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Solid geology of the Schiehallion district Memoir for 1:50 000 geological sheet 55W (Scotland)

J E Treagus contributors P R Thomas, P A R Nell, J M Maclachlan, R A D Pattrick, W J Wadsworth

Bibliographical reference: Treagus, J E. 2000. Solid geology of the Schiehallion district. Memoir of the British Geological Survey, Sheet 55W (Scotland).

This memoir is the product of a mapping contract between the Natural Environment Research Council and the University of Manchester. The interpretations presented are those of the authors.

• Author
• J E Treagus, BSc, PhD, Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL
• Contributors
• P R Thomas, BSc, PhD 7 Bay View, Over Kellet, Carnforth, Lancaster, LA6 1DR
• P A R Nell, BSc, PhD Department of Earth Sciences, University of Liverpool, Jane Herdman Laboratories, Brownlow Street, PO Box 147, Liverpool, L69 3BX
• J M Maclachlan, BSc 65 Brickfield Road, Stonehaven, Kincardineshire, AB39 2LT
• R A D Pattrick, BSc, PhD Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL
• W J Wadsworth, BSc DPhil Department of Earth Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL

London: The Stationery Office 2000. © NERC copyright 2000 First published 2000, ISBN 0 11 884557 8. Printed in the UK for The Stationery Office TJ3860 C6 12/00

The grid used on the figures is the National Grid taken from the Ordnance Survey map. (Figure 2) is based on material from Ordnance Survey 1:50 000 scale maps, numbers 42, 43, 51 and 52. © Crown copyright reserved. Ordnance Survey Licence No. GD272191/2000

(Front cover) Cover photograph: View of Schiehallion from the north shore of Loch Rannoch [NN 6040 5895]; main ridge is made of Schiehallion Quartzite, which dips steeply to the south. The ridge in front is crossed by the Boundary Slide (centre to top right) (D 2717). Photographer: T S Bain

(Rear cover)

Notes

• Throughout the memoir the word 'district' refers to the area covered by the geological 1:50 000 Series Sheet 55W Schiehallion.
• National Grid references are given in square brackets; all refer to 100 km square NN, unless otherwise stated.
• Chemical elements are abbreviated using the standard notation of the Periodic Table of elements.
• Five-figure numbers prefixed S refer to thin sections lodged in the Scottish sliced rock collection of the British Geological Survey (located at Murchison House, Edinburgh); four-figure numbers without prefixes refer to thin sections lodged in the University of Manchester collection.
• Numbers prefixed with D and Z refer to photographs registered in BGS collection.
• The attitude of planar structures, such as bedding (S₀) and schistosity (S₁ etc.), is described in terms of their strike and dip and expressed, for example, as 072/38°S.

Acknowledgement

The production of the 1:50 000 Series Sheet 55W Schiehallion is the result of a contract between the NERC and Dr J E Treagus at the University of Manchester, working with Dr P A R Nell, Ms J M Maclachlan, Dr P R Thomas (then with the University of Strathclyde) and Dr R A D Pattrick. The resurvey, between 1990 and 1994, built upon the first edition of Sheet 55 (1902), making full use of the original 1:10 560 field maps of J S Grant Wilson and E H Cunningham Craig, as well as those of E B Bailey and W J McCallien (1937), which were themselves based on the earlier field sheets.

To the west of the Loch Tay Fault and south of northing [52], the mapping of the Glen Lyon area is that of Nell (1984), with the exception of a small area of new mapping south of northing [45]. To the north of northing [52], west of the fault and stratigraphically down to the top of the Grampian Group, new mapping was undertaken in the 1990–1993 field seasons, principally by Dr Nell; Dr Treagus mapped the Dun Coillich and Strath Fionan areas to the east and north of Schiehallion. Dr Thomas was responsible entirely for the mapping of the Grampian Group, which is based on his earlier work (Thomas, 1966) with subsequent additional mapping, including the A9 road sections in 1979 and further new exposures in 1991–1993. To the south-east of the Loch Tay Fault the mapping is largely that of Ms J Maclachlan in the 1990–1992 field seasons; this work included detailed mapping of the Foss open pit and the Calliachar–Urlar vein system, supervised by Dr Pattrick. Dr Treagus mapped Drummond Hill and the area west of the Keltney Burn. The work was supervised by Drs D I J Mallick and D Gould of BGS.

Use has also been made of 1:24 000 air photography, geochemical data from the BGS Mineral Reconnaissance Programme and tunnel logs held by Scottish Hydro-Electric. In the Foss mine area use was made of mapping by Moles (1985a) and that (unpublished) by Mr A R Burns and by Dr N J Butcher of Dresser Minerals and MI of GB Ltd.

Most of the memoir was written by Dr Treagus, but with substantial contributions by Dr Thomas in Chapters 2 and 3 with regard to the Grampian Group and by Dr Nell regarding the area he had mapped. Dr W J Wadsworth (University of Manchester) was principal author for Chapter 4 (Igneous) and Dr Pattrick for Chapter 6 (Economic). Dr G T R Droop (University of Manchester) identified mineral assemblages for Chapter 5. Considerable use was made of the PhD thesis of Nell (1984) and of preparatory chapters of the thesis by Ms Maclachlan. Dr R A Ixer (University of Birmingham) provided access to information on the mineralogy of the Calliachar–Urlar veins. The memoir was edited by Drs Mallick, Gould and Mr A D McAdam. Figures were produced by BGS Cartography, Murchison House.

Grateful acknowledgement is made to the landowners and people of the area for their help and hospitality. Particular mention must be made of the unstinting assistance given by Dr N J Butcher and the staff at Foss mine (MI of GB Ltd) and for access to data, the opencast and underground workings. Thanks are also due to Bolfracks Estate and Colby Gold plc for access to the Calliachar–Urlar prospect and the supply of samples. Sue Treagus gave unfailing help both in the field and with the manuscript. The following BGS staff are thanked for their encouragement and help throughout the project: C G Smith, D I J Mallick and D Gould, and the help of the late M J Gallagher is also acknowledged.

Preface

Understanding the geological framework of Britain underpins all aspects of sustainable development, exploration for and assessment of resources, the avoidance of hazards, land-use planning, and environmental protection and conservation. Accordingly, Central Government allocates funding to BGS to improve our knowledge of the three-dimensional geology of the UK national domain through a programme of data collection, interpretation, publication and archiving. One aim of this programme is to ensure comprehensive coverage of the UK land mass by modern 1:50 000 scale geological maps accompanied by written explanations of the geology by the year 2005. This memoir on the Schiehallion district of the Central Highlands of Scotland is part of the output from that programme.

The memoir and the 1:50 000 map are based on a resurvey of the Solid geology of the district, undertaken by a team led by Dr J E Treagus, University of Manchester, between 1990 and 1994, under contract with the Natural Environment Research Council. Such contracts stem from a policy of encouraging academics to transfer their knowledge of a particular area of the UK to the public domain. The memoir provides a comprehensive account of a classic area in the central Grampian Highlands based on complete remapping on the scale of 1:10 000. It incorporates information obtained during prospecting, development and mining of base and precious metals, including much geochemical and geophysical work by mining companies. No work was done on the Quaternary geology of the district during this resurvey, and for this aspect of the geology the reader is referred to the previous edition of the memoir, published in 1905.

The memoir emphasises the features of the geology which are of special interest to the understanding of the Dalradian fold-belt. Chief amongst these is the effect of the polyphase deformation, which is apparent not only in the outcrop pattern on the 1:50 000 map but also in almost every rock exposure. Particular attention is given to the evidence for the sequence and geometry of the major folds of the four principal phases. Perhaps the most significant advance has been the correlation of the major folds across the Loch Tay Fault, demonstrating a significant dip-slip displacement on this major fracture.

Other features which are described in detail are: the A9 road section with its unique exposures of sedimentary structures and folds in the Grampian Group; the unusually complete sections through the Appin Group and the world-famous Boulder Bed (tillite) at the base of the Argyll Group; the Aberfeldy baryte-sulphide mineralisation in the Argyll Group and the gold mineralisation in the Southern Highland Group.

Small-scale copper and lead mineralisation in the Urlar Burn was recognised in the late 18th century, but very little exploration occurred until the Aberfeldy baryte deposits and associated lead-zinc-silver mineralisation were discovered in 1975–1978. Foss Mine is now a major baryte producer, but further development has been restricted by the need to preserve the scenic beauty of the area. During the late 1980s, gold mineralisation in the Calliachar–Urlar veins was discovered and evaluated, but no mining development has taken place to date.

The only major construction and development projects to take place in the district were the Tummel–Garry hydroelectric complex of dams in the 1940s and 1950s and the rebuilding of the A9 road in the 1970s. The outstanding scenery and tourist attraction of the district will be a factor when considering future developments.

D A Falvey, PhD Director, British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham. NG12 5GG

Solid geology of the Schiehallion district—summary

The district described in this memoir lies in the centre of the Scottish Grampian fold-belt (part of the Caledonide Orogen) and presents a north to south section across the Dalradian Supergroup from the upper part of the Grampian Group to the lower part of the Southern Highland Group. The Dalradian sedimentary rocks, of late Proterozoic age, together with minor contemporary volcanic rocks, have been deformed in the Grampian and Caledonian orogenies and cut by minor intrusions of late Silurian age.

The northern half of the district is underlain by the psammites (originally feldspathic sandstones and muddy siltstones) of the Grampian Group, followed to the south by the varied schists and quartzites (mudstones, siltstones and sandstones) and metamorphosed limestones of the Appin Group. These, together with the Boulder Bed (tillite) and Schiehallion Quartzite of the succeeding Argyll Group, were deposited in shallow waters, but the quartzites, garnet-bearing schists and hornblende-schists that follow to the south represent deeper-water sandstones, mudstones and volcaniclastic rocks. A stratabound baryte-sulphide ore body, of presumed exhalative origin, was deposited in this sequence. The rugged mountains that expose these rocks give way to gentler hills in the south-east where the Loch Tay Limestone and the turbidites of the Southern Highland Group, intruded by numerous basic sheets, crop out.

These rocks were deformed and metamorphosed during the Grampian Orogeny in the early Ordovician. Both the first and second phases of folding produced tight major (kilometre-scale) folds which trended ENE–WSW, responsible for the striking repetition of the stratigraphy seen on the map, as well as the dominant schistosity and the minor folds seen in outcrop. The amphibolite-grade metamorphism bears witness to deep burial. The ductile deformation caused extreme thinning of the limbs of the major folds, commonly resulting in dislocation and the formation of slides. The most significant of these — the Boundary Slide — occurs near the base of the Appin Group. More basic sheets were intruded after the second folding, and the sequence now lay on the essentially flat and inverted limb of the SE-facing Tay Nappe.

Subsequently, as part of the Caledonian Orogeny, two other significant deformations affected the district, probably at the start of the period of uplift in the later Ordovician. One of these gave rise to the conspicuous deflection in the outcrop pattern of the Boundary Slide in the centre of the district — the Bohespic Antiform/ Errochty Synform fold-pair. The other deformation was probably later, and gave rise to an upright ENE-trending crenulation cleavage with common minor folding in the south; major open folds of this age — especially the Ben Lawers Synform and the Loch Tay Antiform — now control the regional dip pattern. Other more local deformation episodes followed.

The Caledonian evolution of the district was completed by a period of fracturing, associated with minor intrusions and vein mineralisation. The major NNE-trending Loch Tay Fault traverses the district, causing sinistral displacement of some 6 km as well as minor dip-slip and dextral displacements. Many smaller faults of this trend, but also of ENE and NNW trends, also occur; the NNW trend is commonly seen in fractures in the south-east where it is locally associated with auriferous and sulphide-quartz veins. Minor sills and dykes, mostly microgranitic or microdioritic, were intruded during this fracture period. The history of the 'solid' geology as seen in the district is completed by the intrusion of an E–W-trending dolerite dyke of late Carboniferous age in the south-east.

(Table 1) Summary of the geological sequence in the Schiehallion district.

(Front cover) Cover photograph: View of Schiehallion from the north shore of Loch Rannoch [NN 6040 5895]; main ridge is made of Schiehallion Quartzite, which dips steeply to the south. The ridge in front is crossed by the Boundary Slide (centre to top right) (D 2717). Photographer: T S Bain

Chapter 1 Introduction

Location and topography

The Solid geology of Sheet 55W (Schiehallion) of the 1:50 000 geological map series of Scotland, referred to as the Schiehallion district, is described in the following account (Figure 1). The area, of some 545 km², lies in the central Grampian Highlands, in north-west Perthshire, now part of Perth and Kinross Council area (Figure 2). The northern boundary borders the mountainous Forest of Atholl, north of the River Garry and the A9 road. The western margin borders on the Moor of Rannoch to the west, passing just west of the village of Kinloch Rannoch and south to Glen Lyon. The southern margin runs east along the southern watershed of the glen, across the head of Loch Tay and almost to the town of Aberfeldy, near the south-east corner of the map. The east margin runs north from Strath Tay, just to the west of Farragon Hill, across Strathtummel and to the west of Blair Atholl.

The area is divided by three broadly east–west-trending valleys: in the north that occupied by Glen Errochty and part of the River Garry to the east; in the centre, that running from the head of Loch Rannoch along the broad strath of the Tummel; in the south, that comprising the spectacularly steep-sided Glen Lyon passing, near the village of Fortingall, into the broader Strath of Appin and, where the Lyon joins the Tay, into the fertile open valley of Strath Tay. The mountains of the area pass from the gentle, rounded hills of the north and east which rise to 500 m, to the craggy mountains of Schiehallion, Carn Mairg and Carn Gorm in the south and west, rising to above 1000 m.

The general topographical character and much of its detail is closely related to the rock-type and structure (Figure 1), (Figure 2); (Table 1). Thus, the contrast of the rather homogeneous psammites and semipelites of the Grampian Group in the northern half of the district, with the varied quartzites and schists of the Argyll Group in the south is strongly reflected in the topographical pattern. In general, the Argyll Group occupies the mountains south of the Rannoch–Tummel valley, but the buttress of the Beinn a' Chuallaich ridge faithfully marks the folded protrusion northwards of the quartzites. Where the Appin Group, with its limestones and soft graphitic schists, is present in substance between the above-mentioned groups, it gives rise to contrasting gentle contours — commonly with grassy pasture. To the east of the Loch Tay Fault, the green hills north of the Tummel are dominated by Appin Group limestones and schists in striking contrast to the barren moorland to the south. Here, the Argyll Group has a relatively narrow and more subdued expression in the Meall Tairneachan–Farragon ridge; to its south, the bench marking the outcrop of the Loch Tay Limestone is followed by the gentler, poorly exposed hills of the Southern Highland Group schists.

The relationship between individual ridges and valleys and the underlying quartzite/schist geology is generally evident. This is true not only of the major ridges of Schiehallion, Carn Mairg and Carn Gorm and their adjacent glens, but also of many smaller ridges, such as those of Meall Dubh (Strath Fionan), Geal Charn [NN 682 545] and Meall Breac [NN 645 548] west of Schiehallion, and the various quartzite ridges west of Beinn a' Chuallaich and north of Glen Lyon. The three main east–west valleys have less obvious geological control, although the ENE-trending D₃ folding exerts some influence on both valleys and ridges in the southern half of the district. Loch Tummel itself is centred about an east-trending D₃ synformal fold and Glen Lyon is in part controlled by ENE-trending structures in the Ben Eagach and Ben Lawers Schists. The presence of the D₃ Ben Lawers Synform clearly has influenced the ridge between Glen Lyon and Loch Tay, and to the east of the Loch Tay Fault the Meall Tairneachan ridge is again centred about this fold-trace. To the south, the Strath of Appin, Drummond Hill and Strath Tay mark the traces of the three folds of the compound D₃ Loch Tay Antiform.

Faults and joints commonly control parts of the drainage system. The most conspicous example is the course of the Allt Glengoulandie north-east from the farm [NN 765 525] which, together with the deeply incised burn that runs down towards Domnaheiche [NN 792 573], marks the trace of the Loch Tay Fault. The position of this fault is also expressed by well-marked features north of the Tummel and to the west of Drummond Hill in the south, where it coincides with the central section of Loch Tay. Of the minor faults, the Keltneyburn (NNW-trending) and the Frenich Burn (NNE-trending) faults and their associated fractures have the strongest expression in the river pattern, and the section of the River Tummel around Dunalastair Water is probably controlled by the local strong set of faults of ENE trend.

In detail, minor igneous intrusions as well as joint-sets control the outcrop pattern. Sills of amphibolite are commonly the most conspicuous outcrops, especially in the Farragon and Pitlochry Schist formations, and these, as well as the late Caledonian dykes and sills, stand out everywhere from schistose formations. A strong 140°–160° jointing controls the shape of many outcrops, particularly in the Pitlochry Schist and other formations to the south-east of the Loch Tay Fault.

The present landscape has been profoundly modified by the effects of the Pleistocene glaciation and by subsequent river action. The effect of glacial erosion by eastward-moving ice on the three principal valley systems and on the mountain corries is evident from the deep rock basins of the major lochs, the U-shaped valleys and the polished rock surfaces up to 1000 m altitude. Perhaps more profound are the effects on the landscape of the deposition of till and moraine which blanket much of the Garry, Tummel and Tay valleys, but which also reach thicknesses of 20 m or more in tributary valleys above 600 m. The main valleys all contain large areas of alluvium, mostly redistributed glacial sands and gravels, whilst the lower hills, particularly south of the River Tay, are mantled in peat.

History of research

The Schiehallion district has a long history of research, much of which has been fundamental to the understanding of the Dalradian. It was central to the deciphering of Dalradian stratigraphy at the turn of the century and subsequently has become the proving-ground for many of the structural advances in the Grampian fold-belt.

The first edition of Sheet 55 (Blair Atholl) was published in 1902 and the memoir in 1905, based on surveying between 1882–1885 and 1900–1902, at a time when the true stratigraphical succession in the Highlands was largely unknown. From the work of Barrow on Sheet 55 the sequence from the Ben Ledi Grit in the Southern Highland Group (not exposed on Sheet 55W) through to the Ben Eagach Schist was established, although considered to be 'a normal upward succession from south-east to north-west' (Barrow et al., 1905, p.9). The order of the remainder of the succession was confused by the equation of the Ben Eagach graphitic schist with the schist associated with the Blair Atholl Limestone, and with the proposal that the 'quartzite' (including the Carn Mairg and Schiehallion quartzites of the present Argyll Group, as well as quartzite of the Appin Group) was unconformable in relation to underlying rocks. The relationship of the 'Moine' (now Grampian Group) to the other formations was uncertain. The area around Schiehallion, where the true stratigraphical significance of the Argyll Group quartzites might have been established, had relatively little attention.

The interpretation of the structure of Sheet 55 (Geological Survey of Great Britain, 1902), as published in the memoir (Barrow et al., 1905, pp.105–111) was essentially of a synformal, fan-like arrangement of folds based on the variation in dip ((Figure 3)b, section A). The change from horizontal to vertical ENE-striking rocks, from the south-east of the district to the Meall Tairneachan ridge, was the south-east limb of the fan, with open folding related to 'strain-slip' cleavage in the Pitlochry Schist becoming symmetrical and isoclinal towards the axis. The complementary gradual flattening of dips to the north-west was accompanied by folds, now asymmetrical to the north-west. The folds of the axial region and of the north-west limb of the synform were said to be related to cleavage caused by shearing, based on the observation of the increasing destruction of pebbles in the quartzites. It is of interest that the axis of the fan west of the Loch Tay Fault is approximately coincident with that of the subsequently recognised (Bailey, 1922) Ben Lawers Synform (D₃ of the present account). The fan structure east of the Loch Tay Fault became a feature of subsequent interpretations discussed below, although the present account rejects it.

The Loch Tay Fault and its apparent sinistral displacement was recognised in the 1905 memoir, although accounts of the significance and size of this displacement vary from 'great lateral wrench' to 'in addition to vertical displacement, a certain amount of wrench or tortional movement' and from three to six miles (Barrow et al., 1905, pp.9, 105, 133). The other significant advance was the recognition of the divergence from the regional strike, in what is now referred to as the Errochty Synform, north of Kinloch Rannoch; it was attributed to a possible hidden pre-folding intrusion (Barrow et al., 1905, pp.107–108).

Subsequently, after some debate (see review in Bailey, 1925) three papers were published which established the true stratigraphical sequence in the Central Highlands, from the Ben Ledi Grit (youngest) down to the Blair Atholl Limestone and Dark Schist Formation. Most of this sequence has been confirmed during the recent resurvey of the Schiehallion district and has been incorporated into the geological sequence (Table 1).

Anderson (1923) showed, in the neighbourhood of Schiehallion, that the sequence continued down from the Ben Eagach Schist to limestones and graphitic schists, which could be correlated with those at Blair Atholl. He argued cogently (pp.429–432), from the evidence of contained fragments in the Boulder Bed and the succeeding conglomerate, that the sequence has the Blair Atholl Limestone at its base. He also argued (pp.435–438) that the junction between the Struan Flags (Grampian Group) and the Dalradian (Appin Group) was a plane of movement, possibly 'a normal fault of large dimensions which had been subject to intense shear'.

Bailey (1925) revised the area to the east of the Loch Tay Fault and Bailey and McCallien (1937) resurveyed various parts of the sheet (revised Geological Survey field-sheets held by BGS) and essentially established most of the boundaries shown on the present map.

The continuity and repetition of the new stratigraphy led Bailey and McCallien to the identification of the axial traces of many of the folds recognised in the present account. Although minor folds and lineations were used to determine the plunge of these major folds, there is no evidence that fold or cleavage vergence was used, or that there might be a superposition of one minor structure on another. However, the important consequence of Bailey's grasp of large-scale structure was the development of the concept of nappe structure in the Central Highlands, as a consequence of the largely inverted stratigraphy. It is not proposed to discuss the details of Bailey and McCallien's structural interpretation; a detailed review may be found in Treagus (1987) for the Schiehallion area and in Nell (1984) for the Glen Lyon area.

Perhaps the most significant advance by Bailey was his recognition from the outcrop pattern that refolding on a major scale had taken place east of the Loch Tay Fault. His section ((Figure 3)b, section B) shows that he had rejected the fan structure in favour of four isoclinal recumbent folds, facing to the south-east, refolded by upright folds. Significant in the context of the present account, two early folds, the D₁ Creag na h- Iolaire Anticline and the Sron Mhor Syncline, are defined and the upright folding is correlated with the late Ben Lawers Synform. To the west of the Loch Tay Fault, particularly in the Glen Lyon area, Bailey and McCallien (1937) clearly recognised that the outcrop pattern of the Ben Eagach and Ben Lawers Schists was the result of refolding of isoclinal folds ((Figure 3)a, section A). Most significant, in terms of the present account, is their recognition that one of the later folds, the D₂ Ruskich Antiform of this account, was the probable equivalent of the D₂ Ben Lui Fold of the South-west Highlands.

One further feature of Bailey and McCallien's (1937) account, following the work of Anderson (1923), was the recognition of a major dislocation (the Boundary Slide) near the junction of what are now called the Grampian and Appin groups. The authors recognised that this and other slides involved syn-deformational excisions of stratigraphy along the thinned limbs of tight to isoclinal folds. They further recognised that the Boundary Slide and associated folds had been folded later on two subsequent occasions, producing folds which approximate to the D_e Errochty Synform and Bohespic Antiform of the present account.

Rast (1958a) essentially confirmed Bailey and McCallien's (1937) stratigraphy, but presented a very different structural interpretation to the west of the Loch Tay Fault. He recognised two sets of minor folds and cleavages, broadly equivalent to the D₁ and D₂ minor structures of the present account. The first set he related to all the major tight to isoclinal folds (D₁–D₃ of this account), which he considered originally faced towards the north-west ((Figure 3)b, section B). The second set, which he associated with the peak metamorphism (Rast, 1958b), he related to the Errochty/ Bohespic fold-pair (D_e of this account). The relative age of minor folds and cleavages and their vergence was not used to establish the age or geometry of major folds, but this work was significant in its use of minor structures to establish the presence of polyphase deformation in the Dalradian. The Beoil Schist was considered to be a tectonic schist produced in the zone of deformation associated with the Boundary Slide, which was responsible for the removal of a substantial part of the stratigraphical succession.

Ramsay (1959) mapped several sets of minor structures in lower Glen Lyon and considered that a north-facing nappe, related to Rast's first generation of folds, developed a south-facing back-fold, accounting for the inverted flat-belt south of Glen Lyon. This structure was considered to have been affected by a second phase of minor folding during peak metamorphism and subsequently by open folds related to the Ben Lawers Synform.

Sturt (1961) supported the concept of a fountain-like spreading of first generation folds from a central steep belt ((Figure 3)b, section C) east of the Loch Tay Fault. The Sron Mhor Syncline still represented the central upright structure; however, the Creag na h- Iolaire Anticline was now shown increasingly recumbent to its south-east, becoming the core to the recently established Tay Nappe (Shackleton, 1958) represented by the inverted flat belt to the south. To the north-west of the Sron Mhor Syncline, the right-way-up succession faced north-west on minor folds related to the major Loch Tummel Anticline (Harris, 1963). The dominant metamorphic fabric was established to be of a later age, although no major folds of this or subsequent age were recognised.

Papers by Harris et al. (1976) and Bradbury et al. (1979), concerned with the evolution of the Tay Nappe and based on observations to the south and east of the district, are relevant to the present account. These works proposed that the Tay Nappe was a product not only of a D₁ period of recumbent folding, as demonstrated by Shackleton (1958), but more importantly of a second (and local third) phase of simple shear directed towards the east. It was noted that D₂ and D₃ intersection lineations and folds of the flat belt are variably oriented about north–south, while the D₂ stretching fabrics trend dominantly NW–SE. The major folds within the steep belt, including the Creag na h- Iolaire and Sron Mhor folds were identified as D₃, linked to the flat belt by a 'rotation zone' unrelated to minor structures ((Figure 3)b, section D). Northwards from the steep belt the D₃ major folds, commonly reclined, fan into a north- to NW-facing attitude. D₁ and D₂ major folds were not thought to be significant, although the steep belt was seen as a fan of composite S₁–S₃ foliation. It is important to note that the D₃ of the above workers is related to the progressive metamorphism and thus predates the D₃ of the present account.

Nell (1984; 1986) remapped lower Glen Lyon and re-interpreted the structure. Within the district the differences in the mapping of lithostratigraphical boundaries to those of the original map, or to those of Bailey and McCallien (1937), are not fundamental. To the north of Glen Lyon, Treagus (Treagus and King, 1978) established the presence of the Appin Group and re-examined the minor structure evidence for the age of the major folds in this and the Argyll Group (Treagus, 1987). The results of the work of Nell (1984; 1986) are incorporated in the present survey, while those of Treagus (1987) have been elaborated and refined, though without altering the basic interpretation of the structure.

To the north, the Grampian Group, in contrast to the above groups, had been the subject of little research before the investigation of Thomas (1979, 1980). In the original memoir (Barrow et al., 1905) the structural comments include the observation that folds in the River Garry overfold to the north-west, and the recognition of late WNW-trending folds in the Errochty area. Thomas (1979; 1980) subdivided the rocks into a number of formations and presented a detailed structural analysis, which has been refined and incorporated into the present account.

Regional setting

The district to be described occupies a central position in the Grampian fold-belt of Scotland and Ireland, which may be traced some 1000 km from Shetland in the north-east to Connemara in the south-west. The lateral extent of the original sedimentary basin is unknown, but shallow-water shelf deposits of similar age occur in eastern Greenland, Ny Friesland (Spitsbergen) and northern Norway (including late Neoproterozoic tillites and carbonates), as well as in Newfoundland and the eastern seaboard of North America (mostly shallow-water clastics) 5000 km to the south-west, around the arc of the Caledonian–Appalachian orogenic belt. The basin (or basins) is considered to have developed on the south-east margin of Laurentia, as part of the early development of the Iapetus Ocean. The closure of the basin in the Scottish-Irish sector culminated in the early Ordovician Grampian Orogeny (D₁ and D₂ of this account) resulting in a NE-trending thrust/fold nappe pile. Deformation of this age is not recognised throughout the Caledonide belt but the polyphase Penobscotian Orogeny recognised in parts of the Appalachians and the Finnmarkian Orogeny of northern Norway are approximately contemporary.

The Neoproterozoic age of the Dalradian Supergroup is demonstrated by dates of 595 ± 4 Ma (Halliday et al., 1989) for the Tayvallich Volcanic Formation near the top of the Argyll Group and 590 ± 2 Ma for the Ben Vuirich granite (Rogers et al., 1989) discussed below. The outcrop of the supergroup within the Grampian fold-belt of mainland Scotland is bounded by the Great Glen and Highland Boundary faults to the north-west and south-east respectively. The overall structure of the well-studied south-western half of the fold-belt is of a fan-like arrangement of major D₁ folds, from NW-facing in the west to SE-facing in the centre and east. These latter areas are dominated by major and minor structures related to a second deformation and the rocks of the district occupy part of the lower limb of the 60 km wavelength composite D₁/D₂ Tay Nappe. The Ben Vuirich granite, 15 km east of the district, precisely dated by U/Pb methods on individual zircon crystals, is deformed by a second fabric (Tanner and Leslie, 1994) which can be clearly correlated with the D₂ fabrics in the Schiehallion and Pitlochry districts. Amphibolite grade metamorphism, which culminated late in D₂, demonstrates that the rocks of the district were buried to at least 20 km. Basic intrusions in the north-east of the fold-belt dated at 470 Ma (Rogers et al., 1994) and in Connemara dated at 478 ± 3 Ma (Cliff et al., 1996) are thought to shortly postdate this climax.

Subsequent to the D₁ and D₂ phases, the rocks were subject to further activity comprising the Caledonian Orogeny. Ductile folding of regional extent (D_e, D_L, D₃ of this account), related to uplift and cooling were followed by increasingly brittle deformation of more localised significance. This activity, which probably took place through the late Ordovician and Silurian, was concluded by regional intrusion of granites between about 425 and 395 Ma, accompanied or even preceded by major strike-slip faulting on the Great Glen and Highland Boundary faults; a major splay of the latter is the Loch Tay Fault, which traverses the district. These movements, which were concluded by the mid-Devonian, can be related to the final closure of the Iapetus Ocean and the end of the Caledonian Orogeny in Britain.

Depositional history

The district provides an unparalleled traverse across all the principal formations of the Dalradian Supergroup. The oldest rocks, the psammites and semipelites of the Grampian Group, can only be subdivided into formal lithostratigraphical divisions in the south-east of the area of outcrop. The oldest known rocks, attributed to the Atholl Subgroup, are mixed psammites and semipelites with thin quartzites becoming more apparent upwards in the Bruar Psammite Formation. Above, distinct formations of psammite and quartzite have been mapped within the Strathtummel Subgroup with a wealth of sedimentological information, especially in the A9 road sections and the River Garry. The presumed shallow-marine or deltaic deposition of these originally muddy feldspathic silts and sands is deduced from the abundant dune- and ripple- bedding and the restricted range of lithologies. These formations, in the Creag Kynachan– Strath Fionan area (Figure 1), (Figure 4), can be seen to pass up, without any significant sedimentological or structural hiatus, into the Appin Group.

The entire Appin Group metasedimentary rock sequence is well developed in Strath Fionan, but is present elsewhere in a structurally abbreviated form. As shown in (Table 1), the sequence is characterised by a great variety of original sediments: from thin pure sands (Beoil Quartzite) to massive feldspathic, pebbly sands (Meall Dubh Quartzite); from the two dolomitic 'pale' limestones to the 'dark' calcite limestones of the Blair Atholl Subgroup; and from the mud/silt/sand alternations of the 'striped' and 'banded' formations to the two highly graphitic 'dark' mud/silt units. These undoubted shallow-marine sediments record rapidly changing conditions: from the stagnation of the graphitic muds to the open sea conditions indicated by the dolomitic limestones; from the quiet conditions of limestone deposition to the inpouring of coarse sands.

The Argyll Group, as seen from (Table 1), represents even more contrasting sedimentological conditions. The Schiehallion Boulder Bed, interpreted as glacial tillite deposited possibly from grounded ice-sheets, together with the cross-bedded fine sands represented in the Schiehallion Quartzite suggest an inter-tidal shallow-marine environment. However, the advent of the Killiecrankie Schist with its increasing proportion of graded, pebbly sands in the transition into the Carn Mairg Quartzite indicates turbidite deposition in a deepening basin. The graphitic muds of the Ben Eagach Schist contain lenses of pebbly sands and may, in part, be contemporaneous with the two turbiditic formations; in the upper part of this formation, stratiform baryte/base-metal bodies (notably the Foss deposit) were locally developed from exhalative saline brines. The dark muds together with the limestones and sandstones of the Ben Lawers Schist suggest a shallowing, whilst the increasing tuff content of this formation, and of the succeeding Farragon Volcanic Formation, herald the beginning of basic igneous activity in the district.

The succeeding Ben Lui Schist, being the strongly deformed equivalent of the graded pebbly Crinan Grits to the west, marks a return to deposition of perhaps more distal turbidites, while the Loch Tay Limestone heralds a temporary return to shallower and quieter conditions. The massive graded grits and the silts and muds of the Southern Highland Group signify renewed turbidite deposition in a deep basin with evidence from volcaniclastic rocks of contemporaneous basic volcanicity. Pre-deformation, concordant, basic sheets were intruded into most formations from the Grampian Group upwards, but are a particular feature of the Ben Lawers Schist and younger formations; they are now represented by amphibolites and hornblende schists.

Post-depositional history

The outcrop pattern of these Dalradian formations is essentially controlled by major folds related to the four principal deformation phases. The regional NE–SW trend is well seen to the east of the Loch Tay Fault; here, repetitions in the outcrop pattern of the formations south of Loch Tummel are due to two major D₁ folds. To the west of the fault, these tight repetitions become particularly evident north of Schiehallion, where they are due to both D₁ and D₂ major folds. A further phase of deformation, of uncertain, possibly pre-D₃ age, is responsible for the major antiform/synform fold-pair that dramatically twists the earlier fold pattern north of Schiehallion. To the south of Schiehallion, the D₁/D₂ interference pattern is further complicated by ENE-trending folds related to the major D₃ Ben Lawers Synform which crosses the south-east of the district. Similar interference of folds of these three phases causes the complex outcrop pattern to the north of Loch Tummel.

The deformation history of the district commenced (D₁) with four large-amplitude NE-trending folds, possibly with an original close-to-tight style and upright attitude. Minor folds and cleavage of this age survive only locally. Faults, contemporary with low-grade metamorphism, developed on some attenuated fold limbs. The subsequent D₂ deformation dominates the structure of the district, with the development of ubiquitous minor folds and cleavage in most formations. These are related to a stack of tight large-amplitude NE-trending major folds, which originally had flat or gently dipping axial surfaces. These folds developed by vertical shortening and, in the south-east, by top-to-the-south-east simple shear, as part of the inverted limb of the major SE-facing Tay Nappe.

At lower structural levels, now in the centre of the district, the D₂ folding developed by top-to-the-north-west shear, associated with major dislocations (slides) on the thinned limbs of the major folds. One of the dislocations, the Boundary Slide, is located near the base of the Appin Group and is responsible for the excision or extreme thinning of the Appin and much of the Argyll Group in the west.

Locally, there was a late intrusion of basic sheets, cross-cutting D₂ folds, before the climax of the regional metamorphism in the waning stages of the D₂ deformation. Amphibolite facies conditions, producing kyanite in the pelites and hornblende in the basic igneous rocks, indicate a depth of burial in excess of 20 km. This peak metamorphism has been dated elsewhere in the Dalradian as early Ordovician. The formation of these major recumbent folds, part of a regional nappe-pile, is referred to the Grampian Orogeny.

Subsequently, probably starting at the period of uplift in the late Ordovician, the rocks were subject to a sequence of increasingly brittle structures, which are attributed to the Caledonian Orogeny. The Errochty Synform–Bohespic Antiform D_e folds have a profound affect upon the strike of the formations in the northern half of the district. They may be related to early fault movements, although they cannot be certainly dated with respect to D₃. These now-upright south-plunging open folds are associated with angular folds in the Appin and Argyll groups and become progressively tighter downwards into the Grampian Group, although here they are only locally associated with minor structures. The essentially flat-lying rocks were subsequently buckled by three regional-scale folds: the broad, open, Drumochter Dome to the north-west of the district and the Ben Lawers Synform and Loch Tay Antiform that cross the south-east corner of the district. These upright, ENE-trending folds, as well as others of an intermediate scale, control much of the local dip pattern. This D₃ deformation was responsible for the upright crenulation cleavage and minor folds that are features of many schist formations in the south and east of the district. Other deformations are more locally developed; north-trending minor folds and crenulation cleavage (D_L, the Lyon phase), that affect part of the Glen Lyon area, predate D₃ and may be the time-equivalent of the D_e phase; minor folds and a flat-lying crenulation cleavage (D_c, the Chanaich phase), that patchily affect rocks south-east of the Loch Tay Fault, postdate D₃.

The Errochty Synform is postdated by a suite of SE-trending, major- and minor-scale, angular folds (D_t, the Trinafour phase) commonly associated with fracturing.

The closing stages of uplift of the fold-belt, in the late Silurian to early Devonian, were completed by a period of fracturing, closely associated with minor intrusions and vein mineralisation. The major NNE-trending Loch Tay Fault traverses the district with a sinistral displacement of some 6 km, although a component of dextral displacement is also likely and a dip-slip displacement of 0.75 km down to the west can be demonstrated. Many small fractures and faults occur with this NNE trend, and also with ENE and NNW trends; the NNW trend is commonly seen in fractures in the south-east, where it is locally associated with auriferous and sulphide-bearing quartz veins. Calc-alkaline plutonism, ranging from quartz-diorite to granite and of which the Glen Banvie Complex is the only representative in the district, took place during uplift about 425–390 Ma ago. Minor sills and dykes, mostly of microgranitic and microdioritic composition, are associated with this plutonic phase. These intrusions were related to this same period of fracturing; some predate particular fractures, others are intruded into fracture planes. The history of the solid geology of the Schiehallion district was completed with the intrusion of an east-trending quartz-dolerite dyke in the late Carboniferous.

Chapter 2 Dalradian lithostratigraphy

The Schiehallion district provides a complete transect of all the established Dalradian formations of the Central Highlands, without interruption by igneous intrusions or tectonic excisions. Thus, it offers a unique opportunity to describe the principal lithologies, their thickness variations and the nature of their margins; owing to the complexity of the folding this view of the sedimentary prism is less two-dimensional than in adjacent districts. The Schiehallion district, together with the margins of adjacent districts, is the 'type' area for most of the formations of the Argyll and Southern Highland Groups and all of the local representatives of the Appin Group (Table 1; Harris and Pitcher, 1975; Gibbons and Harris, 1994). Many of the formations of the Dalradian of the district have characteristics that are laterally remarkably consistent for many tens of kilometres along strike; correlations with the formations of the district, particularly with the upper Appin Group and the Argyll Group, have been made as far as the Banff coast (Read, 1923; 1936) in the north-east and Dalmally and Islay (Bailey, 1922) in the south-west.

In the Grampian Group of the district, there have been no formally recognised subgroups or formations, although recently Glover and Winchester (1989) have suggested informal subdivisions of the sequence to the north-west of the district. Elsewhere, from Loch Eilde in the west to Struan within the district and to Banffshire in the east, the term 'flags' has been applied to these somewhat monotonous psammites, with the recognition of a more varied succession of quartzites and pelites towards their top. Five formations are recognised in the local succession within the proposed Atholl and Strathtummel subgroups. Particularly fine sections of the lower part of the Group can be examined in the parallel sections of the A9 road and the River Garry; the formations of the Strathtummel Subgroup are also well seen in Strathtummel and on the hills to the south.

The complete Appin Group is well exposed in the compact area of Strath Fionan, with recommended transects in (Figure 4). This remarkable area exhibits not only an unusually varied sedimentary sequence of original limestones, dolomites, sandstones, siltstones, graphitic and non-graphitic mudstones, but also is exceptional in the Dalradian for its wealth and variety of metamorphic minerals and minor structures. The lower part of the Appin Group (the Lochaber and Ballachulish subgroups) has only recently been recognised (Treagus and King, 1978) in the district, the rocks having previously been correlated (Bailey and McCallien, 1937; Rast, 1958a, b) with folded repetitions of younger formations. It has been found necessary to introduce one new formation, the Tullochroisk Semipelite, at the top of the Ballachulish Subgroup. The sedimentological similarities of many of the individual formations with their type equivalents in the Appin area are striking (see descriptions below) but informal formation names, taken from the Strath Fionan area (Figure 4), are used here, because of detailed differences to the type area succession, discussed in the text. Thicknesses are much reduced from those of the Appin area (Figure 6), even in the least deformed Strath Fionan area where it is clear from the preservation of sedimentary structures that this thinning of individual formations is, in part, an original sedimentary feature. Away from the Strath Fionan area, what was an even thinner sequence became the focus of intense deformation, with individual formations reduced to a metre or two in thickness in some places, although the sequence is commonly still intact (Figure 5). The upper part of the Appin Group, the Blair Atholl Subgroup, is well represented in Strath Fionan where, in contrast to the type area, two individual limestones can be demonstrated on either side of the graphitic schists; the 'Banded Group' and 'White Limestone' (Bailey and McCallien, 1937) have been given new names, the Cnoc an Fhithich Banded Semipelite and the Drumchastle Pale Limestone, to conform with modern usage.

The district is the type area for many of the well-established formations of the Argyll Group and all are well developed. Only here is the entire sequence represented; within the western part of the district, however, and throughout much of the South-west Highlands (Roberts and Treagus, 1979), much of the group is missing, with formations below the Carn Mairg Quartzite resting on the thinned Appin Group. To the north-east, in the Pitlochry district (Sheet 55E), the Killiecrankie Schist, Carn Mairg Quartzite and Ben Eagach Schist lose their separate identities (Harris et al., 1978). The continuity from the Appin Group into the spectacular Schiehallion Boulder Bed, through the succeeding Schiehallion Quartzite and into the Ben Lawers Schists can be seen along the south side of Strathtummel, where the sequence has been thinned by deformation, but the most outstanding exposures of these generally thick formations are best seen in the hills between Schiehallion and Glen Lyon. Satisfactory examination of the clean cross-bedded metasandstones of the Schiehallion Quartzite and succeeding gritty pelites and psammites of the Killiecrankie Schists and Carn Mairg Quartzite can only be made in the remote hills north of Carn Mairg (Figure 7a); the graphitic schist of the Ben Eagach Formation, the calc-schists of the Ben Lawers Formation, the volcaniclastic rocks of the Farragon Formation and the succeeding Ben Lui Schist are well exposed on the hills above Glen Lyon (Figure 7b), (Figure 7c). A good single 2 km transect can also be made in the east of the district near Meall Tairneachan, from the Killiecrankie Schist to the Ben Lawers Schist including the famous stratiform baryte-sulphide deposit at the top of the Ben Eagach Schist. (Figure 26), (Figure 27). Another transect to the south of that hill can be made from the Farragon Formation through the Ben Lui Schist to the Loch Tay Limestone and Southern Highland Group.

The psammites and pelites (originally turbiditic grits, silts and muds) of the Southern Highland Group are patchily exposed on Drummond Hill and the hills to the south of Strath Tay (Figure 30), (Figure 31). The Green Beds, which are not well exposed, are volcaniclastic beds now represented by amphibolites within the Pitlochry Schist Formation. They are not necessarily correlated with the better developed 'Green Beds' in higher formations elsewhere. Excellent exposures of the Loch Tay Limestone and the succeeding Pitlochry Schist, as well as the attendant basic sheets, are seen in the Allt Coire Pheiginn, above Fortingall.

This chapter describes the lithological characteristics of the 29 formations briefly defined in (Table 1). Important localities for the type lithology, or significant variations from it, are described. Consideration has also been given to ease of access, where possible. Thicknesses quoted are those measured directly from the map, allowing for dip and duplication by major folds. There are, however, obvious problems in using these apparent thicknesses in a relative or an absolute sense in an area of strong and polyphase deformation. It is estimated that some thin competent formations have been reduced to 1/20th of their original thickness by thinning on the limbs of D₁ and D₂ major folds; this figure is based upon the change in the angle of cross-bed foresets in the Schiehallion Quartzite north-west of Loch Kinardochy. Clearly, incompetent pelitic units will have been reduced even more dramatically. However, compensation has also to be made for thickening of most formations in the district, which are strongly affected by minor and intermediate-scale folds. The D₃ folding in parts of the south and east of the district, and the D₂ folding throughout the district, are particularly significant in this respect. D₁ was a period of major isoclinal folding producing duplication of the succession, although it was also associated with strong stretching of the fold limbs.

Also incorporated in these accounts, where appropriate, are descriptions of the typical structural features of formations, as these are often dependent upon the lithological make-up of the formation and at least as respon-sible for its characteristic appearance. A further discussion and explanation of these structural features will be found in Chapter 3.

Grampian Group

In the original memoir for Sheet 55 (Barrow et al., 1905) the rocks of the Grampian Group were described as gneisses and attributed to a formation known as the 'Strowan' and later 'Struan' flags. This was largely due to the siliceous flaggy nature of the rocks which appeared to the original Survey team of J R Dakyns and J S Grant Wilson to be dipping generally to the south-east and was typified by exposures in the River Garry at Struan. The memoir firmly equates these rocks with the 'Moine Series' of north-west Scotland, mainly on the basis of their lithological similarity. Most workers since then have come to similar conclusions about this apparently vast thickness of rocks of mainly psammitic character which, like the Northern Highland rocks, contain thin layers of calc-silicate rocks and rare concentrations of heavy minerals. However, there is as yet no agreed specific correlation of any part of the successions across the Great Glen Fault.

It was the advent of radiometric dates recording deformation events in the Moine of the North-west Highlands older than any obtained from the Dalradian which sowed doubts in the minds of Highland geologists about the status of the 'Younger' Moine rocks of the Central Highlands. Not only did the latter rocks appear, in places, to pass upwards into the Appin Group Dalradian, but they apparently shared a similar deformational history. Indeed, some claims have been made as to the presence of an unconformity above 'Older' Moine rocks in the northern Central Highlands (Piasecki and Van Breemen, 1979), but conclusive evidence has not yet been found. As a result of these doubts, it has been proposed (Gibbons and Harris, 1994) that the 'Younger' Moine rocks of the Central Highlands be assigned to the Dalradian Supergroup and renamed the 'Grampian Group'.

The systematic coverage of the 'Moine' by the Geological Survey of Scotland began in 1890 when Dakyns and Grant Wilson mapped the original area covered by the 1:63 360 Sheet 55. The publication of the map in 1902 was followed firstly by a paper on 'The Moine Gneisses of the Eastern Central Highlands' by Barrow (1904) and then by a much shorter chapter in the sheet memoir (Barrow et al., 1905), in which Grant Wilson recognised the presence of both overturned and isoclinal folding. However, none of the structural data was recorded on Sheet 55 itself, and no attempt was made to establish a stratigraphy for the 'Moine' of the district. Several lithological types were described, but some confusion over the separation of the Moine from the Dalradian appears on the map. The first clear distinction was made by Anderson (1923) followed later by Bailey and MacCallien (1937). But even Bailey, in his sweep across most of the Dalradian outcrop between Loch Leven and Braemar in the first half of this century, did not attempt to differentiate any lithostratigraphical units in the Moine rocks, concentrating instead on establishing the Dalradian stratigraphy and attempting to understand its structure.

Up until the early 1960s most researchers describing the rocks in the Central Highlands tended to ignore structural and stratigraphical details of the 'Moine' rocks, those in the west being referred to as Eilde Flags after Bailey's (1934) work at Loch Leven, and those in Perthshire as Struan Flags after the work of the Geological Survey of Scotland. Between 1962 and 1965 Thomas (1965) carried out detailed mapping covering the area of 'Moine' rock on Sheet 55W and part of Sheet 54. The recent remapping combines this survey with susequent work on the new A9 road alignment in 1979 and further revision of mapping on a scale of 1:10 000 between 1990 and 1993. Between 1962 and 1990 considerable changes involving afforestation have modified access to exposures and created new ones on tracks built by the Forestry Commission.

Whilst Thomas's work unravelled the complex structural geometry within the Grampian Group, the stratigraphical details could only be established with confidence in the Strathtummel area where relatively simple deformation prevails. The preservation of excellent and varied sedimentary structures has proved most valuable.

Lithologies

Whilst the metasedimentary lithologies of the Grampian Group resemble many of those in the Northern Highlands, such as those in Morar Group and Loch Eil Group, those in the Schiehallion district differ in one major respect: there are very few pelitic schist units greater than 50 m in thickness which are mappable over any distance greater than a few kilometres. The majority of lithologies are psammites and schistose psammites which become truly gneissose in only a few localities. However, there is a variety of psammitic lithologies some of which reflect their sedimentary origin and others which may be more influenced by the metamorphic and deformational history.

The following principal lithologies have been differentiated on the 1:10 000 sheets (symbol in brackets).

Where more pelitic lithologies do occur, they are commonly interbedded with the psammites and quartzites and have only been differentiated where thicknesses exceed 15 m. The individual lithologies are described below.

Quartzite (Q^Q)

Quartzite can be separated from the quartzofeldspathic psammite in the field by its creamy whiteness, which reflects a high quartz content. Biotite, muscovite and oligoclase are only minor constituents, whereas microcline sometimes composes up to 20% of the rock, and quartz makes up around 80% of the whole. Individual grains are difficult to observe in hand specimens, due to recrystallisation of the quartz; hence the rock is very hard and resistant to erosion. It commonly forms ridges which are well exposed.

The most impressive quartzite units lie in a tract south of Glen Errochty, where they define the base of the Strathtummel Subgroup described below. They are particularly well developed on the ridge between Dunalastair [NN 713 590] and Craig Kynachan [NN 762 578], and on the Creag nan Caisean ridge, where they can be seen on the forestry track east of the hydro surge chamber [NN 7739 6004]. Thin quartzites in more strongly deformed psammites, near the top of the Grampian Group but of uncertain attribution, can be traced intermittently from the Allt Druidhe [NN 643 573] in the west to north of Loch Errochty [NN 695 659] and [NN 708 670], from north of Trinafour [NN 710 693] to Dunalastair [NN 716 593] and west of Daloist [NN 779 575] to north-west of Loch Bhac [NN 81 63]. Isolated quartzite bands occur throughout the district, but are less common in the north and north-west.

Psammite (Q)

Much of the Grampian Group in the area comprises psammites interbedded with thin layers of quartzite, semipelite and pelite. All psammites have a larger percentage of mica than the quartzite described above, together with a decrease in feldspar and quartz contents to at least 30% and 40% respectively. Magnetite and clinozoisite are usually present as accessories.

The grain size of the quartzofeldspathic mosaic is typically less than 0.2 mm. Where the maximum dimensions of quartz and feldspar grains is large (up to 1 mm) and the micas remain stunted, the psammite is referred to as coarse psammite; where the quartz and feldspar porphyroblasts are accompanied by large micas, the psammites are in many places so strongly recrystallised that they may be referred to as gneissose psammite. Pebbly horizons are rare. Three varieties of psammite may in many places be distinguished:

Laminated psammite (lmQ)

This lithology may also be coarse grained, but is generally associated with thin biotite lamellae in discrete planes which may be separated by up to several centimetres of psammite. This is the lithology in which the majority of well preserved sedimentary structures are found.

Schistose psammite (sQ^M)

Beds of this lithology are generally more severely deformed than the laminated psammite. Schistose psammite contains compositional layering which may be a product of metamorphic differentiation coupled with intense deformation, rather than sedimentary processes. No sedimentary structures are found in these psammites, but they commonly exhibit a flaggy structure and are rarely associated with isoclinal folding. King and Rast (1956) described the flagginess as an axial-planar structure of the 'Moine Phase', but did not separate them from the 'bedded' rocks. Schistose psammites typically occur in zones of higher strain, but contain a mineralogy very similar to that of other psammitic rocks.

Micaceous psammite (Q^M)

This lithology can usually be distinguished from the above varieties. It cannot be described as quartzite, since the feldspar content in the form of oligoclase and microcline is 10–30% and the presence of up to 25% biotite and muscovite gives the rock a schistose appearance, despite it being thickly bedded.

Semipelite (S)

Thin beds of semipelite are found throughout most of the Grampian Group. They are, as elsewhere, intercalated with the psammites. Their composition is only slightly different from the pelites, but the quartzo-feldspathic constituents form a greater percentage of the total composition. Plagioclase (An₆–An₁₀) is always present, while microcline is an extremely variable constituent. More accessories and rare small garnets are found. In the field, the semipelites may exhibit the original sedimentary structures described below.

Pelite (P)

The character of even the most pelitic of the Grampian Group lithologies is of a quartzofeldspathic granular mosaic lying between or interspersed with platy micas having a strongly planar orientation. The mica content is higher than in the semipelites, reaching 40–50% and the pelitic appearance may also be the result of a finer-grain size in the quartzofeldspathic fabric. Brown biotite is the principal mica, and muscovite is subordinate, unless porphyroblastic in habit. Chlorite can be interlayered with the biotite, and may be the result of retrogressive metamorphism.

The quartzofeldspathic fraction forms a mosaic whose boundaries are mostly triple junctions with no evidence of original clastic grains. Quartz is dominant (more than 40%) in the pelites, and plagioclase feldspar in the oligoclase–albite range (An_6–12) rarely exceeds 10% of the total rock composition. Potassium feldspar in the form of microcline is rarer, except where it appears in a porphyroblastic form. The accessories are sphene and epidote with some apatite and rutile. Garnets occur far less frequently in the Grampian Group pelites than in the younger Dalradian schists.

Few pelitic units are thicker than 4 m; one notable exception occurs at Sithean Beag [NN 625 629], at the western margin of the district, where a quartz-biotite schist over 30 m in thickness is intercalated with thinly bedded psammites. Sedimentary structures are usually unrecognisable in the pelites.

Thin isolated layers of dark iron-rich schist occur at several localities. In hand specimen, they resemble amphibolite bands, but in thin section contain approximately equal amounts of quartz and green biotite (30–40%) together with muscovite (0–13%) and a high percentage of accessory minerals, sphene (7–13%), magnetite (4–9%) and epidote (3–16%). Plagioclase, microcline, garnet and magnetite are porphyroblastic. These heavy mineral bands are rare and usually less than 300 mm in thickness. They occur in the cores of very tight isoclinal folds at East Dail-an-Fhraoich [NN 738 698], and as isolated bands in the northern part of the Garry section near Creag Stalcair [NN 686 721]. Very thin layers have also been found at Dunalastair Falls on the River Tummel [NN 732 593].

Calc-silicate bearing rock (C)

Lenses of calc-silicate rock occur in parts of the River Garry section and A9 road cuts, but they are more difficult to identify away from the clean water-eroded surfaces.

The composition of the calc-silicate ribs depends on the metamorphic grade (Kennedy, 1948; Winchester, 1974), but quartz, brown biotite, albite (An_2–8), and epidote in one of its forms, are always present. Muscovite is rare, except in the more psammitic calc-silicates which approach micaceous psammites in composition. In these bands microcline is more abundant than plagioclase. The other variables in the mineral assemblage are sphene, chlorite, garnet, hornblende, calcite and iron oxide.

In the field the calc-silicate rocks have a less-foliated appearance than the granular psammites since the grain size is larger and mica less developed. The general texture is one of matted dark patches scattered in a pale cream matrix. These rocks form thin layers rarely greater than 100 mm in thickness and are found intercalated with several different lithologies, but are generally absent from pelitic units. Most layers are continuous along the strike for several metres, but some die out abruptly or are lens-shaped within the foliation. Large isolated nodules, with long axes up to 0.5 m lying within the bedding, occur; their ellipsoidal outline does not, apparently, displace the bedding, even after deformation, and they probably originated as concretions. A distinct calcareous layer occurs at Bruar [NN 821 660] where several coarse psammitic bands each about 0.5 m thick, contain free calcite and chlorite interspersed within a granular mosaic. These do not resemble the other calc-silicate bands, and have not been observed elsewhere.

Sedimentary structures

Thomas (1965) recognised numerous localities where original sedimentary structures in the metasedimentary rocks could be clearly and unambiguously observed. These provided valuable information about the way-up of strata and some indications of the environment of deposition. Most sedimentary structures are preserved in laminated psammites, quartzites and semipelites in zones of relatively low strain such as the steep overturned limbs of asymmetric folds.

Trough cross-bedding forms the most reliable way-up indicator since tabular truncations may be formed by tectonic excision, whereas the curved truncations are unequivocal. Dune bedding commonly occurs in association with much smaller ripple laminations, some of which display mica-rich 'flaser-like' trough bases. These can be seen most clearly on the water-worn surfaces of the River Garry, 3 km downstream from Loch Garry [NN 679 716], near the Garry intake dam [NN 703 711]; [NN 708 708], at Dalnamein [NN 758 693] and in Clunes gorge [NN 786 667]. The road cut at Clunes [NN 782 671] also contains these ripples, although they are seen at their best on the southbound dual carriageway of the A9 in the Stalcair road cut [NN 686 717], together with numerous other features of sedimentary origin. The cross-bedded units are frequently accompanied by scour and fill channels, convolutions, slumps and sedimentary dykes.

Sedimentary dykes are not uncommon but rarely more than 100 mm in width and usually persist for less than 1 m across bedding. They were probably formed by the injection of more silty sands into coarse sand where slumping was initiated in the sediment. The best locality is at the salmon leap in the Struan gorge of the River Garry [NN 804 656], 200 m downstream from the railway bridge. Sedimentary structures including dykes are also present on the hill exposures on Creag nan Caisean [NN 776 607] above Tummel Bridge, west-north-west of Loch Bhac [NN 804 629] and north of Glen Errochty [NN 759 649].

The sedimentology of the Grampian Group has been studied recently by Glover and Winchester (1989) at various localities throughout the Central Highlands. Their assessment of the Atholl area concluded that the succession was basically a tidal-dominated deltaic sequence. Thomas (1980) was less specific, arguing that the absence of coarse-grained clasts, the uniform nature of the mineralogy and the abundance of dunes and ripples, channel scours and soft-sediment deformation placed the sediment in a shallow-water marine or deltaic environment influenced by variable conditions of current flow but fed from a relatively mature quartzofeldspathic rich hinterland. Graded beds (Piasecki, 1980) reported in the lower parts of the Grampian Group, probably formed during the earlier stages of basin infill, but in the Strathtummel and Atholl areas no such thinly bedded, graded, rhythmites, nor Bouma units that might be of turbidity current origin, have been identified.

The preservation of cross-cutting sedimentary dykes at high angles to slumped and regular bedding confirms that bedding in at least some parts of the Grampian Group has not suffered high strain nor bedding-parallel dislocation. However, there are wide zones where sedimentary structures are highly deformed or unrecognisable and where intensely developed folding and schistosity are present; this is especially so near the Boundary Slide and also in the core of the major Errochty Synform between Dalnacardoch and Dalnamein. The latter, referred to as the 'Dalnacardoch Banded Zone' by Thomas (1980), is almost devoid of any recognisable sedimentary structures and the apparent bedding structure may in some areas be a transposed metamorphic banding.

A further problem which arises is the distinction between small-scale isoclinal folds of sedimentary and tectonic origin. In some well-exposed and weakly deformed sections tight-to-isoclinal folds are clearly of sedimentary origin, associated with slumped foresets and truncation by higher beds. In more strongly deformed areas some isoclinal folds may well be modified soft-sediment folds even though they have an apparent S₁ or S₂ axial plane schistosity.

Lithostratigraphy

The monotonous nature of the lithologies, together with the local complexity of structure as well as the extremely variable degree of exposure away from the river sections, have all contributed to the difficulty in establishing a comprehensive stratigraphy for the Grampian Group in the district. Unlike the Moine rocks of the Northern Highlands or the Grampian Group of the Laggan area of the Central Highlands (Glover and Winchester, 1989), there are few marker beds or distinctive lithologies such as thick pelitic or striped schists.

The lithologies described above are dominated by psammites, and their schistose and micaceous variants, over most the area of outcrop, commonly interbedded with either quartzites or thin mica-rich units which are irregularly distributed throughout the rock pile. Fortunately, way-up evidence is plentiful away from zones of intense deformation but, conversely, the lack of exposure over moorland areas reduces the effectiveness of stratigraphical correlation.

A line is shown on the 1:50 000 map and on (Figure 1) which separates the area where it has proved possible to erect a lithostratigraphy from that where it has proved impractical, for the reasons given above. The rocks to the east can be divided into an upper Strathtummel Subgroup and a lower Atholl Subgroup. The comparatively well-exposed rocks of the Strathtummel Subgroup to the south of Glen Garry exhibit well-defined formations of psammite and quartzite, which can be tentatively traced into the road and river sections of Glen Garry to the north. These formations pass downwards into the mixed psammites and semipelites of the Atholl Subgroup which are less well exposed north-east of Glen Garry.

The area of undivided Grampian Group to the north-west of the line contains a zone of complex deformation, the Dalnacardoch Banded Zone (Thomas, 1980), which is well seen in mid-Glen Garry (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). This zone of flaggy rocks, which coincides with the axial zone of the D_e Errochty Synform, is partly the product of strong deformation in the tight core of that fold and partly of an earlier zone of strong D₁/D₂ deformation that is folded by the synform. The Boundary Slide Zone, which marks the top of the Grampian Group, makes stratigraphical subdivision and correlation difficult everywhere around the closure and on the west limb of the Errochty Synform. Apart from the excellent Glen Garry sections, exposure is insufficient to allow subdivision or precise correlation with the formations to the south-east of the line. However, Thomas (1980) considered that the structural constraints discussed in Chapter 3 (see axial-traces and cross-section of (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 12)b, indicate that both the older formations of the Strathtummel Subgroup, as well as the underlying Atholl Subgroup are represented.

Undivided Grampian Group

As no formal subdivision of this area is being attempted, there are no type sections. The exposed rocks are dominantly psammites, but laminated psammites, semipelites and rare quartzites and calc-silicate beds, as described above, are all represented. All these lithologies may be seen in the outstanding road and river sections of Glen Garry; elsewhere sections are restricted to burns and isolated hillside outcrops. Two of these latter sections that have yielded useful stratigraphical information, as well as selected localities from Glen Garry, are described.

Creag a' Mhadaidh to Meall Dubh [NN 635 651] to [NN 651 602]

This transect extends from Sheet 54 and provides sections which strongly suggest that the rocks on the west limb of the Errochty Synform may be attributed to both the Strathtummel and Atholl subgroups. On the western margin of the district the well-exposed peak of Creag a' Mhadaidh [NN 635 651] reaches 612 m OD. Here, laminated psammites and semipelites, in which cross-laminations demonstrate the inverted nature of the succession, probably belong to the Strathtummel Subgroup, but they cannot easily be attributed to particular formations. These rocks are involved in the major D₂ Creag a' Mhadaidh Antiform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) which has an overturned north-western limb and forms a plunge culmination on the summit ridge. To the south of and stratigraphically below the Creag a' Mhadaidh psammites, more variable schists outcrop on the Sidhean Beag ridge to the west of the district [NN 626 631]. These resemble schists at the top of the Atholl Subgroup but no exact correlation can be made. The transect continues over the poorly exposed boggy ground below Saunich [NN 641 634] south-east towards Meall Dubh, where occasional outcrops of low-dipping micaceous flaggy psammite give way to more steeply dipping psammites on the flanks of Beinn a' Chuallaich. It is assumed that the D₂ Meall Reamhar Synform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) has been crossed in these micaceous psammites, which probably belong to the Atholl Subgroup. Further south-east, on Meall Dubh [NN 649 602], these psammites are overlain by a feldspathic quartzite of Strathtummel Subgroup affinity before the Beoil Schist of the Appin Group is reached immediately below the crags containing porphyritic felsite sills [NN 651 602].

South Edendon

Exposures on the south side of Glen Garry 2 km north-west of the Trinafour road [NN 710 693] contain some of the few white quartzite beds north-west of the Errochty Synform. Thin but distinct quartzites are interbedded with psammites and are strongly folded by the primary deformation. Being isolated exposures less than 0.2 km2 in area they are difficult to place stratigraphically, but resemble rocks from the uppermost formations of the Grampian Group of Strathtummel.

Glen Garry

Despite the abundance of exposures in Glen Garry, those situated west of Dalnamein [NN 750 696] are difficult to correlate with the Atholl and Strathtummel subgroups and are designated as undivided Grampian Group. The rocks between Dalnamein and Edendon (Figure 1) are severely deformed and are difficult to attribute to any specific formation, especially where they enter the Dalnacardoch Banded Zone (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). Those west of Edendon, on the approaches to the Drumochter Pass, are rich in sedimentary structures and more clearly resemble the formations of the divided Grampian Group. This stratigraphical resemblance, combined with the similarity of structural style, points to a repetition of the section south-east of Clunes (Figure 1), (Figure 12)b. However, the already tenuous distinction of formations in the latter section would not justify the extension of their correlation across the Dalnacardoch Banded Zone.

Details of selected outcrops in the superbly exposed River Garry and the parallel road cuts will be found in the description of the Glen Garry Transect in Chapter 3. Lithologies ranging from metre-thick psammites to thinly bedded laminated psammites and semipelites may be examined in most of the sections. In particular, the laminated psammites, which exhibit cross-laminations and many other sedimentary structures, are well seen at the Stalcair road cut [NN 686 717]. Calc-silicates are abundant in the Wade Stone road cut [NN 698 716]. More strongly deformed cross-laminations are seen in the Edendon road cut [NN 712 708] and psammites seen at the Dalnacardoch road cut [NN 719 705] have a flagginess at least in part of tectonic origin, as the more severely deformed Dalnacardoch Banded Zone is approached. Psammites containing a variety of sedimentary structures appear in the River Garry again near Dalnamein Lodge [NN 749 694] and continue to be well exposed and access-ible to Black Tank Bridge [NN 769 688]. To the south-east of here formations are tentatively assigned to the Atholl and Strathtummel Subgroups.

Subdivided Grampian Group

Two subgroups are recognised: a lower and less well-defined Atholl Subgroup and an upper more readily defined Strathtummel Subgroup (Figure 1), (Figure 12)b.

Atholl Subgroup

In the north-east of the district, the oldest Grampian Group rocks are exposed in the River Garry and its tributaries to the north. Here, they lie down-plunge of the downward-facing Meall Reamhar Synform [NN 784 610], well defined by the outcrop of the base of the Tummel Quartzite on Meall Reamhar. They, therefore, represent an older sequence of rocks, which consists largely of flaggy mixed psammite and semipelite. These rocks are sufficiently distinct from those of the Strathtummel Subgroup above to be assigned to a separate subgroup, the base of which is not seen.

On the west limb of the Errochty Synform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) similar lithologies outcrop in the cores of the folds thought to be the equivalent of the Meall Reamhar Synform and of the Clunes Synform (the Garry Synform) in the north-west corner of the district; these lithologies were attributed to the Drumochter Succession by Thomas (1980). If the correlation of these rocks with the Atholl Subgroup is correct, the rocks exposed in the Drumochter Pass to the west may represent older lithologies than those seen in the Bruar Formation described below.

Bruar Psammite Formation (minimum 600 m)

The upper part of the subgroup, as exposed in the north-east of the district, has sufficiently distinct lithologies to be given formation status, although its base cannot be defined. It is very variable in lithology, containing a few mappable pelites, semipelites and both laminated psammites and thickly bedded quartz-rich psammites and rare quartzites. The type section is in the Bruar Water [NN 8245 6590] to [NN 8206 6825] where the total thickness is about 600 m. Good exposures of this formation also can be seen south-east from the path to the Falls of Bruar [NN 820 663] where cross-bedding, right way up and younging to the south-east (allowing for reversals on minor D₂ folds overturned to the north-west) is commonly displayed. In the rivers Garry and Errochty near Struan [NN 818 654] to [NN 800 658] excellent exposures lie on the regionally inverted north-west limb of the Meall Reamhar Synform (see (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9)) and show abundant sedimentary structures now younging to the north-west.

Further north-west the sequence exposed in the River Garry from the gorge [NN 773 677] (and in the Black Tank road cut in the A9 above) at least as far as the Black Tank Bridge [NN 7663 6878] are considered to belong to this formation, although the position of the boundary with the Tummel Quartzite above is tenuous.

The upper junction with the Tummel Quartzite is best seen on the south-east slopes of Creag nan Caisean [NN 783 609] where good, though less accessible, exposures display cross-bedding, right way up and younging to the south-east, in both formations.

Strathtummel Subgroup

In Strathtummel, the simplicity of structure, the sufficiency of exposure and way-up evidence come together on the steep south-east limb of the Meall Reamhar Synform from the summit of Meall Reamhar to Strath Fionan in the south (Figure 1), (Figure 2). Sedimentary structures confirm an upward-younging succession of strata at least 3000 m in thickness which contains a succession of four formations. These two quartzites and two psammites are sufficiently distictive to map, but have generally gradational boundaries. They constitute the Strathtummel Subgroup immediately below the Appin Group.

In the south-west Highlands, the Grampian Group passes upwards into the Appin Group at the Eilde Quartzite. In the type area north of Loch Leven, the underlying Grampian Group rocks have not been subdivided, but further north-east at Loch Laggan several psammitic and semipelitic formations have been recorded by Glover (Glover and Winchester, 1989; Glover, 1993) beneath the lowest quartzite of the Appin Group. To the south of Loch Leven, at both Glen Orchy and the Black Mount, thick psammitic sequences underlie the lowest quartzite (Binnein Quartzite) of the Appin Group, but older mappable quartzites do not seem to occur (Thomas and Treagus, 1968; Thomas, 1979).

The psammitic rocks of the Strathtummel Subgroup appear to be equivalent to those of both the Glen Spean Subgroup and much of the Corrieyairack Subgroup (Haselock and Evans, 1990), further north.

Tummel Quartzite Formation (1800–10 m)

Above the sequence of psammites, semipelites and pelites of the Bruar Formation, mappable layers of white quartzite up to 30 m in thickness are interbedded with thicker units of laminated schistose psammites which are rich in dune cross-bedding, ripple cross-laminations and soft-sediment deformation structures. The way-up of these structures clearly demonstrates that the numerous quartzite layers are not infolded. The type section is on Creag nan Caisean [NN 783 609] to [NN 784 601], north of Loch Tummel, where this formation is estimated to reach a thickness of 700 m. Sedimentary structures exhibit younging consistently towards the upper boundary with the Tummel Psammite above.

The formation is traceable from the type section to the north-east towards Blair Atholl in the Pitlochry district (Sheet 55E), and to the south-west where it thickens and swings through the River Tummel between Tummel Bridge and Dunalastair Hydro Dam [NN 7215 5912] to continue northwards into the unexposed ground of Drumcroy Hill [NN 72 63].

The Meall Reamhar Synform repeats the Tummel Quartzite Formation on its more gently dipping north-west limb (e.g. on the north flank of Meall Reamhar [NN 78 62]), through the poorly exposed ground of Glen Errochty to the Struan Point crags [NN 788 647] which exhibit good inverted trough cross-bedding. The formation then becomes difficult to trace into Glen Garry south-east of Clunes, where it is believed to be considerably thinned and faulted out. It is seen again in the complex zone north of Clunes on the north-west limb of the Clunes Antiform, although correlation around the hinge-zone of the Bohespic Antiform with the quartzite-rich sequence at Dalinturuaine [NN 76 69] can only be inferred, due to complex polyphase folding and paucity of exposure.

Tummel Psammite Formation (800–200 m)

South of the River Tummel from the eastern hydro dam [NN 771 592] in an arc to the Dunalastair Dam [NN 721 591] the uppermost feldspathic quartzite of the Tummel Quartzite gives way to a dominantly psammitic formation. The laminated thickly bedded psammites contain rare dunes and ripples consistently younging towards the south. The formation is well exposed on the north flank of Creag Kynachan [NN 769 579] through Coille Kynachan [NN 750 585], but the type section is on the south bank of the River Tummel and Dunalastair Reservoir from the weir below Dunalastair Dam [NN 7238 5912] to [NN 7197 5893]; here, regularly bedded quartz-rich schistose psammite with some thin schistose pelite bands dipping steeply south-west, contains uninverted cross-bedding near its junctions with the formations above and below.

The outcrop of the formation on this southern limb of the Meall Reamhar Synform has been traced in poorly exposed ground north of the Tummel, on the west limb of the Bohespic Antiform towards Trinafour and on the east limb north-eastwards towards Blair Atholl. The sequence is also poorly exposed on the northern limb of the Meall Reamhar Synform but is well exposed across the hinge of the Clunes Antiform in the crags between Blairfettie and Kinaldy [NN 761 645] in Glen Errochty. Lithologies seen in the inner core of the Clunes Antiform, in the River Garry [NN 787 667] and in the A9 road section above (Plate 3a), are similar but cannot be assigned to this formation with complete certainty.

Kynachan Quartzite Formation (500–0 m)

The type section on the summit of Creag Kynachan [NN 761 579] shows a thick sequence of massive granular white and cream impure feldspathic quartzites that overlies the Tummel Psammite Formation. Here, the near-vertically orientated rocks reach a thickness of almost 500 m before giving way, once again, to more psammitic rocks to the south. The upper junction with the Kynachan Psammite is gradational in the crags around [NN 763 576].

Unfortunately, lateral variations in the Kynachan Quartzite make it more difficult to recognise away from the type locality. The quartzite is certainly a lens-shaped body possibly thickened by folding. The character of the quartzite changes along strike so that to the west, as the River Tummel is approached, although quartzite still dominates, it becomes more schistose, interbedded with psammitic units and locally attenuated to a thickness of only 100–200 m. The formation continues to thin to the north but can be tentatively identified in sporadic exposures of quartzite north of the Tummel at Dunalastair and in the Allt na Moine Buidhe [NN 720 610] but not in sections further north. Feldspathic quartzites attributed to the formation also occur south of Blairfettie and where the Clunes Antiform meets the Bohespic Antiform surrounding the Torr Dubh Dome [NN 745 632] in Glen Errochty. A lens-shaped outcrop of quartzite around [NN 740 571] in Strath Fionan (Figure 4) is interpreted as the nose of a D₁ anticline of the formation.

The quartzite was originally shown on Sheet 55 in 1902 to be equivalent to the higher Dalradian quartzites, but was separated by Bailey and McCallien (1937) into part of the 'Moine Series' about which they stated that 'the outlines… are so simple that they need not detain us'.

