Maufe (1960) as 'early lamprophyre' sheets. Most are between 1 and 3 m thick; examples greater than 5 m thick are uncommon although a sheet up to 30 m thick occurs in the cliff faces of Coire Ardair [NN 4327 8804]. An appinite plug, 200 m in diameter, is exposed in the Allt Poll-gormack [NN 389 958]. Dykes (dip greater than 60°) and sheets (dip less than 60°) are equally common, the former having a north-east trend while most of the latter dip at a low angle to the southwest. The dykes, controlled by the foliation of the meta-sedimentary country rocks, may have acted as feeders to the sheets. Two exceptionally well-exposed sheets, dipping at 20° to the west-south-west, occur at [NN 343 901]. The lower sheet, 3 m thick, can be traced for 400 m and occupies a planar fracture cross-cutting the foliation of the Leven Schist Formation. Some of the intrusions are irregular with tapering branches. Being readily weathered, the sheets have given rise to waterfalls and are commonly found at the base of overhanging rock faces.

Most of the sheets and dykes are composed of speckled, greyish green and black lamprophyre with pitted weathered surfaces. The thin intrusions are uniformly fine grained with acicular hornblende just visible to the naked eye, and the thicker ones are medium grained and have chilled margins up to several centimetres thick. Irregular and diffuse feldspathic patches and rounded xenoliths of a very dark green to black ultramafic rock are characteristic of the appinite suite. Members of the suite are cut by dykes of the Etive and Ben Nevis swarms, and predate the major granitic igneous complexes. Their temporal relationship to the early pegmatite vein complexes is less clear. The thicker sheets exposed at Coire Ardair cut across pegmatites in the metasedimentary country rock. However, farther south, the thinner lamprophyre sheets are impregnated by pegmatitic veins (e.g. [NN 4365 8444], [NN 4196 8306], [NN 4318 8032] ). In the same area, there are composite pegmatite (core) /lamprophyre sheets [NN 4316 7992] and a lamprophyre sheet chilled against a pegmatite [NN 4371 8176]. Therefore, the field evidence suggests an approximate contemporaneity for the intrusion of the sheets and the pegmatite (Loch Laggan Vein Complex). Cross-cutting relationships show that the appinite suite predates the trondhjemitic veins of the Toman Liath Vein Complex (Figure 19).

Mineralogically and texturally, many of the dykes and sheets are typical talc-alkaline lamprophyres. The majority are spessartites, a few are kersantites and some are sufficiently coarse grained to be described as appinite. In the spessartites ((S82658), (S92503), (S92521)), euhedral phenocrysts of green or brownish green hornblende and rare clinopyroxene are set in a groundmass of anhedral, commonly poikilitic plagioclase with variable amounts of interstitial quartz and potassium feldspar. Renewed growth of the hornblende has given rise to colourless rims and acicular outgrowths penetrating the plagioclase. Biotite is also present. In many examples, the original plagioclase was more calcic than albite-oligoclase and has been partly replaced by calcite and epidote as well as sericite. Accessory phases include opaque ore, sphene and apatite.

The kersantites ((S76895), (S76916)) are more feldspathic, with ragged flakes of biotite in a groundmass dominated by poikilitic, altered plagioclase.

Over most of the area, the spessartite intrusions are massive and their texture is purely igneous. However, south of the Glas Bheinn Appinite, in an area of about 5 km2, they are schistose. It is noteworthy that this area coincides with the Toman Liath Vein Complex (Figure 19) and the occurrence of a late crenulation cleavage in the Leven Schist Formation. The structure is equally well developed throughout the thickness of each intrusion and shows no curvature towards the country-rock contact. It also has a fairly constant east–west strike and steep dip regardless of the orientation of the sheet or dyke. Folding of the intrusion—country-rock contact is rare, but an exceptionally good example is exposed in the Burn of Agie [NN 3720 9158] where a folded dyke crosses the stream in a north–south direction. The schistosity is deflected around slightly flattened xenoliths and is continuous with a crenulation cleavage in the adjoining Leven Schist. The presence of folding and an axial-planar penetrative fabric shows that the dyke has been affected by ductile deformation during shortening parallel to its strike (north–south).

The schistose intrusions contain abundant secondary biotite which shows a very strongly developed preferred orientation ((S76694), (S76702)). Former phenocrysts are represented by pale green hornblende in augen-shaped aggregates elongated parallel to the schistosity. Elongated lenses of biotite with yellow epidote are also present. The former phenocrysts lie in a matrix of equant grains of quartz and plagioclase with aligned flakes of biotite. Both the phenocrysts and the groundmass have been granulated and recrystallised. The resulting tectonite fabric contrasts with the pre-full crystallisation fabric developed in parts of the Glas Bheinn and Allt Dubh appinites.

A number of the rock types forming the dykes and sheets of the appinite suite fall outside the range of lamprophyre described above. Some are ultramafic, comprising pale green amphibole and biotite with only a very small amount of interstitial quartz and feldspar (S81634). An extremely altered rock (S81672) is made up of large crystals of chlorite with wedges of carbonate and talc along the cleavage planes. In another example (S83364), phe nocrysts of hornblende lie in a fine-grained groundmass of chlorite.

Minor intrusions of microgranodiorite are also common, for example cutting the Leven Schist Formation south of the Glas Bheinn Appinite and at Meall Ptarmigan [NN 42 90]. Although some are of aphyric microgranodiorite, many contain large phenocrysts of weakly zoned oligoclase (S81649). A few phenocrysts of pale green hornblende also occur. The groundmass comprises potassium feldspar, plagioclase and quartz with smaller amounts of amphibole and biotite. Small (2 cm) rounded ultramafic xenoliths are common, and locally they are exceedingly abundant (S81640).

Appinitic rocks north-west of the Great Glen Fault Zone

Several appinitic minor intrusions occur south-west of Invergarry, and a large mass of olivine-pyroxenite is exposed at Faichem (Figure 19). The latter has an elongated outcrop and is intruded into Moine psammite adjacent to, and parallel with, the contact between the psammitic rocks and granitic gneiss. The body, some 200 m wide at Faichem, can be traced along strike for more than 1 km before thinning and tapering out. It is a dull grey, coarsely crystalline, apparently homogeneous rock, consisting of clinopyroxene with subordinate amounts of orthopyroxene and olivine as well as varying amounts of amphibole. Plagioclase is a very minor constituent, and accessory minerals include sphene and opaque oxide. Secondary chlorite and carbonate also occur. The contacts with the adjacent country rocks are not exposed but near its margin the intrusion shows no evidence of chilling or brecciation. However, the elongated shape of the outcrop and the occurrence of granitic gneiss along one side only suggest that it may have been emplaced along the trace of a fault trending north-north-west.

Hornblendic rocks near Whitebridge, Glengarry

Rock (1984) described poorly exposed hornblendic rocks in Moine psammites just west of the contact with the granitic gneiss at [NH 279 012] near Whitebridge in Glengarry. They range from concordant hornblendic layers with diffuse margins within the psammites, to irregular patches and breccia-like developments and to discordant veinlets; their hornblende content ranges from 5 to 80 per cent. They are cut by thin reddened vein-like zones containing prehnite and predate leucogranites of the Glen Garry Vein Complex. Rock (1984) noted that both mineralogically and chemically the hornblendic rocks are mostly different to the Moine talc-silicates and amphibolites and interpreted them as probably alkali metasomatised calcsilicate rocks. Highton (1994) showed that similar hornblendic rocks in the southern part of the neighbouring Invermoriston district (Sheet 73W) were clearly intrusive, but of an unusual mafic composition rich in sphene, allanite, zircon and apatite.

Chapter 10 Late granitic intrusions

Late granitic rocks are widely distributed in the Glen Roy district. Two post-tectonic, multiphase granitic intrusions (the Strath Ossian and Corrieyairack granitic complexes) are found south of the Great Glen, whereas roughly contemporaneous granitic vein complexes occur on both sides of and within the Great Glen Fault Zone. Emplacement of the multiphase intrusions occurred during regional uplift, after the main D3 period of ductile deformation of their metasedimentary host rocks (see Pitcher, 1982). These intrusions form part of the Argyll Suite of tonalite-granodiorite-granite plutons within the post-tectonic Grampian Highlands Igneous Province (Stephens and Halliday, 1984; and see Plant, 1986). They are calc-alkaline complexes of apparent I-type geochemistry.

Strath Ossian Granitic Complex

The northern part of the Strath Ossian Granitic Complex (Key et al., 1993) extends into the southern part of Sheet 63W, where it forms a 6 km-wide, vertical-sided (trending 335°) intrusion and underlies an area of about 49 km2 (Figure 19). It was originally defined by Hinxman et al. (1923) as part of the Moor of Rannoch Granite. At its southern end, the Strath Ossian Complex is separated from the main part of the Moor of Rannoch Granite by the north-east-trending Ericht–Laidon Fault (Figure 2). According to Hinxman et al. (1923), the Strath Ossian Complex is either a satellite cupola of the Moor of Rannoch Granite, or an integral part of a single batholith; a narrow north-north-west-trending strip of schists, separating the outcrop areas of the two granites north of the Ericht–Laidon Fault, may be interpreted as a rooted unit or as a floating roof pendant. Work by Anderson (1956), Clayburn (1981) and Henderson (1982) has confirmed the contemporaneity and lithological similarities between the two intrusions, although Clayburn (1981) noted some chemical differences.

The Strath Ossian Complex is poorly exposed in the Glen Roy district, where more than 70 per cent of its outcrop area is covered by an overburden of glacial and glaciofluvial deposits. Good exposures are confined to the relatively high southern crags (Meall Chaorach [NN 383 758]; Meall Dhearcaig [NN 393 747]; Loch Ghuilbinn [NN 424 745] ), and minor crags and watercourses in the north around Meall Luidh Mor [NN 418 797] and between Fersit [NN 352 782], Tulloch Station [NN 355 802] and Roughburn [NN 377 8131 Artificial exposures were created along the A86 during construction of the Loch Laggan Dam (in the quarries immediately north of the dam) and adjacent to the forest track to Fersit [NN 3655 7957].

The geology of this part of the pluton was described by Key et al. (1993). Three main lithologies are variably exposed (Figure 24). Areally, the dominant phase is a medium- to coarse-grained, grey to pink, homogeneous equigranular granodiorite. An earlier mela-granodiorite to quartz-diorite of more variable composition is well exposed to the south-west and, to a lesser extent, in the north-east on the southern flank of Meall Luidh Mor [NN 414 794]. The intervening ground is unexposed, but it is believed that the diorites may form a northeastward trending core to the pluton (Figure 24). The third lithology is a porphyritic microgranite which forms an approximately 30 m wide marginal facies on the western side of the pluton. Both dioritic and granodioritic phases are cut by a suite of 10 cm thick (rarely more than 50 cm thick) microgranite sheets.

The margins of the complex are sharply defined, with a minimal amount of veining of the adjacent country rocks. In contrast, Hinxman et al. (1923) recorded more widespread granitic veining of the wall rocks on Sheet 54.

Quartz-diorite

The early phase comprises mostly quartz-diorites, although a range in composition from diorite through to mela-granodiorite has been recognised (Henderson, 1982). They are typically massive, medium- to coarse-grained rocks, with hornblende and biotite forming 35–50 per cent of the mode. Biotite flakes are intergrown with, and locally enclose, hornblende with both minerals exhibiting minor alteration to chlorite and epidote. Zoned (An33 to oligoclase) plagioclase laths (15–30 per cent) are slightly larger than the other phases ((S76920); (S80055); (S80061)) and may form clusters comprising several crystals. Orthoclase (5–25 per cent) is commonly less altered than plagioclase. Rare clinopyroxene has been recorded as inclusions within hornblende.

The 'diorites' contain unevenly distributed, rounded to angular mafic enclaves, up to 1 m long and rare psammitic xenoliths. They are cut by ubiquitous veins of the granodiorite, which are up to 1 m thick and have sharp contacts with very little signs of marginal re-equilibration.

Granodiorite

The main granodiorite is a massive, grey, well-jointed rock with a relatively uniform grain size (2 to 4 mm), except within 1 m of the pluton's margins where a finer- grained facies may be developed (e.g. at location [NN 4285 7560]). In thin section ((S76899); (S76914); (S80006)), the granodiorite is composed of plagioclase (45–50 per cent), orthoclase (15–20 per cent), quartz (15–20 per cent), biotite, hornblende and sphene, with minor amounts of zircon, apatite, opaque oxide and rare garnet. Clayburn (1981) recorded augite cores to large hornblende blades.

The granodiorite, especially in the north, contains similar mafic enclaves to those seen in the diorites. These enclaves may be shape-oriented, parallel to the surface outline of the pluton and to a weak fluxional fabric developed within granodiorite. Country-rock xenoliths are rare, except in the south-east, and are mostly of psammite. The xenoliths are confined to the margin of the pluton, although in the south-east they occur within a 450 m-wide zone and may be up to several metres in length. In this area, randomly oriented country-rock xenoliths are variably indurated and cut by veins of granodiorite. Rounded clots (less than 20 cm in diameter) of biotite and hornblende, enclosed within haloes of relatively felsic granodiorite, provide a further heterogeneity within the granodiorite.

Feldspar-phyric microgranite

At Fersit there is a marked transition (over several metres) outwards from granodiorite into a marginal feldspar-phyric microgranite. Randomly oriented phenocrysts of plagioclase and orthoclase, typically less than 2 cm and rarely up to 15 cm long, show a gradual increase in size away from the granodiorite, with a concomitant decrease in the grain size of the groundmass, which becomes progressively more granitic. A northsouth-trending fluxional fabric in the groundmass of the microgranite is parallel to the outer contact of the intrusion. Similar feldspar phenocrysts are also developed in rafts (up to 1 m long) of meta-psammitic country rock, as well as within the immediate wall-rock of the pluton. Minor muscovite and corundum have been recorded in the microgranite (Clayburn, 1981).

Microgranite sheets

These microgranites occur as thin, gently dipping sheets, oriented at random. They are white or pink in outcrop depending on the extent of feldspar alteration. In thin section ((S80004); (S80031); (S80058)), they are composed of quartz (up to 45 per cent), orthoclase (up to 35 per cent), plagioclase (up to 25 per cent) and biotite (up to 15 per cent) with minor amounts of hornblende and no sphene or apatite.

Structure

The pluton is deformed by two principal, subvertical, joint sets trending east-north-east and north-north-west (cf. Henderson, 1982), as well as late brittle faults including the east-north-east-trending Laggan Dam Fault. No ductile fabrics have been recorded from any of the lithologies. The east-north-east-trending joints are most intensely developed near to the Laggan Darn Fault and in the northern part of the intrusion, where eastnorth-east-trending microdiorite dykes of the Etive Dyke Swarm are also common. The joints are open (on a millimetre scale) and commonly reddened.

The majority of the faults which cut the intrusion are subvertical with north-east and east-north-east trends, and are denoted by discrete shatter zones up to several metres wide, together with a pronounced reddening (S80080). Sinistral and less common dextral horizontal movement up to several hundred metres can be established along most faults by the offset of the pluton's margins or microdiorite dykes. The amount of vertical displacement on these brittle structures is, however, unknown. Mineralisation along these fault planes is rare; however, calcite (in the hydro-electric tunnel) and barytes [NN 386 779] have been recorded. The Laggan Dam Fault cuts across the whole width of the pluton and is well exposed in the River Spean, downstream from the dam. In contrast, the Lochan na h'Earba Fault is unexposed where it cuts the pluton, and is located by means of a lineament on aerial photography, trending north-eastwards from its exposure at [NN 4332 7720]. North-west- to north-north-west-trending lineaments, identified on aerial photographs in the area of the Strath Ossian Complex, are interpreted as being related to underlying faults or master joints.

Key et al. (1993) demonstrated that the emplacement of the Strath Ossian Complex had a profound effect on the structure of its country rocks. On the western side of the pluton, east-north-east-trending planar fabrics are realigned towards the south-south-east (Figure 25), (for example at Dun Dearg [NN 349 803]. On the eastern side, bedding-parallel fabrics are mostly parallel to the margin of the pluton. The most spectacular effect, however, is disharmonic folding in the hornfelsed semipelitic and pelitic rocks within 200 m of the intrusion. These folds have wavelengths of up to several tens of metres and were superposed upon all regional deformation fabrics.

Isotopic data

Clayburn (1981) recorded a two feldspar whole-rock Rb-Sr isochron of 405 ± 9 Ma (MSWD = 0.015), which is compatible with a zircon age of 400 ± 10 Ma for the main granodiorite phase (Pidgeon and Aftalion, 1978). The pluton is cut by minor intrusions associated with the Etive Dyke Swarm, which have been dated at about 400 Ma (Clay-burn, 1981; Clayburn et al., 1983) or 412 ± 5 Ma (Thirwall, 1988). No examples of either the 439 ± 7 Ma Loch Laggan Vein Complex (Clayburn, 1981) or the contemporaneous lamprophyric sheets, common within the adjacent country rocks, are recorded cutting the Strath Ossian Complex, which therefore postdates these minor intrusions. All the evidence suggests that the Strath Ossian Complex was emplaced during Silurian or early Lower Devonian times. Clayburn (1981) demonstrated that the primary magma of the Strath Ossian Complex was mantle-derived but with some lower crustal contamination. His data (Sr and Pb isotopes) also imply a late-stage interaction between the magma and upper crustal rocks.

Aureole

Key et al. (1993) identified six mineral assemblage zones across the thermal aureole of the Strath Ossian Complex (Figure 24). The main mineral assemblages and pro-grade mineral reactions involved are listed in (Table 9). In general, thermal metamorphism resulted in the breakdown of regional amphibolite facies assemblages (Zone I) and the development of cordierite- and andalusitebearing assemblages within all the pelitic and semipelitic lithologies. Thermal metamorphism of the Grampian Group psammites resulted mainly in the recrystallisation and hardening of these rocks, with only localised development of cordierite and rare andalusite within the more micaceous lithologies.

In Zone II, regional garnet is progressively replaced by rims and pseudomorphs composed of granular foxy red biotite, plagioclase and quartz ( ± opaque oxides). Regional hornblende porphyroblasts in the calc-silicate rocks are pseudomorphed by fine- to medium-grained biotite, quartz, plagioclase and epidote. The next pro-grade mineral change in the pelites is the appearance of cordierite within Zone Ma. Cordierite first appears as small rounded porphyroblasts which replace or overgrow the pseudomorphs after regional garnet. Cordierite is poikiloblastic, is packed with fine inclusions of muscovite, quartz and opaque oxides, and is frequently associated spatially associated with foxy red biotite, the latter being common in all cordierite-bearing rocks. K-feldspar appears in the pelites of Zone Mb, where it coexists with cordierite. The mineralogy of the pelites varies as a direct function of bulk rock composition (see Pattison and Harte, 1985; and Key et al., 1993), with cordierite + andalusite-bearing assemblages occurring at the same metamorphic grade as the above cordierite + K-feldspar- bearing pelitic hornfels. Andalusite forms small prisms and elongate porphyroblasts, 1 to 4 mm long, which contain small inclusions of biotite and quartz within their cores. Andalusite is commonly zoned, with pale pink cores and colourless rims, and is spatially related to foxy red biotite and cordierite. In this zone, andalusite coexists with primary muscovite.

In Zone IV, the next important mineral change is the appearance of cordierite + andalusite + K-feldspar-bearing assemblages in the semipelites and pelites (Table 9). Any muscovite forms anhedral, ragged relict flakes and late, randomly oriented porphyroblasts which overprint thermal metamorphic textures. K-feldspar is common and clearly coexists with andalusite. Minor to rare sillimanite (fibrolite) is observed locally, included within cordierite. Towards the inner part of the zone (adjacent to the contact), granular cordierite is locally/rarely observed enclosing and/or replacing andalusite. Zone V is characterised by the presence of minor fibrous sillimanite (fibrolite) replacing foxy red biotite. Fibrolite appears to have formed, and is associated with the development of late, randomly oriented muscovite porphyroblasts and the retrogression of cordierite and feldspar to sericitic white mica, chlorite and pinite. Based on the work of Kerrick (1987), Key et al. (1993) suggested that fibrolitisation may have occurred in response to the introduction of late magmatic fluids into the adjacent country rocks. This conclusion is supported by the encroachment of Zone V into previously developed pro-grade assemblage zones (Figure 24).

Zone VI is represented by a sequence of semipelitic migmatites which are confined to the innermost part of the aureole, within 150 m of the contact. Two types of migmatite have been recognised (Key et al., 1993): (a) migmatites with diffuse patches (up to several tens of centimetres long) preserving older fabrics; and (b) migmatites consisting of angular, layered semipelitic and psammitic rafts, up to 1 m long and oriented at random, in a semipelitic matrix which exhibits a ductile fabric parallel to the pluton's margin. Type (a) migmatites are found on the eastern side of the pluton adjacent to the main outcrop of the diorites, and as xenoliths in the later granodiorite. Type (b) is observed on the western side of the pluton, notably in the Fersit area. Key et al. (1993) concluded that partial melting and migmatisation occurred relatively early, probably associated with the emplacement of the dioritic phase. Similar migmatites have been described by Pattison and Harte (1988), associated with the emplacement of an early dioritic phase of the contemporaneous Ballachulish Igneous Complex, 40 km to the south-east.

Conditions of thermal metamorphism

Quantitative estimates of P-T conditions in thermal aureoles cannot be made easily, and qualitative methods using a petrogenetic grid are generally more informative (Figure 26). Prograde mineral reactions during thermal metamorphism caused by emplacement of the Strath Ossian Complex have been modelled by Key et al. (1993) (see (Table 9)), based upon observed mineral assemblages and the petrogenetic grid of Pattison and Harte (1985). The presence of cordierite + K-feldspar, andalusite + cordierite and andalusite + K-feldspar pelitic assemblages within Zones Illb to V of the Strath Ossian aureole indicate that temperatures of thermal metamorphism were in the range 650 ± 50°C. Further P-T estimates have been determined using Mg/ (Mg + Fe) in cordierite (Pattison, 1989) and two feldspar geothermometry (Stormer, 1975; Stormer and Whitney 1977; Brown and Parsons, 1981; Haselton et al., 1983). P-T conditions resulting in the development of cordierite- and andalusite-hearing assemblages within the Strath Ossian thermal aureole were in the range T = 600 to 650°C at an estimated P = 3.2 kbars (Key et al., 1993). Partial melting and migmatisation within the pelitic lithologies exposed adjacent to the contact probably occurred at or in excess of 700°C.

Mode of emplacement

Key et al. (1993) concluded that the Strath Ossian Complex was forcefully emplaced into Grampian Group and Appin Group metasedimentary rocks, resulting in the reorientation of previously developed regional deformation structures and fabrics. They suggested that the change in orientation of these structures is consistent with the initial emplacement of the dioritic phase at its northern end, with the subsequent lateral flow of magma towards the south-east. The early part of the intrusion created its own space, as there is very little evidence for wall-rock stoping in the north. However, later emplacement in the southeast was assisted by a considerable amount of stoping. The emplacement of the diorites resulted in partial melting and migmatisation (T 700°C) of the pelites adjacent to the pluton. Subsequent slower emplacement of the granodiorite formed the wider thermal aureole, resulting in the development of cordierite- and andalusite-bearing assemblages (T = 600 to 650°C, P = 3.2 kbars) within the pelitic and semipelitic lithologies. Emplacement of the microgranites was accompanied by the release of late magmatic fluids into the adjacent country rocks, coincident with peak thermal metamorphism in the aureole, causing the growth of feldspar phenocrysts/porphyroblasts and late sillimanite (Zone V) in association with hydrous phases including white mica and chlorite.