Kynachan Psammite Formation (700–0 m)

A further thickly bedded schistose psammite sequence with fewer quartzites overlies the Kynachan Quartzite Formation and shows similar thickness variations along the Kynachan ridge [NN 722 582] to [NN 770 575]. The lower few hundred metres are rich in dune bedding but the formation becomes more banded towards the top, where it takes on a flaggy appearance. The type section, on the south side of Creag Kynachan [NN 751 576] to [NN 751 570], is particularly well exposed, although it can be seen to be considerably thickened by D₂ folding. Another excellent section to the west is the line of crags north-east of Lochan an Daim [NN 722 582] (Figure 4). The upper conformable, and relatively sharp, junction with the lowest formation (Dunalastair Quartzite) of the Appin Group is well exposed between here and Lochan an Daim. Some 800 m to the south-east [NN 725 577], cross-bedding youngs consistently south-west towards the Appin Group junction. The formation can be followed north-west to the Tummel. North of the Tummel, at Dunalastair, psammites that may be representatives of the formation can be seen in the Allt na Moine Buidhe near Mullinvadie [NN 713 611]; to the north of this locality, sections show psammites which probably represent an amalgation of the Tummel and Kynachan Psammite Formations. To the north-east of Craig Kynachan the formation is only well seen in the Allt Kynachan west of Foss [NN 778 575].

Appin Group

Strath Fionan and Errochty

The formations of the Appin Group (1–14, (Figure 4)) are well displayed only within the Strath Fionan area, which has provided the formation names and type localities. They may be conveniently examined in transects A–A′ to C–C′ (Figure 4), within which most of the type localities occur. Beyond Strath Fionan, the equivalent, strongly deformed formations are seen only in a number of key sections close to the boundary of the Grampian Group, e.g. around the closure of the Errochty Synform (Figure 5), to which reference is also made. The Blair Atholl Subgroup rocks which crop out west and east of the Loch Tay Fault, north of Loch Tummel, are treated separately.

Lochaber Subgroup

Good exposures of the first four formations of this Subgroup can be seen on or close to the line of transect A–A′ [NN 723 577] to [NN 721 576] which provides the type section for the Dunalastair Quartzite and Semipelite formations. As can be seen from (Figure 4), the Dunalastair Quartzite and Semipelite are repeated along this transect in what is believed to be a D₁ isoclinal fold-pair, with a narrow belt of Grampian Group psammites in the southern fold core. The Beoil Quartzite and Schist are well exposed to the south-west of this, but the type section of these formations is chosen to the north of, and in continuity with, the Meall Dubh Striped Pelite of transect B–B′.

Dunalastair Quartzite Formation (90–0 m)

These typically rather pink-weathering quartzites and quartzitic psammites are seen as a quite distinct unit within the area of (Figure 4), in sharp contact with the Grampian Group psammites, from due east of Dunalastair [NN 7152 5875] south-east to the ridge at [NN 724 576]. Elsewhere in the area of (Figure 4) and beyond, the formation is patchily exposed. To the north of Dunalastair, as far as the Errochty Synform closure, it can be recognised as a few metres of flaggy white quartz-rich psammite above the feldspathic psammites of the Grampian Group, e.g. in the Allt na Moine Buidhe [NN 7095 6121] and the Allt an Ruighe Fhliuch [NN 7100 6305]; it has not been recognised as a mappable formation on the west limb of the synform. To the east of transect A–A′ similar flaggy quartzites can be traced to Lochan Beoil Chathaiche and as far as the burn at [NN 7609 5670] but in the Allt Kynachan [NN 7775 5723] it is not present within the 3 m zone of strongly deformed rocks (presumed Dunalastair Semipelite and Beoil Quartzite) between the Grampian Group psammite and the Beoil Schist.

The outcrop along and near transect A–A′ [NN 723 577] to [NN 721 577] is the type section. Well-bedded 100–300 mm thick quartzites exhibit slight variations in feldspar content, but are rarely as feldspathic as the adjacent Grampian group psammites. Cross-bedding is evident from feldspar and heavy mineral concentrations, and truncated foresets occur within a few metres of the Grampian Group, which confirm the age relationships. In thin section, the rock exhibits a texture of equigranular quartz enclosing scattered small grains of muscovite, chlorite and plagioclase. The presence of microcline feldspar in a section from one psammitic bed illustrates the transitional nature of this formation with the Grampian Group psammites, which typically contain microcline clasts, and the cleaner, more plagioclase-rich quartzites of the Appin and Argyll groups.

Dunalastair Semipelite Formation (90–0 m)

The distribution of this formation is much as described above for the Dunalastair Quartzite. Good exposures within the area of (Figure 4) can be seen along the ridge crossed by the transect A–A′ on Speirean Ruadh [NN 735 570] and south-west of Lochan Beoil. To the east it is seen in the burn [NN 7609 5670] and in the Allt Kynachan [NN 7775 5723] as a few metres of flaggy semipelite. To the north of (Figure 4) only a few exposures occur e.g. south-east of Torr Mor [NN 710 608] west of the Dunalastair Quartzite, but where the Beoil Quartzite is not exposed the formation is difficult to separate from Beoil Schist.

The type section is seen on the line of transect A–A′ between [NN 722 577] and [NN 721 577]. Although the dominant lithology in this formation is a biotite-muscovite semipelite, its characteristic feature is a ribbed appearance on a variety of scales, owing to the presence of psammitic laminae. This ribbing may be of fine-grained (originally silty) 5–10 mm psammite interbeds or of coarse-grained 10–30 mm quartzofeldspathic or quartzose psammite ribs within the semipelite. The pelites contain the same sigmoidal quartz lenses as the Beoil Schist, but are less rich in muscovite and garnet. Some of the psammitic ribs are similar in appearance to the Dunalastair Quartzite, others are almost as pure as the Beoil Quartzite. Towards the lower margin, these psammites become dominant, although the boundary with the quartzite is quite sharp in the area of this transect and in rare exposure elsewhere. Cross-bedding is apparent in the thicker psammites, but no way-up has been determined. Rare 50 mm-thick beds where carbonate is present in the matrix of the psammites, provide another link with the Appin Group sediments above.

In thin section, the typical semipelite is schistose and comprises biotite–muscovite–quartz–plagioclase–garnet. Some psammites have no obvious feldspar, but always some biotite; cloudy plagioclase contributes up to one third of a psammite or of the psammitic fraction of a semipelite. In many thin sections of this formation calcite is present in the matrix. The texture of the semipelite is dominated by the orientation of the micas in the S₂ schistosity, typically at a distinct angle to bedding.

Beoil Quartzite Formation (30–0 m)

Exposure of this formation is particularly good on the ridge crossed by the transect A–A′ [NN 719 580] to [NN 730 571], but generally within the area of (Figure 4) it can be traced as discontinuous 100–300 mm-thick ribs of pure white quartzite separated by layers of semipelite 10 to 500 mm thick. Thin beds (100–300 mm) of pelite of Beoil Schist aspect and of quartz-feldspar psammite similar to those in the Dunalastair Semipelite also occur within the formation. The type section for the Beoil Quartzite is on the steep south-facing scarp, 400 m west of Lochan Beoil Chathaiche [NN 741 568] and on the ridge 200–300 m to the west, where it shows the maximum thickness; here the formation has gradational boundaries with both the Dunalastair Semipelite and the Beoil Schist.

The boundaries of the formation are defined by the appearance of the ribs of pure white quartzite; between the boundaries, ribs of the quartzite form a very variable proportion (10–50%) of the dominant pelitic, semipelitic and psammitic rocks. The ribs of quartzite are very commonly affected by tight folds (D₁ and D₂; (Plate 4b) with wavelengths of several metres and it is clear that this results in considerable duplication. Feldspar and other impurities commonly produce a lamination on the scale of 1–2 mm and some obliquity of this to the bed margins is apparent; however, no certain cross-bedding in these strongly deformed rocks was detected.

In thin section the characteristic rock is a medium-grained equigranular quartzite usually with a very small amount of scattered muscovite. Sections parallel to the strong quartz rodding, which is a striking feature of this rock, show an elongation of the quartz and an alignment of the muscovite. Some sections show thin (a few millimetres) beds with concentrations of feldspar (untwinned) and heavy minerals. The interbedded semipelites and pelites are identical, in section, to those described in the Dunalastair Semipelite and the Beoil Schist formations.

Outside the area of (Figure 4) the thickness of this formation is typically 2–3 m or thinner, but it is remarkably consistent in its presence above the Grampian Group psammites, commonly as the first recognisable Appin Group formation. For instance, it is identified to the north of (Figure 4) in the Allt an Ruighe Fhliuch [NN 7095 6303] at several localities west of the road north of Glen Errochty e.g. [NN 7224 6618] (Figure 5); also south-west of Loch Errochty e.g. [NN 668 628], as a few metres of rock caught up in the late sills, in the section on the north shore of Loch Rannoch [NN 6495 5887] and in the Allt Druidhe [NN 6423 5693] to the south. To the east of (Figure 4) a metre or two of pure quartzite is seen in a tributary of the Allt Kynachan [NN 7614 5670], while in the Allt Kynachan section [NN 7775 5723] the formation appears to be represented by a few 10–20 mm ribs of quartzite in the strongly deformed semipelites. Thin sections from these quartzites reveal a very strong elongate rectilinear texture to the quartz, with long straight boundaries parallel to S₀, marked by fine-grained muscovite and feldspar.

Beoil Schist Formation (90–5 m)

This formation is well exposed within the area of (Figure 4), forming a prominent topographical feature from the River Tummel to the east end of the Speirean Ruadh ridge [NN 738 567]. The type section is the south-facing scarp west of Lochan Beoil Chathaiche below [NN 741 568] and the ridge to the west of this. The schist is characterised by an unusual concentration of muscovite and the presence of pods of vein quartz. The stratigraphically lower half, apart from these characteristics, is very similar to the Dunalastair Semipelite described above; it is a biotite-muscovite-garnet pelite or semipelite with thin ribs of quartzofeldspathic psammite and rare quartzite ribs towards the lower, well-exposed, gradational boundary. The upper half produces striking muscovite-rich schistosity surfaces studded with 1–8 mm garnets and streaked with quartz veins and pods; it is noticeably less biotitic and bedding is not as obvious. However, in Strath Fionan bedding can always be detected, resulting from subtle changes in quartz and biotite content and rarely from the presence of carbonate and/or graphite. A pronounced topographical hollow separates this formation from exposures of the Meall Dubh Striped Pelite, the upper boundary with which is best seen north of the transect B–B′.

The formation has a number of characteristic structural features, many of which it shares with adjacent formations. Thin quartz veins are subparallel to S₀ but are considered to be strictly parallel to S₁; on close inspection they may often be seen to be at a small angle to S₀ and both are folded by the common D₂ minor folds. Another striking feature is the strong rodding of the quartz veins parallel to D₂ fold hinges (here plunging moderately to the south-east), an effect enhanced by the strongly linear axes of small D₂ folds of the veins. These small folds produce an asymmetric lens-shaped thickening of the quartz veins, seen as discontinuous pods on many surfaces. The dominant surface of the rock is S₂, usually at a detectable angle to S₀ — here some 10° anticlockwise looking down the D₂ axes. Biotite, muscovite, quartz and feldspar are all lineated parallel to the D₂ and S₀/S₂ intersection lineation which is further paralleled by pressure shadows around garnet (and rare feldspar) porphyroblasts.

Thin section examination of the schists of the upper facies shows seams of large (0.8 mm) muscovites with lesser biotite crystals, anastomosing around quartz-biotite augen and the larger garnets. This schistosity (S₂) is oblique to millimetre-thick beds which may be richer in biotite and untwinned feldspar and are rarely graphitic. When large, the garnets are very rich in quartz inclusion trails, commonly exhibiting spectacular curves, and well-developed pressure shadows, both visible in outcrop. The lower facies exhibits a texture and mineralogy very similar to that already described for the Dunalastair Semipelite.

Away from the area of (Figure 4) the formation thins and achieves a more uniform, but less obviously bedded or lineated, flaggy character. It is, however, remarkably persistent (usually together with the Beoil Quartzite) above the Grampian Group psammites (Figure 5). Rocks interpreted as being part of the formation occur in all sections from the Tomintianda Burn [NN 809 606] in the north-east to the Allt Druidhe [NN 642 571] in the south-west. However, in these strongly deformed sections the formation merges imperceptibly into the flaggy semipelites and psammites below and it is often impossible to separate Beoil Schist from the Meall Dubh Striped Pelite which lies stratigraphically above.

An isolated area of banded quartz-muscovite schist with quartz lenses is poorly exposed on Torr Dubh [NN 746 631]. Here a major fold culmination has been produced by the crossing of the Clunes and Bohespic antiforms and the schist in the resulting domal structure must be younger than the surrounding Kynachan Quartzite; it is accordingly attributed to the Beoil Schist Formation.

Meall Dubh Striped Pelite Formation (30–3 m)

This formation is not well exposed. The best exposures, apart from those on transect B–B′ (Figure 4), are in the col [NN 7314 5691] between transects A and B and south of Lochan Beoil Chathaiche around [NN 7468 5640] east of the dyke; otherwise exposures are small and scattered. The junction between the Beoil Schist and this formation is nowhere well exposed. However, on the shoulder near the foot of the Beoil Schist ridge, about 200 m north of transect B–B′ [NN 7387 5680], a bench-like outcrop exposes a continuous section from quartz-veined Beoil Schist, through 3 m of pelite into actinolitic calcareous schist (Meall Dubh Limestone). Graphitic schist, more actinolitic calcareous schist and pelite are exposed below, in what is interpreted to be an isoclinal D₂ fold core. The whole section is evidently strongly deformed and the apparent transitional boundary between the Beoil Schist and Meall Dubh Striped Pelite is at least in part the result of these high strains.

The type section of this formation is at the northern end of transect B–B′ [NN 7386 5661], in a 20 m-long ridge south of the gully that separates it from the exposures described above. Here the delicately striped alternations of fine psammite (meta-silt) and schistose muscovite-biotite pelite are well displayed in a 5 m section. Garnet and feldspar porphyroblasts are characteristic of this lithology, as is grading in the 10–30 mm silty stripes. Although the upper junction with the Meall Dubh Limestone is not well displayed , the youngest beds in the section are calcareous; carbonate-rich rocks occur a few metres to the south, suggesting a transitional boundary.

In thin section of a typical striped pelite, bedding is marked by changes in the proportion of matrix quartz and biotite and the local concentration of muscovite and feldspar (mostly untwinned, but including twinned plagioclase). In addition to porphyroblasts of biotite and garnet, feldspar (untwinned) is commonly present and staurolite occurs in a thin section from exposures at Speirean Ruadh [NN 733 569]. The schistosity, in most exposures axial-planar to folds of S₀, is an intense crenulation (S₂).

Outside the area of (Figure 4), the formation can be identified with certainty only in the Allt na Moine Buidhe [NN 708 612], where 10 m of striped pelites occur east of the Meall Dubh Limestone. As explained above, the high strains at this structural level cause the Beoil Schist and Meall Dubh Striped Pelite to lose their separate identities; where exposure is complete across the succession from the Beoil Schist to the Meall Dubh Quartzite, e.g. the Allt Kynachan [NN 7790 5725], the Allt an Ruighe Fhliuch [NN 7090 6304], exposures near the road north of Trinafour [NN 725 660] (Figure 5) and the Allt Druidhe [NN 6425 5700], pelitic intercalations (probably infolds) in the tremolite schists above mark the position of this formation.

Ballachulish Subgroup

The exposures along the line of transect B–B′ [NN 7386 5661] to [NN 7432 5630] (Figure 4) give the opportunity to examine these six formations in reasonable proximity, although some of the type sections detailed below occur elsewhere in Strath Fionan.

Meall Dubh Limestone Formation (15–3 m)

There is no continuous type section for this formation, but representative lithologies occur in the scattered exposures on the south side of the ridge [NN 7386 5661], and the hollow before the next ridge [NN 7390 5625], on the line of transect B–B′ (Figure 4). Yellow-weathering grey-hearted carbonate, striped calcareous schist and tremolite-actinolite schist are all represented. Biotite or phlogopite porphyroblasts are conspicuous in the latter two lithologies; colourless tremolite and green actinolite occur locally as rosettes but may also be well orientated (parallel to D₂) on the dominant schistosity planes. It is not possible from the fragmentary exposure here or elsewhere to measure a representative section but it is clear that in some sections dominant carbonate is replaced laterally by dominant tremolite schist. Analysis shows the carbonate to be dolomite. Other than these exposures, the outcrop pattern shown in (Figure 4) is based upon scattered exposures between the col [NN 7314 5691] and the burn [NN 742 566].

Thin sections show that carbonate (presumed dolomite), phlogopite, tremolite and quartz occur in many combinations. Actinolite and rare kyanite (in transitional lithologies to the Meall Dubh Graphitic Schist) also occur. Bedding is usually marked by 5 mm concentrations of heavy minerals (especially tourmaline and sphene). Feldspar is not a common constituent.

Outside Strath Fionan, the formation is not well exposed but is persistently represented, albeit by only a few metres of rock, in the sections mentioned above for occurences of the Meall Dubh Striped Semipelite; it is not seen on the west limb of the Errochty Synform. Carbonate is present in the Allt Kinardochy [NN 7808 5725] and the Allt na Moine Buidhe [NN 708 612], but elsewhere the formation is usually represented by tremolitic and calcareous schist. It is probably represented by 1 m of thin alterations of pelite and grey limestone in the Tomintianda Burn section [NN 8085 6073] north of Loch Tummel.

Meall Dubh Graphitic Schist Formation (50–0 m)

This formation is particularly well displayed on the ridge to the east and west of the line of transect B–B′ (Figure 4). The type section is a prominent crag [NN 7409 5645] where the graphitic schist displays 30–100 mm crystals of black kyanite as well as common biotite and garnet and rarer staurolite and feldspar porphyroblasts. Bedding is revealed in cross-sections as semipelite ribs at 100–200 mm intervals, folded with the dominant schistosity axial-planar. Kyanite, biotite and pressure shadows are aligned parallel to the D₂ axes.

Good exposures, showing similar features, can be seen eastwards along the same fold limb to the edge of (Figure 4) and beyond to the wall at [NN 7555 5650]. West and north of transect B–B′, good scattered exposures are seen in the cores of the D₂ fold-pair apparent on (Figure 4), but exposure is very sparse further west.

The lower boundary with the Meall Dubh Limestone is nowhere well exposed, although isolated occurrences with admixtures of graphitic schist and calc-schists show that it must be gradational. The upper boundary is seen along strike to the east at [NN 7422 5640] on the north side of the Meall Dubh Quartzite ridge. Here graphitic kyanite-bearing semipelite and pelite merges into 1 m of transitional muscovitic, rusty, quartzite containing graphitic seams. A better section of this transitional facies is described with the quartzite below.

In thin section the S₂ schistosity planes are more than usually distinct crenulation cleavages, enhanced by the graphitic flakes on the microfolded S₁ planes. Graphite is also concentrated on the biotite-muscovite S₂ planes and around garnet and mica crystals in the quartz-rich lithons. Quartz and quartz-feldspar concentrations mark beds 5–200 mm thick; well-developed grading from semipelite to dense graphitic tops observed in outcrop have been confirmed in thin section. Kyanite is typically 2–3 mm in length and full of graphitic dust; staurolite of similar size and smaller garnets are not graphitic throughout, but contain inclusion trails of graphite and quartz. Albite porphyroblasts, up to 8 mm, have overgrown both staurolite and garnet. Large biotite porphroblasts are well-oriented, but oblique to S₂, and contain curved inclusion trails of graphite dust.

Outside the area of (Figure 4) the formation is poorly exposed in a few sections (see description of Meall Dubh Semipelite for locations), some represented only by a few centimetres of strongly graphitic schist where the quartzite is preserved above. Kyanite and staurolite have not been recorded from these sections.

Meall Dubh Quartzite Formation (180–10 m)

The exposures on Meall Dubh [NN 729 567], although greatly thickened by D₂ folding, provide many clean sections showing the detailed mineralogy, deformation and sedimentary structures of the formation. The type section of the formation is from the east side of the outcrop at [NN 737 567] across the south side of Meall Dubh and down to the road at [NN 7267 5656]. At the easternmost edge of the quartzite, at the contact with the underlying Graphitic Schist Formation, some 20 m of more schistose quartzite are muscovite-rich but non-graphitic and a final 2–3 m of a very characteristic transitional facies to the graphitic schist are developed. This latter consists of finely bedded, usually cross-laminated, fine-grained (silty) quartzite marked by black graphitic laminae. Some of the 20 mm-thick semipelitic beds exhibit grading of their graphite content. The younging is consistently away from the graphitic schist boundary, allowing for the presence of major and minor D₂ folds that affect this boundary.

On transect B–B′ (Figure 4) the upper 20 m of the Meall Dubh Quartzite may be examined on the south bank of the Allt Strath Fionan [NN 7424 5634] where 0.2–0.5 m-thick beds display the characteristic strong lineation and feldspar-rich and heavy mineral laminae; cross-bedding in 0.1 m beds youngs south towards the Strath Fionan Banded Semipelite some 20 m distant. The junction between the two formations is not exposed here, but is present further east [NN 7432 5631] on the south bank; exposure is poor but there appears to be no transition.

The quartzite typically comprises 70–80% coarse-textured quartz with the remainder of pink and milky-white feldspar occuring as 2–10 mm-long rod-shaped clastic grains, commonly concentrated in thin (50 mm) beds. Plate-like aggregates of quartz grains (up to 30 mm in maximum dimension and 5 mm thick) appear to represent highly deformed original clasts and indicate that the original rock was partly conglomerate. The 0.1–0.5 m-thick beds commonly exhibit cross-bedding of feldspathic and red heavy mineral laminae. Foreset angles are typically reduced to 10° or less by deformation, although these strongly folded rocks also exhibit exaggerated foreset angles. One prominent joint surface [NN 7286 5667] shows especially fine examples of cross-beds with ten sets showing truncated high-angled foresets (Plate 1a). Channel structures 0.5 m across occur, e.g. at the roadside [NN 7282 5649], whilst other beds up to 1 m thick, particularly in the middle of the formation, appear to be devoid of any internal sedimentary structure. Cross-bedding on the ridge above the roadside [NN 7275 5654] and to the east youngs south towards exposures of Strath Fionan Banded Semipelite, 6 m away at the road edge.

A characteristic structural feature of the quartzite is the strong linear structure on the bedding/schistosity planes, produced by the elongation of both quartz and especially feldspar grains, which here plunges south-east. The quartz grains and the less elongated feldspar grains form a strong bedding-parallel schistosity. In these exposures of folded quartzite, especially the more schistose lower facies, the schistosity is axial-planar to the folds, which are the regional D₂.

In thin section a recrystallised mosaic of 1–2 mm-long quartz grains generally encloses smaller, more spindle-shaped microcline and plagioclase. Muscovite and heavy minerals are minor constituents, both being locally concentrated on bedding planes. Large aggregates of quartz grains, free of these impurities, define original clasts; even the larger feldspar clasts (10 mm-long) remain as single crystals. The transitional graphitic quartzite shows well-oriented fine biotite in the regional (S₂) cleavage and larger biotite porphryoblasts.

Within the area of (Figure 4), but outside the major D₂ fold-pair of Meall Dubh, the quartzite is moderately well exposed on the ridges that run east from [NN 732 565] and north-west from [NN 727 569]. Good cross-bedding occurs, particularly on the western ridge, but the lower transitional facies is thinner and less well exposed. On the line of transect B–B′, the lower junction crops out [NN 7422 5640] on the north side of the quartzite ridge, where graphitic kyanite-bearing semipelite and pelite merge into 1 m of transitional muscovitic, rusty, quartzite containing graphitic seams.

As the ridges are followed to the eastern and western margins of (Figure 4) and beyond, where the rocks are more strongly deformed, cross-bedding becomes increasingly difficult to identify or interpret and the pebble stretching becomes stronger; good examples of the latter occur, particularly in loose blocks, in the bank south of the Allt Kinardochy [NN 7789 5727]. The quartzite has not been identified beyond this to the east.

To the west, around the Grampian Group margin, the formation can be confidently identified as 10 m of glassy, feldspathic quartzite both in the Allt na Moine Buidhe [NN 7077 6112] and in the Allt Druidhe at the western edge of the district [NN 6424 5694]. It is absent from other exposed sections in the Trinafour area, while it is represented by a few metres of rock along the western outcrop, usually caught up in the porphyry sills, as on the shore and hillside above Loch Rannoch [NN 6500 5884].

Strath Fionan Banded Semipelite Formation (60–0 m)

Exposures of the Strath Fionan Banded Formation along the Allt Strath Fionan ((Figure 4), transect B–B′) are typical of the usual, rather friable, slightly rusty-weathering, muscovitic, interbedded pelite and semipelite. Four metres of schistose carbonate mark the transitional upper junction of the pelite with the succeeding limestone in the steep south bank of the burn. The exposures from [NN 7424 5634] to [NN 7432 5631] along the burn comprise the type section, as they provide the most complete section, although better quality individual exposures are described below.

Typical lithologies occur to the west of the south end of transect B–B′, along the roadside as far as the quartzite junction described above [NN 7275 5656]. Bedding is usually a prominent feature on a 10–100 mm scale and broken or cut specimens reveal a wealth of sedimentary structures, in particular channelled cross-laminations and small-scale grading. Sedimentary structures consistently indicate younging to the south; those by the road [NN 7275 5652] are 2 m from the quartzite junction. Biotite, but not garnet, is usually evident, parallel to a strong D₂ intersection lineation.

In thin section the rocks are typically muscovite-biotite-quartz semipelites and pelites; the quartz is exceptionally fine-grained (0.1 mm) and the rocks are not conspicuously feldspathic nor graphitic, but are rarely calcareous and pyritic. Thin sections of graded units from the bottom of the formation reveal a fine-grained quartz-biotite fabric becoming richer upwards in biotite, muscovite and opaque minerals; cross-laminations are biotite-rich.

Beyond the exposures described above and outside (Figure 4), the formation can rarely be identified with confidence; it is certainly present to the east, on the south bank of the Allt Kinardochy [NN 7789 5727], and north-westwards in the Allt na Moine Buidhe [NN 7076 6110]. North of this, up to the Errochty Synform closure the Strath Fionan Banded Semipelite and the succeeding Pale Limestone are poorly represented (Figure 5). The disappearance of this formation marks the start of a major hiatus on (Figure 1) along the outcrop of the Appin Group on the west limb of the Errochty Synform, between the lower formations of the Appin Group (usually the Beoil Schist, but rarely thin representatives of the Meall Dubh Limestone, Graphitic Schist and Quartzite) and the middle Argyll Group (Killiecrankie Schist or Carn Mairg Quartzite). As explained in Chapter 3, this hiatus is interpreted as tectonic.

Strath Fionan Pale Limestone Formation (20–0 m)

The southern end of transect B–B′ (Figure 4) provides the only opportunity to examine an almost continuous section from the Strath Fionan Banded Semipelite Formation through the Pale Limestone into the Tullochroisk Semipelite Formation at the top of the bank. However, the quality of the exposure is poor and the latter two formations are better seen in the northern end of transect C–C′. This transect [NN 7273 5675] to [NN 7160 5658] has been chosen to permit the examination of all the remaining formations of the Appin Group.

The type section for the Strath Fionan Pale Limestone Formation is at prominent crags south of the road [NN 7275 5638]. Although the base of the formation is not seen here, the crags expose 20 m of white, almost pure, dolomite-tremolite rock. Bedding can usually be detected as 2–3 mm-spaced slightly siliceous laminae; other beds are conspicuously muscovite-rich and phlogopite is a minor constituent. The lower beds (to the east) are noticeably more tremolite rich, some exhibiting rosettes 0.1 m across. Carbonate, tremolite, phlogopite and muscovite all contribute to a strong (D₂) mineral lineation plunging south-east. At the west end of this locality the sharp upper junction with the Tullochroisk Semipelite is seen.

Thin sections show a dolomite fabric (c.3 mm long grains) together with phlogopite, weakly oriented in S₂; tremolite laths are less strongly oriented. Concentrations of quartz, muscovite and phlogopite mark bedding.

Similar rocks are seen at the start of transect C–C′ on the north bank of the burn [NN 7227 5650], although the rocks are more pelitic and the alignment of dolomite and phlogopite in the S₂ cleavage, oblique to the quartz-rich beds, is more evident. The junction with the Tullochroisk Semipelite can be located here to within a metre.

Within the area of (Figure 4), good sections of the Strath Fionan Pale Limestone may be seen at various points down the burn that drains Lochan an Daim e.g. at [NN 7175 5757]; [NN 7156 581] and downstream, but it is not exposed in the Allt na Moine Buidhe and is not present in any section to the north or west. To the east of the exposures described, it is well exposed as 5–10 m of quartz-phlogopite-tremolite white dolomite in the Allt Kynachan, first in the bed of the burn [NN 7707 5689] and then along the steep south bank to [NN 7776 5720]. The formation is noticeably more schistose and phlogopitic in these exposures and in thin section the amphibole is seen to be more actinolitic. The rocks at [NN 7776 5720] have graphitic stylolites developed. Junctions with semipelite above and below are exposed in some places, but the lack of clear bedding and the schistose nature of the rocks indicate strong deformation.

Tullochroisk Semipelite Formation (200–0 m)

This formation is well exposed on or near the line of transect C–C′ (Figure 4) as well as in the bed and banks of the burn draining Lochan an Daim. The sharp lower boundary is well exposed above the type locality of the Pale Limestone [NN 7275 5638] but the lower half of the formation is best seen on the slope and ridge to the south of the Pale Limestone [NN 7227 5670], where it is certainly considerably thickened by the D₂ folding as illustrated by the outcrop pattern of the Pale Limestone (Figure 4). The section in the burn to the south-west [NN 7235 5662] to [NN 7223 5663], is the type section of the lower half of the formation. The upper half of the formation has its type section on the line of transect C–C′, where the upper junction is well seen.

The dominant lithology of the lower half of the formation is a rusty-weathering semipelite with psammitic beds, not unlike the Strath Fionan Banded Semipelite, although the Tullochroisk Semipelite contains a greater proportion of silty and sandy material than the older formation. It is generally a non-graphitic, but pyritic, muscovite-biotite schist, with silty laminae 1–5 mm thick and quartzose psammite ribs 100–200 mm thick. Small-scale cross-laminations and graded bedding have been observed younging south near the lower boundary with the Strath Fionan Limestone. The dominant S₂ planes are oblique to bedding and minor D₂ folds and intersection lineations are common.

Thin sections reinforce the similarities of the lower half of the formation with the Strath Fionan Banded Formation. The dominant lithology is a schistose quartz-muscovite-biotite semipelite with small garnets in some pelitic beds; graphite and carbonate are not significant, but opaque minerals (presumed mainly pyrite) are common. Cloudy feldspar occurs as local concentrations in thin beds and in the thicker psammites.

A feature of this lower part of the formation, however, is the variation that takes place within the semipelite along strike. At the east end of the exposures e.g. at [NN 7278 5636] beds of muscovitic quartz-psammite occur within the semipelite, up to 600 mm thick in a 12 m-long stream section; loose blocks show 200 mm-thick cross-laminated beds. Similar quartz-psammites occur another 80 m south-east, but such a concentration of psammite is certainly not present 100 m along strike to the north-west. At two localities in and around the burn that drains north-west from Lochan an Daim, extremely finely laminated semipelites occur immediately above the Pale Limestone. At the first locality [NN 7169 5759] the more or less quartz-rich muscovite-biotite phyllites exhibit very clear younging to the south-west from 10 mm cross-laminated and graded beds; thin sections show the exceptionally fine-grained minerals (c.0.1 mm) to be strongly oriented in the S₂ cleavage, oblique to bedding. At the second locality [NN 7154 5810] immediately west of the Pale Limestone, the finely laminated beds are here intensely graphitic; more typical semipelites occur on the wooded ridge to the north-west of this locality. At several locations, beds in this formation are strongly pyritic, e.g. underneath the Dark Limestone by the road [NN 7182 5714].

The upper half of the formation is distinctly more graphitic than the typical lithology of the lower half; locally it is as graphitic as the Blair Atholl Formation dark schists above, usually containing small garnet, staurolite and kyanite crystals, but it is characterised by large (c.5 mm) biotite porphyroblasts. Silty laminations (2–3 mm) and psammititc ribs (0.1–0.2 m) are usually present. This change of character is marked by a gully between [NN 7226 5656] and [NN 7220 5673] near the line of the transect C–C′, and by a grey limestone (maximum thickness 8 m) between [NN 7235 5646] and [NN 7278 5636] to the east (above the Pale Limestone type locality previously described). Further east, the limestone cannot be positively identified probably due to poor exposure but all formations are thinned by stronger deformation in this direction. Within the area of (Figure 4), the formation is only seen to the east of the exposures described at [NN 7278 5636] in a steep bank beneath the road at [NN 7454 5622]. Here, 12 m of rusty semipelite, typical of the lower half of the formation, occur beneath the grey limestome of the succeeding formation.

East of (Figure 4) graphitic and rusty semipelite crops out below the forest around [NN 7573 5645] and in several localities around the burn below [NN 7679 5680], west of Tombreck. Thin, very schistose, rusty semipelites also occur on the steep south bank of the Allt Kynachan as far as [NN 7778 5722] between the pale and grey limestones. The 1902 field sheets show exposures of 'siliceous flags' between grey limestones at [NN 7590 5635] in now thickly forested ground. The formation crops out 1 km to the east in the burn [NN 7683 5666], where flaggy semipelites with thin pure quartzites occur at the top of a small gorge in grey limestone. These exposures are interpreted as Tullochroisk Formation lying within a fold closure.

The thin grey limestones and more or less graphitic schists, which are interfolded with the exposures of semipelite and thin flaggy quartzites in this area east of (Figure 4), are interpreted as repetitions of the upper half of the Tullochroisk Formation. However, it is impossible in this poorly exposed area to positively attribute these limestones to this or the succeeding Blair Atholl Formation.

The possibility is discussed below that immediately to the west of the type section, the thin limestone that separates the two halves of the formation branches off the lower limestone of the Blair Atholl Formation and that the upper, more graphitic, part of the Tullochroisk Formation is here only a local development.

As described above, the formation as seen in the north-west part of (Figure 4) is typically represented by a rather laminated silty semipelite in the scattered exposures in the D₁ fold cores east of Tullochroisk. This lithology is typical of the rare exposures of the formation seen to the north, as far as the Errochty Synform closure (Figure 5); the best of these is in the Allt na Moine Buidhe [NN 7074 6109].

Blair Atholl Subgroup

Blair Atholl Dark Limestone and Dark Schist Formation (120–0m)

This formation comprises three members: two grey limestones, enclosing a thicker intensely graphitic schist. Again this formation is most fully developed in central Strath Fionan, where it is well seen on the line of transect C–C′ (Figure 4) between [NN 7200 5665] and [NN 7194 5654], which is the local type section. The two limestones are persistent for some 10 km along strike from Trinafour in the north to Braes of Foss in the east. Both the graphitic pelite and one grey limestone can be traced, progressively thinning, around the Errochty Synform as far as the north shore of Loch Errochty (Figure 5). Three kilometres to the north-west of Trinafour the whole formation is tectonically excised. To the east of Braes of Foss, where the two limestones are tightly interfolded with the schist, the severity of deformation renders it impossible to identify the two limestones separately. The formation also occurs in the closures of the D₁ Beinn a' Chuallaich Anticline and in the D₂ Creag an Earra, Allt Mor and Tom Beithe folds south-east of Braes of Foss, as well as east of the Loch Tay Fault, to the north of Loch Tummel (Figure 1).

The separate identity of the two limestones within the Blair Atholl Formation is important to the structural interpretation of the district, and depends upon mapping and stratigraphical evidence as well as the structural observations which are detailed in Chapter 3.

Grey limestone members

The two limestones are well exposed on the line of transect C–C′ (Figure 4) about [NN 7200 5665] and [NN 7194 5654] respectively. The junctions with the adjacent formations and the intervening graphitic schist are only rarely exposed; they are sharp with little sign of interbedding. Both limestones show essentially the same character; they are grey- to cream-weathering, generally grey-hearted granular calcite 'marbles', each with a maximum thickness of 15 m. The limestones contain a little muscovite and are seldom free of 1–3 mm ribs of quartz and other impurities (Plate 1b) which occur at about 10–200 mm intervals. Local thin beds or patches within the limestones are pure white calcite, free of graphite and other impurites. The laminations commonly exhibit folds of 0.25 m and larger wavelength and truncations usually result on the stretched limbs; such a tectonic cause is thought to be responsible for the simulated cross-bedding that is often observed. A cross-cutting fabric of weakly to strongly aligned calcite grains is another feature of the limestones in Strath Fionan, although elsewhere a bedding-parallel fabric is usual. Both limestones in Strath Fionan contain clearly intrusive sills of amphibolite, 1–2 m thick but strongly boudinaged.

Within the area of (Figure 4) the two limestones may be followed from transect C–C′ via excellent exposures to those in the woods east of Tullochroisk [NN 712 579], where the lower limestone can be followed around a tight D₁ fold-pair. West of Lochan an Daim [NN 715 574] a thin unit of limestone appears to branch south- eastwards from the lower limestone; this disappears in 500 m but within another 500 m it is replaced at the same level by the grey limestone mentioned above within the Tullochroisk Semipelite. If this is so, then the upper, more graphitic, part of the Tullochroisk Semipelite is cut out and the reduction in the total thickness of the formation northwards is explained.

Graphitic schist

Adjacent to the line of transect C–C′ (Figure 4), this is a unit, up to 90 m thick, of black somewhat rusty-weathering, intensely graphitic schist. It makes a strong positive topographical feature, particularly in contrast with the grassy hollows which usually mark the two bounding limestones. Thin laminations, silty ribs or rare calcareous seams usually distinguish bedding, which lies slightly oblique to the dominant muscovite-biotite S₂ schistosity. Grading occurs in 10–30 mm silty beds. Garnets up to 5 mm in diameter are commonly present in certain beds; staurolite of pin-head size and dark kyanite crystals up to 50 mm long are usually evident. The latter are only weakly aligned in the schistosity and the D₂ lineation. Quartz veins, parallel to S₂ and lineated parallel to the D₂ fold axes, and amphibolites, up to 5 m thick, are characteristic of most sections; the latter commonly exhibit boudins which are associated with quartz-feldspar-mica segregations. One of these segregations, near the line of transect, was found to contain crystals of zoisite (XRD identification) up to 100 mm long.

In thin section, S₀ is marked by quartz-rich, not notably feldspathic, 2–10 mm beds with variable mica content; biotite dominates over muscovite and together with the graphite dust may be concentrated at the top of graded units. Opaque minerals are not as prevalent as in the semipelitic formations below, but are concentrated in strings. In the more schistose lithologies, the mica (with graphite) and quartz are segregated into lithons parallel to S₂ and oblique to S₀. Biotite is the dominant porphyroblast, commonly reaching 10 mm; small garnets are ubiquitous and the common staurolite and kyanite reach 2–3 mm; large andesine feldspar porphroblasts commonly enclose both garnet and staurolite.

Other outcrops

Away from the type area ((Figure 4) up to Trinafour in the north) the two limestones and the graphitic schist are seen in most of the stream sections and on Meall na Moine [NN 707 636] and Meall Ban [NN 712 642]; only the lower limestone is reduced in thickness. The upper limestone and graphitic schist of the much reduced formation are quite well exposed further north on the ridges west of the road e.g. [NN 724 659], around the Errochty Synform closure as far as the north shore of Loch Errochty (Figure 5), beyond which the formation is tectonically excised.

To the east of transect C–C′ excellent sections of the Blair Atholl Limestone and Dark Schist Formation also occur in and around the two burns that drain north from Schiehallion to reach Strath Fionan [NN 725 564], but further east the thickness of the formation reduces rapidly and exposure is sparse. A good, convenient exposure occurs in a steep bank below a road [NN 7454 5622], comprising 12 m of rusty semipelite (Tullochroisk Formation) overlain by 6 m grey limestone and 6 m graphitic schist.

All the above-mentioned exposures lie within the right-way-up succession of the lower northern limb of the D₂ Balliemore Antiform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). However, Bailey and McCallien (1937) and Rast (1958a) considered that a single grey limestone was repeated about an isoclinal fold in these outcrops; Treagus and King (1978) and the present work, on the other hand, show that the difference in character and sedimentary structures within the formations below and above can be used to demonstrate clearly the continuity of the succession and the presence of two limestones.

The upper sections in the two burns on the north face of Schiehallion around [NN 723 562], and the ridges on either side, provide outcrops on the southern limb of the D₂ Balliemore Antiform; the two ridges of the upper limestone can be walked westwards to west of Tom na Fuine [NN 709 563], where they close in the Beinn a' Chuallaich D₁ Anticline. Exposures of the lower limestone in the core of this fold occur in the burn to the east [NN 731 560] and ridges to the east of that; however, evidence for the separate identity of these limestones is based on very limited structural observations and not principally on stratigraphical evidence.

Further to the east, the graphitic schist as well as the interfolded limestones are exposed in two burns east of Braes of Foss [NN 752 559], near the Tummel Bridge road north from [NN 784 572] and sporadically in the ground between. In this area there are several parallel limestone units and it is not always possible to distinguish the upper and lower limestone members; the graphitic schist unit between is greatly reduced in thickness and very fissile, and there is commonly a repetition of limestone and schist on a 1–2 m scale. The near-parallelism of bedding and the strong S₂ schistosity in these outcrops indicates that the members are highly deformed and probably interfolded by minor isoclines; however, the possibility of sedimentological changes cannot be ruled out, particularly with the apparent absence from the sequence of the Tullochroisk Formation or the higher Cnoc an Fhitich Formation.

The limestones and graphite schist in the cores of the D₂ Creag an Earra Antiform, Allt Mor Synform and on the east limb of the Sgurran Geal Antiform are less accessible. The limestone of the first is well exposed on the east side of Schiehallion [NN 747 546] and in the Allt Mor, e.g. [NN 7220 5325]. Thick (c. 30 m) limestones in the core of the Allt Mor Synform are well exposed, with some intervening graphitic schist, on two ridges at [NN 758 546] and [NN 754 536], here interpreted as representing the lower and upper limestones respectively; the exposures to the north between [NN 761 548] and [NN 767 557] are now heavily forested, but the exposures recorded on the 1902 field maps suggest alternations of thin limestone and graphitic schist. Poor, but convenient exposures of thin limestones and schist occur near the Tummel Bridge road [NN 781 567] and in the adjacent Allt Kinardochy. Limestone exposures on the east limb of the D₂ Tom Beithe Antiform in the valley [NN 790 573] indicate a single unit at least 50 m thick; it has graphitic schist above and is thus interpreted as the lower limestone.

To the west and north of (Figure 4) the formation is poorly exposed in the core of the Beinn a' Chuallaich Anticline as far as [NN 6905 5885], north of the River Tummel.

Cnoc an Fhithich Banded Semipelite Formation (150–0 m)

There are two typical facies in this formation, well exemplified on and around the southern end of transect C–C′ (Figure 4), which is the type section [NN 7190 5653] to [NN 7167 5652]. The dominant facies is a banded alternation of rusty semipelite with 50–100 mm psammites, in approximately equal proportions, quite similar to parts of the Strath Fionan Banded Semipelite and Tullochroisk Semipelite formations; rarely psammites reach 1.5 m in thickness and appear to be internally featureless.

The other facies is a striking alternation of whiter, quartz-rich psammites with dark biotitic, slightly graphitic, graded pelite, similarly on a 50–100 mm scale; this facies comprises perhaps 25% of the upper part of the formation in the Strath Fionan area, where it probably represents channels in the more pelitic facies. Biotite porphyroblasts aligned parallel to the D₂ lineation are a feature of these rocks. The semipelites of these facies usually exhibit a strong development of micas in S₂, oblique to the bedding surfaces, a marked D₂ lineation and frequent D₂ minor folds.

Both these facies are rich in sedimentary structures; the semipelites and pelites commonly exhibit graded bedding and cross-laminated tops, while the psammites may display grading, channelled bases and, very characteristically, sedimentary dykelets. The latter are almost always injected downwards and are strongly folded. Towards the middle part of the formation thick (up to 4 m) white quartzites appear in the succession, in association with the psammites and rusty semipelites; these also display considerable lateral variations in thickness, but seem to be internally structureless and ungraded.

The lower junction of the formation is not well seen on the line of transect C–C′, but elsewhere, e.g. south of the road 500 m west of Lochan an Daim [NN 712 573], and on the Lassintullich Forestry road [NN 698 570], there is interbanding with thin grey limestone of the Blair Atholl Formation, and the semipelites are calcareous. The upper boundary can be located approximately at the end of transect C–C′ around [NN 716 566], but is better seen to the west on the slopes of Cnoc an Fhithich [NN 708 568], at the western end of Creagan Geur [NN 709 566], and east of the Drumchastle Pale Limestone locality north of the River Tummel around [NN 688 588]; interbanding of the two formations is again evident.

Thin sections of the semipelites show them to be commonly schistose, quartz-rich and to contain both muscovite and biotite, with opaque minerals, rare small garnets, and rare carbonate; plagioclase content varies from 10 to 50%. Biotite is usually present as large porphyroblasts and increases as a matrix mineral at the expense of muscovite in the more feldspathic beds. Of the psammites examined, biotite and muscovite comprise 25% and feldspar comprises up to 30%; feldspar in the semipelites and psammites is dominantly cloudy untwinned plagioclase, but the top of one graded unit has a concentration of small twinned plagioclase and microcline. The white quartzites contain up to 20% plagioclase with a little biotite. The graded pelites are muscovite-quartz rocks with biotite and graphite increasing to the top of the units; twinned plagioclase porphyroblasts may be developed in these beds. Heavy minerals, especially tourmaline, zircon and apatite, are usually concentrated as stringers in this formation.

Apart from the outcrops around the line of transect C–C´, other well-exposed sections on the northern limb of the Balliemore Antiform occur further west on the lower slopes of Creag an Fhithich around [NN 713 567] and near the road around [NN 711 574]. Complementary outcrops on the upper, southern, limb of the antiform occur towards Schiehallion, in the Allt Leathan [NN 7168 5627] and along the ridge to the west, and across the D₁ Beinn a' Chuallaich Anticline on the next ridge around [NN 710 560]. Similar transects across these two folds occur up the two burns to the east which, as mentioned before, also provide excellent sections of the Blair Atholl Formation limestones and schist.

The Cnoc an Fhithich Formation is poorly exposed in the east of (Figure 4); at the east margin, around [NN 7465 5590], it is represented by scarcely a few metres of schistose semipelite. This reduction in thickness, sympathetic to similar reductions in adjacent formations, is maintained around most of the folded boundaries to the east and south-east of (Figure 4) and is attibuted to high strains during both D₁ and D₂ deformations. In contrast, in the now thickly forested ground west of Loch Kinardochy between [NN 768 554] and [NN 763 545] on the east limb of the Allt Mor Synform, the 1902 field maps show a substantial thickening to several hundred metres, but this is probably due in part to thickening by D₃ folding, and in part to the intrusion of numerous amphibolite sheets in this area.

To the west of (Figure 4) the formation is excellently exposed in the core of the D₁ Beinn a' Chuallaich Anticline from the ridge east of the East Tempar track [NN 697 567] to the spine running north from Dunalastair Water [NN 690 590 and northwards]; younging towards the Drumchastle Pale Limestone on the east and west limbs is particularly well displayed by the sedimentary structures in the latter localities. The formation continues to be moderately well exposed on the lower limb of the Balliemore Antiform to the north, e.g. Allt na Moine Buidhe [NN 705 612] and Errochty Water [NN 7245 6480] but exposure is poor as it diminishes in thickness around the Errochty Synform closure and it is nowhere exposed down the western limb of that fold (Figure 5). The tectonic nature of the thinning is confirmed by the absence of sedimentary structures. Elsewhere, the formation is only present, in a strongly deformed state, in the core of the Creag an Earra Synform around [NN 705 535] east of Creag an Earra and on the south limb of the Allt Mor Synform south of Loch Kinardochy around [NN 765 547].

Drumchastle Pale Limestone Formation (30–0 m)

The type section has been chosen outside of the area of (Figure 4), near Drumchastle [NN 6882 5881] where the formation occurs in proximity to its neighbours. The typical calcareous 'honeycomb' rock of the lower part of the Schiehallion Boulder Bed (present 5 m west of the above locality) is separated from the Drumchastle Pale Limestone by two garnet-amphibolite bands. A 5 m-thick bed of biotite calc-pelite at the top of the formation is a transitional facies and is followed eastwards (stratigraphically downwards) and downhill by green tremolite-biotite limestone, pale dolomitic limestone, tremolite-biotite pelite and finally pale buff granular limestone. Much of the limestone has been quarried. The lower junction of the formation with the semipelite and quartzose psammites of the Cnoc an Fhithich Banded Semipelite is unusually sharp at this locality; elsewhere, it is usually marked by transitional calc-muscovite-biotite pelites. The thickness of the formation here is 25–30 m but further up the hill to the north the formation can be seen to thin to 2 m. Thin representatives of the formation may also be seen nearby on the east limb of the Beinn a' Chuallaich Anticline [NN 6894 5990].

Thin sections of the tremolite schist show grains of dolomite, together with phlogopite, tremolite and quartz, weakly oriented in S₂, very similar in make-up to the Strath Fionan Pale Limestone.

This formation is widely distributed within the area of (Figure 4) but exposure is generally poor and there is no good cross-section. However, the formation may be conveniently seen at the end of transect C–C′ described above and is particularly well exposed in the waterfall of the Allt Leathan [NN 7170 5635] above this, on the opposing limb of the Balliemore Antiform. It can also be seen in the core of this fold [NN 7230 5623] if a traverse is being made up either of the two burns that drain north from Schiehallion. It is not well exposed on the south limb of the Beinn a' Chuallaich Anticline at the top of these burns, or elsewhere on the northern shoulder of Schiehallion. The formation is also seen in the closure of the Balliemore Antiform further north around [NN 708 572] south of Crossmount, and exposures at the west end of Creagan Geur [NN 709 566] provide the best examples in proximity to the adjacent formations.

To the east of (Figure 4) and west of the Loch Tay Fault, the formation has been mapped from the few outcrops of white limestone and tremolitic and carbonate schist on the limbs of the various folds of the Argyll–Appin Group boundary; it occurs, together with calcareous Boulder Bed, on the two limbs of the Creag an Earra Fold by the Tummel Bridge road [NN 7834 5714] and in the adjacent Allt Kinardochy [NN 7803 5672]. Convenient exposures of the formation have been made on a new forest road at [NN 7688 5453] in the core of the Allt Mor Synform, again in association with outcrops of Boulder Bed.

To the west of (Figure 4) tremolite schist occurs between banded schists of the Cnoc an Fhithich Formation and calcareous Boulder Bed west of the wall running up from East Tempar e.g. [NN 697 567]. North of the River Tummel the characteristic white limestone and calc-schists are well displayed north of the type exposure of the Drumchastle Formation on the west limb of the Beinn a' Chuallaich Anticline, and on the east limb of that fold [NN 6888 5888]. North of (Figure 4), exposure is very sporadic up to Trinafour, where good exposures occur in the Errochty Water [NN 7243 6488] and the burn immediately to the north [NN 7233 6507]. The northernmost exposures are just within the area of (Figure 5) [NN 7200 6604]. The formation is not seen beyond the Errochty Synform closure.

North of Loch Tummel

West of Loch Tay Fault

The rocks of the area between the Grampian Group and the western branch of the Loch Tay Fault are poorly exposed and affected by minor faulting. They are confidently referred to the Lochaber and Ballachulish subgroups of the Appin Group, although precise correlation is very tentative. Comparison with the abbreviated representatives of these subgroups in the Allt Kinardochy in the area described above is very striking. Bailey (1925) correlated these rocks with part of the Blair Atholl 'Series' or Subgroup. Smith (1980) correlated the rocks with Bailey's Banvie Burn Belt (Glen Banvie Series of Harris and Pitcher, 1975) which occur to the north-east of the district, considered again to be correlatives of the Blair Atholl Subgroup.

The principal section exposed, in the Tomintianda Burn from [NN 8083 6073] to [NN 8091 6059] is described below; the only other section occurs in the Allt Ghramaic west of Loch Bhac [NN 821 624]. Downstream of [NN 8073 6079] 30 m of flaggy (4–20 cm partings) psammites with alternating biotitic semipelite and pelite, interpreted as part of the Kynachan Psammite Formation of the Grampian Group, are highly strained in the Boundary Slide Zone. They are overlain by 3 m of steel grey muscovite-garnet pelite, which is correlated with the Dunalastair Semipelite. This is followed, after a small gap, by 1 m of pure quartzite, which is correlated with the Beoil Quartzite. A 1.2 m-thick grey limestone (possibly Meall Dubh Limestone) is in fault contact with both this quartzite and downstream with a 20 m repetition of flaggy Grampian Group psammites and pelites. The junction with the succeeding 1.5 m garnet-mica pelite is not exposed, but the pelite is followed conformably by 20m of grey limestone. The pelite has a similar appearance to the deformed Beoil Schist/Meall Dubh Pelite in the Allt Kynachan and the limestone would thus most logically correlate with the Meall Dubh Limestone. A further 30 m of faulted, brecciated and veined grey limestone follows downstream which is interpreted to belong to the Blair Atholl Limestone within the Loch Tay Fault zone (see below).

East of Loch Tay Fault

Blair Atholl Dark Limestone and Dark Schist Formation

There are two areas of outcrop of the Appin Group north of Loch Tummel, both belonging entirely to the Blair Atholl Dark Limestone and Dark Schist Formation of the Blair Atholl Subgroup. The first is the large, but very poorly exposed, area north of the Killiecrankie Slide in the north-east corner of the district. The principal outcrops are on Conbhar [NN 8263 6130], in the Allt Bhaic downstream from Loch Bhac [NN 8254 6256] to [NN 8286 6326], and on ridges north of Druim Fada [NN 8353 6314]. These are dominantly of grey crystalline limestone with rare graphitic schist, representative of the Blair Atholl Dark Limestone and Dark Schist Formation. The outcrops of grey limestone that are sporadically exposed in the Loch Tay Fault zone (see Chapter 3), from Loch Tummel shore [NN 8045 5966] to north of Tomintianda [NN 8117 6075], are almost certainly the same horizons displaced to the south-west in the fault lens.

The other outcrop of the Blair Atholl Subgroup is confined to the area between Tomintianda [NN 810 607] and the west side of Creag Mhor [NN 827 603], south of the Killiecrankie Slide. The rocks are poorly exposed in the core of the Balnabodach Fold, and were previously attributed to the Blair Atholl 'Series' by Bailey (1925) and, after remapping, by Smith (1980). Although the discontinuous exposure sometimes makes attribution to a particular formation difficult, most of the rocks may be confidently correlated with the Blair Atholl Dark Limestone and Dark Schist Formation.

It has not proved possible to identify the two (and locally three) stratigraphically distinct limestones of the Strath Fionan area. The area more closely compares with that to the east of the Braes of Foss; of the total exposure in the area, 15% is discrete grey limestone, 55% dark schist, and the remainder an interbanded mixture of the two.

Within the area shown on the map as grey limestone, exposure is moderately good and complemented with numerous sink-holes and areas of good pasture. Good outcrop of discrete limestone units, 4–20 m thick, occurs north of Craig Balnabodach [NN 8255 6055], south-west of Creag Mhor [NN 8265 6028], and around Tomintianda [NN 8120 6061]; these may be the same horizon repeated by folding. Other limestone horizons, 0.5–2 m thick, occur within the dark schists locally on the south slopes of Creag Mhor. The dark grey crystalline limestones with local 10–20 mm graphite-rich schist laminae are identical to those described above.

Numerous small exposures of the dark grey biotite schists mark much of the remainder of the outcrop of the formation. A western area is seen between Tomintianda and Balintuim especially around the track [NN 816 607], and an eastern area around Craig Balnabodach [NN 825 604]. The slightly graphitic schist shows the characteristic rusty weathering, small garnet porphroblasts and poor bedding structures as in the Strath Fionan area already described; however, kyanite and staurolite have not been recognised. Other exposures show a fine interbanding on the centimetre scale of the dark limestone and schist.

Cnoc an Fhithich Banded Semipelite and Drumchastle Pale Limestone formations

About 30% of the exposure consists of a rather different facies from that in the Strath Fionan area; this is a fine interbanding of dark schists and psammites (~1cm-thick) and of creamy weathering dolomitic limestone beds (10–25 cm thick). The carbonate in thin section is a mixture of calcite and dolomite with minor quartz, muscovite and biotite. Beds of this facies in the upper part of the formation exhibit a coarse (0.2–1 mm) gritty quartz texture. Units of this facies occur throughout the formation; good examples are exposed on Creag Mhor cliff [NN 8265 6028] with three distinct units. At this locality there is a bed of some 20 cm thickness of dark limestone with creamy-dolomite streaks, immediately below the Schiehallion Quartzite, which could be a deformed remnant of the Pale Limestone or of Boulder Bed, which are generally missing in this area.

The rocks of this facies also occur as distinct pods and lenses (less than 4 m thick) within the dark schist e.g. north-east of Creag Mhor [NN 8285 6051], and pods of dark limestone and/or schist also occur within units of this facies e.g. [NN 8265 6048]. One lens of this facies, together with infolded Boulder Bed, occupies an isolated D₁ fold-core, some 400 m long centred about [NN 8304 6029] to 400m east of Creag Mhor. Bailey (1925) considered this facies to be part of his 'pale group' immediately beneath the Boulder Bed; if this is the case there must be considerable interfolding and mechanical disruption at the top of the formation.

Sedimentary environments

The sedimentological conditions of the deposition of the Appin Group within the district, as well as in the context of the Dalradian basin as a whole, are discussed below. Many of the suggested sedimentary environments are based upon Anderton (1985).

The original sedimentary contact between the Grampian and Appin groups was suggested to be a unconformity by Grant Wilson (in Barrow et al., 1905, p.9), partly as a result of a misreading of the succession as well as of the mutual ages of the two groups. Anderson (1923, p.436) and Bailey and McCallien (1937, p.105) in their assertion that the approximate boundary between the two Groups is a dislocation (the Boundary Slide), dismissed the possibility of an unconformity on the grounds that there is no conglomerate present and that the (structural) relationships on the two sides are incompatible with that process. Although this study supports the concept of the Boundary Slide, a disconformable surface at about the stratigraphical level of the boundary between the two Groups cannot be dismissed unless the structural discontinuity, and the high strains that are associated with it, can be restored.

The recent resurvey suggests that, although the Boundary Slide Zone is responsible for some thinning and local excision of formations, it is difficult to appeal to mechanical dislocation ('sliding') as a mechanism for the removal of substantial parts of the succession. There has been no mechanical excision of any substantial part of the succession in the Strath Fionan area, but individual formations within the Appin Group are cut out elsewhere to the north and east. In a later section this circumstance is explained by D₁ thinning of part of the succession, further accentuated by D₂ folding and sliding (high strain and local dislocations). However, it is considered that this tectonic thinning was focussed on the ductility contrasts provided by a thinly bedded sequence and one or more disconformities cannot be discounted as part of this scenario.

Two other points are relevant to the palaeogeography of the Appin Group. There is no correlation between loss of sedimentary detail, e.g. cross-lamination, and reduction in thickness, either in the Appin Group or in the lower part of the Argyll Group and the upper part of the Grampian Group on either side. The Schiehallion Quartzite immediately south of the Appin Group of the Strath Fionan area, as well as the Tummel Subgroup to the north, suffer similar but even more dramatic thickness changes; these changes seem unlikely to be solely a result of deformation, but may rather have a common sedimentary origin. These observations are reinforced by more regional comparisons. To the west of the district, the Appin Group shows a dramatic thickness change, but very little facies variation, from the type area to the Strath Fionan area (Figure 6). To the east of the district the variation between the thicknesses seen in the type area and in the Strath Fionan area (and the thinned equivalents of the Allt Kynachan section where affected by high strains) are maintained through the Blair Atholl (Smith and Harris, 1976) and the Braemar (Upton, 1983) areas, as well as the Tomintoul to Banffshire coast outcrop (Treagus and Roberts, 1981). Fundamental facies changes only become apparent in the latter area and become more pronounced in Shetland (Flinn et al., 1972); otherwise the sedimentary character of the Appin Group remains remarkably persistent for several hundred kilometres along strike throughout the Irish and Scottish outcrop. Again, on a regional scale, the Schiehallion Quartzite shows similar thickness variations, usually in sympathy with those of the Appin Group, across the outcrop.

The lowermost formations of the Lochaber Subgroup represent a transition from the poorly sorted, micaceous and feldspathic sandstone and siltstone of the Grampian Group into the cleaner sandstone and mudstone of the Beoil Quartzite and Schist. Cross-sets (in the Dunalastair Quartzite) are smaller, bedding is more regular and the proportion of K-feldspar diminishes. These formations are considered to be deposits on a stable tidal-shelf with the muds representing times of water deepening. Subsequently, through the Ballachulish Subgroup and into the Blair Atholl Subgroup, there is a constant background of thinly bedded silt/mud alternations: Meall Dubh Striped Pelite, Meall Dubh Graphitic Schist, Strath Fionan Banded Semipelite, Tullochroisk Semipelite, Blair Atholl Dark Schist and parts of the Cnoc an Fhithich Banded Semipelite. The silts in this facies contain small (5–10 cm) ripple cross-sets and fine graded beds (a few centimetres thick). The muds at three levels show more or less concentration of carbon and are interbedded with dominantly calcitic limestones (Meall Dubh and the Blair Atholl). The sequence is interupted and ended by two dominantly dolomitic sequences, which in the South-west Highlands contain stromatolites. The mud/silt facies presumably represents a tidal and current-dominated shallow shelf environment punctuated by four periods of low clastic input and stromatolitic(?) carbonate build-up, which on two occasions ended with anoxic conditions, leading to deposition of the present graphitic formations.

One disturbance took place in this quiet environment, the deposition of the pebbly, locally conglomeratic, Meall Dubh Quartzite. Its sedimentary characteristics, the poorly sorted pebbles of plagioclase and potassium feldspar, as well as quartz, and the metre-scale cross-sets, are unique to this formation. The rapid change in thickness within the Strath Fionan area, from 10 to 180 m, is almost certainly in part a sedimentary feature. This occurence, together with similar appearances along strike to the north-east (Beinn a' Ghlo, An Socach, Tomintoul) may represent coarse sand in tide-dominated channels or deltas.

Argyll Group

The description of the formations of this group is treated under the headings of the subgroups. Although a single type locality is given for each formation, detailed description and location are given of the marked lithological variations within several of the formations. The wealth of variation seen in such a small area are partly a consequence of the exceptional strike length visible as a result of the polyphase folding; the dip-slip displacement on the Loch Tay Fault also produces sudden facies changes across it. Since many of the type localities are in remote locations, descriptions are also given of more easily accessible outcrops. The general distribution of the formations is given in (Figure 1).

Islay Subgroup

This subgroup is represented by the Schiehallion Boulder Bed and Schiehallion Quartzite formations. It is seen principally in a curving outcrop pattern for some 25 km from the eastern end of Loch Errochty through Schiehallion (cover photograph) to Loch Kinardochy, where it is displaced north-eastwards to form the north slope of Strathtummel. However, because of duplication by folding, over 95 km of the Schiehallion Quartzite/ Killiecrankie Schist boundary is exposed within the district. Representatives of the two formations may be seen together in several traverses: in the Errochty Water north-west from [NN 724 650], an easily accessible but incomplete section; in the Allt na Moine Buidhe and the slopes and its tributary to the north-west, upstream from the dam [NN 703 614], particularly good for the Quartzite; the latter traverse, extended across the summit of Beinn a' Chuallaich [NN 68 61], includes sections in the Killiecrankie Schist; the track and burn south of East Tempar farm [NN 691 575], which provide the best localities for the Boulder Bed and the lower part of the Schiehallion Quartzite, while the upper part of the Quartzite and lithologies transitional to the Killiecrankie Schist are well seen further south, at the watershed [NN 69 55]; the north face of Schiehallion [NN 730 557], which provides excellent exposures of Boulder Bed and the lower part of the Quartzite; a long traverse of the mountain south from [NN 730 557] to [NN 730 538] provides a section across the upper part of the Quartzite into the Killiecrankie Schist; the Allt Mor upstream from [NN 754 530] gives good though incomplete sections of the formations.

To the north-east of the Allt Mor and east of the Loch Tay Fault, the subgroup is tectonically thinned and poorly exposed. The entire subgroup is not well exposed in any continuous section north of Loch Tummel, although a convenient section on the north shore of Loch Tummel [NN 8303 5963] and extending eastwards into the Pitlochry district does expose the formations and the transition into the Killiecrankie Schist.

Schiehallion Boulder Bed Formation (0–250 m)

This formation has been mapped almost continuously for some 30 km, from its first very attenuated emergence from the zone of attenuation associated with the Boundary Slide near the east end of Loch Errochty [NN 7190 6609] (Figure 5), around the major D₂ Balliemore Antiform and the D₁ Beinn a' Chuallaich Anticline to the Braes of Foss [NN 747 559]. In none of the many good sections available can the formation be demonstrated to be missing; on the two limbs of the Beinn a' Chuallaich Anticline its extreme attenuation is attributed to high strains.

To the south-east, the formation is not exposed on the highly strained south limb of the Beinn a' Chuallaich Syncline to the south-east of Schiehallion, but it can be identified, from its characteristic tremolitic matrix, in the D₂ Creag an Earra Fold around [NN 709 534]. It can then be mapped only discontinuously around the other major D₂ folds between the Allt Mor and Loch Kinardochy, where exposure is generally poor, except for the excellent section in the core of the D₂ Allt Mor Fold; the most easterly exposure is in the Allt Tarruinchon [NN 791 574], within a kilometre of the Loch Tay Fault. The extreme thinning of the formation in some sections may be sedimentological or due to the high strains in this area. To the east of the Loch Tay Fault the formation is again thin (less than 100 m) but it can be traced in discontinuous exposure at the base of the Schiehallion Quartzite; at one locality near Balnabodach [NN 8265 6028] the formation is missing, the Quartzite appearing to rest in conformable contact with the Pale Limestone beneath.

The maximum exposed thickness of the Schiehallion Boulder Bed, in the area around Balmore [NN 705 593], appears to be about 400 m, but some duplication by folding is suspected and there are many amphibolite intrusions here. Away from the areas of clear reduction of thickness resulting from high strains, as in the north-west and east, a thickness of 150–200 m could be regarded as average.

The type section is on the northern slopes of Schiehallion, on the western side of Cnoc nan Aighean [NN 7305 5576] to [NN 7292 5551]. The formation where most fully developed and exposed, as at this type section, may be divided into a sequence of lower calcareous diamictites with dominantly calcareous clasts and an upper sequence of quartz-rich diamictites with a mixture of clasts, including distinctive pink granitic rocks; the two diamictite sequences are generally separated by discontinuous units of quartzite. The lower calcareous sequence, at the type section, is on a steep north-facing slope and commonly obscured by lichen and algae; typically, however, the calcareous clasts are revealed as ellipsoidal cavities. The intervening schistose and feldspathic quartzite only rarely shows bedding; the upper quartzose sequence is superbly exposed at the top of the slope (above the end of a wall), with quartzite and granitic clasts, up to 30 cm in greatest dimension, standing proud of the matrix.

The exposures south of East Tempar farm are especially good and accessible; both calcareous and granitic diamictites are exposed immediately behind the farm [NN 691 575] and up and around the track to [NN 696 564] (Bailey and McCallien, 1937, fig. 6). Other good sections are: in the Errochty Water [NN 723 651] to [NN 724 649]; on Creag Mhor [NN 711 645]; hill-slopes north and south (forested) of the dam in the Allt na Moine Buidhe [NN 703 614] (Plate 2); south of Tullochroisk around [NN 719 571] though exposure is not clean; on the Cnoc an Fhithich ridge especially [NN 710 568] to [NN 724 562]; above Drumchastle [NN 688 588] and scattered exposure to the north; above the Allt Leathan [NN 712 558] on the north face of Schiehallion many loose boulders are seen; on the east shoulder of Cnoc nan Aighean about [NN 742 557] and to the west, accessible clean exposure; in the Allt Mor [NN 7501 5338] to [NN 7518 5333] there is a good section but it is not always accessible; an isolated, but clean, exposure on the track north of Dun Coillich [NN 7634 5442]. The cleanest exposures of Boulder Bed are usually on the widespread large loose boulders of the formation; one particularly accessible and informative boulder is west of Lassintullich, at the gate to a forest clearing north of the road [NN 701 578]. The description which follows is based upon the type section but incorporates observations from some of these other localities, as indicated in the text.

The lower contact with the Pale Limestone is not seen on the type section (Cnoc nan Aighean), but may be established within 10 m in the nearby burn [NN 7226 5569]; this junction is seldom well exposed but, here as elsewhere, appears to be a gradation from the biotite calc-pelite at the top of the Pale Limestone (see also description of type locality for the Pale Limestone). The sequence of lower calcareous diamictites is up to 120 m thick, with individual diamictites (10–15 m thick) separated by beds of calcareous schist free of clasts; the only bedding observed is thin (0.5–12 mm) beds of cream-coloured schistose dolomite, and tremolitic psammite higher up. North of Loch Tummel, e.g. near Balnabodach [NN 8297 6031], such beds have been deflected at the bases of carbonate clasts 10–14 mm long, interpreted as drop-stone structures. The matrix is a calcareous (dolomitic) schist, locally rich in amphibole (radiating blades of white tremolite or pale green actinolite and large hornblende porphyroblasts) and generally rich in phlogopitic biotite. Usually weathering to a rusty-brown, the matrix characteristically shows a regular network of depressions, possibly small carbonate clasts. According to Bailey and McCallien (1937, p.86) lime-rich scapolite and diopside locally occur in the matrix. This is the 'honeycomb schist' of Grant Wilson (in Barrow et al., 1905, p.60).

In thin section quartz and carbonate (dominantly dolomite) comprise up to 50% of the matrix, the remainder being approximately equal proportions of large phlogopite and colourless tremolite with minor muscovite; accessory minerals include zircon, sphene, apatite, tourmaline and iron-ores (not in order of abundance).

The dominant clasts in the lower diamictites are 1–20 cm diameter, disc-shaped fragments of dolomite or schistose dolomite, elongate parallel to the S₂ schistosity. Although commonly completely weathered out, vestiges of the clasts may remain round the rims of the holes. Schist clasts, which are less common, are very elongate (up to 20 cm in length), and the rarer quartzite and granitic clasts are up to 25 cm long but less elongate (ratios of 2:1 or 3:1) and more angular. Grant Wilson (in Barrow et al., 1905, p.60) noted blue quartz fragments. The clasts show no evidence of having been sorted or of having their distribution controlled by water movement. Clasts may form up to 70% of the total rock volume, but over distances of a few metres this proportion diminishes to zero. In exposures north of Loch Tummel some diamictites show evidence of the decrease in the proportion of clasts upwards while in general there is a reduction in the ratio of calcareous to non-calcareous clasts upwards.

In the type section and adjacent exposures a unit of psammitic quartzite, some 50 m thick, intervenes between the two diamictite sequences. Although access and exposure is not good, it appears that 250 m to the east, on the steep face of Cnoc nan Aighean, the quartzite wedges out. In the exposures around the track south of East Tempar Farm, in a similar situation, one 50m thick, 500 m long, wedge-shaped unit of cross-bedded psammite is succeeded by another along strike to the north and at a slightly higher level; exposures in the Allt na Moine Buidhe and Loch Tummel areas similarly suggest discontinuous units of psammite at slightly different levels both between the lower and upper diamictite sequences and between individual diamictites. These psammites are not as clean as the Schiehallion Quartzite above, being sugary and feldspathic, and locally strongly schistose; they are generally massively bedded. Although bedding-plane laminations have been observed, cross-stratification has not been widely detected.

The upper diamictite sequence at the type section is up to 180 m thick, but a D₁ fold pair, identified to the east, is certainly responsible for this thickening; within a kilometre, both to the east and to the west the sequence appears to diminish in thickness to about 80–100 m, which is more typical. The matrix of these diamictites is generally fine-grained, dark, feldspathic psammite, in places slightly calcareous, and variably micaceous; locally it is very hornblendic e.g. at Cnoc na h- Iolaire [NN 703 556]. The matrix usually has a gritty texture as a result of scattered 1–2 mm quartz and pink feldspar grains. Bedding is rarely visible, although beds of metre-thick, clast-free, mica schist and quartzite are present and suggest that several diamictites are present.

Thin section examination shows a fine-grained mosaic of quartz and altered feldspar (some plagioclase twinning) comprising up to 75% of the matrix, the remainder being mostly biotite with variable muscovite and/or calcite aligned in the dominant schistosity. Accessory minerals are as for the calcareous matrix, i.e. zircon, sphene, apatite, tourmaline and iron ores. Bailey and McCallien (1937, p.86) identified aegerine-augite, diopside and scapolite in the matrix locally.

Clasts of fine-grained and coarse-grained grey or pink granite are the most common, followed by pink and white quartzite, schist and dolomite (least common). The pink granite is the nordmarkite of Bailey and McCallien (1937, p.86); the grey granite in (Plate 2) is syenitic. Rare clasts of amphibolite have been recorded by Anderson (1923, p.432) in the Errochty Water exposures. Grant Wilson (in Barrow et al., 1905, p.61) identified scapolite-gneiss in the same exposures, although this could have been a deformed microgranite with a scapolite-bearing matrix, which is the case with the smaller clast in (Plate 2).

Clasts are rarely in contact and spacing appears to be random. No systematic quantitative study of clast proportion to matrix has been made, but in the type section it varies from 70% to zero (measured in a metre square and taking a clast as greater than 5 mm in length). The discontinuous nature of any clean exposure makes discussion concerning changes along strike tentative, but individual diamictites appear to retain their identity for distances of tens of metres, although proportions of clasts within an individual diamictite vary rapidly. Some beds, up to several metres thick between clast-rich diamictites, may be free of obvious clasts (greater than 5 mm in length) but still have the gritty, feldspathic, texture of the diamictite matrix. The maximum length of clasts, which are usually somewhat flattened in the S₂ schistosity, is 30 cm. The rarer schist and dolomite clasts are the smaller, most strongly deformed and consequently apparently rounded. The commoner quartzite and fine-grained granite clasts seldom exceed 10 cm, but may display very irregular, angular and non-flattened shapes; the coarse granite clasts are typically the larger and display similar variations. Many clasts show pressure-shadow fringes of quartz, carbonate and mica. As with the lower diamictites, there is no evidence of sorting by size or composition, although some metres-thick beds are clearly free of granitic clasts.