If an assumed regional pressure gradient of 3.5 km per kilobar is applied, then a depth of emplacement of about 11 km is obtained for the Strath Ossian Complex. This estimate implies considerable uplift since the peak of regional metamorphism, for which depth estimates are from 15 to 18.5 km (see Wells, 1979). Clayburn (1981) concluded that the peak of regional metamorphism occurred sometime prior to 470 Ma, allowing a considerable amount of time for erosion.

The Strath Ossian Complex is thought to have been emplaced along a pre-existing, north-west-trending, fundamental basement structure, the Strath Ossian lineament (Key et al., 1993). The resultant, pronounced, north-west to south-east trend of the pluton is unique amongst the Argyll Suite (Stevens and Halliday, 1984; Plant 1986) of post-tectonic granites, cutting across the regional orogenic grain. Support for this supposition is provided by regional geophysical data (see Chapter 16).

Corrieyairack Granitic Complex

Almost all of this complex lies within the district (Figure 19), underlying the high ground on the south-east side of Glen Roy and the upper reaches of the Spey Valley. Its north-eastern extremity extends for about 2 km along the Spey Valley onto Sheet 63E. At its southern end, the complex comprises a distinctive xenolithic leucogranite in the centre of a vein complex. This granite is the oldest part of the Corrieyairack Granitic Complex (Clayburn, 1981). An aphyric granodiorite, separated from the leucogranite by a narrow sheet of mafic granodiorite, is the dominant lithology, forming a north-eastward trending pluton from Beinn Teallach [NN 360 860] to the Spey Valley. Less common lithologies include pink-weathering granite and microgranite, occurring as veins as well as satellite plugs. There is also a single quartz-porphyry stock cutting the main granodiorite.

The southern leucogranite (and its associated vein complex) was roughly contemporaneous with the early vein complexes described in Chapter 8 (see Clayburn, 1981), i.e. it is a late-tectonic intrusion. However, the later granodiorites are post-tectonic and contemporaneous with the main granodiorite phase of the adjacent Strath Ossian Granitic Complex. The granodiorites postdate the major vein complexes but are cut by microdioritic dykes of the Etive and Ben Nevis swarms.

All phases of the Corrieyairack Granitic Complex are locally well exposed, either as hillside crags or in the numerous watercourses. The summit region of Beinn Teallach provides excellent exposures of all three major lithologies. Lower down the southern slopes of this mountain, there are good sections in the tributaries of the Gleann Glas Dhoire. Farther north, the main granodiorite is well exposed in summit crags, for example on Creag Tharsuinn [NN 365 885], and several watercourses, such as the River Roy, provide sections across its entire width.

Xenolithic leucogranite

This forms an irregularly shaped body, elongate at right angles to the rest of the igneous complex, with a northwest-trending, 5 km long axis. The main lithology is a white, massive, well-jointed, medium- to coarse-grained, locally porphyritic, feldspathic leucogranite with less than 10 per cent of combined biotite, hornblende and muscovite. The granite has an intense red colour where it has been faulted e.g. at [NN 3536 8369]. Elsewhere it weathers to more subtle shades of pink. Diffuse pegmatitic zones are present as well as discrete pegmatite and aplite veins. These phases locally contain magnetite, garnet and muscovite, besides quartz, feldspar and biotite.

The granite is packed with xenoliths of its various metasedimentary country rocks and appinite. All xenoliths vary in size, from several centimetres up to several hundred metres in length. They are cut by granite veins and there are local reaction rims with the large appinite xenoliths. At [NN 3268 8649], [NN 3307 8580] and [NN 3307 8558] in the Allt Glas Dhoire and its tributaries, granitic material has invaded ultramafic appinite as discrete veins, and has permeated its groundmass to produce a bright green and pink hybrid rock. The metasedimentary xenoliths define a ghost stratigraphy, with psammitic xenoliths dominant in the east, and pelitic schist and calc-silicate rock xenoliths (both from the Leven Schist Formation) most common in the west. This feature suggests that the present exposure level is near to the roof of the intrusion. These xenoliths are mostly angular and elongated parallel to any internal planar fabric. They do not appear to be hornfelsed except in the north-centre of the granite.

The eastern and western contacts of the granite with the metasedimentary country rocks are sharply defined, with very little granite veining into the wall rock. However, the granite margin in the south is extremely irregular and its wall rock is strongly veined for up to about 2 km away from the intrusion. The granite is also locally chilled in this area e.g. at [NN 3470 8392] and [NN 3295 8526]. The northern contact with the mafic granodiorite is sharp, with no evidence that either rock is chilled. This contact is well exposed at [NN 3524 8618]; elsewhere it can be located to within about 20 m. All contacts are locally faulted.

The granite consists of zoned plagioclase, quartz, biotite and orthoclase, with minor muscovite, hornblende, sphene, opaque minerals, zircon and apatite, and secondary chlorite, epidote and sericite. It shows signs of post-crystallisation strain and recrystallisation.

Clayburn (1988) discussed the results of isotopic studies of the leucogranite; a lower crustal, high-grade (granulite facies) gneissose source, with Proterozoic and Archaean components, is indicated for this intrusion by its unradiogenic Pb characteristics as well as by its mineralogy. The Sr isotope ratios show that there was very little upper crustal contamination of the magma, which accords with the absence of reaction rims on most of the metasedimentary xenoliths. Clayburn (1988) concluded that the granite was emplaced at the same time as the Loch Laggan Vein Complex, that is at 439 ± 7 Ma during a period of regional uplift and erosion.

Granite veins

Grey-white, pink-weathering, medium-grained granite veins, as well as pink aplitic veins, are common in the southern leucogranite and its country rocks. Aplites always cut granite veins where the two occur together. Both occur as linear veins, less than 50 cm thick in most cases, although 10 and 15 m thick granite veins are present at [NN 3355 8404] and [NN 3257 8466] respectively. The veins form an intersecting network with no preferred orientation, and selvedges of the host rock are locally found in the thicker veins, e.g. at [NN 3246 8423]. Individual veins are locally zoned with coarser-grained centres, and all veins are strongly jointed. Biotite is the only other major mineral phase present besides quartz and feldspar. Quartz forms discrete blebs as well as being interstitial to feldspar. Microcline is the main potash feldspar.

Aphyric granodiorite

This is a massive, equigranular, medium- to coarse-grained, pinkish white rock of remarkably uniform field appearance over its total outcrop of 40 km2. It has a colour index of between 10 and 30 with roughly equal amounts of biotite and hornblende. Oligoclase, in subhedral crystals, is more abundant than potash feldspar which, together with quartz, tends to be poikilitic. Locally the plagioclase is strongly zoned and forms small phenocrysts. The accessory minerals comprise sphene, apatite, zircon and allanite. Angular psammitic and rare semipelitic xenoliths, less than 1 m long, are found only in marginal parts of the granodiorite. More widespread are rounded, fine-grained, dioritic xenoliths or enclaves, which are generally less than 20 cm in diameter, and which preferentially weather to leave shallow pits. In the river section above White Falls [NN 399 932], relatively large (5 m long) rafts of fine-grained mafic rock contain angular country-rock xenoliths. Pink aplitic and granitic veins are also of only local importance, e.g. on the hill north of Loch Spey [NN 42 94].

The granodiorite has sharp contacts with its country rocks, cutting their main planar fabrics at high angles. These fabrics are only disrupted within several metres of the actual contact. On a larger scale, the attitude of the country rocks is not affected, indicating that intrusion was permissive. Good exposures along the south-eastern margin of the granodiorite show that it dips at about 53° towards the south-east. The contact with the mafic grano diorite in the south is sharp at [NN 3498 8666] and [NN 3578 8646], but gradational at its eastern extremity. Where the contact is abrupt, the mafic granodiorite is relatively fine grained, suggesting that it is chilled and consequently younger than the main granodiorite.

No flow foliation has been detected and secondary tectonic structures are completely absent. Joints can be seen in all the exposures of the granodiorite but their orientation has not been analysed.

Mafic granodiorite

This forms an arcuate sheet, approximately 350 in wide at the southern end of the aphyric granodiorite. It is not known why this mafic rock is confined to this area. It is a speckled, white and black, massive, fine- to coarse-grained rock, with no xenoliths. Pink granite veins are uncommon in this granodiorite, which has the same mineralogy (sample (S76895)) as the main granodiorite but with more hornblende (colour index of about 25).

Quartz porphyry

Near Creag a' Bhanain [NN 420 921], the aphyric granodiorite is cut by a large (over 300 m diameter) plug of quartz porphyry. There are abundant loose boulders and a few good exposures of this white rock which contains prominent quartz phenocrysts. A sharp contact is exposed on its western and eastern sides.

Aureole

A thermal aureole can be mapped around the main granodiorite; in the field, it is most obvious in the pelitic lithology of the Leven Schist Formation. Its width is variable, attaining a maximum of about 1.5 km at Creag a' Chail [NN 40 95]. In the outer part of the aureole in the Leven Schist Formation, the partial or complete replacement of garnet by small shreds of biotite is the only indication of thermal alteration, and the rock remains a muscovite-rich schist. At a slightly higher grade, pseudomorphs after garnet consist of cordierite with tiny rounded inclusions of biotite and granules of iron ore. The schist becomes darker in colour and less fissile towards the contact with the granodiorite, although the mica fabric is retained. Andalusite forms large porphyroblasts and the predominant mica is biotite, accounting for the darker colour of the rock. Some laminae are almost mica-free, being composed of quartz, plagioclase, cordierite and potash feldspar. It is clear that muscovite has been consumed in a reaction with biotite and quartz to produce cordierite and potash feldspar. The hornfels in the Allt Chonnal [NN 390 944], up to 80 m from the granodiorite, is free of aluminosilicate and has a very prominent mottled texture. Irregular, dark grey patches up to 40 mm across and comprising about 30 per cent of the rock, are surrounded by pink reaction haloes up to a few millimetres thick. Isolated remnants of hornfelsed schist also occur. The latter consist of biotite, quartz and plagioclase while the dark patches contain a large amount of cordierite and are very depleted in biotite. Muscovite is associated with the biotite and may be a product of the reaction during which biotite was replaced by cordierite. The pink reaction haloes contain no cordierite, almost no biotite but abundant potash feldspar. It appears that the ferromagnesian component of the biotite has been transferred to the dark patches to form cordierite while the potash has remained to be incorporated into alkali feldspar. Within 1 m of the granodiorite, the layering in the hornfels is disrupted and flake-shaped fragments lie in a matrix which may have undergone some melting.

On the south-eastern side of the granodiorite at Meall Ptarmigan [NN 42 90], the entire outcrop of the Leven Schist Formation is strongly hornfelsed, and the pelitic rocks contain abundant sillimanite. In many places, large porphyroblasts of andalusite also occur. Bundles of fibrous sillimanite extend from the matrix into the andalusite (S82245), and in some examples slender prisms of sillimanite occur within andalusite porphyroblasts (S92818). The tremolite schist has been completely recrystallised, and comprises tremolite, phlogopite, plagioclase, quartz and sphene. Diopside is also present in some examples. The tremolite has retained a strong parallel orientation and the rock is fissile, in contrast to the massive character of the adjoining pelitic hornfels.

The semipelite of the Tarff Banded Formation on the east side of Creag a' Chail [NN 41 95] is thermally metamorphosed for more than 1 km from the granodiorite, as shown by the occurrence of andalusite porphyroblasts and the partial alteration of regional garnet to plagioclase and biotite. At 580 m from the igneous contact [NN 4120 9563], high-grade thermal effects are indicated by the presence of corundum as well as andalusite in a quartz-free hornfels. The corundum porphyroblasts show slight marginal retrogression to muscovite. Evidence of partial melting occurs 380 m from the granodiorite [NN 4124 9537], where corundum-hercynite hornfels contains leucosomes. The latter comprise oikocrysts of potash feldspar and quartz, enclosing euhedral tablets of plagioclase as well as small grains of biotite and iron ore.

Anatectic leucosomes are patchily developed within a zone up to 200 m wide, south-east of the granodiorite on Creag a' Bhanain [NN43 91]. Here, the semipelitic rocks of the Grampian Group comprise biotite-andalusite hornfels with irregular lenses, up to several centimetres thick, of coarser grained quartzofeldspathic material in which potash feldspar is micrographically intergrown with quartz. Most of the quartz and potash feldspar forms oikocrysts with euhedral inclusions of oligoclase (S82243). Ragged inclusions of biotite are almost certainly xenocrysts derived from the hornfels. However, a few are euhedral and probably crystallised from the melt. The hornfels contains pinitised cordierite and andalusite but sillimanite is notably absent. In the outer part of the aureole, garnet is replaced by biotite and plagioclase. Regrowth of garnet has occurred in the inner part, where hercynite and corundum have also been recorded.

Andalusite and cordierite, commonly intergrown, occur in hornfelsed psammite of the Dog Falls Formation up to 500 m from the granodiorite. On weathered surfaces, for example south-west of Meall na Harraw [NN 388 912], groups of cordierite grains form dark spots up to 20 mm across, each surrounded by a pink reaction halo depleted in biotite and enriched in potash feldspar.

Emplacement

The xenolithic leucogranite was emplaced with considerable stoping of its country rocks during the late Ordovician or early Silurian periods. Intrusion of the granite, from a lower crustal source, appears to have occurred soon after emplacement of the appinitic complexes in lower Glen Roy. A fundamental fracture trending northwest to south-east may have controlled the emplacement and elongation of all these bodies, as well as that of the later Strath Ossian Granitic Complex. Emplacement of the leucogranite was followed by the generation of its spatially associated and chemically related granite vein complex. All this granitic material may not be genetically related to the spatially associated but younger granodiorites of the Corrieyairack Granitic Complex. Early emplacement of granite is uncommon in the Argyll Suite of intrusions of the Grampian Highlands Igneous Province.

The main granodiorite was emplaced later, after further regional cooling, uplift and erosion over a period of about 20 million years. It was intruded as a discrete pluton which is discordant to the local structure, but which is strongly aligned along the regional orogenic grain. The mafic granodiorite was emplaced as a sheet between the main granodiorite and the southern granite. It is not known whether the isolated quartz-porphyry is a late phase of the same magma as the granodiorites, or if it is an unrelated late intrusion.

Felsite dykes

Felsite dykes are found within the areas of the Etive and Ben Nevis dyke swarms south-east of the Great Glen, and are described as part of these swarms in the next chapter. Cross-cutting relationships between the dykes of different compositions indicate that the felsites are the youngest intrusions. The felsites do occur on Sheet 63E (Dalwhinnie) and it is possible that they are unrelated to the rest of the dykes in the Etive and Ben Nevis swarms, despite their parallel, north-easterly trends.

Toman Liath Vein Complex

This suite of trondhjemitic veins was identified for the first time during the mapping. It comprises pink- or white-weathering veins, which cut the Leven Schists (see Frontispiece) and as members of the appinite suite over an area of about 7 km2 in the upper part of Glen Roy (Figure 19). The veins are well exposed on Toman Liath and in roches moutonees on the hillside to the south-west [NN 366 915]. There are also superb water-washed exposures of the complex in the River Roy, upstream from the Falls of Roy [NN 3699 9223]. Individual veins are commonly up to about 0.5 m thick, although on the west side of Toman Liath there is an exceptionally thick (more than 50 m), steeply dipping intrusion of xenolithic trondhjemite trending east-south-eastwards. Also, a 15 m thick dyke is exposed on Cam Dearg Beag [NN 3671 9387]. This dyke, in common with most of the thinner intrusions, lies parallel to bedding/schistosity in its host metasedimentary rock. Locally, the veins define a ramifying network, with several generations of cross-cutting veins (Plate 11).

The veins cut early folds seen in the Leven Schists, but they are affected by the late ductile deformation which produced east–west trending crenulation cleavages in these metapelitic rocks (Figure 27). The cleavage can be traced into the granitic veins where it forms a penetrative schistosity. In areas affected by the late deformation, the veins are folded, defining concentric structures with wavelengths controlled by vein thickness (Frontispiece). On Glas Bheinn [NN 378 941], the folds are tight and the limbs have been rotated into the extension field, resulting in the disruption of the veins into lenses. Here the crenulation cleavage in the schist has developed into a transposition fabric.

The veins range in composition from trondhjemitic (S83372) to granodioritic (S81653). Most are leucocratic and muscovite-bearing, but biotite, commonly chloritised, is also present in some veins, and a folded vein on Glas Bheinn (S82206) consists of foliated biotite-microtonalite. In foliated microtrondhjemite (S76701), the penetrative fabric is defined by a parallel orientation of muscovite and felts of muscovite are wrapped round irregularly shaped grains of oligoclase. Lenticular patches of quartz tend to be elongated parallel to the muscovite fabric and comprise subgrains showing intense strain. The most common accessory minerals are apatite and sphene. Epidote associated with biotite is seen in some sections, and xenocrystic garnet has also been recorded.

The intrusion of trondhjemite was accompanied by brecciation of the schist, and many of the veins are xenolithic with abundant, small (about 50 mm long), angular flakes of the host schist. The flakes are locally so abundant that the vein margins are ill defined. Examples of slightly later, xenolith-free veins, cutting xenolithic veins, and vice versa, are well displayed in the River Roy (Plate 11). The schist flakes are mostly oriented at random, but are realigned parallel to the penetrative schistosity in strongly deformed veins (Figure 27). No igneous xenoliths were encountered.

Glen Garry Vein Complex

The term is applied to a suite of unmetamorphosed (i.e. post-tectonic) minor intrusions north of the Great Glen, composed chiefly of granodiorite with minor quartz-diorite, tonalite and granite, which occur as irregular bodies and veins ramifying through the country rock (Fettes and MacDonald, 1978). The limits of the complex occur to the north and west of the Glen Roy district, and intrusions of the complex occur in both the Moine psammitic rocks and granitic gneiss. In the Glen Roy district, the veins are only locally abundant and there is no development within the area of an intense ramifying network that has been recorded elsewhere. Associated with the vein network are larger bodies of granodiorite that are sufficiently large to be mapped separately. The largest of these bodies occurs at the northern edge of the sheet, north of Munerigie [NH 269 028].

The general description of the Glen Garry Vein Complex given by Fettes and MacDonald (1978) applies to that part of the complex that crops out within the Glen Roy district. All the veins have sharp, cross-cutting margins and show no evidence of a preferred orientation or attitude. The veins are without any oriented internal fabric and there is no evidence of chilling against the country rocks.

The vein complex rocks are medium to coarse grained and commonly non-porphyritic. Plagioclase, quartz, potash feldspar, biotite and hornblende are the main constituents. Accessory minerals within the veins include sphene, apatite, epidote, carbonates, allanite, zircon and iron oxide; in addition there are abundant secondary micas and chlorite. Hornblende is present in the more basic and intermediate compositions and commonly shows secondary alteration. The plagioclase crystals may be zoned with turbid centres. Fettes and MacDonald (1978) reported that the cores of the plagioclases vary in composition from An26 to An12, with the crystal rims being more sodic in composition. Myrmekitic inter-growths are common. The potash feldspar is generally present as small interstitial grains, although in the more acid rocks it may form large, poikilitic patches, and some of the larger crystals may show perthitic intergrowths.

North of Munerigie Wood, at the northern margin of Sheet 63W [NH 275 035], there is a large, coarse- to medium-grained granodiorite intrusion, considered to form part of the Glen Garry Vein Complex. A smaller, dyke-like granodioritic intrusion occurs in Kilfinnan Burn [NN 272 964] and is probably also part of the vein complex. The granodiorite intrusion north of Munerigie Wood has a maximum width of about 0.5 km and a strike length of approximately 2 km, extending northwards onto Sheet 73W (Invermoriston). The intrusion has an elliptical shape, elongated in an approximately north-north-west to south-south-east direction. At outcrop, the body appears to be mineralogically homogeneous with no obvious compositional layering. It is cut by later aplite veins.

The granodiorite consists of quartz, microcline, plagioclase, and biotite, with less common hornblende. The main accessory minerals present are apatite, zircon and iron oxide. Biotite is commonly altered to chlorite, and hornblende may show some alteration to biotite and chlorite. The plagioclase crystals are zoned and have turbid centres. The potash feldspar is usually orthoclase and myrmekitic intergrowths are common.

Fettes and MacDonald (1978) suggested that the Glen Garry Vein Complex evolved from a quartz-diorite magma under relatively high water-pressure. They suggested that intrusion of the ramifying vein complex may be related to late Caledonian stresses, associated with movement in the Great Glen Fault Zone.

Intrusive breccias

These occur within the area of the Glen Garry Vein Complex and have been recorded in the areas of Sheet 63W and Sheet 73W. The intrusive breccia bodies are chiefly found within an area centred on Loch Lundie [NH 295 035] and intrude both the Moine psammitic rocks and the granitic gneiss. On Sheet 63W, one such intrusive breccia is exposed on Torr a' Chait [NH 2880 0322]. The body is dyke-like in form, with a maximum width of 100 m, extending in a north-north-west to south-south-east direction for some 1200 m; it extends northwards beyond the sheet boundary. The breccia is made up of subangular to subrounded fragments of psammitic rock, less common hornblende schist and, rarely, blocks of granitic gneiss. The rock fragments vary in size from sand-sized particles to large blocks more than 5 m long. Typically the rock fragments are tightly packed and are in contact with each other. Any intervening igneous matrix forms a very subordinate constituent of the rock volume, and in some exposures appears to be absent.

The igneous matrix is very fine grained and of dioritic composition, comprising plagioclase, hornblende, biotite and quartz. The margins of the intrusive breccia are gradational, and consist of a narrow zone in which the country rock is virtually unmoved but fractured and altered. This narrow zone passes rapidly into the intrusive breccia proper, made up of transported rock fragments.

The intrusive breccia bears some resemblance to the explosion breccias of the Kentallen district, associated with the appinites there (Bowes and Wright, 1967). It is regarded as having formed as a result of explosive activity, resulting from the release of gas pressure. Transportation of the fragments of country rock may have been partly by liquid as well as by gaseous phase, with a filter press mechanism resulting in a local concentration of clasts to form a breccia with little or no obvious igneous matrix (see also the description of the breccias associated with the appinites, Chapter 9). The association of these intrusive breccias is uncertain; they have similarities with those associated with the appinites but their matrix material suggests affinity with the Glen Garry Complex.

Granitic rocks in the Great Glen Fault Zone

Igneous rocks in the Great Glen Fault Zone include numerous felsic, granitic and pegmatitic veins intruded into metamorphosed psammitic rocks, and small, coarsely crystalline intrusions of granite and diorite with outcrops that are largely fault-controlled. The age relations between the intrusions are unknown, as is their relationship with intrusive complexes outside the fault zone. All the intrusive rocks are older than the Old Red Sandstone rocks of the Glen Roy district. It is concluded that they are representatives of the late- to post-tectonic granitic suite that have, to a greater or lesser degree, become disrupted and brecciated by movements within the Great Glen Fault Zone.