The smaller clasts (0.25–0.5 mm) seen in thin section are composite quartz-microcline-plagioclase-muscovite clusters scattered through the matrix, while isolated clasts (up to 4 mm) are of similar composition as well as single grains of microcline or of recrystallised quartz.

The upper contact with the Schiehallion Quartzite at the type section is sharp, and the evidence from exposures throughout the district suggests that the contact is not gradational. However, a 10–20 m bed of reworked granitic diamictite is present near the base of the quartzite west of Cnoc nan Aighean [NN 7292 5551]. This junction and the conglomerate are seen again in the East Tempar track exposures [NN 6953 5651], but not well elsewhere. The section on the north side of Loch Tummel [NN 8305 5964] to [NN 8345 5973] is of ambiguous stratigraphical position, but probably includes both the boulder bed and reworked conglomerates at the base of the Schiehallion Quartzite.

Although the Boulder Bed has been mapped over a large area, it should be emphasised that exposure away from the sections already discussed is often not clean and very discontinuous. However, even in the areas of the strongest deformation there is commonly evidence of the lower calcareous diamictite and psammitic beds below the upper diamictite sequence, although it is usually the granitic clasts in the latter that make the formation recognisable. There is no evidence of any substantial change in the character of the formation or its constiituent clasts within the district. No sedimentary structures that can positively relate the formation to its supposed glacial origin have been observed (compare with descriptions of the Port Askaig Tillite in Islay and the Garvellachs by Spencer (1971, 1981)). Possible dropstone structures have been recorded in the type section on Schiehallion, in the Tempar Burn and north of Loch Tummel, but it is difficult to separate the sedimentary deflection of bedding around the clasts from the effects of the strong deformation.

Schiehallion Quartzite Formation (0–800 m)

The distribution of this formation is very similar to that of the Schiehallion Boulder Bed described above. Although, it may be represented in the western part of the district by very thin (metre-thick) beds of impure quartzite that are seen in the Boundary Slide Zone from the Allt Druidhe [NN 643 552] north-east to Loch Errochty, it can first be confidently identified north of the loch [NN 713 662], and around the closure of the Errochty Synform, as it emerges from the zone of attenuation associated with the Boundary Slide (Figure 5). It can then be mapped almost continuously for some 30 km around the major D₁ and D₂ folds to the Braes of Foss [NN 747 559], where the lower boundary and much of the formation is truncated by the Braes of Foss Fault. The very attenuated upper part of the formation can then be traced up Gleann Mor into the core of the Creag an Earra Synform, which extends from Creag an Earra [NN 700 536] for 24 km to the west slopes of Beinn a' Chuallaich [NN 672 625].

The formation has then been mapped another 50 km, almost continuously around the other major folds between the Allt Mor and Loch Kinardochy to north-east of Dun Coillich [NN 773 542] where it is truncated by the Loch Tay Fault. On these highly strained fold limbs the quartzite is commonly only a few metres thick. East of the fault the formation is moderately well exposed for 15km around the folds north of Loch Tummel (Figure 16).

The quartzite outcrop is divided into a lower and upper half in several sections by the Tempar Dolomitic Member; the lower half contains two horizons of discontinuous conglomerate. The type section for the quartzite itself is on the east flank of Schiehallion from Meall nan Aighean [NN 729 555] at the termination of the Boulder Bed section, south-west for 600 m to the Tempar Dolomitic Beds at [NN 725 548], crossing one of the conglomeratic beds [NN 7255 5508] and then, in a very steep section, to the Killiecrankie Schist boundary on the south side of Schiehallion [NN 719 538]. Although this section crosses some clean, weathered exposures of the quartzite, there is very little to choose between one quartzite section and another in terms of internal detail of these rather monotonous beds. The sections in the Tempar Burn [NN 692 569] to [NN 696 559] are not only more accessible but also more instructive, in that they contain the best exposures of one of the conglomeratic horizons [NN 6930 5655] and of the dolomitic [NN 6921 5670] to [NN 6927 5657] beds (Bailey and McCallien, 1939, fig. 6).

Although the Schiehallion sections appear to be the thickest represented on the map, apart from the effect of topography noted above, they are certainly thickened by intermediate-scale folding. The preferred thickest measurement is at Tempar [NN 692 569] to [NN 696 559], where the conglomerate and dolomitic beds can be shown not to be so affected; the true thickness of these steeply dipping beds is about 800 m. This is probably similar to the thickness at the east end of Schiehallion and in the Allt na Moine Buidhe section; the thinning around the Beinn a' Chuallaich Anticline is certainly in part due to high tectonic strains, as is the progressive thinning from the latter locality northwards. As pointed out by Treagus (1987, p.4), it is possible that these thinned sections may have suffered high strains because they were already thin sedimentary sequences.

Other good sections across the Schiehallion Quartzite, to which reference is made below, include the following: the tributary of the Allt na Moine Buidhe [NN 702 614] to [NN 698 619], especially for the conglomerates; the Allt na Moine Buidhe from the dam [NN 703 613] upstream to [NN 694 616] for the dolomitic member and quartzite; Creag Dhubh [NN 699 605] for the conglomerates; Beinn a' Chuallaich [NN 686 618] and Meall nan Eun [NN 688 626] for the quartzite and the Killiecrankie Schist boundary; north-east of Drumchastle [NN 685 594] upstream to [NN 685 598], for the Killiecrankie Schist junction and quartzite. Although good sections are seen across the Creag an Earra Anticline, for instance north of Drumchastle [NN 678 595], south of the Tummel Valley at [NN 680 570] and on Geal Charn [NN 645 545], these are strongly deformed representatives of only the upper quartzitic part of the formation. Other sections in the east of the area are: Cnoc nan Aighean [NN 734 557] south-east to the Allt Ruighe [NN 739 553] for the lower quartzite, crossing conglomeratic beds at [NN 7313 5534]; the dolomitic member in the Allt Ruighe especially [NN 743 553] to [NN 751 557]; any section across the east end of the Schiehallion ridge most accessible around [NN 744 542] for part of the upper quartzite, the width of outcrop of which is much exaggerated by parallelism of dip and topography; the Allt Mor [NN 753 533] to [NN 740 534] on either side of the Schiehallion Boulder Bed core to the Allt Mor Synform, where the dolomitic member is well seen, but there is no conglomerate; on Sgurran Geal [NN 773 558] (now afforested), which shows no evidence of conglomerate in the lower part of the quartzite in this tight fold core, although cross-beds are preserved; a very abbreviated section of the quartzite in one of the highly strained fold limbs in the east can be conveniently seen by the road at Tom Beithe [NN 7821 5685].

East of the Loch Tay Fault, the transitional junction of the quartzite with the Killiecrankie Schist is well exposed in the Frenich Burn [NN 8262 5887] immediately south of the road. Otherwise the formation is best seen near the same boundary south of Meall Urair [NN 8312 6091] (including thin dolomitic member) around [NN 833 605] and in the Allt an Tressait [NN 812 604]; conglomeratic beds are seen north of Croft Douglas [NN 8290 5995] and on the north shore of Loch Tummel [NN 8303 5963]; thin representatives of the dolomitic beds are seen at [NN 8334 6005].

The field characteristics of the quartzite as seen on Schiehallion are of a white to grey, slightly pink-weathering, fine-grained, quartz-rich crystalline psammite. The upper half of the formation is cleaner, with less feldspar and mica, having a more glassy appearance. Bedding is usually well marked by more feldspathic laminae on the scale of 1–5 cm, with thicker partings typically on the scale of 5–40 cm. In the type section and elsewhere on Schiehallion, in the lower half of the quartzite, massive beds, apparently devoid of any internal structure, are seen up to 10 m thick, separated by partings of muscovitic semipelite. Elsewhere cross-bedding of the feldspathic laminae on the 5–30 cm scale may be seen where outcrops are clean and the strain is not high, although the absence of coloured minerals on the foresets, in contrast with the Grampian Group for instance, makes them difficult to detect.

Thin (centimetre to metre) pelite or semipelite beds are not uncommon and calcareous schists are occasionally seen, e.g. on the south slopes of Schiehallion at [NN 7186 5398]. Exposures of an unusual 15 to 20 m-thick sequence of beds of semipelite and finely bedded pelite occur in the Tempar Burn [NN 6938 5640], and similar beds are seen elsewhere [NN 7350 5525] and [NN 7365 5505] on the north-east slope of Schiehallion; these rocks are all at about the same stratigraphical level, probably about 100 m below the dolomitic member and, in the case of the Tempar Burn, 80 m below the upper conglomerate described in the next section. These rocks in the Tempar Burn contain an unusual sequence of original silty and muddy laminated beds: 30–90 cm thick beds exhibit planar, undisturbed millimetre-thick laminations; other thinner beds have discontinuous wedge-shaped centimetre-thick laminae; others have channelled and graded centimetre-thick units; others, 30 cm-thick, have an unusual wavy bedding; there is one 40 cm-thick bed of calcareous schist and limestone.

Conglomeratic beds

There are at least two levels within the Schiehallion Quartzite at which conglomeratic quartzite occurs. The older is no more than 5–10 m thick and is seen only on the north face of Schiehallion 30 m above the Boulder Bed. It can be traced from its most westerly exposure [NN 7155 5552] to two burns [NN 7216 5540] some 500 m to the east and then a further 700 m east, to a 100 m strip of good exposure around [NN 7295 5550], near the top of the wall. These beds are quite certainly not present in the almost continuous exposure on Cnoc nan Aighean, a further 500 m east. They have a matrix typical of the quartzite, but with scattered schist, quartzite and granitic pebbles, very similar to the second unit described below.

Other conglomerates occur discontinuously at a higher, apparently consistent level, close below the dolomitic member, possibly about a third of the way up from the base of the quartzite. This unit is seen from the Allt na Moine Buidhe in the north-west to the Allt Ruighe in the south-east. At the former locality, upstream in the tributary from [NN 700 617] and in the main burn from [NN 6995 6120], a conglomeratic unit occurs about 250 m above the Boulder Bed and is apparently some 150 m thick. It is better seen, although in forest, on Creag na Rainich [NN 6995 6085] and Creag Dubh [NN 6992 6085], to the south, where it is of similar thickness, although duplication by folding is likely. Several small exposures of conglomerate have been identified on Meall nan Eun [NN 683 629] but the horizon has not been seen in the well-exposed sections of the quartzite around the Beinn a' Chuallaich Anticline. These beds re-appear in an excellent section in the Tempar Burn, upstream from [NN 6930 5655]; here, 150 m above the Boulder Bed and 20 m below the dolomitic member, it is 40 m thick, and repetitions by folding can be discounted. The next occurrence to the east is on the north slope of Schiehallion, [NN 723 551] to [NN 729 550], where it has a maximum thickness of 20 m, some 200 m above the lower conglomeratic bed and the Boulder Bed; here, the conglomerate can be seen not to be continued in the outcrops to the east. However, 1 km further to the east, scattered outcrops and loose boulders of conglomeratic quartzite may be followed for another 1 km, at about this same level about 60 m north of the course of the Allt Ruighe [NN 7390 5534] to [NN 7495 5572].

Conglomerate is also seen in the two burns north-west of Meall Ban [NN 702 643] and [NN 709 646]; these occurrences are in a much-thinned Schiehallion Quartzite but probably represent the higher beds. Thin conglomerates and finer pebbly beds in the upper part of the Schiehallion Quartzite, at a higher level above the dolomitic beds, are seen only rarely, e.g. on the east flank of the Schiehallion ridge [NN 743 541] and [NN 744 542], and on the west flank [NN 7186 5398]. These beds contain granitic fragments, and pink feldspar and quartz pebbles up to 16 mm long.

All of the conglomeratic bed outcrops mentioned above have essentially the same character, although no exhaustive investigation of the composition of the matrix and of the clasts, or of the matrix/clast proportions, was undertaken. The field appearance of the quartzitic matrix is very similar to that of the Schiehallion Quartzite itself, but with a somewhat more gritty appearance. This is confirmed in thin section. Of the matrix, 75–80% is composed of 0.5 mm diameter quartz grains, with scattered microcline and altered plagioclase (15%), small biotite flakes (5%) and a very little muscovite making up the remainder. Clasts observed in thin section include rounded grains of microcline up to 3 mm, microcline-plagioclase-quartz-biotite 'granite' grains up to 8 mm long, and very elongate biotite-muscovite-quartz semipelite grains, up to 5 mm in length.

The Tempar Burn exposures are particularly clean and enable the larger clasts to be examined. The rock types and sizes of the various lithological types are much as in the Boulder Bed, described above. Coarse- and fine- grained, grey or pink granitic clasts are the most common, together with pink and white quartzite, schist and carbonate. The significant difference to the Boulder Bed is that there appears to be a stronger bimodal distribution of sizes in the conglomerate, such that the intermediate 1–5 cm sizes are missing; in addition the clasts are less dispersed, although rarely in contact, and they are significantly more rounded. This supports the interpretation of Anderson (1923) that these rocks represent reworked Boulder Bed. Where measured, in the Tempar Burn section, the proportion of clasts to matrix is very variable in exposures of several square metres, from 1% with isolated single clasts, usually granite, to approximately 70%, with crowded clasts of a mixture of rock types, rarely in contact with one another.

The clasts are deformed into disc-shapes within the dominant cleavage, which is very close to the bedding; observation of shape is difficult, but most clasts display approximately circular sections. Deformation appears to be more pronounced than in the Boulder Bed, but this depends on the initial clast shape and orientation after deposition. Of the granite clasts, ratios of greatest length/width, measured on surfaces perpendicular to the bedding, ranged between 10.4:1 and 2.1:1, but were commonly about 5:1. The size (longest dimension) ranged from 2 to 47 cm. Of the schist clasts measured, elongation ratios were from 5:1 to 13:1; sizes ranged from 3 to 18 cm.

Tempar Dolomitic Member

In the type section of the Schiehallion Quartzite east of the summit, and more clearly in the Tempar Burn section, the monotonous sequence of quartzites is broken about 250 m from its base by a 50 m succession of dolomitic limestones, calcareous schists and psammites that Bailey and McCallien (1937, p.87) named the Dolomitic Beds. This member is certainly discontinuous and re-appears at this stratigraphical level at several locations. Its most northerly appearance is north of Loch Errochty, where it can be followed eastwards as a 2–3 m-thick bed of tremolite schist for 500 m from the closure of the Errochty Synform [NN 7131 6655] to [NN 7184 6631] (Figure 5). It reappears in the two burns north-west of Meall Ban [NN 709 647] and [NN 7015 6435] but is best seen in this area in the Allt na Moine Buidhe [NN 6985 6115], where beds of white dolomitic limestone and calcareous schist are exposed over 30 m. Like the conglomeratic beds, the member has not been observed in the Schiehallion Quartzite outcrops around the Beinn a' Chuallaich Syncline, although one exposure of dolomite occurs to the west at [NN 6861 6162].

The type locality for the member is in the Tempar Burn [NN 6921 5670] to [NN 6927 5657] where the full range of varied lithologies described below is well displayed in the burn and its steep banks. The member is almost certainly not present in the well-exposed sections around the summit of Schiehallion, but reappears [NN 716 551] to its east and can be traced as loose boulders to the first good outcrops [NN 725 548]; from here the thickness of the Dolomitic Member certainly increases eastwards to 50 m at about [NN 733 548], before joining the 800 m-wide band of outcrops south of the Allt Ruighe around [NN 742 550]. This latter width is certainly a consequence of folding.

A well-exposed section of the lower part of this band is seen in the Allt Ruighe, where the contact with the Schiehallion Quartzite may be seen [NN 739 553] and where most lithologies are exposed downstream towards Braes of Foss [NN 7510 5575]. This broad band is truncated to the east by the Braes of Foss Fault. The member is next seen on the two limbs of the synform in the well-exposed sections in the Allt Mor for 50 m upstream of [NN 7525 5325] and at [NN 748 535] respectively; the maximum thickness here is 30 m. Otherwise the member is seen only in isolated outcrops and loose boulders of dolomitic limestone and calcareous schist within the quartzite around [NN 768 543] and south-west of Loch Kinardochy [NN 7723 5429] to [NN 7750 5440]; its thickness here is not more than 10 m.

North of Loch Tummel there are small areas of impure dolomitic limestone, calcareous schist and psammite that outcrop throughout the Schiehallion Quartzite. The thickest seen is 4–7 m beside the road near Chapelton of Borenich [NN 8334 6005], and south of Meall Urair [NN 8312 6091] where bedded dolomitic limestones reach 10 m; pyrite is typically very concentrated (locally 50% of the rock). The continuity and exact stratigraphical level of these outcrops is difficult to establish, but they are most probably equivalent to the dolomitic member.

At the type locality, 20 m of schistose, calcareous, Schiehallion Quartzite follows the conglomeratic beds, and is succeeded by a sequence starting with calcareous (carbonate or actinolite-bearing) and non-calcareous pelites, into thin well-bedded (typically 25–50 cm, but reaching 300 cm) pure and impure (tremolitic) dolomites and finally upward into semipelites and very fine-grained psammites. Phlogopite is conspicuous in the calcareous pelites and pyrite, commonly visibly orientated in the cleavage, is locally very enriched. No sequences have been established of the various lithologies; small-scale folding is responsible for some of the repetitions. The top is marked here by 10 m of finely bedded semipelite, in which bedding/cleavage relations are particularly well seen. South of the Allt Ruighe e.g. at [NN 742 547] a 50 m-thick sequence of semipelites is seen at a similar position beneath the upper division of the quartzite, with which it appears to have a gradational boundary. Amphibolites, at this upper boundary particularly, have a sedimentary aspect and probably represent marls.

No sedimentary structures were observed in the dolomitic member, except for a common millimetre-scale lamination and 3 cm-long sedimentary dykelets in some of the semipelites; some beds are noticeably wedge shaped and discontinuous, although some of this can be attributed to tectonic boudinage.

In thin section, the calcareous semipelites and psammites comprise dolomite and quartz (with up to 20% altered plagioclase) and muscovite in various proportions in cm-thick alternations. The calcareous pelites and schistose dolomites contain phlogopite (up to 50%) which is commonly porphyroblastic. One thin section shows quartz segregations (clasts) up to 2 cm long in dolomite. Pyrite may comprise up to 20% of any calcareous lithology, in sections examined from the type locality.

Sedimentary environments

Subsequent to Bailey's correlation of the Schiehallion Boulder Bed with the Port Askaig Conglomerate (in Bailey and Maufe, 1916), Anderson (1923, p.430) discussed the arguments for its glacial origin. He pointed out that the uniform, unbedded nature of the matrix and the haphazard arrangement of the clasts strongly suggested an origin as a tillite. He noted the similarity of the limestone and the quartzite clasts to the Pale Limestone and Banded 'Group', respectively, immediately below, and indeed used their likely derivation from these formations to demonstrate the order of superposition. He further argued that the granitic clasts, which have no likely local source, would be expected to be present in the younger part of a glacial deposit. Anderson also distinguished the two levels of conglomerates, with their clasts of identical lithologies to those in the Boulder Bed, and argued for their derivation from the Boulder Bed by fluviatile or marine erosion. Anderson (1923, p.432) noted that the base of the Schiehallion Quartite closely approaches the Blair Atholl grey limestone in the north of the district, but accepts the possibility that this may be due to either erosion (of the sub-quartzite surface) or to later tectonic movements. Most pertinently he noted that the presence of amphibolite clasts demonstrates a period of igneous activity which predates the boulder bed. The present investigation, however, has failed to reveal clasts of true amphibolite, as against green amphibole-(actinolite) bearing mica-schist.

Other discussion concerning the depositional environment of the 'Boulder Bed' in general has concentrated on evidence from the superior exposures of the Garvellachs or Islay, e.g. Eyles (1989), Eyles and Eyles (1983), Kilburn et al. (1965), Spencer (1971, 1981, 1985). It is generally accepted that the geographically extensive, though discontinuous, distribution of the deposit favours a glacial origin, as opposed to sedimentary slide deposits of a more local character. The general evidence, throughout its outcrop in the Dalradian, of the mixture of locally derived and exotic clasts, their random distribution and size, the lack of bedding within the ill-sorted matrix, the record of dropstone structure, and the particular evidence of other glacially related structures in the South-west Highlands, makes the glacial origin indisputable. The evidence in the Schiehallion district, the massive nature of the diamictites, their inclusion of stratified, sorted sands, and the subrounded shape of some clasts, supports the interpretation of a marginal glaciomarine environment (Edwards, 1986). The local evidence of the thinning and probable absence of the formation in some areas and the inclusion of clasts of at least two of the preceeding formations in the lower diamictites, strongly suggests an unconformable base. The occurrence of discontinuous conglomeratic units (suggesting shallow marine reworking and some sorting in subaqueous outwash moraine) and of possible laminates (suggesting eustatic water movements) lends further support to the glacial environment.

The Schiehallion Quartzite represents a thick blanket of fine sand which, taking into account its outcrop pattern in the Dalradian to the east of the district, was deposited as a series of detached, lens-shaped bodies of several kilometres length along strike. The generally small-scale and oscillatory nature of the cross-sets, and its well-sorted, fine, texture suggest a tide-dominated shallow shoreline; the thickness of the deposit suggests high rates of sedimentation into a steadily subsiding tectonically active continental margin. Its vertical, and presumably horizontal, gradation into the mud-dominated lower Killiecrankie Schist, suggests a change to an offshore environment in a transgressive setting. From the similarities of the clast lithologies to those in the Boulder Bed, the conglomerate beds within the Schiehallion Quartzite are interpreted as being derived by reworking of the Boulder Bed. The reworking resulted in additional rounding of clasts, winnowing to give a bimodal clast size distribution, and a higher proportion of clasts in the rock.

The occurrence of the Dolomitic Member within the Schiehallion Quartzite is unique to the Schiehallion district. However, the sequence bears many similarities to the Bonahaven Dolomite Formation of the South-west Highlands (Fairchild, 1980), which lies between the Port Askaig Tillite and the base of the Jura Quartzite. The sedimentology of the Bonahaven Dolomite Formation can be summarised as: laminated dolostone, dolomitic sandstone, and layered alternations of fine-grained dolomitic siltstone and variably dolomitic mudstones; beds are commonly lenticular; the uppermost muddy part passes gradationally into the quartzite above. Features of this formation (that have not been identified in the Schiehallion district, presumably because of deformation and recrystallisation) are stromatolites, cross-bedding, oolites, wave-ripples and dolomite intraclasts. These features led Fairchild (1980) to interpret the environment as supratidal to subtidal, in a wave-dominated nearshore area, subsequently becoming landward of a barrier island with washover sands interbedded with lagoonal muds.

Easdale Subgroup

The subgroup comprises the Killiecrankie Schist, Carn Mairg Quartzite, Ben Eagach Schist, Ben Lawers Schist and Farragon Volcanic formations. Each formation is described under two sub-headings, to the west and to the east of the Loch Tay Fault, as there are significant facies variations on the two sides. On the west side of the fault, the subgroup forms a central spine from Loch Errochty south and eastwards to form the rugged hills on both sides of Glen Lyon in the south of the district. To the east of the fault, the subgroup forms the hills south of Loch Tummel, rising southwards to the Meall Tairneachan watershed. Both areas are moderately well exposed.

Killiecrankie Schist Formation (0–900 m)

The formation was named by Bailey (1925) from exposures in the Pass of Killiecrankie, which occur in the Pitlochry district (Sheet 55E) district as a continuation of those east of the Loch Tay Fault in the Schiehallion district. It had, however, previously been identified by Anderson (1923) in the Schiehallion district, which provides a superior type section (described below) in that it is not so obscured by intrusive amphibolites, and that it presents both its lower boundary with the Schiehallion Quartzite as well as its upper boundary with the Carn Mairg Quartzite (Figure 7a), thus providing continuity with the type areas for those formations.

This formation is a sequence of well-bedded pelites, semipelites and psammites, usually containing small garnets and mostly slightly graphitic and kyanite-bearing. The boundaries with the quartzites below and above are normally gradational. The rocks across these transitional boundaries usually provide good evidence of younging (cross-bedding at the lower boundary, graded bedding at the upper) and are important structurally in providing critical cleavage and minor fold relationships, that can be used to establish the age of the major folds. Locally observed sharp boundaries suggest a mechanical contact. Amphibolites are commonly seen in the Killiecrankie Schist; most are intrusive but others, bedded and associated with calcareous schists, are clearly metamorphosed tuffs and/or marls.

The outcrop pattern of the belts of Killiecrankie Schist (Figure 1) defines the position of virtually all the major D₁ and D₂ closures in the central area of the district, as well as the Bohespic/Errochty fold-pair. For this reason, and because of their structural importance to which reference will be made in Chapter 3, particular attention is given below to the location and definition of these belts.

West of Loch Tay Fault

The principal outcrop of this formation, to the west of the Loch Tay Fault, is essentially in three belts controlled by the major D₁ and D₂ fold closures wrapped around the Bohespic Antiform (Figure 1).

The main outcrop runs from the Loch Tay Fault at Glengoulandie [NN 763 525], west along Gleann Mor to Meallanan Odhar [NN 680 535], then swinging north around the Bohespic Antiform to Craig Varr [NN 670 590], east of Kinloch Rannoch. The formation tends to erode more readily than the adjacent quartzites, but the exception to that is the outcrop on Dun Coillich [NN 763 537] at the east end of this belt; the east and south flanks of this hill provide some of the most accessible exposures, particularly on the slope above the path that runs north of the Allt Mor for 1 km from Glengoulandie Farm [NN 762 523] to [NN 754 531]. Excellent refolding and grading is exposed on clean joint surfaces. Gorge sections in the Allt Mor itself to the south of this and to the west upstream from [NN 748 534] are not always accessible, nor as informative.

The lower boundary with the Schiehallion Quartzite can be established in the burn on the north side of Dun Coillich [NN 7675 5420] and is seen on the west limb of the Allt Mor Synform [NN 7476 5342]; the boundary on the east limb is a fault. The central sector of this easternmost Killiecrankie Schist outcrop contains an unusually high proportion of gritty psammites containing excellent grading, heralding the Carn Mairg Quartzite lithologies; these are seen both on Dun Coillich and to the south on Na Craigean [NN 752 522] south-west of the Allt Mor. At the latter, amphibolites and calcareous schists are particularly evident and there is one thin limestone. The upper boundary with the Carn Mairg Quartzite is poorly exposed.

A section from the Geal Charn ridge [NN 686 534] along the watershed via Meallanan Odhar [NN 680 532] to Coire Cruach Sneachda [NN 6777 5266] provides the best exposed and the type section for the area (Figure 7a). All the rocks dip steeply to north-east or south-west, but do not all young consistently southwards. A clear vergence-reversal across an antiformal D₂ fold is indicated by S₂/bedding relationships on Meallanan Odhar [NN 680 532] and, although later folds are absent or minor, a reversal in the facing on S₂ around [NN 678 529] indicates an additional antiformal D₁ core.

In the north of the section, very thinly bedded clean, white, fine-grained quartzites in the Schiehallion Quartzite (cross-bedding younging south on Geal Charn around [NN 687 540]) pass into dark layered semipelites and thin-bedded psammites containing several 8 cm concordant fine-grained garnet amphibolite layers. The boundary with the Schiehallion Quartzite Formation is unusually sharp, and a transitional zone only 10 m thick at [NN 6859 5398] contains several fine-grained quartzite ribs. Some 2–10 cm beds of gritty feldspathic quartzite are present at some nearby localities [NN 6880 5365]; [NN 6884 5347], but the lowest part of the formation is dominated by biotitic semipelite, in places graphitic and with kyanite, but mostly rust-stained. Southwards, concordant fine-grained garnet amphibolite layers, displaying a centimetre-scale amphibole/feldspar striping, are folded by minor D₂ folds and become thicker and more numerous around [NN 683 536] and on Meallanan Odhar [NN 680 530]. Many of these layers to the south contain blebs of feldspar and there were probably sills as well as ashy beds. Further south several units of coarse to very coarse feldspathic quartzite or 'gritty' granule psammite are present. Bed thicknesses vary from 15 cm in the finer rocks to over 1 m in the coarsest units, and they are separated by thin quartz-muscovite schists. These coarse-grained units higher in the Killiecrankie Schist locally resemble the Carn Mairg Quartzite and form part of the transition to it, but are thin, have clasts mostly less than 4 mm, and contain more pelitic material. With a general coarsening, involving the loss of the finer intervening semipelitic material and the appearance of some truly pebbly material, the gradation into the Carn Mairg Quartzite is complete in the Allt Coire Cruach Sneachda [NN 6777 5269] (Figure 7a).

One folded 20 m-thick unit of a fine-grained glassy quartzite occurs on the western slopes of Geal Charn [NN 6758 5441], at an unusually high stratigraphical position in the Killiecrankie Schist; other pure quartzite units lie close to the Schiehallion quartzite. Several such mappable quartzites are present in the core of the Beinn a' Chuallaich Anticline, south of Schiehallion [NN 70 54]; [NN 709 536], as well as north of Drumchastle [NN 680 593] and elsewhere.

The width of outcrop of the formation diminishes northwards and exposure is less good, except for the Creag Mhor ridge [NN 677 551] and, north of Strathtummel, on the south face of Craig Varr [NN 670 590]. This outcrop represents the core of the Allt Mor Synform and the highly strained boundaries with the Ben Eagach Schist and the Carn Mairg Quartzite are seen to the west and east [NN 666 590] and [NN 672 590]; at the former boundary the Carn Mairg Quartzite has probably been removed by sliding, although facies variations cannot be discounted. Exposures of the formation are locally rich in graphitic kyanite, but sedimentary structures were not observed. This belt thins and is only sparsely exposed to the north, as far as the trace of the Errochty Synform [NN 670 610].

The second belt of outcrop, occupying the core of the Beinn a' Chuaillaich Syncline, commences in the east with scattered exposure on the south flank of Schiehallion. The closure of the syncline is poorly defined [around the 73 easting] but improves westwards [between 72 and 70], where the boundary with the Schiehallion Quartzite is best exposed on the north fold limb which shows interbeds of the two lithologies, as well as interbedded gritty quartzites higher in the sequence. Sedimentary structures are difficult to interpret in these strongly folded schistose quartzites. Good exposure is seen again across the watershed on Geal Charn around [NN 688 550], and south of West Tempar [NN 683 564], where the boundary is sharper and can be located on both limbs. Along the northern slopes of Strathtummel, good sections are seen of the two boundaries, e.g. near Drumchastle [NN 679 595] to [NN 685 594], on the summit of Meall Breac [NN 675 607], and in burn sections in the intervening ground. Along the west flank of Beinn a' Chuallaich, however, good exposures are restricted to the top of the ridge and the burns draining the watershed southwards, around [NN 680 601] and northwards [NN 678 628] to [NN 681 643]; the latter give continuous exposure across the Errochty Synform to the Carn Mairg Quartzite boundary on its north-western limb. Sedimentary structures, both cross-bedding and grading, can be used at many of the localities given above to confirm the age relationships at boundaries and facing within the formation.

The third belt, occupying the core of the D₂ Balliemore Antiform, branches from the belt described above on the south-eastern slopes above Loch Errochty. The closure of the D₁ Beinn a' Chuallaich Anticline, as defined by the boundary with the Schiehallion Quartzite around [NN 695 645], can only be traced from isolated exposures. However, good and accessible sections are seen in the Errochty Water downstream of the dam [NN 715 656] to [NN 719 652] and on the hill south-west of the dam [NN 708 651]; here, in the core of the Errochty Synform, folds are well displayed with some sedimentary detail, and amphibolites are common. The burn draining Loch na Caillich provides nearly continuous exposure, from transitional lithologies with the Carn Mairg Quartzite at [NN 700 652], up to the boundary with the Schiehallion Quartzite at [NN 6928 6308]. The former exposures, on the side of Loch Errochty, show dramatic folds and boudins of quartzose schist and amphibolite while the somewhat graphitic schist of the exposures in the burn commonly carries conspicuous kyanite.

To the south, the boundary with the Schiehallion Quartzite on the west limb of the antiform can be established east of Meall nan Eun [NN 690 627] and further good exposure on this limb occurs east of Beinn a' Chuallaich summit [NN 687 618] and further south around [NN 690 606]. The eastern boundary with the Schiehallion Quartzite is hidden south of Loch na Caillich, but can be placed in scattered exposure between Loch na Caillich [NN 693 627] and the Allt na Moine Buidhe [NN 693 616] and on Tom na Moine [NN 691 610] from scattered exposures. The tight D₂ minor folds in the closure of the antiform are well displayed on top of the crags north of Auchtibart [NN 693 590]; quartzites at the junction contain cross-bedding.

Two further belts of outcrop of the Killiecrankie Schist, defining further D₂ fold cores, branch from these belts. Sedimentary structures are not seen in these more highly deformed rocks. The first, a narrow and poorly exposed belt from the Allt Mor at [NN 742 534] to Domnaheiche [NN 796 580], is highly strained and the boundaries with the Schiehallion Quartzite are usually quite abrupt, or the transition takes place within 1 or 2 m. The pelites are more graphitic than those previously described, and the transitional rocks contain quite pebbly psammites; it is difficult to know how far this is due to original facies changes. The most useful exposures are: along the forest track from [NN 760 555] to the road, where the south-eastern margin is seen, with platy quartzites and remarkably deformed amphibolites only 10 cm thick; around the burn [NN 765 560] to [NN 767 562] where both margins are seen; the burn at [NN 7648 5613] with a particularly good exposure of slid margin with Schiehallion Quartzite; by the roadside north of [7815 5685]; the Allt Tarruinchon [NN 791 574] to [NN 790 576], a good section, with exceptionally garnetiferous schist and amphibolite on the south-eastern margin.

The Ben Eagach Schist/Carn Mairg Quartzite belt of Craig Varr [NN 678 557] to [NN 673 601] is separated from Schiehallion Quartzite to its east by the second, very narrow belt of Killiecrankie Schist, at most 50 m thick. Exposure of this highly strained sliver is limited to crags around the burn [NN 6758 5926] and southwards from the gully [NN 6775 5705]. This belt probably joins the belt to its east, described above, around an inferred closure of the Schiehallion Quartzite in the core of the D₂ Creag an Earra Fold around [NN 670 623], as it is folded about the Errochty Synform. The fold core forms a poorly exposed narrow belt of Killiecrankie Schist on the west limb of the synform [NN 670 626] to [NN 656 590], terminating in a closure beneath Loch Rannoch. The best exposures are on the crags north of Kinloch Rannoch around [NN 6555 5904] where the two highly strained boundaries with the Cairn Mairg Quartzite can be established.

The Killiecrankie Schist in the core of the Allt Mor Synform on Craig Varr, described above, similarly forms a parallel belt on the west limb of the Errochty Synform, which closes on the southern slopes of Loch Rannoch on Meall Dearg about [NN 652 564]; the extension of this closure further south by Bailey and McCallien (1937, plate II) and by Rast (1958a, plate III) is precluded by exposures of pink feldspathic Carn Mairg Quartzite. Exposures of banded garnetiferous semipelitic schist (mostly with kyanite) form narrow north–south belts on both sides of Glen Sassun [NN 6592 5321]; [NN 6584 5351]; [NN 655 546] to [NN 6567 5489]; [NN 6553 5496]. Some parts are locally pyritous, and the rocks appear to be D₁ cores of the Killiecrankie Schist. Numerous exposures of pink feldspathic quartzite in the intervening ground show that they do not connect with the outcrop of Killiecrankie Schist on the south of Loch Rannoch [NN 656 571].

Good exposures of the flaggy semipelite and psammite in this core are seen in crags on the north side of the loch [NN 657 592] to [NN 659 602], and in the burn to the north [NN 661 604]; [NN 6588 5967] where the south-eastern boundary is well defined. A good cross-section is seen on the south side of the loch around [NN 656 572].

Thin section examination reveals the following mineral assemblages and textures in the dominant semipelitic lithology. The quartz and feldspar, which comprise up to 50% of the rock, are more or less elongated between the phyllosilicate fabric of muscovite and biotite. Muscovite is usually dominant over biotite. Plagioclase, where weathered and detectable, comprises up to 10% of the psammitic fraction; twinned plagioclase and microcline are only evident in thin (centimetre-scale) psammitic beds within the semipelites. Garnet is common, usually as subhedral crystals up to 5 mm in diameter, but rarely, e.g. above Auchtibart [NN 693 590], as innumerable minute crystals 0.1–0.2 mm across. Kyanite is found only where there is a concentration of graphite in the thin laminae. A light dusting of graphite is not uncommon but the rocks could not be described generally as graphitic in the district. Pyrite and zircon are the most common accessories.

In thin sections of the psammitic rocks there are extremely variable proportions of minerals, but the following percentages are typical: quartz (60–80), muscovite (10–20), feldspar (5–10) biotite (2–5). The quartz and feldspar (altered and untwinned) form a granoblastic matrix of 0.1–2 mm equant grains, becoming more elongate in the S₂ schistosity in more micaceous beds. Feldspar pebbles, up to 3 mm across, are altered or twinned andesine or microcline.

Extremely muscovite-rich pelites, up to a metre or two thick, are seen throughout the formation; in thin section these are commonly graphitic and rarely calcareous.

East of Loch Tay Fault

The Killiecrankie Schist Formation forms one principal outcrop on the south slopes of Strathtummel, on what will be called the Craig Loisgte ridge, from the Loch Tay Fault east to the Frenich Burn (Figure 1). An extension of this outcrop occurs on the north side of Loch Tummel in the core of the D₂ An Tulach Fold; a minor, poorly exposed, belt occurs to the south of Conbhar [NN 816 610] to [NN 835 613].

The main belt is well exposed on Craig Loisgte [NN 8017 5670], Dubh Chnocan [NN 7988 5705] (Plate 1c), and in the Allt Tarruinchon [NN 7995 5616] from the Carn Mairg Quartzite boundary to the Loch Tay Fault; the Frenich Burn provides an almost continuous section, though difficult of access, across the entire formation. The principal lithologies are very similar to those of the area west of the Loch Tay Fault: quartzose mica-schist, pebbly quartzite, muscovite-rich feldspathic psammite and muscovite-rich schist with minor calcareous rocks. However, here, as at the eastern end of the area west of the Loch Tay Fault, there is a dominance of quartz-rich lithologies, with a greater proportion of pebbly psammites. Although usually slightly graphitic, true graphitic schists have only been observed, as discrete thin beds, towards the top of the formation. The upper boundary of the formation is more transitional in nature and thus more difficult to define east of the Loch Tay Fault. Thin section characteristics are as described above.

The thickness of the more or less schistose quartzite beds varies from 20 to 400 cm, commonly with 1–5 cm partings of muscovite-rich pelite. In the upper part of the formation particularly, the thicker beds are generally the coarsest, with rounded granules of quartz (90%) and more angular feldspar reaching 5 mm diameter. Graded bedding is spectacularly displayed, e.g. on the west side of Dubh Chnocan [NN 799 570], on Creag Loisgte [NN 802 567] and north-eastwards, and in the Allt Tarruinchon [NN 5616 7995]. Load-casts occur at the bases of these beds and some may be interpreted as flute-casts. Discontinuous laminae of graphitic pelite, suggesting flaser bedding, occur at the base of some graded beds.

There are two types of calcareous rocks. Most common are beds of a distinctive grey-green biotite-tremolite schist, 0.7–2 m thick, e.g. east of the Frenich Burn [NN 8317 5804] and Meall Urair [NN 8324 6086]. The second is a finely bedded (1–4 cm) amphibole-rich, cream-coloured carbonate, calcareous quartzite rock. Lenses of such calcareous rocks are seen immediately beneath the Carn Mairg boundary in the Allt Tarruinchon [NN 800 562].

Two belts of the Killiecrankie Schist Formation are exposed north of Loch Tummel, around Strathtummel village and on Meal Urair [NN 832 609]. The rocks are characteristic of the transition to the Schiehallion Quartzite: schistose quartzite (rarely pebbly), with minor graphitic pelite and calcareous schist.

Carn Mairg Quartzite Formation

West of the Loch Tay Fault (0–900 m)

The principal, and best-exposed, belt of outcrop is that south of the Meall Garbh–Carn Mairg–Meall nan Eun–Meall Crumach watershed, comprising the steep northern slope of Glen Lyon. Of these watershed hills, both Meall Garbh [NN 647 516] and Carn Mairg [NN 684 512] are particularly well exposed as is the Creag Mhor–Beinn Dearg ridge at the east end of Glen Lyon [NN 713 489].

This belt is a product of major D₂ and D₃ folding of the upper inverted limb of the D₁ Chesthill Syncline (cross-section, (Figure 17)). A second belt of Carn Mairg Quartzite, which occupies the folded core of the D₁ Meall Garbh Anticline, straddles the western end of Glen Lyon within the district, running from a second Meall Garbh [NN 640 490] east to south of Balintyre Farm [NN 688 472]. A small outcrop of the formation in the core of a D₃ antiform occurs at the eastern entrance to Glen Lyon, north of Culdaremore [NN 729 473] (Figure 7b).

The formation as seen in these outcrops essentially comprises massive white, pebbly quartzites and rusty gritty psammites, with local pink feldspathic quartzites and calcareous schists. Carn Mairg [NN 6848 5125] is the type locality and provides the type section [NN 6836 5207] to [NN 6823 5075]. The lower boundary [NN 6836 5207] (Figure 7a) is well exposed and is marked by the appearance of the massive pebbly quartzites in an 80 m-wide transition zone from the impure quartzites and semipelites of the Killiecrankie Schist Formation. A 50 m-wide lens of black graphitic pelite, identical to the Ben Eagach Schist, occurs 200 m above the base of the Carn Mairg Quartzite [NN 682 519] to [NN 677 523] (Figure 7a). It is more poorly exposed than the surrounding quartzites, and can be traced for only 1.5 km, but is substantially thickened in places by folds of the Glen Lyon set. In general, the quartzitic parts of the Carn Mairg Quartzite are over-represented in hillside exposures compared with the softer, recessive-weathering pelites, as the complete exposure in some stream sections reveals.

The remainder of the section comprising massively bedded (0.04–2 m thick) pure quartzite is exposed on the ridge through the Carn Mairg summit and SSW to the Ben Eagach Schist boundary. These rocks are made up largely of stretched and flattened quartzite pebbles, although up to 10% of the rock may comprise angular, white feldspar grains. The quartz pebbles are typically 1–1.5 cm long; the feldspar grains are less deformed and typically less than 1 cm across. Thin seams of muscovite-rich, and commonly graphitic, pelite separate the beds.

In thin section the quartz pebbles have irregular margins, commonly sheathed by envelopes of mica and are seen to be strongly elongated in the dominant S₂ schistosity. The feldspar grains are dominantly microcline with lesser microcline-perthite and generally untwinned plagioclase. The coarser quartzites are usually richer in muscovite than biotite, the micas comprising less than 5% of the rock. The pebbles are set in a granular mosaic of quartz (85%) and feldspar, micas and accessories. Purer, medium-grained quartzites contain little feldspar but are usually rich in pyrite, e.g. as on Creag Dhearg [NN 6869 4833].

The west end of the Creag Mhor ridge, especially the burn [NN 702 478] and the hill, Meall na Moine [NN 705 480], above Chesthill, provide good convenient sections of the typical lithologies. The latter provides good localities for variations within the principal facies, e.g. rare biotitic and garnetiferous schist pebbles (original mud-flakes?) [NN 7079 4735]; [NN 7103 4779]; more common garnetiferous calc-siltstone pebbles at [NN 7052 4760] and pebbles as long as 5 cm seen at a few localities e.g. [NN 7056 4781]; [NN 7020 4786]; [NN 7017 4837]. A section of exceptionally undeformed (but inverted) graded beds is seen at [NN 702 479]. Bed thickness varies from 2 cm to over 2 m, the bases of the thicker graded beds showing the coarsest grains; the tops, commonly of fine psammite, are overlain by laminated graphitic pelite and siltstone. Good Bouma sequences with flaser and wavy bedding may be observed. Some of the finer quartzites show ripple cross-lamination, e.g. in the Allt Muilinn [NN 7213 4960]. Meall na Moine [NN 7065 4800] also provides exposure of the calcareous facies described below. Very convenient exposures of typical massive pebbly quartzite are seen by the road at the entrance to Glen Lyon [NN 7290 4744] (Figure 7b).

Impure, non-pebbly quartzites are seen everywhere among the pebbly beds but in the Meall Garbh–Balintyre belt they comprise the majority of the outcrop. They vary from massive dark gritty quartzites, with a high biotite content and quartz and feldspar grains up to 9.5 mm across, to flaggy garnetiferous schistose psammites, again biotitic and commonly highly pyritiferous. In thin section up to 30% of these strongly cleaved rocks consists of mica, aligned in the penetrative S₂ schistosity; quartz (about 55%), together with the remaining feldspar, forms a granoblastic mosaic, although larger grains, enveloped in mica, appear to be detrital. Good exposure of the more pebbly facies is seen in this Meall Garbh belt, south of the river opposite Balintyre [NN 688 472] and in the Invervar Burn which also gives a section across the less pebbly facies, as well as the calcareous rocks described below. Calcareous schists are found in thin beds throughout the formation but in this belt exceptionally thick (up to 30 m) beds of calcite-garnet-biotite-hornblende schist are exposed on the hill above Invervar [NN 670 489], and similar muscovite-biotite-chlorite-tremolite schists, which in thin section show calcic plagioclase, quartz and calcite, in equal proportions.

The upper boundary with the Ben Eagach Schist is not well exposed at the southern end of the type section on Carn Mairg [NN 6825 5069]. A better impression of this very variable, but usually very transitional, boundary is seen in the Allt Muillin [NN 7245 4946], on Coille Dhubh [NN 7087 4755] or on Creag Dhearg [NN 6958 4865]; these localities have the added advantage of sections that can be continued across the Ben Eagach Schist and into the Ben Lawers Schist. The positioning of this boundary is arbitrary; the bottom of the Ben Eagach Schist has been mapped where the micaceous, graphitic pelites of that formation dominate over quartzite beds. The Carn Mairg Quartzite, in its coarser facies, is less pebbly at this margin.

Beds of unusually coarse, highly feldspathic, pink quartzite are seen east of the summit of the northern Meall Garbh [NN 647 519]. Thin sections show that half of the rock is made of twinned, turbid laths of microcline in a matrix of finer quartz and clear microcline. Plagioclase has replaced some of the microcline laths, and magnetite, intergrown with goethite, occurs as 1 mm cubes and on the grain boundaries. Similar beds are widespread further north and are encountered in Glen Sassunn [NN 653 540] around the summit of Meall Breac [NN 646 548], and extend to the south [NN 657 571] and [NN 655 575] (roadside quarry) and north [NN 658 591] of Loch Rannoch, interleaved with banded amphibolite. Another strip of pink feldspathic quartzite with magnetite cubes forms a belt up to 150 m wide in the Killiecrankie Schist near Glenmore Bothy [NN 7058 5258] to [NN 721 528]. This appears to be a D₁ antiformal fold core.

Concordant amphibolites, mostly garnetiferious and banded on a centimetre or larger scale with more quartz, biotite- or carbonate-rich varieties, are scattered throughout the Carn Mairg Quartzite. They may be individually 10 cm to 10 m thick and are usually delicately interbanded with the adjacent schistose quartzite at their margins, as seen south [NN 6570 5718] and north [NN 658 591] of Loch Rannoch; they are particularly numerous near the Boundary Slide. Some of the amphibolites are slightly calcareous and may be the higher-grade equivalents of the calcareous schists to the south. This suggests that some of the amphibolites may have originated as marls, although a volcaniclastic origin is more likely. Unusually thick and coarse examples are seen close to the watershed west of Cairn Mairg [NN 6663 5225] which, like other rare amphibolites with sharp margins in the formation, are interpreted as intrusive.

The remaining outcrop of the Carn Mairg Quartzite in the district continues north from the Cairn Mairg belt as a number of rapidly diminishing and bifurcating belts, which terminate north of Kinloch Rannoch in the zone of complex D₂ closures folded in the Errochty Synform. The main outcrop continues through the generally poorly exposed ground of Glen Sassunn towards crags on the north-facing slopes south of Kinloch Rannoch. Here a sharp boundary between the pink feldspathic quartzites and the Killiecrankie Schist is exposed [NN 6570 5718] but the upper junction of schistose quartzite with Ben Lawers Schist, e.g. in the burn south of Creag an Fhithich [NN 6655 5663], has been strongly sheared with some loss of strata, and has also probably been subject to late faulting. Much of the intervening schistose quartzite is interbedded with delicately banded, quartz-rich amphibolite, e.g. in the next burn to the north-west [NN 6655 5690]. A sharp, but strongly deformed, junction with the Ben Eagach Schist is seen in the adjacent belt on Creag an Fhithich [NN 6688 5738]; the boundaries of this belt become more transitional to the south, and its boundaries with the Ben Eagach Schist are well exposed in the Innerhadden Burn from [NN 6694 5585] to the waterfall [NN 6700 5591].

The quartzite, in general terms, here and further north, is very schistose, slightly gritty and more feldspathic than in the type locality; it is also strongly folded and more or less highly strained. However, cross-bedding and graded bedding are rarely preserved and the change in character is certainly, in part, one of facies variation. Thin concordant amphibolites, mostly only centimetres thick, are commonly present. The formation is well exposed in a series of crags north of Kinloch Rannoch [NN 654 591] in its pink feldspathic facies, but more typical exposures occur to the east [NN 661 592] and especially in a burn at [NN 662 591], in the various D₂ fold cores; the latter exposure provides a particularly good section in the burn which runs along the Ben Eagach Schist boundary. The boundaries with both the Killiecrankie and Ben Eagach Schists are here transitional in the sense that lithologies are interleaved, but this is certainly as much an effect of tight folding as it is of sedimentary gradation. To the east the formation has been removed by sliding between the Killiecrankie and Ben Lawers Schists on the west side of Craig Varr [NN 6693 5895] and is reduced to 15 to 60 m on the two limbs of the next D₂ antiform at [NN 6725 5908] and [NN 6757 5926]; these latter, remarkably preserved, slivers of the formation can also be traced south of the glen on either side of Creag Mhor Lodge [NN 677 576]. However, here, the Carn Mairg Quartzite is usually impure and is represented by two lenses of gritty and feldspathic quartzite on either side of the fold core. No pebbly rocks are present close to the Killiecrankie Schist, and the younger rocks in the core of the fold consist of a heterogeneous semipelitic sequence of rust-stained fine psammites, graphitic schists, amphibolites and some thin pebbly quartzite beds. There is no major development of pebbly rocks between the Killiecrankie Schist and what more closely resembles a semipelitic transition to the Ben Eagach Schist; the Killiecrankie Schist itself to the east is particularly thin, and that to the west is unusually graphitic. These changes appear to be the result of local facies variations.

The closures of the various belts of the formation around the Errochty Synform between Craig Varr and Creagan Breac [NN 670 610] and to the west are based on extremely sparse exposure. Above the Boundary Slide, an extremely tight reclined fold-core of Ben Lawers Schist closes northwards at Loch Errochty, and is enveloped by graphitic schist and flanked by two belts of Carn Mairg Quartzite. The upper belt is over 200 m thick near the Loch Rannoch Hotel [NN 654 590], and contains pebbly quartzites, numerous thick units of amphibolite as well as some pink feldspathic quartzites. Northwards this belt thins to 60 m, and it is mostly represented by feldspathic quartzite [NN 6525 5937] to [NN 6692 6264], although for most of this distance it maintains a white quartzitic edge adjacent to the Killiecrankie Schist. This lower stratigraphical boundary is well constrained by exposure, but the upper boundary to the west with the Ben Eagach and Ben Lawers schists is concealed by drift and a thick porphyry sill. For 1800 m northwards the belt is unexposed, but the lower boundary can be located at Colrig [NN 6787 6415], and a complete section from the Ben Lawers Schist through to the Killiecrankie Schist is exposed in the nearby burn [NN 681 644] and subsidiary streams to the east. The quartzites and psammites here are not particularly feldspathic, locally gritty, and interbedded with thin graphitic schists.

The lower westermost belt, underlying the core of Ben Lawers Schist, can be traced from the Allt Druidhe with a thin skin of Ben Eagach Schist to its west [NN 6422 5689] northwards to crags west of the Loch Rannoch Hotel [NN 651 589]. It can then be traced with certainty for over 1 km along the eastern edge of the attenuated Boundary Slide Zone succession. No exposure of the quartzite is then present to the west of the Ben Lawers Schist between [NN 650 600] and [NN 666 624], east of Carn Fiaclach. However, tight D₂ cores of quartzite, flanked by graphitic schist, are present within the Ben Lawers Schist [NN 6636 6208]; [NN 6580 6138]; [NN 6572 6100]; [NN 6512 6018] and another on the north shore of Loch Rannoch [NN 6520 5884]. These infolds suggest that the quartzite was formerly present as a continuous belt before it was excised by minor thrust faults and minor porphyry sills. The Carn Mairg Formation is finally seen in the burns on the south shore of Loch Errochty [NN 693 649]; a good section of this 50 m-thick belt of schistose pebbly quartzite, with a sharp upper junction with the thin Ben Eagach Schist together with much amphibolite, is seen in the Riuighe nan Saoraich burn [NN 674 635].

East of Loch Tay Fault (900 m)

The formation occurs in one principal belt of outcrop from Tom Phobuill [NN 778 543] east-north-east to south of Lick [NN 834 578]; to the south of this, at the east edge of the district, the quartzite is repeated about the D₁ Sron Mhor Syncline and it crops out in the core of the D₁ Creag na h- Iolaire Anticline [NN 827 553]. The formation is generally not well exposed but the principal sections are: in the Allt Tarruinchon from the Killiecrankie Schist boundary [NN 7994 5615] upstream almost to the Ben Eagach Schist boundary at [NN 8020 5520]; on the south face of Meall Damh towards the Ben Eagach boundary [NN 7973 5517] and the north slope of Bad Garbh [NN 8191 5602] close to the Killiecrankie Schist boundary (both the latter localities exhibiting abundant graded bedding to demonstrate the stratigraphical order); the Frenich Burn across the whole formation [from NN 8259 5768] to [NN 8260 5615]; the Creag na h- Iolaire crags exposing the core of the D₁ Anticline, but not the Ben Eagach Schist boundary from [NN 8268 5528] to [NN 8262 5567].

The dominant lithology is massive pebbly quartzite, essentially the same as that already described west of the fault. Graded bedding is particularly obvious in the coarser pebbly beds, 10–200 cm thick, near the base of the formation; generally the grading is not well marked, being indicated by subtle variation in grain size within discrete beds of increasingly fine fractions, within 50–300 cm thick units. Sharp erosive bases, channelling and soft-sediment slumping have all been observed in these units and they commonly alternate with laminated, nongraded, impure quartzites, semipelites and pelites. These features are best observed on the Creag na h- Iolaire crags (Plate 3b).

Impure schistose quartzites, similar in appearance to those described west of the fault, are common throughout, but dominate near the lower and upper boundaries. These quartzites, and particularly the thin (up to 4 cm) interbedded graphitic and nongraphitic pelites, are usually pyritiferous and mostly garnet-bearing. In the Allt Tarruinchon [NN 7986 5581] a 10 m-thick, bedded sequence of graphite schist and granular psammites is repeated downstream in a number of folds, in which it is seen to wedge out laterally [NN 7987 5586].

Thin calcareous schists are commonly present in the succession and rarely become thick enough (5 m) to be represented as a bed, e.g. in the Frenich Burn [NN 8246 5736]. The component minerals are calcic plagioclase + quartz (50%), calcite (30%), garnet-muscovite-biotite (20%). Thin (10–20 cm) limestones of granular calcite are rare. The coarse, pink, feldspathic, magnetite-bearing quartzites described west of the Loch Tay Fault have not been observed to the east.

Ben Eagach Schist Formation

West of Loch Tay Fault (0–150 m)

The characteristic lithologies of this formation are seen in three belts of outcrop in and to the north of Glen Lyon, on the three limbs of the D₁ Chesthill Syncline and Meall Garbh Anticline (Figure 1), (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). The sinuous outcrop pattern of these belts is a result of D₂ and D₃ refolding. The central and southern belts merge south of the glen to form the principal area of outcrop and a small outcrop area occurs in the east end of the glen north of Culdaremore (Figure 7b).

The remainder of the principal outcrop lies in the north–south belts from the Cairn Mairg watershed to the south-west slopes of Beinn a' Chuallaich, controlled by the D₂ Creag an Fhithich, Innerhadden and Craig Varr folds. Three minor strips of outcrop occur in the west: the one that runs from the south of Loch Rannoch to Loch Errochty is part of the attenuated Boundary Slide Zone sequence; the others, west of Glen Sassunn [NN 650 540] and north of Loch Rannoch [NN 653 590], are thin envelopes to Ben Lawers Schist outcrop in minor D₂ fold cores. Other small outcrops occur as fold cores within the Carn Mairg Quartzite.

As mentioned above, the boundary between the Carn Mairg Quartzite and the Ben Eagach Schist is a complete transition between the the two lithologies and in the course of the 1:10 000 mapping it was found possible to separate, in much the Glen Lyon area, a transitional facies. On the 1:50 000 map this transitional facies outcrop is incorporated within the Ben Eagach Schist, but it is briefly discussed here as it one of the most useful lithologies for the observation of D₂ and other minor structures.

The rocks of the transitional facies are characterised by rusty-weathering micaceous quartzites, in which white, mostly coarse, feldspar clasts are visible, with graphitic schist interbeds varying in thickness from centimetres to metres. The quartzites vary from fine (silty) to coarse, differing from the impure quartzites of the Carn Mairg Quartzite in their high proportion of graphitic schist. The pure micaceous graphitic pelites of the Ben Eagach Schist proper are less dominant. Thin beds of pure glassy or slightly feldspathic quartzite are unique to the transition zone and it is also the location of some stratabound sulphide mineralisation south of Dericambus [NN 6767 4630] discussed in Chapter 6 (Figure 29). The transitional facies fails in the northern of the three belts around Creag Mhor from the south side of Creag Dhearg [NN 6947 4860] northwards to Meall na Aighean [NN 7064 4967], so that Carn Mairg Quartzite passes abruptly into Ben Eagach Schist. Exposures of massive pebbly quartzite appear locally within the transition zone, e.g. in Gleann Muillin [NN 7101 4981], and in general this zone appears to be the locus for small facies changes. The typical transitional facies can be seen in most of the crags on the south side of Glen Lyon, represented as Ben Eagach Schist on the 1:50 000 map between eastings 65 and 70]; two burns up from [NN 6958 4673] and [NN 6781 4731] offer good sections, both exposing the interbedded pure quartzites and the eastern burn exposes a calc-schist at the Ben Eagach Schist boundary.

The Ben Eagach Schist proper is a very variable sequence of lithologies from the typical graphitic, soft, dull leaden-grey to black schist with variations to a rusty, feldspathic and micaceous quartzite interbedded with the graphitic schist. The graphitic schist is characterised in the field by the presence of pyrite (hence its rusty weathering), by the presence of small garnets and by the common development of several sets of crenulations. In thin sections of the graphitic schist felted muscovites, full of graphitic dust, together with very sparse feldspar and quartz grains, form a phyllitic cleavage (S₂) usually affected by later crenulations. In the more quartzose lithologies quartz and feldspar form a polygonal mosaic between these cleavage domains, marking a tight S₂ crenulation structure. Larger grains (up to 0.5 mm long) of biotite, muscovite and chlorite and garnet (up to 1.5 mm across) are usually present and accessories include common pyrite with ilmenite, apatite, zoned tourmaline, clinozoisite, calcite, actinolite and rutile. Porphryoblasts of kyanite, staurolite and feldspar are only rarely seen.

The impure quartzites, which occur in beds 10–30 cm thick, rarely exceeding 1 m, are interlaminated with the graphitic schist, and are distinctly finer grained than those in either the Carn Mairg Quartzite or in the transitional facies described above. They are not gritty, nor can clasts of feldspar be detected; the thinner beds are the finest grained. However, thick (up to 25 m) beds of pure, coarser, rarely pebbly, quartzite do occur locally within the graphitic schists, chiefly west and south of Creag Mhadaidh [NN 717 470].

Calcareous garnet schist beds (up to 3 m thick) are found on Creag Dhearg [NN 6969 4854] and very thin (10 cm) beds of actinolite schist are common throughout. Some parts of the formation near the Ben Lawers Schist boundary are so rich in actinolite as to become carbonaceous amphibolites, e.g. Creag Ghlas [NN 6420 4979], not unlike the Ben Lawers Schist lithology itself. These amphibolites are up to 30 m thick and of probable sedimentary origin. Other amphibolites in the formation have sharp margins and normal amphibolite mineralogy and are undoubtedly intrusive.

A thin, commonly graphitic, limestone normally occurs at the boundary with the Ben Lawers Schist, particularly in eastern Glen Lyon. This is best seen around Woodend where several thin beds of a pink calcitic sandy limestone are present [NN 7038 4736] and 40 cm of graphitic marble [NN 7078 4735], but it can be seen elsewhere south of Glen Lyon [NN 6471 4597]; [NN 6976 4618].

Where limestones are not developed at the upper boundary with the Ben Lawers Schist, a transition zone, some 2 to 20 m thick west of the Loch Tay Fault, but up to 100 m thick to its east (see below), is present. In this zone the graphite held in the muscovite-rich schists is gradually lost and there is a concomitant increase in the amount of chlorite and carbonate.

Exposure is good throughout Glen Lyon, although not always clean. Sections of the graphitic schist can be seen on many of the crags on the south of Glen Lyon e.g. around [NN 693 468] and [NN 663 475] (Creag Dhubh) and below the transitional facies in burn sections [NN 6958 4673]; [NN 6782 4733] (Figure 29). Clean and convenient sections are seen in the River Lyon below Woodend [NN 715 476] (Figure 7b), where limestones are also seen. However, the type section is chosen in the Allt Muillin [NN 7288 4929] to [NN 7244 4949], giving a complete section through from the Carn Mairg Quartzite boundary, including the transitional facies, to the Ben Lawers Schist. Good sections are also seen in continuity with the Ben Lawers Schist in the area of (Figure 7c) [NN 644 467] to [NN 648 479].

In the remainder of the outcrops, north of Glen Lyon, the formation is thinner, ranging from 100 m commonly to no more than a few metres and there is generally only a narrow zone of transition from the formations above and below. These effects are undoubtedly in part owing to higher strains, although seen in conjunction with the facies changes in the Carn Mairg Quartzite, sedimentary thinning and diachronous facies changes cannot be ruled out. Lithologies, including very abbreviated equivalents of the transitional facies, are as described above. Exposure is poor through the areas east and west of Glen Sassunn, except in burn sections; sections across the whole formation are seen in the Innerhadden Burn below [NN 668 555] and [NN 671 570]. Further north, good sections are seen on Creag an Fhithich [NN 668 574] (but like the Innerhadden Burn section containing many amphibolite sheets), east and west Craig Varr [NN 669 590] and [NN 672 591], and in the two burns north of Kinloch Rannoch around [NN 6625 5938] and [NN 6634 5915]. Mapping of other parts of these belts and of those to the west is based on very scattered exposure; barely 10 m-thick sections are seen on the limbs of the fold either north of Kinloch Rannoch above the Loch Rannoch Hotel [NN 6518 5894] and [NN 6523 5907] or in the Allt Druidhe [NN 6422 5634]; similar sections are seen in the westernmost belt on Spioran Poll Bath [NN 6681 6263], but 20 m are seen in a burn [NN 682 645], south of Loch Errochty, the latter with much interlayering with amphibolite sheets.

East of Loch Tay Fault (250 m)

The outcrop here consists of one belt from the Loch Tay Fault, south of Tom Phobuill [NN 777 542], to Doire Leathan [NN 820 558], where it splits into the three limbs of the Sron Mhor–Creag na h- Iolaire D₁ fold pair; to the west of the Frenich Burn Fault the nose of the latter fold is defined by the Ben Eagach Schist–Ben Lawers Schist boundary.

Lithological variation is as for the area to the west of the fault, with two important differences. Firstly the characteristic transitional facies, described west of the fault, contains a significant body of Carn Mairg Quartzite facies, and the proportion of non-pebbly, and rarely pebbly, quartzite in the Ben Eagach Schist above is higher, with the pure graphitic schist being confined to the uppermost levels. Secondly, these upper levels are distinguished in the east by the presence of the Foss stratabound baryte-sulphide horizons.

Meall Damh and a quarry [NN 793 551] to its west provide an excellent section through the upper part of the transitional facies, with Carn Mairg and Ben Eagach lithologies equally represented, although fine quartzites dominate the lower levels and pebbly varieties the upper. The pebbly facies is part of a lens-shaped unit of coarse quartzite, some 30 m above the base of the Ben Eagach Schist, which has been mapped from the Allt Blair Rainich [NN 7885 5480] across Meall Damh [NN 7973 5517], where it reaches a maximum thickness of 70 m, to Doire Leathan [NN 8107 5557]; sporadic exposures allow it to be traced a further 500 m eastwards, but the unit cannot be traced to the east of the Frenich Burn Fault, although other thick massive quartzites occur at this level. Graded bedding in 3 m-thick beds demonstrate that this unit is a true stratigraphical intercalation of Carn Mairg Quartzite lithology.

The Allt Glengoulandie [NN 7785 5415] to [NN 7805 5416] gives a convenient section across the transition facies into graphitic schist, but the upper boundary is faulted. To the east, the lower boundary with the Carn Mairg Quartzite is seen on the south slopes of Meall Damh [NN 7973 5517] and in the Frenich Burn [NN 8245 5652]. The latter gives intermittent exposure across the formation up to [NN 8195 5485], across the two D₁ closures. The upper boundary with the Ben Lawers Schist in these folds is only poorly exposed in this burn and its tributary to the west [NN 8127 5510] and across to Ciochan a' Chop [NN 8088 5524] (Figure 26). In the tributary [NN 8127 5510] the boundary is marked by a thin impure limestone and heavy pyritisation, and the transitional facies south of here contains thick fine-grained quartzites with strong pyritisation e.g. around [NN 815 551]; otherwise the baryte–sulphide mineralisation of the southern limb of the Creag na h- Iolaire Anticline is not evident, except for minor occurrences of quartz–celsian beds reported below these quartzites around [NN 813 550]. Thin amphibolites are also seen at this level in these graphitic schists. The best exposures of the formation generally are above here, in the nose of the Creag na h- Iolaire Anticline, on the north-facing slopes of Creag an Chanaich around [NN 813 547] and towards the track to the north.

On the southern limb of the Creag na h- Iolaire Anticline, trackside exposures [NN 8063 5472] and for 200 m east, show that the transition from graphitic schist to the muscovite schists of the Ben Lawers Schist takes place over a few metres. Here also are the most westerly signs of the baryte-sulphide mineralisation which develops fully to the east, within the 100 m-thick graphitic schist facies at the top of the formation (Figure 26). Details of this mineralisation, which occurs at several horizons within a zone some 80 m thick at its maximum, are given in Chapter 6. Between this locality and the Frenich Burn Fault, some 8 m of the graphitic facies of the Ben Eagach Schist are seen above the mineralisation, in sharp contact with the Ben Lawers Schist especially along the track between [NN 8122 5457] and [NN 8137 5450]. The bulk of the visible mineralisation starts above two discontinuous thin amphibolites, of probable volcaniclastic origin [NN 8129 5458] and is well displayed eastwards in road cuts and opencast pits as far as [NN 8170 5450] (Figure 27). The boundary may also be observed in the Frenich Burn east of the fault at [NN 8196 5491], where the mineralisation occurs only a few centimetres below it, and can be located in drainage ditches and the trackside exposures further east; pebbly quartzite beds [NN 826 549] appear again close to the Ben Lawers boundary, above the baryte mineralisation.

Ben Lawers Schist Formation

West of Loch Tay Fault (200 m)

The principal outcrop occurs in three belts in the Glen Lyon area with only a minor area of outcrop in the three belts centred on Kinloch Rannoch (too small to be shown separately from the Ben Eagach Schist on (Figure 1)). The major area is on the south slopes of Glen Lyon, part of the outcrop related to the D₃ Ben Lawers Synform to the south. At the western margin of the district a second belt is in the core of the D₂ Ruskich Antiform (Figure 7c), while the third, northern, belt occupies the core of the D₁ Chesthill Syncline. Exposure is generally good in the southern and western belts, but is patchy in the north.

The formation is an extremely variable sequence of calcareous schists, semipelites, and quartzites, together with minor amphibolites; it is nongraphitic and commonly contains pyrite. The formation is difficult to characterise, but is probably dominated by rhythmically bedded calcareous schists and quartzites, the beds commonly a few centimetres thick, but varying from millimetres to several metres. The beds are commonly lens-shaped and discontinuous, features which can only partly be attributed to the commonly observed boudinage and tight minor folding (Plate 3c), (Plate 4c). Sedimentary structures have not been observed. The quartzites are generally massive, characteristically up to 30 cm thick, rich in ferroan calcite giving the rock a yellow, pitted, appearance. They are most dominant in the upper part of the formation, where they may locally be pebbly (plagioclase and minor microcline). An exceptionally pebbly bed of quartzite, rich in blue quartz clasts, is seen 1 km west of Boreland Farm [NN 704 447].

As mentioned above, the pelitic rocks are more dominant in the lower half of the formation. A wide transition zone at the base of the formation of weakly calcareous, nongraphitic semipelites is seen north of Slatich [NN 631 483]. Eastwards on the south slopes of Meall Garbh [NN 641 485], the boundary is of normal layered calcareous pelites in sharp contact with with the Ben Eagach Schist (Figure 7c). The pelites are usually muscovite-rich, with some chlorite lending a characteristic green colour to the rocks. Where the pelites are rich in amphibole, biotite becomes the dominant mica; the amphiboles have frequently grown across the bedding and schistosity planes and may develop as garbenschiefer up to 8 cm long. Calcite rhombs, small garnets and pyrite cubes are frequently seen in the pelites and semipelites, and pods of vein quartz are usually present. However, it has not proved possible to erect a stratigraphy within the formation (compare with Elles, 1926). In the southern belt, where the metamorphic grade is lower, the formation has the aspect of a soft grey-green phyllite, and hornblende is a less common constituent.

Thin limestones are seen, particularly in the lower third of the formation. Thin (40 cm or less) micaceous and graphitic limestones are particularly common at the lower boundary of the formation, e.g. on Creag Mhadaidh [NN 72 47] (Figure 7b). Beds of orange-coloured, purer, nongraphitic limestone are seen elsewhere, e.g. a wide zone south-east of Meall na Moine [NN 7039 4736]. Individual beds are up to 40 cm thick but commonly occur in alternating sequences, with pelites and amphibolites, up to 5 m thick. The beds are slightly gritty and structures suggestive of cross-bedding and slumping have been observed.

Pyrite, and less frequently chalcopyrite and pyrrhotite, are common constituents. Smith (1977) noted a particular enrichment some 10–400 m below the Farragon Formation. The only noticeable concentration within the district is above Roromore [NN 6390 4672] ((Figure 7c); Chapter 6).

Thin sheets of amphibolite are found throughout the formation. Some of these are massive, coarse grained and show no indication of being gradational, or interbedded with the adjacent sedimentary rock; some are discordant to the bedding and are clearly intrusive. However, other concordant amphibolites, equally as massive at their centres, do show an apparent interlayering with sedimentary rock at their margins or may grade into heavily hornblendic schists. Beds of quartzite occur rarely within these bodies and, in one instance above Invervar [NN 6491 5032], there is a limestone. In the upper part of the formation, a lamination of amphibole-rich and quartzofeldspathic layers in some sheets is reminiscent of the Farragon Volcanic Formation above. An origin as volcaniclastic or marly sediments for these bodies is indicated.

In thin sections of the quartzites, quartz, plagioclase (andesine-labradorite) and calcite in varying proportions form a granular mosaic, which in the pelites or semipelites are mixed with, or alternate in thin beds with, the (S₂) schistosity-forming phyllosilicates and hornblendes. Most of the pelites and semipelites are texturally dominated by the strong S₃ crenulations, around which the phyllosilicates are rotated and locally recrystallised. Garnet, actinolite, clinozoisite, idocrase and microcline have been found locally as porphyroblasts; apatite, rutile, sphene, zircon and opaques are common accessories. The limestones consist of equant mosaics (1–2 mm) of ferroan calcite or dolomite (90%), quartz (10%) and minor epidote/clinozoisite. The amphibolites of suspected volcaniclastic origin are not significantly different in their mineralogy from those of certain intrusive origin; coarse hornblende occurs in a fine matrix of biotite, clinozoisite, hornblende and plagioclase.

Good clean exposure of the lower part of the formation and its boundary with the Ben Eagach Schist is seen in spectacular sections in the River Lyon downstream from [NN 7217 4767] (Figure 7b) and on Creag Mhadaidh [NN 720 470]. The more chloritic facies of the southern belt is best seen on Creag Mhadaidh [NN 710 460]. The type section is chosen above Roromore (Figure 7c), where a small burn [NN 6385 4686] to [NN 6404 4642] allows virtually the whole formation to be examined, together with sections in the Farragon Formation below and the Ben Eagach Schist above; it also includes the pyrite-rich horizon as well as amphibolites.

In the belts of Ben Lawers Schist to the north of the Meall Garbh–Carn Mairg watershed, only the lower parts of the formation are seen, of which a large proportion are amphibolite sheets; exposure is generally poor, although most of the variations described above can be observed. In the long thin outcrop of the Creag an Fhithich Syncline, most of the exposed rock in the south is amphibolite, but good exposure of the formation, with several thick quartzites, is seen in the Innerhadden Burn downstream of [NN 665 547] to the disturbed Ben Eagach Schist boundary [NN 668 555]. Scattered exposure is seen again on Creag an Fhithich [NN 667 574], although again much is amphibolite. The belt immediately east of the Boundary Slide Zone, not previously recognised, is well exposed at the following localites: in the Allt Druidhe [NN 6421 5635] to [NN 6423 5686]; between the north shore of Loch Rannoch [NN 6528 5882] and good exposure on the hill above; east of Meall Dubh [NN 651 602] to [NN 653 602]; on the crags south and north of Carn Fiaclach [NN 659 614]; [NN 6675 6260]; in the burns [NN 6743 6347] and [NN 6833 6463] to [NN 6865 6486]; above Loch Errochty [NN 6905 6490].