The granitic and pegmatitic veins are commonly 0.2 to 0.3 m wide although locally, larger discordant bodies occur. The veins have more or less parallel-sided, sharp contacts with the host rocks and no evidence for chilled margins has been observed. The veins are either white or salmon-pink at outcrop, and the more coarsely crystalline pegmatitic bodies show rapid transition to granitic veins of smaller crystal size without any evidence of a boundary between the two rock types.

In thin section, the granitic veins are seen to consist of orthoclase and quartz, with plagioclase occurring as a subordinate constituent. Myrmekite is locally present and the plagioclase is strongly sericitised. Muscovite is the main mica and may show replacement by chlorite. These granitic veins have suffered brittle deformation and in nearly all outcrops they are brecciated. Like the host psammitic rocks, they readily break along closely spaced fractures. That the shape and attitude of the veins is recognisable does imply that they may not have suffered the same degree of intense brittle deformation as the host psammitic rocks, which perhaps suggests that the veins were intruded some time after the commencement of deformation by Great Glen faulting.

A relatively large granitic body is exposed on Creag nan Gobhar [NN 313 982], and forms an elongated outcrop that has a maximum width of about 300 m, and extending in a north-east to south-west direction for approximately 1 km. The north-western boundary of the intrusion is defined by a north-east- to south-west-trending fault; the other boundaries, although not well exposed, are interpreted as the original, unconformable contact of the overlying Old Red Sandstone sedimentary rocks (Figure 29)b. The intrusion is a pink, coarsely crystalline granite. At outcrop, the presence of abundant quartz and pink feldspar is obvious, and dark minerals appear to be absent. The intrusive body shows no internal fabric, is slightly brecciated, and includes many small fractures and narrow joints. It is cut by pink pegmatitic bodies and finely crystalline aplite veins.

In thin section, quartz, potash feldspar and plagioclase are the dominant mineral constituents. Rare micas are completely pseudomorphed by chlorite. The granitic rocks show some degree of cataclasis, presumably as a result of movement within faults of the Great Glen. The degree of deformation, however, is less than that recognised in the adjacent psammitic rocks to the north-west of the body.

The eastern boundary of the granitic intrusion appears to pass into a coarse breccio-conglomerate, presumably of Devonian age, made up of large blocks of granite in contact with each other, the interstices between the blocks being infilled by fragmented granite, quartz and pink feldspar clasts. Bedding has not been identified and the breccio-conglomerate can he difficult to distinguish from the original intrusive body; it represents a coarse scree deposit, laid down on the slopes of a geomorphological high formed by the granite intrusion. The granite is thus regarded as predating Old Red Sandstone sedimentation.

To the south-west of the granite is a highly faulted dioritic intrusion with an outcrop that is entirely bounded by faults. At outcrop, the intrusion is brecciated and altered, with many strong fractures and shear zones infilled with chloritic material that imparts a dark colour to the rock. It is made up chiefly of plagioclase showing a varying degree of alteration, subordinate quartz and biotite, the biotite being almost completely pseudomorphed by chlorite and iron oxide. The quartz displays undulose extinction.

The dioritic body shows evidence of intense brecciation caused by fault movement, and appears to be more deformed by such movements than the granite body of Creag nan Gobhar. The relationship between the two intrusions is unknown. They occur on either side of the same north-east-trending fault; they may represent different phases of the same intrusive body (as in the Strath Ossian Granitic Complex), displaced by faulting, or they may be relicts of two separate intrusions now located near to each other as a result of fault displacement.

Chapter 11 Late Caledonian and Permo-Carboniferous dyke swarms

Two major dyke swarms, associated with the late Caledonian Etive and Ben Nevis plutonic complexes, were recognised around the turn of the century, during the primary survey of the Ben Nevis–Glen Coe area (Sheet 53). Subsequent mapping traced both swarms along their north-easterly strikes onto sheets 54W and 62E; their full extent south of the district is shown on figure 18 of the second edition of the Sheet 53 memoir (Bailey and Maufe, 1960). The present mapping has delineated, for the first time, the north-eastern limits of both swarms (Figure 19). Isolated dykes, possibly related, occur beyond the north-eastern limits of both swarms, but over 99 per cent of the dykes fall within the limits shown. The Etive Swarm is the larger of the two swarms, with a width of about 20 km and a strike length of about 110 km; these are roughly double the equivalent figures for the Ben Nevis Swarm. The present mapping has also established that they are separated in the Glen Roy district by a well-exposed tract of land, about 1 km wide, through Tulloch [NN 355 803] and Sgurr Innse [NN 290 750]. The north-eastern limit of the Etive Swarm roughly coincides with the trace of the Laggan Dam Fault.

Bailey and Maufe (1960) provided a full description of both swarms, and Anderson (1935, 1937) presented detailed accounts of each swarm in the areas adjacent to their source plutonic complexes. This early work established that the dyke swarms arc lithologically similar, with porphyritic microdiorites, referred to as porphyrites, and aphyric microdiorites as the main rock types. Less common are more basic lamprophyric dykes and more acidic quartz-porphyritic dykes.

Members of the microdiorite suite, which are abundant over a large area farther west (Smith, 1979), are uncommon in the Glen Roy district north-west of the Great Glen, where only a single microdiorite dyke was recorded.

The final phase of igneous activity that is represented by the intrusion of camptonite-monchiquite dykes of probable Permo-Carboniferous age north of the Great Glen.

Etive Dyke Swarm

This great swarm of calc-alkaline dykes extends into the southern part of the Glen Roy district, with the main concentration of dykes found in the rugged countryside overlooking Loch Treig. A dramatic reduction in the number of dykes occurs north-eastwards across the area of the Strath Ossian Granitic Complex, so that it is possible to define a north-eastern limit to the dyke swarm some 54 km from the centre of the Etive mass. In the Loch Treig area the dykes trend between north-east and east-north-east (mean trend 051°), in contrast to the north-east to north-north-east (mean 038°) trend of the dykes immediately south of the district. On Sheet 63W, the swarm comprises subvertical dykes; no associated gently dipping sheets were found.

In detail, dyke margins are not perfectly planar and opposite contacts are rarely parallel. Dyke margins have thin chills. Bifurcations are common; for example, the 20 in-thick dyke exposed for several hundred metres down the middle of the River Spean at Inverlair Falls has several splays. The porphyritic dykes are up to 60 m thick, but mostly from 1 to 10 in in thickness, whereas the aphyric intrusions are only up to about 5 m thick. A good cliff section on the steep slopes west of Loch Treig, [NN 3359 7505] shows vertical pinching out of a 10 m thick porphyritic dyke. Individual dykes may be traced for up to a kilometre in some well-exposed areas. It is easier to trace the thicker dykes between exposures; it is impossible to be absolutely certain that thin (i.e. less than 5 m thick) dykes seen in exposures more than 100 m apart along strike are one and the same dyke. The thickest dykes maintain their thickness for most of their strike-lengths before rapidly tapering out. Very thin dykes locally follow master joints, side-stepping via intersecting joint planes. In many cases, the dykes are concordant to the main planar fabric in the host metasedimentary rock because the dyke swarm is parallel to the regional orogenic grain. However, in the Loch Treig area, where the country rock has a complex fold history, the dykes cut across structures in the metasedimentary rocks. In this well-exposed area it is also possible to show that the dykes are offset across faults. For example, the thick, easterly trending dyke at locality [NN 3536 7606] is offset several metres by a fault trending north-eastwards. However, an exposure of brecciated and severely altered granodiorite adjacent to a north-north-east-trending fault in the Fersit area [NN 3734 7855] contains a 1 m-thick, fresh, north-north-east-trending dyke which thus appears to postdate the faulting.

The majority of the dykes in the district are porphyritic microdiorites in which hornblende and plagioclase occur as the main phenocryst phases. Within this suite, a complete gradation exists between intrusions in which hornblende is the principle phenocryst phase (lamprophyric microdiorites) to those in which plagioclase is dominant (e.g. feldspar-phyric microdiorites). Intrusions holding clinopyroxene or biotite as phenocrysts are uncommon (roughly 6–10 per cent of the swarm). Aphyric microdiorites are rare, as arc more acid lithologies which include quartz porphyries and porphyritic microgranodiorites. The latter are confined to the north-western edge of the swarm near Inverlair. There are also rare felsites throughout the outcrop area of the swarm which may represent the acidic end-member of the main suite (the felsite dykes have parallel trends to the Etive microdioritic dykes), or they may be related to a more regional development of this lithology, unrelated to the Etive Swarm. Bailey and Maufe (1960) make no reference to felsites within this swarm. No composite or multiple dykes were found in the Glen Roy district.

Detailed work in the Etive area has established that several pulses of dyke injection occurred, concomitant with the emplacement of the successive components comprising the Etive plutonic complex (Anderson, 1937; Bailey and Maufe, 1960; D I Smith, personal communication). On Sheet 63W, it is not possible to demonstrate a relative chronology of dyke emplacement; no crosscutting relationships occur between any dykes. Individual dykes cut the Strath Ossian Granitic Complex (no dykes are observed to terminate at the margin of this mass), as well as the early lamprophyric sheets, e.g. at locality [NN 3376 7516]. The dykes of the Etive Swarm, are not cut by any other igneous intrusion.

All the porphyritic microdiorites are massive rocks with the thicker intrusions forming prominent crags, most spectacularly east of Loch Treig at Creag Dhearg [NN 3539 7602]. No xenoliths have been noted in most of the dykes, although a 5 m-thick dyke exposed east of Loch Treig [NN 3521 7500] contains angular, fine-grained, mafic xenoliths up to several centimetres long. In hand specimen, the dykes are fine-grained, pink-grey rocks speckled by green-black mafic or white feldspar phenocrysts up to 13 mm long. Secondary alteration is clearly visible, even in hand specimens; pitted weathered surfaces are due to breakdown of the alteration products of the mafic phenocrysts. The total phenocryst content varies up to about 30 per cent of the rock volume, and there is a gradation from rocks in which mafic phenocrysts dominate to those in which feldspar is dominant.

In thin section, the ubiquitous strong secondary alteration of all primary minerals is clearly seen. Plagioclase phenocrysts are euhedral to subhedral, with concentric zoning sometimes visible. Microprobe analyses indicate that these phenocrysts range in composition from oligoclase to andesine, but with no consistent chemical zonation. Amphibole phenocrysts occur as brown prismatic laths or, less commonly, as green, more acicular crystals. Microprobe analyses of sample (S80098) show that the brown amphibole is magnesio-hastingsite. Less common biotite phenocrysts are smaller than the two main phenocryst phases, and form ragged flakes. Augite forms subhedral phenocrysts with minor inclusions of groundmass material. Quartz forms rounded phenocrysts in rocks containing subhedral potash feldspar phenocrysts (see Bailey and Maufe, 1960 ). These phenocrysts are set in a groundmass of plagioclase and untwinned feldspar laths with interstitial quartz, traces of sphene and tiny apatite crystals. No primary mafic phase remains in the groundmass. Chlorite, epidote and carbonate are the most common secondary minerals, and can occur as clusters containing an opaque phase. The feldspars are heavily sericitised as well as containing albite overgrowths. Microlitic textures are seen in some sections, e.g. in sample (S80064).

The aphyric microdiorites are dominated by subhedral plagioclase (An30) laths, with some hornblende and interstitial quartz. Secondary alteration to epidote, sericite, chlorite and very fine-grained haematite is variably developed.

In hand specimen (e.g. (S80638)), the porphyritic microgranodiorites are dominated by euhedral, pink-weathering feldspar phenocrysts. Most of the feldspars are strongly zoned plagioclase, with minor potash feldspar phenocrysts.They are accompanied by minor green amphibole and ragged biotite phenocrysts, set in an equigranular groundmass of quartz and strongly altered feldspar, with accessory apatite. The phenocrysts make up about 30 per cent of the modes. Locally, biotite forms pseudomorphs after amphibole. Microprobe analyses of the amphibole from sample (S80110) indicate that it is an actinolitic hornblende. Sericite and albitic overgrowths replace feldspar, with chlorite and carbonate also occurring as secondary phases.

In hand specimen, the felsites are fine-grained, pink-weathering, leucocratic rocks, flecked by biotite and feldspar microphenocrysts. Thin sections reveal strongly altered aphanitic groundmasses of feldspar and quartz, with secondary sericite, carbonate, chlorite, epidote and haematite. The microphenocrysts are equally altered; in sample (S80067), plagioclase laths are clustered to give a glomeroporphyritic texture.

Clayburn et al. (1983), using mineral-whole rock Rb/Sr isotope analyses, showed that the whole Etive plutonic complex was emplaced over a short time period, at around 400 Ma. Thirlwall (1988) suggested a slightly older age of 412 ± 5 Ma for this igneous activity, using the same dating technique but on different members of the complex. These late Silurian or early Devonian ages date the emplacement of the dyke swarm; Bailey and Maufe (1960) discussed the genesis of this swarm in relation to the emplacement of the major plutons of the central complex. The existence of the dyke swarm indicates a tensional upper crustal regime at the time of their emplacement, with the dyke trends reflecting the orientation of the principal horizontal stresses. The dyke swarm has a sigmoidal shape, with both extremities trending north-east to east-north-east in contrast to the middle section which trends roughly north-north-east to north-east ((Figure 28), and see Anderson, 1937). The regional change in the stress field implied by these changes in orientation may be related to tear movements along the major faults, such as the Great Glen and Ericht–Laidon faults, bounding a transtensional block into which the dyke swarm was emplaced. Watson (1984) showed that the dyke swarms were emplaced during the late orogenic stage, dominated by sinistral strike-slip motion. She noted that north-eastward movement of the whole Grampian block along its diverging bounding faults (the Great Glen and Highland Boundary faults) would have caused extension normal to these faults to facilitate the development of north-eastward trending dyke swarms.

Bailey and Maufe (1960) deduced that dyke emplacement resulted in a 30 per cent crustal volume increase in the centre of the swarm on Sheet 53. A similar calculation for the Loch Treig area indicates that dyke emplacement led to crustal extension of 6–10 per cent.

Ben Nevis Dyke Swarm

Intrusions identified as components of the Ben Nevis Dyke Swarm extend into the central part of the district (Figure 19). A discrete north-eastern limit to this swarm is placed some 30 km from the Ben Nevis Igneous Complex. Most of the dykes exposed on Sheet 63W are subvertical with pronounced north-east to south-west trends, although intrusions cropping out at the north-eastern extremity of the swarm show east-north-east trends. Most dykes cannot be traced away from single exposures with confidence, although some of the thicker dykes have been traced for several hundred metres along strike. Continuous exposure in the River Spean at Monessie [NN 2967 8100] means that one dyke can be followed for about 300 m. In rare instances, the intrusions take the form of more gently inclined sheets, such as the intrusion exposed in the River Roy at Roybridge [NN 2714 8140] which has a dip of about 50° towards the south-east. Individual intrusions are discordant to planar fabrics in the host metasedimentary rocks, and all have chilled margins.

The Ben Nevis Swarm comprises similar lithologies to the Etive Swarm, although aphyric microdiorites are more common. No composite or multiple dykes were found. Amygdales are locally seen, for example in a porphyritic microdiorite at locality [NN 3176 9289], in which epidote, calcite and a small amount of pyrite make up the amygdales. The abundance of dykes is significantly less than in the Etive Swarm, and their emplacement resulted in crustal extension of less than 1 per cent in the Glen Roy district. Individual dykes are mostly less than 5 m thick, with porphyritic dykes tending to be thicker than aphyric dykes. The thickest dyke is a feldspar porphyry exposed on Carn Dearg [NN 410 893] which is 50 m thick.

Emplacement of the Ben Nevis Dyke Swarm was roughly contemporaneous with that of the Etive Swarm. The only known cross-cutting relationship between two dykes in the Ben Nevis Swarm occurs on Creagan Breac [NN 380 900] where a felsite cuts a porphyritic microdiorite. Consequently, it is not possible to establish a chronology of dyke emplacement within the dyke swarm, except to say that the felsitic rocks are late intrusions. Like those of the Etive Swarm, the dykes are not cut by any other igneous phase, but are seen cutting various members of the appinite suite as well as all phases of the Corrieyairack Granitic Complex. The dykes are locally faulted; a shattered dyke is exposed in the River Roy [NN 2885 8263].

The dyke rocks can be divided into a dioritic-granodioritic group and an acidic group. The first group forms a continuous series ranging from lamprophyric rocks to feldspar-phyric microgranodiorites. The acidic group forms a distinct suite ranging from aphyric rocks (felsite) to feldspar porphyry, and possesses characteristic spherulitic textures.

Good examples of relatively fresh larnprophyric micro-diorites include (S76699), (S76705), (S76709) and (S82645); most other samples are strongly altered. In hand specimen they are grey or brownish grey, fine grained and appear aphyric, although close examination shows much acicular amphibole forming microphenocrysts. Thin sections show that they are composed of hornblende and plagioclase with accessory opaque minerals and apatite. Primary sphene is notably absent. A small amount of colourless pyroxene occurs in (S76699). All contain chlorite and carbonate pseudomorphs, probably after pyroxene phenocrysts. The hornblende is brown, euhedral and prismatic to acicular in habit, and is present as small phenocrysts as well as in the groundmass. In (S76699) it is partly replaced by biotite, but usually it is altered to chlorite and sphene. Microprobe analyses of amphiboles in (S80652) indicate that they are kaersutite and titanian-potassian-pargasite. Plagioclase forms subhedral laths; phenocrysts are uncommon or completely absent. The plagioclase in the aphyric dyke (S82660) exposed in the Allt Bohaskey [NN 3063 8776] is arranged in spherulites. In sample (S76705), small prisms of euhedral brown hornblende occur as inclusions in poikilitic andesine(An37) which also contains inclusions of epidote. A small amount of interstitial quartz is invariably present. Some of these rocks could be described as spessartites, but many contain a few feldspar phenocrysts. In others, feldspar phenocrysts are more abundant and the dividing line with the types described below is arbitary.

Fresh, unaltered feldspar-phyric microdiorites are grey, with hornblende and feldspar easily visible macroscopically. In thin section, a porphyritic texture is typical, with plagioclase, brown hornblende and biotite phenocrysts. Pyroxene cores to amphibole phenocrysts are preserved in (S76723). Euhedral plagioclase phenocrysts consist largely of labradorite; zoned laths have oligoclase rims. Hornblende phenocrysts are also euhedral and may be colour-zoned in shades of brown. They are variably replaced by chlorite and sphene, the latter occuring as irregular grains with a tendency to be elongated parallel to the c-axis of the parent amphibole and commonly associated with carbonate, epidote and opaque minerals. Microprobe analyses of the amphibole from (S82422) indicate that it is edenite or edenitic hornblende. Minor biotite phenocrysts are euhedral and partially chloritised. The groundmass consists of subhedral andesine showing normal zoning to oligoclase. Shreds of biotite, commonly chloritised, and minor quartz occur interstitially. Accessory minerals include opaque minerals and apatite.

Pale grey microgranodiorite dykes are recognised, in which feldspar and biotite phenocrysts are visible in hand specimens. In thin section (S82664), the feldspar phenocrysts are seen to be euhedral oligoclase. Small numbers of amphibole phenocrysts, almost completely altered to biotite and carbonate (S82664) or chlorite and sphene (S76686), have been recorded. The ground-mass is made up of plagioclase, quartz and potassium feldspar. In (S81666), xenocrystic quartz and plagioclase phenocrysts are mantled by a narrow zone which is composed of a micrographic intergrowth of quartz and potash feldspar. Similar intergrowths occur elsewhere in the groundmass. Primary accessories include zircon and apatite. Late interstitial sphene, epidote and carbonate also occur.

The spherulitic feldspar porphyries have a characteristic brick-red colour. The outer parts of each dyke commonly show a regular planar structure lying parallel to the dyke walls; an associated close jointing causes the rock to break into thin slabs. Invariably, feldspar phenocrysts are intensely sericitised, although normal zoning outwards from andesine to oligoclase is preserved in (S82225). Euhedral biotite phenocrysts are altered to dark masses, probably of haematite, and muscovite. Phenocrysts of potassium feldspar and ti-quartz occur in (S82225), but are generally absent. The groundmass is very fine grained, with euhedral feldspar microlites in a matrix having a spherulitic texture (e.g. in (S76689)). The spherulites have a radiating texture defined by haematite inclusions which produce the distinctive brick-red colour. Spaces between the spherulites are infilled with fans of white mica and coarsely crystalline carbonate (S82219).

The spherulitic felsites are an almost aphyric variety of the spherulitic feldspar porphyries. The late dyke on Creagan Breac [NN 3791 9015] is fine grained and pinkish in colour. In (S76722), a few small, euhedral crystals of plagioclase, B-quartz and chloritised biotite are set in an aphanitic felsic groundmass with a spherulitic texture. The rock is thought to be a denitrified glass which contained minor microphenocrysts.

Alteration of the dykes

Petrographic examination of all the Ben Nevis and Etive dykes presents something of a paradox, in that these youngest rocks in the metamorphic terrain are seen to be the most altered. It is thought that the dykes were deuterically altered as they cooled during the Devonian Period. The presence of the alteration minerals as primary infillings to vesicles in some Ben Nevis dykes supports the idea that the alteration is due to late magmatic fluids. Clayburn et al. (1983) also concluded, from their isotopic studies of the main Etive Complex, that the rocks acted as open systems with regard to fluid movement well into Devonian times. Some of the late alteration seen in the country rocks may also be due to this late fluid release spreading from the dykes along open brittle fractures.

Microdiorite suite north-west of the Great Glen Fault Zone

Only one microdiorite dyke was recorded north-west of the Great Glen, at locality [NN 2705 9915]. The rare occurrence of microdiorite confirms the eastern limit of the numerous microdiorite intrusions north of the Great Glen as proposed by Smith (1979). In the hydro-electric tunnel connecting Loch Garry to Loch Oich, only 16 microdiorite dykes were recorded within a tunnel length of some 5 km.

Camptonite–monchiquite dykes

Ten intrusions of this Permo-Carboniferous suite have been mapped. They are possibly members of the Eil–Arkaig Swarm which is found within the Moine psammitic rocks to the west of the Glen Roy district (Rock, 1983). They occur as dykes, rarely more than 2 m thick, which have an approximate east–west strike. In hand specimen the rocks are dark grey-green to black, aphanitic, and phenocrysts up to 5 mm in size are common locally. At outcrop, the dykes may show a pockmarked surface due to the preferential weathering of the phenocrysts. Many of the dykes are cut by thin carbonate veinlets.

The phenocrysts in these igneous rocks consist of olivine, pseudomorphed by serpentine, chlorite, iron oxide and carbonate, and clinopyroxene which is partially replaced by actinolitic amphibole or chlorite, or pseudomorphed by carbonate. The phenocrysts occur in a groundmass consisting essentially of an intergranular matrix of amphibole, feldspar, clinopyroxene and biotite, in which the main accessory minerals are apatite and iron oxide.