East of Loch Tay Fault (200 m)

The outcrop of the formation is in one broad 2 km-wide belt from the Loch Tay Fault at Glengoulandie [NN 765 520] to Meall Tairneachan [NN 809 544], where it splits into a narrow northern tongue which marks the core of the D₁ Sron Mhor Syncline towards Lick [NN 832 570] and a southern belt on the south limb of the D₁ Creag na h- Iolaire Anticline. Exposure is particularly good in the craggy areas around Meall Tairneachan, but patchy elsewhere.

Lithological variation is much as for the area west of the fault described above, but here the lower 100 m or so are consistently muscovite-rich schists, not noticeably quartzitic or hornblendic. Only examples of the best exposures are given below. The slopes above Glengoulandie are poorly exposed, but useful traverses across the whole formation, the width of which is variable due to major D₁, D₂ and D₃ folding (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 14) can be made as follows: from Meall Gorm [NN 793 548] to Creag Chean [NN 796 530]; from the east side of Meall Tairneachan [NN 809 544] to Tom an Oir [NN 806 537] and south to the Farragon Formation [NN 800 529]; from the mine at Creag an Chanaich [NN 813 545] south and south- west to the Farragon Formation [NN 8175 5375]; from the Ben Eagach Schist boundary near the track at [NN 825 550] south along the cliff of Creag an Loch to the Farragon Formation at [NN 826 544].

A more convenient, but not always accessible, locality for the upper part of the formation and its boundary with the Farragon Formation is in the gorge of the Keltney Burn [NN 7669 5164]. The outcrop which forms the core of the Sron Mhor Syncline (see also Ben Eagach Schist, above) can be examined on Ciochan a' Chop [NN 808 552], in the Frenich Burn [NN 826 561] and on the hill to the north-east [NN 832 568].

Farragon Volcanic Formation (80–150 m)

This formation is exposed as a continuous 100–150 m thick unit across the district east of the Loch Tay fault, marking the northern limit of the Flat Belt. West of the fault there is only minor outcrop of the thinner representatives of the formation in Glen Lyon, in the core of the Ruskich Antiform (Figure 7c). Exposure of the formation is generally good in burns and on the higher crags. The descriptions below apply to the outcrop east of the Loch Tay Fault, where the type section is chosen as the ridge south of Creag Chean between [NN 798 527] and [NN 801 528]; at the Glen Lyon outcrop excellent exposures, though of a more limited range of lithologies, are more easily accessible above and east of Slatich around [NN 640 477].

In any one section the formation is a very variable sequence and is equally variable from section to section; this is illustrated in (Figure 8), where the succession in 11 of the best sections is illustrated. The formation is defined by the first appearance below, and the last appearance above, of fine-grained, generally layered, amphibolite. A typical section is dominated by this lithology but may contain varying proportions of coarse-grained amphibolite, garnet-mica schist and thinly bedded milky-white quartzite. These lithologies are interbedded on scales from tens of metres to tens of centimetres; the schists, the fine amphibolites and the quartzites also are gradational one into the other. Other features of the formation are: the presence of feldspar and quartz clasts in some beds of all three lithologies; pyrite is not more common than in the other formations below, but is in a few places concentrated in centimetre-thick beds; the formation does not contain the calcareous lithologies of the Ben Lawers Schist, nor is it noticeably graphitic.

The amphibolites, all of which are conformable, are of two types: coarse-grained (massive) and fine-grained (banded or massive). The coarse-grained variety has in every respect the same appearance as amphibolites within other formations, described below; the units have massive interiors while their margins are schistose and their contacts sharp; they are interpreted as pre-metamorphic intrusive sheets.

The finely laminated amphibolites dominate in units which may be up to 20 m thick, although discrete beds 5–10 m thick alternate with thin schists and quartzites. The lamination is due to commonly thicker (0.5–10 cm thick) hornblende-rich layers alternating with thinner (0.1–1.0 cm) pale pink to white quartzofeldspathic layers; the layers are in some places sharp, but usually gradational and, rarely, graded from a sharp quartz-rich base to a hornblendic top. Laterally the individual layers are remarkably persistent over several metres, although disrupted beds and lens-shaped variations do occur. This lithology may grade, vertically, into hornblendic quarztite or into unlayered amphibolite. The delicate lamination is usually affected by small-scale tight to isoclinal folds (D₂), which in the Glen Lyon exposures can sometimes be seen to fold earlier isoclines (Plate 5).

Clasts of white quartz and of pink or white plagioclase, 2–10 mm long, are common in the fine amphibolites, usually scattered, but in some places concentrated in single quartzofeldspathic beds; in the latter case the layering is augened about the clasts. Common features of these amphibolites are garnet porphyroblasts with marked quartz pressure shadows and a strong hornblende lineation (Plate 8). In thin section the melanocratic bands are fine grained (0.1–0.5 mm) comprising 70% hornblende (and minor Ca-poor amphibole) and the remainder dominantly plagioclase with variable quantities of fine-grained quartz, clinozoisite, calcite and chlorite with sphene, magnetite, pyrite, apatite, rutile and zircon as accessories. The plagioclase in the pebbles and matrix is usually untwinned. The leucocratic bands are more or less feldspathic quartzites, with a little muscovite and clinozoisite.

A striking feature of many exposures of the formation is the interbedded white quartzites. Individual beds are commonly less than 0.5 m thick but do rarely reach 15 m. Internally, they are bedded on a centimetre or thicker scale, sometimes with mica-schist or amphibolite interbeds a few centimetres thick. Tight to isoclinal folds, with a wavelength of tens of centimetres, characterise exposures east of the Loch Tay Fault. The quartzites commonly contain rounded quartz and white or pink feldspar clasts (3–30 mm across). Some of the feldspar clasts are microcline and a few lithic quartz-albite clasts have been identified in thin section. The quartzites comprise mainly fine-grained quartz with plagioclase, few phyllosilicates and rare K-feldspar; rare millimetre-thick beds rich in K-feldspar are seen and others show centimetre-scale grading from coarse quartz to fine quartz with phyllosilicates. Pyrite is locally concentrated in beds up to 2 cm thick.

Mica-schists, apart from the thin interbeds with the above lithologies, occur in places as discrete beds up to 10 m thick. Some are muscovite- or biotite-chlorite-hornblende-rich, but others are garnet-mica schists identical to the Ben Lui Schist above. Calcite may be a minor constituent. Interbedded (less than 3 cm thick) quartzites, usually pyritiferous, are very common. These schists may also contain scattered quartz and feldspar clasts and some beds appear to consist of closely packed clasts up to 15 mm long of highly deformed quartz, and rare feldspar.

All of the above features may be seen in the type section south of Creag Chean between [NN 798 527] and [NN 801 528] and in most of the other sections indicated in (Figure 8). However, as indicated, it is difficult to trace a sequence of the various lithological variations described from one section to another, indicating rapid facies changes with lens-like interfingering, possibly enhanced by the boudinage resulting from the high strains these rocks have undergone. As an example, a 300 m-long, 10 m-thick lens of quartzite within schists and amphibolites can be determined from mapping between [NN 7853 5229] and [NN 7904 5244]; amphibolites [NN 775 518] and [NN 7710 5175] show a similar tapering. West of the area represented by (Figure 8), less complete but more convenient sections are seen in the Keltney Burn [NN 767 515] (not always accessible) and the Allt Coire Pheiginn [NN 756 505].

At the west end of the part of Glen Lyon within the district, exceptionally clean exposures of the formation may be examined above the road 1 km west of Ruskich [NN 6385 4773], for 400 m to the east and several hundred metres up the hill to the north (Figure 7c). The formation here is only 20–30 m thick and is exclusively composed of finely layered amphibolite, which reveals excellent examples of folding and refolding (Plate 5); the adjoining exposures provide a complete section into the adjacent formations, the boundary with the Ben Lawers Schist being gradational over several metres but that with the Ben Lui Schist sharp. The burn south of the River Lyon [NN 6385 4687] also provides a section. On the south limb of the Ben Lawers Synform a few scattered exposures of banded amphibolite are seen south-west of Boreland [NN 7142 4444], but a burn immediately south of the district, west of the Fearnan Hotel [NN 7100 4418], provides a good section through some 100 m of finely banded amphibolites and quartzites, as well as poor sections in the adjacent formations.

Sedimentary environments

From the shallow water deposition of the Schiehallion Quartzite, the onset of the presumed deeper water during the deposition of the Killiecrankie Schist was certainly transitional, as shown by the interdigitation of the original sand and more silty muds of the two lithologies. However, the passage is fairly rapid if viewed in terms of the few tens of metres in which the change takes place. The transition into the yet deeper-water facies of the Carn Mairg Quartzite is more gradual, with beds of pebbly turbiditic sand becoming generally more evident towards the upper boundary of the Killiecrankie Schist Formation. However, there is clear evidence of this coarsening becoming more marked towards the east of the Schiehallion district, with the two areas of pronounced coarsening in the centre, the interbedded pebbly beds of the areas south-west (Meallanan Odhar) and south-east of Schiehallion (Dun Coillich), possibly representing two small fans or channels. This would seem to be part of the regional pattern of the general pebbly nature of the upper part of the formation east of the Loch Tay Fault, as seen in the district and continued further east in to the Pitlochry district (Harris et al., 1978).

The lithologies of the Cairn Mairg Quartzite and of the Ben Eagach Schist formations, and their sedimentary environments, are closely related to those of the Schiehallion Quartzite and the Killiecrankie Schist formations. The evidence of basin-deepening seen in the transition from Schiehallion Quartzite into the less well-sorted, muddy and slightly carbonaceous Killiecrankie Schist is emphasised by the first appearance of graded pebbly turbidites. The increasing steepening and instability of the basin floor, associated with probable faulting in the South-west Highlands (Anderton, 1977), and here associated with the appearance of volcaniclastic deposits, continues into the Carn Mairg Quartzite with its massive pebbly turbidites which, in the Ben Eagach Schist, interfinger and channel within the encroaching euxinic carbonaceous muds.

There is a consistent pattern in the district in the manner in which pebbly facies become more dominant in the east. This is first seen in the possible fan-like distribution of pebbly facies in the Killiecrankie Schist, to the south and east of Schiehallion, and continues with the eastward coarsening of the Carn Mairg Quartzite in the Glen Lyon area and to the east of the Loch Tay Fault; the distinctive pink pebbly facies of Meall Garbh and the Loch Rannoch area also suggests discrete fan deposits. This trend continues into the Ben Eagach Schist east of the Loch Tay Fault, with the eastward thickening of the transitional quartzitic facies, the persistence of the Carn Mairg pebbly facies, and the possibly fault-controlled hydrothermal activity related to the baryte-sulphide mineralisation.

The Ben Eagach Schist is marked by the first major appearance in the Argyll Group of graphitic pelite, and the Ben Lawers Schist by the non-graphitic calcareous rocks. Deposition of a channelised pebbly facies may well have continued later in parts of what was otherwise a relatively sediment-starved basin, and consequently the boundary between the Carn Mairg Quartzite and the Ben Eagach Schist is essentially a facies boundary rather than a time plane. Similarly the first appearance of calcareous turbidites marking the start of the Ben Lawers Schist may also have been diachronous.

A significant, basinwide, change in sedimentary environment came with the onset of the fine quartzites, carbonates, pelites and presumed volcaniclastic rocks of the Ben Lawers Schists; this assemblage is slighty anticipated in the thin calcareous schists and volcaniclastic rocks of older formations in the subgroup. The evident shallowing at this time, interpreted as subtidal in the South-west Highlands (Anderton, 1977), shows signs of a return to the deeper basinal conditions with the incoming of noncalcareous quartzites and pelites in the Farragon Formation which herald the turbidites of the Ben Lui Schist Formation above; evidence of major volcaniclastic sedimentation at this level is now clear.

Crinan Subgroup

Ben Lui Schist Formation (500–800 m)

The principal outcrop of this formation is a kilometre-wide belt, to the east of the Loch Tay Fault, stretching 10 km north-eastwards from Fortingall. The formation is generally well exposed and the following localities offer the best sections: good stream sections across the entire formation in the Allt Coire Pheiginn [NN 7640 5034] to [NN 7562 5054] and the Keltney Burn [NN 7674 5063] to [NN 7673 5148] (not always accessible); the almost continuous crag exposure between the Loch Tay Limestone and the type section for the Farragon Formation, west of Loch Farleyer [NN 804 519] to [NN 800 527], which is chosen as the type section for the area, although the boundary with the Loch Tay Limestone is not exposed; good, but partial exposure, on Creag Odhar [NN 776 510], Creag nan Cop [NN 786 517] and Creag Brollachain [NN 822 536] and east to the south slope of Creag a' Mhadaidh [NN 830 538].

At the west end of that part of Glen Lyon that lies within the district, the Ben Lui Schist outcrops in the core of the D₂ Ruskich Antiform (Figure 1), (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). Exposure is good on the hillside above Slatich [NN 639 478] to [NN 639 486], but limited to the lower part of the formation (Figure 7c). The formation is also represented, to the south of Boreland around [NN 718 446], on the south-eastern limb of the Ben Lawers Synform; however, it is not exposed in this area. The description below applies principally to the outcrop east of the Loch Tay Fault.

The formation can be generally characterised as a garnet-quartz-mica schist or semipelite, and is a rather monotonous sequence of dominant semipelites alternating with pelites and psammites on scales from centimetres to several metres. There are minor graphitic pelites, calcareous pelites and psammites; amphibolites are not common and all appear to be intrusive. The formation, which outcrops on the hinge zone of the D₃ downbend from the Flat Belt to the south, has suffered high strains in comparison to the formations above and below. This has obscured the undoubted pebbly nature of some of the semipelites and psammites as well as contributing the quartz veins and pods that characterise much of the formation. Since the formation is less affected by later crenulations than most other pelitic formations in the district, its petrography, with respect to the nature of S₂ and of garnet growth, is important in the understanding of the structural and metamorphic development of the rocks.

The dominant semipelite lithology is very consistent in its mineralogy. Quartz, muscovite, biotite and feldspar (An_35–40), generally in that order of importance, form a penetrative (S₂) schistosity which can usually be seen, in the field and in thin section, to be oblique to bedding. The quartzofeldspathic components (up to 20% feldspar in thin section) mostly form a granoblastic matrix in the S₂ microlithons but, in the semipelites as well as the psammites, highly flattened detrital grains of quartz, up to 1 cm long, are commonly present; smaller, rounded, grains of both quartz and feldspar, up to 2 mm across, are also scattered throughout the rock. Feldspar porphyroblasts (twinned plagioclase An_35–40) highly charged with inclusions, are rare in thin section. Quartz is also the dominant component of the thin (1–5 cm), strongly rodded, veins which parallel the fabric.

Phyllosilicates may form up to 30% of the rock. Muscovite (with minor chlorite) is dominant in the pelites, in some places to the exclusion of biotite, as well as in the semipelites, although biotite may dominate over muscovite in the psammites. Concentrations of biotite commonly mark bedding and it usually forms coarse flakes up to 2 mm across, aligned in S₂. Small muscovite flakes parallel to S₁ are commonly obique to, or define folds within, the S₂ cleavage domains. Common accessories are graphite, ilmenite, apatite, rutile, zircon and tourmaline. Pyrite and other opaques are usually concentrated on bedding in the semipelites and thin (cm) stratabound concentrations of pyrite cubes are common in the psammites.

Deep red almandine garnets (1–15 mm diameter) are ubiquitous in the pelites and semipelites, concentrations usually picking out the bedding. Exceptionally, thin (1–2.5 cm), pink, quartz-rich beds in the semipelites, comprising up to 60% minute (0.1 mm) garnets, can be traced for several metres, in many places defining tight folds. Isolated pods of quartz-garnet rock are also common. In thin section, the garnets are subhedral and form augen within the S₂ fabric, with well-developed quartz-chlorite pressure shadows. Their most important feature, in this and similar lithologies, is the preservation within them of inclusion trails representing the S₁ fabric which, as discussed in Chapter 5, suggests pre- to post-S₂ growth. Post-S₂ kyanite porphyroblasts have been identified immediately to the west of the exposures in Glen Lyon, while 5 mm porphyroblasts of staurolite have been identified in thin sections from both the Glen Lyon exposures and from graphitic pelites south of Creag nan Cop [NN 785 517].

Other lithologies include white quartzites, graphitic schists and calcareous rocks. Very pure quartzites with little feldspar, up to 70 cm thick, are common near the lower stratigraphical boundary. Elsewhere, quartz-rich psammites up to a metre thick occur throughout the formation, but, apart from rare suggestions of grading into the semipelites with which they are interbedded, they are of very uniform thickness and show no internal sedimentary structure. Strongly deformed pebbles, up to 1 cm long, of quartz and feldspar can be discerned at the base of some units. Rare (10–50 cm) beds of coarse pebbly (quartz, plagioclase and quartz–plagioclase rock fragments in thin section) schistose quartzite are seen throughout; one occurence is immediately above the Farragon Formation in the south bank of the Allt Coire Pheiginn [NN 7553 5045].

Thin beds of graphitic schist are not uncommon in the formation. The occurrence mentioned above, south of Creag nan Cop [NN 785 517], is associated with thinly bedded hornblende schists and calcareous schists. Thin (< 0.5 m) calcareous schists, calcareous quartzites and gritty limestones are uncommon generally, but more common in the upper parts, towards the Loch Tay Limestone boundary. That upper boundary is transitional and, although fixed at the first occurrence of obvious limestones in the field, the upper few tens of metres of Ben Lui Schist are interbedded with slightly calcareous 'silty'-looking schists; these are best seen in the two burns at Garth Castle, above their confluence [NN 7644 5030]. This junction is also seen in the Coire an Easain [NN 7946 5175], but is otherwise poorly exposed.

Tayvallich Subgroup

Loch Tay Limestone Formation (200–300 m)

This formation extends from the Loch Tay Fault, north of Fortingall, north-eastwards in a 300–400 m-wide, 20 km-long, belt. It is poorly exposed. The most complete and type section for the district is in the Allt Coire Pheiginn from [NN 7639 5033] to [NN 7666 5017], which provides continuous exposure with the adjoining formations; the section in the Allt Odhar from the Loch Tay Fault [NN 7386 4856] to [NN 7406 4820] is cleaner, although not always easily accessible, and does not expose the lower boundary. A small roadside quarry on the B846 [NN 7683 5036] together with adjacent hillside exposures form the most accessible locality, while exposures within the formation at Creag Odhar [NN 7889 5120], and at the lower boundary at Coire an Easain [NN 7946 5175] and west of Loch Farleyer [NN 805 519] are close to the Ben Lui Schist type section. The formation comprises mainly well-bedded pure and impure (originally muddy, silty and sandy) limestones; thin quartzites and volcaniclastic amphibolites comprise a minor proportion and there are locally thick intrusive amphibolites.

The pale-buff weathering, grey-hearted, pure limestones are composed of 90% or more calcite; scattered equigranular, rounded quartz grains and minor opaques account for the remainder. Interbedded with these, on the scale of 10 cm or more, are the impure limestones, which form the majority of the succession; these comprise up to 80% calcite with various proportions of quartz, muscovite, biotite and clinozoisite (in general order of importance); all the limestones are slightly graphitic.

Thin (2–10 cm) ribs of 'silty' limestones and calcareous quartzites are particularly noticeable near the two boundaries and are more especially a feature of the upper transition into the Pitlochry Schist Formation, and contain increasing proportions of fine-grained quartz and muscovite. The finer 'silty' rocks are marked by a well-developed S₂ cleavage which, combined with the presence of 5–10 cm cross-laminations, provides useful facing and structural information; these relationships can be seen in the Allt Coire Pheiginn [NN 766 502] and in the Allt Odhar, near the upper boundary [NN 7406 4826]. This upper boundary is particularly transitional with alternations of limestones and calcareous schists with semipelites of the character of those in the Pitlochry Schist above.

Thin quartzites, particularly, impart a ribbed appearance to many exposures, e.g. west of Loch Farleyer [NN 804 519]; they comprise about 80% quartz and a little plagioclase (An_50–55); calcite and muscovite comprise the remainder. Graded bedding in these units is confirmed in thin section from the decrease in both size and proportion of quartz to mica. Thin (< 7 cm) garnet-biotite schists are also encountered less commonly throughout the succession.

Conformable amphibolites are a common feature of the formation, particularly evident in the continuous burn sections. Some of these are thick (10–40 m), e.g. in the Allt Odhar [NN 739 485] and, although they have an increasingly schistose texture towards their margins, the boundaries with the adjacent limestone are sharp. The petrography of these bodies is the same as that of other intrusive sheets in the district (Chapter 4). However, there are many thinner amphibolites, ranging from a few centimetres to a metre or two in thickness, e.g. in the Allt Coire Pheiginn [NN 7648 5022] and the Keltney Burn [NN 7672 5060] which usually show one or both margins to be not only schistose, but to grade into or to be interlaminated with the sedimentary rocks. These bodies contain increasing amounts of tremolite, epidote, calcite and quartz towards their margins, in addition to the usual feldspar and hornblende, and are interpreted to be of volcaniclastic origin.

Southern Highland Group

This group is represented only in the south-eastern corner of the Schiehallion district, east of the Loch Tay Fault, within the Flat Belt of the Tay Nappe. The only formation represented is the Pitlochry Schist, although green bed horizons are poorly exposed in the core of the D₃ Loch Tay Antiform across the Strath Tay in the south- east corner of the district (Figure 1), (Figure 14). Barrow et al. (1905) attributed the rocks stratigraphically above the 'Green Beds', only poorly exposed in the Schiehallion district, to the Ben Ledi schists and grits. Bearing in mind the undoubted diachronous nature of boundaries in the Southern Highland Group and the consequent uncertain status of the various formation names, the original name Pitlochry Schist is retained for the time being for all the metasedimentary rocks exposed in the district, above the Loch Tay Limestone.

Pitlochry Schist Formation (over 300 m)

This formation is moderately well exposed on the hills south of Kenmore and the prominent mass of Drummond Hill, between the valleys of the Lyon and Tay. The formation, as a whole, is inverted in the Flat Belt (Figure 14). The best sections south of Kenmore are in the two burns which meet at Tombuie Cottage [NN 7895 4465] and the hill to the west (Figure 31), although much of the exposure is of conformable amphibolite sheets and post-Caledonian intrusions; the Urlar Burn at the east margin of the district upstream from [NN 826 451], also gives good sections (Figure 30). On Drummond Hill the forestry roadcuts provide useful sections, but the crags are very often difficult of access owing to the afforestation and steep faces. To the north-west of the Flat Belt the formation begins to bend down with increasingly steep dips, in a belt stretching 20 km north-eastwards from Fortingall. In this belt, better sections, giving a more representative view of variations in the formation, are seen in the Allt Odhar (or Fortingall Burn) upstream from Glen Lyon House [NN 736 471], and the Keltney Burn [upstream from 774 491] which offer almost continuous exposure through the lower half of the succession from the Loch Tay Limestone boundary. The latter section is taken as the type section for this (stratigraphically) lower half of the formation, which can be complemented with the Camserney Burn as the type section for the upper half. Crags around the track north from the Camserney Burn [NN 8080 5100] give good exposures which can be followed on to Loch Tay Limestone exposures [NN 8035 5188] and through to the type section of the Ben Lui Schist [NN 804 519] to [NN 800 527]. Crags on the hills also give sections across lower parts of the succession, e.g. above Blairish [NN 760 490], and to the north-east of Coshieville around [NN 782 500]. The forestry road-cuts in Dull Wood east of [NN 795 495] provide good detail of the upper parts of the successsion.

The lower parts of the formation comprise the same lithologies, and have essentially the same characteristics, as those seen in the Ben Lui Schist, that is quartz-mica schists with variations into micaceous pelites and quartz-rich psammites, interbedded on the scale of centimetres to metres. However, partly due to less intense deformation, the pebbly nature of many of the beds of psammite becomes evident some 100 m into the Pitlochry Schist Formation. Downstream from this level in the Keltney Burn [NN 768 500], and especially in the Camserney Burn, from [NN 8080 5100] to the topmost beds seen around [NN 8145 4965], the thickness (20 cm up to 1 m) and frequency of these schistose pebbly quartzites (grits) increases. These beds, where cleanly exposed, show convincing (usually structurally inverted) graded bedding; cross-laminations in the upper silty 10 cm can rarely be detected; the uppermost few centimetres of such beds are in places a graphite-rich schist. These features are generally apparent in the principal exposures mentioned above and on the Drummond Hill exposures identified below.

Apart from this coarsening-upward trend, the variations in the thinly bedded alternations of garnet-mica pelite and semipelite, fine quartz psammite and pebbly psammite are impossible to characterise. In a few places it is apparent that the graded schistose grits are separated by laminated (centimetre-scale) mica-schist/fine psammite (original silt/mudstone) beds. These 1–5 m intervals contain many beds, up to 10 cm thick, of more or less graphitic schist; some of these beds are intensely graphitic and many are lensoid in cross-section.

One particularly interesting feature of some intensely graphitic 5–20 mm beds is that they are seen, in thin-section, to comprise up to 80% small apatite grains, which together with graphite usually grade upwards; other components of these beds are fine-grained quartz, feldspar, muscovite, biotite, carbonate, monazite, allanite and tourmaline. Good sections of thin pebbly psammites, garnet and graphitic schists including some of these apatite-rich beds are seen on Drummond Hill in trackside exposures [NN 7482 4506], in a 400 m section west of [NN 7508 4530], on crags, above Black Rock [NN 7628 4589] and elsewhere [NN 7781 4731]. Thin sections show these garnet schists to be exceptionally rich in ilmenite (Plate 9d).

Another unusual feature of this formation, observed in a few places, are 1–2 cm-thick semipelites crowded with tiny (0.005–0.1 mm) garnets. These garnets are slightly richer in MnO and CaO (electron-microprobe analyses) than the usual almandine garnets. Beds of this type are conveniently seen in roadside crags [NN 7833 4474], south of Kenmore.

The petrography of the quartz-mica schists and of the psammites is essentially the same as that described above for the Ben Lui Schist. Pebbles are most commonly seen in thin section to be recrystallised quartz and altered plagioclase, with rare microcline (pink in outcrop) and plagioclase/quartz lithic fragments. Garnets are common, with biotite in most lithologies, and twinned plagioclase porphyroblasts are also seen. Both the pebble clasts and porphyroblasts exhibit quartz-chlorite pressure shadows.

Green Bed units

In distinction to the many thick, sharp-margined, coarse-grained amphibolites of clear intrusive origin in the formation, some thin (20–300 cm) laminated amphibolites, usually associated with grits and chlorite-schists, are clearly of sedimentary origin. The principal upper unit is discussed later but there are many occurrences of these volcaniclastic beds at a number of lower stratigraphical levels within the Pitlochry Schist; they are thin and probably discontinuous. The principal exposures of these are in the Allt Odhar below the footbridge [NN 7363 4729] (here rich in pyrite), in the Keltney Burn [NN 7679 5007] (only 200 m into the formation) and in the Urlar Burn, limited to one 20 m-thick unit [NN 8239 4485] and the adjacent Craig Hill (Figure 30).

The most complete succession, on Craig Hill, is seen on a traverse up the hill on the east side of the wall [NN 8223 4666] to [NN 8202 4600]. Here there are three units, each comprising interbedded laminated amphibolite, amphibolitic grits and chlorite-rich pelites; graded bedding in the intervening gritty psammites demonstrates the continuity of the succession. The thick laminated gritty amphibolites, also seen on the slopes above the Urlar Burn north-west of [NN 8256 4542], and in the Urlar Burn itself [NN 8239 4485], are composed of rhythms of 0.2–10 mm-thick, white quartzofeldspathic layers alternating with thicker (2–100 mm) chlorite-amphibole-(biotite) schist. The upper layers commonly show evidence of slumping, in the disruption and discontinuity of the layering. Small (2–4 mm) pebbles of quartz, plagioclase and quartz–feldspar fragments are scattered throughout and are locally concentrated and exhibit graded bedding. The pebble content diminishes upwards and the overlying quartzite is usually slightly amphibolitic. The highest (lowermost stratigraphically) Green Bed on Craig Hill is followed upwards by a 20 m-thick amphibolite. The (inverted) amphibolite, which has a rather diffuse contact with the Green Bed, is at first laminated (although often disrupted and discontinuous) and contains thin beds of graded quartz, feldspar and lithic clasts. The amphibolite is massive in its centre and has a fine-grained, flinty, sharp contact with the psammite above. This pattern, seen elsewhere in the formation, e.g. in the Keltney Burn [NN 7679 5007], suggests that the massive sheets may be lava-flows followed immediately upwards by ashy sediments.

The highest stratigraphical occurrence of Green Beds is in the extreme south-east corner of the district, on the two shallow-dipping limbs of the D₃ Loch Tay Antiform. In the adjacent Pitlochry district (Sheet 55E) this unit extends for over 10 km eastwards on the two fold limbs and thus, although poorly represented in the present district, does appear to represent the thickest and most persistent unit of the Green Beds. Barrow et al. (1905) regarded the other occurences described above as isoclinal fold repetitions of this principal green bed unit; graded bedding, both in the adjacent sediments and in the green bed grits themselves, shows that this is not the case.

The principal exposure of the main green bed unit is in the Camserney Burn upstream of Crachan Farm [NN 8153 4958]. Unfortunately, the best exposure is that in a rather inaccessible gorge section [NN 8142 4967] to [NN 8144 4982]. The rocks examined showed the same assemblages of banded amphibolites, amphibolitic grits and chlorite schist as in the units previously described. Beds of the Pitlochry Schist, of younger age than the Green Beds, are exposed for some 100 m below the main Green Bed in the Camserney Burn as far as [NN 8148 4960]. Exposure on the south side of the Strath Tay, where afforestation has limited access, is confined to the small burns draining north into the River Tay e.g. [NN 8175 4753].

In thin section the fine-grained amphibolites comprise hornblende-biotite schists alternating with quartzofeldspathic laminae with minor hornblende and biotite. Pebbles (up to 5 mm) of quartz, feldspar and quartz-feldspar fragments are seen in the matrices of both types of layering, sometimes so concentrated as to form 90% of the rock. Small garnets are commonly present, together with accessory iron ore and carbonate.

Sedimentary environments

The sequence of the Ben Lui Schist together with the Loch Tay Limestone and the Pitlochry Schist represents another basin-deepening event. The early, less psammitic and non-pebbly, facies of the Ben Lui Schist might represent a distal equivalent of the coarse turbiditic Crinan Grits of the South-west Highlands, with a subsequent increasing input of coarse turbiditic sand into the district through the Ben Lui Schist continuing into the Pitlochry Schist. These sediments differ from those of the Carn Mairg Quartzite in the dominance of quartz in a consistently muddy matrix; however, the proportion of feldspar does increase upwards into the Pitlochry Schist. Also in contrast with the earlier formations, the Ben Lui and Pitlochry schists contain no evidence of any noticeable changes of thickness of beds on an exposure scale or of facies variations within the outcrop in the district.

The notable interruption in this clastic input is the time represented by the Loch Tay Limestone. Although never entirely free of a fine sand or muddy component, the well-bedded non-magnesian carbonates are rarely gritty and the coarse input below and above ceases and recommences quite suddenly. These changes may be related to the resumption of igneous activity, marked to the west of the district by contemporary volcanism at the base and at the top (the Tayvallich volcanics) of the carbonate succession, and within the district by the volcaniclastic rocks (the Green Beds) in the Pitlochry Schist.

Chapter 3 Structure

The structure of the district is treated under the headings of three sub-areas: the Grampian Group (which occupies the northern half of the district) and the remaining area, under two headings — the Appin and Argyll groups to the west of the Loch Tay Fault and the Appin, Argyll and Southern Highland groups to the east of the Loch Tay Fault. Within each sub-area the structural details are discussed under the headings of the three regional deformations D₁, D₂, D₃ and of the more localised phases of Lyon (D_L), Chanaich (D_c), Errochty (D_e) and Trinafour (D_t), as appropriate. The Faults and Fractures section deals with the late brittle phase as it affects the whole district. Correlation is made between the structure of the Grampian Group and the remainder of the district and between the two sides of the Loch Tay Fault. Finally the structure is put into its regional context. An outline of the structural history of the district is given at the end of Chapter 1. Axial-traces of the major folds are shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and representative minor structures of the D₁ and D₂ phases on (Figure 10) and of the D₃ phase on (Figure 11). Fault traces are shown on (Figure 13). Cross-sections where the major fold axial-traces are identified will be found on (Figure 12b), (Figure 14), (Figure 17) and (Figure 18), as well as those on the published 1:50 000 Series map.

Deformation history: evidence and summary

It is important to establish the basis for the interpretation that follows, since the Schiehallion district holds the key to much of the regional structural history, and has been the subject of so many, often conflicting, interpretations (Bailey and McCallien, 1937; Rast, 1958a; Ramsay, 1959; Sturt, 1961; Bradbury et al., 1979; Thomas, 1980; Nell, 1986; Treagus, 1987). Of exceptional importance in the district are the large-scale interference patterns produced by the four principal phases of deformation; the relative mutual age of the these, and of the more local phases, has additionally been established from the usual principles of congruent and superimposed minor structures.

First deformation D₁

Minor structures of D₁ age cannot generally be used to establish the age or geometry of major folds. D₁ minor folds are not well developed in many lithologies and are commonly obscured elsewhere by the intensity of D₂, making observations of plunge and vergence difficult. For similar reasons S₁ cleavage is only locally seen to be discordant to bedding, in D₁ minor fold hinges. Where D₂ is not strong, S₁ is seen as a bedding-parallel schistosity in all lithologies except the purest quartzites. Thus, establishing the identity and the geometry of major D₁ hinges is primarily dependent on the recognition of fold outcrop patterns which are deformed by D₂. Removal of the effects of the post-D₂ major folds would indicate an overall north-east to east-north-east strike of bedding and of the major D₁ axial-surfaces, in common with that elsewhere in the Dalradian. However, the original dip of these surfaces is more contentious, as discussed at the end of this chapter.

In the Grampian Group no major D₁ folds are identified, although a substantial intermediate-scale fold can be identified in one section in the River Garry and some of the clean road-cuttings on the A9 reveal clear examples of minor D₁ folds with superimposed D₂ structures.

In Glen Lyon the lithostratigraphical repetitions of the Argyll Group formations most clearly demonstrate the isoclinal D₁ Chesthill Syncline and Meall Garbh Anticline and the folding of them by the D₂ Ruskich Antiform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and (Figure 17); the relatively open nature of this latter fold makes its association with the regionally dominant D₂ minor folds and schistosity particularly clear. The Farragon Volcanic Formation in the closure of the antiform also provides rare examples of minor-scale D₁ folds folded by those related to D₂ (Plate 5). In the Schiehallion–Kinloch Rannoch area at a higher (pre-D₃) structural level, the lithostratigraphical repetitions of the Appin and Argyll Group formations are attributed to both D₁ and D₂ tight major folds (Figure 18); although no large-scale interference patterns are produced, major D₁ closures can be identified from their non-congruent relations with the D₂ minor structures. Thus the D₁ age of the Beinn a' Chuallaich Anticline north of the River Tummel is particularly easy to establish from the superposition of D₂ minor structures in the Appin Group formations; the complementary Beinn a' Chuallaich Syncline can similarly be attributed to D₁ from the superposed D₂ minor structures in the Killiecrankie Schist Formation. Similar criteria have been used to attribute a D₁ age to the two smaller-scale Innerhadden and Creag an Fhithich fold-pairs in the Kinloch Rannoch area. Other intermediate-scale D₁ folds are apparent from local stratigraphical repetitions cross-cut by D₂ cleavage. The thinning of the common limb of the Beinn a' Chuallaich fold-pair is attributed to D₁ sliding, although high D₂ strain cannot be excluded. A similar thinning takes place on the common limb of the D₁ Conbhar fold-pair east of the Loch Tay Fault, regionally attributed to the Killiecrankie Slide.

To the east of the Loch Tay Fault (Figure 14) similar criteria have been applied in the identification of the D₁ Meall Urair Anticline to the north of Loch Tummel and of the Sron Mhor Syncline and Creag na h- Iolaire Anticline to the south. The latter fold is unique in the area in that an axial-planar S₁ fabric is present in the Carn Mairg Quartzite Formation of its core (Plate 3b). Intermediate-scale D₁ folds are refolded by D₂ folds in the complex outcrops patterns seen in the Appin and Argyll Group formations north of Loch Tummel. The Conbhar fold-pair with its attendant slide, on rather fragmentary evidence, is crossed by S₂.

Second deformation D₂

The attribution of a D₂ age to most of the remaining tight-to-isoclinal major fold closures in the district (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) depends, apart from their deformation of D₁ structures, on their congruency with D₂ minor structures. The dominant D₂ minor structure in the pelitic and semipelitic lithologies is an S₂ crenulation cleavage. The intensity of this cleavage varies, according to lithology and structural position, from a sub-penetrative schistosity to a 5 mm-spaced cleavage. At the former end of this spectrum it may rarely be mistaken for a D₁ fabric, although thin section inspection will always resolve ambiguity. On the other hand, it can be very similar in field appearance to crenulation cleavages produced in association with D₃ and locally with the D_L and D_c phases. However, cleavages associated with the latter two phases have very particular orientations and it is only locally that problems have been encountered with distinguishing S₂ from S₃. In garnet-bearing lithologies, pressure shadows associated with augening of S₂ around the porphyroblasts are often diagnostic (Plate 5a; Chapter 5); in thin section the relationship of S₂ to the porphroblasts is unambiguous and S₃ (if present) can always be seen to deform an earlier crenulation ((Plate 8c); Chapter 5). Thus S₂ can be confidently correlated as the dominant fabric through all formations (except in the purest quartzites where it is only locally developed), from the Grampian Group in the north to the Pitlochry Schist in the south.

D₂ minor folds are common in all the well-bedded lithologies and again their morphology depends on the bed-thickness, lithology and structural context (Plate 1b) and (Plate 1c), (Plate 3a), (Plate 4a) and (Plate 4b). Distinction from D₁ folds is only difficult in some of the thinly bedded carbonates (e.g. Blair Atholl Formation limestones) and quartzites (e.g. certain levels within the Schiehallion Quartzite Formation) where S₂ is not well developed. Distinction from D₃ folds is only a difficulty where there is local ambiguity in the distinction of S₂ referred to above. D₂ is also characterised by a stretching lineation in pebbly lithologies (such as those within the Pitlochry and Ben Lui Schists, the Carn Mairg and Meall Dubh Quartzites) and commonly by a quartz-rodding; both structures are commonly subparallel to D₂ fold hinges. These features are not associated with minor folds of other phases.

Thus there is no ambiguity concerning the D₂ age of the major folds whose axial-traces and names are shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and which can be identified on the cross-section (Figure 18), and on the sections below the published map. Representative readings of the minor fold plunge and vergence which have been used to identify the major antiforms and synforms are shown on (Figure 10). The most significant of these folds, to the west of the Loch Tay Fault, in descending structural order are the Ruskich Antiform of Glen Lyon, the probable equivalent of the Sgurran Geal Antiform in the north-east, the Allt Mor Synform, the Craig Varr Antiform and the Creag an Earra Synform defined south and west of Schiehallion, and the Balliemore Antiform to their north and east (The farm originally named Balliemore [NN 7021 5936] is now called Balmore). On the north limb of the latter fold, the right-way-up formations of the Appin Group pass downwards into the Grampian Group in the Strath Fionan area, although elsewhere the junction rocks are strongly attenuated or disrupted by high D₂ strain in the Boundary Slide Zone. In the Grampian Group (Figure 12b) the Meall Reamhar Synform restores the rocks to the regional inverted attitude with the Clunes Antiform at a lower structural level. The latter fold and its companion synform are equated with the Creag a' Mhadaidh Antiform and the Garry Synform in the undivided Grampian Group in the north-west of the district. Many D₂ folds of intermediate scale (hundreds of metres in amplitude) are also locally important throughout the district.

To the east of the Loch Tay Fault (Figure 14) four major folds are recognised. In descending order from south to north these are the Meall Tairneachan Antiform in the centre, the Craig Loisgte Synform south of Loch Tummel, and the An Tulach Antiform and Balnabadoch Synform to the north. Since these folds are strongly affected by D₃ folding they have neither a dominant synformal nor antiformal geometry, but to be consistent with the nomenclature of the folds to the west of the Loch Tay Fault the synform/antiform names are adopted. The presumed equivalence of these folds across the fault is shown in (Figure 18).

Major D₂ fold limbs are commonly associated with zones of high strain which, when formations are excised, may be termed slides. The most obvious example of these is the Boundary Slide.

Lyon phase (D_L)

This phase is of local importance in Glen Lyon and in the Killiecrankie Schist outcrop to the south of the Allt Mor, but is not responsible for any major structure in these areas. In these areas the small- and intermediate-scale folds generally have westerly vergence and east-dipping axial-surfaces. Cleavage is an axial planar crenulation.

Errochty phase (D_e)

The large-scale deflection of the Grampian/Appin Group boundary, caused by the south-plunging, west-verging Bohespic Antiform/Errochty Synform fold-pair, is the most dramatic major structure in the district (Figure 12a). The Bohespic Antiform particularly clearly folds the axial-traces of both D₁ and D₂ major folds in the Grampian, Appin and lower Argyll groups to the north of Schiehallion; this fold tightens downwards within the Grampian Group, but it rapidly opens out into the higher groups south of Schiehallion. The Errochty Synform is a very tight structure where seen in the Grampian Group in Glen Garry and at the Appin Group boundary at Trinafour, but opens out upwards in the Argyll Group towards Kinloch Rannoch. Neither fold has an identity in Glen Lyon.

Minor structures associated with this phase are very variably developed. In the Grampian Group, the axial zone of the Errochty Synform is marked by a strong schistosity and tight minor folds (the Dalnacardoch Banded Zone), but only a locally developed crenulation cleavage and open minor folds are associated with the Bohespic Antiform. In the higher groups, in the axial zone of the Errochty Synform, near Kinloch Rannoch, conjugate sets of minor folds associated with crenulation cleavages, are attributed to this phase. No minor structures of this phase have been identified to the east of the Loch Tay Fault.

Third deformation D₃

The identification of major folds of the age of the regional D₃ is perhaps the most significant difference from previous interpretations of the district, particularly in the Glen Lyon area and the area east of the Loch Tay Fault. The D₃ major folds, being generally more open in style than D₂, are detected from the change in local and regional S₂ dip patterns rather than from stratigraphical repetitions. Most D₃ major folds can be substantiated from the vergence relations of the S₃ cleavage and D₃ minor folds (see (Figure 11)), although in the more competent lithologies and especially in the areas of weaker D₃ these minor structures may not be developed. The steep dip and ENE strike of the axial-planes of minor D₃ folds and of S₃ is not greatly affected by subsequent local phases, so these minor structures may often be identified from their attitude alone.

The axial-traces of the major folds of D₃ age are shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and in cross-section on (Figure 18) as well as below the published map. To the west of the Loch Tay Fault they are confined to the south and east of Schiehallion. The most important of these folds is the Ben Lawers Synform on the south margin of the district; it is responsible for the change in the regional dip pattern from the Flat Belt, south of the district, to the overall southward dip of the formations to its north. This fold is clearly identified from changes in vergence of minor structures to the north-west of Fearnan, from whence it can be traced WSW to the type locality 4 km south of the district. At the east end of Glen Lyon a tight D₃ fold-pair, the Culdaremore Antiform and the An Stuc Synform, and related intermediate-scale folds to the west deflect the axial-traces of the major D₁ and D₂ folds (Figure 7b); these folds are accompanied by strongly developed minor structures.

North of Glen Lyon the effect of D₃ diminishes, producing only gentle variations to the southerly regional dip and weak minor structures to the south of Schiehallion. Minor structures of D₃ age, south of the Allt Mor to the south-east of Schiehallion, appear to be hardly affected by the curvature of the Bohespic Antiform (see discussion below), but are not represented to the west. To the east of Schiehallion an intermediate-scale fold-pair in the Loch Kinardochy area, which deforms a D₂ axial-trace, is the northernmost expression of D₃ deformation. Minor structures of probable D₃ age occur only locally in the Appin and Grampian groups to the north of Schiehallion, and the regional flattening of the dips in the extreme north-west of the district towards the Drumochter Dome is regarded as a possible consequence of D₃.

On the east side of the Loch Tay Fault (Figure 14) the Flat Belt, represented by the Southern Highland Group in the south of the district, is affected in the south by the gentle Creag an Fhudair Synform and to the north by the Loch Tay Antiform. The Flat Belt terminates here at the Creag Chean Synform (the equivalent of the Ben Lawers Synform) which can be traced, from changes in dip and minor structure vergence, within the outcrop of the Ben Lawers Schist (Plate 4c). North of this, D₃ is largely responsible for what has come to be called (perhaps misleadingly) the Tummel Steep Belt; this belt of variably dipping Argyll Group Formations is affected by the Dubh Chnochan Antiform and the Loch Tummel Synform; these folds are quite open in profile and the south limb of the former fold is corrugated by rolling intermediate-scale folds; the latter fold is particularly clear from the opposing dips of S₂ south and north of Loch Tummel, restoring the regional structure to a south-easterly dip in the north-east of the district. Minor structures are strongly to weakly developed in the variable lithologies, but the Killiecrankie Schist south of Loch Tummel offers very clear examples of the interference of minor D₂ and D₃ folds (Plate 1c).

There is no direct evidence of the age relations between the D₃ and D_e phases, since the minor and major folds of the two are largely confined to the south and north of the district respectively. The assumption that the D₃ phase is the earlier by Treagus (1987) rested with the intense development of crenulation cleavage and of ductile minor folds of that phase, in contrast with their general absence in association with D_e. However, this effect may be attributable to the restriction of the intense development of the D_e phase to the Grampian Group, at a low structural level, where the contrasting lithologies and pelitic formations, in which D₃ minor structures are found, are rare. The possibility that the D₃ phase might postdate D_e had, therefore, to be entertained during the re-mapping, and this raised the question that the D_L phase minor structures of Glen Lyon, which certainly pre-date D₃ and which are geometrically sympathetic to Errochty/Bohespic fold pair, might be temporally related to the D_e phase. The conclusion has been that, on balance, the evidence favours the view that the D_e and D_L structures are of the same age and that they are postdated by D₃.

Creag an Chanaich phase D_c

A flat-lying crenulation cleavage, with associated minor folding, affecting bedding and S₂, is developed locally in steeply dipping rocks (Plate 4b). For this latter reason, it is not usually developed together with D₃ structures, but is, in a few instances, superimposed upon S₃ cleavage. Its morphology in the field and thin section is very similar to that of S₃ and it is assumed to be not very far removed in time from that phase. It is developed particularly east of the Loch Tay Fault in the Ben Eagach Schist, but is also seen west of the fault in the Killiecrankie Schist north of the Allt Mor. No major structures are known.

Trinafour phase D_t

Several major folds plunging SE or SSE, with rather angular profiles, are developed in the flaggy Grampian Group rocks on the west limb of the Errochty Synform. Two of these, the Trinafour Monoform and the Croftnagowan Synform, strongly deform the Errochty Synform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). Minor chevron folds and fracture zones are commonly associated with the major structures. These structures cannot be directly dated with respect to the D₃ or D_c phases, although their angular style suggests they are the latest of all the phases; they are offset by the the NNE-trending faults.

Fractures and faults

The area is transected by one major NNE-trending fault, the Loch Tay Fault, one of a series of subparallel fractures that affects the Grampian fold-belt. Like most of the series it has suffered both sinistral strike-slip as well as dip-slip displacement. There are numerous other fractures with a broadly NE trend and small sinistral and dextral displacements, which are particularly evident in the centre of the district (Figure 13). Locally, fractures and faults of other trends are dominant; for example, in the south-east of the district a NW-trending fracture set is strongly developed, whilst a cluster of ENE-trending faults has been mapped in Strathtummel near Dunalastair. As discussed later, these faults are closely associated in time to the late-Silurian dyke suite and to mineralisation.

Grampian Group

The earliest structural observations in the Group were those of Grant Wilson in the memoir to Sheet 55 (Barrow et al., 1905), who had recognised the presence of both minor folding and lineations. Although neither was recorded on Sheet 55 (Geological Survey of Great Britain, 1902), both isoclinal and overturned (asymmetric) folding are described in the memoir, and figures 2 and 4 therein clearly recognise that the overturned folds frequently verge to the north-west. This vergence was perhaps contributory to the concept of the synformal structure of the Dalradian, the Grampian Group being situated on the north-west limb (Figure 3a). Otherwise the memoir contains no discussion of major folding, except the recognition of a swing in strike (part of the Errochty Synform) which was attributed to a hidden pre-folding intrusion.

Bailey and McCallien (1937) pioneered the concept of major polyphase folding in the Dalradian with their recognition of the swing in strike of the Dalradian/ 'Moine' boundary (the Boundary Slide). Their explanation for the formation of the 'Schiehallion Twist' (now the Errochty Synform/Bohespic Antiform) differs from that of the present account, but certainly was based on observations of the deflection of earlier schistosities and folds. However, their work contains no evidence of any detailed structural study in the Grampian Group below the Boundary Slide. Rast (1958), in his development of the concept, had used only limited data from the Grampian Group.

The detailed structural geometry of the Grampian Group in the district has since been studied by Thomas (1965; 1979; 1980), and has been extended in the present work. By mapping minor structures, intersecting fabrics and bedding orientations over the district, a structural history has been evolved. This consists of five phases of deformation. Although, in general, events agree with those found in the southern part of the district (Treagus, 1987; Nell, 1986; and below), notable differences are apparent. Axial-traces of the major folds of the two most significant phases (D₂ and D_e) are shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9); minor structures of the D₂ phase are shown on (Figure 10).

D₁/D₂ minor folds and deformational fabrics common to all of the Dalradian of the Central Highlands are well represented and no deformation fabrics which might be related to an earlier event (compare Piasecki, 1980) were seen. The minor folds and fabrics related to the D₂ deformation dominate the rocks, and are sympathetic to three major folds, the Meall Reamhar Synform, the Clunes Antiform and the Clunes Synform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and (Figure 12b). These folds and fabrics, where unaffected by later folding, have the regional NE-trend.

The dominant major fold, the Meall Reamhar Synform, traceable over 15 km, has been the subject of some controversy. It was originally interpreted as the downward-facing closure of the Atholl Nappe (Thomas, 1965; 1980), a major structure thought to be responsible for the total inversion of the Grampian Group south-east of the Drumochter Dome. However, in the light of work south of the Boundary Slide (Treagus, 1987) and due to the sympathetic vergence of abundant D₂ minor folds of the same style, it is now regarded as the synformal companion of the D₂ Balliemore Antiform that affects the higher groups in the south of the district. However, the presence of D₁ folds of significant magnitude cannot be excluded and a possible subsidiary D₁ closure may be inferred (Lindsay et al., 1989) in one locality in the River Garry where facing on D₂ folds is reversed over several hundred metres.

The D_e, or Errochty phase, is responsible for the major NNE-trending Errochty Synform/Bohespic Antiform fold pair, which dramatically folds the D₂ axial-traces and the Boundary Slide. This phase is locally associated with crenulation fabrics and minor folds and, in the axial zone of the Errochty Synform, with penetrative fabrics.

The D₃ (Ben Lawers) phase of deformation does not appear to affect the Grampian group to the same extent as it does the groups to the south. No major D₃ folds are evident, although upwarping of the Drumochter Dome, to the north-west of the district, is possibly related to this deformation. It is worth noting that major-scale D₃ folds of similar geometry to the Ben Lawers structures in the south and east of the district are found in the Grampian Group throughout the Ben Alder and Ossian areas to the north-west of the Drumochter Dome (Thomas, 1979). Some minor upright crenulations in the district are attributed to this event but there is no direct evidence in the Grampian Group for the relative timing of the De and D₃ events.

A set of more angular SE-trending folds, developed on various scales in the flaggy psammites, have an important influence on the final geometry of the outcrop. They are referred to here as the Trinafour phase (D_t) since they are typified by the Trinafour Monoform of Glen Errochty; these folds clearly postdate the Errochty Synform.

Well-exposed areas which have contributed most significantly to the structural understanding of the district are: the ridge running west from Creag Kynachan [NN 76 57], south of the Tummel; the road sections and crags within the Tummel Forest, including Meall Reamhar [NN 78 61] and the Creag nan Caisean ridge [NN 77 60]; most importantly, the unrivalled sections by the A9 road and river in Glen Garry. The latter sections are so important in the interpretation of the structure that they are described in the separate Glen Garry Transect.

D₁ and D₂ minor structures

A pre-D₂ bedding-parallel schistosity, S₁, is developed throughout the district in the Grampian Group. Only rarely is the fabric axial-planar to folds of bedding and it is sometimes difficult to differentiate genuine D₁ folds from slump folds. The rarity of small-scale D₁ folds means that they cannot be used generally to define any major structure of this age, nor can any statement be made about their axial trend. Intersection lineations between S₀ and S₁ are rarely observed and no D₁ stretching lineation has survived the syn-D₂ recrystallisation. However, tight D₁ folds facing downward to the south-east can be seen on the A9 road cuts [NN 7114 7083]; [NN 785 670] and in the River Garry [NN 7121 7065] west of Edendon Water; an exposure behind the Errochty Power Station [NN 7725 5932] offers a rare opportunity to observe tight D₁ minor folds, which here show a considerable variation in plunge.

D₂ folds are by far the most common structures, refolding the first schistosity S₁. Most minor D₂ folds are tight and generally plunge at low to moderate angles in a Caledonoid (NE–SW) direction, where unaffected by the subsequent deformation (Plate 3a). The dominant vergence is towards the NW, on the long limbs of the major D₂ folds (see discussion of major folds, below). The SE-dipping S₂ is usually developed as a strong penetrative fabric, axial planar to these folds, but rarely appears as a crenulation of S₁ within pelitic lithologies. The S₀/S₂ intersection lineation and mineral stretching lineations are parallel to the local D₂ fold hinges. The typical attitudes of selected D₂ minor structures are shown on (Figure 10).

Specific localities where the geometry of typical D₂ minor stuctures may be examined, especially in relation to the major folds, are given under the headings Major Structures and Glen Garry Transect.

Boundary Slide Zone

Generally, as the boundary with the younger groups of the Dalradian is approached, the S₂ fabric intensifies and D₂ folds tighten; the psammitic and semipelitic lithologies develop a flagginess in which lithological variation and sedimentary structures are progressively less obvious. As discussed above, these effects are considered to be the effect of a zone of high strain located at this boundary, the Boundary Slide Zone, which generally culminates in a tectonic discontinuity, the Boundary Slide. These features may be studied in several sections on the west limb of the Errochty Synform. The Allt Druidhe [NN 642 566] offers a convenient section (400 m up from the road) in flaggy psammites and quartz-rich psammites that probably represents a very compressed section of the upper Strathtummel Subgroup and includes post-tectonic felsite sills and continues upwards across the slide into a section of abbreviated Appin and Argyll Group rocks, discussed later in this chapter. The crags on the west side of Meall Dubh [NN 650 603] and Carn Fiaclach [NN 661 621] offer similar sections. A section in an unnamed burn at the south-west end of Loch Errochty [NN 668 645] to [NN 673 637] exhibits a particularly good transition into the slide zone.

On the east limb of the Errochty Synform, sections in the Allt an Ruighe Fhliuch [NN 716 630] to [NN 709 630] and the Allt na Moine Buidhe [NN 713 610] to [NN 709 612] show highly strained, but probably unbroken, sections from the higher Strathtummel Subgroup formations (the Kynachan Quartzite and Psammite) into the Appin Subgroup. In comparison, the sections south of the River Tummel around the hinge-zone of the Bohespic Antiform, e.g. north-east of Lochan an Daim [NN 724 583] to [NN 720 580], show the complete passage from the Grampian Group into the Appin Group without evidence of exceptionally high strains (Figure 4). To the east of Creag Kynachan [NN 76 57], however, there is a rapid return to high strains at the top of the Grampian Group, with the Slide located in very highly strained Appin Group rocks above. These relationships are well illustrated in the Allt Kynachan; the lower end of this section [NN 777 576] near Daloist exhibits Kynachan Psammite with typical south-younging cross-sets and D₂ minor structures; the section upstream [NN 779 574] to [NN 779 573] illustrates the rapid transition into centimetre-layered flags, continuing into the highly strained representatives of the Dunalastair Quartzite and Schist at the base of the Appin Group.

Post-D₂ minor structures

Errochty phase (D_e)

Strong Se crenulation fabrics are seen locally in relation to the Errochty Synform and Bohespic Antiform. These fabrics are penetrative in association with reclined minor folds plunging steeply to the SSE on Craig Kynachan [NN 76 57] on the east limb of the Bohespic Antiform. Axial fabrics associated with the closure of the Bohespic fold are generally weak elsewhere but they may be most readily observed on the A9 [NN 769 690] near the Allt Crom Bruathaich. Distribution is patchy, restricted largely to the exposed hinge zone of the major antiform. Minor folds here plunge 16°–25° to 150°–170° related to strong axial-planar cleavage dipping 40° towards 080°. Well-developed fabrics, near the axial trace of the antiform, are seen above Blairfettie farm [NN 753 648] in Glen Errochty.

The Errochty Synform becomes extremely tight in the Grampian Group so that the Se fabrics are represented by a zone of highly strained banded psammites. These flaggy rocks dominate the Glen Garry road and river exposures from Edendon Water to Dalnamein Lodge (Figure 1), (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). This Dalnacardoch Banded Zone (Thomas, 1979) is at least 3 km wide in the River Garry. Near East Dail-an-fhraoich [NN 7375 6983], close to the axial trace of the Errochty Synform, exposures show D₁/D₂ isoclines refolded about reclined D_e folds plunging 10°–30° to 140°–170°. Brecciated early schistosities, here, are welded in zones less than a metre thick contained within the banding and there is little doubt that movement on Se was widespread.

On the west limb of the synform, roadside exposures near Dalnacardoch [NN 7183 7055] again show D_e minor folds, here associated with boudins folding tight D₁/D₂ isoclines; the early folds and S₂ schistosity are almost completely transposed into the Se banding. D_e minor folds and fabrics are otherwise not generally well developed.

The Dalnacardoch Banded Zone extends northwards into the Forest of Atholl and southwards into the burn sections near the Trinafour road [NN 72 68], where it passes around later Trinafour phase folds into the Appin Group rocks in the closure of the Errochty Synform (Figure 6).

Ben Lawers phase (D₃)

D₃ minor crenulations are rarely developed but have the characteristic ENE trend and steep axial-surfaces. They are seen overprinting S₂, at Eas Chliabhan [NN 733 593] on the River Tummel.

Trinafour phase (D_t)

This phase (D_t) is characterised by fracture cleavage, minor box-shaped folds and kink bands, developed particularly north of Glen Errochty. The cleavage and axial-surfaces of the folds strike south-east and dip steeply or moderately south-west; the box-shaped folds plunge gently to the SSE. Minor structures of this generation are particularly well displayed on the steep limb of the Trinafour Monoform, e.g. on Meall Dail-chealach [NN 704 675]. Further south, the fabrics become less obviously brittle in style so that crenulation cleavages of this phase have been recognised locally as far south as Creag Kynachan [NN 76 57], where they cut the eastern limb of the Bohespic Antiform.

A late stage of brittle fault-associated deformation created minor localised folds and kinks. Many such small faults, presumably associated with the Loch Tay Fault movements, are seen to cut the NE-trending dykes and sills; these features are discussed more fully in the Fractures and Faults section at the end of this chapter.

Major structures

Axial-traces of major fold structures are shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9).

First deformation (D₁)

No major folds of this generation are recognised. However, changes in facing of minor D₂ folds on the steep north-west limb of the Clunes Synform discussed below (see also Garry Gorge locality of Glen Garry Transect) suggest that intermediate scale D₁ folds of wavelengths of at least tens of metres must be present.

Second deformation (D₂)

Three major folds are recognised in the east of the Grampian Group, in descending structural order: Meall Reamhar Synform, Clunes Antiform and Clunes Synform (see Section 2 below published map and (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 12b). The latter two folds are thought to be equivalent to the Creag a' Mhadaidh Antiform and the Garry Synform, respectively, in the west of the district.

Meall Reamhar Synform

This fold is clearly revealed between Glen Garry and Strathtummel, in the east of the district, both from the repetition of the Tummel Quartzite about the core of Bruar Psammite and from the sympathetic vergence of the S₂ cleavage and of the minor folds on its two limbs. The structural relationships can also be studied on the continuation of the south-east limb into the higher formations on Creag Kynachan south of the Tummel. On the south-east limb the steep (60°–80° SE) dips and upward younging beds (cross-sets) are well displayed in laminated psammite beds within the Bruar Psammite and the Tummel Quartzite formations on Meall Reamhar [NN 784 618], north of Meall Dubh [NN 791 616] and around Creag nan Caisean [NN 778 607]; the somewhat gentler dipping (30°–60°SE) inverted beds on the north-west limb are seen on the crags 300 m north-west of the summit of Meall Reamhar, near the track [NN 766 614] and crags [NN 760 610] north of Easter Bohespic. Good upward younging is seen in the Kynachan Quartzite and Psammite on Creag Kynachan [NN 760 576].

The axial-planar, penetrative S₂ and sympathetic minor folds, as well as 10 m (and upward) wavelength folds, can be seen in these exposures. The plunge of fold axes and S₀/S₂ intersections is moderate to the north-east on the north-west limb of the fold, but are steeper on the south-east limb (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). Plunges on the Kynachan ridge are consistently steep to the south-east.

Similar stratigraphical and minor structural evidence for the Meall Reamhar Fold is well displayed further north-east in the River Garry upstream from Struan [NN 809 654] and in the Bruar gorge [NN 820 661] on its north-west and south-east limbs, respectively. Here again, D₂ plunge is moderate (20°–40°) north-east and downward-facing evidence on the SE- and NW-vergent minor folds on the two limbs is plentiful. From this evidence the major synformal hinge can be located east of Old Struan [NN 815 655] in the River Garry. More detail is given in the Struan and Bruar localities of the Glen Garry transect. The Tummel Quartzite on the north-west limb appears to be faulted out of the river section by a major south-south-east-trending fault seen near Drochaid na h-Uinneige [NN 790 665]. The fold cannot be easily traced further north, for lack of exposure.

Westwards from Meall Reamhar, the Meall Reamhar Synform may be traced fairly confidently around the D_e Bohespic Antiform from the repetition of the Tummel Psammite about the core of Tummel Quartzite, to just north of Glen Errochty, although exposure is very variable. There is a change in plunge of minor D₂ folds and S₀/S₂ intersections to low angles to the south (Figure 10) on this steep limb of the D_e Errochty Synform, rather than the northerly plunges that might be expected from the later flexure by the D_e folding; this relationship suggests a regional steepening of the D₂ plunge, and is also seen in the Appin Group at a higher structural level (see below). This gentle southerly plunge supports the suggestion from the narrowing of the Tummel Quartzite outcrop, that the major fold also plunges to the south.

At the expected position of the axial-trace of the fold in Glen Garry near East Dail-an-fhraoich [NN 739 699] flaggy psammites are involved in the D_e Dalnacardoch Banded zone, and D₂ minor folds are probably represented by strongly deformed isoclines of indeterminate plunge and vergence (see also Edendon localities of the Glen Garry transect).

On the west limb of the Errochty Synform, the Meall Reamhar Synform cannot be positively identified in the Edendon Bridge area [NN 716 706] of Glen Garry, where the rocks lie on the western edge of the Dalnacardoch Banded Zone. To the south, the position of the axial-trace of the Meall Reamhar Fold as shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) is surmised from sparse stratigraphical evidence for repetition of the Strathtummel Subgroup about a core of Bruar Psammite; further circumstantial evidence for a synform here is the presence of the D₂ Creag a' Mhadaidh Antiform further west, as discussed below. Minor fold and S₀/S₂ intersection plunge information, which might be related to the Meall Reamhar Synform on this west limb of the Errochty Synform, is more sparse and largely confined to scattered crag and burn outcrops north-east of Loch Errochty. There is, however, some indication of a similar pattern of plunge variation, across the hinge, to that noted in the east of the district. The south-east limb is strongly affected by the D₂ Boundary Slide Zone to the south-west of Loch Errochty and by the Dalnacardoch Banded Zone (D_e) to the north-east, but plunges show considerable variation from low to moderate angles to the north-east to reclined attitudes. The north-west limb (as far as the axial-trace of the Creag a' Mhadhaidh Antiform) exhibits gentle plunges to the north-east in the Glen Garry section and in the Allt Maraig [NN 665 678] and the Allt Sleibh [NN 657 663] north-east of Creag a' Mhadaidh [NN 634 650]. To the south-west of this hill (in the Loch Rannoch district) plunges change to low angles to the south-west, a change also seen in both the Grampian Group and in adjacent Argyll Group formations from localities immediately north and south of Kinloch Rannoch, e.g. Meall Dubh [NN 649 600] and two kilometres south of Loch Rannoch (Figure 10). This D₂ plunge variation is also discussed later in this chapter.

Clunes Antiform

This is the most easily demonstrated of the major D₂ folds of the district. The closure of the fold is dramatically displayed in both river and road (Figure 21) sections in Glen Garry (Garry and Struan localities of Glen Garry transect) where the Tummel Psammite is involved in a major antiformal hinge over some 120 m [NN 783 670] to [NN 790 663]; quartz-rich psammites exposed in the Garry below Clunes Lodge may represent the thinned representative of the Tummel Quartzite on the steep limb (Figure 1), (Figure 12b). The north-western, steep (60°–70°) SE-dipping limb youngs upwards. Sedimentary structures are generally well preserved on the long, steep-dipping, right-way-up limbs of the minor folds, whereas way-up criteria are more difficult to interpret on the gentler inverted limbs, due to higher strains. This may indicate that D₁ structures had a steep orientation prior to the rotation of the gentle limbs during the second deformation. The inversion of the south-eastern gentle (20°–30°) SE-dipping limb of the major fold is similarly most obvious from the evidence of cross-bedding and ripple-drift bedding on the short steep limbs of minor folds.

Thus the fold clearly displays downward facing of the sympathetic tight minor folds, which plunge constantly at a low angle to the north-east (Plate 3a). SE-vergent, neutral and NW-vergent minor folds are particularly well displayed across the closure from north-west to south-east. Cross-cutting penetrative S₂ cleavage planes intersect both hinges and limbs and deform the earlier S₁ foliation and rare D₁ isoclines. Subparallel linear structures (S₀/S₂ intersections, mineral stretching, minor fold hinges) are particularly strongly developed in the hinge zone of the fold, including the famous 'water-pipe' mullions (Barrow et al., 1905, p.68) so spectacularly displayed in the river [NN 786 668].

The axial-trace of the Clunes Antiform can be followed into Glen Errochty over a distance of about 5 km until it is wrapped tightly around the Bohespic Antiform at Torr Dubh [NN 745 633]. Here, the interference between the two antiforms brings the Kynachan Quartzite back to outcrop with, poorly exposed, possible Kynachan Psammite forming a dome with a core of muscovite-pelite, which is interpreted as the Beoil Schist Formation of the Appin Group; the Dunalastair Quartzite and Semipelite are missing, for reasons of poor exposure or stratigraphical thinning, but there is no evidence of high strain in the rocks. The minor folds on the east of this domal core, plunging 20°–30°E, veer to plunge 10°–18°N on its north-west side.

The axial trace of the fold is inferred to continue due north, to a tight antiformal closure exposed in an isolated zone in the River Garry over some 500 m, about [NN 751 695] near Dalnamein Lodge; here the rocks are affected by downward-facing D₂ minor folds, plunging 10°–20° north-east. To the east, river exposures at Dalinturuaine [NN 760 692] continue to show NW vergence of D₂ minor folds, which are here more steeply plunging to the ENE, but still downward-facing. To the west of the antiformal closure, in the river exposures towards East Dail-an-fhraoich [NN 738 698], the rocks become increasingly flaggy as they are involved in the D_e Dalnacardoch Banded Zone, and D₂ minor folds are probably represented by strongly deformed isoclines of indeterminate plunge and vergence (see also Dalnamein and Dalnacardoch localities of the Glen Garry transect).

Creag a' Mhadaidh Antiform

There is a marked similarity between the geometry of the Clunes Antiform, and that of the Creag a' Mhadaidh Antiform north-west of Loch Errochty, both folds being downward facing and overturned to the north-west. It is geometrically possible that the two folds are the same structure refolded about the Errochty/Bohespic fold-pair (Figure 12b). At the type locality, Creag a' Mhadaidh in the Craiganour Forest, 750 m to the west of the district [NN 634 650], the plunge of the minor fold hinges is at low angles (5°–15°) south-west. At the east margin of the district, the fold has a culmination so that plunges are gently towards the north-east, similar to the change noted above for the Meall Reamhar Synform and the Argyll Group rocks to the south-east. The fold can be traced from the summit for over 8 km to the north-east through to the River Garry [NN 7090 7075] and the road cut above, described further in the Glen Garry transect; this is at a point approximately 10 km north-west of the outcrop of the Clunes Antiform in the road section. At these very accessible road and river localities it is possible to observe the vergence and downward facing of the gently ENE-plunging minor folds on the two limbs and in the hinge-zone. The fold has not been traced further north.

Clunes Synform

Some 700 m upstream from the position of the Clunes Antiform, a tight D₂ synform occurs in the River Garry [NN 779 671]. On the north-west gentle limb, which dips 35°–40° SE, minor folds axes are subhorizontal but are less common than on the south-east limb; the consistency of the way-up evidence is also difficult to establish, for reasons mentioned above. Further upstream [NN 774 676] this north-west limb is interrupted by a D₂ fold-pair which contains some minor folds which, for a few hundred metres, face upwards; clearly undetected D₁ folds must be present. Way-up evidence is limited in the gorge above and in the adjacent road-section as the axial-trace of the D_e Bohespic Antiform is approached (see below), so the extent of this change of facing and the magnitude of the D₁ folding is unknown.

The fold cannot be traced around the Bohespic Antiform to the west and it cannot be positively identified at the expected return position in Glen Garry; D₂ minor folds near this point south-east of Dalinturuaine [NN 764 689] plunge at high (reclined) angles NE, where they are increasingly affected by deformation related to the Bohespic Antiform. The fold cannot be traced to the north of Glen Garry, but on the west limb of the Errochty Synform the analogous structure appears to be the Garry Synform.

Garry Synform

Some 1100 m upstream from the position of the Creag a' Mhadaidh Antiform in the River Garry, the hinge of a major synform occurs both in the river [NN 699 714] and in the road above (see Stalcair cut of Glen Garry transect). The right-way-up south limb of the ENE trending fold dips 50°–60° to the south, whilst the north limb has a low (10°–20°) dip to the north; thus the fold appears to be more open than the other D₂ folds described. The minor folds, which plunge at low angles to the ENE and WSW, are particularly well developed in the hinge-zone and on the north limb and show abundant evidence of downward facing on the S₂ cleavage, which dips some 30° SSE. The fold has been traced onto the hills north of the Glen where, on Creag Stalcair [NN 677 729] and Meall nan Ruaig [NN 698 727], its axial-surface has a low dip to the south-east; the general flattening of dips to the north-west of the district is related the Drumochter Dome originally considered by Thomas (1979) to be an early structure but now thought to be of D₃ age.

Errochty phase (D_e)

Figure 9 shows the axial-traces of the major Errochty Synform/Bohespic Antiform fold-pair, and (Figure 12)a (see also Thomas, 1980; Treagus, 1987) shows their geometry viewed down-plunge. (Figure 12)b shows their interference with the non-coaxial D₂ folds. These D_e folds, which control the strike swing through the northern two-thirds of the district, are south-plunging, west-verging, have a wavelength of more than 30 km, and have an amplitude of some 8 km, as defined by the Appin/Grampian Group boundary. Although they clearly fold the axial-traces of the major D₂ folds, their age relationship to the D₃ deformation cannot be demonstrated in the Grampian Group, owing to the weak development of the latter phase in the north of the district.

Errochty Synform

This fold becomes extremely tight as it is traced north into deeper levels within the Grampian Group. Although a single major synformal closure cannot be identified in the Glen Garry exposures, minor D_e structures in the Dalnacardoch road-cut and the Dail-an-fhraoich river section leave little doubt as to the intensity of the deformation. From the evidence of the minor structures in the Glen Garry section, discussed above, the major fold there may be judged to be tight to isoclinal, plunging about 20° to 150° with an axial surface dipping 40° to 080°. South of Glen Garry the trace must be deflected by two folds of the Trinafour Phase (see below) but it cannot be identified positively until the Appin Group boundary is approached north of Loch Errochty (Figure 6). Here, where the synform can be readily defined from the bedding readings around the Grampian/Appin Group boundary, it is more open (interlimb angle 50°), plunging 25° to 176° with an axial surface dipping 55° to 105° (latter two readings rotated to allow for later deformation).

Bohespic Antiform

This complementary fold to the east can be more easily identified within the Grampian Group. From the north, it can first be identified in the records of the Bruar–Edendon hydro-electric tunnel [NN 775 722] and it can be clearly seen 300 m south of there as a single fold in the Allt Chireachain [NN 775 719], where the trace trends about 005° and the hinge plunges 20° to 140°. It can be identified again for 1 km on open moorland south of Meall Dubh-ghlas [NN 769 693] from the change in bedding/schistosity attitudes; the interlimb angle in these areas is less than 40°. The hinge is completely exposed on the A9 at the Allt Crom Bhruathaich cut [NN 769 690] (see Glen Garry transect), from where it can be traced into the Garry gorge [NN 786 667]. The major hinge here plunges about 20° to 160° and the axial-plane cleavage dips 40° to 070°. Complex fold structures in poorly exposed ground mask its continuation to the south but it is clearly picked up again north of Blairfettie [NN 755 648] where it trends SSW and plunges gently SSE with an axial-plane cleavage dipping 35°–50° to 085°. It then runs into the Torr Dubh dome where it interferes with the D₂ Clunes Antiform.