Chapter 12 Devonian (Old Red Sandstone) rocks in the Great Glen Fault Zone

Old Red Sandstone (ORS) sedimentary rocks of Devonian age occur in only two separate outcrops within the Great Glen Fault Zone (Figure 30). One outcrop is immediately south-east of Loch Oich, and forms a narrow unit on the lower south-eastern slopes of the Great Glen that can be traced from Aberchalder to Laggan, a distance of some 8 km. The outcrop is fault-bounded and continues beyond the limits of the present mapping. The sedimentary rocks show intense brecciation as a result of movement within the Great Glen Fault Zone.

The second outcrop of ORS rocks occurs immediately north-west of the Glen Buck Fault, in a 1 km-wide fault-bounded block that extends south-westwards from near Leitir Fhearna [NN 325 995] to the south-eastern slopes above Loch Lochy, at the western margin of the sheet (Figure 30). This outcrop is cut by subvertical faults of Great Glen trend, but the ORS sedimentary rocks have not suffered much cataclasis and the rocks are virtually undeformed.

The presence of ORS sedimentary rocks in this part of the Great Glen has been known for many years and their outcrop was recorded on the 3rd edition of the 1:625 000 geological map of the United Kingdom. Kennedy (1946) correlated the rocks, on lithological grounds, with the Middle ORS of the Moray Firth area, and Holgate (1969) supported this correlation by lithological comparison with the ORS exposures at Mealfuarvonie [NH 457 223] on the north-west side of the Great Glen. Eyles and MacGregor (1952) described the intense brittle deformation of the ORS sedimentary rocks within the Great Glen Fault Zone, but recognised that the sedimentary rocks forming the outcrop north-west of the Glen Buck Fault (Zone B of Eyles and MacGregor) were for the most part relatively undeformed. They recognised that the most common rock types were conglomerate and sandstone although Holgate (1969), in correlating with the ORS of Mealfuarvonie, considered that 'massive conglomerates were not obviously represented'.

Lithology

Conglomerates and strongly coloured, red-brown sandstones are the most common rock types in both outcrops, with pebbly sandstones occurring as impersistent lenses. Mudstones have not been recorded. The sedimentary rocks have been examined for microfossil content, but with negative results. The rock types are well displayed in the virtually undisturbed sequence in the fault block bounded by the Glen Buck fault and another fault in the Great Glen Fault Zone (Figure 29) and (Figure 30).

The main rock types are a very coarse basal conglomerate which passes upwards through a thin, transitional contact into red sandstones containing interbedded impersistent pebble horizons. A coarse granite scree deposit forms a limited, but distinctive unit that has been recognised on the south-eastern slopes of Creag nan Gobhar [NN 313 978].

The conglomerate is a very coarse, poorly sorted rock, containing abundant rounded to well-rounded cobbles and boulders of various sizes. It occurs in two separate outcrops, north-east and south-west of Creag nan Gobhar [NN 318 982], separated by a poorly exposed, north-east-trending fault (Figure 30). In both outcrops, rounded clasts more than 15 cm in diameter are very common, and boulders up to 30 cm or more occur. The most readily recognisable rock types, forming the coarse cobbles and boulders, are pink granite, siliceous psammite, psammitic gneiss, quartzite and vein quartz, rock types that can be readily compared with adjacent outcrops of igneous and metamorphic rocks. The large clasts may be in contact with each other, but are more commonly separated by a matrix of red, gritty sandstone. In outcrop, the matrix is usually very subordinate volumetrically to the cobble and boulder clasts. Bedding has not been observed in the conglomerate, but there are occasional, thin, impersistent lenses of bedded red sandstone.

That the conglomerate is a basal conglomerate is confirmed by the exposure of the unconformable contact with underlying metamorphic rocks north-east of Creag nan Gobhar ((Figure 30), locality A). Even in the few outcrops in which it can be seen, the unconformity has a variable attitude, and it is evident the conglomerate was deposited on an undulating landsurface. Mapping indicates that the unconformity is inclined to the south-west at less than 30°. In a few places, and especially where they are cut by quartz veins, the metamorphic psammitic rocks show red staining due to deposition of iron oxide in crevices below the unconformity. The conglomerate is estimated to have a true thickness of between 100 and 130 m. To the south-west of Creag nan Gobhar, the basal unconformable contact of the conglomerate is not exposed.

The contact of the conglomerate with the overlying red sandstone is not exposed, but there is an increase in the proportion of sandstone intercalations in the upper parts of the conglomerate, indicating a rapid transition from conglomerate to a dull red sandstone with pebble layers. The pebble beds are impersistent and decrease in number so that the highest member of the succession is a dark red, fine-grained sandstone. This upward passage from pebbly sandstone to sandstone is well displayed on the southeastern slopes of Creag nan Gobhar where an interbedded sequence of pebbly beds, 5 to 20 cm thick, are interbedded with thin, red, 2 to 3 cm thick sandstones. Locally, impersistent beds of coarse sandstone occur. This sequence has a low dip (c. 20°). The interbanded coarse sandstone with pebbly beds is overlain by fine-grained, red-purple sandstones that are remarkably uniform in composition, with no obvious bedding or sedimentary structures. This highest sandstone is well exposed on the summit of Creag nan Gobhar where it is broken by a well-developed set of sub-vertical, closely spaced joints, trending north-east to southwest subparallel to the Great Glen. These red sandstones are downfaulted to the south-east and exposures of the fine-grained sandstone occur in the Allt an Lagain, Allt Beinne Baine, Allt an t-Sidhein [NN 299 961] and Allt a' Choilich [NN 289 951]. South-west of the Allt a' Choilich, the outcrop is fault-controlled, and is made up chiefly of coarse, dark green and purple conglomerate.

A coarse, granite breccia is exposed south-east of the granite outcrop, to the south-west of the summit of Creag nan Gobhar [NN 312 978] (Chapter 10). The breccia is clast-supported and consists of coarse angular blocks of pink granite, with the interstices between the boulders infilled by fragmented granite, quartz and pink feldspar. The deposit is unbedded. The granite boulders forming the breccia are identical to the adjacent exposed granite, and the breccia is interpreted as a very coarse scree deposit, laid down on the lower slope of a geomorphological high formed by the intrusion.

The fault-bounded outcrop of ORS sedimentary rocks on the south-east side of Loch Oich does not exceed 300 m in width. It includes numerous fault zones and crush rock, and all the sedimentary rocks are brecciated, sheared and intensely shattered by movement in the Great Glen Fault Zone. The rocks readily break into joint- bounded fragments, and different rock types can be very difficult to distinguish in outcrop. The Calder Burn provides the best continuous section through the ORS outcrop, but other exposures occur along the line of the former railway south-east of Loch Oich and on the lower slopes of Coille Leitir Fhearna [NN 315 997]. Such sections confirm that the ORS of this outcrop includes coarse conglomerates, sandstones and coarse sandstones with pebble-bearing horizons. The conglomerate is well exposed in the lower reaches of the Calder Burn, south of Aberchalder Lodge [NH 342 033], where it includes rounded boulders with diameters of 15 cm or more, some reaching diameters of 30 cm, chiefly made up of pink granite, vein quartz and psammite. The boulders are matrix-supported, similar to the conglomerates in the other ORS outcrop described above, but the original lithology is disrupted by narrow (less than 10 cm wide) shears, infilled by green chlorite(?), which fragment the conglomerate and trend parallel to the Great Glen. Despite the intense fragmentation the conglomerate is lithified, and the Calder Burn flows through a steep-sided gorge cut into it.

Exposures immediately south of the Calder Burn [NH 341 029] indicate that impersistent bands of sandstone and arkosic grit occur interbedded with the conglomerate, but the amount of exposure available is insufficient to show whether the conglomerate is a basal unit unconformably overlying older metamorphic rocks, or if it is succeeded by finer-grained sedimentary rocks. A stream section at the southern end of Loch Oich [NN 306 984] exposes highly fractured conglomerate containing large, rounded, pink granite boulders, that is succeeded to the south-east by dark red sandstones with impersistent grit horizons. These rocks are directly comparable to the undeformed ORS rocks exposed in the outcrop immediately adjacent to the Glen Buck Fault.

Conditions of deposition

The basal conglomerates or breccio-conglomerates include rounded boulders and cobbles of rock types that can be readily identified with adjacent rock units. The numerous pink granite boulders appear to be identical to the granite of Creag nan Gobhar. 'Exotic' rock fragments have not been identified, and it is likely that the basal conglomerate is made up of material derived from the immediate hinterland of the area of deposition. The basal sediments were laid down on an uneven topography within an area of deposition that was probably controlled by active fault scarps, and it may be that no, or very limited, deposition took place south-east of the Glen Buck Fault or north-west of the Great Glen Fault. Between the bounding faults, it is probable that fault-controlled geomorphological highs occurred, and the coarse scree deposit adjacent to the granite south of Creag nan Gobhar indicates that the granite mass formed a topographic high at the time of deposition of the basal sediments. The deep red pebbly sandstones and sandstones represent finer-grained sediments, deposited on the coarse, poorly sorted, unstratified scree material.

There is no evidence to confirm that all the ORS sediments of the two separate outcrops were laid down in one area of deposition that was subsequently disrupted into two separate outcrops by fault movement. The relatively undisturbed sedimentary rocks immediately adjacent to the Glen Buck Fault are in marked contrast to the intensely deformed sediments adjacent to the Great Glen Fault, and the differences in deformational state throw light on the history of fault movements within the Great Glen Fault Zone (see Chapter 13).

Chapter 13 Faulting

Numerous faults occur throughout the Glen Roy district. Generally, their presence is indicated by a reddening of bedrock and gullying due to the friable nature of the faulted rock. It is possible to trace many of the faults as lineaments on aerial photographs. The majority are minor structures which can be recognised in stream sections, commonly by the horizontal offset of dykes. It is rarely possible to ascertain any vertical component of movement. There is ample evidence for repeated movement along faults, and consequently fault trends do not necessarily reflect the regional stress pattern in operation during any single phase of faulting.

Great Glen Fault Zone

The Great Glen Fault Zone, which divides the district into two parts, is by far the most important fault structure in the area of Sheet 63W (Plate 12). This major fracture operated as a transcurrent fault during the later stages of the Caledonian Orogeny, and has subsequently been re-activated, not only during the Devonian, but also during later periods. The direction, scale and timing of movements remain the subject of current research, but the net displacement is thought to be sinistral and to exceed 100 km (Johnstone and Mykura, 1989).

The Great Glen structure is not a single fault plane but a zone, between 1.5 and 2.5 km wide, bounded by two parallel faults of unknown but probably large displacement. Many faults and much associated crushing occur within the zone, which is asymmetrically placed in relation to Loch Lochy and Loch Oich (Figure 30). Most of it lies to the south-east of the Great Glen, and includes outcrops of Old Red Sandstone as well as metamorphic and igneous basement rocks. The zone was probably the site of a narrow graben within which the Old Red Sandstone was deposited. The contrast between intensely shattered sandstone and conglomerate immediately south-east of Loch Oich, and the undeformed, flat-lying, Old Red Sandstone adjacent to the Glen Buck Fault, implies that post-ORS movements were confined to the north-west side of the zone, coinciding with the valley of the Great Glen. The relationships in the south-east part of the zone are particularly well shown at locality A (Figure 30) where unbroken conglomerate rests unconformably on brecciated psammite.

The effects of repeated movements can be studied in the Calder Burn [NH 345 032] and in the steep gullies to the south-east of Loch Lochy. In the Calder Burn ((Figure 30), locality B), the main features include innumerable fractures, brecciation, subvertical faulting with and without gouge, granulated cataclasite and pseudotachylite. All the exposures contain numerous planes of varying orientation that intersect and break the rock into fragments of varying size. Most fracture surfaces are coated with chlorite or, less commonly, calcite. The fractures thicken and pass into narrow brecciated zones, usually not more than a few centimetres wide and infilled with granulated rock. Such zones of brecciation show a more regular orientation than the thin fractures, with the main trend being north-east to south-west, parallel to the Great Glen. Faults up to 1 m thick containing grey clay gouge with angular fragments of rock are common. The trend of the faults is dominantly north-east to southwest, parallel to the Great Glen, and nearly all are subvertical. Adjacent to the major faults, straight linear shears have developed, not more than 2 cm wide and separated by thin bands of psammite. These shears, which impart a foliation to the rock, are steeply dipping. Thin zones of cataclasite occur; they are up to 1 m thick, have sharp contacts with the adjacent rock, and may include, or be adjacent to, thin bands of dark grey-black pseudotachylite.

South-east of Loch Lochy (Figure 30), locality C, the mylonitic rocks associated with the Eilrig Shear Zone have been affected by later brittle deformation, converting them to cataclasites with microfractures 'cutting the mylonitic foliation. In some exposures, the cataclasite is cut by an irregular network of quartz veinlets, and kink-folded foliation has also been recorded. It is also cut by numerous, steeply inclined crush zones of soft breccia and clay gouge. It is likely that the formation of cohesive cataclasite predates the deposition of the Old Red Sandstone, which has only been affected by later movements producing fault gouge and general shattering.

Large gullies, being actively eroded at the head of the Allt a' Choilich [NN 291 947] (Figure 30), locality D), provide spectacular exposures of many steeply inclined late faults cutting a variety of metamorphic rocks which are all in a shattered and crumbly condition. Some of the faults are oblique to the trend of the Great Glen and comprise crush zones, up to 1.5 m thick, containing abundant, powdery graphite. The crush rock is friable and has a lenticular structure due to the presence of graphite-coated 'buttons' of vein quartz and decomposed rock. This occurrence of graphite was recorded by Heddle (1901) who described it as a vein, about 3 feet wide, from which about 2 tons were raised in 1825 (see also Chapter 15). Recent work indicates that the `veins' consist of fault gouge, and although graphitic phyllite occurs in the adjacent Glen Buck Formation, the mechanism of graphite enrichment is not clear.

Sronlairig Fault

The Sronlairig Fault has been traced east-north-eastwards from its junction with the Glen Buck Fault (Figure 30) to the north-east margin of Sheet 63W. The fault is well exposed in the Allt Innis Shim [NN 329 984], Allt na Larach [NN 340 987] and Allt Lagan a' Bhainne [NH 392 012]. It is vertical or subvertical, and is marked by a crush zone, 1 to 2 m wide. Kink folds occur adjacent to the fault. The displacement of the boundaries of lithological units indicates that there has been a net sinistral displacement along the fault of 4 to 5 km.

Glen Gloy Fault

The outcrop of the Glen Gloy Quartzite shows a sinistral offset of about 100 m across a north-north-west- trending fault exposed in a stream on the north side of the glen [NN 302 937] (Figure 30), locality E. The north–south-trending branch of the fault (Figure 30) is also exposed. A fault zone exposed along the Allt Neurlain [NN 30 92] to the south of the glen, although not precisely in alignment with the faults to the north, is probably an extension of the same structure. The stream has eroded a ravine along the fault, and broken rocks, mainly psammite with some semipelite, are well exposed in the stream bed. The fault zone is at least 10 m wide and comprises several subvertical planes. Kink folds occur in the micaceous lithologies, and an irregular sheet of lamprophyre (appinite suite) appears to be offset sinistrally by about 40 m. At [NN 3034 9289] (Figure 30), locality F, the foliation in crushed semipelite to the east of a fault plane shows curvature indicating sinistral movement along the fault, which is marked by a few millimetres of gouge. At [NN 3042 9258] (Figure 30), locality G, gouge up to 2 m thick contains lenses of carbonate. The carbonate, yellowish brown on exposed surfaces, is probably ankeritic and was brecciated and cut by veinlets of pink calcite before being finally reduced to lenses in a matrix of dark grey fault gouge.

It has been claimed that recent movement manifested in a 40 m sinistral offset of drainage patterns and a 0.5 m dextral displacement of rock surface morphology, has occurred along the fault (Ringrose, 1987, Fenton and Ringrose, 1992). The evidence in the Allt Neurlain is controversial, but detailed levelling in Glen Roy indicates that movement on a possible south-south-eastward extension of the Glen Gloy Fault could have been responsible for a vertical dislocation of late glacial lake shorelines (Sissons and Cornish, 1982). Both Ringrose (1987) and Sissons and Cornish (1982) claimed to have identified fault scarps in Glen Roy. However, it has subsequently been proposed that these surface disturbances are not true fault scarps but are due to block movements along pre-existing fractures above an active fault (Fenton and Ringrose, 1992). It is interesting to note that a seismic event, intensity 5 MSK (Medredev-Sponheuer-Karnik intensity scale) with an epicentral location in Glen Roy, occurred as recently as 25 December 1946 (Musson et al., 1987).

Glen Fintaig Faults

Large east-south-east-trending gullies on the south side of Glen Fintaig [NN27 88] mark the traces of two parallel faults, 130 m apart. Both dip to the north-northeast at between 70° and 80°, and cut the boundary between the Glen Fintaig Semipelite and the Beinn Iaruinn Quartzite, which is offset sinistrally. An exposure of the northern fault [NN 2710 89815] shows brecciated quartzite up to 2.5 m wide. A layer of fine-grained, haematitic crush rock with carbonate-filled cavities occurs along both sides of the breccia, and the fault zone is bounded by smooth rock surfaces with slickensides plunging at 15° to the west-north-west.

The southern fault is also exceptionally well exposed, and its structure can be studied in the bed of the stream [NN 2684 8813]. Red-stained breccia, varying from 1 to 5 m thick, is bounded by smooth rock surfaces. Veinlets of coarse-grained carbonate cut clasts of semipelite and quartzite but the carbonate is also crushed, indicating that fault movements predate and postdate the deposition of carbonate. The final movements probably took place along marginal layers of haematitic gouge up to several centimetres thick (Plate 13).

Chapter 14 Quaternary

Little has been published on the Quaternary of the northern half of the district, but the southern half lies within the general area of the so-called Parallel Roads of Glen Roy, glacial lake shorelines which have excited interest from the eighteenth century onwards, giving rise to some 90 publications since their description by Pennant (1776). Recognition of the Parallel Roads as former shorelines of ice-dammed lakes depended on acceptance of the glacial theory, first put on a firm basis in the area by Agassiz (1840) and extended by Jamieson (1863; 1892). Publications since 1970 have concentrated on the glaciers of the Loch Lomond Advance or Readvance (LLR) that dammed the lakes; the lake shorelines themselves; fluvial and glaciofluvial fans and deltas associated with the lakes; palaeoseismicity; and biostratigraphy in Glen Roy and Glen Spean. Though there is little dating control within the district, most of the deposits and minor landforms can be correlated geomorphologically with the two phases of Late Devensian glaciation established elsewhere in Scotland (Table 10).

Pre-Late Devensian

The district's drainage pattern was probably largely developed in the late Tertiary, but the broad outline can be traced back to the early Devonian (Hall, 1991; Mykura, 1991). The high peaks near or more than 900 m above OD in the north-east, in the Creag Meagaidh area and on the southern border of the district, rise above generally high, undulating ground (Figure 1), with topographic basins in Glen Garry (adjacent to the Great Glen), Glen Spean, upper Glen Roy and the upper Spey valley, and A' Ghraidhleag (Tarff-Brein watershed). Such open basins stand in contrast to the deeply incised, U-shaped valleys such as middle and lower Glen Roy. Glacial breaches include the Gloy-Roy watershed, the watershed to the Spey at the head of Glen Roy, and Glen Spean. Many streams that used to flow north-eastwards have been diverted westwards to the Great Glen as a result of watershed migration in Glen Spean. Several valleys (Corrie Yairack, middle Glen Tarff, Glen Turret, Corrie Ardair) terminate upstream in steep, box-like corries that may have originated from a combination of glacial and fluvial erosion. Other corries in the area occur particularly on the north and east faces of the hills; they were presumably formed by repeated episodes of glacial erosion (Boulton et al., 1991).

Structural control of drainage on the medium scale is evident in the precipitous lower sections of Glen Tarff and Glen Buck, which are graded to the base level provided by the deep trough of the Great Glen. The middle section of Glen Tarff has been excavated on the south-westerly continuation of the Sronlairig Fault. East of Loch Oich, erosion by ice flowing roughly north-eastwards, parallel to the foliation of the rocks, has resulted in an ice-moulded landscape that controls the courses of many minor streams. Though many faults are marked by small thicknesses of soft crush rocks, an exceptionally extensive area of such material is present on the east side of the Great Glen Fault Zone, where it extends from the middle part of the hillside upwards to the top of the ridge (6 on (Figure 31)). The preservation of such easily erodeable rock in an area that was probably glaciated many times during the Pleistocene is difficult to explain: it may be related to factors such as weak glacial erosion near the junction of ice streams from west and south of the Great Glen (see below), to the retention of a cover of Devonian rocks until a late stage in the geological history, and to incipient toppling of strata following glacial unloading (see p.102).

Deep decomposition that probably partly predates the last glaciation has affected three major appinitic bodies in the Glen Roy drainage area, as well as, very locally, the main granodiorite of the Corrieyairack Granitic Complex and Dalradian country rock (Figure 31). The two more northerly appinite intrusions (Allt Dubh [NN 370 960] )and Glas Bheinn [NN 375 935]) are associated with subdued topography (4 and 5 on (Figure 31)), but in the more southerly intrusion, near Achavady [NN 297 867], the decomposed rock occurs on the side of the U-shaped valley of Glen Roy itself and decomposition is thus likely to postdate much of the incision (2 on (Figure 31)). The physical decomposition is accompanied by the replacement of ferromagnesian minerals by clay minerals, particularly vermiculite. Some 800 m north-west of the summit of Meall Ptarmigan [NN 426 904] (3 on (Figure 31)), the upper surface of the Corrieyairack granodiorite, below till, is decomposed or partly decomposed to a gruss (little-weathered granite sand), to a depth of more than 3 m. An area of decomposed schist at the southern margin of the district (1 on (Figure 31)) has not been investigated in detail.

Elsewhere in Scotland, granite grusses have been assigned to the Pleistocene, and clayey grusses (with greater chemical alteration) to the tropical conditions of the Middle Miocene (Hall 1991). In the present instance, a Pleistocene age is suggested for both the gruss and the decomposition of the appinites, as the remnants of deep Tertiary weathering would probably have been removed by glaciers early in the Pleistocene.

Late Devensian and Flandrian

Minor landforms and superficial deposits

In the absence of contrary evidence, all glacial deposits within the district are ascribed to the Dimlington Stadial ice sheet glaciation and the later, much smaller Loch Lomond Readvance (LLR) ice cap and valley glacier glaciation (Table 10).