The position of the SSW-trending trace becomes more difficult to pinpoint towards the Boundary Slide since the plunge increases and the major fold opens out, although numerous minor folds, plunging 40°–70° to 110°–160°, outcrop on Creag Kynachan [NN 760 575]. Near the trace of the Appin/Grampian Group boundary, bedding readings give a fold plunge of 56° to 174°, an interlimb angle of 50° and an axial-surface 66° to 122°. Within the Appin Group the fold becomes box-like (Treagus, 1987) before rapidly dying out in the Argyll Group in the south of the district.

Ben Lawers phase (D₃)

A gentle D₃ upwarping more open than the Ben Lawers Synform may be responsible for the flattening of dips of the D₂ axial-planes in the north-west of the district. It may thus have contributed to the final shape of the Drumochter Dome, which lies to the north-west of the district, although its shape may be of more fundamental origin (see discussion at end of this chapter).

Trinafour phase (D_t)

The SE- or SSE-trending major folds of this phase have developed on the lower, west, limb of the Errochty Synform particularly in the flaggy rocks near and above the Boundary Slide. They appear to die out rapidly upwards on the upper east limb of the synform. Three significant open folds can be traced over several kilometres. The Allt Culaibh Antiform on Creag Stalcair [NN 684 724], an open fold with a gently curving hinge-zone and an 8 km wavelength, plunges gently south-east to deflect the trace of the Errochty Synform around [NN 72 67], from where it tightens to an angular hinge seen on the crags [NN 733 660]; further south-east the fold dies out on the hillside above Auchleeks House [NN 742 650]. The Meall na Leitreach Synform is tightest near the edge of the district south-east of Meall na Leitreach [NN 64 69], diminishes in amplitude south-eastwards towards Sron Chon [NN 690 663] and appears to die out as it approaches the Appin Group north of Loch Errochty; however, its relationship with the Croftnagowan Synform and the Sron Chon Monoform (see below) is not clear. The Allt Sleibh Antiform which trends SSE down the Allt Sleibh [NN 665 662] is smaller in extent and opens out as it approaches Loch Errochty.

The most significant structures of this phase are the Trinafour Monoform and the Croftnagowan Synform, which are responsible for signicant deflections of the Errochty Synform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). The steep limb of the Trinafour Monoform is almost vertical and its hinge-zones are quite angular on the north-west limb of the Errochty Synform. Its amplitude increases to the south-east from Meall a' Bhiord [NN 688 702] to reach maximum development of 1.5 km on Meall Dail-chealach [NN 710 670]. On the south-east limb of the Errochty Synform, the upper hinge- zone becomes more gently curving near Trinafour [NN 715 646], before dying out to the south-east, 7 km down plunge. The axial plane of this NE-vergent fold is moderately inclined to the south-west so that the steep limb outcrops progressively further north-east at higher elevations. The logs of the Garry–Errochty Tunnel (BGS files) shows that the steep limb of the monoform beneath does not extend vertically for more than 250 m. The Croftnagowan Synform, which is the complememtary fold to the south-west, is open and hardly evident on the north-west limb of the Errochty Synform but becomes more angular as it crosses to the south-east limb at Croftnagowan [NN 712 640], beyond which it dies out rapidly into the Grampian Group.

Conjugate sets of smaller monoforms are also seen, such as that [NN 681 667] to [NN 675 672] west of Sron Chon. These plunge almost southerly, commonly in association with subparallel fracture zones running along the line of steep dips. One of these lines passes through Loch Meall na Leitreach [NN 638 689] and can be traced to the Allt Poll Dubh-ghlas [NN 647 670] over 2.5 km to the south-east just on the west of the district.

Glen Garry transect

Glen Garry forms one of the best and most accessible sections of the Grampian Group rocks in the Central Highlands. Not only does the river bed itself have 75% exposure but many of those exposures are above water level much of the year due to the hydro-electric scheme intakes in both the River Garry and its northern tributaries. The upgraded A9 road also now provides excellent rock cuts that complement those of the river.

The following 12 localities have been chosen to give as complete as possible section through the three major D₂ folds as they are repeated about the later Errochty Synform/Bohespic Antiform fold-pair. The description includes information about other aspects of the geology. It starts in the north-west and follows road and river down towards Blair Atholl. See (Figure 1) for location and (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) for line of section relating to the cross-section of (Figure 12)b.

Access is from lay-bys on the new A9 road, from which the river exposures may usually be also reached. However, the old A9 road from Calvine to Dalnamein is still open and allows easy access to the river section, particularly at Clunes. The minor road from the A9 at Dalnacardoch Lodge to Trinafour also gives access to the old A9 and river sections, from Edendon to Dalnamein.

1. Stalcair cut [NN 683 717] to [NN 686 717]

The third lay-by [NN 683 717] on the south-bound section of the dual carriageway descending from the Drumochter Pass provides access. At the lay-by, on the gently dipping NW limb of the Garry Synform, inverted beds face down to the SE on S₂; bedding and lenses of calc-silicate are well seen. Roadside exposures 200 m east of the lay-by, by a pylon buttress, display laminated psammitic schists with trough ripple-laminations and larger dune beds, as well as slump structures, in the cores of a D₂ fold-pair; these D₂ minor folds, plunging gently towards 070°, are downward facing approaching the hinge-zone of the Garry Synform to the south-east and show a strongly developed S₂ cleavage, 070° to 30°S, which is axial planar to the folds in the schists and schistose psammites.

2. Wade Stone cut [NN 694 716] to [NN 699 715] and river section

Lay-bys (the fourth on the southbound section of the dual carriageway; the second on the northbound section) provide access to the exposures. Near the southbound lay-by [NN 694 716], a high rockface on the curve of the road cuts obliquely across the hinge-zone of the Garry Synform. The section east of the lay-by is still in the flat limb of the synform, but east from a retaining wall alternating flat and steep (40°SE to vertical) limbs are seen, associated with D₂ folds plunging 20° towards 070°. Cross-bedded laminated psammitic schists face down on the gently east-dipping S₂ schistosity. This schistosity is also developed in many of the numerous calc-silicate lenses found here. Post-D₂ microcline porphyroblasts may be evidence of later metasomatism.

Shallow, and later steep-dipping, 2–5 cm thick, sulphide-rich granitic and quartz veins, cross-cut by two steep NE-trending microdiorite dykes and NNE-trending vertical faults and shatter zones, are seen here, as well as in the section to the west of the lay-by. At the start of the hinge-zone of the Garry Synform, the granitic and quartz veins, concordant with bedding, appear to be folded, with a steep sheet-dip, by the D₂ folds. Of special note is the NE-trending fault exposed in the Allt an Stalcair [NN 690 717] associated with calcite–haematite veining and the molybdenum-bearing pegmatitic veins in the gorge upstream; 30 m to the west of the vein, in the road-cutting, a 3 m-thick sill of biotite-rich diorite is exposed.

The northbound lay-by [NN 697 715], nearer to the River Garry, gives access to the river section as well as the section described above. Good cross-bedding, facing down to the south on the minor folds, is again seen, beneath the railway bridge [NN 6995 7131]; the hinge-zone of the Garry Synform can be located immediately west of the bridge, whilst 1 km downstream the hinge of the major D₂ Creag a' Mhadaidh Antiform can be traced [NN 7090 7075] plunging 10°–14° towards 070°–080°.

3. Edendon cut [NN 712 708]

The next lay-by southbound is immediately east of the axial-trace of the major D₂ closure of the Creag a' Mhadaidh Antiform. At the west end of the rock cut [NN 7114 7083] to the west of the lay-by, very tight, possibly D₁, folds in the gently SE-dipping psammites appear to verge north-west and to face down to the south-east as indicated by deformed current ripple-laminations. Granitic and quartz veins in these rocks again appear to be affected by the D₂ folding. At the burn that cuts this section, a strike-slip fault trending 024° brings in a more psammitic lithology with tight NW-vergent D₂ folds plunging 16° towards 075°, seen at the lay-by section to the south-east. The tight, presumed D₁ folds can also be seen in the River Garry upstream from the junction with Edendon Water [NN 7121 7065].

In the Edendon Water to the east of the lay-by [NN 716 707], three sills of basic microdiorite are well exposed; these are cut by a NW-trending fault occupied by one of the granitic veins together with late veins of carbonate.

4. Dalnacardoch cut [NN 718 705]

Half a kilometre south-east of the lay-by mentioned in the previous locality an apparently regular sequence of flaggy psammites dips between 25° and 30°E in the deep southbound road cut. This is the west margin of the Dalnacardoch Banded Zone, which can be shown to extend 3 km in the River Garry [NN 716 706] to [NN 748 694] to just west of the Dalnamein exposures, described below. This zone, a product of the strong deformation associated with both the D₂ Boundary Slide and with the tight D_e Errochty Synform, must contain the trace of the fold which is the equivalent of the D₂ Meall Reamhar Synform to the south-east (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 12b). At the north-west end of the highest section [NN 7183 7055], very tightly refolded isoclines occur which are interpreted as D₁ or D₂ folds folded by tight folds of the Errochty phase; further south-east into the cutting the isoclinal folds lie within an apparently simple Se banding. No way-up criteria have been observed in this section. This zone also contains gneissose lithologies, in which boudins, of presumed D_e age, are quite common. Large (5 m) kink bands have deformed the Se banding.

The closure of the near-isoclinal D_e Errochty Synform is thought to pass through the Garry near East Dail-an-fhraoich [NN 7375 6983] where exposures of primary (D₁ or D₂) isoclines are refolded about D_e folds plunging 10°–30° towards 140°–170°. The river here also exposes a number of interesting crush zones parallel to layering in which angular schistose clasts are preserved in narrow breccia bands less than 1 m in thickness. Access to this and the next locality is easiest from the minor road to Trinafour, which leaves the A9 at Dalnacardoch Lodge, and then the old A9 road to the south of the new road.

5. Dalnamein river exposures [NN 749 694] to [NN 769 687]

This section illustrates structures on the common limb between the D_e Errochty and Bohespic closures, in which the evenly dipping psammitic flags of the Dalnacardoch Banded Zone give way to more open D₂ once again. At the bend in the River Garry west of Dalnamein Lodge [NN 749 694], more varied lithologies than those apparent in the Zone are affected by tight NW-vergent D₂ folds which have reverted to a Caledonoid trend, plunging 15° towards 035°, and contain sedimentary structures which show the folds to be downward-facing to the south-east.

This isolated unit of folds is separated from the next zone of structures to the south-east by 300 m of unexposed river bed and no road cuts. However, intermediate exposures can be examined in a narrow gorge in the Allt a' Chireachain [NN 7539 7055], accessible by foot from the old A9. Here, strongly folded (D₂) quartzites and pelites are interpreted as overlying a platy ductile thrust-zone, the strike of which trends south towards the unexposed confluence with the River Garry.

In the River Garry eastwards of the Allt a' Chireachain confluence [NN 759 694], there are again good exposures with psammitic rocks containing numerous sedimentary structures. The succession is still inverted, but the folds are now reclined on the western limb of the Bohespic Antiform (De), with most plunging towards 080° at 40°–45°, all the way to the confluence with the Allt Crom Bhruthaich [NN 769 687]. Exceptionally fine examples of inverted slumped cross-bedding can be seen just above the Black Tank footbridge [NN 7663 6878], together with complex D₂/De polyphase folding downstream from the bridge towards the axial trace of the Bohespic Antiform, which is inferred to cross the gorge below the confluence with the Allt Crom Bhruthaich [NN 769 687].

6. Allt Crom cut [NN 769 690] to [NN 768 690]

These exposures can be approached from a lay-by (southbound) [NN 772 685] to their east, or as a continuation of the walk from the previous locality. Just to the west of the Allt Crom Bhruthaich bridge a cutting [NN 7688 6896] to the north of the road exposes the hinge of the Bohespic Antiform in three dimensions. Plunges of both major and minor folds here are in the range 16°–25° towards 150°–170°, but strong axial-plane cleavage appears to be restricted to the hinge zone where it dips at 40° to 080°. Earlier isoclines and the S₂ schistosity and intersection lineation can be observed folded by the antiform, but no way-up criteria have been observed in these strongly deformed rocks.

7. Black Tank cut [NN 772 682] to [NN 774 678]

This cut is accessible from a northbound lay-by at the west end. The rocks in this deep cutting are on the east limb of the Bohespic Antiform. D₂ minor folds opposite the lay-by plunge 8° towards 215° and verge south-east on the steep limb of a large D₂ antiformal closure which can be detected by careful observation of S₀/S₂ relations and dip changes about one-third of the way along the north face, to the east of the lay-by. Cross-sets are once again apparent, but determination of way-up is difficult. One cross-set on the steep limb, near the antiformal hinge, is inverted, supporting the existence of D₁ folds in the section, as discussed below. At the south-east end of the cut [NN 7743 6789] an 8 m-thick red quartz-feldspar porphyry sill in flaggy psammite is cut by later faulting. Opposite the northbound lay-by, the rock face is controlled by regularly dipping planar structures 1–3 m apart with a dip of 52°–55° towards 270°–290°. Some joint faces are occupied by thin pegmatitic sheets, rich in pyrite; steep fault planes display slickensides, some indicating dip-slip and others involving strike-slip final movements. Several of these discontinuities caused wedge failures of the face during construction.

8. Garry gorge [NN 769 687] to [NN 774 676]

Below the junction with the Allt Crom Bhruthaich the River Garry plunges into a gorge which is only accessible when water levels are very low. The tight hinge of the Bohespic Antiform probably passes through here about [NN 770 685] but is difficult to detect.

An accessible section at the lower end of the gorge can be reached from the old A9 road at about [NN 772 681]. At [NN 771 679] the hinge of the D₂ antiform, seen in the road-cut mentioned above, can be identified; cross-bedding in laminated psammites is demonstrably inverted in the north-west, steep SE-dipping, limb just north of the hinge, whilst the gently dipping limbs of the NW-vergent D₂ folds on the flat limb for some 50 m south similarly face up to the north-west (Lindsay, 1989). However, further south-east in this section the facing on the D₂ minor folds changes to downwards to the south-east, a relationship which dominates the remainder of the Clunes section (see below). The implication of this change of facing is the presence of an earlier D₁ closure, which cannot be precisely located. Whether this is a major D₁ closure or merely one closure of a medium-scale fold-pair cannot be ascertained owing to the paucity of way-up evidence west of the Garry gorge. In the exposures between here and those of the Clunes gorge (see below) there is a change in D₂ vergence implying the presence of the major D₂ Clunes Synform [near 780 671].

9. Clunes cut [NN 782 671] to [NN 789 667] and gorge [NN 785 668]

Both river and road localities, accessible from lay-bys in the road section and from the old A9 road here demonstrate the presence of the Clunes Antiform, a major D₂ structure the probable equivalent of the Creag a' Mhadaidh Antiform to the north-west. The cross-bedded and ripple-laminated psammites, quartzites and semipelites are stratigraphically located near the transition between the Atholl and the Strathtummel subgroups. In the road locality, SE-vergent minor folds on the north-west limb of the antiform, can be seen climbing towards the hinge (Plate 3a) which lies near the south-east end of the cut (marked by a conformable SE-dipping microdiorite dyke) (Figure 21). The minor folds, which show few variations on a plunge of 10° towards 045° are related to a strong axial-plane cleavage, dipping 20°–30° towards 120°. Small-scale D₁ isoclines with an S₁ axial-plane schistosity are seen locally (especially at the north-west end of the cutting) to be folded by D₂, but their vergence and facing are not clear.

The folds in the river are commonly associated with strong parallel linear structures, the famous 'water-pipe' mullions (Barrow et al., 1905, p.68) [NN 784 667]. The antiformal hinge is located some distance down the Clunes gorge [NN 787 667] and the SE limb, with its NW-vergent folds, can be examined further downstream.

Several vertical and inclined dykes cut the both the road and gorge sections (Figure 21). Dark hornblendic microdiorites, grey felsites and pink quartz-feldspar-porphyritic felsites can be seen. The thickest of the microdiorites, best seen in the river just upstream of the mullions, contains xenoliths made up from a variety of metamorphic rocks, especially quartzite, and some which appear not to be from normal Grampian Group lithologies, as well as granitic and dioritic fragments. Downstream [NN 790 664], large faults cut the river section and bring in lithologies which can more readily be assigned to the Bruar Formation of the Atholl Subgroup.

10. Struan river exposures [NN 802 657] to [NN 808 655]

Flaggy psammites dominate the River Garry for 1 km below the junction with the Allt a' Chrombaidh [NN 790 664]. D₂ folds verging NW on the gently dipping inverted limb of the Clunes Antiform, with amplitudes of up to 10m, have been entrenched by the river from under the twin bridges at Calvine [NN 802 657], downstream almost as far as Struan Church [NN 808 655]. Access is possible from the right bank 200 m south-east of the Calvine-Trinafour road at the 'Salmon Leap' [NN 8040 6563]. The sedimentary structures of the types seen at Clunes are further enhanced by the presence of a number of well-preserved sedimentary dykes, washouts and slump structures. The rocks here are assigned to the Bruar Formation not far below the Strathtummel Subgroup boundary.

11. Bruar exposures

The major closure of the D₂ Meall Reamhar Synform must pass through the River Garry some 500 m downstream from its confluence with the Errochty Water [NN 815 655], and its upper limb is exposed intermittently from here as far as the west entrance to Blair Castle grounds [NN 839 659] before the Boundary Slide is reached. Exposure is much better in the Bruar Gorge and is accessible via the paths to the Falls of Bruar from the car park on the old A9 [NN 8230 6608]. The good exposure in the river bed e.g. [NN 8204 6618]; [NN 8235 6595] both upstream and downstream of the railway bridge, provide evidence of an upward-younging succession on the steep SE-dipping limb of the Meall Reamhar Synform. From above the upper falls, the micaceous flaggy psammites of the undivided Atholl Subgroup give way to the much more variable lithologies of the Bruar Formation downstream. These consist largely of thickly bedded psammitic schists with more thin pelitic and semipelitic layers than the Strathtummel Subgroup. Thin impure quartzites are also present and in places interbedded micaceous layers provide the consistent way-up evidence. Rare D₂ minor folds are SE vergent.

12. Pitaldonich cut [NN 833 654]

The final exposures in the transect occur in a low road cut on the A9 east of Pitaldonich. At the west end [NN 8310 6550], upward-younging SE-dipping psammites and quartzites probably form part of the upper Tummel Quartzite Formation. A thin bed of garnetiferous pelite has a schistosity (S₂) with lower dip to the east than bedding; the psammite above contains a later (S₃?) cleavage, steeper than bedding, next to a 10 m-wide porphyritic microdiorite dyke. To the south-east the relationship of bedding to the low-dipping early schistosity continues for a further 50 m, but the units become progressively thinner and more regularly dipping towards the south-east as they come within 1 km of the supposed position of the Boundary Slide. This part of the cut may be within the Tummel Psammite Formation of the Strathtummel Subgroup.

Appin, Argyll and Southern Highland Groups, east of Loch Tay Fault

The present mapping has afforded the opportunity, following initial work by Treagus (1987; 1999), to establish the basis for a radical re-interpretation of the evolution of the major structure of this district. This interpretation has much in common with that of Bailey (1925) but is fundamentally at variance with the subsequent interpretations of Sturt (1961) and of Bradbury et al. (1979). The structure this side of the fault lacks the late complexity of the area to the west and has been the focus for many attempts to unravel the structural history of the Dalradian north-west of the 'Flat Belt'. Both Barrow (Barrow et al., 1905) and Bailey (1925) recognised the fairly abrupt change in dip, from subhorizontal to near-vertical, that takes place between the River Tay and the Farragon–Ben Eagach ridge (Figure 3a, section B). Bailey, aware that the Flat Belt is inverted, was concerned, as subsequent workers have been, to relate this to the evident tight folds of the complex belt to the north-west in what has come to be called the Tummel Steep Belt.

Bailey (1925) recognised, from the stratigraphical repetition of the Ben Lawers Schist to Cairn Mairg Quartzite outcrop pattern, two important tight, upright, folds in the Steep Belt: the Creag na h- Iolaire Anticline and the Sron Mhor Syncline. Bailey regarded these folds as part of a system of isoclinal, originally recumbent, folds; these were later regarded as part of his SE-facing Iltay Nappe system (Bailey, 1934). Sturt (1961), similarly correlated them in age with the (then presumed D₁) Tay Nappe of Shackleton (1958). He regarded the Creag na h- Iolaire Anticline as the original upright core of the Tay Nappe itself, which becomes recumbent to the south, the Flat Belt forming its inverted limb; the Sron Mhor Syncline was regarded as the equivalent to the Loch Awe Syncline of the South-west Highlands, forming a central structure between the SE-facing Tay Nappe and the NW-facing Tummel Anticline (Figure 3a, section C).

Thus, to Bailey, the transition from flat to steep belt was part of the flexuring associated with late folding of the early nappe structure, whereas to Sturt it was the transition from an upright to a recumbent attitude in the original nappe structure. An important result of the present work is that it can now be demonstrated that the steepening is a result of the late D₃ flexuring, but, also, that, although the Creag na h- Iolaire/Sron Mhor fold-pair is D₁, it is not central to the fountain-like D₁ major structure which Sturt proposed. An essential element to this re- interpretation is the recognition of the extension of the D₃ Ben Lawers Synform through the district (Creag Chean Synform of this account; (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9)).

Indeed, folds of D₃ age are the key to the overall structure. Two other major upright folds of this age, north of and complementary to the Creag Chean Synform, are now recognised: the Dubh Chnochan Antiform and the Loch Tummel Synform; these three folds are essentially responsible for the steepening of dips in the so-called Steep Belt.

The other new, and dominant, elements in the present interpretation, are structures, both minor and major, of D₂ age. Minor structures of this age have been recognised by Stringer (1957) and Bradbury et al. (1979) as being dominant in, and partly responsible for, the Flat Belt associated with the Tay Nappe. Four major folds of D₂ age play an important role in the structure of the Steep Belt area (Figure 14); these are named from north-west to south-west, upwards in the D₂ pile, the Balnabodach, An Tulach, Creag Loisgte and Meall Tairneachan folds.

An entirely different interpretation of the Flat- to Steep Belt relationship has been made by workers revising the Pitlochry district (Sheet 55E). Bradbury et al. (1979) proposed that the Steep Belt was a fan of a composite S₁–S₃ foliation, associated with major D₃ folds (including the Creag na h- Iolaire/Sron Mhor fold-pair); this D₃, however, is related to progressive metamorphism and thus is not the D₃ recognised in the present account. The transition from the Steep into the Flat belt was regarded as a 'long-lived rotation zone' of all the S₁ to S₃ fabrics (Figure 3a, Section D).

In the account below, the geometry of the minor and major structures of each of the three phases D₁–D₃ mentioned above will be described together with locations where they best be examined, particularly with a view to justifying their mutual ages. A locally developed fourth phase (D_c) responsible only for minor structures is also described. The youngest structures are described first, so that with the aid of (Figure 15), the geometry of the older rotated structures may be better understood. Axial traces of major folds are shown in (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and minor structures of the the D₂ and D₃ phases in (Figure 10) and (Figure 11) respectively. A cross-section of the area in addition to that below the published map is given in (Figure 14).

Creag an Chanaich phase (D_c)

Minor stuctures of this generation are developed sporadically throughout the Steep Belt. A generally flat-lying crenulation cleavage is commonly evident in incompetent lithologies, particularly in the graphitic schists, where the S₂ cleavage is steeply dipping. Where finely bedded lithologies are affected, symmetrical folds with amplitudes of up to several metres may be developed, giving rise to local reversals of dip; such folds will be open to tight in style with generally subhorizontal ENE–WSW-trending axes. No major folds of this age are recognised.

The age relationship of these structures to D₃ is not always unambiguous. Thus in exposures of the transitional Ben Lawers Schist/Ben Eagach Schist/Carn Mairg Quartzite lithologies seen on or near tracks west and east of Foss Mine at Creag an Chanaich [NN 815 545], the flat Sc cleavage commonly alternates in beds with the steep S₃ cleavage. Here, however, it can be locally established that the latter is the earlier structure, usually more strongly developed than the former. In other lithologies, for instance in the pelites and semipelites of the Ben Lawers Schist south of the mine, crenulation cleavages commonly show many intermediate attitudes between the upright S₃ and the flat S_c; as S₃ does demonstrably fan into quite flat-lying attitudes, the attribution of a cleavage to the S_c phase is not unequivocal. For instance, the thin-bedded limestones and calc-schists of the Blair Atholl Formation on Craig Balnabodach [NN 822 604] show folds with a strongly developed, gentle to moderate NW-dipping crenulation cleavage which, although identified as S_c, could well be strongly fanning S₃.

Third deformation (D₃)

The evidence from the geometry of the major D₃ folds suggests that the pre-D₃ attitude of S₂ (and generally of S₀ and S₁) in the district was essentially flat lying. This D₂ regional sheet-dip has been corrugated by the five major D₃ folds, shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and (Figure 14). Each of these major folds is discussed below, together with localities where associated minor structures (Figure 11) supporting the major structure may best be examined.

To the south of the district the Flat Belt is gently corrugated by several such upright ENE-trending folds. The most northerly of these is the southernmost D₃ fold in the district, the Creag an Fhudair Synform, whose limbs rarely dip more than 20° with a gentle ENE plunge. Its presence is emphasised by the outcrop pattern of the amphibolites on Hill Park and Creag an Fhudair, where associated crenulations and open minor folds can be seen on the two limbs [NN 7918 4412] and [NN 7814 4448]. Open neutral folds with a weak upright crenulation cleavage can be seen in the hinge area [NN 7820 4411].

To the north, the Loch Tay Antiform is a more significant structure, with dips on its northern limb steepening to 50° NW as the hinge of the Creag Chean Synform is approached; the outcrop of the Farragon Volcanic Formation is taken for descriptive purposes as the northern limit of the fold. This fold is particularly apparent from the closure of the Green Beds in Strath Tay, south of Dull, although this closure is not exposed. To the west the axial trace splits into a triple fold, two antiforms marked by the eastern ends of Glen Lyon and of Loch Tay, separated by the synformal mass of Drummond Hill. This latter synform is well marked by the outcrop pattern of amphibolites on Drummond Hill which, in exposures at the western end [NN 726 453], shows a plunge of up to 30° towards 275° and, at the eastern end [NN 779 478], 25° towards 254°. As shown in (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and (Figure 14), the Drummond Hill synform itself splits into three component folds; limbs rarely dip more than 30°.

The geometry of the Loch Tay Antiform overall is best appreciated by comparing the gentle south-east dips of the south-east limb near the road to Tombuie Cottage [NN 7814 4448], below the amphibolite, with the steeper north-west dips below similar amphibolites in the Keltney Burn [NN 771 497]. Minor structures, seen in many exposures of incompetent beds, such as in the Pitlochry Schists and the schistose parts of the Loch Tay Limestone, are open asymmetrical folds with local development of upright ENE-trending crenulation cleavage; minor folds and crenulation lineations plunge at low angles to the WSW or ENE. A convenient exposure for examining minor D₃ and refolded D₂ structures in the steep short limb of an intermediate-scale D₃ fold is by the road at Blairish [NN 7651 4860].

The succeeding fold to the north is the Creag Chean Synform, which has not been previously identified, due no doubt to its lack of expression in the outcrop pattern of formations because of the self-cancelling effect of the D₂ antiform within the same outcrop (Figure 14), (Figure 15a). Moreover, as the trace of the fold lies entirely within the Ben Lawers Schist, changes in the sheet-dip of bedding are obscured by the disharmonic folding caused by the varied and incompetent nature of its lithologies. For descriptive purposes the limit of the fold to the north is taken at the northern boundary of the Ben Eagach Schist.

From the southern margin of the Ben Lawers Schist outcrop, the finely bedded calc-schists, calc-quartzites and metalimestones, which dip up to 50°NW, develop increasingly strong S₃ crenulation cleavage related to steeply SE-dipping axial planes of intermediate- and small-scale folds, with amplitude up to several tens of metres. Folding and refolding (D₁, D₂ as well as D₃ and D_c) renders measurement difficult but it becomes evident that the sheet-dip of S₀/S₂ decreases towards the centre of the Ben Lawers Schist outcrop and that the clear southerly vergence of D₃ folds in the south (Plate 4c) becomes neutral and eventually dominantly northerly. In the well-bedded lithologies near the northern boundary with the Ben Eagach Schist, the northerly vergence of minor folds becomes evident and, in the transitional lithologies to the Ben Eagach Schist, especially where the stratabound baryte is present, the S₃ vergence relations are particularly well displayed (Figure 11), (Figure 27). At this boundary, the bedding dips generally steeply SSE, and S₃ and D₃ axial planes dip at moderate to steep angles to the NNW. Folds of all scales, as well as S₀/S₃ intersection lineations, plunge at shallow angles to the WSW, although locally to the ENE. A stereographical plot of poles to bedding for the synform in a traverse around Creag Chean shows a mean plunge of 10° to 254°.

It is important, in terms of the regional structure, to note that the inward-dipping boundaries marking the two limbs of the Creag Chean Synform have been interpreted by all previous workers as a progressive steepening and overturning towards the north of a SE-younging succession (Figure 3a), Sections C and D. The stratigraphical relationships are explained below as a consequence of the refolding of a major D₂ fold by the Creag Chean Synform within the Ben Lawers outcrop (Figure 14).

The structures on the southern limb of the synform across the hinge zone, within the Ben Lawers Formation, are well displayed in two traverses: in the Keltney Burn upstream from [NN 7677 5161] and south-east of Glengoulandie Farm upstream in the two burns [NN 768 520], where the beginning of the hinge zone is seen; the broad axial zone can be best examined from Meall Gorm [NN 793 548] to Creag Chean [NN 796 530] from where it can be traced west to Meall Dubh Mor [NN 7813 5293] (Plate 4c) or east to the south side of Creag an Oir [NN 8070 5408]. The north limb at the Ben Eagach boundary is best seen in the west of the area in Coire Screabaig [NN 7805 5416] or in the Ben Eagach Schist outcrop in and south-east of a quarry [NN 7850 5513]. Three traverses give sections in which the synform may be demonstrated: south of Meall Tairneachan [from 800 531] via Creag an Oir [NN 806 540] to the east side of the hill [NN 809 544] from the Farragon Volcanic Formation [NN 8175 5375] north to the mine at Creag an Chanaich [NN 813 545] and similarly along the cliff at Creag an Loch [from 826 544] up to the Ben Eagach boundary at the track. Exposures of the stratabound baryte and quartz–celsian rock in the mine track at Creag an Chanaich [NN 8122 5457] to [NN 8137 5450] give a fine section through a D₃ fold-pair some 80 m in wavelength, verging south with attendant S₃ cleavage and minor folds; D₂ structures are clearly deformed by D₃ in this section, as discussed below and in Chapter 6 (Figure 26), (Figure 27), (Figure 28). Another large D₃ fold-pair occurs in a traverse from the mine road [NN 8122 5457] north to the Frenich Burn [NN 8112 5497].

The next major D₃ fold to the north is the Dubh Chnochan Antiform. For the purposes of description this will be treated as the area of Carn Mairg Quartzite and Killiecrankie Schist on the south side of Loch Tummel. In the western part of this area, the antiform is defined by the contrasting moderate to steep SSE dips along the Carn Mairg Quartzite/Killiecrankie Schist boundary on the south limb and the steep northerly dips at and close to the Killiecrankie Schist/Schiehallion Quartzite boundary south of Loch Tummel, on the north limb. In between, around Craig Loisgte and Dubh Chnochan, the Killiecrankie Schist forms a broad arch, corrugated by several intermediate-scale folds of neutral vergence. To the east of the Frenich Burn the rocks are affected by a possible post-D₃ deflection in the outcrop associated with much faulting; this, together with poor exposure, makes the delineation of the antiform trace less easy in this direction.

As mentioned above, the Ben Lawers/Ben Eagach/ Carn Mairg formations on the south limb dip steeply south and the graphitic schists show excellent sympathetic fold and cleavage vergence towards the antiform to the north, e.g. on the mine track [NN 8122 5457] to [NN 8137 5450] (Figure 11). The steep southerly dip is also well displayed at the Carn Mairg Quartzite/ Killiecrankie Schist boundary, e.g. in the Allt Tarruinchon [NN 7995 5618] and in the Frenich Burn [NN 826 577]. In the Ben Eagach and Ben Lawers formations in the cores of the D₁ Creag na h- Iolaire Anticline, e.g. north-east of Meall Tairneachan down the Frenich Burn West [from 8105 5480] and the Sron Mhor Syncline (exposed in the Frenich Burn [NN 824 557] to [NN 825 565]), the pelites are dominated by the upright S₃ crenulation cleavage, striking north-east. The sequences are affected by 10 m- wavelength and smaller-scale folds verging north and plunging gently south-west or north-east; this folding is strikingly clear in the transition to the Carn Mairg Quartzite to the north in the Frenich Burn [NN 8248 5653]. From here towards Lick [NN 835 580], as mentioned above, the strike of formations, as well as of the S₂ and S₃ cleavages, is deflected for 2 km into a NNE direction.

The Carn Mairg Quartzite of the D₁ Creag na h- Iolaire Anticline to the south around [NN 825 554]) is similarly cross-cut by the S₃ cleavage in semipelitic beds, but here, as elsewhere where earlier folding causes the strike of bedding to be at a high angle to S₃, the D₃ crenulations plunge steeply to the south-west.

Generally the Carn Mairg Quartzite is not strongly affected by D₃, but, in contrast, bedded quartzites and pelites of the Killiecrankie Schist, which occupy the core of the antiform, clearly display intermediate- and small-scale folds as well as the S₃ cleavage. These features, together with exceptionally clear graded bedding and refolds of D₂ folds, are well displayed on the crags of Craig Loisgte [NN 802 567] and Dubh Chnochan [NN 799 571] (Plate 1c), (Plate 4a). Similar features in the broadly neutral-verging folds are seen in the Allt Tarruinchon, upstream of [NN 793 570]. Here, D₃ plunge varies between 20° both to the ENE and WSW.

The northern limb of the antiform is less well exposed. However, exposures on the forest track e.g. [NN 822 585], in the Frenich Burn [NN 827 587] and above Lick [NN 831 587] show steep north-dipping bedding with D₃ minor-fold and cleavage vergence to the south, with plunges gently to the south-west.

The hinge-zone of the Loch Tummel Synform is hidden by the loch and the fold is described by reference to the Appin and Argyll Group rocks on its northern south-dipping limb, exposed north of the loch. The existence of the synform is clear enough from the contrast of the moderate northerly dips already referred to on the south side of the loch, with the steep southerly dips seen on the north side, e.g. Allt an Tressait [NN 816 601]. The synform can be further appreciated from the curvature of formations on the north side as they close around the WSW-plunging hinge. This latter feature is continued into the Pitlochry district (Sheet 55E) in the closure of the Schiehallion Quartzite/Killiecrankie Schist boundary at the east end of the loch around [NN 850 595] and possibly by the refolding of the Carn Mairg/Killiecrankie Schist core of the Sron Mhor Syncline around [NN 890 600] (Figure 16). These patterns indicate that the axial trace trends towards 085°.

Although the fold is quite tight (interlimb angle of about 70°), the S₃ cleavage and D₃ minor folds are not strongly developed. These features can be best seen in the Killiecrankie Schist near the fold core, e.g. Allt an Tressait [NN 816 599], and in the Blair Atholl limestones and schists on the loch shore [NN 822 598]. Elsewhere, they are patchily developed on Craig Balnabodach [NN 825 604] and Meall Urair [NN 831 609].

First and second deformations (D₁ and D₂)

Although it is difficult to quantify the first deformation, the second is regarded as the more important, not least in the sense that it was responsible for four major tight folds which cause the complete inversion of the stratigraphy over large parts of the district. Minor D₂ fold-pairs can be identified throughout the district in most lithologies and are generally tight-to-isoclinal folds of near similar style. Interlimb angles vary with lithology, 20°–30° being typical of pelites and up to 80° where thick psammites are involved. The short limbs of the folds vary from tens of centimetres to tens of metres in length (Plate 1c), (Plate 4a).

The regional S₂ schistosity, which is axial planar to these folds, is a fine penetrative schistosity although it is clearly a crenulation of an earlier cleavage especially visible on the short limbs of D₂ folds. S₀/S₂ intersection lineations and D₂ hinges show variable orientations (Figure 10) but generally have a north–south trend in the south of the district, becoming more north-easterly and easterly towards the north. (Figure 15a) has been constructed to illustrate, particularly, the effect of the D₃ folding in the centre of the district on the D₂ minor structure orientation. When the effect of folding by the D₃ major folds, discussed above, is removed (Figure 15b) it appears that this swing in trend is an original D₂ feature; its origin is considered later. D₂ fold and cleavage vergence varies across the district, according to its position on the major D₂ fold limbs. These changes have been the principal tool for determining which of the folds revealed by the outcrop pattern of formations are of D₂ age, in the usual manner of structural analysis (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 10). Thus north of Loch Tummel the repetitions of the Schiehallion Quartzite can be attributed to the Balnabodach and An Tulach Folds; the Craig Loisgte Fold, structurally above, is less well defined, but analysis of vergence patterns has been the sole method of establishing the presence of a fourth fold, the Meall Tairneachan Fold, whose axial-trace lies entirely in the Ben Lawers Schist Formation in the south of the Steep Belt (Figure 14). These major structures are referred to as antiforms and synforms, as if they originally closed up to the north-west or down to the south-east, in spite of the fact that D₃ refolding, of course, has locally reversed this relationship.

Generally, S₁ cleavage cannot be separated from bedding surfaces, and D₁ minor folds or S₀/S₁ intersections are rarely able to be identified. S₀/S₁ and S₂ surfaces very commonly contain a variety of mineral lineations. A weak mica lineation is common in pelites, whilst S₀/S₂ planes in psammites and subparallel quartz veins usually contain a strong quartz rodding. Amphiboles and garnet pressure shadows commonly exhibit a strong preferred orientation (Plate 8). These lineations vary in orientation in a similar manner to that of the fold hinges and S₀/S₂ intersections (Figure 10). In fact, the two types of lineation are usually subparallel; where they can be separated, especially in the south of the district, the stretching lineation lies a few degrees anti-clockwise of the intersection lineation.

In the account below, the D₂ structures are described in a traverse from south to north. In this way the style and attitude of the minor structures can be related to the various lithologies and to the major folds. The cross-section of the present structure (Figure 14) together with the three parts of (Figure 15) will help in the understanding of the D₂ structure. The axial-traces of the major D₁ and D₂ folds are shown in (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), and minor structure orientation and vergence in (Figure 10). Mention will also be made in these descriptions of the rare instances of minor stuctures which may be attributed to D₁. The major D₁ folds will be treated separately.

Pitlochry Schist of the Flat Belt

The most southerly exposed section in the district is that in the Allt a' Bhealaich upstream from Tombuie Cottage [NN 789 447]. The schistose grits and pelites, although affected by D₃ folding, may be treated as essentially horizontal; in a few places they reveal tight to isoclinal fold-pairs [e.g. one with a 3 m short limb at [NN 7919 4412], which trend consistently between 345° and 020° and have a westerly vergence. In the absence of folds, the cleavage vergence can be used to establish the same relationships. Reversals of vergence over distances of 10 m or so indicate the presence of substantial intermediate- scale D₂ folds. Lineations represented by mica elongation, quartz rodding (possibly quartz pebble elongation) and amphibole orientation are difficult to separate from the fold hinge and intersection lineation, but may in places be seen to be up to 15° anticlockwise. Garnet pressure shadow elongation shows the same relationship, orientated about 350°; asymmetry of the latter is top to the SE (Plate 9b). Similar relations can be observed in the crags beneath the amphibolite sills further north around [NN 780 445], at a lower D₂ structural level. Centimetre-thick micaceous beds crowded with minute garnets (coticules) from these exposures exhibit pre-D₂ isoclinal folds (presumed D₁) and inclusion trails in the garnets suggest early D₂ growth.

Across strike to the north-west the next well-exposed sections are on Drummond Hill. Here, the exposure is generally disappointing, in being inaccessible or dirty, but there are good sections on tracks and crags e.g. [NN 745 452] around [NN 7500 4525], in the crags above Black Rock [NN 763 459], at, [NN 7715 4765], and a traverse from [NN 7770 4800] up to the fort at [NN 7791 4765] and across to [NN 7777 4726]. The structural relationships are generally the same as described above, except that the fold, and S₀/S₂ intersection and extensional lineation trends are more consistently to the NNE, commonly to 030°, when restored to presumed pre-D₃ horizontal attitude. Elongate, carbonate-filled amygdales in amphibolites show a north-west to NNW trend, as do well-developed pressure shadows around garnets; asymmetry of the latter is top to the south-east.

Many of the exposures mentioned, especially at Black Rock, give good sections through west-verging D₂ folds, several metres in amplitude. The style of folding of beds of various competence, together with that of S₁ and parallel quartz veins, the variation in the morphology of S₂ (from penetrative in pelite to spaced in psammite) can be studied. No significant change in orientation of D₂ structures could be detected from the east to the higher D₂ structural level at the west end of Drummond Hill.

Further north, in tracks and cliffs in Dull Wood [NN 800 495] are good sections of 10 m wavelength west-verging, downward-facing D₂ folds. A poor section in the lower Camserney Burn down from [NN 814 500], being nearest to the presumed outcrop of the Green Beds, is the lowest D₂ structural level observed. Both sections show a range from 010° to 035° of D₂ fold and intersection lineation directions.

The continuation of the traverse into the north-western outcrops of the Pitlochry Schist marks the beginning of the steepening of dips on the south-east limb of the Creag Chean Synform. Good sections through these rocks, up to the Loch Tay Limestone, is seen in the Allt Odhar above Glen Lyon House [NN 737 471] together with adjacent crags, crags above Blairish [NN 760 490], in the Keltney Burn above the road [NN 774 492], in the crags north-east of Coshieville [up from 780 500], and in the upper Camserney Burn [up from 814 500]. As the beds steepen to the north-west, the N–NNE D₂ lineations pitch steeply to the north-east, as would be expected; however, on rotation back to the presumed pre-D₃ horizontal plane, the trend in all the above-quoted sections is between 030° and 050°, with local variation as far as 065°. There is no apparent along-strike or structural-level control on these variations, but rather the more extreme swings seem to be associated with more pelitic lithologies. Biotite and amphibole lineations as well as garnet pressure shadows show a more north–south orientation than the S₀/S₂ intersection, in places 20°–30° anticlockwise in the same outcrop.

Throughout the Pitlochry Schist outcrop it has been possible to determine, and confirm in thin section, an angular relationship between bedding and S₁. However, the infrequency of these observations and the lack of facing evidence does not allow these readings to be used to deduce anything significant about the D₁ structural geometry.

Downbend in the Loch Tay Limestone, Ben Lui Schist and Farragon Volcanic Formations

Sections continuing north-west from the above, across these three formations, are seen in the Allt Odhar [NN 741 482] (Loch Tay Limestone), Allt Coire Pheiginn [NN 767 502] to [NN 754 505] (all formations), Creag nan Cop [NN 7855 5175], Coire an Easain [NN 7911 5235] and the line of crags to Creag an Loch [NN 8240 5412] (Ben Lui Schist to Farragon Volcanic Formation).

The westerly D₂ vergence is particularly convincing at the Pitlochry Schist/Loch Tay Limestone junction, with small-scale folds and S₂ cleavage well developed. Generally, the Ben Lui Schist is a monotonous semipelitic sequence, and the impression is that there is a high concentration of strain in the formation. Thus S₂ is pervasive and folds are very tight with boudinaged long limbs. The dominant structure is commonly a quartz-rodding, affecting S₁, S₂ as well as quartz veins, parallel to both surfaces; this lineation, which is also in some places a strong pebble elongation lineation, is parallel to the D₂ intersection lineation discussed below. Refolded D₁ isoclines and and S₀/S₁ relations can be observed occasionally, but too infrequently to be used for structural analysis.

In the Farragon Volcanic Formation, tight to isoclinal D₂ folds are commonly well developed in the thinly bedded quartzites and amphibolites and show consistent overall westerly vergence, although reversals are not uncommon. Refolded minor D₁ isoclinal closures are not uncommon in thin quartzites, e.g. on Creag an Loch [NN 8241 5405].

These sections exhibit the most interesting variations in D₂ fold and S₀/S₂ attitude. Beds in these sections dip as much as 70–80° NW, but there is a very clear distinction between the steep north-east pitch, in the competent quartzites and thinly bedded amphibolites of the Farragon Volcanic Formation, and the gentler pitch in the incompetent beds of the other two formations below. When rotated to a horizontal position, this variation shows that, in the incompetent schists and schistose limestones, the D₂ lineations have swung to 050°–065° continuing the trend seen in the Pitlochry Schist, whilst in the competent psammites it remains 025°–035° and in the highly competent Farragon Volcanic Formation it has reverted to 010–020°. In the Farragon Volcanic Formation there is a clear distinction between this and the amphibole, quartz elongation and garnet pressure shadow lineation (Plate 8) which, when rotated, have a N–NNW orientation.

Meall Tairneachan Fold: Ben Lawers Schist to Killiecrankie Schist

The presence and geometry of this, the uppermost of four major D₂ folds, is dependent on the contrast in geometry of minor structures described above with those seen in these formations to the north. In all the sections described below proximity to formation boundaries, usually reinforced by sedimentary structures, means that direction of younging can be assumed, and in every section D₂ fold and cleavage vergence is to the south on gentle to moderate SW-plunging axes (Figure 10), (Figure 15a). This reversal of plunge from that on the south side of the Ben Lawers outcrop, is clearly due to folding about the D₃ Creag Chean Synform. However, restoration of these directions to the presumed pre-D₃ horizontal attitude shows that the swing of D₂ axes previously noted has continued northwards and has now a north-east or NNE trend (Figure 15b).

The significance of the change of D₂ vergence, from one margin of the Ben Lawers Schist to the other, is that a major D₂ fold must outcrop within the main outcrop of that formation. Although reversals of vergence are constantly seen within the formation, no statistically significant change was found that would assist in determining the precise position of the major D₂ fold. However, consideration of the plunge of the D₂ intersections requires that the fold must be an antiform outcropping to the north of the Creag Chean Synform trace (Figure 14), (Figure 15a), in the general geographical area of Meall Tairneachan. It is, however, clear from the regional parallelism of formation boundaries that the major fold hinge cannot parallel the (original) north to NNE trends of the minor folds and S₀/S₂ intersections discussed above; thus an original north-east trend is assumed for the Meall Tairneachan Fold (Figure 15b) which, subsequent to deformation by the D₃ Creag Chean Synform, assumes a gentle south-west plunge (Figure 15a).

Important sections are: the Ben Eagach/Ben Lawers Schist boundary particularly in the Allt Glengoulandie [NN 7805 5420], on the mine track [NN 8066 5471], and on Creag an Chanaich from [NN 8109 5461] east along the track; at the Ben Eagach Schist/Carn Mairg Quartzite boundary and the schistose parts of the latter formation e.g. the quarry at [NN 7850 5513], in the Allt Tarruinchon from [NN 795 555] down to the Killiecrankie Schist at [NN 7995 5618], across the Creag na h- Iolaire Anticline especially [NN 8268 5514] (also D₁ minor folds), and in the Frenich Burn [NN 8247 5652]. Boudinage of psammitic beds and of quartz veins is a common feature of the Ben Lawers Schist (Plate 3c). Axes are parallel to D₂ minor fold axes, and S₂ augens the boudins; however, the boudins are certainly, in part, a consequence of synsedimentary deformation and of D₁.

The section along the track at Creag an Chanaich [NN 8123 5462] to [NN 8136 5448] is perhaps the most instructive in the whole area (Figure 27), (Figure 28); here the clear stratigraphy of the baryte/quartz-celsian beds at the top of the Ben Eagach Schist enables an intermediate-scale (short limb 150 m) D₂ fold-pair to be mapped out, supported with consistent minor-scale evidence, plunging moderately down to the south-west and verging south-east. The fold-pair is seen on the short limb of a D₃ fold-pair and cross-cut by S₃ cleavage. Exposures and drill-core information in the open pits to the north-east have been used to distinguish a further D₂ fold-pair, folded by the same D₃ antiform as seen in the track (Figure 27).

Killiecrankie Schist south of Loch Tummel

The section in (Figure 14) shows that the traces of three major D₂ folds should be present within this outcrop, which is also the site of the D₃ Dubh Chnochan Antiform. In view of the poor outcrop over much of this area, all that can be said is that there are very clear reversals of D₂ vergence and many intermediate-scale folds are apparent (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 10); (Plate 1c), (Plate 4a). In the well-exposed sections on the west side of Craig Loisgte [NN 802 567] there is more vergence to the north than to the south, suggesting that a D₂ fold with a synformal geometry, the Craig Loisgte Fold, (closing down to the south) has been crossed (Figure 14). On Dubh Chnochan [NN 799 571] a return to southerly vergence is dominant in the flat-lying rocks in the hinge area of the D₃ antiform, suggesting the presence of the next lowest antiform (closing to the north); this fold would be the An Tulach Fold, named from the north side of Loch Tummel. Information within the outcrop of the Killiecrankie Schist to the east is insufficient to establish the traces of this and the next lower D₂ major fold, the Balnabadoch Fold, also named from the north side of Loch Tummel.

The most important localities on the north limb of the Dubh Chnochan Antiform are in the track exposures in Frenich Wood west of [NN 822 585] and in the Frenich Burn [NN 827 587]. These exposures are at or close to the boundary of the Killiecrankie Schist with the Schiehallion Quartzite on the lower limb of the Craig Loisgte Fold. Cleavage relations here show a northerly vergence and a gentle westerly plunge of the intersection with bedding.

Plunges of minor folds and intersections are at low angles to the WSW, or more rarely to the ENE in this area. This suggests that on restoration to their pre-D₃ attitude, the lineations would show a further continuation of the swing from a north trend in the south of the district, now to an ENE direction (Figure 15b). The major Craig Loisgte Fold is assumed to have a gentle south-west plunge, subparallel to the minor structures. There is no clear evidence in these rocks of a D₂ extension lineation. Pebble alignment appears to be parallel to the fold hinges, but is regarded as an unreliable indicator of the finite extension direction.

D₂ folds north of Loch Tummel

Although exposure is patchy in this area, the outcrop of the Schiehallion Quartzite makes it clear that two major folds are present, the An Tulach Fold and the Balnabodach Fold with cores of Killiecrankie Schist and Blair Atholl limestone and schist respectively (Figure 16). The outcrop pattern of the Balnabodach Fold is complicated by the refolding of two intermediate- and one major-scale D₁ folds, discussed below, clear evidence that this fold is of D₂ age. Minor structure evidence for the D₂ identity and the geometry of these two folds is very restricted, but the key relationships seen at stratigraphical boundaries on their three limbs (upper to the south, middle and lower to the north) is as follows (Figure16).

The upper limb is not well exposed. Vergence is dominantly north in the Killiecrankie Schist in the Allt an Tressait [downstream from 816 601], but the Schiehallion Quartzite boundary is not exposed; shore exposures of Blair Atholl Formation eastwards from [NN 821 597] show D₂ closures folded by D₃, but no consistent D₂ vergence. On the middle limb, Blair Atholl Formation limestones near the Schiehallion Quartzite above the track on Creag Mhor [NN 8262 6015] show southerly cleavage vergence. On the lower limb, consistent north vergence of S₂ and minor folds is seen on Creag Mhor from the Blair Atholl/Schiehallion Quartzite boundary at [NN 8272 6023] across the Boulder Bed, which is repeated in intermediate-scale D₁ cores to [NN 8310 6038], and at the Schiehallion Quartzite/Killiecrankie Schist boundary on Meall Urair [NN 8330 6085].

The plunge of the minor D₂ folds and cleavage intersections is at low angles to the east and west, which suggests once again, on restoration to a pre-D₃ horizontal attitude (Figure 15b), that the minor fold/intersection direction had swung here to an almost due east trend. However, the outcrop pattern of formations around the gently WSW-plunging D₃ Loch Tummel Synform requires that the major D₂ fold plunge should be to the east (or at least less steep to the west than the D₃ plunge); on the south limb of the Loch Tummel Synform, the major D₂ hinges should plunge more steeply west than the gentle D₃ plunge (see discussion below).

A further D₂ major antiformal fold is shown on the cross-section (Figure 14), to the north and structurally below the Balnabodach synformal fold; this is shown from purely regional considerations, discussed at the end of this chapter. The trace of this fold may well cross the Blair Atholl Formation in the north-east corner of the district, but exposure is very poor. In the area of Sheet 55E Bailey recognised an antiform in the same structural position, confirmed and named the Beinn a'Ghlo Antiform by Smith and Harris (1976).

D₁ major folds

Creag na h- Iolaire Anticline and Sron Mhor Syncline

These two folds have played a key role in previous interpretations of the area, but their geometry and age in the deformation sequence has not hitherto been demonstrated; the presence of both is clear from the outcrop pattern, but their plunge and limb dip (and hence their definitions as synform/antiform) is less obvious. However, the present work shows that the S₂ and S₃ cleavages are superimposed on both closures and hence their identity as D₁ is clear.

This age is confirmed from D₁ minor structures in the Carn Mairg Quartzite well exposed in the core of the Creag na h- Iolaire fold on the line of crags [NN 8267 5518] to [NN 8263 5566], particularly on the steep east-facing slope. The bedding in the quartzite on the south limb of the anticline has a south-east strike and steep south-west dip in contrast with the east–west strike and steep north dip of the north limb. A well-developed steep S₁ cleavage is associated with intermediate- scale D₁ folds and fold and cleavage vergence change from north on the south limb to south on the north limb within a neutral zone about mid-outcrop (Plate 3b). These observations make it clear that the closure is indeed an anticline upward-facing to the west; S₁/S₀ intersections parallel to a quartz-rodding show plunges 20–30° WSW. Two representative readings illustrate these relationships on (Figure 10).

In psammites, the steep S₁ (commonly exhibiting upward-facing on graded bedding) is associated with a pebble flattening, while the interbedded semipelites and graphitic pelites, where they are strongly refracted into flat attitudes, are dominated by S₂ and S₃ with minor folds developed in thicker incompetent beds; flat-lying Sc cleavage is also locally strong. S₂ is seen to cross-cut individual 10 m wavelength D₁ folds, dipping more gently to the south than S₀ and S₁.

No such D₁ data can be observed on the two Ben Eagach/Carn Mairg boundaries defining the Sron Mhor Syncline [NN 8264 5584] and [NN 8248 5654]; the near parallel steep dips of bedding only demonstrate that this is a near-isoclinal fold.

The axial traces of the two folds cannot be identified in the main Ben Lawers Schist outcrop to the west of Meall Tairneachan and, although the boundary with the Ben Eagach Schist which defines the two closures can be mapped approximately, no significant D₁ data has been found. Localities on the track [NN 8066 5471], in the Frenich Burn West [NN 8132 5520], and south-east of Ciochan a' Chop [NN 8095 5515] define the position of this boundary on the two limbs of the anticline; bedding is steep but strongly folded and crossed by a penetrative S₂ and locally by S₃ crenulation cleavage, which verge south and north respectively across both boundaries. The boundary of the Sron Mhor Syncline as defined by the Ben Eagach and Ben Lawers Schists is best observed in the Frenich Burn [NN 8257 5600] to [NN 8261 5617], where it is dominated, and consistently crossed, by S₂ and S₃ cleavages, thus supporting the D₁ age of the fold.

It is clear from the cross-section in (Figure 14) that the plunge of the major D₁ fold-pair is not co-axial with, and is more curvilinear than, the D₃ major folds. Thus, although the hinge of the Creag na h- Iolaire Anticline in the Meall Tairneachan area is approximately parallel to the moderate south-west plunge of the D₃ folding, the outcrop of Ben Eagach Schist to the east of the Loch Tay Fault at Glengoulandie [NN 770 528] is interpreted as the reappearance of the hinge, now with an easterly plunge. This plunge, to be compatible with the outcrop pattern in Glen Lyon to the west of the fault (see later discussion), would be very steep. The persistent narrow outcrop of the Ben Lawers Schist in the core of the Sron Mhor Syncline between Ciochan a' Chop [NN 8095 5515] and the Frenich Burn [NN 8257 5600] and its widening from here to the east margin of the district suggests a low plunge to the west in the west, and a similar low plunge to the east in the east; the westerly plunge reasserts itself to the east of Sron Mhor in the Pitlochry district [NN 846 578], where the Ben Eagach Schist closes around the Ben Lawers Schist.

North of Loch Tummel

The complex outcrop in this area is explained by the axial-traces on (Figure 9, (Figure 14) and (Figure 16), in terms of a D₂ fold-pair folded about the D₃ Loch Tummel Synform, with earlier repetitions of stratigraphy produced by a major D₁ anticline with two intermediate-scale folds on its southern limb. (Figure 16) is an attempt to reconstruct the outcrop pattern and axial traces of the folds of the three phases in unexposed ground, incorporating some evidence from the Pitlochry district. The age of the D₁ folds is established entirely on the basis of the repetitions of Schiehallion Quartzite and older formations being crossed by the S₂ cleavage. Exposures exhibiting this relationship on either side of the trace of the major Meall Urair Anticline are seen at [NN 8273 6048] (Schiehallion Quartzite/Blair Atholl Formation boundary) and on Meall Urair [NN 832 609] (Schiehallion Quartzite/Killiecrankie Schist boundary); the folded Schiehallion Quartzite/Boulder Bed/Blair Atholl Formation boundaries on the limbs of the intermediate-scale fold-pair, are seen cross-cut by the cleavage in a traverse near the path from [NN 830 603] north-east to [NN 8330 6044]. As will be seen on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) the Meall Urair Anticline, as defined by the unexposed termination of the Schiehallion Quartzite/ Killiecrankie boundary south of Strathtummel, faces downwards on a gentle east plunge on the north limb of the Loch Tummel Synform and is predicted to face upwards on a westerly plunge, as a result of this D₃ folding, on the south limb. Taking into account the D₂ folding on the north limb, outcrop patterns east of Meall Urair (Pitlochry district) indicate that the D₁ hinge has curved through the horizontal to be downward-facing to the west. When restored to its pre-D₃ attitude (Figure 15b), this latter attitude becomes an ENE trend, whereas the plunges seen elsewhere suggest an original curvature to a more ESE trend. It must be emphasised that these D₁ plunges are entirely deduced from the reconstructed outcrop pattern of formations and are not directly observed. To maintain structural consistency, a companion D₁ Meall Urair Syncline is shown at a higher structural level on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 14) and (Figure 15b); the trace of this fold does not crop out in the district.

Two further major D₁ folds are also shown on the section (Figure 14) at a lower structural level, to the north of the Meall Urair Anticline. The first of these, the Conbhar Syncline, is recognised from the repetition of the Blair Atholl Formation to the north of the Killiecrankie core of the Meall Urair Anticline. The Schiehallion Quartzite does not occur at this boundary and this, together with other excisions in the Pitlochry district led Bailey (1925) to propose a major slide, the Killiecrankie Slide, in this position. Very limited observations, owing to poor exposure, suggest that D₂ vergence remains constant across the northern boundary of this closure, south-east of Conbhar [NN 829 613]. This fold and its necessary, but unexposed, companion the Conbhar Anticline, are downward-facing to the south but with unknown plunge. The presumed slide boundary (the regional Killiecrankie Slide), which excises the Schiehallion Quartzite from the common limb of the fold-pair, is unexposed in the district; however, the reconstruction in (Figure 14) suggests it has a top-to-the-north-west sense of shear and, as the pattern of its outcrop is folded by upright D₃ folds, it could be D₁ or D₂ in age. As discussed later, this fold-pair, with its attendant slide is precisely analagous to the Beinn a' Chuallaich fold-pair, and its slide of probable D₁ age, to the west of the Loch Tay Fault. (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and (Figure 14) also show the trace of the D₂ Beinn a' Ghlo Antiform projected from the Pitlochry district as well as the anticipated depth of the Boundary Slide.

Summary of structural history east of the Loch Tay Fault

Three major D₁ fold-pairs have been identified, the Creag na h- Iolaire Anticline and the Sron Mhor Syncline in the centre of the area and the Meall Urair and Conbhar fold-pairs north of Loch Tummel. An attempt has been made to restore the attitude of these folds pre-D₂ in (Figure 15)c (see discussion at end of this chapter), but all that may reliably be said is that post-D₂ they would have been, like the D₂ folds themselves, NW-verging and facing down to the south-east at a low angle (Figure 15b); as discussed above there is evidence of quite significant curvature of the fold hinges. Four major D₂ folds are identified, which pre-D₃ are predicted to have been recumbent, NW-verging within an overall inverted succession (Figure 15b), part of the regional Tay Nappe; a regional curvature pattern of the D₂ minor structures is apparent, but this is not mirrored by the major fold hinges, which have generally a south-west plunge. The present disposition of the formations is broadly controlled by the five major, upright, NE-trending, SW-plunging D₃ folds. The latest phase is D_c which, while it may have corrugated steep-dipping formations, has not significantly affected the outcrop pattern. Major faulting also does not contribute significantly to the structure, but minor faulting and fracturing is discussed under that general heading for the whole district, later in this chapter.

Appin, Argyll and Southern Highland Groups, west of Loch Tay Fault

As outlined above in Chapter 1 (History of Research), the major folds of this area were essentially identified in the work of Anderson (1923), Bailey and McCallien (1937) and Rast (1958a). Their axial-traces were further elaborated by Nell (1984; 1986) and Treagus (1987), who also assigned them to a sequence of deformation phases, based on observations of congruency of minor structures. This sequence, understandably, differs from that implied by Bailey and McCallien (1937), who did not use minor structure information. It also differs, however, from that of Rast (1958a), who was one of the pioneers in the use of minor structures in the Dalradian. This difference arises from the fact that Rast (1958a) attributed all the major tight-to-isoclinal closures to his first deformation phase, whereas it is shown below that they may be assigned to both a D₁ and a D₂ phase, and locally to a D₃. Although Rast (1958a) recognised a second set of minor structures, he did not relate these geometrically to these major isoclinal structures, but rather associated them with the more open Bohespic/Errochty fold-pair. It is shown below that these latter folds deform both the minor and major D₁ and D₂ structures.

The rationale for the deformation sequence used here has been given in the introduction to this chapter. The sections below concentrate on the detailed geometry of the minor and major structures and localities where they may be examined. The structures are described under the headings of the D_c, D₃, D_e, and D_L phases and the D₂ and D₁ phases treated in reverse order. The last two of these phases are dealt with together in a number of sub-areas, from the base of the Appin Group upwards, in order to give continuity with the description of the Grampian Group structure above. Axial-traces of the major folds are shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9); representative minor structures of the D₂ and D₃ phases respectively are shown on (Figure 10) and (Figure 11). Cross-sections of the district, in addition to that below the published sheet, are given in (Figure 17) and (Figure 18).

Creag an Chanaich phase (D_c)

The effects of this phase are limited to the area of Killiecrankie Schist in the Dun Coillich area (Figure 2) immediately west of the Loch Tay Fault. Here, the steeply dipping, thinly bedded semipelites and pelites are locally affected by a low-dipping crenulation cleavage which cross-cuts D₃ minor folds. The minor folds produced are variable in attitude but generally have a low plunge to the east and west. These structures are well seen on west-facing joint surfaces around [NN 7685 5346]. No major folds are recognised. Other structures possibly of this age are discussed under the D_e structures.

Third phase (D₃)

The most significant folds of this phase are in the south of the district, notably the Ben Lawers Synform, and the Culdaremore Antiform and An Stuc Synform (Figure 7b), (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 17) which can be traced on the south side of Glen Lyon. A number of unnamed folds, shown further north, can be traced continuously northwards from the Loch Tay Fault as far as [NN 745 500] south of the Allt Coire Pheiginn, to the west boundary of the district on the east slopes of Carn Gorm [NN 640 500]. (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) and the sections, (Figure 17) and (Figure 18), illustrate how these folds clearly refold D₁ and D₂ axial-traces. In terms of the cross-sectional effect of these folds on the D₂ axial-traces, the Ben Lawers Synform is the dominant structure, with a wavelength of some 3 km, although it is relatively open (interlimb angle of 100°); the other folds, of some 1–1.5 km wavelength, are tighter (interlimb angle of 50°) but essentially are parasitic, corrugating the northern limb of the Ben Lawers Synform, as it rises gently to the north.

Farther north, an antiform/synform pair is less well defined in the Killiecrankie Schist, with accompanying minor structures, from the Loch Tay Fault as far west as grid line 70; in the extreme north-east, between the Loch Tay and Allt Mor faults, the Loch Kinardochy fold-pair is the most northerly manifestation of this phase; these folds refold the D₂ Sgurran Geal Antiform but minor structures are weakly developed.

All D₃ folds, major and minor, trend ENE–WSW, plunging gently in both directions. Minor folds, generally tight in style, with a related pervasive crenulation lineation, are well developed in the Ben Lawers and Ben Eagach Schists with a well-marked axial-planar crenulation cleavage. The plunge of the minor structures (including the S₀/S₃ intersection lineation) is shown in (Figure 11); generally this is to the east in western Glen Lyon and to the west in eastern Glen Lyon, compare (Figure 7c) and (Figure 7b). Axial planes and cleavages fan from vertical in the upright Ben Lawers Synform through to 70–60°S north of Glen Lyon, but return to steep southerly dips in the Killiecrankie Schist. The vergence changes of the D₃ minor folds and of S₀/S₃ (Figure 11) are particularly clear and important in establishing the position and geometry of the major folds.

These minor structures can be observed in any rock exposure in Glen Lyon but the following are the more instructive and accessible. A traverse of the well-developed minor structures just to the north of the hinge of the Ben Lawers Synform can be made across the crags east of Lochan Creag a' Mhadaidh [NN 711 459]; the hinge area is seen around the two burns draining towards Boreland about [NN 714 453]. The Culdaremore Antiform closure, folding the D₂ repetition of the Ben Eagach Schist/Ben Lawers Schist boundary, is well seen above Culdaremore [NN 722 471] to [NN 715 467], while minor folds related to the An Stuc Synform are exposed in spectacular sections of the Ben Lawers Schist in the River Lyon downstream of [NN 722 477] (Figure 7b). Refolding of D₂ minor structures is seen in all these sections but D₂/D₃ fold interference patterns are best seen in the Killiecrankie Schist of Dun Coillich, e.g. above the Allt Mor path [NN 762 525] and on the east side [NN 767 536]. Superposition of D₃ minor folds on D₁ and D₂ minor structures is seen in many schist exposures in eastern Glen Lyon, e.g. on Creag Dhearg [NN 695 486] and Creag Mhor [NN 714 485], and the effect of large-scale D₁ folds on D₃ major and minor structures is described below, on the south side of Carn Mairg [NN 688 505].

Errochty phase (D_e)

The major folds are the Bohespic Antiform and Errochty Synform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). These folds are particularly clear from the outcrop pattern of the formation boundaries north of Glen Lyon; the folds are shown in profile in (Figure 12)a. The antiform defined by the base of the Appin Group is a broad open fold plunging 56° towards 174° with no single axial surface, but an approximate limb- bisector plane dips 66° towards 122°. At the level of the D₂ axial-traces within the Killiecrankie Schist, south and south-east of Schiehallion, it becomes more of a box-shaped fold plunging 55° towards 142°. Minor structures which appear to be associated with the major fold are seen in the Blair Atholl Subgroup semipelites, especially in the Cnoc an Fhithich Banded Semipelite in a zone trending WSW on its western limb and towards the hinge zone, e.g. exposures near Lochan an Daim [NN 711 572] and the north-east ridge of Geal Charn, above [NN 693 566]. The minor folds, which are developed only in thinly bedded quartzites, pelites and semipelites, verge east and plunge 30°–50° SSE, sympathetic to the major antiform, and have an east-dipping widely spaced, axial-planar, crenulation cleavage.

The Errochty Synform, at the Grampian/Appin Group boundary, is a tight fold (interlimb angle 50°) plunging 25° towards 176° with an axial surface dipping 55° towards 015° (Figure 6). The fold remains tight at the level of the folded traces of the D₁ and D₂ folds on Craig Varr, where the steeply east-dipping Ben Eagach Schist/Killiecrankie Schist on the east limb [NN 669 588] can be contrasted with the shallow east-dipping Ben Eagach Schist/Carn Mairg Quartzite on the west limb, which is well seen in the burn above Kinloch Rannoch [NN 6627 5905]. South, on Creag an Fhithich [NN 668 572], the computed plunge has reduced to 11° towards 170°. The syncline can still be defined from the contrasting shallow and steep easterly dips in Glen Sassunn, on its west [NN 653 540] and east [NN 663 546] limbs respectively.

At the boundary of the Carn Mairg Quartzite and Ben Eagach Schist, running from Meall Garbh [NN 650 517] to Meall Crumaich [NN 740 500] north of Glen Lyon, the major D_e folds have a negligible effect.

Minor structures probably related to the Errochty Synform are seen on its east limb on Craig Varr [NN 670 590, and 676 592] and northwards, in the thin schistose Carn Mairg Quartzite on the two limbs of the D₂ Craig Varr Antiform. At least two sets of minor folds are developed in the Ben Eagach Schist core of this fold, which itself is folded by a late N–S-trending monoform. The monoform, with a 300 m-wide short limb dipping gently east, is evident from the outcrop pattern of the mantle of thin Carn Mairg Quartzite and Killiecrankie Schist. N–S-trending, west-verging minor folds with upright axial surfaces are geometrically sympathetic to the larger monoform. Other minor folds that postdate S₂ are east-verging and have gentle east-dipping axial surfaces in the flat limb of the monoform. Both sets of minor folds are associated with spaced crenulation cleavages and, since they are not seen to interfere, they might be interpreted as conjugate sets of D_e structures. However, the folds with low-dipping axial surfaces may belong to another locally developed phase or be representative of the D_c phase seen east of the Loch Tay Fault.

There is no evidence from outcrop patterns or minor structures for the age relationship of between the D_e and D₃ phases or between D_e and the D_L phase described below. Neither set of minor structures related to the D₃ and D_L phases is developed close enough to the outcrop pattern deflections associated with the Bohespic or Errochty folds to be unambiguously dated in relation to them. On the balance of evidence, the D₃ minor structures in the Killiecrankie Schist south of Schiehallion are not affected by, and thus postdate, the swing in strike resulting from the Bohespic Antiform. It will be argued below that the D_L minor structures of the Glen Lyon and Allt Mor areas may be related to the D_e major structures discussed above.

Lyon phase (D_L)

This phase is associated with minor folds and a strongly developed axial-planar crenulation cleavage, particularly in the schists of eastern Glen Lyon and in the Killiecrankie Schist south and south-east of Schiehallion. No major folds are developed. In Glen Lyon, this phase is responsible for considerable deflections in strike of lithostratigraphical boundaries in the area between Carn Mairg and central Glen Lyon (Figure 1 and north end of the cross-section of (Figure 17)), with the Ben Lawers and Ben Eagach schists infolded into the Carn Mairg Quartzite, the effects of which can be seen north of the Allt Coir' Chearcaill [NN 680 502]; [NN 688 505]. S₃ cuts across these cores and mutual interference between the D₃ and D_L sets of folds causes considerable variation in the plunge of both. Regionally, the axial traces of these D_L folds, which trend north to NNE and verge west, can be followed across the major D₁ Chesthill Syncline and affect the upper limb of the D₂ Ruskich Antiform (Figure 7c).

On a minor scale, in the Ben Lawers and Ben Eagach Schists of eastern Glen Lyon, e.g. on Creag Dhearg [NN 695 486] and Creag Mhor [NN 714 485], a locally intense crenulation cleavage has the same north to NNE trend and dips steeply east, axial-planar to the west-verging folds. Tight to open minor folds and a strong crenulation lineation plunge generally east or south-east, but considerable variation occurs owing to the influence of strong D₃ folding of these D_L minor structures. In the Killiecrankie Schist outcrops south of the Allt Mor e.g. [NN 752 522] the D_L cleavage and minor folds are folded by D₃ folds to give flatter dips and more reclined attitudes than normally associated with these folds.

North of the Allt Mor, structures possibly of this age are developed locally. These are seen, firstly, in the Blair Atholl Limestones of the SSW-striking limb of the Allt Mor Synform [ridge running north from 759 542]) and the E–W-striking limbs of the D₂ folds in Strath Fionan [NN 7245 5633] and ridge east of [NN 739 560]. Here a strong calcite/mica fabric, crossing the D₂ folds in an anticlockwise sense giving a SSE- to SE-plunging intersection lineation, is axial-planar to rare minor folds with low-dipping axial surfaces which verge west or south-west; the adjacent pelites are only locally affected by this phase, with a spaced crenulation cleavage imprinted on S₂. However, structures with a similar geometry developed in the Beoil Quartzite enclosed in the very micaceous Beoil Schist in their curving outcrop between Lochan an Daim [NN 722 571] and Lochan Beoil Chataiche [NN 750 567] in Strath Fionan, are interpreted as D_c structures; minor folds plunging moderately to steeply south-east are associated with a low-dipping crenulation cleavage and verge west; they clearly fold D₂ minor structures (Plate 4b).

Effect of D₃ and D_e folds and high strains on D₂ minor structures

The D₃ major folds of Glen Lyon, as discussed above, have a clear superimposed relationship on the axial-traces of the D₁ and D₂ major folds. The plunge of the minor D₂ folds and S₀/S₂ intersections is clearly dispersed about the D₃ major folds but, as discussed for the area east of the Loch Tay Fault, there is an original curvature of the D₂ axes from a north–south attitude south of the district, on the south limb of the Ben Lawers Synform, to a progressively more ENE-trend to the north of Glen Lyon (compare with (Figure 15)b).

To the north of the Schiehallion area the effect of D₃ diminishes but the effect of the refolding of D₁ and D₂ major stuctures at a high angle by the major D_e folds is profound. As shown by Treagus (1987), the removal of the effect of the major Bohespic/Errochty fold-pair reveals that the minor and major D₂ axial-trend curved, as seen on original pre-De horizontal surfaces, from an E–W trend in the west (Loch Rannoch to Loch Errochty) to SE–NW in the east (Strath Fionan area). The complexity of the D₂ fold-trend is heightened by the curvature of the hinges and intersection lineations in areas of high strain. The effect of this where plunges are normally low, for instance, is to steepen them; this is seen in the Loch Rannoch area as a result of high strains adjacent to the Boundary Slide. In the Strath Fionan area, where plunges are steep to the south-east, the effect of high strains on the attenuated limbs of D₂ major folds is to rotate folds through a down-dip position to produce plunges to the south-west. These effects may cause difficulty in the interpretation of vergence.