Till and associated deposits

North of the Tarff valley, bedrock is at or near the surface over a considerable area, but elsewhere till is widespread except on the higher hills (see Sheet 63W). It is also present at many localities underlying blanket peat. For the most part, it is a stiff, poorly sorted deposit formed from nearby bedrock, being richer in clay and silt in areas of petite, and sandier where psammite and quartzite predominate. Faceted and striated cobbles and boulders occur at some localities. In a few places, it contains a significant but minor proportion of rock debris and boulders from farther afield. The surface of the deposit is commonly smooth or gently undulating. Laterally, till passes into hummocky glacial deposits. Within areas occupied by the LLR glaciers, much of the till is probably little-modified lodgement till, deposited directly from active ice. Outside the LLR limit on the highest ground (Figure 32), such as on the north side of the Creag Meagaidh range, the till surface has been extensively modified by periglacial processes, including solifluction and the formation of stone polygons and lobes. At other localities, particularly where valley sides are steep, lodgement till locally passes into or is overlain by loose gravelly diamicton that may be accompanied by sorted gravel, sand and silt. Such deposits locally exceed 30 m in thickness (Figure 31). Their association with morainic ridges and kettle holes suggests formation by paraglacial rather than periglacial processes.

Hummocky glacial deposits

General

Hummocky glacial deposits include both well-defined moraines and assemblages of ridges and mounds in which no obvious order can be discerned. They vary from sharp mounds and ridges to landforms that merge into the sheets of till. Their form is a function of the lithology of the underlying bedrock, the lithology of the deposit, and exposure to periglacial conditions following deposition. Thus, predominantly sandy, gravelly and bouldery moraines retain steep slopes, whereas those formed of matrix-dominated diamicton tend to have softer outlines. These modifications probably date chiefly from the period immediately following deglaciation, before the ground was stabilised by vegetation. However, outside the LLR limit, gelifluction during the cold climate of the Loch Lomond Stadial (when the ground was again unprotected by a closed vegetation cover) has probably further subdued the outlines of the landforms, particularly those with a high content of silt.

Lateral and terminal moraines

The maximum position of the LLR glaciers is marked at many localities by terminal and lateral moraines, and similar features also occur locally in other parts of the district that were glaciated during the earlier Dimlington Stadial (see glacial history, p.103). Natural sections are uncommon, but are sufficient to show that the moraines are formed of boulder gravel, till, loose gravelly diamicton, and sorted gravel and sand. To a greater or lesser extent, the lithology is related to the underlying bedrock; thus the terminal moraines of the LLR at the west end of Loch Laggan are formed chiefly of poorly sorted boulder gravel derived from the Strath Ossian Granitic Complex. Although terminal moraines formed of imbricate boulders, which are almost certainly push-moraines, have been identified at the site of one former LLR glacier near Loch Roy (xiv on (Figure 32)), most seem to have been formed at the ice margin by paraglacial processes, including sediment gravity flow and meltwater deposition (see also Benn, 1991). The presence of lateral moraines that are prominent on the north side of Beinn Chlianaig [NN 29 79], below the LLR maximum, suggests that the Spean glacier remained active during retreat.

Hummocky moraines

Hummocky moraines are widespread, both inside and outside the LLR limit. In places, their passage into adjacent sheets of till is abrupt, but in others there is a gradual lateral transition and any boundary between the two is notional. Such moraines can be broadly classified into four associations in the district.

1. Aligned hummocks passing into chaotic mounds. In upper Glen Roy [NA. 38 91] and [NN 41 91], bouldery mounds and ridges display a crude east-north-east to west-south-west orientation that probably reflects the position of former ice margins. They pass laterally into bouldery hummocks in which no such orientation is evident. Similar crudely aligned mounds can also be seen immediately within the LLR limit at localities where well-marked lateral and terminal moraines are otherwise absent, for example south of Glen Spean [NN 29 76]. Associations of this type seem to have been formed from both active and inactive ice, some from basal debris deposited behind the ice front and others at the ice front itself, where the debris would have been carried to a higher level in the zone of compression and subsequently deposited as debris flows (flow tills) and water-sorted gravel, sand and silt (Peacock, 1984; Benn, 1991). Hummocks adjacent to the Abhainn Ghuilbinn [NN 44 81] are elongated south-west to north-east in the direction of ice movement and may be drumlins.

2. Thick (greater than 10 m) diamicts, associated with kettled drift and ridges of gravel and/or diamict that form V-shaped patterns pointing down-valley. This association occurs in Corrie Yairack (B on (Figure 31)) and adjacent to the Allt Coire Ardair (C on (Figure. 31)) [NN 450 880] and downstream for nearly 3 km]. In both valleys, the ridge summits accord with smooth, in places almost horizontal, valley profiles; thus, the V-shaped orientation and the association with kettle holes suggests that the ridges represent infillings of supraglacial streams and that the relief has subsequently been inverted as the ice melted. Both valleys seems to have been deglaciated early and base levels for the deposition of debris may have been successively lower glacial lakes dammed by ice in their lower reaches.

3. Thick diamicts associated with surficial ridges aligned roughly parallel to stream courses, for instance in the AIlt na Larach (A on (Figure 31)). As in (2), the ridges seem to be the infillings of structures in the ice (supra-glacial streams or crevasses?), with inversion of relief as the ice decayed. It can be inferred that the thick till-like deposits here, and at other localities where obvious ridges are absent, are paraglacial, having been formed by mass wastage of glacial deposits on the valley sides, with subsequent redeposition either in the valley bottom or on the surface of a decaying glacier (Eyles, 1983; Harrison, 1991).

4. Kettled hummocks and ridges near the source of the River Roy [NN 41 89 and 41 90] and at Luib-chonnal [NN 38 93] are partly formed of gravel, and in the latter case are accompanied by small eskers. Both examples seem to be associated with inactive ice and paraglacial redistribution of sediment (see above), and in the former case, the final wasting of Dimlington Stadial glaciers in the corries of the Creag Meagaidh massif.

Meltwater deposits

These comprise glaciofluvial and glaciolacustrine deposits, inclueling those laid down as sheets, as ice contact deposits (with a moundy, kettled surface), and as deltas and subaqueous fans. The largest areas underlain by meltwater deposits are associated with the maximum positions and retreat stages of the LLR glaciers. In Glen Spean, deltas that were formed in the 260 m Parallel Road lake, at or near the maximum positions of LLR glaciers, are situated near Moy and at Roughburn (A and B in (Figure 32)). They comprise topset, foreset and bottomset beds that fine distally from gravel to fine-grained sand. Much of the Treig delta (C in (Figure 32)), which was deposited against a retreating ice margin, is irregular and pitted with kettle-holes, and there are only a few remnants of the original, gently sloping surface. The Allt nam Bruach delta (D in (Figure 32)) was partly deposited in a small lake at about 290 m, and subsequently in the 260 m Parallel Road lake. Fine-grained sand, in the form of steep-sided ridges and mounds with a few kettle-holes in the area around Murlaggan [NN 317 812] (E in (Figure 32)), was laid down close to the confluence of the Spean and Treig glaciers. At F (Figure 32), in the valley of the Allt Leachdach, a terraced and kettled fan with small eskers is graded to the 260 m level.

The mass of sediment that nearly blocks Glen Roy immediately south of the LLR limit (G in (Figure 32)) comprises foresets of gravel and sand, interbedded in places with diamicton (Peacock and Cornish 1989). No topsets have been seen and the deposit is interpreted as a subaqueous outwash fan.

The gently sloping Turret outwash fan (H in (Figure 32); 1 in (Figure 33) and (Table 12)) is backed by terminal moraines and clearly marks a former ice front. Its origin is otherwise controversial. Sissons and Cornish (1983) suggested that it was deposited subaqueously in the 260 m Parallel Road lake of the rising sequence, and that it marks the LLR maximum of the Gloy glacier, a view supported by Lowe and Cairns (1991). However, as the sedimentology of the gravels resembles that of subaerial outwash fans, there are difficulties with this interpretation and a pre-LLR age has also been suggested (Peacock, 1986; Peacock and Cornish, 1989).

In lower Glen Garry and between Loch Oich and Loch Lochy in the Great Glen, much of the valley floor is underlain by gently undulating, locally kettled, often well-sorted outwash gravel that was laid down during the later retreat stages of the LLR glaciers. At I (Figure 32), a small delta near the head of the valley of the Allt an Lagain was deposited in an ice-dammed lake that drained over the neighbouring col (380 m above OD) into Glen Buck.

Fine examples of eskers, deposited at or immediately following the maximum of the LLR, can be seen adjacent to the Kilfinnan Burn [NN 267 963] and on the south side of Glen Spean [NN 37 79]. Eskers formed during the retreat of the ice of the Dimlington Stadial occur at a high level some 300 to 400 m south of the summit of Beannain Beaga [NN 455 915], where they are associated with a suite of meltwater channels (p.103).

Lake shorelines

The well-known Parallel Roads are shorelines of ice-dammed lakes (Table 11). The principal lakes, the shorelines of which are outlined on the 1:50 000 topographical map, drained across cols that controlled their levels (Figure 32). All are associated with the maximum or near-maximum positions of LLR glaciers, and in Glen Roy were formed in a rising sequence (260, 325 and 350 m) and a falling sequence (325 and 260 m) as the ice advanced and retreated. A shoreline (Table 11) on the east side of Glen Buck [NH 34 01] relates to a small lake that accompanied the ice-marginal drainage in this area. The shorelines have commonly been formed by erosion at the hack and gravel deposition at the front, the volume of deposit and width being greater where the fetch is longer or where fluvial deposition has occurred (Sissons 1978). At some localities, both major and minor Roads are cut in rock, a fact explained by the combined action of frost and wave action at lake level (Sissons, 1978). However, this seems to have been greatly assisted by prior breakage due to slope deformation and by frost action earlier in the Loch Lomond Stadial.

Fans and deltas

The remains of deltas deposited in the lakes that formed the Parallel Roads occur at many localities in Glen Roy at the exits of side streams, good examples being preserved for instance at 325 m on the course of the Burn of Agie [NN 367 918] and 260 m at the mouth of the Allt na Reinich [NN 331 905] (Figure 33). Subaqueous or subaerial origins have been proposed for other fans in Glen Roy (Sissons and Cornish, 1983; Peacock, 1986; Peacock and Cornish, 1989), based on their apparent relationship to the Parallel Roads and on their landforms and sedimentology (see (Table 12)).

Head, scree and in-situ frost-shattered debris

In common with other parts of Scotland (Ballantyne, 1984), the higher hills bear the record of severe periglaciation during the Loch Lomond Stadial. In general, this takes the form of a thin layer of frost-shattered bedrock which passes into head and scree on steep slopes. Though screen are present within the Loch Lomond Readvance limits, they are generally less prominent than outside such limits, particularly on corrie walls. Inactive boulder lobes occur on ground over 700 m underlain by psammite and granite such as Corrieyairack Hill [NN 428 997], Geal Charn [NN 44 988] and the Creag Meagaidh range. In the last of these, the boulder lobes are particularly prominent on the south side of Cam Liath [NN 472 904]. Gelifluction terraces also occur on the highest mountains and the occurrence of stone polygons on till surfaces at higher levels has already been noted (p.98). However, meltwater channels that predate the Loch Lomond Stadial are still well preserved at these high levels and the amount of downhill transport indicated by the boulder lobes seems to have been small. Active periglacial features are at present confined to small gelifluction lobes, for instance those on the ridge [NN 425 888] above Creag an Lochain.

Slope deformation

Deformation of flaggy bedrock has taken place to depths of over 6 m (and in places over 30 m) on slopes on the west side of Glen Gloy, upstream from Glen Fintaig and, to a small extent, on the west side of Glen Roy (Figure 32) (Peacock and May, 1993). The east side of both glens is less affected. In the former, the effect of the creep has been to reverse the dip of strata at the surface from east-south-east to west-north-west. Rocks affected by creep in Glen Gloy are truncated by the 355 m and, less certainly, by the 260 m Parallel Roads in Glen Roy; here the relationship of the deformation to the 325 m and 350 shorelines has not been determined. The slope deformation is thus of Late Devensian age, predating the formation of the Parallel Roads of the rising sequence (and probably the Loch Lomond Stadial), but postdating the removal of glacier ice. It is interpreted as a response to glacial unloading at the end of the Dimlington Stadial and to high cleft water pressure associated with ice retreat and permafrost.

Landslips and possible palaeoseismicity

The major landslips in the area are chiefly in Glen Gloy and Glen Roy (Figure 32) and (Table 13). The slopes of these valleys are also the site of many incipient slips that are difficult to distinguish in some instances from the effects of large-scale slope deformation mentioned above, as well as many landslip fissures that postdate the such deformation. From detailed levelling in Glen Roy and upper Glen Gloy, Sissons and Cornish (1982) reported the presence of differentially uplifted blocks in which the Parallel Roads are tilted at gradients of up to 4.6 m/km. They associated some of the landslips with these dislocations, which they attributed to stress release following rapid drainage of the lakes (Table 11). These authors also found that the Parallel Roads reached their highest altitudes at or near the LLR limit in Glen Roy. Ringrose (1989) and Davenport et al. (1989) suggested that earthquakes accompanied reactivation of a major fault in Glen Roy and Glen Gloy during lake drainage, but no supporting evidence for this view was found during the recent geological survey.

In the lower part of the deposits of the Alltnaray landslip (3 on (Figure 32) and (Table 13)), there has been recent but undated movement on a fault scarp some 2 to 3 m high and dipping into the hillside, which has caused the diversion of a stream. In the winter of 1989/90, exceptionally heavy rain resulted in reactivation of a small landslip immediately north of this slip, with down-slope movements of up to 1 m, and also led to the occurrence of a debris flow at the base of the Brunachan slip in Glen Roy (6 on (Figure 32) and (Figure 5) on (Table 13) which blocked the public road. The debris flow probably resulted from ponding of drainage in the older landslip, followed by failure as water pressure increased.

Peat

Large areas of ground between Glen Roy, Glen Buck and the Corrieyairack Pass are covered in blanket peat commonly 2 m thick, for instance on Leacann Doire Bainneir [NN 295 945], arid over 3 m thick on the south slope of Beinn Bhan [NN 320 960]. Deposits of peat are also widespread on the south side of the upper Spey valley, extending into the corries on the north side of the Creag Meagaidh range. Elsewhere, patches of peat are common in districts underlain by hummocky glacial deposits, but peat is rare or absent where rock is at or near the surface, as on the north side of Glen Tarff. A layer of stumps of Scots Pine (Pinus sylvestris) is common in the peat, usually close to the base of the deposit. The maximum height at which such boles have been seen is about 650 m above OD in the valley of the Allt Coire Bhanain [NN 438 908].

Alluvium and alluvial terrace deposits

Narrow alluvial haughs or flats are common at the margins of most streams, but broad stretches of alluvium are confined to the major rivers, such as the River Garry upstream of Invergarry. Alluvial terraces are common in the valleys of the rivers Roy and Spean, particularly impressive examples being present between the Falls of Roy and Braeroy (Figure 33). Most are underlain by gravel, but the floodplain alluvium of the River Spean below Roybridge [NN 273 810] is chiefly fine-grained sand, from which a former cover of peat has been largely removed during agricultural improvement.

Indicators of ice movement
Striations

Apart from the availability of exposure, the distribution of striations and glacial grooves is controlled chiefly by (1) the intensity of glaciation (generally greater in valley floors than on high ground), (2) the lithology of the underlying bedrock, and (3) the degree of weathering and frost shattering since deglaciation. Thus striae are relatively common within the limit of the LLR, for instance north of Loch Treig, but uncommon elsewhere, particularly on higher ground that was subject to periglacial conditions during the Loch Lomond Stadial. In these latter areas, only grooves are usually preserved on exposed bedrock. With these qualifications, the distribution of striae and grooves suggests that ice movement immediately prior to deglaciation was in a north-north-easterly direction in the north of the area, and to the east on the east side of the district (Figure 32). As the ice retreated, there was a general eastward movement in Glen Spean and across the Roy–Spey col, and, at a still later stage, a north-eastwards movement across Beinn Chlianaig [NN 303 775] in the south-west of the area. Late local movements of ice are suggested by crossing striae at the mouth of the Burn of Agie [NN 365 919] and striae directed downstream in Corrie na Reinich [NN 341 899].

Meltwater channels

Meltwater channels at high levels occur on the eastern boundary of the district. Those on the ridges on both sides of the Corrieyairack Pass were probably incised as streams that flowed initially on the surface of the ice sheet, but cut down to bedrock as the ice retreated; their trend supports the evidence of the striations for a north-north-eastwards or north-eastwards movement of ice across this part of the district at that period. At a slightly later stage, water overflowed into Corrie Yairack from ice situated to the north-west of the pass, suggesting early deglaciation of this enclosed, southward-facing valley. A plexus of channels between Beinnain Beaga [NN 456 914] and the ridge to the south was formed as ice in the Spey valley became separated from that in Glen Spean during the general westward decline of the ice sheet. Slightly later, water from the Glen Spean glacier overflowed across the east ridge of Sron a' Ghoire [NN 455 880] into the valley of the Allt Coire Ardair, again suggesting early deglaciation of an enclosed valley.

Erratics

Erratics from west of the Great Glen (granitic gneiss and Moine pelitic gneiss) and Devonian sandstones from within the Great Glen itself can be found up to the limit shown on (Figure 31). Their distribution suggests that, at one time, ice from Glen Garry penetrated some 10 km east of the Great Glen. Farther south, the eastward carry of erratics of granitic gneiss extends a little beyond the LLR limit, again indicating an early advance of ice into the area from west of the Great Glen.

Within the district itself, the distribution of granodioritic boulders of the Strath Ossian/Rannoch Pluton and the Corrieyairack Granitic Complex and boulders of Leven Schists provides evidence for a westerly, northwesterly and northerly movement of ice from a centre to the east or south-east of Loch Treig (Figure 31). Boulders of Strath Ossian Complex rocks are common within Coire Ionndrainn [NN 26 86], at the western margin of the district, and occur well within the LLR limit in Glen Roy. Farther east, they can be found at 1050 m near the summit of Creag Meagaidh and occur abundantly in the lower part of the valley of the Allt Coire Ardair. Erratics from the Corrieyairack mass are also widely distributed east of the granodiorite outcrop. Their marked southerly limit (Figure 31) probably represents the approximate margin of eastwards flowing ice in the upper Spey valley during the later stages of deglaciation. The occurrence of boulders of feldspathic quartzite from the Inverlair Psammite Formation, which crops out in Glen Spean immediately west of the Strath Ossian Complex, near to the eastern end of Loch Laggan on the adjacent Sheet 63E, supports the evidence from striations for a late eastward movement of ice in this area.

Late Devensian and Flandrian history

Dimlington Stadlal

From the evidence of erratics, striations and meltwater channels, it is clear that there were complex ice movements comprising (1) an invasion by ice from west of the Great Glen into the western part of the area, (2) a north-westerly movement of ice from ground east of Loch Treig and (3) a north-north-easterly and easterly movement of ice during ice-sheet deglaciation, followed by more localised movement as ice became confined to the valleys. As movements (1) and (2) have left no record of striations or roches moutonnées, their relative ages are uncertain. If however, the LLR is regarded as indicating conditions at an early stage in the development of a Scottish ice sheet (Sissons, 1979b), then (1) can be taken as earlier than (2). This would accord with the view that the main centres of ice accumulation in Scotland migrated from north to south during the course of the Dimlington Stadial glaciation (Sutherland, 1984).

There is limited evidence that glacier ice remained active at times during the retreat of the Dimlington Stadial glaciation, with local, probably topographically induced stillstands. On the south side of the Spey valley, between Shesgnan [NN 440 953] and Drummin [NN 470 955], recessional moraines adjacent to the river are accompanied by small eskers on the valley floor. Aligned mounds and ridges, probably reflecting former ice margins, have already been noted in upper Glen Roy (p.98). Terminal moraines and an outwash fan at the mouth of Glen Turret have been interpreted either as belonging to the LLR maximum or to a pre-LLR event (Table 12). Moraines by the 'West' Allt Dearg above Braeroy (Figure 33) may indicate the former presence of a small side-valley glacier, though its attribution to the Dimlington Stadial rather than to the Loch Lomond Stadial is uncertain. Other features of glacier retreat, such as the formation of thick diamicts during the final stages of glacier wastage and the early deglaciation of Corrie Yairack and Corrie Ardair, have been discussed above.

Windermere Interstadial

No organic deposits attributable to the Windermere Interstadial have yet been identified in the Glen Roy district. It is possible, however, that some of the deposits associated with the final disappearance of the Dimlington Stadial ice were formed early in the interstadial.

Loch Lomond Stadial

During the stadial, glaciers of the LLR entered the district from the west and south. Many of the tongues and glaciers (i to xv in (Figure 32)) have been detailed by Sissons (1979a), and only additions and modifications are mentioned here. In general terms, ice from west of the Great Glen advanced up Glen Gloy, Glen Roy and Glen Spean, to be joined by various glaciers from the south, in particular from the Treig valley. Lakes dammed by the advancing ice were formed at levels of 355 m in Glen Gloy, 260 m in Glen Spean, and successively at '260 m, 325 m and 350 m in Glen Roy as the ice approached its maximum (see above). In the north, the Glen Buck tongue (i on (Figure 32)) is delineated by a massive moraine on its south-eastern side where it descends into the glen; its retreat led to the formation of a lake that overflowed north-eastwards into the Tarff valley. The Lagain tongue (ii on (Figure 32)) also dammed a small lake that drained to the north-east. In the Glen Gloy area, current opinion suggests a terminal position either near Alltnaray [NN 2815 9135], where there is a small subaqueous fan, or in the Turret valley. In Glen Gloy, the presence of rocks affected by slope deformation, which would almost certainly have been removed by a valley glacier, supports the former view. The maximum position of the LLR tongue in Glen Roy (vi) is marked by a Drift limit a little north of the subaqueous fan deposits in the valley bottom, and the tongue in Glen Glas Dhoire (vii) is partly bounded by a terminal moraine on its south side. The Treig glacier formed a piedmont tongue in Glen Spean, outlined by prominent terminal and lateral moraines to the north and east.

In contrast to the above, the Creag Meagaidh range supported only small corrie glaciers. Two of these (xii and xiii, (Figure 32)) are questionable, being suggested only by the occurrence of fresh rock faces in contrast to scree covered cliffs elsewhere in the neighbourhood. No evidence has been found to support the possibility that (xiii) could have been a large LLR valley glacier (Sissons, 1979a). The terminus of the Lochan Uaine glacier (xiv) is marked by low moraines of imbricate boulders. A very small LLR glacier (xv) in the corrie west of Sron Garbh Choire is suggested by the presence of a massive moraine below a cliff of little-weathered rock that contrasts with the loose, scree-covered rock on either side; a low ridge immediately to the north probably relates to the retreat stage of a Dimlington Stadial glacier (hut see Sissons, 1979a for an alternative interpretation).

Flandrian

Following the maximum of the LLR, the ice retreat was accompanied by catastrophic lowering of the Parallel Road lakes, particularly the 260 m lake in the Spean valley, and the rapid terracing of deltaic and lacustrine deposits as drainage became established on present lines (Sissons, 1979c and 1979d). It is possible that gorges in lower Glen Roy and Glen Spean were either formed or deepened at this time. The presence of lakes in the Great Glen during ice retreat is shown by exposures of interbedded diamicton, gravel and laminated silt along the forestry roads on the valley side above the northern end of Loch Lochy, and the final stages of deglaciation are represented by the kettled outwash gravels in lower Glen Garry and on the floor of the Great Glen at the northern end of Loch Lochy and at the north-eastern end of Loch Oich. Biogenic sediments, preserved in a number of lake basins within and outside the LLR limit, represent the Loch Lomond Stadial/Flandrian transition, and a vegetational succession probably beginning about 10 000 BP (Lowe and Cairns, 1991). Subsequent Flandrian events include the growth of blanket peat, the deposition of alluvium by rivers and streams, the gullying of steep slopes, and the terracing of existing alluvium and glacial meltwater deposits. In Glen Roy and Glen Gloy, gullying is active on slopes underlain by fractured rock showing deep gravitational creep, and is accompanied by the deposition of small alluvial fans.