First and second deformations (D₁ and D₂)

The effects of the D₂ deformation dominate the pelitic and semipelitic rocks of the whole district, with a pervasive schistosity and widely developed minor folds. The plunge and vergence of minor structures (Figure 10) are of particular importance in establishing the age, geometry and position of the major D₂ folds. Thus the obvious repetitions of the stratigraphy in this area have thus been identified to be the consequence of four major D₁ folds (from the superposition of the D₂ minor structures) and nine significant D₂ major folds (from the congruence of the minor structures); tight major folds in Glen Lyon have similarly been identified as D₃ in age from their deformation of these D₂ minor structures.

D₁ minor folds are rare except in certain thin-bedded formations such as the Beoil Quartzite (Plate 4b) and Blair Atholl limestones of Strath Fionan, and the Farragon Volcanic Formation rocks of Glen Lyon, where they are isoclinal (Plate 5). Intermediate-scale isoclinal D₁ folds are seen in those same areas as well as in the massive Schiehallion Quartzite north-east of Schiehallion. The first cleavage is generally parallel to bedding except rarely in massive semipelitic lithologies such as the last-mentioned locality.

Axial-traces of major folds of the two phases are shown in (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), while minor D₂ structure (folds and S₀/S₂ intersections) are shown in (Figure 10). Cross-sections of the structure, as well as that below the 1:50 000 Series map, are shown in (Figure 17) and (Figure 18).

After an initial discussion of the Boundary Slide Zone, the major structures of these two phases are treated from the base of the Appin Group upwards in four sub-areas. The sub-areas are (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9):

1. D₂ Balliemore Fold, and the D₁ Beinn a' Chuallaich Anticline and Syncline in the Appin Group and the Islay Subgroup, around the Bohespic Antiform.
2. D₂ Creag an Earra, Craig Varr, Allt Mor and Sgurran Geal folds in the same Appin and Argyll Subgroups around the Bohespic Antiform at a higher structural level.
3. D₂ and D₁ major folds principally in the Easdale Subgroup in the west of the district and in the core of the D_e Errochty Synform.
4. D₂ Ruskich Antiform, and the D₁ Chesthill Syncline and Meall Garbh Anticline in the Easdale and Crinan subgroups in the south of the district.

Boundary Slide Zone

As recognised by Bailey and McCallien (1937), a zone, approximately co-incident with the boundary between the Grampian and Appin groups, is the locus throughout the region for exceptionally high D₂ strains. This high strain is manifest generally from the extreme attenuation of formations, and the intense development of the S₂ schistosity (as well as the tightening of the D₁ and D₂ folds) giving rise to a zone some 500–2000 m wide of exceptionally platy, schistose rocks. In view of the fact that high strains and excision of formations is evident during, D₁ elsewhere in the district (see below), it is very likely that the high viscosity contrast at the Grampian/Appin group boundary would also have been the locus for D₁ movements. In the west of the district, from the south shore of Loch Rannoch to north shore of Loch Errochty [NN 709 660], and in the east along the Allt Kynachan [NN 775 570], formations are excised within the zone and the Boundary Slide is identified on the 1:50 000 map. Specific localities, where features of the zone may be examined, are treated from west to east.

At the western edge of the district, the Allt Druidhe [NN 6423 5693] and the north shore of Loch Rannoch [NN 6495 5887] offer sections from the psammites of the Grampian Group across very attenuated formations of the Appin Group, from the Beoil Schist possibly to the Strath Fionan Banded Semipelite. There is then an hiatus of the stratigraphy before banded semipelites with amphibolites, possibly representing the Killiecrankie Schist, and thin quartzites representing strongly deformed Carn Mairg Quartzite, are seen; no single plane of movement nor movement indicators have been identified. Poor sections can be seen along the zone as far as Loch Errochty which suggest a similar hiatus, although the outcrop of the slide is usually obscured by the post-tectonic microdiorite sills.

It has been chosen not to draw a slide plane between Loch Errochty and the sections in Strath Fionan, discussed below. Discrete planes of movement may still be present, within or at the margins of formations, and many formations are reduced to a metre or two in thickness but there is no major stratigraphical hiatus (Figure 5). To the north of Loch Errochty some poor sections, e.g. north of the track at [NN 712 663], show that some limestones and graphitic schists of the Blair Atholl Formations are beginning to be represented, and the next good sections, close to the road near Maud Loch [NN 725 658] and in the Errochty Water [NN 726 647], show that only the Strath Fionan Banded Semipelite and Pale Limestone are missing or extremely attenuated, and formations in the Lochaber and Ballachulish Subgroups are only represented by a few metres of rock; semipelitic and pink quartzite beds are present, which could be the very attenuated representatives of the Dunalastair Semipelite and Quartzite. However, the Grampian Group rocks that abut against the Appin Group are poorly known from Trinafour to Loch Rannoch (but are probably the Tummel Psammite) and it is possible that a slide exists between them and the Appin Group rocks, although facies variations cannot be discounted.

The next continuous sections south of Meall na Moine are in a burn at [NN 7100 6305] and in the Allt na Moine Buidhe upstream of [NN 7095 6120] where complete, but still very abbreviated, stratigraphical sections appear to be present.

There are no continuous sections from the last locality until the slopes north-east of Lochan an Daim [NN 720 577] in Strath Fionan, which as far as [NN 750 565] gives sections of the complete succession (Figure 4; Chapter 2). Sedimentary structures and details of bedding can be seen here in psammites, where none can be detected to the north-west, although pelites such as the Beoil Schist appear, from the intensity of the S₂ schistosity, to have suffered high strain.

To the east of Strath Fionan, severe attenuation and eventual excision of formations in the Appin Group become apparent. There are good, complete, though not always accessible, sections in the Allt Kynachan [NN 757 568] to [NN 7780 5723]; the main excision here, as on the west side of the district, is of some of the lowermost Lochaber Subgroup formations, especially the Dunalastair Quartzite and Semipelite, although locally other formations are missing.

The best sections of the slide zone are in Strath Fionan [NN 719 580] to [NN 750 567]. Apart from a weak biotite lineation pitching steeply south-east, there is no evidence of a strong extensional strain in the pelites. Parallel to this direction garnets, particularly within and adjacent to thin amphibolites, show a weak asymmetry of quartz pressure shadows, top to the north. The muscovite-rich Beoil Schist is distinctive in its 4–5 mm long sigmoidal quartz segregations and thin quartz veins (which are parallel to S₁ and folded by D₂ folds); the tails on the segregations, if they are taken to be analagous to deforming rotating porphyroclasts, also show top-to-the-north shear in D₂. On the other hand, where clasts are present in psammites (especially in the Meall Dubh Quartzite, see Chapter 2) the felspars are highly elongate L-tectonites (typical X:Y:Z ratios are 4.4:1:0.25). These are particularly well seen in the quartzites above the slide in the Allt Kynachan [NN 7780 5723]. Sheath folds, indicative of a high D₂ strain and representing plunge curvature of over 90°, are well developed in the Beoil Quartzite north and north-west of Lochan an Daim [NN 7209 5755].

D₂ Balliemore Antiform and D₁ Beinn a' Chuallaich Anticline/Syncline pair

The Balliemore Antiform is perhaps the most instructive fold in the district. The axial-trace is clear from the repetition of the Schiehallion Quartzite/Killiecrankie Schist boundary between Loch Errochty [NN 695 645] and Dunalastair Water [NN 695 587] and then, as the fold is refolded about the Bohespic Antiform, to Strath Fionan [NN 725 563] from the repetition of Blair Atholl Limestones about the Schiehallion Boulder Bed. Over this area, the convergence of the boundaries makes it clear that this is a S- to SE-plunging antiform of the inverted succession. In addition, the sympathetic D₂ fold- and cleavage-vergence data (Figure 10) make it abundantly clear that this is a major D₂ fold. Axial plunge from Glen Errochty to the Tummel is gently south, but around the Bohespic Antiform increases to 20–30° south and east and thence to 50–60° towards the Loch Tay Fault. As explained above, this pattern although largely attributable to the folding by the Bohespic Antiform, represents an original SSE–NNW D₂ trend (on a subhorizontal plane).

Minor structures are well seen on the upper limb of the fold on the east flank of Beinn a' Chuallaich, but less easily on the lower limb, as a result of poor exposure and high strains. The Allt na Moine Buidhe [NN 710 612] to [NN 699 611] gives a section across the lower limb, while the crags above Auchtibart [NN 693 590] provide an excellent view of the hinge zone; D₂ minor structures on the upper limb, cutting across the D₁ stratigraphical repetition of the Beinn a' Chuallaich Anticline, are well seen in a traverse north-westwards from the last locality to [NN 688 595]. In the highly strained schists around the thinned Schiehallion Quartzite on this limb, D₂ folds show exceptional curvature of between 60° and 70°. In the Killiecrankie Schist further west, steeply plunging boudins of syn- to post-D₂ age are developed.

The antiform is particularly clear from the outcrop pattern of the semipelites of the Appin Group in the Strath Fionan area, shown in detail in the western half of (Figure 4), in which minor structural features are well developed. Some of the structural characteristics of these formations have already been described in Chapter 2. The traverse A–A′ of (Figure 4) [NN 723 577] to [NN 721 577] provides good examples of cleavage- and minor fold-vergence to the south-west, on a moderate south-east plunge, in Beoil Schist and Dunalastair Semipelite. The repetition of the Dunalastair Quartzite is crossed by this vergence and thus must represent a D₁ fold-pair which, if it is facing up, must plunge to the west. In addition to well-developed D_c and D₂ minor folds, D₁ minor folds may also be seen in the thin Beoil Quartzite beds, along strike from here; these are crossed by S₂ and in rare instances are observed to be refolded by minor D₂ folds (Plate 4b). D₁ minor folds have locally strongly curving hinges but otherwise plunge west and verge north.

Intermediate-scale fold-pairs of D₁ and D₂ age can be seen on the lower limb. The former is evident from the doubling of the Blair Atholl Formation limestones around Lochan an Daim [NN 717 574] and the return hinge at Tullochroisk [NN 712 584]. Isoclinal folds are well displayed on the roadside knoll [NN 7166 5736], refolded by tight D₂ folds, and S₂ cleavage consistently crosses the closures in the adjacent pelites, particularly east of Tullochroisk. The closures of the fold-pair must plunge steeply north. An intermediate-scale D₂ fold begins to be evident at this locality and increases in amplitude towards Meall Dubh, where it is evidently an easterly plunging antiform from the folded quartzite/graphitic schist junction [NN 732 567]; sub-penetrative S₂ axial-planar cleavage is strongly developed in the transitional facies here. To the east, the Ballachuliish Subgroup boundaries are tightly folded in the hinge zones of the D₂ pair; D₂ minor structures plunge 40°–60°E.

A section which provides an opportunity to observe vergence changes across the major trace is from the roadside quarry at [NN 7263 5649] to the SSW in exposures in and around the two burns which descend the lower slopes of Schiehallion. Good south-verging relationships on 30°–34°SE plunges are seen in the roadside exposures of the Strath Fionan Banded Semipelite and in the graphitic schists upstream, e.g. at burn junction [NN 7252 5641]. In the graphitic schists, between the ridges of the repeated limestone which mark the D₁ Beinn a' Chuallaich Anticline core, north-verging fold and cleavage vergence are well seen e.g. [NN 7236 5608]. They are also found in the Cnoc an Fhithich Banded Semipelite above e.g. waterfall at [NN 7232 5586], and below, in the westerly burn [NN 7227 5618]; S₀/S₁ intersections are noticeably steeper (over 60° SE) on this upper limb. Both D₁ and D₂ minor folds are developed in the grey limestones in these exposures (Plate 1b); they are not easily distinguished except in instances, where they interfere [as seen at 7218 5612]. The persistent anticlockwise late calcite fabric, attributed above to D_L, transects these folds.

To the east of Strath Fionan towards the Loch Tay Fault, the antiform becomes extremely tight and its trace less easy to identify between the poorly exposed ridges of Blair Atholl Formation limestone. D₂ vergence relationships become difficult to establish as the angle between S₀ and S₂ narrows. Plunges steepen progressively to 50°–56° in this direction (Figure 10) and locally are down-dip. The fold can be substantiated west and east of Braes of Foss by comparing vergence at localities on the lower limb with that on the upper, e.g. a locality [NN 7456 5621] below the road with one [NN 7461 5592] in graphitic schist, and on either side of the track at [NN 7680 5671] in semipelites in and near the Allt Kynachan.

The presumed D₁ age of the Beinn a' Chuallaich Anticline/Syncline pair is dependant of the observation that D₂ vergence is consistent across their traces. Such observations have already been documented above for the anticline in the Strath Fionan area, where merging of the two ridges of Blair Atholl limestone on its two limbs south of Cnoc an Fhithich [NN 710 563] first makes the fold apparent in the east of the district. The D₁ age can be similarly verified to the north of the Dunalastair Water, in the Blair Atholl Formation within the core defined by the repetition of the Schiehallion Quartzite and its eventual closure on Beinn a' Chuallaich [NN 682 622], which is consistently crossed by the S₂ cleavage, as described above. Localities where younging can also be seen are on the north-east ridge of Geal Charn [NN 691 553] and on Meall nan Eun [NN 710 560] on the west and east limbs respectively. The closure in the Schiehallion Quartzite on Beinn a' Chuallaich is dominated by D₂ folds and rodding which were mistakenly interpreted by Bailey and McCallien (1937, p.137) as indicative of the plunge of the major fold. Minor D₁ folds or intersection lineations are in fact rarely seen and in view of the high D₁ and D₂ strains will not be representative of the plunge of the major structures.

The trace of the Beinn a' Chuallaich Syncline is clear on the map from the repetition of the Schiehallion Quartzite about the core of Killiecrankie Schist that runs from the south side of Schiehallion [NN 728 537] to the west flanks of Beinn a' Chuallaich [NN 680 630]. This boundary has particularly well-developed D₂ minor structures in its transitional facies and these can be seen consistently crossing the two limbs of the D₁ core at locations south and north of Strathtummel, e.g. above West Tempar [NN 687 565]; [NN 690 548] and above Drumchastle Farm [NN 6801 5933]; [NN 6845 5938]. On the south side of Schiehallion, the upper limb is greatly attenuated where the Blair Atholl Formation graphitic schist and limestone reappear; the Schiehallion Quartzite and intervening formations are reduced eastwards to a few tens of metres thickness and complete excision of certain formations cannot be excluded. Some of this strain is certainly attributable to D₂; curvatures of up to 70° on D₂ hinges can be established e.g. at [NN 720 534]. This is the Schiehallion Slide of Bailey and McCallien (1937) and its significance is discussed further below.

In the massive schistose Schiehallion Quartzite of Meall nan Aighean [NN 729 555] eastwards to the southern slopes of Cnoc nan Aighean, there are folds of tens of metres in wavelength which have, in their semipelitic beds, a penetrative axial-planar cleavage. These folds, of variable plunge to the east and to the west, are not sympathetic in vergence to those of D₂ age in adjacent formations, but have the southerly vergence of D₁ folds compatible with the downward-facing Beinn a' Chuallaich Antcline to the north.

On the east side of Schiehallion, the syncline trace is displaced by the Braes of Foss Fault Complex at about [NN 746 540] from the Schiehallion Quartzite into the Blair Atholl Formation. Thereafter, its course within the limestones and graphitic schists is poorly constrained, but its trace must be close to its companion anticline near Tombreck [NN 776 571] where it is folded about the Balliemore Antiform.

The attitude of the major D₁ axes is only constrained by the fact that no interference patterns are seen in the area affected by the Balliemore Antiform and Beinn a' Chuallaich folds and that the only D₁ minor folds seen (on Beinn a' Chuallaich) plunge steeper to the south than those of the D₂ phase. This subparallel (but steeper) south-east plunge in the vicinity of Schiehallion is further supported by the regional supposition that the D₁ fold-pair faces down to the south on the upper limb of the Balliemore Antiform (Figure 18).

The traces of the D₁ fold-pair to the east of Braes of Foss are very speculative, based on very limited outcrop information and the supposition that the D₁ pair should be folded by the D₂ Balliemore Antiform. The repetition of the Tullochroisk Formation and one grey limestone in the burn west of Tombreck [NN 7680 5671] on the lower limb of the Balliemore Antiform suggests that a pair of D₁ closures, crossed by S₂, may be present between here and the Allt Kynachan. Logically, if the plunge of the D₁ Beinn a' Chuallaich fold-pair is steeper to the east than that of D₂ axes on the upper limb of the Balliemore Antiform, the D₁ plunge would be shallow to the north- east on the lower limb. The interference outcrop pattern of formations in the lowest part of the Blair Atholl subgroup and the upper part of the Grampian Group might be that shown on the published map in the kilometre square [NN 75 56]; the fold-traces could cross from the Appin Group, into the Grampian Group in an area of no exposure around [NN 765 567]. Further folding of these D₁ hinges about the Bohespic Antiform could create a shallow north-west plunge to the west, and thus might be expected to re-enter the Appin Group; the D₁ fold-pair might be the same as that which affects the Dunalastair Quartzite north-east of Lochan an Daim [NN 72 57] (Figure 4).

D₂ Creag An Earra, Craig Varr, Allt Mor and Sgurran Geal folds

No distinction could be made in this area generally between the D₂ hinge and S₀/S₂ intersection direction and the various mineral lineations, in association with these folds. A quartz/feldspar rodding is present in most exposures of the Schiehallion Quartzite which is clearly, in part, a feldspar pebble stretching; amphibolites, especially well seen in the Killiecrankie Schist, exhibit a strong hornblende lineation; well-developed quartz/ carbonate pressure shadows to large garnets are quite common. In general north of the River Tummel, D₂ lineations are subhorizontal or plunge shallowly to the north; to the south the plunge is shallow to the south, steepening increasingly to the south-east as it is folded about the Bohespic Antiform. Plunge variations occur in association with local high strains on the fold limbs; these variations are particulaly strong on the two limbs of the Craig Varr Fold.

The D₂ Creag an Earra Fold can be traced from the repetition of Killiecrankie Schist about a core of Schiehallion Quartzite from about 2 km south of Loch Errochty [NN 677 630], to the south side of Schiehallion [NN 700 540], where the Blair Atholl Formation occupies the core up to the Loch Tay Fault on the south side of Loch Tummel. The trace and D₂ age is first well defined to the north-east of Craig Varr where D₂ minor folds and S₂ vergence can be seen to be sympathetic to the major fold at [NN 6752 6073] and [NN 6766 5927] on the Schiehallion Quartzite/ Killiecrankie Schist boundary of the east and west limbs, respectively. D₂ axes and lineations plunge at low angles to the north and south. Similar vergence relations can be seen south of the River Tummel at [NN 6820 5665] and [NN 6775 5671]. At both locations on the west limb, the Killiecrankie Schist is reduced to a few metres thickness; as the reduction is crossed by D₂ minor structures, it is attributed largely to D₁. These D₂ minor structures now plunge at low angles to the south, increasing (to the south-east) towards Creag an Earra.

To the north-east of Creag an Earra [NN 700 540], the core of Blair Atholl Formation within the Schiehallion Quartzite is mantled by extremely attenuated representatives of the intervening formations, again with sympathetic D₂ fold and cleavage vergence on the two limbs. To the east in Gleann Mor [NN 723 532]–[NN 745 538], the Schiehallion Quartzite returns in an inlier in the core resulting from the refolding of a pair of intermediate D₁ folds. D₂ lineations and axes plunge consistently to the east. The Schiehallion Quartzite on the upper limb is again reduced to a few metres in thickness.

Across the Braes of Foss Fault, the trace cannot be defined precisely in the Blair Atholl Formation. Here the D₂ minor structures plunge steeply to the south or south-east on the NE-striking S₀/S₁ planes, in places going through a down-dip pitch. East of Braes of Foss, vergence change can be confirmed by comparison of the 's'-vergence in the graphitic schists around [NN 758 560] with the 'z'-vergence in the Killiecrankie Schist at [NN 7650 5612]. Consistent vergence on the lower limb is not easily established but good vergence on the upper limb can be seen on either side of the very attenuated Schiehallion Quartzite at [NN 7650 5614] and by the roadside [NN 7833 5714].

The Craig Varr Fold is again well defined by the repetition of the Carn Mairg Quartzite (although locally extremely thinned) around a core of Ben Eagach Schist and amphibolite, north of Craig Varr [NN 672 600] to [NN 669 594], north of the River Tummel; in the east it is defined by the repetition of Schiehallion Quartzite about a core of Killiecrankie Schist from the Allt Mor [NN 740 530] to the Loch Tay Fault. In the Craig Varr area vergence changes, of both the gentle N-plunging D₂ folds and of the cleavage/bedding relations, are seen particularly well in the Ben Eagach Schist and Killiecrankie Schist on the two sides of the very attenuated Carn Mairg Quartzite [NN 672 590], and to the south on Creag Mhor [NN 676 568] where plunges change to gentle to the south. The closure of the boundaries support the culmination of the major hinge plunging to the north and south across the Tummel Valley.

The trace of the fold within the Killiecrankie Schist to the south-west of Schiehallion is demonstrable from vergence reversals, e.g. on either side of the track south-west of Geal Charn [NN 677 540] and on Meallanan Odhar [NN 680 532] (Figure 7a). In the latter area reversals of facing on S₂ demonstrate the presence of intermediate scale D₁ folds. Reversals of vergence on the Killiecrankie Schist/Schiehallion Quartzite boundary north-east of the Allt Mor, on the lower limb (detailed above) can be compared with those on the upper limb, at Tom Beithe [NN 7817 5686] (roadside and burn to west), and south-east of the Braes of Foss [NN 759 556], on the forest track. The plunge of the D₂ minor structures varies, as for the Creag an Earra and Allt Mor folds, from gentle south to steep to the east, as the strike is followed from south of Strathtummel to the Allt Mor; from here north-eastwards the minor structures are steep either side of a down-dip pitch direction.

The Allt Mor Synform is well defined from the outcrop pattern of the core of Killiecrankie Schist within the Ben Eagach Schist on Craig Varr [NN 670 590] where vergence changes are well seen. The west limb is highly attenuated, so that the Carn Mairg Quartzite is commonly reduced to a thickness of less than one metre and on the west side of Craig Varr is demonstrably excised [NN 6693 5897]. This thinned formation is well seen in the Innerhadden Burn [NN 6719 5722] where folding by rootless D₂ folds shows that much of the thinning must be attributed to D₁. Vergence changes are less easily established in the core south of Dalchosnie [NN 673 572], and within the broad outcrop of Killiecrankie Schist south of Schiehallion. The trace can be established again within the Killiecrankie Schist south of the Allt Mor [NN 7320 5245] (although here it is succeeded southwards by a companion antiform) before it runs into the well-constrained core of Schiehallion Quartzite in the Allt Mor [NN 7520 5327] to [NN 7460 5340]. Although the Schiehallion Quartzite/Killiecrankie Schist boundaries are faulted, vergence changes are particularly well displayed in the older formations within the core; minor folds are exceptionally well displayed in the dolomitic beds. To the north-east, the D₂ age of the fold is substantiated by comparison of the vergence relations on the lower limb detailed above, with that in the upper limb in the graphitic schists in the Allt Kinardochy [NN 7795 5656] near the quartzite boundary.

The Sgurran Geal Antiform is only certainly identified in the east of the district, between the Allt Mor Fault [NN 7530 5322] across the Killiecrankie Schist on Dun Coillich [NN 762 536] and into the Schiehallion Quartzite core on Sgurran Geal [NN 774 558]. The trace is ill defined within the Killiecrankie Schist of Dun Coillich because, although D₂ minor folds are exceptionally well displayed on the south and east slopes, e.g. above the Allt Mor path [NN 762 525] and to the east around [NN 767 536], the area critical to the vergence change on the north slopes [NN 763 540] is unexposed. In the south and east, the minor folds plunge moderately east on the ESE-striking beds and verge predominantly north; poor exposures in the north-east near White Bridge [NN 7705 5405] show consistent southward vergence. Across the quartzite core of Sgurran Geal, comparison of vergence can be made between the exposures on the two limbs in graphitic schists at [NN 7795 5656] and [NN 7761 5578] on the lower and upper limbs respectively, close to the quartzite margin. As shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), the continuation of the trace to the south, after it is broken by the Loch Tay Fault [NN 77 54], is considered to be folded by a D₃ synform, where rationally it becomes the equivalent of the Ruskich Antiform, discussed below.

It has been noted above that the Schiehallion Quartzite is greatly reduced in thickness or even excised completely from the limbs of the three D₂ folds described above, which make up the upper limb of the Beinn a' Chuallaich Syncline. The thickening in the D₂ hinge-areas is due to the effect of plunge on the duplicated quartzite. In Chapter 2 it has been estimated that the thickness of the quartzite on Schiehallion and to the north is about 800 m (cover photograph); in these sections, on the lower limb of the Beinn a' Chuallaich Syncline, cross-bedding is common within beds of conglomerate and pelite, as well as the dolomitic beds. The two limbs of the Beinn a' Chuallaich Anticline are both thinned near the hinge-zone on the Beinn a' Chuallaich sections, but the lower limb of this fold then returns to thicknesses of about 800 m in the Allt na Moine Buidhe.

It is suggested, therefore, that the thinning described above can be attributed to original D₁ extension of the upper limb of the D₁ Beinn a' Chuallaich Syncline, in places accompanied by excision or 'sliding'. Although it is difficult to reassemble the D₁ fold in three-dimensions, or to determine the D₁ extension direction, it is certain that the extension of this 'long' limb of the fold-pair was top towards the south or south-east, in the context of the its post-D₂ orientation. The possible significance of this is further discussed at the end of this chapter.

D₂ and D₁ major folds on the west limb and in the core of the D_e Errochty Synform

The interpreted continuation of the axial traces of the D₁ and D₂ folds, discussed above, around the hinge and down the west limb of the Errochty Synform (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), is largely controlled by the stratigraphical repetition of formations (Schiehallion Quartzite to Ben Lawers Schist). The four D₂ folds, the Balliemore Antiform, the Creag an Earra Synform, the Craig Varr Antiform and the Allt Mor Synform can be seen on the slopes north of Loch Rannoch [NN 650 592] to [NN 659 593]. Exposure elsewhere is patchy. The rocks on this limb are strongly deformed in D₂, such that S₂/S₀ angles are extremely close and, as explained above, fold hinge and intersection lineations become very variable and difficult to measure, and consequently vergence changes become increasingly ambiguous.

The trace of the Balliemore Antiform passes down into the Carn Mairg Quarztite on the south shore of Loch Errochty [NN 693 649] and into the Ben Eagach and Ben Lawers Schist which make a remarkably parallel-sided belt for over 14 km into the Loch Rannoch district (Sheet 54E). The minor fold pattern suggests that north of Loch Rannoch the 'antiform' has become synformal with a low plunge to the south, but steepening to its south; due to high strains the minor structures tend to plunge down-dip to the south-east near the Boundary Slide (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9).

The traces of the D₁ Beinn a' Chuallaich fold-pair both enter the outcrop of the Killiecrankie Schist immediately east of the Errochty Synform trace and, when folded about that fold, remain within the long thin outcrop of that formation between Drumcroy Hill [NN 675 632] and Loch Rannoch, where they should pass into the Carn Mairg Quartzite outcrop to the south; there is no evidence that the axial traces cross boundaries here (or the outcrop pattern elsewhere to the east), were it to be assumed they cross the trace of the D₂ Creag na Earra Synform (see below). It is likely that the fold-pair is in effect self-cancelling.

The trace of the D₂ Creag na Earra Synform also passes from the Schiehallion Quartzite east of Carn Fiaclach [NN 670 623] into this Killiecrankie Schist outcrop, its presence confirmed by the changing D₂ vergence data (Figure 10) across the Killiecrankie Schist outcrop west of Kinloch Rannoch [NN 6556 5915]. The outcrop pattern again suggests a low plunge to the north-east at the Schiehallion Quartzite closure; exposure is poor here and minor structure plunge observations are very scarce, but north and south of Kinloch Rannoch D₂ minor structures plunge at very variable, shallow and steep, angles to the south.

The D₂ Craig Varr Antiform passes from the Ben Eagach Schist near the Errochty Synform hinge at Creagan Breac [NN 670 612] into the long parallel-sided outcrop of Carn Mairg Quartzite on its west limb; the Allt Mor Synform similarly must be contained within the parallel-sided outcrop of Killiecrankie Schist which closes on Meall Dearg to the south of Loch Rannoch [NN 652 564]. The two folds are presumed to plunge at low angles to the north-east, but the plunge of the Allt Mor Synform must be steeper than 17°, the slope of the hillside. Minor structure vergence changes confirm the presence of these closures, but plunges are very variable, from low to steep angles to the north-east. D₂ boudinage of competent beds and of amphibolites is a common feature of all these major fold limbs; axes plunge steeply down-dip.

The outcrop of Carn Mairg Quartzite which makes up the upper limb of the D₂ Allt Mor Synform folded in the hinge of the D_e Errochty Synform, is interrupted by two infolded belts of Ben Eagach and Ben Lawers Schists. On the east limb of the Errochty Synform one belt of Ben Eagach Schist runs from one closure in Glen Sassunn [NN 670 550] around the hinge to close again north-west of Kinloch Rannoch [NN 673 588]; the second belt closes in the south on the west flank of Carn Mairg [NN 660 516] but is lost in the amphibolite mass south of Kinloch Rannoch [NN 666 577] in the hinge of the Errochty Synform. These folds (with the necessary complementary closures), the Innerhadden and Creag an Fhithich fold pairs, must be upward-facing D₁ synclines and anticlines; they are crossed by the S₂ cleavage, north of Kinloch Rannoch [NN 660 591] and in the Innerhadden Burn [NN 673 573] to [NN 663 546], and their closure pattern suggesting northward plunges at their southern ends, shows that their hinges must be at a high angle to the adjacent D₂ hinges.

D₂ Ruskich Antiform, D₁ Chesthill Syncline and Meall Garbh Anticline

These folds dominate the structure of the Glen Lyon area south of the Carn Mairg watershed. Here the axial-trace of the tight Ruskich Fold, which is particulary evident from the fold core of Ben Lui Schist just west of the district , crosses a three-fold repetition of the Ben Eagach Schist, which represents the two D₁ isoclinal folds (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9), (Figure 17). D₂ minor and intermediate-scale structures can be seen throughout Glen Lyon, but the following localities concentrate on establishing the age and location of the important Ruskich Antiform and its cross-cutting relations to the D₁ fold-pair to the east.

At the western boundary of the district in Glen Lyon, the Farragon Volcanic Formation closes around the Ben Lui Schist [NN 640 470], in the core of the Ruskich Antiform (Figure 7c). To the south-west, on Creag Roro [NN 633 463], the sheet-dip of this boundary is 030°/40°SE; to the north, the same boundary, above Slatich [NN 632 483], has a sheet-dip of 079°/60°S; the major hinge plunges almost due east at 30°. Minor D₂ structures with opposed vergence can be seen at these two localities. The north limb between the hinge and the last locality is considerably corrugated by both D₂ and D₃ intermediate-scale folds, well exposed on the hillside north-east of Slatich [NN 640 477]. This locality offers the opportunity to examine not only minor D₃ structures superposed on both D_L and D₂ (all plunging east), but also rare refolded isoclinal D₁ minor folds developed in the finely bedded quartzites and amphibolites of the Farragon Volcanic Formation (Plate 5).

The fold-core can be traced east to the tight closure of Ben Lawers Schist in Ben Eagach Schist, south of Inverinain; south vergence can be seen on this boundary in the burn [NN 6541 4737] on the north limb, neutral folds in the Ben Lawers Schist [NN 6620 4752], and north vergence in the Carn Mairg Quartzite [NN 6577 4702] on Creag Dhubh. D₃ interference is evident in all exposures. In the intervening ground eastwards to Balintyre [NN 687 478] the Carn Mairg Quartzite core of the D₁ Meall Garbh Anticline is crossed; the trace can be established from neutral vergence around [NN 681 479] and vergence changes for example between [NN 6842 4870] and [NN 685 471] on the two limbs. On Creag Dhearg [NN 695 485] the trace crosses the belt of Ben Lawers Schist occupied by the D₁ Chesthill Syncline. D₂ relations across the D₂ hinge and on the two limbs are best seen in the Carn Mairg Quartzite, as the schist outcrops are greatly complicated by D_L and D₃ structures; on Meall na Moine [NN 702 480] good northward cleavage-vergence in inverted graded beds gives way to a zone of changing vergence, and around [NN 694 491] to dominantly south vergence, in the Allt Linntich.

The trace of the D₂ Antiform is less precisely defined to the north-east, but the Allt Muilinn (Allt Odhar) provides a good section. At the Ben Eagach/Ben Lawers Schist junction at [NN 7285 4929], D₂ folds are neutral to north-verging, whereas at the eastern end of the tongue of Ben Eagach Schist [NN 7174 4973] a clear southerly sense is seen. D₂ folds and intersections plunge east generally but are dispersed by ubiquitous D₃ folding. Towards the Loch Tay Fault the hinge of the antiform can be loosely identified from changing vergence in exposures by the track at [NN 736 496] contrasting with the southward vergence in the Carn Mairg Quartzite south of Coire Pheiginn [NN 743 506] and the northward vergence in the Allt Odhar near the Loch Tay Fault around [NN 7357 4877].

The trace of the D₁ Chesthill Syncline is evident elsewhere from the core of Ben Lawers Schist with its mantle of Ben Eagach Schist running from the west margin of the district [NN 640 505] in the Allt Lairige, to the mouth of Glen Lyon [NN 726 475]. Covenient localities in the latter area, where the consistent D₂ north vergence on the two D₁ limbs at the Ben Lawers/Ben Eagach junction can be observed, are by the River Lyon [NN 7206 4764], and to its south [NN 7051 4699]; the hinge zone passes through Macgregor's Leap [NN 723 476] (Figure 7b).

The trace of the Meall Garbh Anticline within the Carn Mairg Quartzite, from Meall Garbh at the west margin of the district, passes north of Balintyre around the D₃ An Stuc Synform into the Ben Eagach Schist outcrop towards the south-west. A good section across the pebbly core of the Carn Mairg Quartzite outcrop exhibiting consistent south vergence is seen in the Invervar Burn [NN 6644 4839] to [NN 6634 4860]. The Carn Mairg Quartzite core of the anticline can also be traversed on the north-west slope of Creag Dhubh [NN 660 472] showing consistent north vergence of D₂ minor structures, now on the south limb of the Ruskich fold. A major D₁ anticline, subsidiary to the Meall Garbh Anticline is responsible for the isolated outcrop of Ben Lawers Schist south-west of Caldaremore (Figure 7b).

On (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) the trace of the Ruskich Antiform is shown recrossing the Loch Tay Fault south-west of Glengoulandie [NN 762 522] to be folded about a D₃ antiform/ synform pair in the Killiecrankie Schist, and to link with the Sgurran Geal Antiform near the Allt Mor Fault about [NN 753 532]. The position of this trace is mostly circumstantial, but a major D₂ trace occupied by unusually pebbly psammites can be identified in this position on Na Craigean [NN 753 522].

To the west of the district (Nell, 1984; Treagus, 1987) in Glen Lyon, the D₁ fold-pair can be traced around a D₂ synform/antiform fold-pair above the Boundary Slide (Figure 18). These latter two folds are the equivalents, at a deeper structural level, of the Creag an Earra and Balliemore folds discussed above.

The D₁ Chesthill Syncline and the Meall Garbh Anticline (and their equivalents east of the Loch Tay Fault) must have considerable plunge variations. The folds must have a steep plunge in the north-east where they are cut by the Loch Tay Fault. Immediately to the west of the fault the Ben Eagach Schist closes in the Ben Lawers Schist in the closure of the Meall Garbh Anticline, and yet to the east of the fault, at Glengoulandie, a small area of Ben Eagach Schist is interpreted to be the repetition of this closure. The westerly downthrow of the fault (Figure 18) means that the plunge must be almost parallel to the fault plane. To the east of the fault, in the core of the Creag na h- Iolaire Anticline (the equivalent fold to the Meall Garbh Anticline) the plunge on the same boundary is moderate to the south-west; in the same area, the Sron Mhor Syncline (the equivalent fold to the Chesthill Syncline) has gentle plunges both to the south-west and north-east.

Summary

The D₂ Clunes fold-pair and Meall Reamhar Synform in the Grampian Group are separated by a zone of high strain (the Boundary Slide Zone), with local excision of formations, from the pile of D₁ and D₂ major folds affecting the remaining groups above and to the south east (Figure 18). The isoclinal D₁ folds, the Chesthill Syncline and Meall Garbh Anticline of Glen Lyon and the Beinn a' Chuallaich fold-pair of the Schiehallion area, are folded by four major D₂ folds of decreasing tightness away from the Boundary Slide Zone. In the area north of Schiehallion these early folds were affected by the major, south-plunging, D_e Errochty/Bohespic fold-pair, whereas in Glen Lyon they were affected by ESE-trending upright D₃ folds, including the major Ben Lawers Synform. As has been argued for the D₁ and D₂ major folds east of the Loch Tay Fault, it is probable that these D₁ and D₂ folds were originally (pre-D₃) recumbent and SE-facing, within an overall inverted succession, part of the regional Tay Nappe. Minor D₂ (and probably D₁) folds show considerable curvature when the effects of later deformation is removed, but both D₁ and D₂ major folds would have been north-east trending and north-west verging.

Faults and fractures

One major fracture, the Loch Tay Fault, transects the Schiehallion district and there are a number of less extensive faults which have a significant effect on the geology (Figure 13). These latter are named and briefly described below as well as the geometry and significance of other sets of minor faults and fractures.

Loch Tay Fault

This major fracture zone can be traced across the district with an almost undeviating trend of 030°, from Fearnan [NN 720 444] at the southern margin to [NN 836 641] the north-east. The trace of the fault and observations of component fractures suggest that overall it has a near-vertical dip. Exposures of the zone are dealt from south to north.

A strong topographical feature on the east side of the valley that marks the west end of Drummond Hill is a clear expression of the fault zone. Fault breccia with minor pyrite is seen on 180° joint faces along this feature, from near Easter Achtar [NN 7237 4568] to [NN 7254 4600]. The fault does not make a strong feature north of Glen Lyon, but is next seen in the Allt Odhar (also known as Allt Muillinn and Fortingall Burn) [NN 7383 4857]. Here the fault zone is 20 m of silicified carbonate breccia, the south-east margin of which is marked by a planar surface, strongly quartz-veined and pyritised, with a trend of 033°. The north-west margin of the zone is marked here by 2.5 m of schistose gouge which trends 047°/70°NW in contact with the unbrecciated Killiecrankie Schist of the country rock. Fractures trending 080° within the zone have slickensides pitching 30°W, whilst the Loch Tay Limestone to the south-east is affected by small sinistral faults averaging 020°/70°E, brecciated fractures trending 030° with vertical slickensides and fractures trending 160° with apparent dextral movement. Mineralisation adjacent to the fault here is described in Chapter 6.

The fault is again seen in the Allt Coire Pheiginn [NN 7509 5064] where a 50 m long bend in the burn is marked by loose boulders of a cream-coloured carbonated breccia of schist and quartzite. The Ben Lawers Schist to the south-east is strongly fractured and locally pyritised. A pronounced gully trending 040° continues the line of the fault to the north-east. The line of the fault further north-east is constrained within a 20 m gap by exposures in the burn at [NN 7553 5117] and is similarly constrained in the Allt Mor [NN 7621 5215]. Between the Allt Coire Pheiginn and the Allt Mor, exposures of quartzose garnet-mica schist containing amphibolite to the south-east of the fault are interpreted as representing a lens of Killiecrankie Schist some 1200 m long and 250 m wide, contained by a fault branching east from the main line of the fault. In the Allt Mor, the Killiecrankie Schist appears undisturbed to the north-west of the fault zone, whilst the Ben Lawers Schist to the south-east is carbonated and strongly disturbed by fracturing, gouge zones and kink-folding for 60 m downstream; 20 mm-long pods of pyrite in quartz-carbonate veins have been deformed by late movements on the schistosity.

From the Allt Mor to Tom Phobuill [NN 7767 5441] the main fault is strongly expressed by the straight course of the Allt Glengoulandie. However, at least two other fault braids branch south-east of the fault; these enclose a lens some 3 km long and up to 400 m wide, with a north-western component of Blair Atholl Formation grey limestone and graphitic schist and a south-eastern component of mixed quartzite, quartzose schist and pebbly quartzite. Although the latter three lithologies are shown on the map as one unit, they probably represent fault braids of Schiehallion Quartzite, Killiecrankie Schist and Carn Mairg Quartzite, respectively. Near the main fault boundary, in the quartzose schist of the fault lens in the banks of the Allt Glengoulandie between [NN 7684 5296] and [NN 7695 5322], a series of lenses of quartzite breccia, each up to 6 m thick, is contained within phyllonitic quartz-schists which dip 45°–50° south-east; there is evidence of repeated brecciation and late carbonation. To the north, the limestone and graphitic schist of the lens, surprisingly little disturbed, are seen on the south-eastern bank of the Allt Glengoulandie [NN 7714 5352], within a few metres of the Killiecrankie Schist to the north-west of the main fault. To the south-east of the burn, scattered exposures of non-pebbly quartzite between [NN 7678 5265] and [NN 7706 5302] are heavily brecciated and slickensided; slickensides here and elsewhere in the fault lens show a great variety of attitudes on joint surfaces, which may be a result of adjustments to a history of movements on the major fault.

The limestone of the lens, with minor graphitic schist, is well-exposed in two quarries at Tom Phobuill [NN 7768 5420], [NN 7778 5445]; bedding and joint surfaces exhibit a variety of slickenside orientations but the rocks are mostly unbrecciated. The fault branch that separates the limestone from the outer component of pebbly quartzite is well seen in the burn at [NN 7774 5419], where a river bluff of quartzite exhibits a brecciated fault surface (020°) with slickensides pitching 25°N; the adjacent limestone contains 010° and 070° fractures, with small dextral and sinistral displacements respectively. Another prominent brecciated fracture (020°/85W°) 20 m upstream separates the brecciated pebbly quartzite of the fault lens from the Carn Mairg Quartzite with graphitic schist of the country rock to the south-east, although the latter is still brecciated with locally pyritised fractures.

To the north of Tom Phobuill, the main fault is marked by a strong feature and photolineament to the south-east of Loch Kinardochy and its position can be identified in the Allt Blair Rainich [NN 7833 5577], to within 30 m, between exposures of veined and fractured grey limestone and 100 m of veined and fractured schist and quartzite upstream. Further north, the fault lies along the course of the burn for 300 m north-east of [NN 7891 5694]; 2 m of quartz breccia occurs in the burn [NN 7884 5682] but only loose blocks elsewhere. In the Allt Tarruinchon 30 m of Blair Atholl Formation dark limestone, affected by angular folds and fractures and forming the north-west wall of the fault, is terminated by the main fault zone seen at a river cliff [NN 7912 5730]; two thick quartz veins (trending 020°/85°W and 035° vertical) bound a 3 m-thick slice of strongly fractured limestone followed by a few metres of highly sheared graphitic schist (possible Blair Atholl Formation dark schist). The Killiecrankie Schist of the south-east wall, exposed 10 m further upstream, is not disturbed.

North of Loch Tummel, brecciated grey limestone of Blair Atholl aspect is seen east of a grey microdiorite dyke on the north shore near Bruchbane [NN 8034 5967] to [NN 8051 5966]. These bedded, but strongly fractured and carbonate-veined, limestones can be followed in a 200 m-wide zone north-east to [NN 8104 6035], to the burn east of Tomintianda between [NN 8097 6060] and [NN 8104 6061] and in many small exposures as far as [NN 8126 6093]. The strongly fractured limestone is almost certainly the equivalent of the Blair Atholl limestones in the Conbhar area to the east, displaced sinistrally into the fault zone. Rare exposures of quartzite and psammitic schist are seen in the zone north of Tomintianda.

The zone is bordered to the west by a dyke of porphyritic microdiorite which, on the shore [NN 8034 5967], appears to have been intruded into a breccia marking the western edge of the fault zone. The 1 m-wide breccia immediately to the east of this 5 m-thick dyke is terminated to the east by a steeply SE-dipping fault plane with subvertical slickensides. The dyke appears to define the western contact of the fault zone, which is regarded as the principal fault plane. It is seen in the Allt a' Bheithe [NN 8092 6061] with the NE-striking Ballachulish subgroup rocks to the west at [NN 8173 6180], and south-eastwards at [NN 8209 6245] where 5 m of breccia occurs to its east and Appin Group limestone and schist 20 m to its west. To the west of Tomintianda between [NN 8113 6056] and [NN 8133 6083] the disturbed limestone occurs close to the east-striking formations east of the fault zone. Further west for 100 m north-east of [NN 8130 6088] a strong topographical change of slope apparently marks the continuation of the eastern margin of the fault zone, which probably narrows northwards to reunite with the straighter western margin. The continuation of the fault to the east edge of the sheet is marked only by a well-expressed topographical hollow.

Frenich Burn Fault

Some 3 km east of the Loch Tay Fault, the Frenich Burn Fault lies between Loch Farleyer [NN 813 529] and Frenich [NN 827 590]. It trends 010°, has a near vertical dip and an apparent sinistral displacement of boundaries. No direct observation has been made of the fault plane or of rocks in the fault zone. The vertical axial plane of the Sron Mhor Syncline is displaced sinistrally some 500 m near the middle of the fault outcrop and, since the width of the Ben Lawers Schist in the fold core is not greatly changed across the fault, any dip-slip component is not significant here.

The displacement appears to diminish rapidly northwards as there is no evidence of the fault affecting the Appin subgroup boundaries north of Loch Tummel. Movement is probably transferred to three splay faults to the east of the main fault north of [NN 820 553]; these faults are closely associated with the change in strike of formations east of the Frenich Burn. To the south, the change of stratigraphical level of the mineralised horizons in the Ben Eagach Schist between a locality [NN 8179 5464] on the west wall and one [NN 8188 5489] on the east wall, requires a component of dip-slip movement, as well as the obvious sinistral strike-slip. The projected line of the fault to the south accounts for apparent sinistral displacements of 300 m of the Farragon Volcanic Formation and of 140 m of the Loch Tay Limestone at Loch Farleyer. The fault has been tentatively extended further south to a NNW-trending fault that crosses the Camserney Burn at [NN 8128 5017] and a further 3 km south to the NW-trending fault south of the River Tay, which is exposed on Bolfracks Hill [NN 8212 4715] and continues towards the Urlar Burn as a marked feature on the aerial photographs.

Another fracture plane evident from the aerial photographs and which has a similar NNE–SSW to N–S curvature occurs about 1 km east of the Frenich Burn Fault, running close to the eastern margin of the district near Clach an Fhuarain [NN 835 570] to [NN 828 525]. There is no certain evidence of offset of boundaries, but south of Lochan Lairig Laoigh [NN 8283 5422] it apparently offsets the Farragon Volcanic Formation and Loch Tay Limestone some 100 m in a sinistral sense. The deflection in the strike of the amphibolite sheets at Weem Hill [NN 828 516] could mark its southern termination.

Urlar Burn Fault

The NE-trending Urlar Burn Fault can be identified for 1.5 km across the south-eastern corner of the district, and continues for a further 2.3 km in the Pitlochry district (Sheet 55E). In the Urlar Burn [NN 8226 4456] to [NN 8273 4520], where the fault zone is well seen, it is some 3m wide with a trend of 030°, the component anastomosing fractures dipping 70°–85° SE (Figure 30). Fracture surfaces in the zone are commonly brecciated and slickensides pitch at low angles to the north. Repeated quartz veining and brecciation can be observed before late carbonation associated with minor pyrite. Just east of the district [NN 8314 4577], the fault is marked by a 5–30 m wide zone of country rock breccia cemented by cream-coloured dolomite, comprising up to 80% of the rock.

There is no direct evidence for the sense of displacement on the fault, but in the Pitlochry district the outcrop of the gently south-dipping beds has an apparent sinistral displacement of about 1 km, although a small dip-slip movement, down to east, could be responsible. The common low-pitching slickensides, on the other hand, certainly indicate one phase of near strike-slip movement.

The fault zone is host to two of the late microdiorite dykes and is crossed by the Late Carboniferous dolerite in the Urlar Burn [NN 8261 4509]. The mineralised 140°-trending fractures, discussed further in Chapter 6, are spatially related to the fault but its relationship to the fracturing and the mineralisation is not clear. The fractures might be interpreted as tensional features related to the fault movement; there is no evidence that individual veins cross the fault, indeed the distribution of the veins on the two sides of the fault might be interpreted as the result of a sinistral displacement. The microdiorites appear not to be affected by the SE-trending fractures.

Keltney Burn Fault

This fault, exposed in the Keltney Burn between [NN 7690 4980] and [NN 7674 5025], has the 160° trend common to many fractures in the adjacent rocks (see below). The stream gorge, up to 6 m wide, is occupied by several vertical, massive, pink calcite veins, locally up to 3 m wide; these alternate with screens of country rock, in places silicified and brecciated. The carbonate veins, which have sharp undeformed margins, invade the breccia and fill other 160°-trending fractures in the country rock and thus appear to postdate the main fault movements in a tensional phase. A steeply dipping amphibolite sill [NN 7678 5009] has an apparent dextral displacement of 60 m, but no movement indicators were observed. It is likely that the Keltney Burn Fault bends northwards into the fault trending 020°, in the burn at [NN 7671 5061]. Another 170°-trending fault to the west, at Garth Castle [NN 7643 5033], similarly shows an apparent dextral displacement and is seen in a photolineament to curve northwards into a 010° trend.

The spectacular NE-facing scarp to Drummond Hill [NN 780 477] has many fracture-controlled faces trending about 140° and it is possible that the scarp feature is controlled by a curving extension of the 160° Keltney Burn Fault. Another major 150° fracture-controlled face is seen to the south-west [NN 7715 4760]; both these features, and a third between them, can be identified as strong lineaments on aerial photographs, but there is no direct evidence of fault activity. A south-easterly continuation of this 150°-trending fracture set is seen in two faults, which displace amphibolites, to the east of Kenmore [NN 780 452] to [NN 795 454].

Allt Mor Fault

The Allt Mor Fault is one of a series of important fractures newly mapped to the west of, and parallel to, the Loch Tay Fault. It can be located to within one metre in the south bank of the Allt Mor [NN 7530 5322], where kink folds plunging steeply north-east in the Killiecrankie Schist are juxtaposed against some 15 m of shattered Schiehallion Quartzite to the north-west. In the latter, vertical joints trending 020° and 048° contain subhorizontal slickensides. Brecciated and kinked Killiecrankie Schist, 100 m to the north-east, has a major joint face 030°/75°SE which may represent the attitude of the fault in this area. Further north-east [NN 7555 5357] a major joint face trends 025°/65°W.

To the north-east from [NN 759 543], the fault appears on aerial photographs to curve, between exposures of Blair Atholl Limestone and Schiehallion Quartzite, into a north–south trend, parallel with the strike of the limestones to its west. Further on at [NN 761 550], the photolineament returns to a north-east trend, cutting obliquely across formations north of Loch Kinardochy, but eventually curving into parallelism with the country rock near Daloist [NN 7800 5723]. One locality of brecciated quartzite [NN 7752 5655] and brecciated and slickensided limestone in a gully [NN 7752 5681] provide the only field evidence for the fault in this part of its proposed course.

Displacement on the fault is difficult to quantify, although it is clear that the lack of a precise match of boundaries across the fault implies a component of dip-slip movement. This is most evident from the contrast between the east–west strike of the Schiehallion Quartzite/Killiecrankie Schist south of Dun Coillich and its NNE trend to the west of the fault. A sinistral displacement of about 1 km, to connect the boundary on the two sides, would also bring the slight bend in strike of the limestones to the west of the fault into alignment with the D₃ fold-pair to its east. However, a net dip-slip movement would also be required to account for the change in intensity of D₃ folding on the two sides; since the D₃ folding increases in intensity regionally up-plunge to the east, the downthrow would seem to be on the east side.

Four faults are postulated between and parallel to the Allt Mor and Loch Tay Faults. These are principally based on lineaments from the aerial photographs, supported by some major joint surfaces with a NE trend, e.g. on the south-west side of Dun Coillich [NN 7565 5325], and localised brecciation in the Killiecrankie Schist. The evidence for movement is from the offset of formations south of Loch Kinardochy which, on the basis on very limited exposure information, suggests apparent sinistral strike-slip components in the displacements on the four faults; the pattern could be satisfied by small (tens of metres) dip-slip movements, down to the east.

To the south of the Allt Mor, the parallel fractures to the south-east of the fault, as well as the main fault, are postulated on the basis of photolineaments to curve SSW, and possibly to be splays from the Loch Tay Fault (Figure 13).

Braes of Foss Fault

A single north-trending fracture or a number of closely spaced fractures between [NN 7457 5400] and [NN 7448 5550], must be responsible for the remarkable contrast between the generally E–W-striking Schiehallion Quartzite of the Schiehallion mass and the N–S-striking Blair Atholl Limestone to its east. It is certain that a major component of dip-slip movement is required to account for the disappearance of the 2 km-wide Schiehallion Quartzite outcrop to its west. The principal structure within this outcrop is the antiformal, east-plunging, downward-facing, D₁ Beinn a' Chuallaich Syncline. Before faulting, the thickened northern limb (greatly enhanced by D₁ and D₂ folding) might have been expected to close with the thin southern limb, perhaps a kilometre to the north-east. Thus a displacement of several hundred metres, down to the east, would be required to entirely remove the outcrop of the quartzite from this east side of this fault, the Braes of Foss Fault.

Further north the fault swings north-east, parallel to the strike of the Schiehallion Quartzite and Tempar Dolomitic Member in the Allt Ruighe nan Coireachan, with scattered exposures of Blair Atholl Formation to the south-east. The northern boundary of the Schiehallion Quartzite terminates against the fault at Braes of Foss [NN 7527 5575] from whence the fault is presumed to continue in parallelism with the NE-striking limestones and graphitic schist of the Blair Atholl Formation on its two sides; a line of boulders and exposures of silicified and brecciated limestone between [NN 7595 5608] and [NN 7625 5621] may define its course. Near Tombreck [NN 7744 5676] the fault appears to join the Allt Mor Fault discussed above. It seems likely that the movement on both faults northeastwards from here is transferred into bedding-parallel movement in the flaggy Appin Group rocks near the Boundary Slide, and eventually into the Loch Tay Fault zone, at the eastern edge of the district.

To the south, the movement on the fault is probably dispersed on to a number of N- to NNE-trending, steeply-dipping fractures between the Loch Tay Fault [NN 7470 5000] and the east end of Schiehallion [NN 7457 5400]. In the Allt Mor betweeen [NN 7461 5333] and [NN 7494 5337], some 15 such fractures can be seen, many associated with brecciation and strike-parallel slickensides. Since the faulting occurs largely within the Killiecrankie Schist, displacements are difficult to quantify. The north-trending fractures are locally offset by NW-trending faults. One important NE-trending splay has been traced, in part as a photolineament, from the Allt Mor [NN 7396 5323] 4 km WSW to Meall nan Eun [NN 7020 5115]. Another fracture associated with the Braes of Foss Fault [NN 7470 5000], may be a splay from the Loch Tay Fault itself. It is thus possible that the Braes of Foss Fault, together with the proposed southerly extension of the Allt Mor Fault, may be regarded as branch faults of the Loch Tay Fault, possibly developed in the dip-slip phase discussed below.

Innerhadden Burn Fault

The Innerhadden Burn Fault can be traced intermittently for some 12 km from the Innerhadden Burn [NN 6694 5677] northeast to the Kinloch Rannoch–Trinafour road [NN 7015 6182]. In the burn between [NN 6694 5677] and [NN 6727 5731] the vertical fault, trending 040°, is associated with brecciation and strike-parallel slickensides. Here, it offsets two sets of microdiorites (south-dipping sill and NE-striking dyke), but is itself intruded by a dark grey basic microdiorite, which contains xenoliths and screens of country rock. A dextral strike-slip movement of some tens of metres is evident from the offset of near vertical amphibolites on the south-east slopes of Beinn a' Chuallaich e.g. [NN 694 602].

Other fracture sets

It is clear from the above descriptions that the principal fracture set is that subparallel to the NNE-trending Loch Tay Fault. Like the main fault, these faults commonly show evidence of sinistral displacement, together with a dip-slip component, which is mostly down to the east. Many less extensive faults and fractures with a range of attitude from north to north-east can be established and some of the more significant of these faults are identified on the map, particularly in the centre of the district between Glengoulandie and Kinloch Rannoch, in the Trinafour area in the north, and in the Glen Lyon area in the south. Many exhibit small (metres) apparent sinistral and dextral displacememts but dip-slip associated with tensional opening may be just as likely; many of these are occupied by the NE-trending, late Silurian/ early Devonian microdiorites which show no evidence that they have undergone deformation. The dykes evidently have either been intruded during tensional opening of the fractures or postdate movement, e.g. on the north-west ridge of Schiehallion [NN 712 549] mentioned below. Some of the microdiorite sheets are themselves affected by NE-trending faults. Further discussion on the timing of the various igneous sheets in relation to fracturing will be found in Chapter 4.

In the north of the district many small faults are particularly well seen in the A9 road-cuts and in the River Garry. These have trends from north-east through to NNW and are usually associated with gouge and shattering and are commonly occupied by thin carbonate sheets and quartz veins. Examples are in the Edendon Water [NN 717 717] and in the road section above [NN 712 708], in the Wade Stone road-cut [NN 697 716] (particularly good examples of the features mentioned above, including faults occupied by microdiorites), and in the Allt an Stalcair [NN 690 717] (associated with a 10 m-wide zone of pink carbonate veins and breccia). Many faults are associated with open angular folds which plunge gently SE–SSE; these are clearly seen in the River Garry near Dalnacardoch [NN 72 70], at Clunes [NN 7895 6650] and in the A9 roadcut [NN 774 680], where a fault cuts a porphyrite dyke. Down-dip slickensides can often be observed, e.g. in the Wade Stone road-cut [NN 697 716]. Throws appear to be of only a few metres, but the monotonous lithologies make even these apparent displacements difficult to quantify.

In the area north of Lochan an Daim [NN 715 575] the north-east-trending fracture set appears to fan around into a distinct concentration of faults with an ENE trend. These, from their displacement of minor folds in the Blair Atholl Formation limestones, can be shown to be dip-slip. Dip-slip displacements can be certainly demonstrated in some instances elsewhere: the N to NNE-trending faults on Creag an Earra [NN 693 536], which displace sills; and a major north–south fault, displacing down to the west, on Creag an Fhithich [NN 666 573]. On the other hand, the zone of NE-trending fractures containing microdiorites near the summit of Schiehallion [NN 712 549], displaces vertical amphibolites.

The unusual 115°-trending fault south of Meall Urair [NN 8327 6047] appears to be dip-slip, down to the SW, from its displacement of fold-traces.

The pit opened for the recovery of baryte [NN 815 545] (Figure 27) gives an excellent indication of the intensity of fracture and fault development. For instance, at the east end of the pit, over a distance of 50 m, some 25 fractures were recorded (data from Dresser Minerals); these mostly trend NNE but with a strong north to north-west subset. Apparent displacements range from 2 to 10 m; displacement of fold-traces demonstrate net sinistral movements, whilst others are dip-slip down to the east.

The apparent telescoping of the stratigraphy at the Boundary Slide in the west of the district is thought to have been in part caused by low-angle east-dipping thrusts, which cut out part of the stratigraphical succession. These faults, best seen in the Allt Druidhe [NN 642 570], are in places associated with brecciation and appear to be earlier than the intrusions and the NE-trending faults. East-dipping thrusts are also seen in the Bruar Water and in the Allt a' Chireachain [NN 754 705].

Small movements on bedding are apparent throughout the district, but are particularly demonstrable in the Allt Mor [NN 745 535], in the E–W-striking formations south of Glen Lyon, and on the N–S-striking bedding and schistosity on the west side of Craig Varr [NN 669 591].

A strong 140°-trending fracture set is developed east of the Frenich Burn from Lick Hill [NN 834 549] north to Loch Tummel and in the south-east of the district, roughly south of grid line [49] and east of the Loch Tay Fault, there is strong set of fractures with generally south-east-trend; this fracture set is not so pervasively developed elsewhere. In the south-eastern area, the fractures are particularly noticeable in amphibolites, and are responsible for the form of exposures over much of the outcrop of the Pitlochry Schist. They are particularly well seen on Drummond Hill and in all the principal crags and burns of the area. The only area where this fracture set is strongly developed north-west of the Loch Tay Fault is in the Killiecrankie Schist on Dun Coillich [NN 762 537].

In the Urlar Burn, the quartz-carbonate veins in this 140° fracture set carry the auriferous sulphide mineralisation described in Chapter 6 (Figure 31). None of the fractures show more than a few centimetres (usually dextral) displacement and those carrying quartz veins clearly show from the fabric that the associated opening was tensional. Major fractures south-east of Kenmore, on Drummond Hill and in the Keltney Burn are expressions of this set, some of which also show a component of dextral displacement. As mentioned above, the Keltney Burn faults trending at 160° appear to veer into a NNE trend as they approach the Loch Tay Fault; this is also seen in a NNW-trending fault in a gully north-west of Fortingall [NN 7373 4719] which to the north turns to a NNE trend, parallel to the adjacent Loch Tay Fault. In the area south-east of Kenmore, the microdiorites are affected by SE-trending fractures and offset by faults of that trend e.g. [NN 7773 4440]. The grey microdiorite which occupies part of the Loch Tay Fault zone on the north shore of Loch Tummel [NN 8034 5968] is affected by two strong sets of joints at 050° and 140°.

Fracture history

As stated by Treagus (1991), the lack of a precise match of formations across the Loch Tay Fault clearly shows that the net movement is not simply the sinistral one usually quoted. (Figure 18) is a composite cross-section along the plane of the fault, using information from both its east and west sides, by projecting the D₂ and D₃ folds along plunge on to the fault walls. The match of the fold axial planes across the fault has been made with the minimum vertical separation (which is about 750 m maximum, diminishing to zero northwards) down on the west side. The horizontal component (restored on (Figure 8)) is more tightly constrained and, using the vertical axial plane of the Ben Lawers Synform as a control, is about 6.25 km. At the north end the constraints on the values of these displacements are particularly uncertain. The down-to-the-west movement is consistent with the occurrence of the Loch Tay Limestone and Ben Lui Schist in the Flat Belt on the west side of the fault, to the south and north of Loch Tay; there the vertical axial-trace of the Loch Tay Antiform is estimated to have been displaced sinistrally by some 7.5 km with a net vertical displacement of about 1 km (Treagus, 1991).

Treagus (1991) pointed out that the major fault lens of Blair Atholl Subgroup limestone, with an outer sheath of probable Schiehallion Quartzite, Killiecrankie Schist and Carn Mairg Quartzite to the east of the straight fault at Glen Goulandie, suggests a dextral component in the movement history. In the simplest scenario (ignoring any dip-slip component) this would involve about 3 km of dextral strike-slip, which could have pre- or postdated the sinistral component of about 10 km. If the sinistral displacement came first, this would have taken place on the curving fault trace to the east of the Glen Goulandie lens as well as that to the east of the presumed Killiecrankie Schist lens further to the south; the subsequent dextral movement would have been on the straight section of the fault up Glen Goulandie. Initial movement on such a braided fault, with subsequent movement on a straighter course through the braids, is well documented, e.g. Woodcock and Schubert, 1994. Slickensides on this fault, and the relationship of igneous rocks to other faults in the Grampian Highlands (Treagus, 1991), certainly suggest a complex movement history.

Generally, no clear age relationships could be established between the various fracture sets discussed above. There is no evidence of sinistral displacements on the 140°-trending fractures, which would be required if they were interpreted as 'Riedel shears' related to the sinistral movement on the 025°-trending Loch Tay Fault. It would be more likely that they were tensional joints if related to that movement. Aerial photographs and satellite images show that the distribution of the SE-trending fracture set is not displaced by the major NE-trending fault set, although changes of orientation are seen as the fracture set approaches the faults, which might suggest some contemporeity. A regional NW–SE tensional phase may have postdated the sinistral movements, accounting for the dip-slip movement on the Loch Tay and other faults and permitting the intrusion of the microdiorite dyke swarm; alternatively, the microdiorite intrusions might have been emplaced in tensional fractures contemporary with the major sinistral fault movements. A later history of dextral displacement on the Loch Tay Fault might be suggested to have then instigated the dextral movement on the SE-trending fractures and certain splay faults. The late dextral movement on the Great Glen Fault occurred in the late Carboniferous (Rogers et al., 1989).

Structural synthesis

In this section the information presented above is drawn together in order to provide a coherent geometrical, as well as kinematic, view of the deformation history of the whole district. An attempt is also made to integrate the structures with those known from adjacent districts and to put this into the context of Grampian fold-belt. The various cross-sections presented in (Figure 12), (Figure 14, 15), (Figure 17) and (Figure 18), as well as those accompanying the map are relevant.

The latest structures developed in the district are the faults and fractures, the possible sequence of which has been discussed above. The latest certain movement is the N–S extension which permitted the intrusion of the E–W-trending Late Carboniferous dyke in the south of the district. This is one of the northernmost of a swarm of dykes that extends some 70 km to the south into the Midland Valley. An earlier NW–SE extension associated with the intrusion of the Silurian to early Devonian igneous sheets into NE-striking tensional fractures may also account for the dip-slip movements on some of the NE-trending major faults. This is probably predated by the north–south shortening, proposed by Anderson (1942) to account for the principal sinistral movements on the major faults. The latest fold phase appears to be D_t, the angular folds of which are more akin to structures related to 'brittle' deformation; the box-shaped geometry of these folds suggests a NE–SW shortening.

The Loch Tay and parallel faults in the district are the most easterly of a family of NE- or NNE-trending faults affecting the Dalradian as far west as the Great Glen Fault. All these faults have a history of major sinistral strike-slip with minor net dextral and dip-slip movements in the Silurian to early Devonian (Treagus, 1991; 1999). At its southern end the Loch Tay Fault appears to be a splay of the Highland Boundary Fault, which has a similar history of Ordovician to Silurian strike-slip and Devonian to Lower Carboniferous dip-slip movements.

The latest folds of a truly 'ductile' nature are those of the localised D_c phase, which produces minor folds and cleavage in the more incompetent lithologies and is probably responsible for local changes in dip of steeply inclined competent formations. This phase only significantly affects rocks now east of the Loch Tay Fault, which would at the time have been deeper than those to the west (Figure 18). The flat axial-planes and crenulation cleavage of this phase suggests a vertical shortening, possibly related to the late Ordovician uplift (Dempster, 1985. Folds and cleavage with the D_c geometry appear not to have been documented outside the district, although the author has recognised similar structures in the Tyndrum and Tomintoul areas.

The next oldest deformation phase is assumed here to be D₃. The major upright ENE-trending folds on the two sides of the Loch Tay Fault can be reasonably correlated (Figure 18) in terms of a sinistral movement. If the sense of vertical displacement deduced for the fault is correct, D₃ was more strongly developed at higher levels in the crust (now west of the fault). The Ben Lawers Synform is the most significant of these folds, marking the transition from the 'Flat Belt' to the south into the more complex belt of D₁ and D₂ folding of the district (sometimes erroneously called the 'Steep Belt'). Folds and cleavage of D₃ age are recognised throughout the Flat Belt (Stringer, 1957) and indicate a widespread NNW–SSE shortening of the orogenic belt. Although this is apparently inconsistent with uplift, the effect of the D₃ folds on the presumed subhorizontal D₂ axial traces (Figure 15a, b) is to 'stair-step' them downwards to the south. Thus the Drumochter Dome to the north of the district, with its absence of minor folds and cleavage, may be seen to represent the uplifted centre of the belt with the D₃ folds descending through the district and the Flat Belt, terminating at the Highland Border steep belt. Episodic uplift presumably continued between D₂ and D₃. The growth of biotite indicates only a moderate depth of burial during D₃ in the district.

The D_e phase is represented by the major Errochty/ Bohespic fold-pair (Figure 12a) and is here correlated with minor D_L phase structures of Glen Lyon. No structures of this age have been recognised to the east of the Loch Tay Fault, but three other major folds of similar geometry are developed south-west of the district towards Tyndrum (Treagus, 1987, p.7). Minor folds of post-D₂ age and of similar geometry to those in Glen Lyon are widely recognised in the Flat Belt (Stringer, 1957; Nell and Treagus, 1994). This deformation indicates an E–W shortening along the length of the orogenic belt with a top-to-the-west shear component. The growth of garnet indicates that the rocks of the district were deeply buried, possibly to about 10 km, at this time. Treagus (1987) discussed the possibility that the Loch Tay, and other major faults outside the district, might have been initiated as early dextral shear-zones on the limbs of the D_e fold-pairs.

The correlation of the major D₁ and D₂ folds on the two sides of the Loch Tay Fault is shown on (Figure 18) (see figure description for names). To the east of the fault, four major D₂ folds are identified together with two major D₁, NW-verging fold-pairs within the pile (Figure 14). These six folds can be correlated with reasonable confidence with those to the west of the fault, if the downthrow of some 0.75 km of the west wall is allowed. At a lower level than that seen on the east side of the fault are the D₂ Balliemore Antiform (above the Boundary Slide), Meall Reamhar Synform and Clunes Antiform (affecting the Grampian group below).

The D₂ folds are predicted to have been recumbent within an overall inverted succession (Figure 15b), part of the regional Tay Nappe, before being affected by D₃. The present plunge variation of the major D₂ folds, north-east in central Glen Lyon and south-west to the east of the Loch Tay Fault, would at that time have represented a gentle curvature of the hinges of the NW-verging fold-pile. The fold pattern suggests that a near-vertical pure shear was responsible for the initiation of the folds imposed on a steeply dipping limb of a major D₁ fold 'proto-Tay Nappe') whose axial plane lay to the north-west of the district (Figure 15c). This pile of D₂ folds is least modified in the slightly NW-vergent folds seen in the lower Argyll to Grampian Group rocks in the north of the district then at a relatively low level. Simple shear, top-to-the-south-east, was, however, the dominant mechanism during D₂ at higher levels, possibly increasing above the level of the axial trace of the Meall Tairneachan Fold (Figure 15b) and now seen in the Argyll and Southern Highland groups in the south of the district. Thus, the whole district can be regarded as part of the flat inverted rocks of the regional D₂ Tay Nappe. The D₂ phase, which dominates the later stage of the Grampian Orogeny, postdates the intrusion of the Ben Vuirich granite of the Pitlochry district (Sheet 55E), dated at 590 Ma (Tanner and Leslie, 1994). The garnet/kyanite mineral assemblage in Glen Lyon and Schiehallion suggests depths of about 35 km at this time (Wells and Richardson, 1979).

Somewhat contradictary to the SE-directed simple shear mentioned above is the top-to-the-north-west shear of the Boundary Slide Zone. The high D₂ strains in the slide zone are considered to be largely a product of pure shear as strongly developed shear-sense indicators are not present. However, the sense of the simple shear component is to the north-west and it is more strongly developed than in the adjacent rocks outside the shear zone. The north-west shear is attributed to the exceptionally strong attenuation of this short limb (of the D₂ Balliemore/Meall Reamhar fold-pair), whereas it is the long limbs of the higher D₂ folds which are subject to the SE-directed shear associated with the Tay Nappe.

The component D₁ major folds of the 'proto-Tay Nappe' which lay to the north-west, originally verged north-west and are assumed to have been upright (Figure 15c). The axes would have been strongly curving, mostly at a high angle to the subsequent D₂ axes. This accounts in part for the large apparent amplitude of these folds on the section in (Figure 18) and in particular for the 'double closures' of the Meall Garbh/Creag na h- Iolaire Anticline and Chesthill/Sron Mhor Syncline. Some substantial thinning of 'short' D₁ fold limbs, with excision of formations took place before D₂, possibly contributing to the abbreviation and excision of formations at the Boundary Slide zone. Treagus (1987, fig. 9) has suggested that the D₁ 'proto-Tay Nappe' was the south-easternmost fold of the three major upright D₁ folds (including the Islay Anticline and Loch Awe Syncline) which dominate the fold-belt in the Grampian Orogeny. This phase, which also probably postdated the Ben Vuirich granite (Tanner, 1996) as well as the Lower Cambrian Leny Limestone (Tanner, 1995), resulted from NE–SW shortening. The evidence of the fine grain-size of the S₁ fabric (particularly in the inclusion trails in D₂ porphyroblasts) suggests that the rocks, both in the Schiehallion district and elsewhere in the orogenic belt, were at a high level in the crust at the time of the D₁ deformation.

Chapter 4 Igneous rocks

Igneous rocks form a relatively small component of the geology of the Schiehallion district, and mostly comprise pre-tectonic sills and post-tectonic (late Caledonian) minor intrusions, together with at least one Permo-Carboniferous dyke. The only substantial body of igneous rock is the Glen Banvie Complex, part of which is exposed in the extreme north-east corner of the district.

The earliest evidence of magmatism in the district is that of the volcaniclastic sediments, discussed above. Probably contemporary with these are an extensive series of early amphibolites that have been intruded as pre-tectonic basic sills into formations above the Grampian Group. A smaller group of amphibolites were intruded as dykes and sills at a later stage in the deformation history.

The late-Caledonian minor intrusions have been grouped into a number of distinct categories, partly on field evidence (distribution, attitude and rare cross-cutting relationships) and partly on petrological characteristics, but these divisions are to some extent arbitrary and the precise sequence of events is uncertain. However, comparison with neighbouring areas suggests that the Glen Banvie Complex and the late-Caledonian minor intrusions were broadly contemporaneous, and represent an episode of calc-alkaline igneous activity of late Silurian to early Devonian age. For convenience, the Glen Banvie Complex is described separately from the minor intrusions, despite the probable overlap in terms of magmatic affinities and emplacement history. The intrusions contain no evidence of having been involved in the folding of schistosities seen in the country rocks. The main groups of minor intrusions are described in probable chronological order, including the quartz-dolerite dyke, believed to be an outlying member of the Late Carboniferous suite of tholeiitic dykes and sills, exposed mainly in the Midland Valley of Scotland. These various episodes and categories of igneous activity are summarised in (Table 2).

Amphibolites

All formations in the Appin, Argyll and Southern Highland groups contain amphibolites. The origin of some of these is probably sedimentary, others are undoubtedly volcaniclastic, while those considered here are igneous intrusions. The criteria for an intrusive origin are the thickness of these bodies (metres to tens of metres), the uniformity of lithology, and the sharp nature of their junctions. Some amphibolites are of ambiguous origin, particularly thin (centimetres-thick) concordant sheets or lenses, occurring especially in the Appin Group pelites. Such bodies may have been originally metres in thickness in areas of high strain and, in contrast to beds of similar thickness of clear sedimentary or volcanic origin, they have sharp margins; these factors suggest that such bodies may, in fact, have been intrusive. Also, some sheets of several metres thickness, especially in the Carn Mairg Quartzite in the west of the district and throughout the Ben Lawers Schist, appear to show gradational margins and an intrusive origin is not unequivocal. An extrusive origin cannot be excluded for some of the sheets, as metamorphism and deformation are likely to have obscured the contact relations and pillow structures which can be used as discriminating criteria in lower-grade parts of the Dalradian.

The intrusions can generally be attributed to one of two groups, based upon difference in texture, metamorphic fabric and relations to the D₂ events. The first (early) group are concordant schistose sills, affected by the D₂ fabric, whereas the second (late) group have only a marginal schistosity and an interior with more randomly oriented amphibole and relict feldspar phenocrysts. Sheets of the second group are commonly discordant and may cut across D₂ folds. It must be emphasised, however, that attribution of a sheet to one of the two groups is not always clear-cut. Some thick sheets attributed to the early group have an internal fabric very similar to that of the late group; other sheets attributed to the early group on the basis of their concordance and strong fabric may, in fact, be late sheets which have suffered high strains, possibly as a result of their very concordance.

Early amphibolites

Representatives of this group of amphibolites are broadly concordant, in bedded rocks, although locally margins of the thicker bodies can be seen to transect bedding. They occur in all formations from the Beoil Schist upwards. In the field the thinner sheets, typically 2–5 m, are uniform, foliated, fine-grained amphibole–feldspar rocks. The amphibole needles, typically 2 mm long but in some cases up to 5 mm long, show a strong preferred orientation, within the fine-grained feldspathic matrix. The foliation and lineation are parallel to the D₂ schistosity and extension lineation in the host rocks. Biotite is common and the rocks are commonly garnetiferous with a density and size increase towards the margins. Rarely a faint cm-scale striping may be present, as a result of the relative concentration of amphibole and feldspar. In the thicker sheets, up to 60 m, there may be a coarsening of texture and the loss of foliation and lineation towards the centre, with the amphiboles, up to 30 mm long, becoming porphyroblastic and more randomly oriented. In these thicker bodies, especially in the Pitlochry Schists, the outline of relict feldspars pseudomorphed by carbonate-epidote-plagioclase are discernible. Even in some thinner (1 m-thick) sheets discrete, elongate, blebs (up to 1 cm long) of similar composition are seen. Carbonate-filled, ellipsoidal holes (up to 1 cm long) near the margins of some sheets may be interpreted as amygdales.

In thin section typical foliated amphibolite is seen to consist of sub-oriented amphibole prisms (approximately 50%) with minor biotite and (in some cases) scattered garnet, together with granular quartzofeldspathic material. Quartz is always present and predominates in some examples, but in others plagioclase is dominant. Patches of skeletal or spongy iron oxide are also present. Sections from the interior of the more massive examples in the Pitlochry Schist tend to be dominated by randomly oriented amphibole, associated with altered plagioclase, quartz and large patches of granular sphene.

No amphibolites have been observed in the Grampian Group. The lowest sheets, in the Beoil Schist, are remarkable in that some vary in thickness from as little as one centimetre to 20–30 cm. These variations can sometimes be seen in a single sheet, usually accompanied by boudinage, e.g. in Strath Fionan [NN 7272 5716], [NN 7378 5680] and some of the thickness variation is undoubtedly due to high strains near the Grampian/Appin Group boundary. In the pelites of the Appin Group above, thicknesses range up to a metre or two and the common features described above are best displayed in the Blair Atholl Formation graphitic schist, e.g. south of Lochan an Daim [NN 7198 5660]. A sheet seen south of the road in Strath Fionan e.g. [NN 7338 5630] shows good outlines of small (1 mm) feldspars in a matrix of fine granular amphiboles.

Of the amphibolites in the Argyll Group, those in the dolomitic beds of the Schiehallion Quartzite are particularly numerous and rich in sulphides, e.g. Allt Ruighe [NN 7454 5529], and common, but unremarkable, throughout the remaining formations. Thicknesses vary from 1 to 10 m and there appears to be no significant stratigraphical or geographical concentration of intrusions. Clean sections can be examined in the forest tracks to the west and south of Loch Kinardochy [NN 7593 5562]; [NN 7714 5454]; at the latter locality and the hill-slope above, relict feldspar (now epidote-plagioclase) phenocrysts appear to be preserved. Only one sheet, on the southern slopes of Beinn a' Chuallaich [NN 682 610], has been observed to be folded by tight folds, possibly D₁ in this instance.

Exceptionally thick masses of amphibolite are present in the Appin and Argyll Group formations in the core of the Errochty Synform south of Kinloch Rannoch [NN 666 575], east of Crossmount [NN 707 580] and along the Boundary Slide Zone between Loch Errochty and Loch Rannoch; these bodies appear to be several tens of metres thick, but it is likely that they consist of a stack of many thinner sheets. The 1902 edition of Sheet 55 shows large masses west of Loch Kinardochy [NN 766 550] and south-west of Meall Garbh [NN 640 514], but the present work has shown these occurrences to be groups of thin sheets. A thin kilometre-long body near the top of the Ben Lawers Schist swells to about 80 m at Coire Chille [NN 788 524] and, at its western termination, cuts across individual beds in the schists, e.g. west of Creag Chean [NN 7879 5256].

The number and thickness of amphibolites increases significantly in the Southern Highland Group. Thin amphibolites are very common in the Loch Tay Limestone, but become increasingly common and thicker (up to 60 m) in the Pitlochry Schist. The latter thickness is for the sheet west of Tombuie Cottage, which is well exposed on the ridge [NN 783 444] (Figure 31). This and other nearby sheets usually have fine-grained margins of tremolite-chlorite-talc-magnetite grading into foliated amphibolite which may have an unfoliated centre; carbonate is usually present and quartz veins and pods common. Although broadly concordant, the margins of these sheets can in places be seen to be discordant. Some thinner sheets are composed entirely of the talc-chlorite schist, which was once in demand for building stone; old quarries are seen at Balmore [NN 821 506] and Bolfracks Hill [NN 833 473]. Good sections of amphibolite sheets are provided in and around the Allt Odhar upstream of [NN 736 472], in the Keltney Burn upstream of [NN 7710 4964], in Dull Wood [NN 799 496], in the Camserney Burn upstream of [NN 813 502], and on Drummond Hill. On the latter, at least six individual sheets are exposed; sections on the north-east of the hill [NN 7770 4820] to [NN 7781 4888] and to the north of the track at [NN 7792 4679] show marginal chills and apparent amygdales. In the Allt a' Bhealaich [NN 7901 4447] (Figure 31) a contact is exposed above sediments which exhibit inverted graded bedding; the contact sediments are hard and flinty as if baked by the intrusion.

The cross-section east of the Loch Tay Fault (Figure 14) suggests that some eight individual sheets, many of which may be discontinuous, may be identified from data in the various sections, making up some 50% of the total thickness (1200 m) of the Pitlochry Schist. Craig (in Barrow et al., 1905, p.73) considered that there was probably only one sill in the Pitlochry Schist, repetition by folding accounting for the several outcrops. Repetition by D₁ and/or D₂ folding cannot be discounted, although no sheet has been observed to pass round a hinge; where way-up criteria have been observed close to a sheet boundary, it has invariably been inverted.

Late amphibolites

The primary distinction of this group of amphibolites from those discussed above is a difference in field and thin-section texture. This difference was hinted at in the first edition of the memoir to Sheet 55W with references to small intrusions west and south of Trinafour as 'either in the form of hornblende-schist or more massive epidiorite' (Barrow et al., 1905, p.86). Rast (1958b) also recognised the difference in texture between the two groups, and noted that some of the later group cut across fold closures (see below).

In the field the extreme margins of these amphibolites show a foliation and lineation of amphibole, indistinguishable from that of the earlier group, but the fabric rapidly becomes more random towards the interior of the sheets, with large relict feldspar phenocrysts (Plate 6), typically 8 mm but up to 20 mm long. The texture usually has an ophitic appearance. The schistosity in the margins is of a similar intensity to that of S₂ in the country rock schist, but in many instances has a markedly different orientation.

In thin section these rocks are strongly to weakly foliated amphibolites with possible relict feldspar phenocrysts. The matrix consists of approximately 50% of sub-oriented amphibole prisms, associated with granular quartzofeldspathic material, mostly quartz. Sphene or rutile, although relatively minor, is ubiquitous, whereas opaque minerals are scarce. The original phenocrysts were probably plagioclase, since they now consist of granular aggregates of clinozoisite, associated with minor muscovite, quartz and carbonate. The schistose margins to these sheets commonly contain large (up to 20 mm) spongy garnets, which may deflect the surrounding fabric.

Critically, amphibolites with these textures are frequently discordant not only to the adjacent bedding in the host rocks, but also to folds of D₂ age; they are locally folded by folds of D₃ and D_L age. They usually display branching from sills into cross-cutting dykes and vary in thickness from one to 20 m; they tend to occur in groups of closely associated sheets. They are present in a broad belt from the east end of Loch Errochty [NN 717 659], across Meall na Moine [NN 700 636] to [NN 706 636], down the east flank of Beinn a' Chuallaich, in the Tempar area [NN 691 565], across the top of Schiehallion [NN 711 550] to the hill, WSW of Glengoulandie [NN 750 525]. They have not been recorded to the east of the Loch Tay Fault nor in the Grampian Group or in rocks younger than the Killiecrankie Schist.

Three sheets exposed in the north tributary to the Allt na Moine Buidhe [NN 7022 6141] illustrate the cross-cutting relationship to D₂ schistosity in the host schists and quartzites; the sheets dip gently west (while the beds dip steeply east) and have an east-dipping fabric which appears not to be concordant with S₂ in the adjacent rocks. Similar relations can be seen on the top of Meall nan Eun [NN 685 625]. A sheet which runs north-east for 2.3 km from Drumchastle [NN 690 589] to [NN 693 613] cuts across the major D₁ Beinn a' Chuallaich Anticline and D₂ Balliemore Antiform. A 20 m-thick concordant sheet occurs at Tempar, but in Strath Fionan [NN 724 571], on Schiehallion [NN 711 550] and near Glengoulandie [NN 750 525] the sheets dip gently east or south-east, discordant to bedding. Some of this difference in attitude can be accounted for by the later folding about the Bohespic Antiform.

In forest track exposures at Lassintullich [NN 6985 5732] to [NN 6976 5718] two late amphibolites are well exposed and show an approximately axial-planar relationship to D₂ folds. The northern one, 10 m thick, has a marginal schistosity which appears to be parallel and contiguous with the S₂ in the adjacent schist; garnets occur both in the margins and in the schistose centre of this sheet. In the southern, 1 m-thick, sheet hornblendes have a strong orientation parallel to the D₂ schistosity and lineation in the schists.

There is no firm evidence of the age of any of the early amphibolites, or of many of the late amphibolites, with respect to the D₁–D₂ deformation history. Nell (1984) drew attention to the similarity in mineralogy and imposition between the talc-chlorite schists seen at Balmore [NN 821 506] and Bolfracks [NN 833 473] and the serpentinite sheets in Glen Lyon, south of Roroyere [NN 612 473] in the Loch Rannoch district (Sheet 54E) which are folded by the D₂ Ruskich Antiform, but post-date the D₁ inversion of the host-rocks. This suggests, together with the difficulty of attribution of some sheets to the early or late groups, that some sheets (particularly those in the Pitlochry Schist) may be post-D₁ in age and that there may have been several periods of pre- to syn-D₂ intrusion in the district.

With respect to the late amphibolite sheets which transect D₂ folds, if their marginal fabrics were of D₂ age, they would have to have been intruded later than the principal development of the D₂ folds but before the peak of the strain and of metamorphic conditions. However, the fabric in some instances is at a high angle to the local S₂. Although the schists in the areas that contain the sheets do not have a D_L fabric, the marginal fabric is compatible in attitude with that expected for D_L. If it is a D_L fabric it must have developed preferentially in the weak, hydrous, rocks at the margins of these sheets. The relationships seen in the sheets at Lassintullich appears to suggest that these sheets were injected along the axial-planes of D₂ and that the D_L fabric (not developed in the country rock) is coincidentally parallel to S₂.

Glen Banvie Complex

Although this is one of the smallest of the Newer Granite intrusive bodies in the Scottish Highlands, it exhibits many of the features characteristic of the group as a whole. It occupies an approximately oval area of 0.7 km2, 5–6 km north-west of Blair Atholl, only part of which is within the Schiehallion district; it has been described in detail by Holgate (1951). Exposures are good only in the Banvie Burn and its tributary, Allt an t-Seapail (Table 3) and (Table 4))." data-name="images/P999975.jpg">(Figure 19), although boulder fields help to constrain internal boundaries. The junction with the adjacent country rock (psammites of the Grampian Group) is similarly poorly constrained except for contacts in these streams e.g. [NN 8430 6964]; [NN 8398 6910]; [NN 8341 6949].

Holgate identified three principal components of the Glen Banvie Complex: appinitic diorite, tonalite, and granodiorite.

These occupy, respectively, the north-eastern marginal strip, a roughly central belt and the south-western half of the intrusion as currently exposed, and are believed to represent a chronological sequence of intrusive events within the complex. Only the granodiorite component is exposed in the district, but for completeness the whole intrusion is considered in this account which is largely based on Holgate's (1951) study, but with additional field and petrographical information.

The appinitic diorite appears to have a steep external contact, probably faulted, with unmetamorphosed psam-mites to the north [NN 8430 6964] and north-east [NN 8433 6957]. The diorite and its internal contact with the tonalite are well exposed in the Allt an t-Seapail [NN 8427 6961] where the contact is sharp, steep and intrusive. No chilling is evident, but the age relationships are clearly indicated by the veins of tonalite locally penetrating the appinitic diorite. The granodiorite, in turn, shows an intrusive relationship to the tonalite, with granodiorite veins invading the tonalite, and enclosing xenoliths of it [NN 8401 6940]. Aplitic granite veins, apparently associated with the granodiorite, occur in the adjacent tonalite and similar veins are also found in the psammites close to the external margin of the granodiorite [NN 8400 6908]. This margin is steep where exposed in the Banvie Burn at the south-eastern edge of the complex, but Holgate suggested that the granodiorite is generally dome-like, and may be more extensive laterally than its present outcrop would indicate, especially on its southern margin.

The appinitic diorites are typically dark in colour, and are characterised by abundant mafic minerals, giving a colour index in the range 50–70. Holgate described pyroxene-rich (black) and hornblende-dominant (green) varieties, but the average rock type contains both minerals, with amphibole considerably in excess over pyroxene. The crystals tend to be euhedral or subhedral, and exhibit prominent zoning, with brownish green cores of pargasitic hornblende, and narrow, paler green, rims of edenitic hornblende. Clinopyroxene (Mg-rich augite) occurs as relict cores to many of the amphibole crystals. Holgate also referred to rare orthopyroxene crystals, and to serpentine–magnetite aggregates which he interpreted as pseudomorphs after olivine. Interstitial plagioclase (sodic labradorite zoned to oligoclase) is always present, and is generally accompanied by alkali feldspar and quartz, in places forming rudimentary micrographic intergrowths. Apatite is a common accessory mineral; biotite and magnetite occur locally as minor constituents. Most of the appinitic diorites are moderately altered, with hornblende (and augite) partially replaced by secondary amphibole, and the feldspars containing patchy aggregates of secondary muscovite (sericite), clay minerals and rare epidote. Secondary calcite may be present.

The tonalite is distinctly finer grained and more leucocratic than the appinitic diorite. In hand-specimen it is bluish or pinkish grey, with rare small phenocrysts of plagioclase, and may have a faint lamination caused by subparallel orientation of tabular feldspars and prismatic hornblendes. More distinctive is the widespread occurrence of spheroidal mafic clots, up to 5 mm in diameter, comprising aggregates of hornblende and biotite crystals. The tonalites are petrologically uniform throughout their exposed extent. Plagioclase is the dominant mineral, generally comprising about 50% modally. It tends to form subhedral crystals, ranging in size from 4 to 5 mm across, effectively small phenocrysts in the more typical 'groundmass' crystals averaging 0.5 mm across. The plagioclase crystals are all zoned, some in oscillatory fashion, and the overall composition range is An₃₅ to An₃. Minor alkali feldspar (microcline-perthite) is also present. Anhedral quartz crystals are similar in size to the smaller plagioclase crystals, and make up 15–18% modally. Of the mafic minerals, hornblende (edenitic) is more abundant than biotite; together they comprise about 25% modally of a typical tonalite. They occur both as scattered anhedral crystals and as irregular to sub-rounded clusters in which large primary hornblendes appear to have recrystallised to a fine-grained aggregate of hornblende and biotite. Sphene, magnetite, primary (?) epidote, apatite and zircon occur as minor accessories, in order of decreasing abundance. Alteration effects are present in biotite, which is partly replaced by chlorite, and in plagioclase, where turbid aggregates of secondary muscovite (sericite), epidote and clay minerals are commonly present. Minor interstitial calcite also occurs locally.

The granodiorites are generally coarser grained and slightly more leucocratic than the tonalites; they are essentially non-porphyritic, and are typically creamy grey in hand-specimen, except in proximity to the tonalite, where the feldspars are distinctly reddish. Plagioclase is the principal mineral (55–60% modally) and forms subhedral crystals of rather variable size, up to 10 mm in length. The cores typically display faint oscillatory zoning, while the margins are normally zoned. The overall composition range is An₂₅ to An₆. Quartz is the next most abundant mineral (around 20% modally) and forms crystals over a wide size range from clusters of large anhedral crystals up to 5 mm across individually, grading to a fine mosaic of interlocking quartz and plagioclase crystals. The general texture of the granodiorite is therefore seriate rather than truly porphyritic. Microcline-perthite (up to 12% modally) is also associated with the smaller plagioclase and quartz crystals; it tends to be interstitial to sub-poikilitic. The dominant mafic mineral is brown biotite, which forms subhedral crystals up to 4 mm in length. Hornblende (edenitic) is slightly less abundant than biotite, and forms smaller and more euhedral crystals. The colour index of the granodiorites is generally in the range 10–15. The minor accessory minerals are sphene, epidote, apatite and magnetite. Alteration is largely restricted to the feldspars, which develop turbid areas of secondary muscovite and clay minerals.

Holgate noted gradations to slightly more mafic and finer-grained granodiorites as the contact with the tonalite (and appinitic diorite) is approached. He also described a group of granodiorite boulders of this type (near the north-western margin of the complex) which contain abundant xenoliths, including appinitic diorite and local country rock, and rare mica-schists and amphibolites. Xenoliths are scarce elsewhere in the Glen Banvie Complex.

Whole-rock chemical analyses (and modal analyses) of five rocks from the Glen Banvie Complex were presented by Holgate (1951, table 1). These comprise two varieties of appinitic diorite, tonalite, and two varieties of granodiorite (normal type and slightly more mafic marginal variant). Three new whole-rock analyses (including trace-element data) of typical appinite, tonalite and granodiorite are presented here (Table 3). In addition, electron microprobe analyses of the mafic minerals in the main components of the Glen Banvie Complex are given in (Table 4).

Minor intrusive suite

The greatest concentration of the minor intrusions is in the north-western two-thirds of the district, bounded to the south-east by the Loch Tay Fault and to the south by the Carn Mairg watershed. They are dominantly NE–SW- trending steeply dipping dykes, and regionally appear to be the south-eastern termination of the swarm which has the Glen Tilt Complex and the Cairngorm Granite as its foci. To the east of the Loch Tay Fault the virtual absence of intrusions is particularly noticeable, although a marked increase is seen at the southern margin of the district south of the Lyon–Tay valley.

As remarked by Barrow et al. (1905) for the Blair Atholl district, the intrusions cannot be usefully categorised as either (discordant) dykes or (concordant) sills; some are controlled in their attitude by the bedding or schistosity, but mostly by a NE–SW-oriented fracture set. In this account the intrusions will be referred to generally as sheets, with the term dyke used only for the NE-trending, generally discordant sheets of the dominant swarm and the term sill used when a sheet is broadly concordant.

Typically the sheets range in thickness from one to 20 m, although one multiple intrusion west of Kinloch Rannoch [NN 6425 5700] reaches 60 m. Commonly they may be followed for several kilometres, making a noticeable topographical contrast with the usually softer sedimentary rocks. The sheets were accurately delineated as near-vertical dykes or, locally, concordant sills in the original survey, but several discordant sheets dipping at a low angle to the south have been mapped in the resurvey; two are particularly large, centred south [NN 685 565] and north [NN 660 603] of Kinloch Rannoch. The only stock-like intrusive body in the district is the Glen Banvie Complex described above.

Field relationships indicate a number of distinct igneous events, chiefly from cross-cutting relationships; although these are consistent, it is acknowledged that they may be of no more than local significance. The rock-types and field relations associated with these events are summarised in (Table 2).

Grey porphyritic microdiorite (fP)

The 60 m-thick sheet west of Kinloch Rannoch (the Beinn a' Chuallaich 'sill' of Flett (1905)), which runs for 12 km from the Allt Druidhe [NN 6425 5700] to [NN 7100 6700] north of Loch Errochty, is a very distinctive grey porphyritic microdiorite. It is a planar, concordant body, dipping at low angles south-east, parallel to the regional schistoisity. Abundant zoned grey or red feldspar phenocrysts, up to 15 mm long, sometimes accompanied by rounded quartz xenocrysts, are set in a fine microdioritic groundmass. Prominent exposures of the sheet occur north of Loch Rannoch [NN 649 594], all along the crags to Colrig [NN 677 643] and on the south-east slopes of Sron Chon [NN 692 657] north of Loch Errochty. In these exposures the sheet is seen to be locally discordant to bedding and to transgress upwards and southwards from the psammites of the Grampian Group into the Appin Group schists above. The thin northern extremity of the sheet follows the schistosity around the hinge of the Errochty Synform in the Appin Group to north of Errochty Dam [NN 719 665]; however, the sheet is unfoliated and it is clear that it is not folded by the Synform. The sheet is cut by, and contributes numerous xenoliths to, a microdiorite dyke of the main suite, 300 m north-east of the outcrop in the Allt Druidhe [NN 6445 5725].

Above West Tempar [NN 6830 5652], a 3 m-thick north– south vertical sheet of grey porphyritic microdiorite is cut by a branching, gently dipping sheet of main suite microdiorite, which in turn is cut by a NE-trending fault 600 m to the south-east. Elsewhere rocks similar in appearance to the grey porphyritic microdiorite only occur as a few thin NE-, N- or NW-trending sheets south of Tempar as far as Glen Lyon [NN 675 485].

A grey porphyritic quartz-bearing microdiorite dyke, of similar field appearance to the above, is exposed intermittently for 4 km immediately to the west of the Loch Tay Fault. It is best exposed on the north shore of Loch Tummel [NN 8034 5968] where the eastern contact of the strongly fractured, 12 m-wide dyke against shattered Blair Atholl Formation grey limestone is seen to trend 018° and dip at 80° to the east. Another grey dyke of porphyritic microdiorite, although not conspicuously quartz-bearing, occurs in grey Blair Atholl limestone south of Tombreck [NN 7665 5632] and may be a continuation of the above; the 1902 field-slips for Sheet 55 show this dyke extending over 2 km from [NN 7695 5648] south-west to [NN 755 554]. The field relations suggest that these grey porphyritic microdiorites pre-date the principal movements on the Loch Tay Fault.

Geal Charn breccias

On the upper slopes of Geal Charn [NN 688 549] several grey porphyritic microdiorite dykes are closely associated with a cluster of breccia-pipe like features (Figure 20). The field relationships suggest that both the dykes and the breccias occupy NNW-trending fractures and have a genetic relationship. Northwards [NN 6879 5508] a 10 m-wide dyke is exposed and breccia crops out 35 m to the south. In the track at [NN 6868 5500] an outcrop of breccia has the porphyritic dyke material as a matrix to the quartz schist fragments; rare xenoliths of dyke material also occur in the breccia. The breccias are cut by an ENE-trending fault, which to the south-west is occupied by a felsic microdiorite sheet of the main suite.

The areas of breccia vary from a few metres across to one irregularly shaped body 150 m across [NN 6863 5480]. The latter contains large angular blocks, up to 40 cm across, of pink quartzite with vuggy quartz and K-feldspar, as well as quartz-schist. One pipe [NN 6852 5555] is polymictic with amphibolite and mica-schist clasts. The matrix is mostly cominuted country rock and, although quartz veining and silicification are present, much of the quartz is late and vuggy. The breccias lack evidence of extensive coeval hydrothermal activity and therefore appear to be the result of gas fluidisation and subsequent collapse, the gases being derived from the dyke magma.

Similar breccia bodies associated with the late Caledonian intrusive suite have been recognised, especially in the Appin area and at Cruachan Cruinn (Platten and Money, 1987). Many of these are sub-volcanic pipes, whereas the linear, fault-controlled Geal Charn breccias may reflect a deeper level of formation.

Appinitic diorites (P)

Several coarse-grained, dark, mafic sheets are seen in the Grampian Group in the north-west of the district. A number of them, closely spaced and gently south-east dipping, can be traced from Edendon Water [NN 7157 7073] to the vicinity of Loch Con and possibly are continuous with similar sheets north and south of Loch Rannoch. At the Loch Con locality one of these sheets 3 m thick and subhorizontal, exposed on the main peninsula [NN 6882 6793], is cut by a 3–4 m-thick steep NE-trending felsite dyke, which contains small xenoliths of the appinitic diorite. This diorite, and another at [NN 6908 6791], contain irregular pink feldspathic veins, which appear to be segregation veins, unrelated to the later felsite. Another appinitic sheet forming a prominent peninsula further east [NN 6907 6793] also contains the pink veins. At the Edendon locality, in the river bed, 100 m upstream from the bridge [NN 7157 7073], three closely spaced sheets of appinitic diorite (one 2 m and two 4 m thick) are cut by a 130°-trending fault, which is occupied by granitic quartz–feldspar veins; on the adjacent A9 road-cuts e.g. [NN 698 715], it is possible to see that these veins predate the microdiorites exposed there. One dark appinitic diorite exposed in the A9 Wade Stone road-cutting [NN 6884 7171] is typically a gently dipping thin (1–2 m) sheet. Such sheets commonly show evidence of fractionation, particular evident in sheet I (Figure 4) at the Iron Bridge [NN 7091 5851] south of Dunalastair Water, where the upper part is relatively feldspathic.

Pegmatitic granite and quartz veins

In the A9 road-cuttings and in the River Garry, between eastings [68] and [77], thin (1–20 cm) quartz–feldspar veins traverse the Grampian Group psammites. Evidence from the new A9 road-cut shows that the distribution of these veins is more extensive than that shown on the original (1902) edition of Sheet 55; however, no detailed investigation of the veins has been undertaken in this resurvey nor are the veins sufficiently thick to be shown on the map. The veins vary from granitic to pure quartz veins, sometimes consisting of pegmatitic granite margins (rich in large pink feldspar) with quartz-rich interiors, perhaps representing successive pulses of increasingly hydrothermal aspect. White feldspar is also present locally. The veins contain a little muscovite and are commonly associated with pyrite and sometimes other sulphides (see below).

The veins are particularly well exposed in the road-cut east of the Allt an Stalcair [NN 695 716] (see Wade Stone road-cut of Glen Garry transect in Chapter 3). Here, they mostly dip at low angles to the north-west or south-east, although other near-vertical veins cross-cut these and there may be several generations. As noted above, at least one of these vein-sets is later than the appinitic diorite sheets in the nearby Edendon Water and in the Wade Stone road cut, and elsewhere veins predate the microdiorite dykes wherever the two are seen together; the veins also predate faults with small displacements, which are commonly exposed in these cuttings. As the veins are probably related to the granites of the Monadhliath area to the north of the district, these relationships are useful in establishing at least a local time sequence between the granites and the suites of minor intrusions.

One curious feature of the granitic veins in the Wade Stone road-cut [NN 6971 7158], and locally elsewhere, is that some of them appear to be folded by D₂ folds. However, in thin section the veins have no fabric that might be expected of such deformation and it is concluded that this is imitative intrusion along the already folded bedding.

Microdiorite suite (P^D and F)

The great majority of the igneous sheets comprise microdiorite suite rocks which present a wide range of textures and colours. In the field and on the map, these are generally described as either microdiorite (PD) ranging from relatively mafic to felsic, or felsite (F) for the more extreme felsic rocks. Petrographical examination allows a more precise nomenclature, which is discussed below. Two localities illustrate many aspects of this range: the Clunes road-cut on the A9 (Figure 21) contains eight sheets, within 600 m of continuous exposure, ranging from the more hornblende-rich to more felsic varieties; at Strath Fionan (Figure 4) are many of the varieties and field relations which are referred to below. Other notable localities are: south of Kenmore on Drummond Hill; south of Loch Rannoch where a variety of sheets occur; any road-cuts and river exposures along Glen Garry (see Glen Garry transect, Chapter 3).

Clear cross-cutting relationships are rare but a small number of east-trending sheets are cut by the NE-trending sheets which comprise the remainder of this suite. These earlier sheets mostly dip at low angles to the south and are generally more mafic than the majority of the suite. Some examples of this earlier set have been mentioned above: north-west and south-east of Kinloch Rannoch centred about [NN 660 603] and [NN 685 565] respectively; on Creag an Earra [NN 695 537], Geal Charn [NN 684 550] to [NN 683 558], Innerhadden [NN 668 569] to [NN 671 561] and in Strath Fionan [NN 7300 5654] to [NN 7426 5666].

The east-trending, south-dipping mafic microdiorite in Strath Fionan ((Figure 4), sheet V) is a particularly fine example of one of this earlier set; sheet VI is similar, but more altered. The weathering of the feldspar in sheet V causes the 1 cm-long acicular hornblende prisms to be well displayed. At its eastern end [NN 7426 5666], where it is cross-cut by a vertical NE-trending porphyritic microdiorite dyke ((Figure 4), sheet VII), the earlier sheet is deflected into a north-east trend and is apparently displaced some 40 m northwards. This locality supports the view that the early east–west sheets have been cut by NE-trending faults and fractures which have subsequently been occupied by the later members of the microdiorite suite. Additional circumstantial evidence that the east–west sheets are earlier than the remainder of the swarm is that they all belong to the more basic end of the microdiorite suite spectrum.

A similar relationship can be seen with the sheet at the east end of Loch Rannoch [NN 660 603] which, as previously noted, cross-cuts the early grey porphyritic microdiorite; this sheet, like at Creag an Earra [NN 695 537], is itself cut by NE-trending faults. Most of the remainder of the microdiorite suite has been emplaced in fractures with this north-easterly orientation, to form the principal dyke-swarm; where movement has occurred on these fractures the dykes, in many places, can be shown not to have been affected.

Most of the microdiorite sheets are more steeply dipping than this earlier set and may be collectively described as dykes. These are NE-trending and range in composition from mafic to felsic, as described below. Over 50 of these dykes can be counted in a south-east to north-west traverse across the district. Many of the dykes that traverse the Grampian Group have not had their petrography investigated in detail in the present resurvey. These were originally designated as felsite and porphyry on Sheet 55 (Geological Survey of Great Britain, 1902). On the present map, those dykes that cannot be precisely attributed are designated F (microgranitic rocks, unclassed). However, recent investigation of those dykes exposed in the Glen Garry road and river sections show that while some are felsic members of the microdiorite suite, many are more mafic microdiorites.

In the field two distinctions can be made within the microdiorite suite. Firstly, the more mafic dykes (PD) are dark grey-green, and some, on weathering, may reveal well-developed hornblende prisms while others contain discernible biotite. An example of the hornblendic variety from the east-trending set has been described above in Strath Fionan ((Figure 4), sheet V), whilst another, of the NE-trending set, occurs in the Clunes road-cut ((Figure 21), sheet VII). The prominent dyke west of Lochan Beoil, which is exposed in the Allt Strath Fionan ((Figure 4), sheet VII), is a good example of the biotite-bearing variety.

At the other extreme are the felsites (F). Some felsites are fine grained, generally non porphyritic and, whitish grey, pale yellow or pink; a striking feature is a closely spaced, foliation-like jointing parallel to their margins, in some cases accompanied by a rudimentary columnar jointing perpendicular to those surfaces and in others by a strong flow-lineation on the joint surfaces. Other felsites are noticeably porphyritic, with prominent white or grey feldspar and, less commonly, quartz. Good examples of both varieties of felsite are seen in the south of the district; the phenomena associated with the fine-grained type are particularly well developed in a 20 m-thick sill-like sheet south of Kenmore [NN 7772 4439], while the porphyritic type is exposed on the south shore of Loch Tay [NN 7732 4402], where a 3 m-thick sill occurs below a thinner, grey, fine-grained felsite. Other good sections occur along the forestry roads on Drummond Hill. These include a 6 m-thick, fine-grained, pink concordant sill [NN 7493 4523], a porphyritic felsite immediately to the east [NN 7510 4531] and an interesting sill [NN 779 468] showing a pilotaxitic texture in thin section (see below), with a disturbed upper contact and very porcellanous texture evident in the field. A good example of the felsites is a distinctive pink sheet which forms a prominent feature at the south end of the Black Tank road-cut [NN 7742 6791]; a similar sheet in the A9 Clunes road-cut (Figure 21 sheet V) is a 12 m-wide, vertical, pinkish grey quartz-porphyry. In the Allt Druidhe [NN 6473 5646], a felsite which exhibits the characteristic foliation jointing and the flow-lineation cuts through a more mafic microdiorite.

Between these two extremes is a range of fine- to medium-grained microdiorites, varying in colour from pale green to brown, sometimes pink or red, and porphyritic to aphyric in texture. The petrographical descriptions given below support the impression in the field that there is a complete spectrum of composition and texture from mafic to felsic types. However, on the 1:50 000 map an arbitrary distinction has been drawn, between felsites (F), the obviously felsic rocks described above, and the remaining more mafic microdiorites (PD).

Examples of the main varieties of the 'intermediate' microdiorites are as follows: north-east of Tombuie Cottage [NN 7917 4501]; on Drummond Hill [NN 7510 4531] above the pink felsite previously described [NN 7293 4523]; a pyroxene-bearing microdiorite in the River Lyon, below Achloa [NN 7662 4849], in Strath Fionan ((Figure 4), sheets II, III, VIII–X) where the many examples include the type 'Strowan (now Struan) Porphyry' of Flett (the Schiehallion dyke in Barrow et al., 1905, p.119) [NN 7263 5660] with feldspar and hornblende phenocysts in a red matrix (sheet III).

The Clunes road-cut [NN 782 672] to [NN 788 667] (Figure 21) contains dykes which range from the more mafic example (sheet VII) mentioned above, through sheets II and VI, varieties of the typical darker hornblende-bearing microdiorites, to the paler leucocratic varieties of sheets I and III. Sheet V is a pink-grey microdiorite with scattered quartz xenocrysts. In the road-cut, sheet IV is a coarse-grained felsite and clearly xenolithic. In the river section [NN 784 668], sheet IV has split into two branches dipping steeply south-east; the lower is approximately 10 m thick separated from its 4 m thick neighbour by a thin screen of metasediment, as exposed on the north-east side of the river, this screen appearing to widen on the south-west side. A third sheet downstream at this locality is almost vertical, and approximately 25 m thick. The microdiorite of all three sheets here is essentially similar, with a pinkish-grey colour, abundant small phenocrysts of pink feldspar and green ferromagnesian material, and rare xenoliths. There is also evidence of multiple intrusion, with internal chilled margins developed locally. The xenoliths are not abundant (much less than 1% by volume of the intrusions), but are very variable in size, shape and composition. Most of them are metasedimentary, probably of local derivation, and range up to 20 cm across (rarely 40–50 cm). Many of them are angular in shape, especially those with well-developed bedding or cleavage characteristics, but the quartzites frequently tend to be more rounded. The obvious igneous inclusions also tend to be ovoid in shape, and range from relatively coarse-grained granitic material to quite mafic diorite or appinitic diorite.

Two exposures of thin, dark grey, basic microdiorite dykes occur near Woodend, in Glen Lyon. The first dyke is 1 m thick, trends approximately NNE across the bed of a stream [NN 7098 4701], and is deeply and spheroidally weathered. A second, thinner dyke can be traced for 170 m with a similar orientation, and is intruded along one of a set of minor faults, with the same trend, on the east bank of the River Lyon [NN 7123 4722]. Neither show any evidence of faulting after intrusion. Two other occurences of similar, dark grey basic microdiorite dykes occupy NNE-trending faults. One, seen in the Innerhadden Burn [NN 6716 5732], is 2 m wide and pyritiferous; the other seen in the Allt a' Bhealaich [NN 7871 4500], south of Tombuie Cottage, consists of two dykes 3 m and 1 m wide, also mineralised.

Petrology

Terminology

The late Caledonian minor intrusions vary in composition from mafic (rarely ultramafic) to extremely felsic, although the dominant rock type is microdiorite. The terminological situation is confusing, partly because of the gradational nature of the variation represented, and partly because of changing nomenclatural fashions. Flett (in Barrow et al., 1905) recognised three main groups, namely porphyrites (diorite-porphyrites), porphyries (granite-porphyries) and lamprophyres, including the specific types kersantite, vogesite and spessartite. The terms felsite and quartz-porphyry (or acid-porphyry) have also been applied to minor intrusions of granitic composition. The term appinite has generally been used to describe relatively coarse-grained mafic diorites, such as the earliest component of the Glen Banvie Complex, and in this account the term appinitic diorite is retained for the group of relatively early and relatively coarse- grained mafic to ultramafic minor intrusions found mainly in the north-western part of the district.

Recent descriptions of Caledonian minor intrusive suites have tended to stress the gradational nature of the compositional spectrum represented, and follow Smith (1979) in referring to the microdiorite suite which encompasses the complete range from mafic to felsic compositions. On this basis, many rocks previously identified as lamprophyres would be called melamicrodiorites, and the more acidic types would be termed felsic porphyrites. Peacock et al. (1992), in describing the Caledonian minor intrusions of the Glen Affric district, have modified Smith's scheme in recognising the category of leucomicrodiorite, complementary to melamicrodiorite, and have proposed a quantitative mineralogical basis for this subdivision. They suggest that melamicrodiorites should be identified as having more than 50% mafic minerals, and the leucomicrodiorites as having less than 25% mafic minerals. However, the continued use of the term felsic porphyrite to describe the more acidic (silicic) members of the suite (quartz-microdiorites and microgranodiorites) is inconsistent, since it appears to exclude non-porphyritic varieties of these compositions. It would seem more appropriate to avoid the terms porphyrite and porphyry altogether, and to use the prefix porphyritic where necessary. Further, the distinction between leucomicrodiorite and felsic porphyrite is not explained, but presumably relates to the quartz content and the proportions of plagioclase and K-feldspar. The problem is that the progression towards more leucocratic rocks is generally accompanied by an increase in both quartz and K-feldspar, so that the overall composition may not remain strictly within the diorite field. In any case, these subtleties are not apparent in the field, where the colour change from dark green or dark grey, to pink or pale grey, and, in extreme cases, the appearance of quartz phenocrysts, are the best indicators of more acidic composition. Such rocks have traditionally been termed felsites or porphyritic felsites (including quartz-porphyries) in the field, and in this account the term felsite is retained for the most felsic (leucocratic and quartz-bearing) minor intrusions. For convenience, they are regarded as part of the microdiorite suite, although strictly they approach microgranite in composition. On this basis it is useful to subdivide the suite into three groups, namely melamicrodiorites (formerly lamprophyres), microdiorites and felsites, although it is recognised that all gradations exist, and some rocks are difficult to classify precisely because they lie on the boundaries of categories.

The classification scheme applied to these minor intrusions is summarised as follows:

The melamicrodiorites tend to be aphyric or microporphyritic, but the microdiorites and felsites are commonly mildly porphyritic. Highly porphyritic rocks (i.e. phenocrysts content > 30%) are unusual, with the exception of the grey porphyritic microdiorites.

Whole-rock chemical analyses (major and trace elements) of selected members of the minor intrusive suite (and related material from the Glen Banvie Complex) are presented in (Table 3), and their overall coherence in the form of a typical calc-alkaline trend when plotted on a AFM diagram is shown in (Figure 22). Microprobe analyses of representative ferromagnesian minerals (pyroxene, amphibole and biotite) are presented in (Table 4).

Grey porphyritic microdiorite

Although it is difficult to prove the coherence of this group in terms of precise age relationships, a combination of field and petrographical characteristics provides consistent evidence of its validity. In hand-specimen the rocks are all grey and highly porphyritic, with plagioclase the dominant phenocryst phase, but generally accompanied by subordinate hornblende, and rare quartz, which may be xenocrystic. The plagioclase phenocrysts are typically white, but take on a pink tinge in the more altered varieties.

In thin section, these porphyritic microdiorites consist of large euhedral plagioclase crystals (generally about 5 mm across, but up to 1 cm locally), many of which display prominent oscillatory zoning (mostly within the andesine range), and smaller euhedral hornblende prisms. These are set in a fine-grained, equigranular groundmass of feldspar (some plagioclase, but much of it indeterminate because of alteration), quartz, hornblende and biotite. Quartz is sufficiently abundant in the groundmass for some of these rocks to be classified as microgranodiorites but most are quartz-bearing microdiorites. Minor accessory minerals include apatite, zircon and sphene. Most examples are extremely altered, with sericitised feldspars and chloritised hornblendes. Epidote and carbonate are also present in the most altered varieties. An unusually fresh example occurs on the Trinafour–Loch Con track [NN 714 663].

The thickest intrusive sheet in the district is well exposed between Loch Rannoch and Loch Errochty [NN 6425 5700] to [NN 7100 6700]. The typical rock is conspicuously porphyritic in hand-specimen, with pink or white plagioclase phenocrysts set in a pale grey groundmass. The groundmass comprises plagioclase, biotite and hornblende, as well as micrographically intergrown K-feldspar and quartz. Secondary chlorite, epidote, muscovite and clay minerals are widely developed. Rare quartz phenocrysts (or xenocrysts?) have also been recorded (Flett, in Barrow et al., 1905), and this evidence, together with the relatively quartz-rich groundmass, indicates that the Beinn a' Chuallaich sheet is transitional between microdiorite and felsite, i.e. microgranodioritic.

The granitic rocks associated with the Geal Charn breccias resemble the grey porphyritic microdiorites, superficially at least. Locally the breccias consist of quartzitic country rock fragments in a porphyritic microgranodiorite matrix consisting of abundant phenocrysts of plagioclase, quartz, hornblende and biotite in a granular quartzofeldspathic groundmass. The plagioclase phenocrysts exhibit well-developed oscillatory zoning. The porphyritic microgranodiorite passes laterally into more homogenous and generally coarse-grained granodiorite or quartz-diorite. This consists of large subhedral plagioclase crystals (up to 1.5 cm in length and commonly displaying oscillatory zoning), hornblende, biotite and interstitial quartz. Minor accessory minerals include apatite, zircon, sphene and opaque oxide.

The superficially similar grey porphyritic rock in the Loch Tay Fault, exposed on the north side of Loch Tummel [NN 8034 5968], is characterised by relatively abundant quartz phenocrysts, as well as plagioclase and altered ferromagnesian minerals (hornblende and biotite), set in a granular quartzofeldspathic groundmass. The plagioclase phenocrysts do not show the well-developed oscillatory zoning which is characteristic of the other grey porphyritic rocks.

Appinitic diorite

The appinitic minor intrusions comprise one of the more distinctive and apparently coherent groups within the Caledonian suite, not only in terms of their distribution and field relationships (see above), but also their mafic (or ultramafic) and relatively coarse-grained lithology. In hand-specimen they are dark green in colour, and have a characteristically lustrous appearance, resulting from abundant large hornblende crystals or secondary sheet silicates such as talc or chlorite replacing primary ferromagnesian minerals.

From thin section evidence it is clear that there are two main components of what has been mapped as appinitic diorite. One is essentially ultramafic (apparently feldspar-free), consisting of abundant euhedral to subhedral crystals of a primary ferromagnesian mineral, now totally replaced by talc and other secondary sheet silicates. The primary mineral was believed by Flett (in Barrow et al., 1905, pp.130–131) to have been olivine, and the rocks were therefore described as peridotites. However, the characteristic magnetite-filled fractures of altered olivines are not present, and it is perhaps more likely that the original rocks were pyroxenites (possibly orthopyroxenites). Associated with these ferromagnesian pseudomorphs are varying proportions of edenitic hornblende, heavily chloritised biotite and interstitial areas of almost isotropic chlorite which tends to enclose the hornblende crystals and may represent original poikilitic clinopyroxene crystals. Other minerals include colourless secondary amphibole (which forms overgrowths on the primary hornblende crystals), sphene, opaque oxide and carbonate. Excellent examples of these ultramafic varieties of appinitic diorite are those in Edendon Water [NN 7157 7073]. Similar sheets occur in the Loch Con area [NN 69 68], where Flett (in Barrow et al., 1905, pp.130–131) recognised distinct hornblende-rich and biotite-rich varieties. However, an unusually fresh example from the shore of Loch Con [NN 6893 6806] contains approxinately equal amounts of hornblende and biotite, associated with talc pseudomorphs possibly after subhedral orthopyroxene which comprise approximately 50% of the rock.

The other main variety of appinitic diorite is less mafic, and is characterised by the presence of both hornblende and biotite, but the absence of pseudomorphs after olivine or pyroxene. The hornblende and biotite tend to be euhedral–subhedral, and typically comprise between 50 and 70% modally, with hornblende more abundant than biotite (Plate 7)a. They are accompanied by highly altered feldspar (some of it recognisably plagioclase) together with quartz and carbonate. The carbonate is particularly patchy in distribution, and locally forms distinct veinlets. In addition, large apatite needles are relatively abundant, and there are rare sphene crystals. These hornblende-rich appinitic minor intrusions have many features in common with the appinitic diorite of the Glen Banvie Complex, and it may be significant that both are relatively early members of their respective magmatic suites.

Good examples of these appinitic sheets are those described above in the Loch Con area. The prominent 3 m-wide sheet consists of abundant euhedral hornblende crystals and minor biotite, set in a matrix of altered plagioclase, quartz and carbonate, together with scattered apatite needles. Minor pink veins in the main appinitic sheet at this locality are believed to be segregation veins rather than offshoots from the later microdiorite that also cuts it. A similar (but more heavily altered) appinitic sheet [NN 6907 6793], also contains pink veins which are dominated by equigranular subhedral feldspar crystals (mostly plagioclase) with minor quartz, biotite, magnetite and sphene. Texturally and mineralogically these veins are unlike any of the normal microdiorites. Similar material forms the upper part of an appinitic diorite sheet at the eastern end of Dunalastair Water [NN 7091 5851] (Figure 4), and consists of an interlocking mosaic of plagioclase crystals, completely altered hornblende (and biotite) crystals and minor interstitial quartz; sphene and abundant apatite needles are also present.

Further north in A9 roadcuts close to the Wade Stone [NN 6884 7171], the top of a horizontal sheet of hornblende-rich appinitic diorite is characterised by patchy variations in grain size, and the local development of pink, carbonate-rich areas and quartz ocelli in an otherwise black or greenish black, biotite-rich rock. In thin section, large euhedral biotite crystals predominate over euhedral hornblende crystals which have been partly replaced by aggregates of smaller biotite crystals; together they comprise about 50% of the rock. The mafic minerals are set in a matrix of highly altered feldspar (some of it recognisably plagioclase), together with quartz and carbonate. The carbonate is particularly patchy in distribution, and locally forms monomineralic veinlets. In addition, large apatite needles are relatively abundant (compared with most meladiorites in the area), and there are a few large sphene crystals.

Pegmatitic granite veins

In thin section the granitic material of these veins consists of large irregular microcline crystals (some containing patches of quartz in micrographic intergrowth with the feldspar), smaller anhedral plagioclase and quartz crystals, and subhedral muscovite crystals. The microcline is extremely fresh, but the plagioclase exhibits extensive sericitisation. The only accessory minerals are opaque oxide and sulphides (Chapter 5). The grain size is rather variable and, although the rocks are generally pegmatitic in appearance, aplitic patches occur locally.

Microdiorite suite

Melamicrodiorite

The melamicrodiorites are relatively fine grained and appear black or dark green in hand-specimen. A typical example from the A9 road cut, west of the Allt an Stalcair [NN 674 718] is dominated by euhedral pargasitic hornblende crystals, some of which would qualify as microphenocrysts, but there appear to be all gradations to much smaller groundmass crystals. These strongly prismatic hornblendes commonly show a preferred orientation parallel to the intrusion contacts, and are interpreted as the result of flow during crystallisation (Plate 7)b. Some of the hornblende-rich melamicrodiorites tend to contain scattered phenocrysts of Ca-rich clinopyroxene (magnesian augite, typically Mg₄₆Ca₄₆Fe₈). Sheet IV from Strath Fionan [NN 7212 5615], (Figure 4) contains abundant pyroxene phenocrysts set in a fluxion-textured groundmass, dominated by hornblende but with some pyroxene present. Other pyroxene-bearing varieties occur in Strath Fionan, notably sheet V [NN 7326 5666] (Figure 4). Apart from a scattering of magnetite crystals, the remaining constituents in these rocks (less than 50% modally) are felsic, but mostly too fine grained and heavily altered to resolve. In the freshest examples, plagioclase appears to be the dominant feldspar, but it is very turbid, and is typically associated with secondary epidote. Small amounts of quartz may occur interstitially, probably accompanied by K-feldspar, although this has not been proved. Secondary chlorite and calcite are generally present. A slightly less mafic intrusion (transitional between melamicrodiorite and normal microdiorite), containing scattered augite phenocrysts (Plate 7)c, occurs at Achloa, on the River Lyon [NN 766 485].

A small, but distinctive group of 'basaltic' microdiorites was identified by Flett (in Barrow et al., 1905, p.117) as intermediate between lamprophyres (melamicrodiorite) and more typical microdiorites. Although he described them as kersantites, Flett commented on their textural resemblance to porphyritic (feldsparphyric) basalts. Good examples of this type of minor intrusion occur near Tombuie Cottage, south-east of Kenmore [NN 7917 4501], on the A9 near the Wade Stone [NN 692 719] and in the Innerhadden Burn [NN 670 568]. The Tombuie Cottage rock comprises plagioclase, augite and competely altered orthopyroxene or olivine phenocrysts, in a groundmass of interlocking plagioclase, brown amphibole and opaque oxide crystals. The Innerhadden rock lacks plagioclase phenocrysts but contains talc pseudomorphs of what may have been original olivine, as indicated by the euhedral crystal shapes and the pattern of magnetite-filled fractures.

Microdiorite

The less mafic varieties of microdiorite are rather variable in appearance, ranging from dark grey to pale grey, and commonly having a pinkish weathering. There seem to be all gradations from melamicrodiorites to leucomicrodiorites, as the proportion of felsic to mafic constituents increases. Many of these rocks contain feldspar phenocrysts, but they are rarely abundant, and aphyric varieties are by no means uncommon. Mafic phenocrysts occur in a few examples.

Plagioclase is always the principal mineral in the microdiorites, and the interlocking subhedral crystals dominate the texture of these rocks (Plate 7)d. It is always heavily altered to an aggregate of epidote, muscovite and clay minerals, and it is the secondary haematite associated with this alteration which is responsible for the characteristic pink mottling of many microdiorites. In the fresher examples the plagioclase crystals are seen to be strongly zoned, and the composition range is from sodic labradorite (notably the plagioclase phenocrysts) to almost pure albite. Alkali feldspar and quartz are also significant components of the more felsic microdiorites, (leucomicrodiorites) and they may form micrographic intergrowths. With increasing abundance of these constituents, the rocks grade into felsites.

The mafic minerals are commonly completely replaced by chlorite, but in the fresher examples they consist of hornblende or biotite (or both), in the form of subhedral crystals interlocking with the feldspars. Fresh augite is rare, but when present it is significantly more Fe-rich (Mg/(Mg + Fe) = 0.70–0.75) than in the melamicrodiorites (Mg/(Mg + Fe) = 0.85). Opaque oxides, both magnetite and ilmenite, are common accessory minerals; apatite and sphene are also typically present as minor constituents. In addition to the secondary minerals already described, secondary calcite in ubiquitous.

The microdiorites are the most abundant and widespread of the late Caledonian intrusive suite, and are particularly prominent in the A9 road and the river sections in Glen Garry ((Figure 21) and Glen Garry transect, Chapter 2), and in Strath Fionan (Figure 4). Some specific intrusions and rock types merit more detailed description.

One of the more distinctive types of felsic microdiorites is that belonging to the 'Strowan Porphyries' of Flett (in Barrow et al., 1905, p.118), which take their name from Struan on the River Garry. The best known example is sheet III in Strath Fionan [NN 7263 5660] (Figure 4). The rock is pink, with dark green or black blotches, probably representing original phenocrysts of hornblende, and brick-red feldspar phenocrysts. In thin section the dark patches consist largely of chlorite but with rare relict hornblende, and the feldspar phenocrysts are plagioclase, although intensely altered. The groundmass is dominated by interlocking subhedral plagioclase crystals, heavily altered for the most part, and hornblende crystals now largely replaced by chlorite and epidote. Interstitial quartz is relatively abundant and locally forms micrographic intergrowths with murky K-feldspar. Accessory magnetite and apatite are also present.

Felsite

As with the microdiorites, the felsites may appear grey or pink in hand-specimen, and the only safe distinguishing feature is the presence of quartz phenocrysts, or very abundant quartz in the rock as a whole. In fact, most of the true felsites are uniformly pink in colour, whereas the apparently similar pink microdiorites tend to be more mottled.

A typical porphyritic pink felsite contains euhedral feldspar phenocrysts and slightly embayed quartz phenocrysts in equal abundance. These are set in an equigranular groundmass of quartz, K-feldspar and muscovite (Plate 7)e. Micrographic intergrowths of quartz and feldspar are commonly present; mafic minerals are very scarce or absent. A good example of this type of felsite is an 8–9 m-thick sheet exposed in the A9 Black Tank roadcut [NN 7742 6791]. An example of a felsite which is more transitional to microdiorite is the prominent sheet close to the north-western margin of the Glen Banvie Complex [NN 8360 6954] (Table 3) and (Table 4))." data-name="images/P999975.jpg">(Figure 19). This rock is strongly porphyritic, but with quartz phenocrysts significantly less abundant than plagioclase phenocrysts, and a groundmass of plagioclase, K-feldspar, biotite and quartz. The biotite forms partly chloritised, euhedral microphenocrysts, and there are larger mafic phenocrysts, now completely chloritised, which may represent original hornblende crystals.

Excellent examples of aphyric (or mildly porphyritic) felsite sheets are well-exposed along and above the forestry track on Drummond Hill between [NN 746 452] and [NN 752 454]. These subhorizontal sheets are very pale and fine grained, and consist of a felsic aggregate of feldspar needles and quartz, some displaying a trachytoid texture.

In general, felsites are considerably less abundant than microdiorites, but they are equally widespread in occurrence. All these features reinforce the impression that the microdiorite suite represents a spectrum of compositions from mafic to felsic.

Late Carboniferous quartz-dolerite dyke

A dolerite dyke, believed to be one of the northernmost representatives of the Late Carboniferous intrusive suite which is well developed in the Midland Valley of Scotland, crosses the district. This E–W-trending vertical dolerite dyke, 10 m wide, is exposed in the Allt a' Bhealaich, below Tombuie Cottage [NN 787 450] (Figure 31), and may be the same intrusion as the 3 m-wide, ENE-trending dolerite dyke in the Urlar Burn [NN 825 450] (Figure 30). The dolerite is considerably fresher than any of the late Caledonian minor intrusions, and consists of interlocking subhedral plagioclase crystals (with bytownite cores and margins zoned towards sodic andesine), two clinopyroxenes and ilmenite (Plate 7)f. The dominant pyroxene is augite which forms anhedral crystals, locally subpoikilitic towards the plagioclase. Pigeonite forms smaller crystals, each of which tends to be located at the core of a single augite crystal. Very minor quantities of micrographically intergrown quartz and K-feldspar, associated with apatite, chlorite and calcite, occur in the interstices of the plagioclase-pyroxene framework. These are believed to represent the silica-rich residual magma, developed by in situ crystal fractionation, and are characteristic of the quartz-dolerite suite as a whole. A chemical analysis of the quartz-dolerite dyke is presented in (Table 3); when plotted on an AFM diagram (Figure 22) it is quite distinct from the late Caledonian trend. Microprobe data on the pyroxenes (Table 4) show these to be co-existing Ca-rich (Ca₃₆ Mg₄₈ Fe₁₆) and Ca-poor (Ca₉ Mg₆₂ Fe₂₉) monoclinic pyroxenes, similar to those in quartz-dolerite dykes in the Midland Valley (Armstrong et al., 1985).

Chapter 5 Metamorphism

The Dalradian sedimentary successsion, together with the pre- to synorogenic igneous intrusions, has undergone regional metamorphism in the lower amphibolite facies. The metamorphism was first described by Barrow et al. (1905) in the memoir that accompanied the earlier edition of Sheet 55. Subsequently, aspects of the metamorphism were treated by Elles and Tilley (1930) in their review of the structure and metamorphism of the Highlands, and by Bailey and McCallien (1937) in their re-appraisal of the structure of the Schiehallion district. Textural criteria for the relationship between mineral growth and structural events were examined for the Schiehallion area west of the Loch Tay Fault by Rast (1958b) and east of the fault by Sturt and Harris (1961).

Maps of metamorphic zones were presented by Elles and Tilley (1930), Bailey and McCallien (1937) and Rast (1958a); the diagram in the margin of the 1:50 000 map is modified after these and after Nell (1984), in the light of new observations.

In recent years there have been several attempts to estimate P/T conditions in the Central Highlands. Wells and Richardson (1979) in their investigation sampled garnet-kyanite-plagioclase-quartz and garnet-biotite assemblages at two unspecified localities in the district, one near Kinloch Rannoch (probably the Killiecrankie Schist) and one near Schiehallion (probably the Meall Dubh Graphitic Schist of Strath Fionan). They determined pressures of 9–12 kbar and temperatures of 550°–620°. Wells (1979) using plagioclase-hornblende-quartz-garnet-epidote assemblages from the Grampian Group in the district, determined pressures of 11.3 kbar and temperatures of 555° ± 50°. Moles (1985a) used a variety of geobarometers from the baryte/base metal horizon, and its host rock, at Foss Mine and estimated pressures of 8–10 kbar and temperatures of 540° ± 580°. Bendall (1995) used hornblende-garnet assemblages from the Farragon Volcanic Formation and the Ben Lawers Schist at Slatich [NN 634 477] and Boreland [NN 717 450] to estimate peak metamorphic temperatures of 638° and 583°, respectively.

Field characteristics

Pelites and semipelites, which occur in all formations throughout the district, are characterised by the presence of garnet and biotite. The former mineral is particularly prominent in pelites in the Pitlochry, Ben Lui, Killiecrankie and Beoil schists. Garnet is only absent in the Ben Lawers and Ben Eagach schists to the south of Glen Lyon. Biotite is the principal phase defining the ubiquitous S₂ schistosity, usually together with macroscopic muscovite and (retrogressive) chlorite.

To the west of the Loch Tay Fault, in the extreme west of the district and north of Kinloch Rannoch and Schiehallion, kyanite is commonly evident in the field in the graphitic formations (Ben Eagach, Blair Atholl and Meall Dubh schists) as well as locally in the Ben Lui and Killiecrankie schists. Large blue crystals, in association with quartz veins and pods, are commonly seen in the Killiecrankie and Blair Atholl graphitic schists. Staurolite is less visible in the field, but commonly accompanies kyanite south of Strath Tummel. To the east of the Loch Tay Fault, kyanite has not been recorded and staurolite only in graphitic parts of the Ben Lui and Pitlochry schists. Neither kyanite nor staurolite has been recorded in the Grampian Group of the district.

Other porphyroblastic minerals visible in the field include plagioclase (especially in the Appin Group pelites and in the Ben Lawers Schist), microcline (especially in the Grampian Group semipelites), muscovite (in the Beoil Schist) scapolite (adjacent to amphibolite, especially in Blair Atholl limestones) and pyrite (especially in the Tempar Dolomitic Member). Chlorite and plagioclase, together with quartz, occur as coarse pegmatitic veins and pods, especially in the Blair Atholl, the Meall Dubh and the Beoil schists of Strath Fionan; zoisite crystals, up to 100 mm in length, have been recorded in such pods in the Blair Atholl graphitic schist south of Lochan an Daim [NN 720 566].

Hornblende is commonly a prominent mineral, particularly in the pre-orogenic igneous sheets, but also in the Ben Lawers Schist, except in the outcrop south of the River Lyon. It is also evident in parts of the Farragon Volcanic Formation. Amphibole, hornblende (edenite) as well as tremolite-actinolite, is usually evident in the calc-schists of the Tempar Dolomitic Member, in the Green Beds, as well as in the impure limestones that occur in many formations. The calcitic marbles of the Blair Atholl Subgroup, well seen on the south side of Strath Fionan, commonly exhibit a strong calcite fabric oblique to bedding. Although this is usually sympathetic to individual D₂ minor folds and the sense of cross-cutting of S₂ in the adjacent schist, a later calcite fabric may also cross-cut such folds and appears to be unrepresented in the schists.

In the psammites, especially where impure, the D₂ schistosity and lineation usually dominate the structure. These features, especially the stretched pebbles, are strongly developed in the Meall Dubh Quartzite of Strath Fionan and more locally in the pebbly beds of the Carn Mairg Quartzite and Pitlochry Schist. Exceptionally, the schistose quartzite of Meall nan Aighean [NN 730 555] has preserved cleavage axial-planar to D₁ folds, where protected by the envelope of the more massive Schiehallion Quartzite.

Certain field-based relationships between the time of mineral growth and the minor structures, discussed in Chapter 3, can be established. In the pelites and semipelites, the ubiquitous biotite, together with muscovite, is dominantly parallel to S₂ surfaces. In a few cases, for example in the semipelites of the Pitlochry and Killiecrankie Schists, fine-grained muscovite together with biotite, can be detected on discrete S₁ surfaces, crenulated by S₂. Both minerals are rotated into parallelism with the later crenulation cleavages. Biotite, especially when porphyroblastic, usually shows a linear orientation on S₂, subparallel to D₂ minor folds and the S₀/S₂ intersection lineation.

Garnet porphyroblasts are generally mantled by the S₂ surfaces and commonly exhibit quartz pressure-shadows. Straight or S-shaped inclusion trails are present in many large (> 10 mm) garnets. These features are particularly developed in some pelites (especially where graphitic) in the Pitlochry, Ben Lui and Killiecrankie formations and in the Appin Group formations of Strath Fionan. Garnets in amphibolites in the Farragon and Beoil Schist formations commonly display distinctly asymmetric pressure shadows (Chapter 3; (Plate 8)).

In the amphibolites, and especially in the Farragon Formation, large (> 5 mm) hornblendes lie within the S₀/S₂ foliation and locally show strong preferred orientation subparallel to D₂ fold hinges. On the other hand, smaller hornblendes are clearly folded around the D₂ hinges, which suggests that there was an amphibole fabric originally parallel to S₁. In the fold hinges, large hornblendes are more rarely seen to be aligned parallel to the axial-surfaces. In the Ben Lawers Schist, exceptionally large (up to 60 mm) hornblendes cut across S₂, or are randomly arranged along S₂, but like all other hornblendes are folded by later fold phases.

As with amphibole, kyanite occurs both as small (< 5 mm) crystals locally parallel to S₂ and larger crystals (typically 15 mm but up to 100 mm in the Meall Dubh Schist of Strath Fionan) randomly arranged on S₂ as well as cross-cutting it.

Thin section characteristics

The essential textural relationships and mineralogy of formations have been described in Chapter 2 and those of the pre- to synorogenic intrusions in Chapter 4. Below, a more detailed account is given of the textural and fabric relationships which have been used, in conjunction with the field observations, to establish both the structural history discussed in Chapter 3 and the timing of metamorphic events relative to that history.

Pelitic and semipelitic rocks

Biotite and muscovite

These are the common schistosity-forming minerals, together with secondary chlorite. As illustrated in (Figure 23), column A, the development of S₂ shows all stages from an open to a tight crenulation of S₁ (slide number 2544; 2811; (Plate 9d)) with mica recrystallisation on limbs, to a sub-penetrative fabric where S₁ is hardly detectable (2546; (Plate 9a) and (Plate 9b)). In the semipelites, and the graphitic pelites particularly, fine-grained muscovite defines S₁ planes oblique to, or folded between, the coarser biotite and muscovite that marks the dominant S₂ planes (2582). Coarse biotite also commonly has recrystallised subparallel to the S₁ muscovite, giving rise to an anastomosing fabric (2582). In the more psammitic semipelites, isolated and less well-oriented biotite crystals may mark S₂.

Biotite also occurs as small, early, syn-D₂ porphyroblasts. These are especially well developed in the Meall Dubh Striped Pelite (2390) and Graphitic Schist of Strath Fionan, where they are oriented parallel to S₂ and contain straight or slightly curved inclusion trails of S₁, oblique to the penetrative S₂ fabric. Biotite also occurs as more randomly oriented syn-D₂ porphyroblasts (up to 0.5 mm long) in the pelites of the Ballachulish and Blair Atholl subgroups (2929), where they preserve open microfolds of graphite trails of both bedding and S₁.

Garnet

Garnets exhibit certain characteristics throughout the district, as observed in the field. They are commonly augened by the S₂ fabric, which shows the variety of microfolding of S₁ and they have pressure shadows (usually of quartz) elongated in S₂ (slide number 2843). The larger porphyroblasts (> 0.5 mm) usually contain inclusion trails of the S₁, dominantly of quartz and opaque minerals and less commonly of mica, noticeably finer grained than the external S₁ or S₂ fabrics (2843, 2847, 2811; (Plate 9a), (Plate 9b), (Plate 9c). In some instances the internal S₁ can be followed into the external microfolded S₁ fabric. In the Pitlochry Schist, ilmenite crystals parallel to S₁ and folded between S₂ planes, can be traced into the internal inclusion trail fabric; individual ilmenite crystals are bent as they cross the garnet margin (2811, (Plate 9d)). Rarely, in the Ben Lui Schist (2795; (Plate 9a)) and in the Pitlochry Schist, bedding is marked by concentrations of quartz inclusions within garnets; the S₁ fabric is marked by the obliquity of the elongate inclusions to the bedding.

The implied time relationships of growth of garnet to deformation history, suggested by these trails, has been extensively discussed in the literature (see review by Passchier and Trouw, 1995) and that in the Dalradian of the Schiehallion district and adjacent areas has received particular attention (e.g. Harte and Johnson, 1969; Rast, 1958b; Sturt and Harris, 1961; Phillips and Key, 1992). It is not proposed to examine the great variety of fabric and garnet relations in the rocks of the district but, as discussed below, the majority of these relations indicate, unambiguously, garnet growth, before, or at various stages through the D₂ deformation. Observations have been made mostly on garnets cut perpendicular to the local D₂ microfold lineation.

The variations in matrix textures, as a result of garnet growth, and of inclusion trail patterns within the garnets are dependant on the time of growth with respect to the D₂ deformation, as illustrated in (Figure 23), columns A–C. The commonly observed textures and patterns illustrated in (Figure 23), column D, are interpreted to be the result of slow continuous growth of garnet (syntectonic) throughout D₂.

The principal variations in textures and inclusion patterns, and their interpretation, are as follows.

1. Straight, slightly curved towards the margins (slide number 2811). This indicates post-D₁ growth (but possibly early D₂) before microfolding fully developed. Rarely, such garnets contain fine-scale bedding, to which the S₁ trails may be slightly oblique (2795, (Plate 9a)). The S₁ trails are commonly at a high angle to the external S₂, whereas the S₁ fabric outside exhibits either a distictly smaller angle to S₂, or is microfolded, or is destroyed by the D₂ recrystallisation (2817, (Plate 9c)).
2. Microfolded, indicating growth of garnet after early D₂ microfolding had developed. The included S₁ microfold pattern is always less intense than that between the S₂ planes outside the porphyroblast (2777, (Plate 9b)). Rarely, an S₂ fabric is associated with the internal microfolds. This pattern only occurs in the south of the district.
3. S-shaped, with slighty diminishing curvature towards the margins (2847; 2777, (Plate 9b)). This common form indicates early D₂ growth of garnet as microfolding of the S₁ fabric was developing; the relative rotation of the garnet with respect to the external S₁ fabric produces a complex spiral pattern, which in most cuts gives an S-shaped pattern, but other patterns occur (Powell and Treagus, 1970).

Some porphyroblasts have distinct rims which contain a different inclusion pattern, and/or a coarser size, from that of the core, suggesting two distinct growth periods within the D₂ period (2328; 2777, (Plate 9b)). Rims containing no inclusions, and apparently truncating S₂, are common in the Glen Lyon area; they are interpretated as an indication of post-D₂ growth. The size of inclusions commonly coarsens towards the rim, or within certain parts of the S-shaped patterns (2847); this might suggest growth during the coarsening of the S₂ matrix, but it may also be the result of a cut-effect through the complex shape of the S-shaped spiral of inclusions.

Pressure-shadows may be symmetrical (2847; 2777, (Plate 9b)) or curving and asymmetrical with respect to the external S₂. When asymmetrical, and viewed to the north-east or north (the common D₂ microfold directions), the curvature is that commonly interpreted as indicating a top-to-the south-east or south sense of shear (2788N; (Plate 8)).

Spessartine has been tentatively identified in the Killiecrankie Schist on the south side of Dunalastair Water [NN 6939 5903] and in the Pitlochry Schist near Tombuie Cottage [NN 7828 4479] (2778), where small pink crystals are crowded in thin beds in the manner of coticules.

Kyanite

In the graphitic schists of the Appin Group, the large (up to 30 mm long) kyanites overgrow small garnets, which themselves contain fine straight or slightly curved inclusion trails of S₁ (2328). The strongly microfolded S₁ in the external rock fabric, and more rarely the coarse S₂ fabric, is preserved in the kyanites, which are thus clearly of post-D₂ age. Most kyanites have grown with a random orientation; a strong preferred orientation, locally present on S₂, appears to be a result of mimetic crystallisation.

Staurolite

Like kyanite, with which it is commonly associated in the Blair Atholl graphitic schists, staurolite partly mantles garnet (slide number 2319). Small staurolites are common in the Meall Dubh Striped Pelite and the Blair Atholl sugroup pelites; they truncate large S₂ biotite and exhibit no pressure shadows or augening of S₂, in contrast with the garnets of similar size in the same rock. They have preserved microfolds of S₁ marked by trails of elongated quartz grains, and rarely the coarse S₂ fabric is also overgrown (2328). Within both staurolite and kyanite, the microfolds of S₁ are generally better preserved than those outside the porphyroblasts, where S₂ recrystallisation must have outlasted porphyroblast growth.

Feldspar

Plagioclase (oligoclase) porphyroblasts, up to 5mm across, are common within the graphitic schists, particularly the Ben Eagach and those within the Appin Group. They commonly include small garnets and more rarely small kyanite and staurolite grains (slide number 2319). They preserve D₂ microfolds of S₁, as trails of fine graphite (2328) or quartz (2420) which, being coarser than that in adjacent garnet, suggests later growth of the feldspar. However, the plagioclase growth appears to have occured before the final recrystallisation of the external fabric, in which the quartz, biotite and graphite grains making up S₁ are coarser. Plagioclase porphyroblasts, up to 10 mm across, are a feature of the late amphibolites (Plate 6); they are interpreted as recrystallised phenocrysts.

Microcline occurs as porphyroblasts, up to 5 mm across, in the Meall Dubh Schist (2319); it includes both kyanite and garnet and is not augened by S₂ and thus appears to entirely postdate D₂ and the other porphyroblast growth. Microcline also occurs as porphyroblasts in the semipelites of the Grampian Group (2386), but the timing of growth is not clear.

Accessory minerals

Apatite, sphene, zircon and iron sulphide (possibly pyrite) are common in the Grampian Group and are present in most pelites and semipelites above, together with tourmaline, graphite and rutile in various proportions. Zoisite occurs in the Blair Atholl graphitic schist. Certain beds in the Pitlochry Schist of Drummond Hill (slide number 2809) are rich in ilmenite (Plate 9d) and 1–10 mm thick graphitic beds are almost totally made up of small apatite grains (2835), whilst others are exceptionally rich in apatite, sphene and zircon (2711).

Amphibolites and volcaniclastic rocks

In the strongly foliated amphibolites, large hornblende crystals (5–10 mm) generally form a pentrative S₂ fabric, but in some rocks small (0.2 mm) hornblende crystals define an S₁ fabric which is folded by the D₂ microfolds (slide number 2815). At the hinges of the D₂ folds, larger hornblendes (0.5 mm) are axial planar to the folds, although some of these have also recrystallised mimetically around the fold hinges (2794). The larger hornblendes commonly contain inclusion trails of a quartz fabric, which is finer than that outside. Garnets in the same rock contain similar size quartz inclusions and a few of fine hornblende. These suggest that fine-grained, possibly actinolitic hornblende that grew during S₁ is preserved in the garnet, but was coarsened outside during D₂. The large hornblendes show a similar variety of inclusion trail patterns to those in the syn-D₂ garnets.

Epidote and clinozoisite are common constituents of the bedded calcareous amphibolites (interpreted as tuffs) that occur in most formations in the Argyll Group, particularly in the Farragon Formation (2796) which is of clear volcaniclastic origin, as well as in the Green Beds. Assemblages with these minerals contain combinations of hornblende, chlorite, biotite, garnet, plagioclase, quartz, biotite and muscovite. Fine inclusions of epidote within garnet (2794) and large hornblende (2954) suggest that the mineral first grew during D₁. Rarely clinozoisite forms D₂ porphyroblasts with fine inclusion trails. These beds are particularly rich in accessory minerals; magnetite, zircon, apatite, sphene and pyrite are common.