Chapter 15 Economic geology

Metalliferous minerals

The Glen Roy district has no history of mining of metallic minerals, and no sign of this form of mineralisation was found during the present fieldwork. However, chemical analyses of stream sediments from the district (British Geological Survey, 1987; 1990) revealed three metalliferous anomalies:

  1. A combined As and Bi anomaly over the northern part of the Strath Ossian Granitic Complex, which extends as a smaller anomaly north-westwards into poorly exposed ground south of the Corrieyairack Granitic Complex.
  2. A combined Cr and Ni anomaly over the three large appinites in the lower part of the Glen Roy. Petrographical studies indicate that these elements are present in silicate, and not sulphide or oxide phases. Cr is mostly present in clinopyroxene, and Ni in olivine.
  3. A combined Cu and Sn single sample anomaly on the north-western edge of the Strath Ossian Granitic Complex, north of Fersit.

Gold assays, carried out on hand specimens of carbonate-bearing schists of the Leven Schist Formation from locality [NN 257 788], gave only between 60 and 90 ppb Au. A similar range of low gold values was obtained from hand specimens of massive and brecciated Glencoe Quartzite in the vicinity of the Innse Appinite [NN 276 753]. Even lower gold assays were obtained from seven hand specimens of typical, highly altered dykes from the Etive and Ben Nevis swarms (all less than 20 ppb Au), and from three samples of altered pebbly rock from within, or adjacent to, the Eilrig Shear Zone (up to 32 ppb Au). Cidu and Edmunds (1990) reported a single value of 1.2 ppb Au in the water of a stream draining the Achavady Appinite. This concentration of dissolved gold is anomalous with respect to other sampled streams in Glen Roy.

Sand and gravel

Although there are widespread occurrences of sand and gravel in the district (see Chapter 14), none of these deposits is presently being worked. In the past, several have been worked to supply local needs. For example, the Loch Lomond Readvance terminal moraine near Moy [NN 432 822] and the deltaic gravel mounds around Fersit [NN 334 786] were quarried during the construction of Laggan Dam and adjacent forestry roads. Due to their good drainage, the sand and gravel deposits underlie much of the better quality agricultural land in the district. This will limit the extent of any future quarrying, as will the fact that the deposits are often found in National Nature Reserves such as Glen Roy. Large sand and gravel quarries immediately west of the district, at the mouth of Glen Spean, presently supply local needs.

Limestone and dolostone

Limestone beds within the Ballachulish Limestone Formation have previously been worked on a small scale on the north side of the River Spean in the Inveroy area, immediately west of the district, to meet local requirements for lime (Robertson et al., 1949). Both high-quality limestone (Muir et al., 1956) and impure dolostone (Hickman and Wright, 1983) beds occur in the formation throughout its whole outcrop north of this river. Poor exposure means that a drilling programme would be required to fully assess the reserves of limestone and dolostone in the district.

Building stone

The main granodiorite of the Strath Ossian Granitic Complex has previously been worked from two quarries in the Glen Spean area [NN 3715 8093], [NN 3645 7960]. Dimension stone from these quarries was used in the construction of Laggan Dam. Unworked reserves remain in both quarries for use as dimension stone or aggregate.

Other industrial minerals

The large appinitic complexes of Glen Roy weather to produce soils enriched in vermiculite and hydrobiotite. Gully erosion reveals these distinctive, golden-brown, flaky minerals, as for example at locality [NN 3740 9345] in the Glas Bheinn Complex. The soils do not appear to be thick enough to be of commercial interest, although the presence of apatite with vermiculite means that the soils contain some phosphate and may be suitable as a potting medium. Graphite is present in a series of east-west or east-north-east to west-south-west trending vertical veins, exposed in the steep walls of a ravine at the head of the Allt a' Choilich [NN 293 946] on the upper southeastern slopes of the Great Glen. The veins, which are up to 60 cm thick, are found in shattered quartz-mica-schists. Individual veins can be traced up the entire 30 m or so height of the ravine, but their horizontal extent is not known because of poor exposure. The graphite occurs in three forms: as thick coatings on shatter surfaces in the schists; as friable or earthy graphite; and as solid, massive graphite confined to well-defined veins (only a single, 25 cm-thick vein of this type was found).

Water

Good quality, unpolluted water is found throughout the district in the numerous rivers, streams, lochs and lochans, as well as in the large reservoirs formed by damming the Spean and Treig rivers. The dam on the River Treig has increased the original 6.24 km2 surface area of a former Loch Treig (volume of 394 X 106 m3). A new lake has been created on the River Spean, which when full extends along Glen Spean from Roughburn to Moy. The dams, completed in the early 1930s, form a central part of the water supply for hydro-electric power generation at the aluminium smelter in Fort William. Water from the headwaters of the Spey, Loch Laggan and the two reservoirs is supplied to Fort William by means of tunnels. These tunnels connect the two reservoirs, and the main tunnel extends westwards from the valve station on Loch Treig across the south-western part of the district. Stream water from 11 south bank tributaries of the River Spean is fed into the tunnel west of Loch Treig through intake shafts, e.g. on the Allt Laire [NN 3275 7773]. The total catchment area of the scheme is about 785 km2.

Water analyses carried out during the region stream sediment sampling programme (British Geological Survey, 1987; 1990) show that surface water in the Great Glen is slightly alkaline [pH 7.7–8.0) with conductivities (SEC) of between 160 and 245 ps/cm. This compares with conductivity (SEC) values of 31 to 90 ps/cm for the Spean-Roy drainage in which water pH was measured at between 6.2 and 7.7.

During 1987 and 1988, a detailed hydrogeochemical survey of a 255 km2 area around Roybridge (between [NN 230 760], [NN 230 960], [NN 400 960], and [NN 400 760]) was conducted to investigate the relationships between stream baseflows, stream sediments and bedrock geology. This area was selected in view of its relatively pristine environment where pollution effects remain minimal. The results of the survey are reported separately (Edmunds and Key, 1996). Several distinctive features may be seen in the hydrogeochemistry relating to inputs from rainfall, extent of buffering of input acidity by bedrock, the clear inprint of several lithologies in the baseline chemistry, and, very locally, the effects of man's activities.

Concentrations of chloride range from 2 to greater than 12 mg/1, reflecting the input from rainfall which has been modified by evapotranspiration. Higher concentrations of chloride and other ions are found in areas covered by thick drift, and in some afforested areas. Highest concentrations are to be found along Strath Spey.

The whole region is characterised by water of low alkalinity (typically below 20 mg/1 HCO3, (Table 14)) and this maintains pH values below 6.5 over a wide area. The pH and HCO3 distributions reflect the neutralising capacity of the bedrock, and a distinct contrast can be observed between the mainly granitic and psammitic terrains and areas underlain by more basic rocks and carbonate-bearing lithologies (Table 15).

The regional and local geology are quite well defined by the hydrogeochemistry. Over 25 different elements or parameters of major, minor and trace element patterns have been used to characterise particular formations or groups of lithologies. For example, the Ballachulish Limestone Formation is distinguished clearly by both high Ca and Mg anomalies, and the highest molybdenum values occur in streams on the Corrieyairack Granitic Complex (Table 15). The most distinctive hydrogeochemical pattern in the region is given by Ba, Mg, Sr and K which, either separately or together, pick out a north-west to south-east lineament (Figure 34). The lower Glen Roy appinites are the main source of the lineament; the hydrogeochemistry reveals the full outcrop of the appinites beneath drift. Water chemistry is, therefore, a useful aid in the mapping process, notably where exposure is poor, since the groundwater baseflow represents a sample in three dimensions of the shallow bedrock/superficial deposits.

Invergarry and Roybridge are supplied with drinking water from small reservoirs in adjacent high ground. The reservoirs result from damming the Aldernaig Bum [NH 296 020] (Invergarry) and the Allt Beinn Chlianaig [NN 276 802] (Roybridge). There are plans to supply Roybridge with water from the large borehole complex in Quaternary gravels at Gairlochy. Springs and shallow private boreholes supply water to other local communities. For example, Brae Roy Lodge has a gravity-fed water supply from a spring controlled by a large landslip on the steep eastern slope of Glen Roy. Very little work has been undertaken on studying aquifers in the solid and drift deposits, or on the effects of the major faults, such as the Great Glen Gault, on deep groundwater circulation. The thickest of the glaciofluvial deposits described in the previous chapter have the potential to be important aquifers. For example, a well-field has recently been established in thick gravels along the River Lochy which will supply good quality water to Fort William (Johnstone and Rennie, 1991).

Chapter 16 Geophysics

Aeromagnetic and gravity data provide an important source of information for regional geological studies. Although general studies of these data have been made for the Grampian region (Hall, 1978), no published information is available for the Glen Roy district itself.

Aeromagnetic surveys in the Glen Roy region were flown at a mean terrain clearance of 305 m, with east–west flight lines at 2 km intervals and north–south tie lines at 10 km intervals. The data were collected in analogue form during 1962/63 and subsequently digitised by BGS (Smith and Royles, 1989). The gravity data coverage is approximately 1 station per 2 km2.

A magnetotelluric (MT) traverse across the Great Glen Fault Zone at Glen Loy [NN 15 81], undertaken by the University of Edinburgh (Meju, 1988), provides information on the broad-scale lateral variation in conductivity within the deeper crust. This work identified highly resistive (2000–100 000 ohm metre) upper crust either side of the Great Glen Fault Zone which appears to thicken in the vicinity of the zone. The lower crust is distinctly more conductive, with resistivity values in the range 50–400 ohm metre, comparable to the results of previous MT studies in northern Scotland (Mbipom and Hutton, 1983; Kirkwood et al., 1981). Meju (1988) described the Great Glen Fault Zone as a steep, narrow (about 1 km wide), conductive zone which continues down to the lower crust. Comparable near-vertical conductors alongside the Great Glen Fault Zone have also been recognised and tentatively interpreted as shear zones.

No borehole or seismic data exist, except for the Lithospheric Seismic Profile in Britain (LISPB) refraction line (Bamford et al., 1978; Barton, 1992), located about 60 km to the north-east of Sheet 63W. Within the Grampian Region, LISPB has identified three crustal layers comprising Caledonian belt metamorphic rocks (velocity 6.1–6.2 km/s) above Pre-Caledonian basement (greater than 6.4 km/s), underlain by the lower crust (around 7 km/s). The crust thickens from 28 km at the Great Glen Fault Zone to 35 km at the Highland Boundary Fault. The structure of the Great Glen Fault Zone was not resolved by the seismic data.

Ground geophysical surveys have been conducted primarily in the southern part of the district (Chacksfield, 1992), to assist geological mapping in areas of poor exposure. Total field magnetic surveys have traced the contact of the Leven Schist Formation over poorly exposed ground near Roybridge. At Labvan [NN 445 790], Very Low Frequency (VLF) electromagnetic surveys have mapped the contact between the Meall Cos Charnan Semipelite and Creag Meagaidh Psammite formations, which have contrasting electromagnetic signatures.

Rock physical properties data (primarily outcrop and laboratory magnetic susceptibility measurements) have been obtained for the main rock types in the region. These have proved useful in identifying and discriminating between magnetic and non-magnetic schists in the field.

Approximately 123 line km of ground magnetic data, 48 line km of VLF data, and over 2700 magnetic susceptibility measurements from 269 localities are held in digital files at BGS Keyworth.

Physical properties of rocks

Physical properties of the main rock types, compiled from BGS data, are presented in (Table 16) and (Table 17). The magnetic susceptibility is a measure of the ease of magnetisation of a material, which usually reflects its magnetite content. The most magnetic rocks are from the Leven Schist Formation and a magnetic unit within the Loch Treig Schist.

Density data show a significant density contrast of about + 0.1 Mg/m3 between Appin Group schists and Grampian Group psammites. Another important density contrast of - 0.05 Mg/m3 occurs between Grampian Group psammites and Central Highland Complex gneiss (not outcropping in the Glen Roy district). Granitic rocks show density values ranging from 2.68 to 2.73 Mg/m3.

Magnetic data

The reduced-to-pole, greyscale, shaded-relief, aeromagnetic anomaly map of Glen Roy (Figure 35) was compiled from digital data interpolated onto a 0.5 km grid and further interpolated to produce an image based on a 0.1 km cell size. Illumination from the north-west highlights features which have a Caledonian trend. Anomaly amplitudes are shown as contours and anomaly gradients as relief. The reduction to the pole transformation was applied, assuming induced magnetisation, to centre anomalies over their sources. In general, the sharper the relief, the closer the source of the anomaly is to the surface.

The regional aeromagnetic data indicate a broad positive anomaly along the Great Glen Fault Zone with the anomaly maxima on the northern side. The broad wavelength of the anomaly implies magnetic basement at depth which becomes progressively shallower towards the Great Glen Fault Zone, culminating along the northwestern side of this structure.

The West Highland Granitic Gneiss may be the source of the local, shorter wavelength anomalies south of Invergarry. Magnetic traverses at Balma Glaster [NN 285 969], near Laggan, have identified granitic gneiss at outcrop with magnetic susceptibility values up to 30 X 10−3 SI units.

Prominent, north-east- to south-west-trending, short-wavelength, positive aeromagnetic anomalies, with amplitudes locally in excess of 300 nT, occur to the south-east of the Great Glen Fault Zone (shown as high relief on the shaded-relief image). These are caused by the Leven Schist Formation which exhibits outcrop magnetic susceptibility values up to 78 X 10−3 SI units, in marked con trast to the weakly magnetic surrounding metasedimentary rocks. The dominant magnetic mineral is magnetite, which in thin section is seen to form euhedral crystals cutting the S1 and S2 fabric, and was therefore developed after the main period of ductile deformation.

The south-eastern margin of the Leven Schist Formation has a distinct magnetic signature. On ground magnetic profile A–A′ (Figure 36), an abrupt change in the anomaly pattern from a 'quiet' magnetic signature over the Loch Treig schists to highly magnetic Leven schists occurs immediately above the Innse Quartzite. This contact has proved a valuable magnetic marker which can be mapped for several kilometres either side of Glen Spean. The north-western margin of the Leven Schist Formation, adjacent to the Ballachulish Limestone, is less clearly defined due to the transitional nature of the boundary and complex folding.

Within the Leven Schist Formation, a zone of weakly magnetic schist occurs in the core of the Stob Ban Synform. Since the younging direction is towards the core of the synform, the Leven Schist Formation appears to become less magnetic towards the top of the sequence. The ground magnetic anomaly pattern over the Stob Ban Synform is reflected on the aeromagnetic image where, in the south-west part of the district, the anomalies over the fold limbs bifurcate, indicating a fold plunge towards the south-west. Magnetic evidence suggests that the north-western fold limb is continuous as far as Ben Nevis, but the south-eastern fold limb is less extensive.

Most of the Loch Treig Schist and Quartzite Formation, including the Glen Coe and Stob quartzites, has low magnetic susceptibility values and does not produce a significant aeromagnetic anomaly. Similarly the Eilde Flags have a flat magnetic signature which is typical of nearly all other psammites in the region, except the Dog Falls Psammite which is strongly magnetic.

Similar prominent positive aeromagnetic anomalies west of Loch Treig, in the extreme south of Sheet 63W and on adjacent Sheet 54W, are caused by an infold of Leven Schist Formation rocks and the underlying 200 m of the Loch Treig Schist Formation. The latter exhibits magnetic susceptibility values up to 45 X 10−3 SI units and ground magnetic anomalies with amplitudes in excess of 1500 nT (profile A–A′ (Figure 36)). The magnetic unit within the Loch Treig Schist Formation appears to be restricted to the Loch Treig area.

The Corrieyairack (and Allt Crom) granitic intrusions produce aeromagnetic anomalies with amplitudes over 200 nT, but only the north-western part of the Strath Ossian Complex has a significant magnetic anomaly within Sheet 63W. Here, the western margin of the pluton near Glen Spean is clearly defined on detailed aeromagnetic images. Elsewhere, the intrusion produces a series of smaller, local, magnetic anomaly highs, which are abruptly terminated to the south-east by the Ericht–Laidon Fault. The Moor of Rannoch Granite, to the south, produces a broad magnetic high over most of its outcrop and has a magnetic signature markedly different from that of the Strath Ossian Complex.

Porphyritic microdiorite dykes of the Etive and Ben Nevis swarms give local ground magnetic anomalies of 100–200 nT, but are not of sufficient volume to influence the main aeromagnetic anomalies in the region.

Gravity data

The contour Bouguer gravity anomaly map of Glen Roy and surrounding area (Figure 37) was compiled from data in the National Gravity Databank. Data were reduced using a density of 2.7 Mg/m3 and gridded at 0.5 km. The outline geology was compiled from mapping within Sheet 63W, and from the BGS 1:250 000 series geological maps 'Argyll' and 'Great Glen'.

The negative Bouguer anomaly values in the Glen Roy district form part of a regional gravity low which extends over most of the Grampian Highlands. The complex anomaly pattern, however, shows that most of Sheet 63W is situated on a gravity high, located, for the most part, immediately south-east of the Great Glen Fault Zone.

Within this feature, local gravity highs occur over the outcrop of the Leven Schist Formation, with a maximum centred over the core of the Stob Ban Synform. A measured density contrast of + 0.1 Mg/m3 between the Appin Group schists and surrounding Grampian Group psammites suggests that the schists within the core of the fold would have to be at least 700 m thick in order to produce the 3 mGal residual anomaly.

The gravity high (outlined approximately by the −11 mGal contour) suggests a higher-density body (presumably pre-Caledonian basement) at depth. The north-east to south-west trend of the gravity contours adjacent to the Great Glen Fault Zone suggests that this basement is displaced by the fault zone. The south-east margin of the gravity high is defined by a steep gravity gradient (feature 1 on (Figure 37)), located over the south-east margin of the Leven Schist Formation. This gradient continues towards the north-east, where it transects the surface geology and may represent a deeper crustal feature running parallel to the Great Glen Fault Zone.

A second prominent feature (2), signified by a flexure in the anomaly contours, is located along the south-east margin of the infold of Leven Schist Formation on the north-west side of Loch Treig. This feature, together with a similar feature (3) located along the north-west side of the Ericht–Laidon Fault, may represent a progressive, step-like thickening of lower density material (presumably psammite) towards the south-east. This may, in part, be fault related, thus implying vertical movement as well as known lateral displacement in the region. Most of the main mapped faults, however, appear to have very little gravity expression, which is more consistent with strike-slip movement.

Although the Bouguer anomaly trend is predominantly Caledonian, features perpendicular to this also cross the region. A north-west- to south-east-trending gravity trough (4) over the north-west part of the Strath Ossian Complex suggests a marked deepening of the granodiorite body at this locality. This feature is an extension of a north-west-trending lineament defined by aligned appinites in Glen Roy, the leucogranite at the southern end of the Corrieyairack Granite Complex, and by an alignment of anomalous stream sediment and water chemistry values (Chapter 15). The lineament is referred to as the Strath Ossian Lineament. Elsewhere, a north-north-west to south-south-east gradient (5) in the extreme south-west corner of the district transects the outcrop of the Leven Schist Formation, and has a similar trend to trans-Caledonian basement lineaments recognised elsewhere in the Scottish Highlands (Fettes et al., 1986).

The Allt Crom Granitic Complex and most of the Corrieyairack Complex have a very little gravity expression, and must therefore be either very thin or have a small density contrast with the surrounding country rocks (no reliable density data are available for these intrusions). The prominent gravity lows in the extreme south-west of the region are related to granites at Ben Nevis and Mullach nan Coirean, which may be connected at depth.

References

Most of the references listed below are held in the libraries of the British Geological Survey at Murchison House, Edinburgh, and Keyworth, Nottingham. Copies of the references can be purchased from the Library subject to the current copyright legislation.

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ANDERSON, J G C. 1937. The Etive Granite Complex. Quarterly Journal of the Geological Society of London, Vol. 93, 487–252.

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Appendix 1:10 000 and 1:10 560 maps (Solid and Drift)

The maps at 1:10 000 and 1: 10560 covering, wholly or in part, the Solid geology in 1:50 000 sheet 63W are listed below with the names of the surveyors (G C Clark, R M Key, F May, D I Smith and A J Highton) and the date of survey. Dr Peacock completed the mapping of the Drift for all these sheets during 1990 and 1991. The maps are not published but most of them are available for consultation in the BGS office, Murchison House, Edinburgh where photocopies can be purchased.

National Grid sheets
NH 20SE (Munerigie) GCC, AJH 1982–84, 1989–90
NH 30SW (Invergany) GCC, AJH 1982–83, 1989–99–90
NH 30SE (Glen Tarff) GCC 1988–89
NH 40SW (Glen Tarff, upper) GCC 1988– 89
NH 40SE (Meall Caca) GCC 1989
NN27 NE (Cruach Innse) RMR 1989
NN 27SE (Stob Ban) RMK 1989
NN 28NE (Glen Fintaig) RMK, FM 1988–90
NN 28SE (Roybridge) RMK 1986–89
NN 29NE (Laggan) GCC 1989–90
NN 29SE (Auchivarie) FM, DIS 1979, 1990
NN 37NW (Loch Treig Dam) RMK 1987–89
NN 37NE (Fersit) RMK 1986–87
NN 37SW (Stob a'Choire Mheadoin) RMK 1987–88
NN37SE (Chno Dearg) RMK 1987
NN 38NW (Brunachan) RMK, FM 1988–89
NN 38NE (Beinn Teallach) RMK, FM 1986–89
NN 38SW (Glen Spean) RAIK 1986–88
NN 38SE (Laggan Dam) tustx 1986–88
NN 39NW (Creag nan Gobhar) GCC, AJH 1988–90
NN 39NE (Poll-gormack) GCC, AJH 1988–90
NN 39SW (Brae Roy) FM 1986–90
NN 39SE (Luib-chonnal) FM 1986–88
NN 47NW (Meall Cos Charnan) RMK 1986–87
NN 47NE (Beinn a'Chlachair) RMK 1987
NN 47SW (Loch Ghuilbinn) RMK 1987
NN 47SE (Ben Alder) RMK 1987
NN 48NW (Creag Meagaidh) RMK, FM 1986–89
NN 48NE (Aberarder) RMK 1986
NN 48SW (Lochan an Mire) RMK 1986–87
NN 48SE (Binnein Shuas) PAM 1986–87
NN 49NW (Corrieyairack) GCC, RMK 1988–90
NN 49NE (Gairbeinn) GCC, RMK 1988–90
NN 49SW (Loch Spey) FM 1986–90
NN 49SE (Glenshirra Forest) RMK, FM 1986–90

COUNTY SHEETS (clean copied and available for study in Murchison House, Edinburgh)
INVERNESS 82SW Bishop and Smith (1) 1958–60
INVERNESS 127NW Johnstone and Wright (2, 3) 1953–54
INVERNESS 127SW Wright (3) 1953–54
INVERNESS 142SW Carruthers (3) 1912–14
INVERNESS 152NW Bailey, Grant-Wilson; 1908–12;
Johnson (3) 1956
INVERNESS 152NE Carruthers (3) 1910–12
INVERNESS 153NW Carruthers (3) 1910–14
INVERNESS 153NE Carruthers and Hinxman (3) 1911–14

Amendments to the above County maps by GCC (1), FM (2) and RMK (3) have been included in the 1:50 000 map.

Appendix 2 List of Geological Survey photographs for the Glen Roy district (Sheet 63W)

Copies of these photographs are deposited for public reference in the Library of the British Geological Survey, Edinburgh. Prints are available on application. The photographs belong to Series D.

No. Description
D4156 Looking south-west along the summit ridge of the 'Grey Corries' towards Aonach Beag
D4157 Sgurr Innse from the west with Stob Coire Easainn in the distance
D4158 Uppermost part of the Leven Schist Formation, Allt Ionndrainn
D4159 Upstream end of Monessie Gorge, Glen Spean
D4160 Looking south towards Stob Coire na Ceannain
D4161 Fine-scale internal disruption within the Ballachulish Limestone Formation, Allt Coire Ionndrainn
D4162 Asymmetric folds of early planar fabrics in pelitic schists of the Leven Schist Formation, Loch Treig
D4163 Asymmetric folds and early quartz veins in pelitic schists of the Leven Schist Formation, Loch Treig
D4164 Interference folds within pelitic schists of the Leven
Schist Formation, Loch Treig
D4165 Looking west from Loch Treig at Meall Cian Dearg
D4166 Veins of the Loch Laggan Vein Complex cutting Grampian Group (Loch Laggan Psammite Formation) psammitic rocks, Rubha na Magach
D4167 Veins of the Loch Laggan Vein Complex cutting Grampian Group (Loch Laggan Psammite Formation) micaceous psammites, Rubha na Magach
D4168 Sedimentary structures in graded micaceous psammites of the Grampian Group (Loch Laggan Psammite Formation) cut by veins of the Loch Laggan Vein Complex, Rubha na Magach
D4169 Intersecting pegmati tic and granitic veins of the Loch Laggan Vein Complex, cutting Grampian Group (Loch Laggan Psammite Formation) micaceous psarnmites, Rubha na Magach
D4170 Thinly bedded micaceous psammites of the Creag Meagaidh Psammite Formation, Craigbeg, A86 road cutting
D4171 Breccia zone in the appinite at. Achavady with disrupted quartzite xenoliths.
D4172 Huge metaquartzite blocks in a xenolithic appinite, Achavady.
D4173 Massive brecciated white quartzite cut by ultramafic appinite sheets and feldspathic appinites with angular ultramafic clasts, Achavady
D4174 Quartz-carbonate veins cutting laminated limestones and calc-silicates of the Ballachulish Limestone Formation, Allt Coire Ionndrainn
D4175 Asymmetric folds in the Leven Schist Formation, Allt Ionndrain
D4176 Asymmetric folds in the Leven Schist Formation, Allt Ionndrainn
D4177 Small-scale bimodal planar cross-bedding in thickly bedded psammites of the Brunachan Psammite Formation, Allt Coire Ionndrainn
D4178 Strongly disrupted quartz-carbonate veins in calcareous rocks of the Ballachulish Limestone Formation, Allt Coire Ionndrainn.
D4179 Lenticular bedding in feldspathic: quartzites of the Inver-lair Psammite Formation, Inverlair Gorge, River Spean
D4180 Interbedded psammites and quartzites of the Inverlair Psammite Formation, Inverlair Gorge, River Spean
D4181 Dewatering structures in feldspathic quartzites of the Inverlair Psammite Formation, Inverlair Gorge, River Spean
D4182 Sedimentary structures preserved in the Inverlair Psammite Formation, Inverlair Gorge, River Spean.
D4183 Inverlair Gorge, River Spean.
D4184 Uppermost part of the Glencoe Quartzite, locality [NN 274 758]
D4185 Interbedded white quartzite and grey quartz-mica schists in the Glencoe Quartzite, locality [NN 274 757].
D4186 Brecciated Glencoe Quartzite adjacent to the Innse Appinite, locality [NN 275 754].
D4187 Brecciated Glencoe Quartzite adjacent to the Innse Appinite, locality [NN 275 754].
D4188 Veined and brecciated quartz-mica-schists in the innermost part of the contact aureole of the Strath Ossian Granitic Complex, Fersit.
D4189 Disharmonic folds in quartz-mica-schists in the aureole of the Strath Ossian Granitic Complex, Fersit.
D4190 Mafic xenoliths in the main granodiorite phase of the Strath Ossian Granitic Complex, Laggan Dam.
D4191 The massive granodioritic phase of the Strath Ossian Granitic Complex, quarried to provide building blocks for the Laggan Dam, Laggan Dam
D4196 Looking south-west towards Aonach Beag along the summit spine of the 'Grey Corries'.
D4197 Aonach Beag viewed from the 'Grey Corries' which have extensive scree deposits derived from frost-shattered quartzite of the Glencoe Quartzite.
D4198 Well-bedded quartzite of the Glencoe Quartzite exposed along the summit ridge of Stob Choire Claurigh viewed from Stob Coire na Ceannain.
D4199 Stob Ban (eastern slopes) viewed from Stob Coire Easain.
D4200 North from Stob Choire Claurigh towards Stob Coire na Ceannain with summit exposures of well-bedded quartzite of the Glen Coe Quartzite
D4201 Looking south-west towards Aonach Beag along the summit spine of the 'Grey Corrics'
D4455 Folded granite veins of the Toman Liath Vein Complex in Leven Schist Formation rocks, Glen Roy
D4612 Granite with extremely abundant xenoliths of Leven Schists and vein quartz, Toman Liath Vein Complex, River Roy, 1 km E of Falls of Roy
D4613 Granite with extremely abundant xenoliths of Leven Schists and vein quartz, Toman Liath Vein Complex, River Roy
D4614 Leven Schists cut by veins of pink granite, Toman Liath Vein Complex, River Roy
D4615 Leven Schists with veins of pink granite, Toman Liath Vein Complex, River Roy
D4616 Folded granite veins of the Toman Liath Vein Complex cutting Leven Schists, River Roy.
D4617 Bedding in the Leven Schists, River Roy.
D4618 Veins of pink granite of the Toman Liath Vein Complex, intruded into Leven Schists, River Roy.
D4619 Cross-bedding in psammite (Grampian Group), River Roy
D4620 Bedding surface in psammite (Grampian Group) with biotite-chlorite pseudomorphs after porphyroblastic hornblende, River Roy
D4621 Tight folds of bedding in psammite, Grampian Group, River Roy
D4622 Tight folds of bedding in psammite, Grampian Group, River Roy
D4623 View north in Glen Roy from the tourist viewpoint
D4624 Allt Brunachan Fan, Glen Roy
D4625 Landslip February 1990, Glen Roy
D4626 Landslip February 1990, Glen Roy
D4627 Landslip February 1990, Glen Roy
D4628 Landslip February 1990, Glen Roy
D4629 Landslip February 1990, Glen Roy
D4640 Lochan a Choire/Coire Ardair
D4615 Leven Schists with veins of pink granite, Toman Liath Vein Complex, River Roy

Figures, plates and tables

Figures

(Figure 1) Topography and place names of the Glen Roy district.

(Figure 2) Regional geological setting of the Glen Roy district.

(Figure 3) Simplified geological map, showing areas of stratigraphical successions described in the text.

(Figure 4) Stratigraphical relationships in the Grampian and Appin groups of Area 2.

(Figure 5) Representative measured sections in Grampian Group formations in the eastern part of the district. Subfacies (a) lenticular-bedded psammite and quartzite (bed thickness of 20–50cm), trough cross-bedding and channels can be recognised; subfacies (b) thinly bedded psammites with cross-lamination; and subfacies (c) thin, parallel-bedded psammites.

(Figure 6) Lithofacies variations within the Creag Meagaidh Formation east of Beinn a' Chaorainn.

(Figure 7) Generalised Appin Group lithostratigraphy.

(Figure 8) Measured section through the Glencoe Quartzite, Cruach Innse.

(Figure 9) Measured sections through the Ballachulish Limestone Formation in the south-west of the Glen Roy district.

(Figure 10) Regional structures south-east of the Great Glen Fault Zone and west of the Corrieyairack and Strath Ossian granitic complexes.

(Figure 11) Cross-sections south-east of the Great Glen Fault and west of the Corrieyairack and Strath Ossian granitic complexes (see (Figure 10) for location).

(Figure 12) Regional structures cast of the Corrieyairack and Strath Ossian granitic complexes.

(Figure 13) Minor structures in the Leven Schist Formation as seen in a rock pavement at Loch Treig [NN 343 760].

(Figure 14) Schematic section through the Tarff Synform north of Cam Leac (location of section is shown on (Figure 10)).

(Figure 15) Reactivation of SI surfaces during D2 deformation on the limbs of the Inverlair Antiform.

(Figure 16) Metamorphic zoning near the Eilrig Shear Zone.

(Figure 17) Apparent variation in style of the D1 folds across the Glen Roy district.

(Figure 18) Pressure–temperature diagram constructed for the Grampian and Appin groups.

(Figure 19) Distribution of igneous rocks in the Glen Roy district. (Rocks of uncertain affinity are omitted).

(Figure 20) Traverse through the Achavady Appinite, west bank of the River Roy.

(Figure 21) Plots of geochemical data from the Achavady Appinite.

(Figure 22) Sketch map of the Glas Bheinn Appinitic Complex.

(Figure 23) Sketch map of the Allt Dubh Appinite.

(Figure 24) Simplified geological map of the northern part of the Strath Ossian Granitic Complex.(Figure 24) Simplified geological map of the northern part of the Strath Ossian Granitic Complex.

(Figure 25) Simplified structural map of the northern part of the Strath Ossian Granitic Complex and its aureole.

(Figure 26) Petrogenetic grid for thermally altered pelitic rocks in the aureole of the Strath Ossian Granitic Complex.

(Figure 27) Diagram showing the relationship of trondhjemeitic veins to structures in the Leven Schist at Toman Liath.

(Figure 28) Distribution of the Etive and Ben Nevis dyke swarms.

(Figure 29) Gross-sections through the Great Glen Fault Zone (see (Figure 30) for location).

(Figure 30) Great Glen Fault Zone and other faults in the north-west part of the Glen Roy district.

(Figure 31) Distribution of thick till and diamicton, erratics and decomposed bedrock.

(Figure 32) Loch Lomond Readvance glacial limits, meltwater channels, striae, glacial lake sediments and major glaciofluvial delta and fan deposits.

(Figure 33) Selected Quaternary landforms and deposits in middle and upper Glen Roy.

(Figure 34) Distribution of strontium in streamwaters in the south-west of Glen Roy district.

(Figure 35) Reduced-to-pole greyscale shaded-relief aeromagne tic anomaly map of the Glen Roy district (outlined) and surrounding area. NW illumination. Contours at 100 nT intervals represent anomaly amplitudes. Anomaly gradients are shown as relief. A–A′ is line of profile in (Figure 36).

(Figure 36) Ground magnetic profile A–A′ in the south-west of the district (see (Figure 35)).

(Figure 37) Bouguer gravity anomaly map of the Glen Roy district and surrounding area.

Plates

(Geological succession) Geological sequence in the Glen Roy district.

(Plate 1) Turbidites of th Loch Laggan Formation, showing concordant calc-silicate pods in basal psammites and convoluted upper parts of the graded beds Cut by veins of the Loch Lagga: Vein Complex. North shore of Loch Laggan [NN 458 849] (D4168).

(Plate 2) Thinly bedded micaceous psammites of the Creag Meagaidh Psammite Formation. Individual beds are weakly graded. A86 roadside exposure [NN 491 819] (D4170).

(Plate 3) Dewatering structures in feldspathic quartzites of the Inverlair Psammite Formation with undisturbed lenticular bedding in lower units (by hammer). Inverlair Gorge [NN 341 304] (D4181).

(Plate 4) Ice-scoured pavements of pelitic schists of the Leven Schist Formation showing interference folds of layering defined by metamorphic minerals. Loch Treig valve station [NN 344 760] (D4164).

(Plate 5) Minor 'D2' folds defined by disrupted dolomitic limestone beds and an early foliation in pelitic schists of the Leven Schist Formation at the boundary with the Ballachulish Limestone Formation. Allt Ionndrainn [NN 275 836] (D4175).

(Plate 6) Tight folds of bedding in Grampian Group psammites on the western limb of the Stob Ban Synform [NN 3306 9090] (D4622).

(Plate 7) Intersecting pegmatite and granite veins of the Loch Laggan Vein Complex cutting the Grampian Group psammites. Shore of Loch Laggan [NN 478 849] (D4169).

(Plate 8) Schist breccia within the Glas Bheinn Appinitic Complex. Igneous clasts lie in a foliated garnet-bearing micaceous matrix (TS2437).

(Plate 9) Felsic appinite exposed in Allt Dubh. Mafic and ultramafic streaks define a foliation that predates full crystallisation (TS2438).

(Plate 10) Breccia within the aureole of the Allt Dubh Appinite. Micro-granite containing dark micaceous flakes infills spaces between clasts of psammite and talc-silicate layers (TS2439).

(Plate 11) Toman Liath Vein Complex. Pink granite veins intruding Leven Schist Formation pelitic schists with non-xenolithic veins cut by later xenolith-rich veins [NN 3699 9223] (D4618).

(Plate 12) Great Glen Fault Zone from south-west of Loch Lochy (D2128).

(Plate 13) Fault exposed in bed of watercourse south of Glen Fintaig. The fault zone, composed of brecciated semipelite and quartzite, is 1–4 m thick and dips to the NNE at 78°. The zone is bounded by smooth rock surfaces and a marginal layer of red haematitic gouge can be seen below the hammer [NN 2684 8813] (TS2440).

(Front cover) Cover photograph: Dissected alluvial fans of Allt Feith Bhrunachan on south-east side of Glen Roy at Brunachan [NN 318 896]; upper parallel roads visible above on shoulder of Leana Mho. (D4624) (Photographer: T S Bain)

(Rear cover)

(Frontispiece) Folded veins of the Toman Liath Vein Complex cutting pelitic schists of the Leven Schist Formation. An axial planar schistosity in the veins extends into the schists as a crenulation cleavage. River Roy [NN 3700 9225] (D4616).

Tables

(Table 1) Summary of Grampian Group lithostratigraphy north-west of the Corrieyairack Granitic Complex.

(Table 2) Summary of Grampian Group lithostratigraphy in the southern part of the district.

(Table 3) Summary of Grampian Group lithostratigraphy in the south-eastern part of the district.

(Table 4) Published mean whole-rock analyses of Grampian Group rocks from the south-east of the district.

(Table 5) Measured sections through the Leven Schist Formation.

(Table 6) Mean whole-rock analyses of the main Appin Group lithologies.

(Table 7) Pressure-temperature estimates from Appin Group and Grampian Group rocks.

(Table 8) Representative analyses of the Achavady Appinite.

(Table 9) Mineral assemblage zones within the aureole of the Strath Ossian Granitic Complex.

(Table 10) Summary of Quaternary history and processes.

(Table 11) Parallel Roads (lake shorelines) data.

(Table 12) Fans and deltas in upper and middle Glen Roy.

(Table 13) Major landslips.

(Table 14) Mean values of stream water chemical analyses.

(Table 15) Hydrogeochemical data on the main lithological units.

(Table 16) Summary of magnetic susceptibility measurements for the main rock types.

(Table 17) Summary of density values for the main rock types.

Tables

(Table 1) Summary of Grampian Group lithostratigraphy north-west of the Corrieyairack Granitic Complex

Formation Thickness m Lithology Sedimentary structures Comments
March Burn Quartzite Formation 90 Feldspathic quartzite with subordinate interbedded psammite — Thins to NE; can be traced round Creag a' Chail fold; is lateral equivalent of part of the Inverlair Formation
Brunachan Psammite Formation 550 Psammite with interbedded quartzite at base Trough cross-bedding, ripple cross-lamination Thins to the NE
Beinn Iaruinn Quartzite Formation 1260 Feldspathic quartzite with subordinate beds of quartzite Cross-lamination Thins to NE; at Creag a' Chail only 120 m thick
Glen Fintaig

Semipelite Formation

 1600 Predominantly semipelite, with subordinate beds of psammite. Quartzite uncommon but upper 500 m made up of interbedded semipelite and quartzite Cross-bedding Becomes increasingly psammitic to the NE; tapers Out east of Creag a' Chail fold
Tarff Banded Formation > 2500 Mixed assemblage of interbedded psammite and semipelite. Quartzite beds occur locally. Calc-silicates Cross-bedding Tapers out to SW; lateral equivalents are psammites and semipelites of Auchivarie Psammite and Glen Fintaig Semipelite
Glen Cloy Quartzite up to 270 Colour-laminated feldspathic quartzite with occasional thin beds of psammitc and semipelite Trough cross-bedding, flaser bedding, cross-laminated ripples Tapers out to NE
Auchivarie Psammite Formation 330–>500 Grey psammite, with thin beds of semipelite. Calc-silicates present — Passes laterally to NE into interbedded semipelites and psammites of Tarff Banded Formation
Allt Goibhre

Semipelite Formation

 -, _ 400 Chiefly semipelite, with grey micaceous psammite. Gale-silicates Trough cross- lamination Tapers out to NE. Lateral equivalent is sequence of psammites identical to those of Auchivarie Psammite
Glen Doe Psammite Formation 2700 Grey psammite with occasional thin beds of semipelite Cross-bedding Probably lateral equivalent of Loch Laggan Psammite Formation. Occurs only east of Creag a' Chail Synform
Coire nan Laogh Semipclite Formation 500 Mixed assemblage of interbedded psammite and semipclite, with a basal unit of gneissose semipelite at least 100 m thick — —

(Table 2) Summary of Grampian Group lithostratigraphy in the southern part of the district (areas 3 and 4)

Formation Approximate thickness (in metres) Lithology Sedimentary structures Comments
Dog Falls Psammite Formation 500 Micaceous psammite, semipelitic schist, rare quartzite Bedding-parallel lamination, grading Gradational passage laterally and upwards into basal Appin Group schist formations. Intertidal, water deposits (proximal facies)
Eilde Flags Formation 300+ Flagg micaceous psammite, thin quartzite and semipelite interbeds Rare cross-bedding Gradational passage up from Inverlair Psammite and up into Appin Group. Lateral equivalent of Dog Falls Psammite, in part
Inverlair Psammite Formation 170–700 Psammite, feldspathic quartzite, semipelitic schist interlayers Locally common (notably in south): cross-bedding, rippling, scouring, slumping Rare white calc-silicate pods. Fluviodeltaic sands at the end of a period of basin shallowing
Meall Cos Charnan Semipelite Formation 0–750 (strong tectonic influence) Semipelitic gneiss with a pelitic gneiss member (160 m thick) Obliterated by  intense contact  metamorphism Strongly hornsfelsed. Laterally equivalent to Clachaig Formation
Clachaig Semipelite and Psammite Formation 0–820 Semipelitic schist, thinly laminated micaceous psammite, minor quartzite Bedding-parallel lamination, minor cross-bedding and ripple lamination White calc-silicate pods only locally common. Deposition in shallowing basin, or on a shelf, possibly infilling a major channel
Creag Meagaidh Psammite Formation 150–765 Micaceous psammite with or without semipelite bands, massive psammite, rare quartzite Graded beds, bedding-parallel lamination White calc-silicate pods only common in south-east. Part of sequence formed as stacked submarine channel fills, other parts as more distal turbidites
Ardair Semipelite Formation 400–800 Semipelitic schist with psammitic interbeds Graded beds, cyclic interlayering of semipelite and psammite. Tabular beds with rare cross-cutting beds White calc-silicate pods common. Deposition in submarine fan within low- to medium-energy environment as thick turbidites
Loch Laggan Psammite Formation (lowest exposed unit) 2500 + Medium- to thickly bedded micaceous psammite with internal pelitic laminations. Massive psammite Bedding-parallel lamination, beds mostly tabular, minor channelling, cross-bedding, convolute bedding, scouring, rippling, common grading Base of unit not exposed. White, calcsilicate pods common. Equivalent to the Glen Doe Formation of Haselock et al. (1982). Turbidite sequence within prograding shelf environment

(Table 3) Summary of Grampian Group lithostratigraphy in the southeastern part of the district

Formation Approximate thickness (in metres) Lithology Sedimentary structures Comments
Mixed unit without calc-silicate rocks + 100 Semipelitic gneiss, micaceous psammite, amphibolite, quartzite Obliterated by ductile tectonic fabrics Tectonic contacts preclude correlation of these units with a specific named lithostratigraphic formation (or even group)
Mixed unit with calc-silicate rocks 0–200 Calc-silicate rock, amphibolite, semi-pelitic schist and gneiss, quartzite Obliterated by ductile tectonic fabrics This unit may be equivalento the Ord Subgroup of Winchester and Glover (1988)
Semipelite 200 (Core of synform) Semipelitic gneiss Obliterated by tectonothermal fabrics Main development in Sheet 63E. Uppermost unit in south-east
Psammite and quartzite 800 Micaceous psammite, feldspathic quartzite, semipelite interlayers and laminations Bedding-parallel lamination, rare cross-bedding, grading Strongly deformed within the in the Ossian–Geal Charn Steep Belt. Probably laterally equivalent to the Inverlair Formation

(Table 4) Published mean whole-rock chemical analyses of psammites and semipelites from the Grampian Group in the south-east of the district

Loch Laggan Psammite Formation

Ardair Semipelite Formation

 Creag Meagaidh Psammite Formation Inverlair Psammite Formation
A Semipelites x s.d. x s.d. x x
 SiO2(%) 60.10 2.36 56.01 4.46 62.77 62.63
TiO2 1.04 0.41 1.09 0.19 1.06 1.09
A12O3 17.64 1.56 20.95 3.01 17.08 16.32
Fe2O3 1.47 0.38 1.48 0.37 7.03 1.35
FeO 4.76 1.20 5.39 0.80 4.81
MnO 0.14 0.08 0.11 0.03 0.10 0.10
MgO 2.35 0.37 2.38 0.48 2.78 3.30
CaO 3.52 1.50 1.49 0.70 1.85 1.88
Na2O 3.20 0.59 2.58 1.02 2.56 2.17
K2O 3.24 1.09 5.30 1.36 4.71 5.72
P2O5 0.40 0.21 0.29 0.11 0.23 0.20
LOI 1.23 0.42 1.65 0.66 1.61
Ba(ppm) 814 308 1531 523 994 1007
Cr 60 14 76 12 51 64
Nb 17 4 17 2 19 21
Ni 31 7 39 16 28 22
Rb 147 28 188 24 173 204
Sr 369 68 271 114 286 249
Y 40 21 45 9 42 49
Zr 371 351 288 74 362 472
No. of analyses 9 14 27 32
B Psammites
SiO2(%) 69.01 4.73 72.64 6.29 72.85 72.03
TiO2 0.66 0.10 0.54 0.19 0.61 0.70
Al2O3 14.28 2.15 12.72 2.72 12.80 12.77
Fe2O3 1.15 0.34 0.88 0.39 0.27 3.14
FeO 3.49 0.74 2.76 1.10 3.45
MnO 0.09 0.04 0.09 0.04 0.07 0.05
MgO 1.75 0.41 1.52 0.44 1.32 1.81
CaO 1.97 0.97 2.24 0.71 2.18 2.20
Na2O 2.99 0.74 3.51 0.69 2.95 3.11
K2O 2.92 0.98 1.73 0.73 2.48 3.26
P2O5 0.17 0.06 0.15 0.05 0.16 0.06
LOI 0.96 0.27 0.60 0.19 0.86
Ba(ppm) 716 255 417 246 581 776
Cr 43 8 39 11 45
Nb 15 2 16 2 16 15
Ni 26 6 24 7 19
Rb 110 24 78 34 91 84
Sr 296 67 347 82 291 179
Y 24 5 20 6 30 26
Zr 209 43 226 77 236 698
No. of analyses 14 17 4 24

Data for the Loch Laggan Psammite and Ardair Semipelite formations are from Okonkwo (1989); data for the Creag Meagaidh Psammite and Inverlair Psammite formations are from Winchester and Glover (1988).

(x: mean; s.d: standard deviation).

(Table 5) Measured sections through the Leven Schist Formation

Allt Ionndrainn
Top (Transitional passage into Ballachulish Limestone Formation)
70 m  Porphyroblastic garnet schists
50 m  Fine-grained quartz-mica schists
150 m  Porphyroblastic garnet schists
40 m  Fine-grained quartz-mica schists
330 m  Porphyroblastic garnet schists with rare quartzite seams
80 m  Fine-grained quartz-mica schists
10 m  Porphyroblastic garnet schists
25 m  Fine-grained quartz-mica schists
510 m  Porphyroblastic garnet schists
Allt Leachdach
Top (Transitional passage into Ballachulish Limestone Formation in the core of the Stob an Synform)
550 m  Quartz-mica schists with marble seams up to 2 m thick (significant tectonic thickening)
800 m  Fissile, laminated quartz-mica schists (tectonically thickened)
90 m  Porphyroblastic garnet schists
300 m  Fissile, laminated quartz-mica schists
490 m Variably garnetiferous quartz-mica schists with thin carbonate-bearing schist seams at top of the unit

(Table 6) Mean whole-rock analyses of the main Appin Group lithologies

Metapelite1 Metapelite2 Marble3 Slate3 Slate1
(Loch Treig Schist and Quartzite Formation) (Leven Schist Formation) (Ballachulish LimestoneFormation) (Ballachulish Slate Formation) (Ballachulish Slate Formation)
No. of analyses 20 20 8 36 ?
SiO2(%) 62.9 60.57 18.48 56.24 55.7
TiO2 1.07 8.81 0.05 0.61 0.59
Al2O3 19.6 23.02 2.68 23.66 22.7
Fe2O3* 5.6 6.01 2.44 7.08 7.5
MnO 0.09 0.08 0.02 0.03
MgO 2.1 1.87 7.83 3.63 4.1
CaO 0.9 0.83 32.86 0.80 1.0
Na2O 1.8 1.92 1.20 1.1
K2O 5.6 4.78 0.45 3.99 3.6
P2O5 0.21 0.12 0.08 0.04 0.04
Ba(ppm) 1260 917 97 594 521
Cr 16 93
Nb 25 21 12 11
Ni 18 24 4 27 27
Rb 220 206 16 158 145
Sr 132 146 406 93 83
Y 58 57 9 31 31
Zr 543 310 60 191 173
Samples collected south of Sheet 63W Samples collected in Glen Roy - Glen Spean Samples collected in Allt Ionndrainn–Glen Spean Samples collected south of Sheet 63W Samples collected south of Sheet 63W
  • * = total iron
  • Sources:
  • 1: Lambert et al. (1981)
  • 2. Lambert et al. (1982)
  • 3. Hickman and Wright (1983)

(Table 7) Pressure-temperature estimates obtained from Appin Group pelitic schists and calc-silicate rocks, and Grampian Group semipelitic schists and calc-silicate rocks

[G &S]

[H & C]
Sample N Tmin Tmax x s.d. min max x s.d. min max x s.d.

Appin Group: pelitic schists
(S82456) 18 390 580 516 48 6.3 8.0 7.2 0.6
(S76868) 15 450 500 501 29 6.2 7.9 7.1 0.5
(S82420) 8 480 545 503 21 6.2 7.7 6.9 0.6
(S93793) 14 440 595 520 56 6.6 8.6 7.6 0.5 5.3 9.2 7.5 1.1
(S93791) 12 420 582 523 42 6.4 8.5 7.4 0.6 6.1 9.1 6.9 2.1
(S93795) 12 512 555 527 22 7.6 8.9 8.3 0.4 7.0 9.2 8.3 0.6
(S93790) 6 525 615 604 81 7.9 9.3 8.5 0.6 8.0 10.4 9.2 0.9

Appin Group: calc-silicate rocks
(S80634) 6 475 575 527 30

(garnet - biotite pairs) (estimated P = 7.5 kbars)
(S80634) 6 501 570 525 21

(garnet - hornblende pairs) (estimated P as above)
(S76869) 14 507 620 557 32

(garnet - hornblende pairs) (estimated P as above)

Grampian Group: semipelitic schists
(S92599) 13 460 605 556 42 6.7 10.4 8.5 1.0 5.7 10.5 8.6 1.4
(S92566) 6 475 530 500 17 7.3 9.0 7.9 0.6 6.7 8.7 7.5 0.7

Grampian Group: calc-silicate rocks
(S92601) 10 510 580 549 22

(garnet - biotite pairs) (estimated P = 8.0 kbars)
(S92601) 10 503 596 561 39

(garnet - hornblende pairs) (estimated P = 8.0 kbars)
(S92565) 6 330 395 362 23

(core garnet - biotite pairs) (estimated P = 5.0 kbars)
(S92565) 9 385 525 455 45

(rim garnet - biotite pairs) (estimated P = 8.0 kbars)
(S92565) 14 489 557 530 21

(garnet - hornblende pairs) (estimated P = 8.0 kbars)
  • Temperature estimates (PC) were obtained using Ferry and Spear (1978) and Hodges and Spear (1982) and pressures (in kb) using the geobarometers of Ghent and Stout (1981) [G&S] and Hodges and Crowley (1985) [H&C]. Temperature estimates for the talc-silicate rocks were also obtained using the garnet-hornblende geothermometer of Graham and Powell 1984).
  • N-number of determinations
  • x-mean
  • s.d.-standard deviation

(Table 8) Representative analyses of the Achavady Appinite

Felsic appinite MX 1 Appinite ACY 13 Mafic appinite ACY 9 Hornblende lamprophyre ACC 12
SiO2 61.39 50.58 46.28 44.78
TiO2 0.22 0.57 0.81 1.12
Fe2O3 18.85 10.91 9.20 13.52
MnO 0.04 0.15 0.14 0.17
MgO 2.90 13.31 16.42 10.13
CaO 3.71 7.17 8.30 8.87
Na2O 7.00 1.89 0.99 2.38
K2O 2.71 4.02 4.38 3.47
P2O5 0.10 0.50 0.40 0.90
LOI 1.24 1.97 3.02 4.72
Total 100.96 99.91 99.32 99.12
V ppm 36 125 142 144
Cr 174 1165 1479 399
Co 11 42 54 35
Ni 53 303 327 200
Cu 119 16 22 51
Zn 23 67 60 80
Rb 40 96 106 75
Sr 2992 644 264 897
Y 8 15 11 22
Zr 99 69 35 230
Nb 3 4 3 12
Mo 3 2 nd 2
Ag 0 5 3 2
Ba 1181 1579 1403 1152
La 22 15 12 70
Ce 28 36 24 163
Pb 33 17 3 13
Th 7 5 3 13
U nd nd 1 0
As 0 0 nd 1
W 2 0 1 2
Bi 0 0 0 nd
  • nd - not determined
  • ACYI Zoned phenocrysts of plagioclase in a groundmass of hornblende, biotite and altered plagioclase. Sparse quartz and potash feldspar.
  • ACY13 Phenocrysts of amphibole in a feldspathic groundmass with rounded grains of colourless clinopyroxene.
  • ACY9 Phenocrysts of green amphibole surrounded by, and with inclusions of, mica and clinopyroxene. Small amount of interstitial feldspar.
  • ACY12 Blades of brown-green hornblende in a fine-grained groundmass of altered feldspar, mica and hornblende.
  • All the above samples were collected from exposures in the River Roy.
  • Analysis by XRF at BGS, Keyworth with I.OI determined by weight difference at 1000°C.

(Table 9) Mineral assemblage zones identified within the thermal aureole of the Strath Ossian Granitic Complex (Key et al., 1993)

Zone 1— [Regional]
Representative mineral assemblages: (see text)
Diagnostic: Gt + Bio ± St. Retrogressive: Chl
…outer limit of aureole…
Zone II — [Biotite Zone]
Breakdown of regional garnet within the pelites (Zone Ila) and hornblende within the talc-silicate rocks (Zone III)).
Representative mineral assemblages: Pelites: (a) Qz + Mu + Bio + Pl; (b) Qz + Mu + Bio + PI + relict regional Gt; (c) Qz + Mu + Bio + Pl ± Ksp (Grampian Group). Psammites: (a) Qz + Bio + Pl + Mu ± relict regional Gt; (b) Qz + Bio + Pl. Calc-silicates: (a) Qz + Pl + Bio + Ep ± relict Hb.
Diagnostic: Bio. Relict: ± regional Gt. Retrogressive: Chl + Mu.
…cordierite-in…
Zone IIIa — [Lower Cordierite Zone]
Reactions: cordierite-in reaction: [1] 4 Gt + 2 Mu + 3 Qz = 2 Bio + 3 Crd
Representative mineral assemblages: Pelites: (a) Qz + Bio + Mu + Crd + Pl; (b) Qz + Bio + Mu + Crd
+ PI + rare relict regional garnet; (c) Qz + Bio + Mu ± P1. Psammites: (a) Qz + Bio + Mu + Pl; (b) Qz + Bio+ Mu + Crd ± Pl. Cale-silicates: no examples.
Diagnostic: Crd + Bio. Relict: ± relict regional Gt*. Retrogressive: minor late Mu porphyroblasts.
…(cordierite + K-feldspar)-in and andalusite-in isograd…
Zone IIIB — [Upper Cordierite/Lower Andalusite Zone]
Reactions: (cordierite + K-feldspar)-in Reaction: [2] 6 Mu + 2 Bio + 15 Qz = 3 Crd + 8 Ksp + 8 H2O (continuous)
Diagnostic: Crd + Bio + Ksp. Absent: regional Gt. Retrogressive: ± minor late Mu porphyroblasts. Reactions: andalusite in Reaction: [3] 2 Mu + 3 Crd = 7 Qz + 8 And + 2 Bio + 3 H2O (continuous) Diagnostic: Crd + And + Mu. Absent: regional Gt. Retrogressive: ± minor late Mu porphyroblasts.
Representative mineral assemblages: Pelites: (a) Qz + Bio + Crd + Mu + PI + Ksp; (b) Qz + Bio + Crd + And + Mu + Pl;
(c) Qz + Bio + Crd + And + Mu + Pl + minor Ksp. Psammites: (a) Qz + Bio + Mu + P1; (b) Qz + Bio + Crd + PI ± Ksp. Cale-silicates: no examples.
…(andalusite + K feldspar)-in isograd…
Zone IV — [Upper Andalusite Zone]
Reactions: muscovite – quartz breakdown reaction: [4] Mu + Qz + = And + Ksp + H2O (discontinuous)
Representative mineral assemblages: Pelites: Qz + Crd + And + Ksp + Bio + Pl. Psammites: Qz + Bio + Crd + Pl ± Ksp. Cale-silicates: no examples.
Diagnostic: And + Ksp ( + Crd, Bio). Absent: Mu + Qz* Retrogressive: ± minor late Mu porphyroblasts.
…sillimanite-in…
Zone V — [Sillimanite – Muscovite Zone]
Zone defined by the presence of sillimanite (replacing biotite) and late muscovite within the pelitic assemblage, and as such cannot be interpreted, with any degree of certainty, as a prograde metamorphic zone.
Diagnostic: Sill + late Mu porphyroblasts. Retrogressive: chlorite + sericitic Mu.
…melt-in isograd…
Zone VI — [Migmatite Zone]
Partial melting and localised migmatite development.
granite contact
  • * represents possible metastable phases
  • And—andalusite; Bio—biotite; Chl—chlorite; Crd—cordierite Ep—epidote; Gt—garnet; Hb—hornblende; Ksp—k-feldspar; PI—plagioclase; Qz—quartz; Mu-muscovite; Sill—sillimanite; St—staurolite

(Table 10) Summary of Quaternary history and processes

Flandrian (Holocene) (0–10 000 years BP)
Formation of blanket peat; alluvial terraces and deposits; biogenic lacustrine deposits; small landslides; gullying of hill slopes; formation of small alluvial fans; hillwash; small solifluction lobes on the highest ground.
Late Devensian (10 000–20–26 000 years BP)
Loch Lomond Stadial (10 000–11 000) years BP. Loch Lomond Readvance valley glaciers in south and NW parts of the district, small corric glaciers elsewhere; glacial lake shorelines and deposits; glacial and glaciofluvial deposits; landslides; slope deformation; extensive periglaciation.
Late-glacial (Windermere) Interstadial (11 000–13 000 years BP). No dated deposits. Valley side adjustment (slope deformation, fracturing and possible landslip) immediately following deglaciation.
Dimlington Stadial (13 000–26 000 years BP). Ice-sheet glaciation extending to mountain tops; extensive glacial, glaciofluvial and paraglacial deposition.
Pre-Late Devensian (more than 26 000 years BP)
Excavation of major glacial troughs and corries; deep decomposition of schist, appinite and granite.
(dates in years BP refer to the radiocarbon timescale)

(Table 11) Parallel Roads (lake shorelines) data (see (Figure 32) and (Figure 33)

Glen Spean 260 m Widespread west of Loch Laggan; related to col at 260 m east of Loch Laggan

Glen Roy

 350 m Related to col at 350 m and drainage into the River Spey
334 m Minor shoreline, formed on rising sequence of lakes
325 m Related to col at 325 m in Glen Glas Dhoire

Glen Gloy

 355 m Related to col at 355 m at head of Glen Gloy
295 m Minor shoreline

Glen Fintaig

 426 m Minor shoreline
416 m Minor shoreline
Allt an Lagain 380 m Delta related to col at 380 m
Glen Buck 220 m Shoreline related to col at 220 in into Glen Tarff

(Table 12) Fans and deltas in upper and middle Glen Roy see (Figure 33)

1 Turret Glaciofluvial outw-ash hacked by terminal moraines. Coarsening upward succession of sandy to bouldery gravel overlain and underlain by glaciolacustrine silts (P; PC). Bedding roughly parallel to fan surface. Interpreted variously as subaqueous and deposited in 260 m lake of rising sequence (SC), and subaerial, probably pre-LLR (P).
2 Burn of Agie Fluviatile gravel on lake silts on fluviatile gravel. Interpreted as evidence for subaerial deposition pre-and postdating the 325 and 350 m lakes.
3 Canal Burn Apex of fan of fluviatile gravel interbedded with fossil soils, resting on laminated silts; toe (at 260 m above OD) of gravel overlying lacustrine deposits (base not seen). Interpreted as delta in 260 m lake (SC), but as toe has been eroded by the River Roy, this is uncertain (P).
4 East' Allt Dearg 'Local gravel on lake silts on gravel on rock (at one locality). Possible delta in 260 in lake (SC); possible subaerial origin for gravel underlying lake deposits (P).
5 Reinich Apex of fan below a delta of 325 m Parallel Road. Lower part overlain by lake silts. Possible lacustrine deposits at base, but these may be part of a more general suite of sands and gravels in this part of Glen Roy that almost certainly predate the LLR. Fan interpreted as subaqueous (SC), subaerial or subaqueous (P).
6 Brunachan Apex at 270 m; cut by 260 m Parallel Road. Lower part overlain by lake silts. Poorly sorted imbricate boulder gravel on north side of stream; possible paraglacial deposits on south side. Interpreted as subaqueous (SC) and subaerial with deposition on the north side and erosion on the south by (P).
7 Upper Turret Gravel. Possible delta of 325 m lake (PC).
8 Mouth of Allt a' Chomlain Ice-contact deposit. Possible subaqueous fan.
9 Allt a' Chomlain Delta of 325 m lake (PC).
  • References: Peacock, 1986 (P), Peacock and Cornish, 1989 (PC), Sissons and Cornish, 1993 (SC).
  • LLR–Loch Lomond Re-advance

(Table 13) major landslips

Locality notes
Glen Tarff South side of valley. Rockfall
Glen Fintaig North side of valley. On major joints, foliation and unknown slip plane (Watters, 1972; Peacock and Cornish, 1989). Postdates Parallel Roads, related to 426 m shoreline?
Alltnaray South-east side of valley. Complex Toppling? Upper part cohesive rock, lower part debris with recent movement on scarp dipping into hillside. Lower part predates 355 m Parallel Road.
Braeroy South-east side of valley. Rock sag, and toppling? Accompanied by brittle folds. Postdates Parallel Roads
Brunachan West side of valley. Toppling? Postdates Parallel Roads. Associated with crustal movement; Roads locally rise 3 m above normal (Sissons and Cornish, 1982).
Coire Chouplaig East side of valley. Predates Parallel Roads of falling sequence.
Braigh Bac East side of valley. Possible old slip predates Parallel Roads of falling sequence; upper part more recent.
(unnamed) East side of valley. Lower part partly incorporated in LLR terminal moraine; younger upper part. Associated with crustal movement (Sissons and Cornish, 1982).
Achavady West side of valley. Lower angle. Recent movement. Much debris of weathered appinite.
Cranachan North-west side of valley in superficial deposits. Immediately south of termination of 350 m Parallel Road. Associated with dip in shoreline altitudes (Sissons and Cornish, 1982).
(unnamed) North-west side of valley, incipient, in creep. Cut by 355 m Parallel Road.

(Table 14) Summary of chemical analyses of 512 surface water samples collected from the south-west part of the district and adjoining areas

Range of values Mean value
pH 4.9–12.4 7.2
SEC (μs/cm) 1–420 41
Na (mg/1) 1.1–66.4 3.5
K 0–4.4 0.4
Ca 0.3–24.6 3.7
Mg 0.2–9 1.1
HCO3 0–218 14
SO4 0–7.3 2.2
Cl 0–20.1 4.2
Si 0–5.5 1.3
Sr (μg/l) 0–563 29.1
Ba 0–127 10.4
Li 0–4.1 0.7
B 0–817 10.1
Fe 0–10032 328
Mn 0–1224 32
Cu 0–57.4 1.5
Zn 0–65 5.6
Al 0–518 74

Analyses carried out at BGS, Wallingford.

(Table 15) Hydrogeochemical data classified according to the main lithological units.

Grampian Group north GrampianGroup north Appin Group Ballachulish Limestone Achavady Appinites Corrieyairack Granitic

Complex

 Strath Ossian GraniticComplex
No. of samples 120 66 137 64 29 24 43
SEC(μs/cm) 47 28 35 59 61 27 38
PH 7.42 6.70 6.95 7.20 7.30 6.73 6.83
Na(mg/1) 3.30 2.70 3.19 3.4 4.17 2.55 3.19
K 0.42 0.20 0.27 0.46 1.55 0.15 0.31
Ca 4.96 1.46 2.52 6.03 4.90 1.59 3.43
Mg 0.92 0.45 0.77 1.84 2.13 0.58 2.13
HCO3 21.3 6.09 10.1 21.6 25.4 6.16 11.3
SO4 2.29 1.99 1.95 2.65 2.20 1.46 1.71
CI 4.07 3.47 4.41 4.03 4.28 3.23 4.98
Sr(μg/l) 0.025 0.12 0.027 0.270 0.116 0.045 0.028
Ba 0.0090 0.005 0.0094 0.011 0.050 0.015 0.009
B < 0.06 < 0.006 < 0.006 0.008 > 0.065 < 0.006 < 0.006
Si 1.06 0.99 1.00 1.14 2.05 0.842 1.93
Fe 0.087 0.156 0.44 0.259 0.574 0.189 1.06
Mn 0.0047 0.034 0.037 0.018 0.090 0.010 0.063
Al 0.049 0.095 0.072 0.084 > 0.032 0.082 0.128
Cu < 0.002 < 0.002 < 0.002 < 0.002 > 0.002 < 0.002 0.002
Zn 0.0050 0.0047 0.0051 0.0070 < 0.0070 < 0.004 0.006

(Table 16) Summary of magnetic susceptibility measurements for the main rock types.

Magnetic susceptibility (K) x 10 − 3 SI
Mean Min. Max. Standard deviation No. of sites Total readings
Metasedimentary rocks
Appin Group
Ballachulish
Limestone Formation 0.23 0.16 0.33 0.07 4 45
Leven Schist Formation 12.99 0.16 78.22 15.99 39 443
Loch Treig Schist and Quartzite Formation 0.20 0.15 0.24 0.06 2 25
Loch Treig Schist and Quartzite Formation (magnetic unit) 29.24 6.26 51.56 22.66 3 44
Grampian Group
Quartzites 0.07 0.07 0.07 0.00 1 12
Psammites 0.43 0.02 3.68 0.82 19 208
Semipelites 0.26 0.22 0.32 0.03 7 87
Eilde Flags 0.67 0.10 3.68 1.16 9 113
Igneous rocks
Felsites 0.45 0.02 1.32 0.49 6 73
Porphyritic microdiorites 11.77 2.15 28.28 11.39 4 39
Strath Ossian
Granitic Complex 10.46 2.80 31.47 10.6 8 96
Corrieyairack
Granitic Complex 3.01 1.73 4.44 1.36 3 33
Appinites 5.22 0.66 9.79 6.46 2 20

(Table 17) Summary of density values for the main rock types

Saturated density (Mg/m3)
Age and lithology Mean Min. Mean No. of sites No. of samples
Sedimentary rocks
Devonian
Sandstones 2.66 2.62 2.71 3 11
Metasedimentary rocks
Appin Group
Quartzites 2.68 2.68 2.68 1 2
Limestones 2.69 2.69 2.69 1 1
Schists 2.78 2.75 2.87 6 21
Grampian Group
Psammites 2.69 2.65 2.75 7 23
Central Highland Migmatites
Gneiss 2.74 2.71 2.78 4 13
Loch Eil Group
Psammites 2.73 2.69 2.76 2 9
Igneous rocks
Strath Ossian Granitic Complex 2.70 2.68 2.73 2 25