Calc-silicate rocks

Impure calcareous rocks, as exemplified by the Tempar Dolomitic Beds and the Ben Lawers Schist, provide a range of lithologies between the calcareous amphibolites discussed above through the calcareous schists and psammites to almost pure dolomites. Many of these assemblages are combinations of the usual pelite- and psammmite-forming minerals together with calcite and/or dolomite, and with additional proportions of tremolite/actinolite, zoisite and clinozoisite. Of note is the occurence of margarite, e.g. in the Meall Dubh Limestone (slide number 2460) of Strath Fionan, and of wollastanite, vesuvianite and diopside with calcite and quartz at Foss [NN 7828 5722] (2957). The latter locality is at the predicted position of a slide within the Blair Atholl Formation. Vesuvianite is also seen as 5 mm-long, randomly oriented crystals containing fine inclusion trails, in the calcareous Ben Lawers Schist of the Meall Tairneachan area.

Hornblendes show the same characterists as described for amphibolites; tremolite blades commonly contain delicate sigmoidal S₁ inclusion trails of graphite (2327).

Growth of minerals relative to the post-D₂ deformation

Both biotite and muscovite recrystallised parallel to SL, Se, Sc and S₃ crenulation cleavage planes and cut across minor fold hinges of these ages (Plate 4b and c).

Curved inclusion trails in the margins of some garnets in the Glen Lyon area appear contiguous with D_L crenulations, but there is no indicatation that garnet growth lasted into D₃.

All the other porphyroblast phases discussed above, with the exception of plagioclase, clearly predate all the later crenulations, including those of the D_L phase which are strongly developed in the Glen Lyon area. The S₂ schistosity, which augens many of these porphyroblasts, is folded by the later phases (e.g. (Plate 4b), (Plate 4c) and slide 2817, (Plate 9c)) and individual bladed forms may be broken or folded. Large plagioclase porphroblasts, in the Ben Eagach Schist of the Glen Lyon and Meall Tairneachan areas have overgrown D₃ crenulations, but their age with respect to D_L or D_c is not clear.

Chapter 6 Economic geology

The metalliferous mineralisation of the Schiehallion district comprises both pre-tectonic stratabound mineralisation, and epigenetic mineralisation which postdates the metamorphic and folding events. A fuller, detailed account of the mineralisation can be found in Pattrick and Treagus (1997).

Stratabound mineralisation

Aberfeldy baryte–sulphide mineralisation

The existence of extensive stratiform mineralisation in the Ben Eagach Schist north of Aberfeldy was established by the BGS in 1975–1978 during regional geochemical exploration (Figure 24), (Figure 25). Soil, rock and overburden (till) surveys in 1976–1978 and the drilling of 11 boreholes in 1977 delineated the mineralisation which was recognised as having major economic potential as a source of baryte and base-metals (Coats et al., 1980; Smith et al., 1984; Hall, 1993). The mineralisation is divided into three concessions; the mineral rights to the Western Sector (Foss East and Foss West) and Eastern Sector (Duntanlich) (Butcher, 1991) were acquired by Dresser Minerals in 1979, while the Central Sector (Ben Eagach, (Figure 24)) is owned by the Edradynate Estate. Only the Foss area occurs in the Schiehallion district, and is described here. An underground mine and opencast pit were opened at Foss in the Western Sector in 1984; these are operated by MI of Great Britain Ltd and produce approximately 50 000 tonnes per annum of drilling grade baryte for use in the North Sea oil industry. The Western Sector (Figure 26) contains a reserve of 10 Mt baryte, including 2 Mt at Foss Mine. Further exploration focussed on the Ben Eagach Schist and revealed several horizons enriched in barium (as baryte and Ba-silicates) (Coats et al., 1984; Gallagher et al., 1989).

The mineralisation has been extensively studied, notably in reports and publications by the BGS (Coats et al., 1978; 1980; 1981; Parker, 1980) and in the most detailed work on the Foss area, the PhD thesis of Moles (1985a), whose comprehensive account is based on field data, early mining information and drill core (BGS and Dresser Minerals). The most recent summary of Dalradian stratiform mineralisation is given by Hall (1993).

Mineralised horizons

The Aberfeldy mineralisation is largely hosted by the Ben Eagach Schist which crops out on the ridge of high ground between the Tay and Tummel valleys north of Aberfeldy (Figure 24), (Figure 25). The total extent of the mineralisation revealed so far is 7 km, from Creag an Fhithich (Duntanlich) [NN 873 567] in the east to Meall Tairneachan [NN 807 547] in the west (Figure 24). The mineralised horizons are mainly situated on the southern limb of the D₁ Creag na h- Iolaire Anticline (Figure 26), although extensive D₂ and later D₃ folding have complicated the outcrop pattern and cause considerable problems for the miners. The total thickness of mineralised rock reaches 80 m, and the maximum thickness of baryte is 15 m, in Frenich Burn East [NN 8196 5492]. The mineralised horizons are hosted by graphitic, muscovite schists and are largely close to or at the contact with the overlying Ben Lawers Schist. The high-grade mineralisation is characterised by conformable layers of pyritiferous baryte and quartz-celsian (Ba-rich feldspar) rock, although barium-enriched muscovite schist and sulphide-bearing graphitic schists are volumetrically the most abundant rock types. Massive sulphide layers are rare but significant components; the thickness and relative importance of the mineralised lithologies varies considerably along strike and down dip.

The baryte is the mineral of prime economic interest, forming several conformable units. The occurrence of quartz-celsian rock is closely related to the baryte horizons but is more extensively developed. Natural exposure of the mineralisation is generally poor, the most significant natural outcrops occurring on Creag an Chanaich [NN 8122 5457], in Frenich Burn East and on Creag an Loch [NN 8267 5504]. In the area of Foss Mine, the mineralised horizon has been extensively exposed in the opencast workings [NN 815 545], the access road on Creag an Chanaich and the road leading to Creag an Loch. The adits to the underground workings are in the north-eastern face of Creag an Chanaich [NN 8139 5450] and these have exposed the mineralisation at depth to the west. In addition, the mineralisation was encountered in exploration and development boreholes drilled by the BGS and Dresser Minerals.

Foss West

Western exposures

In the Foss West area, the two main mineralised horizons are called the Upper and Lower Mineralised horizons. Their most westerly expression is seen in limited exposures on the (southern) margin of the mine track. The Lower Mineralised Horizon is represented by an outcrop of quartz-celsian rock and white, crystalline baryte [NN 8072 5473], and the Upper Mineralised Horizon by an exposure of pyritic, graphitic schist and quartz-celsian rock [NN 8073 5468]. To the west of these outcrops, the Ben Lawers/Ben Eagach Schist contact is clearly defined and the overlying Ben Lawers Schist is well exposed to the south-east. The Upper and Lower Mineralised horizons were exposed in exploration trenches in the peat-filled depression between Meall Tairneachan and Creag an Chanaich but only surface debris now remains [NN 8092 5465]. The Upper Mineralised Horizon forms a 9 m-wide outcrop on the east side of the peat-filled hollow [NN 8095 5461] where massive baryte, with pyritic layers, grades upwards into cherty lithologies. Galena and sphalerite are present in the baryte and quartz-celsian. The Lower Mineralised Horizon is exposed 60 m to the NNE, where it comprises massive, white sugary baryte at least 3 m thick.

Creag an Chanaich

On the western slope of Creag an Chanaich, the Upper Mineralised Horizon is represented by massive baryte, overlain by quartz-celsian rock [NN 8109 5461]. It is also exposed for 80 m on the ridge of Creag an Chanaich [NN 8114 5459] to [NN 8122 5457] where the 3.5 m baryte containing pyritic layers is overlain by 1 m of siliceous Ben Eagach Schist, and then by quartz-celsian rock. On the steep, east slope of Creag an Chanaich, the surface trace of the Upper Mineralised Horizon is controlled by D₂ and D₃ folding, resulting in the outcrop being subparallel to the hairpin bends of the mine track as it descends the slope, providing extensive outcrops of the mineralised succession (Figure 27). In the extensive outcrop in the west wall of the first decline in the track [NN 8128 5457] to [NN 8131 5453], the Ben Lawers Schist includes a 3 m-thick calc-hornblende schist (metabasite). This intersects the decline at a steep angle and is succeeded down the track by 1 m of baryte in an envelope of quartz-celsian rock (2 m below and more than 3 m above). However, as the result of a D₃ fold axis affecting a D₂ fold, the mineralised horizon swings parallel to the track and the baryte layer climbs back up the track side on the southerly limb of the D₃ fold (Figure 27). Consequently, the quartz-celsian rock forms the lower 50 m of the exposure in this decline until, at the hairpin bend in the road [NN 8131 5453], the baryte and quartz-celsian rock form the steeply dipping core of a D₂ fold which takes the surface trace of the Upper Mineralised Horizon down the slope to intersect the track on the lowest decline.

On this lowest decline, the Upper Mineralised Horizon is again well exposed in the western wall of the track, in a disused adit [NN 8135 5455] driven along the core of a D₂. fold (Figure 27). The baryte layer is 4 m thick, massive white or grey. In the upper part of the adit the baryte is on the inverted limb of the fold and dips at 42° to 325° (NW). The fold core is exposed in the floor of the northern side of the adit and the baryte, which here dips steeply, must become approximately horizontal just beneath the adit. A 10 cm-thick layer of massive pyrite is present at the stratigraphical base of the baryte in the north wall of the adit. The quartz-celsian envelope is well exposed and extensively oxidised. The combination of a northerly dip on the lower limb of the D₂ fold and the descending track results in the baryte cropping out in the track wall 40 m to the south of the upper adit, where a further (lower) adit has been driven into the baryte in a D₃ anticlinal structure. Further down the track the Ben Eagach/Ben Lawers Schist contact is exposed [NN 8137 5450]. A structural synthesis of these exposures in the mine track is shown in (Figure 28).

The Lower Mineralised Horizon is also present on Creag an Chanaich, although it thins progressively eastwards. Outcrops on the top of the ridge [NN 8122 5464], in a trench by the track [NN 8124 5463] (in a D₃ fold core) and at the sharp bend in the track [NN 8133 5457] comprise pyritic quartz-celsian rock and sulphide-rich schist with a heavily weathered calc-hornblende schist (metabasite) at the base. Drilling and mining beneath Creag an Chanaich revealed the 3-D folded nature of the mineralised horizon and the complex interference of D₂ and D₃ folds (Figure 28).

Foss opencast pits

The Upper Mineralised Horizon is offset by a pair of NNE-trending, left-lateral faults whose surface trace is obscured by mine buildings and plant (Figure 27), (Figure 28). These faults also separate the underground workings from the opencast pits. The more easterly of these faults is exposed in the western end of the workings and contains an altered basic Caledonian dyke [NN 8144 5446]. To the east of the fault, the mineralised horizon has been mined opencast in several pits which are exploiting the thickest, near surface occurrences of the baryte horizons (Plate 10a). In the south-western part of the opencast workings, exploitation has focussed on baryte rock in the core of an eastward-plunging D₂ synform. On the southern limb of this synform, the Ben Lawers Schist, muscovitic graphitic schist (Ben Eagach Schist) and the quartz-celsian rock are well exposed in the pit wall; the Ben Eagach Schist is also exposed in the core of the synform [NN 8147 5448]. A D₃ antiformal structure causes the baryte and quartz-celsian rock to crop out again to the north [NN 8152 5450] and exploitation revealed that the original BGS exploration Borehole No. 2 (Coats et al., 1981; (Figure 26), (Figure 28)) was drilled into an overfolded D₂ antiform, complementary to the D₂ synform in the south-east of the opencast. To the east, the D₂ synform closes [NN 8159 5446], and the outcrop of the Upper Mineralised Horizon is complicated by small NE-trending faults and D₂/D₃ folding. The baryte in the eastern end of the opencast workings [NN 8171 5443] has a steep southerly dip with Ben Lawers Schist at the upper contact, and it thins rapidly towards the Frenich Burn fault.

Creag na h- Iolaire anticline — northern limb

On the northern limb of the Creag na h- Iolaire Anticline, the mineralised horizon is poorly represented. However, a 40 cm-thick layer of pyritic quartz-celsian rock, hyalophane cherts and green (Cr-Ba) mica is present within the Ben Lawers Schist in Frenich Burn West [NN 8112 5497] (Coats et al., 1981); this horizon can be traced 350 m to the ENE (Moles, 1985a; (Figure 26)).

Foss East

Frenich Burn East

The Frenich Burn Fault (Figure 26), (Figure 27) causes 250 m left-lateral displacement of the mineralised horizons. East of the Frenich Burn Fault, a 30 m-wide mineralised zone, including a 15 m-thick baryte layer, is exposed in the Frenich Burn East [NN 8196 5493]. The mineralisation occurs very close to the junction between the calc-chlorite Ben Lawers Schist and the graphitic Ben Eagach Schist. This horizon is underlain by a basal chert and/or thin (20 cm) layer of metabasite (calc-hornblende schist), the latter in places containing abundant carbonate and pyrrhotite. The basal 3 m of the mineralised horizon comprises layers of quartz-celsian, pyrite, carbonate and massive baryte. This passes up into 15 m of baryte and 5 m of quartz-celsian rock. There is a 20-cm bed of graphitic schist (Ben Eagach) separating the top of the horizon from the Ben Lawers Schist. Conglomeratic fragments and breccia textures are present in discrete layers in the baryte, quartz-celsian rock and intervening schists in the mineralised section. Clasts of pure baryte in sulphide-rich baryte, fragmented quartz-celsian rock and quartz-celsian schists are most common at the base of the mineralised horizon. The baryte and basal chert also exhibit minor folding. These features were interpreted as evidence of synsedimentary reworking (intraformational breccias and slumping) by Moles (1985a). Minor faulting, perhaps associated with the Frenich Burn Fault, has disrupted the baryte horizon cropping out in the Frenich Burn East. Trenching has revealed the thick baryte layer to persist for 50 m either side of the burn but, both to the east and down dip, the horizon splits into several conformable units.

Frenich Burn East to Creag an Loch

To the east of the Frenich Burn East (Figure 26), the surface expression of the Frenich Burn baryte rapidly splits into four mineralised horizons of quartz-celsian ± baryte. These are exposed in the drainage gully to the track; the lowermost horizon [NN 8218 5497] is hosted by graphitic schist while the uppermost horizons are contained within calc-muscovite schist (Ben Lawers in character). At Creag an Loch, a north–south-trending, track-side outcrop [NN 8258 5503] includes at the base a sulphidic quartz-celsian and cherty schist with quartz-celsian layers, while the top of the zone is a 50 cm-thick bed of sulphide (pyrite, galena, sphalerite)-bearing quartz schist. The lowermost part of the horizon thickens eastwards to form a 7 m-thick outcrop of subvertical baryte in an east-facing crag [NN 8267 5504] which is underlain by 2 m of quartz-celsian rock. On the top of the crag, and at the northern end of its east face, beneath the mineralised zone, a 20 cm layer of metabasite (calc-biotite schist) is exposed. To the east of the massive baryte outcrop, a northerly trending fault displaces the mineralised horizons.

Correlation of mineralised units

Moles (1985a) recognised seven mineralised horizons in Foss (denoted M1–7). Two quartz-celsian horizons (M1 and M2), found in borehole intersections to the east of Frenich Burn East, are only recorded from this area. M3–7 form the thick baryte in Frenich Burn East, which splits eastwards with M4 or M5 and M7 dying out; the presence of the metabasite beneath the baryte at Creagan Loch, Frenich Burn East and the Lower Mineralised Horizon in Foss West suggests they are all M3. The laterally extensive Upper Mineralised Horizon possibly correlates with M4 and/or M5 in Foss East.

Lithology and mineralogy

Barium-rich rocks

The baryte is light grey to white in colour and has bedding-parallel, centimetre-scale layering due to concentrations of sulphides. Pyrite is the dominant sulphide, although sphaleritic layers are common, with galena and, rarely, chalcopyrite also present. Magnetite layers are common in zones poor in sulphides, while micaceous layers are rarely developed. Conformable, and lenticular dolomite and cherty layers are present and cross-cutting baryte veinlets are common close to fault zones.

The quartz-celsian rock is generally grey and cherty and ranges from fine-grained through to a granular rock in which the celsian attains a crystal size of up to 4 cm. The rock is mainly composed of quartz and celsian but the presence of variable amounts of carbonate and sulphides (pyrite and lesser sphalerite) in layers gives the rock a striped appearance. True cherty (silica-rich) rocks are exposed beneath the main baryte of the Upper Mineralised Horizon in the adit at Foss West [NN 8134 5454] and in Frenich Burn East (Coates et al., 1981; Moles, 1985a). Brecciated quartz-celsian rock at these localities is interpreted by Moles (1985a), as tectonic in origin. Thin sections reveal the quartz-celsian rock to comprise either layers of quartz and celsian or intimate mixtures of the two minerals. Relic cymrite, BaAl₂Si₂O₈.H₂O, is present in the celsian (Fortey and Beddoe-Stephens, 1982; Moles, 1985b), and there is a restricted occurrence of cymrite replacing celsian along grain margins. Analyses of Ba-rich silicates are reported by Coats et al. (1980; 1981), Fortey and Beddoe-Stephens (1982) and Moles (1985a; b). In the series KAlSi₃O₈–BaAl₂Si₂O₈, the mineral names can be defined by barium content (mol.% of Ba + K + Na), such that celsian is Ba > 50% (or Cn 50–100) and hyalophane is 25 < Ba < 50 (or Cn 25–50). Orthoclase with Ba < 25 mol.% is termed barian feldspar. EPMA reveals a whole range of compositions for hyalophane and celsian (Table 5) and many celsian analyses are close to the end-member, BaAl₂Si₂O₈. Cymrite analyses (Fortey and Beddoe-Stephens, 1982; Moles, 1985b) reveal it to be near the barium end-member, and muscovite in the mineralised zone contains BaO concentrations ranging up to 11 wt% (Coats et al., 1981; (Table 5)). The quartz-celsian rock of the Aberfeldy mineralisation represents the world's largest known concentration of barium-containing feldspars.

Sulphides

Pyrite is by far the dominant sulphide mineral, occurring either disseminated throughout the mineralised lithologies or as 100 µm–2 mm euhedral (cubic) grains in layers in the baryte and quartz-celsian rock. Rarely it forms concentrated layers, as at the base of the Upper Mineralised Horizon in the upper adit at Foss and in the open pit. Pyrrhotite is commonly found as small (10 µm–1 mm) grains, either disseminated or as thin layers in the quartz-celsian rock; it is commonly replaced by pyrite. Sphalerite is a common constituent of the mineralised horizons (Moles, 1983), present as layers of pale yellow/brown or red/brown (Zn,Fe)S in the baryte and quartz-celsian. Massive sphalerite (+ pyrite and galena) layers were encountered in boreholes at Creagan Loch. Inclusions of galena (100 m in diam.) are found widely throughout the mineralised zones, but chalcopyrite is rare. Moles (1985a) recorded the presence of tennantite.

Carbonates

Dolomitic rocks occur in the mineralised zones in Foss East at the base of the Ba-rich successions at Frenich Burn East and Creagan Loch [NN 8270 5503]. These contain sulphides, celsian and rutile. BGS borehole 3 intersected a 4 m brecciated unit of dolomitic layers at the base of the baryte beneath the Creagan Loch outcrop.

Host rocks in the mineralised zone

In addition to the main Ba-enriched mineralised horizons, much of the mineralised zone comprises graphitic or muscovite schists, some of which are barium enriched by up to 1 wt % (Coats et al., 1980). The schists are usually pale green-grey (Moles, 1985a) due to barium in muscovite (Plate 11a). Adjacent to the mineralised horizons, the graphitic schist is commonly silicified, as on the surface outcrop on Creag an Chanaich.

Geochemical studies

Extensive sulphur isotopic analyses have been carried out on the baryte and sulphides from the Aberfeldy mineralisation (Pattrick et al., 1979; Willan and Coleman, 1983; Moles, 1986; Hall et al., 1989; 1991; Scott et al., 1991; see also Hall, 1993). These indicate that the sulphate sulphur source was late Precambrian sea water. Oxygen and strontium isotope analysis of the baryte, quartz and celsian (Hall et al., 1991; Hall, 1993; Fisk, 1986) and lead isotope analysis of galena, baryte and celsian (Swainbank et al., 1981; Swainbank and Fortey, 1981) support an origin by mixing of hydrothermal fluids and sea water while the source of the ore components is in the older Dalradian lithologies and the sub-Dalradian basement.

Formation of the Aberfeldy mineralisation

The Aberfeldy mineralisation is considered to be a SedEx-type (sedimentary exhalative) deposit (Coats et al., 1980; Willan and Coleman, 1983; Russell et al., 1984; Moles, 1985a; Hall, 1993; Hall et al., 1989), forming from the exhalation of metal-laden mineralising fluids onto the seafloor. The baryte resulted from the mixing of Ba carried in the reduced hydrothermal fluids with sulphate in the relatively oxidised seawater; Ba also reacted with the aluminosilicate-rich seafloor muds to produce barian clays and cymrite. Exhalative silica precipitated as cherts in the aluminosilicate muds and recrystallised during metamorphism. The mineralised horizons are seen as successive exhalative hydrothermal pulses emanating onto the seafloor, the variation in the mineralised horizons being a function of both intensity of hydrothermal activity and the distance from exhalative centres. The recognition of exhalative centres is difficult; the thickest units of baryte and sulphide concentrations (Frenich Burn East, Upper Foss Adit and Creagan Loch) may indicate proximity to emanative centres (Moles, 1985a; Hall, 1993). Cymrite is seen as the sedimentary precursor to the celsian, the transition taking place during metamorphism with some reversion to cymrite during retrograde metamorphism (Moles, 1985b).

Other stratiform mineralisation

Pyrite horizon (Ben Lawers Schist Formation)

In Glen Lyon, the conformable 'Pyrite Horizon' (Smith, 1977; Smith et al., 1977) is exposed in a small tributary [NN 639 467] near Roromore. The pyritiferous outcrop is 120 m wide and is situated close to the top of the Ben Lawers Schist, at its contact with the overlying Farragon Volcanic Formation [NN 6387 4687]. Upstream, the amphibolites have a sharp contact with an interbedded sequence of quartz schist, biotite schist and quartzite, which changes further upstream into typical calc-chlorite-mica schist with interbedded quartz schists and quartzites of the Ben Lawers Schist. The calc-chlorite-mica schists and quartz schists contain disseminated euhedral pyrite and the quartzites contain stringers and conformable layers of pyrite; sulphide concentrations can reach 3% by volume in centimetre-scale layers. The calc-chlorite schists have metal concentrations of 5–75 ppm Cu, 80–120 ppm Zn and 20 ppm Pb (Smith et al., 1977). The 'Pyrite Horizon' also crops out in a stream near Boreland, Loch Tay, [NN 715 447] but the mineralisation is very restricted. The Ben Lawers Schist to the east of the Loch Tay Fault near Glengoulandie [NN 7715 5183] contains anomalously high pyrite concentrations, suggesting another outcrop of the Pyrite Horizon.

Allt Odhar horizon (Pitlochry Schist Formation)

An amphibolite exposed in the Allt Odhar [NN 7361 4723] contains a conformable, 2 m-thick horizon rich in pyrrhotite and pyrite, and containing minor rutile and chalcopyrite. The mineralisation is best exposed on the west bank of the stream, the pyrite concentration decreasing to the north-east. The presence of this enrichment of sulphide in the amphibolite, its conformable nature and the common occurrence of chalcopyrite indicates a hydrothermal origin for the these sulphides, possibly representing a very poorly developed volcanogenic massive sulphide deposit.

Dericambus

The Transitional Quartzite Member at the base of the Ben Eagach Schist contains stratabound and podiform Zn-Pb mineralisation in Glen Lyon, near Dericambus (Coats and Pease, 1984; (Figure 29)). It is found close to the Inverinain Burn [NN 657 465] and a farm track [NN 6573 4710], but is best developed over a 200 m section of a stream cutting across the strike [NN 6767 4630] to [NN 6763 4647] and in crags to the west [NN 6737 4630] to [NN 6758 4630]. The host lithologies are quartzites with quartz-biotite schist intercalations. In the mineralised succession, there are micaeous layers (10 cm thick) enriched in pyrrhotite and sphalerite with associated microscopic galena and chalcopyrite. The conformable sulphides are associated with sulphide-bearing carbonate and quartz pods exposed in the stream, the latter containing sulphides [NN 6762 4647]; [NN 6761 4646]. The conformable nature of the sulphides and the presence of monomineralic layers of sphalerite and galena suggest the stratabound mineralisation to be exhalative in origin but it is of limited economic interest.

Braes of Foss

The lowermost 100 m of the Tempar Dolomitic Member (within the Schiehallion Quartzite) contains anomalous sulphide and is exposed in the Allt Ruighe nan Coireachan and adjacent outcrops [NN 7501 5577] to [NN 7381 5526]. The local succession is of thinly bedded (0.5–5 m) pelites, semipelites and quartzites, many of which exhibit orange-coloured weathering, being more or less dolomitic; some beds are tremolite-marbles. In addition to conspicuous sulphide in the carbonate units, the pelites, semipelites and an amphibolite [NN 7453 5528] are enriched (up to 5% locally) in pyrite with minor chalcopyrite and pyrrhotite. Nearby [NN 7470 5541] pyrite is present as 1–5 cm pods in thin concordant quartz-carbonate veins. The Drumchastle Pale Limestone is exposed in a stream 5 m north of the road bridge near to Braes of Foss farm [NN 752 557]. The white limestone contains layers of 20% pyrrhotite with minor chalcopyrite and microscopic pentlandite.

Epigenetic mineralisation

Vein mineralisation is widespread throughout the district and several generations of hydrothermal activity have been recognised. Historically, lead-copper veins in the Urlar Burn [NN 824 449] were trialed for 'copper pyrite', galena and 'blende' during the late 18th century, following a mineral survey carried out on behalf of the Marquis of Breadalbane (Thost, 1860; Wilson and Flett, 1921). However, recent gold exploration in the area, carried out by Colby Gold plc (Mason et al., 1991), has revealed several gold-bearing vein structures between the Urlar and Calliachar Burns, and also near Tombuie Cottage [NN 7895 4475]. As the Urlar–Calliachar veins appear to be related, they are described together, although the latter overlap into the Pitlochry district (Sheet 55E).

Calliachar–Urlar vein suite

A 7 ppm gold anomaly in a heavy mineral pan concentrate from the Calliachar Burn near to its confluence with the Urlar Burn was recorded in the BGS Regional Geochemical Survey; this led to the discovery in 1988 of a very limited amount of auriferous quartz-carbonate gossan in the Calliachar Burn [NN 8400 4520] by Colby Gold plc. Trenching (Plate 10b) and top of bedrock sampling revealed 14 mineralised structures to the west of the Calliachar Burn, under the peat/till cover and traceable as far as the Urlar Burn (Figure 30). A subsequent drilling programme (400 m) was carried out to determine the continuation of the veins to depth (Mason et al., 1991). Gold grades averaging 8.81 g/t over a strike of 87.5 m were recorded; 700 g of gold was produced in 1991 from an excavation into the top of a gossanous vein, and was sold as jewellery and small ingots. The historical veins in the Urlar Burn were also trenched [NN 8230 4490], but no drilling was undertaken. Due to the presence of the peat and lodgement till, exposure in the Calliachar–Urlar area is limited to the two burns and hill crests and the veins are particularly poorly exposed due to the weathering of sulphides. However, the trenching (now restored) and drilling by Colby Gold plc revealed the vein characteristics.

Geology

The veins are hosted by the Pitlochry Schist Formation which comprises a succession of gritty psammite, biotite-rich psammite, quartz-mica schist, thin micaceous pelite, Green Beds and amphibolites; the bedding has a dip of less than 20°, its attitude controlled locally by open flexuring due to the D₃ Calliachar Burn Antiform. A major feature is the NE-trending Urlar Burn Fault [NN 832 460] (Figure 30) which has a left-lateral and vertical displacement. It varies from an anastomosing series of minor, breccia-filled faults that dip SE to ESE at approximately 70° to 85° [NN 8270 4530] to a 5 m-wide zone of brecciated mica-schist and quartzite, cemented by massive carbonate, quartz and pyrite. Low-angle slickensides (plunging 7°–10°N) indicate late movement was essentially horizontal. The rocks adjacent to the fault are commonly traversed by carbonate veins and spotted by orange dolomite [especially around 8317 4590] where the fault is marked by a 5 to 30 m wide zone of fine-grained, creamy, dolomite-cemented breccia of which the carbonate forms 80% of the rock. Stained sections revealed the majority of the carbonate to be ferroan dolomite, although late irregular veins of whiter ferroan calcite occur throughout the sequence. The country rocks, especially amphibolites, within 3 m of the fault are bleached mainly due to the breakdown of ferromagnesian minerals to sericite and pale 'hydrothermal' chlorite. The Calliachar-Urlar area is traversed by a set of near-vertical fractures trending 140° to 160°, representing a master joint set related to left-lateral movement on the Urlar Burn and related faults. Some small dextral movements present [e.g. in Black Burn 827 461], are indicated by slickensides with shallow NW and SE plunges; alteration of the type associated with the Urlar Burn Fault is present in the wallrocks adjacent to the joints. Three types of minor intrusion are present in the Calliachar area (Figure 30):

1. pink felsite [NN 8258 4504] which is cut by the Urlar Burn Fault
2. altered porphyritic microdiorite [NN 8260 4512] which postdates the fault but is altered
3. Permo–Carboniferous quartz-dolerite [NN 8260 4508]

Vein controls and alteration

The mineral veins occupy the 140°–160°-trending master joints and the local development of the veins was controlled by the host lithology; the competent gritty quartzite, biotite-rich quartzites, finely banded green beds and unbanded chlorite-quartz schists are well-jointed and good hosts for mineralisation. In the more pelitic lithologies and massive or micaeous amphibolites, the fractures are poorly developed, discontinuous or absent, although bedding-parallel quartz segregations (flats) are common. Wallrock alteration is most extensively developed in the mafic country rocks (around the Calliachar suite). Close to the veins there is complete sericitisation of the feldspars, chloritisation of biotite and breakdown of the amphibole and garnet to chlorite; haematite and TiO₂ are common by-products of biotite breakdown. Fine-grained carbonate and quartz + carbonate, base-metal sulphide veinlets are also present. Further from the veins is a zone rich in carbonate, present as spots and thin wispy veinlets where calcic plagioclase is sericitised but alkali feldspars less affected. An outer alteration halo (up to 20 m from the veins) is characterised by chloritisation of garnets and amphiboles, with biotite unaffected.

Calliachar veins

Only two veins were recognised at outcrop during mineral exploration. The gossanous (pyrite–goethite) top of Vein 1 (Figure 30) occurs in the eastern bank of the Calliachar Burn [NN 8400 4520] but cannot be traced in the adjacent stream bed. There is limited exposure of Vein 3 in the Calliachar Burn [NN 8415 4545] and its strike extension was proven by trenching to the north-west (Figure 30). Detailed information was, therefore, largely obtained from samples from trenching and drilling operations. The veins vary from a few centimetres to 2 m wide. Quartz is the dominant mineral, euhedral crystals being evidence of open space growth; vuggy cavities containing euhedral quartz or siderite are widespread. Angular fragments of wallrock are cemented by quartz and sulphides, although large-scale breccias are absent. The main sulphides (which form coarse aggregates up to 10 cm) present are pyrite, galena and sphalerite, with minor chalcopyrite and arsenopyrite. Samples derived from trenches or outcrop are severely weathered (gossanous) and the original sulphides have undergone advanced or complete oxidation. Drilling revealed that the zone of oxidation in the host rocks extended down to 3 to 7 m below surface but sulphides in the more permeable veins were oxidised to at least 10 m.

The oxidised surface zone in trenches in Vein 3 and Vein 6 [NN 835 456] (Plate 10b) comprise quartz, and siderite, goethite, limonite and haematite. Polished thin-sections reveal the goethite to be concentrically layered with the precursor pyrite forming atoll-textured relics. Electrum is present typically as small (1 to 20 µm) inclusions and veinlets, and exceptionally as inclusions up to 90 µm in vugs in the limonite. Other supergene minerals in the gossanous material are jarosite, cerussite, anglesite, pyromorphite, covellite, spionkopite, malachite, native copper and scorodite. Partially oxidised and fresh sulphide-rich vein material was produced from trenching and drilling (especially Vein 1 and Vein 6), revealing a complex paragenesis. The general sequence of sulphide mineral precipitation was pyrite ? arsenopyrite + gersdorffite ? sphalerite ? galena + chalcopyrite with extensive replacement by the later sulphides. In addition the pyrite contains inclusions (1 to 50 µm) of chalcopyrite, galena, sphalerite, pyrrhotite + chalcopyrite + pentlandite, 60 µm aggregates of pyrrhotite + mackinawite + cubanite + sphalerite + galena + chalcopyrite, and marcasite overgrowths and intergrowths. Gersdorffite ((Fe,Co,Ni)AsS) + tetrahedrite ((Cu,Ag)₁₀(Zn,Fe)₂ (As,Sb)₄S₁₃) + chalcopyrite + sphalerite + polybasite ((Cu,Ag)₁₆(Sb,As)₂S₁₁) intergrowths form as relicts in arsenopyrite. Electrum precipitation is associated with the galena and it commonly develops at galena/pyrite margins (Plate 11b), in fractures and as inclusions (10–120 µm) in pyrite with galena and chalcopyrite. The galena contains rare (10 µm) grains of hessite (Ag₂Te) with rims of benleonardite (Ag₈(Sb,As)Te₂S₃) and native tellurium + hessite. Tetrahedrite-group minerals form as (20–50 µm) inclusions in major sulphides. The coarse-grained sphalerite contains abundant, oriented inclusions of chalcopyrite (chalcopyrite disease). Very rarely, the Calliachar veins have very high concentrations of gold. These centimetre-scale 'pods' (encountered during exploration) comprise massive galena + electrum, the latter forming grains up to 6 mm; these pods are associated with carbonate-rich wallrock.

Electron probe microanalyses (EPMA) of minerals from the Calliachar and Urlar veins can be found in Pattrick and Treagus (1997) and Ixer et al. (1997). A selection of the electrum analyses is reproduced in (Table 6). Analyses of the electrum (Table 6a) associated with the primary sulphides reveal a range of gold content from 60–72 wt % Au and minor amounts of Hg. The gold in the oxidised surface ores had a similar range of gold/silver ratios, indicating no increase of fineness in the in situ oxidised ores. The sphalerite in inclusions in pyrite is low in iron, whereas the massive sphalerite in the core from Vein 6 contains approximately 2 wt% Fe and 0.2 wt% Cd. EPMA (Table 6c) of placer gold in the Calliachar Burn revealed it to, in some cases, have a similar composition to the vein electrum (P1/2; detrital grains) and is others to have Au-enriched rims (P3/4).

Urlar veins

They are 0.1 to 3 m thick and are best developed adjacent to the Urlar Burn between [NN 8230 4450] and [NN 8280 4510] where they form inclined, impersistent veins typically 30 cm across, hosted by a gritty quartzite. Limited dump material is present, although trenching during exploration exposed one vein on the west side of the Urlar Burn [NN 8231 4493]; this trench is now restored but the vein remains exposed in a small tributary to the Urlar Burn. The vein material largely comprises quartz with sporadically developed patches of early pyrite, coarse- grained galena ± chalcopyrite with minor sphalerite (+ chalcopyrite disease) and arsenopyrite. Marcasite, gersdorffite, cobaltite (CoAsS) and cubanite are present, replacing or intergrown with the major sulphides. The galena is associated with or contains common inclusions (10 to 200 µm) of bournonite (CuBiSbS₃), altaite (PbTe), hessite (Ag₂Te) with a benleonardite (Ag₈SbTe₂S₃) rim, and coloradoite (HgTe), and rare (< 10 µm) inclusions of electrum, wehlite (BiTe), melonite (NiTe₂) and volynskite (AgBiTe₂). The galena contains 'bunches' of blueish laths which appear to have been exsolved from the host. Aggregates (200–300 µm) of hessite and altaite are found in pyrite and the altaite contains distinctive elongate pink inclusions. A 10–500 µm-wide rim to some of the the galena contains a complex secondary mineralogy of Fe-hydroxy-oxide (goethite), lead tellurate (PbTeO₄), soft colourless Fe-Pb-tellurate, hessite, enargite, chalcopyrite, bornite, native silver (1 to 3 µm) and grains (20 to 70 µm) of electrum. There is a late generation of ferroan dolomite veins.

Alteration of galena to anglesite and the breakdown of chalcopyrite to covellite, goethite, limonite and amorphous CuS, is extensive; the rhombic gersdorffite and arsenopyrite are commonly pseudomorphed by goethite and erythrite. Euhedral muscovite is associated with development of goethite in vugs.

EPMA of the rare primary electrum in the galena and pyrite showed it to be gold-rich with 72 wt.%Au (Table 6b) while the electrum in the altered rim of the galena contains up to 23.5 wt.% Hg and has probably developed from the breakdown of the primary electrum and tellurides. The sphalerite contains 4 wt.% Fe and 1.4 wt.% Cd and the galena contains 0.0–0.3 wt.% Ag and 0.0–1.3 wt.% Bi. Analysis of 30 ? 200 µm pink laths in the altaite reveal them to be Bi-Pb-tellurides with a formula of (Bi,Pb)_1-xTe_1+x, where x is ² 0.1 and Bi>>Pb. The bluish laths in galena are Pb-Te-S phases with minor Bi, and maybe a fine (sub-micron) scale intergrowth with PbS.

Vein genesis

Fluid inclusions indicate at least three generations of mineralising fluid, one at ² 300°C containing ∽ 4 wt% NaCl, a second at ≈ 180°C with 8 wt% NaCl and a third, late fluid at ² 140°C; CO₂ and small amounts of CH₄ and N₂ are present in the first two inclusion suites. Oxygen and hydrogen isotopic data are consistent with a magmatic source of the fluids with some input from Devonian (the assumed age of the mineralisation) meteoric water (Smith, 1996). Sulphur isotopic data reveal the sulphur source was probably the Dalradian metasediments, although the Calliachar values are in the range for the magmatic sulphur of the region (Curtis et al., 1993; Lowry et al., 1994). ³He/⁴He ratios derived on fluid inclusions in vein sulphides reveal a mantle component in the fluids, with evidence of a high level of crustal helium in the Urlar galenas (Ford, 1994). The isotopic data, therefore, indicate the primary mineralising fluid was magmatic in character with a significant mantle component. The lack of evidence (field and geophysical) of spatially related Caledonian intrusions of significant size indicates that such fluids must have originated at considerable depth. Hydrous fluids of mantle/magmatic character may have been generated at the crust–mantle boundary during the end-Caledonian melting event with the Urlar Burn Fault acting as the conduit for these rising fluids, the mineralisation occurring at relatively high levels in the crust in suitable dilated fractures. The mineralisation at Calliachar–Urlar has similar characteristics to hydrothermal gold mineralisation world-wide; the sericitic-propyllitic alteration + carbonation and the geological setting are typical of mesothermal veins. The Calliachar–Urlar mineralisation also shares many characteristics with the gold mineralisation at Cononish, near Tyndrum some 45 km to the west.

Tombuie veins

The area around Tombuie Cottage [NN 7898 4468] (Figure 31) contains several mineralised fractures (and boulders) which have a spatial association with the Tombuie Fault. This fault is poorly exposed in the Allt a' Bhealaich [NN 7873 4495], trending NNE and has an 'apparent' left-lateral and vertical displacement, plunging 20°SW. The rocks adjacent to the fault are highly fractured and affected by a set of steeply dipping joints trending 130° to 150°, which commonly contain minor mineralisation e.g. at [NN 7863 4507]. These fractures have sharp, undeformed margins and the vein constituents grew into open space, with vuggy quartz common. Quartz and carbonate are the main constituents of the veins which also contain pyrite and minor amounts of chalcopyrite, galena and sphalerite. In places 'net' veining forms pseudobreccias. In the burn to the west of the Allt a' Bhealaich [NN 7877 4517] is a quartz + pyrrhotite vein with minor pyrite and chalcopyrite. In 1989 trenching by Colby Gold plc revealed further vein structures containing electrum, galena, chalcopyrite and pyrite in the same vicinity [NN 791 451] and [NN 788 445].

Veins exposed in the Allt Odhar

Carbonates of the Loch Tay Limestone Formation exposed in the stream [NN 7390 4856] are host to thin (2 to 10 cm), laterally persistent sulphide + carbonate veins trending approximately 180°. The veins comprise pyrite containing relicts of pyrrhotite, calcite and quartz, and minor arsenopyrite; late galena veinlets (1 to 5 mm) occupy the same fractures. Polished thin-sections showed the euhedral pyrite to be brecciated and filled with fibrous carbonate; chalcopyrite is present as inclusions in the main sulphides and forms as 'chalcopyrite disease' in sphalerite. Late carbonate and quartz veins crosscut the sulphide minerals; clay minerals are associated with the galena.

Quartz + carbonate + sulphide veins 1 to 20 cm in width, and occupying joints trending 140° to 180°, are well developed in the river section [NN 7362 4726] to [NN 7362 4732]. They comprise quartz, calcite and pyrite, with minor chalcopyrite. Veins (1 to 10 cm thick) adjacent to exposed felsites [NN 7396 4790] comprise mainly quartz-carbonate and contain 0.3 to 1 cm pyrite pods and fine-grained, disseminated pyrite with trace amounts of chalcopyrite. A fault breccia containing abundant pyrite which disrupts the base of an amphibolite [NN 7384 4730], contains clasts of quartz, cemented by later generations of calcite + pyrite + chalcopyrite, quartz + pyrite + chalcopyrite and fine-grained quartz and calcite.

The Loch Tay Fault zone is exposed in the Allt Odhar [NN 7385 4857] and comprises a 1–3 m-wide zone, trending 033°, containing a very high concentration of pyrite as anastomosing veins, pods and disseminations in a brecciated, calcareous rock. This zone is exposed for 30 m where the stream follows the fault plane. Polished thin-sections reveal the pyrite to be fine-grained, zoned, brecciated aggregates, intergrown with quartz and cemented by later pyrite + quartz.

Quartz + pyrite + molybdenite veins

These veins are common in the Allt an Stalcair [NN 6908 7183] and adjacent A9 road sections. They range from 10 to 50 cm in width and generally dip between 35° and 50°N and strike between 033° and 090°. They comprise massive quartz containing disseminated aggregates of pyrite and rare molybdenite. K-feldspar is developed along the vein margins while the adjacent wallrock is heavily chloritised; muscovite may be present in the vein quartz. Polished sections reveal the pyrite to be intergrown with marcasite and fractured and replaced by minor amounts of galena; arsenopyrite is also present. Some 300 m to the east in the northern side of the A9 road cutting [NN 6939 7179], steeply dipping (80°S) irregular quartz + feldspar + chlorite veins contain pyrite + galena aggregates up to 5 cm across; these cut a quartz + K-feldspar vein (170°/35°E). Polished sections of the sulphide-rich ore revealed inclusions of chalcopyrite (10 to 100 µm) and sphalerite (5 to 20 µm) in the galena and pyrite, while covellite is a breakdown product of the chalcopyrite.

Carbonate veins

Carbonate veining, weathering to an orange-brown colour, is common in the area. It varies from minor veining associated with small fractures (joints) to large (metre scale) carbonate veins developed in faults. The latter are best developed in the Urlar Burn Fault [NN 8317 4575] to [NN 8320 4600], the Keltney Burn Fault [NN 7689 4980], the Loch Tay Fault in the Allt Coire Pheiginn [NN 7510 5064] and in the Allt an Stalcair [NN 6905 7175]. In the Allt an Stalcair, the carbonate occupies a 178°-trending fault (dipping 64°E) that cuts Grampian Group psammites and forms a zone up to 10 m wide composed of early massive pale pink ferroan calcite and late dark red haematitic calcite. Both wallrock and early pale pink carbonate form as clasts in breccias and the carbonate zone/fault margins are chloritised. In the Keltney Burn [NN 7689 4980] to [NN 7673 5024], a 160°-trending fault is occupied by a zone of massive pale pink/green ferroan dolomite in veins up to 3 m wide, alternating with silicified and carbonated breccia containing clasts of country rock; again darker red carbonate veins cut the the earlier, paler calcite. In the Allt Coire Pheiginn, carbonate veining is found within 150 m of the Loch Tay Fault. Adjacent to the Fault, carbonate-cemented quartz-rich Ben Lawers Schist and massive carbonate veins occur; the latter are cut by a set of smaller carbonate veins. The carbonate is all ferroan dolomite.

Other vein mineralisation

In the southwards-flowing stream, 900 m NNE of Drumchastle Farm [NN 686 599], sporadic pods and veins (20 cm to 2 m wide) occupy a near bedding-parallel fault (trend 170°) in steeply dipping (70°E, trend 176°) quartzites and amphibolites. They contain quartz and carbonate with pyrrhotite, pyrite and chalcopyrite in grain aggregates up to 5 cm across. The veins are commonly discontinuous and rarely exceed 15 cm in thickness. The carbonate appears to postdate the quartz. In the Allt Choire Leathanaidh [NN 668 708], small irregular and poorly exposed veins comprising haematitic carbonate are present in the bed and banks of the stream.

Industrial minerals and bulk resources

Beoil Schist mica

Dalradian mica schists are a potential source of industrial mica and the Beoil Schist and Ben Lawers Schist have been examined with a view to commercial exploitation (Harris and Turner, 1971; Moir and Henley, 1967). The Beoil Schist is particularly attractive because of its high muscovite content and relative homogeneity. In the Harris and Turner study, samples were collected from north of Trinafour [NN 725 655], south-east of Dunalastair [NN 722 574], from the Allt na Moine Buidhe [NN 708 612] and near Kinloch Rannoch [NN 651 589]; Moir and Henley examined a 7 ton composite sample. The mica content was found to be typically 35 to 55 wt%, with 25 to 30 wt% muscovite and 10 to 20 wt% biotite; mineral separation was effective when the rock was ground to 30 mesh. The quality was deemed suitable where bulk physical properties were important, e.g. lubricants and insulation, but the high iron content of the muscovite would prohibit its use where colour was important, e.g. in paper manufacture (Harris and Turner, 1971).

Dunalastair talc

In the north-western bank of a small tributary burn 300 m south of the River Tummel at Dunalastair [NN 7154 5807], the Strath Fionan Pale Limestone comprises a very friable talc-tremolite-carbonate rock. The talc forms greater than 90% of the rock in places, considerably higher than any other outcrop of the limestone, but the deposit is of limited extent and of no significant commercial value.

Garnet

Regional metamorphism has led to the extensive development of garnets in many lithologies and represent a potential source of abrasives. Large tonnages of garnetiferous rock are developed in the pelitic and semipelitic horizons in the Killiecrankie Schist, Pitlochry Schist and Ben Lui Schist, e.g. as exposed in the Allt Coire Pheiginn west of Garth [NN 765 505] to [NN 754 506]. The Beoil Schist is not only highly micaceous but is also highly garnetiferous, e.g. in Strath Fionan [NN 725 573]; the high concentration of mica would facilitate garnet separation.

Carbonates

The Blair Atholl Limestone has been exploited in the past as a source of lime with a small quarry at Lochan an Daim [NN 715 574], and at Tomphubil [NN 777 545] where the quarry and lime kilns are now a tourist attraction. Major quarrying activity into this limestone is currently (1997) taking place to the east at Shierglas Quarry, Blair Atholl [NN 882 639] in the Pitlochry district (Sheet 55E), producing road aggregate, including coated stone. There is also a small disused quarry in the Loch Tay Limestone adjacent to the B846 [NN 768 504]. The only potential future use for these carbonates in the Schiehallion district is as a local source of aggregate, or lime for agricultural purposes.

Building stones and aggregate

Historically, the dark green talc-chlorite epidiorite in the Pitlochry Schist Formation has been quarried on Bolfracks Hill [NN 833 473] and, as it was easily dressed, used as a dimension stone, for example in many buildings in Aberfeldy, Kenmore and at Taymouth Castle. The Green Beds have also been used as dimension stone in the past; the paler green, tuffaceous rocks was quarried near Aberfeldy and is found in many local buildings in the Pitlochry district. The large felsite north of Kinloch Rannoch, the granodiorite at Glen Banvie and the massive amphibolites near Trinafour and east of Loch Tay are potential sources of aggregate. The massive and homogeneous nature of the Schiehallion Quartzite and Grampian Group Quartzites, for instance adjacent to the A9, makes them potential source of durable dimension stone, though they would be hard to work.

Sand and gravel

Strath Tay between Kenmore and Aberfeldy, and to a lesser extent Lower Glen Errochty and Glen Garry east of Calvine, are potential sources of sand and gravel. These valleys contain extensive glacial, reworked glacial and alluvial sediments, with a wide range of particle sizes present.

Future exploitation

The designation of part of the district as one of outstanding natural beauty and the strength of the environmental lobby suggests that it is unlikely planning permission would be obtained for new sites of bulk resource quarrying, except in a national emergency. There is likely to be little local demand for these resources and exploitation would also involve prohibitive costs for upgrading the transport infrastructure through difficult terrain, except adjacent to the A9 and A827. The baryte exploitation and market have been an exception.

Information sources

Further geological information held by the British Geological Survey relevant to the Schiehallion district is listed below. It includes published material in the form of maps, memoirs and reports and unpublished maps and reports. Also included are other sources of data held by BGS in a number of collections, including borehole records, mine plans, fossils, rock samples, thin sections, hydrogeological data and photographs.

Searches of indexes to some of the collections can be made on the Geoscience Index System in BGS libraries. This is a developing computer-based system which carries out searches of indexes to collections and digital databases for specified geographical areas. It is based on a geographical information system linked to a relational database management sytsem. Results of the searches are displayed on maps on the screen. At the present time (1997) the datasets are limited and not all are complete. The indexes which are available are listed below:

• Index of boreholes
• Topographical backdrop based on 1:250 000 scale maps
• Outlines of BGS maps at 1:50 000 and 1:10 000 scale and 1:10 560 scale County Series
• Chronostratigraphical boundaries and areas from BGS 1:250 000 maps
• Geochemical sample locations on land
• Aeromagnetic and gravity data recording stations
• Land survey records

BGS maps

• Geology maps
• 1:625 000
• United Kingdom (North Sheet) Solid geology, 1979; Quaternary, 1977
• 1:250 000
• Sheet 56N 06W, Argyll, Solid geology, 1987
• Sheet 56N 04W, Tay–Forth, Solid geology, 1986
• 1:63 360
• Sheet 46 Crianlarich, Solid and Drift, 1900 (out of print)
• Sheet 47 Crieff, Solid and Drift, 1888 (out of print)
• Sheet 55 Blair Atholl, Solid and Drift, 1967 (out of print)
• Sheet 64 Kingussie, Solid and Drift, 1913 (reprinted 1964)
• 1:50 000
• Sheet 54E Loch Rannoch, Solid, 1975
• Sheet 55W Schiehallion, Solid, 2000
• Sheet 55E Pitlochry, Solid, 1977
• Sheet 63E Dalwhinnie, Solid, 2000
• 1:10 000
• The surveyors of the component 1:10 000 sheets included wholly or partly in Sheet 55W were P A R Nell, J E Treagus and J M Maclachlan (University of Manchester), and P R Thomas (University of Strathclyde, now retired.) All maps are solid only. The maps are not published but are available for consultation in the British Geological Survey Library, Edinburgh, and also at Keyworth and the London Information Office in the Natural History Museum, South Kensington, London. Dyeline copies may be purchased from the Sales Desk.

Geophysical maps

• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1996
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1996
• 1:625 000
• Aeromagnetic map of Great Britain (and Northern Ireland), North sheet, 1965
• Bouguer anomaly map of the British Isles, Northern sheet, 1986
• 1:250 000
• Sheet 56N 06W, Argyll, Aeromagnetic anomaly, 1981; Bouguer gravity anomaly, 1979
• Sheet 56N 04W, Tay–Forth, Aeromagnetic anomaly, 1981; Bouguer gravity anomaly, 1979
• 1:50 000
• Geophysical information maps; these are plot-on-demand maps which summarise graphically the publicly available geophysical information held for the sheet in the BGS databases. Features include:
• regional gravity data: Bouguer anomaly contours and location of observations
• regional aeromagnetic data: total field anomaly contours and location of digitised data points along flight lines
• gravity and magnetic fields plotted on the same base map at 1:50 000 scale to show correlation between anomalies
• separate colour contour plots of gravity and magnetic fields at 1:125 000 scale for easy visualisation of important anomalies
• location of local geophysical surveys
• location of public domain seismic reflection and refraction surveys
• location of deep boreholes and those with geophysical logs
• Geochemical atlases
• 1:250 000
• Point-source geochemical data processed to generate a smooth continuous surface presented as an atlas of small-scale colour-classified digital maps.
• Argyll, 1990
• East Grampians, 1991
• Geochemical Survey Programme data are also available in other forms including hard copy and digital data.
• Hydrogeological maps
• 1:625 000
• Sheet 18 (Scotland) 1988.
• Groundwater vulnerability (Scotland) 1995.

BGS books

Memoirs, reports and papers relevant to the Schiehallion district arranged by topic. Most are either out of print or are not widely available, but may be consulted at BGS and other libraries.

• General geology
• British regional geology
• The Grampian Highlands, 4th Edition, 1995.
• Memoirs
• The geology of the country around Blair Atholl, Pitlochry and Aberfeldy (Sheet 55), 1905.
• The geology of Upper Strathspey, Gaick and the Forest of Atholl (Sheet 64), 1913.
• The geology of Corrour and the Moor of Rannoch (Sheet 54), 1913.
• Solid geology of the Schiehallion district (Sheet 55W), 2000.
• BGS Reports
• Metalliferous minerals
• Strata-bound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland: Preliminary report. MRP Report, No. 26, 1978.
• Strata-bound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland: Final report. MRP Report, No. 40, 1981.
• Bulk minerals
• Scottish mica schist as a possible source of ground mica. IGS Report, No. 71/10, 1971.
• Industrial mineral potential of andalusite and garnet in the Scottish Highlands. MRP Report, No. 142, 1997.
• Sand and gravel resources of the Tayside Region. IGS Report, No. 77/6, 1977.
• Economic geology
• Economic geology of the Schiehallion district, Central Highlands of Scotland. BGS Technical Report, No. WA/96/89, 1997.
• Geophysics
• A review of detailed airborne geophysical surveys in Great Britain. MRP Report, No. 136, 1995.

Documentary collections

Borehole record collection

BGS holds collections of records of boreholes which can be consulted at BGS, Edinburgh, where copies of most records may be purchased. For the Schiehallion district the collection consists of the sites and logs of the seven boreholes listed:

Material collections

Geological Survey photographs

Some 42 photographs illustrating aspects of the geology of the Schiehallion district are deposited for reference in the libraries at BGS, Edinburgh, and BGS, Keyworth; and in the BGS Information Office, London. The photographs were taken at various times over the last century. The photographs depict details of the various rocks and sediments exposed either naturally or in excavations and also some general views. A list of titles can be supplied on request. The photographs can be supplied as black and white or colour prints and 2 x 2 colour transparencies, at a fixed tariff, from the Photographic Department, BGS, Edinburgh.

Photograph numbers from the Schiehallion district are as follows: C. 377–401, 470–479; D. 2714–2718, 3722, 3724; Z. 00035–00061.

Petrological collections

The BGS petrological collections for the Schiehallion district consist of about 240 hand specimens and thin sections of the metamorphic and igneous rocks in the district. Information on databases of rock samples, thin sections and geochemical analyses can be obtained from the petrological collections manager, Mineralogy and Petrology Group, BGS, Edinburgh. This includes selected rock specimens and thin sections representative of the part of the sheet surveyed by P R Thomas, which have been donated to the BGS collection.

Rocks collected by P A R Nell, J M Maclachlan and J E Treagus during the 1989-94 resurvey are presently held at the University of Manchester, though critical specimens have been transferred to BGS. Information on these specimens should be requested through BGS.

Palaeontological collections

No fossils have been recorded from the district.

References

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

Anderson, E M. 1923. The geology of the schists of the Schiehallion district. Quarterly Journal of the Geological Society of London, Vol. 79, 423–445.

Anderson, E M. 1942. The dynamics of faulting. (Edinburgh: Oliver and Boyd.)

Anderton, R. 1977. The Dalradian rocks of Jura. Scottish Journal of Geology, Vol. 13, 135–142.

Anderton, R. 1985. Sedimentation and tectonics in the Scottish Dalradian. Scottish Journal of Geology, Vol. 21, 407–436.

Armstrong, M, Paterson, I B, and Browne, M A E. 1985. Geology of the Perth and Dundee district. Memoir of the British Geological Survey, Sheets 48W, 48E, 49 (Scotland).

Bailey, E B. 1922. The structure of the south-west Highlands of Scotland. Quarterly Journal of the Geological Society of London, Vol. 78, 82–127.

Bailey, E B. 1925. Perthshire tectonics: Loch Tummel, Blair Atholl and Glen Shee. Transactions of the Royal Society of Edinburgh, Vol. 53, 671–698.

Bailey, E B. 1934. West Highland Tectonics: Loch Leven to Glen Roy. Quarterly Journal of the Geological Society of London, Vol. 90, 462–523.

Bailey, E B, and McCallien, W J. 1937. Perthshire tectonics: Schiehallion to Glen Lyon. Transactions of the Royal Society of Edinburgh, Vol. 59, 79–118.

Bailey, E B, and Maufe, H B. 1916. The geology of Ben Nevis and Glen Coe and the surrounding country (explanation of geological Sheet 53), 1st edition. Memoir of the Geological Survey of Great Britain (Scotland).

Barrow, G. 1904. Moine gneisses of the east central Highlands and their position in the Highland sequence. Quaterly Journal of the Geological Society of London, Vol. 60, 400–444.

Barrow, G, Grant Wilson, J S, and Cunningham-Craig, E H. 1905. The geology of the country around Blair Atholl, Pitlochry and Aberfeldy (Sheet 55, Scotland). Memoir of the Geological Survey of Great Britain (Scotland).

Bendall, C. 1995. A geochronological, structural and metamorphic study of parts of the central and SW Dalradian. Unpublished PhD thesis, University of Manchester.

Beveridge, R, Brown, S, Gallagher, M J, and Merritt, J W. 1991. Economic geology. Chapter 16 in Geology of Scotland, Craig, G Y (editor). (London: The Geological Society.)

Bradbury, H J, Harris, A L, and Smith, R A. 1979. Geometry and emplacement of nappes in the Central Scottish Highlands. 213–220 in The Caledonides of the British Isles — reviewed. Harris, A L, Holland, C H, and Leake, B E. (editors). Special Publication of The Geological Society of London, No. 8.

Butcher, N J. 1991. The Duntanlich Proposal. In Exploration in the environment, Abstracts of 9th International conference 'Prospecting in areas of glaciated terrain'. (Edinburgh, BGS/NERC.)

Cliff, R A, Yardley, B W D, and Bussy, F R. 1996. U-Pb and Rb-Sr geochronology of migmatisation and metamorphism in the Dalradian of Connemara, Western Ireland. Journal of the Geological Society of London, Vol. 153, 109–120.

Coats, J S, Fortey, N J, Gallagher, M J, and Grout, A. 1984. Stratiform barium enrichment in the Dalradian of Scotland. Economic Geology, Vol. 79, 1585–1595.

Coats, J S, and Pease, S F. 1984. Strata-bound Zn-Pb mineralisation at Dericambus, Lower Glen Lyon, Scotland. In Excursion guide to stratabound mineralisation in the Middle Dalradian of the Grampian Highlands. Symposium Volume 'Prospecting in areas of Glaciated Terrain'. (Glasgow: University of Strathclyde.)

Coats, J S, Smith C G, Fortey, N J, Gallagher, M J, May, F, and McCourt, W J. 1980. Stratabound barium-zinc mineralisation in Dalradian Schist near Aberfeldy, Scotland. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied Earth Science), Vol. 89, B110–122.

Coats, J S, and five others. 1978. Stratabound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland: Preliminary Report, Institution of Geological Sciences, Mineral Reconnaisance Programme Report, No. 26, 43pp.

Coats, J S, and six others. 1981. Stratabound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland:Final Report, Institution of Geological Sciences, Mineral Reconnaisance Programme Report, No. 40, 116pp.

Curtis, S C, Pattrick, R A D, Jenkins, G T R, Boyce, A J, Fallick, A E, and Treagus, J E. 1993. A stable isotope and fluid inclusion study of fault related mineralisation in the Tyndrum area, Scotland. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied Earth Science.), Vol. 102, 39–47.

Dempster, T J. 1985. Uplift patterns and orogenic evolution in the Scottish Dalradian. Journal of the Geological Society of London, Vol. 142, 111–128.

Edwards, M B. 1986. Glacial environments. 416–438 in Sedimentary environments and facies. (Second Edition), Reading, H G (editor). (Oxford: Blackwell Scientific Publishing.)

Elles, G L. 1926. The geological structure of Ben Lawers and Meall Corranaich (Perthshire). Quarterly Journal of the Geological Society of London, Vol. 82, 304–331.

Elles, G L, and Tilley, C E. 1930. Metamorphism in relation to structure in the Scottish Highlands. Transactions of the Royal Society of Edinburgh, Vol. 56, 621–646.

Eyles, C H. 1989. Glacially and tidally-influenced shallow marine sedimentation of the Late Precambrian Port Askaig Formation, Scotland. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 68, 1–25.

Eyles, C H, and Eyles, N. 1983. Glaciomarine model for upper Precambrian diamictities of the Port Askaig Formation, Scotland. Geology, Vol. 11, 692–696.

Fairchild, I J. 1980. Sedimentation and origin of a late Precambrian 'dolomite' from Scotland. Journal of Sedimentary Petrology, Vol. 50, 423–446.

Fisk, S. 1986. An oxygen isotope study of siliceous rocks associated with stratabound mineralisation in Scotland and Ireland. Unpublished PhD thesis, University of Strathclyde.

Flinn, D, May, F, Roberts, J L, and Treagus, J E. 1972. A revision of the stratigraphic succession of the East Mainland of Shetland. Scottish Journal of Geology, Vol. 8, 335–343.

Ford, J L. 1994. Noble gases in ancient mineralising fluids. Unpublished PhD thesis, University of Manchester.

Fortey, N J, and Beddoe-Stephens, B. 1982. Barium silicates in stratabound Ba-Zn mineralisation in the Scottish Dalradian. Mineralogical Magazine, Vol. 46, 63–72.

Gallagher, M J, and six others. 1989. Stratabound barium and base metal mineralisation in Middle Dalradian metasediments near Braemar, Scotland. British Geological Survey, Mineral Reconnaisance Programme Report, No. 104, 24pp.

Geological Survey of Great Britain. 1902. Blair Atholl: Scotland, Sheet 55, Solid and Drift Geology, 1:63 360 series. (Southampton: Ordnance Survey for Geological Survey of Great Britain.)

Gibbons, W, and Harris, A L. 1994. A revised correlation of Precambrian rocks in the British Isles. Special Report of the Geological Society of London, No. 22.

Glover, B W. 1993. The sedimentology of the Neoproterozoic Grampian Group and the significance of the Fort William Slide between Spean Bridge and Rubha Cuilcheanna, Inverness-shire. Scottish Journal of Geology, Vol. 29, 29–43.

Glover, B W, and Winchester, J A. 1989. The Grampian Group: a major Late Proterozoic clastic sequence in the central Highlands of Scotland. Journal of the Geological Society of London, Vol. 146, 85–97.

Hall, A J. 1993. Stratiform mineralisation in the Dalradian of Scotland. 38–101 in Mineralisation in the British Isles. Pattrick, R A D and Polya, D A (editors). (London: Chapman and Hall).

Hall, A J, Boyce, A J, Fallick, A E, and Hamilton, P J. 1989. Isotopic evidence of the depositional environment of late Proterozoic stratiform barite mineralisation, Aberfeldy, Scotland. 28th International Geological Congress, Abstracts, Vol. 2, 2.10.

Hall, A J, Boyce, A J, Fallick, A E, and Hamilton, P J. 1991. Isotopic evidence of the deposition of late Proterozoic stratiform barium mineralisation, Aberfeldy, Scotland. Chemical Geology (Isotope geology section), Vol. 87, 97–114.

Halliday, A N, Graham, C M, Aftalion, M, and Dymoke, P. 1989. The depositional age of the Dalradian Supergroup: U-Pb and Sm-Nd isotopic studies of the Tayvallich Volcanics, Scotland. Journal of the Geological Society of London, Vol. 146, 3–6.

Harris, A L. 1963. Structural investigations in the Dalradian rocks between Pitlochry and Blair Atholl. Transactions of the Edinburgh Geological Society, Vol. 19, 256–278.

Harris, A L, Baldwin, C T, Bradbury, H J, Johnson, H D, and Smith, R A. 1978. Ensialic basin sedimentation: the Dalradian Supergroup. 115–138 in Crustal evolution in northwestern Britain. Bowes, D R, and Leake, B E (editors). Special Issue of the Geological Journal, No. 10.

Harris, A L, Bradbury, H J, and McGonigal, N H. 1976. The evolution and transport of the Tay Nappe. Scottish Journal of Geology, Vol. 12, 103–113.

Harris, A L, and Pitcher, W S. 1975. The Dalradian Supergroup. 52–75 in Correlation of Precambrian Rocks of the British Isles. Harris, A L, and others (editors). Special Publication of the Geological Society of London, No. 6.

Harris, A L, and Turner, D C. 1971. Scottish mica schist as a possible source of ground mica. Report of the Institute of Geological Sciences, No. 71/10. 9pp.

Harte, B, and Johnson, M R W. 1969. Metamorphic history of Dalradian rocks in Glens Clova, Esk and Lethnot, Angus, Scotland. Scottish Journal of Geology, Vol. 5, 54–80.

Haselock, P J, and Evans, R H S. 1990. Discussion on the Grampian Group: a major late Proterozoic clastic sequence in the Central Highlands of Scotland. Journal of the Geological Society of London, Vol. 147, 732–734.

Holgate, N. 1951. The Glen Banvie igneous complex of Perthshire. Quarterly Journal of the Geological Society of London, Vol. 106, 433–460.

Ixer, R A F, Pattrick, R A D, and Stanley, C J. 1997. The geology and genesis of gold mineralisation at Calliachar-Urlar Burn, Scotland. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), Vol. 106, B99–108.

Kennedy, W Q. 1948. On the significance of thermal structure in the Scottish Highlands. Geological Magazine, Vol. 85, 229–234.

Kilburn, C, Pitcher, W S, and Shackleton, R M. 1965. The stratigraphy and origin of the Portaskaig Boulder Bed Series (Dalradian). Geological Journal, Vol. 4, 343–360.

King, B C, and Rast, N. 1956. Tectonic styles in the Dalradians and Moines of parts of the Central Highlands of Scotland. Proceedings of the Geologists' Association, Vol. 66, 243–269.

Lindsay, N G, 1989. Contrasts in Caledonian tectonics of the northern and Central Highlands of Scotland. Unpublished PhD thesis, University of Liverpool.

Lindsay, N G, Haselock, P J, and Harris, A L. 1989. The extent of Grampian orogenic activity in the Scottish Highlands. Journal of the Geological Society of London, Vol. 146, 733–735.

Lowry, D, Boyce, A J, Fallick, A E, and Stephens, W E. 1994. Genesis of porphyry and plutonic mineralisation systems in metaluminous granitoids of the Grampian Terrane, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 85, 221–237.

Mason, J, Pattrick, R A D, and Gallagher, M J. 1991. Auriferous structures in the Upper Dalradian near Aberfeldy, Scotland. In Exploration in the environment, Abstracts of 9th International conference 'Prospecting in areas of glaciated terrain', Edinburgh, BGS/NERC, pp 47–49.

Moir, D N and Henley K J. 1967. Flotation of Scottish mica schist. Report Number CR 190 (MST), Warren Spring. 27pp (unpublished).

Moles, N R. 1983. Sphalerite composition in relation to deposition and metamorphism of the Foss stratiform Ba-Zn-Pb deposit, Aberfeldy, Scotland. Mineralogical Magazine, Vol. 47, 487–500.

Moles, N R. 1985a. Geology, geochemistry and petrology of the Foss stratiform baryte-base metal deposit and adjacent Dalradian metasediments, near Aberfeldy. Unpublished PhD thesis, University of Edinburgh.

Moles, N R. 1985b. Metamorphic conditions and uplift history in central Perthshire: evidence from mineral equilibria in the Foss celsian-barite-sulphide deposit, Aberfeldy. Journal of the Geological Society of London, Vol. 142, 39–52.

Moles, N R. 1986. Geological setting and the origin of the Foss celsian-barite-Zn-Pb sulphide deposit, central Scottish Highlands. Irish Association of Economic Geologists, Annual Review, 34–40.

Nell, P A R. 1984. The geology of lower Glen Lyon. Unpublished PhD thesis, University of Manchester.

Nell, P A R. 1986. Discussion on the Caledonian metamorphic core: an Alpine model. Journal of the Geological Society of London, Vol. 143, 723–728.

Nell, P A R, and Treagus, J E. 1994. Discussion on a pre-D₂ age for the Ben Vuirich Granite. Journal of the Geological Society of London, Vol. 151, 1045–1048.

Parker, M E. 1980. VLF electromagnetic mapping of the stratabound barium-zinc mineralisation in Dalradian schist near Aberfeldy, Scotland. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied. Earth Science), Vol.89, 123–129.

Passchier, C W, and Trouw, R A J. 1995. Microtectonics. (Berlin: Springer.)

Pattrick, R A D, Coleman, M L, and Russell, M J. 1979. Published discussions of papers presented at meeting on Mineralization in Northern Britain, University of Strathclyde, May 1979. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied. Earth Science), Vol. 88, 184–189.

Pattrick, R A D, and Treagus, J E. 1997. Economic geology of the Schiehallion district, Central Highlands of Scotland. British Geological Survey Technical Report, No. WA/96/89.

Peacock, J D, Mendum, J R, and Fettes, D J. 1992. Geology of the Glen Affric district. Memoir of the British Geological Survey, Sheet 72E (Scotland).

Phillips, E.R, and Key, R.M. 1992. Porphyroblast–fabric relationships: an example from the Appin Group in the Glen Roy area. Scottish Journal of Geology, Vol. 28, 89–101.

Piasecki, M A J. 1980. New light on the Moine rocks of the Central Highlands of Scotland. Journal of the Geological Society of London, Vol. 137, 41–59.

Piasecki, M A J, and van Breemen, O. 1979. The 'Central Highland Granulites': cover-basement tectonics in the Moines. 139–144 in The Caledonides of the British Isles — reviewed. Harris, A L, Holland, C H, and Leake, B E (editors). Special Publication of the Geological Society of London, No. 8.

Platten, I M, and Money, M S. 1987. Formation of late Caledonian subvolcanic breccia pipes at Cruachan Cruinn, Grampian Highlands, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 78, 85–103.

Powell, D, and Treagus, J E. 1970. Rotational fabrics in metamorphic minerals. Mineralogical Magazine, Vol. 37, 801–814.

Ramsay, D.M. 1959. The structure and metamorphism of Glen Lyon. Unpublished PhD thesis, University of Glasgow.

Rast, N. 1958a. The tectonics of the Schiehallion Complex. Quarterly Journal of the Geological Society of London, Vol. 114, 25–46.

Rast, N. 1958b. Metamorphic history of the Schiehallion complex, Perthshire. Transactions of the Royal Society of Edinburgh, Vol. 64, 413–431.

Read, H H. 1923. The geology of the country around Banff, Huntly and Turriff (Lower Banffshire and north-west Aberdeenshire). Memoir of the Geological Survey, Scotland, Sheets 86 and 96 (Scotland).

Read, H H. 1936. The stratigraphical order of the Dalradian rocks of the Banffshire coast. Geological Magazine, Vol. 73, 468–475.

Roberts, J L, and Treagus, J E. 1979. Stratigraphical and structural correlation between the Dalradian rocks of the SW and Central Highlands of Scotland. 199–204 in The Caledonides of the British Isles — reviewed. Harris, A L, Holland, C H, and Leake, B E (editors). Special Publication of the Geological Society of London, No. 8.

Rogers, G, Dempster, T J, Bluck, B J, and Tanner, P W G. 1989. A high-precision U/Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup. Journal of the Geological Society of London, Vol. 146, 789–798.

Rogers, G, Paterson, B A, Dempster, T J, and Redwood, S D. 1994. U-Pb geochronology of the 'Newer' gabbros, N E Grampians. In Caledonian terrane relationships in Britain. Symposium abstracts, British Geological Survey, Keyworth, Nottingham.

Russell, M J, Hall, A J, Willan, R C R, Allison, I, Anderton, R, and Bowes, G. 1984. On the origin of the Aberfeldy celsian + barite + base metal deposits, Scotland. 159–170 in Prospecting in areas of glaciated terrain, 1984. (London: Institution of Mining and Metallurgy).

Scott R A, Polya, D A, and Pattrick, R A D. 1991. Origin of sulphur in metamorphosed stratabound mineraliation from the Argyll Group Dalradian of Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 82, 91–98.

Shackleton, R M. 1958. Downward-facing structures of the Highland Border. Quarterly Journal of the Geological Society of London, Vol. 113, 361–392.

Smith, C G. 1977. Investigations of stratiform sulphide mineralizations in part of the Dalradian of central Perthshire, Scotland. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), Vol. 86, B50–51.

Smith C G, Gallagher, M J, Coats, J S, and Parker M E. 1984. Detection and general characteristics of stratabound mineralization in the Dalradian of Scotland. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied. Earth Science.), Vol.93, B125–133.

Smith, C G, and six others. 1977. Investigation of stratiform sulphide mineralisation in parts of central Perthshire. Institution of Geological Sciences, Mineral Reconnaisance Programme Report, No. 8, 88p.

Smith, D A. 1996. Fractures and mineralisation in the Scottish Dalradian. Unpublished PhD thesis, University of Manchester.

Smith, D I. 1979. Caledonian minor intrusions of the Northern Highlands of Scotland. 683–697 in The Caledonides of the British Isles —reviewed. Harris, A L, Holland, C H, and Leake, B E (editors). Special Publication of the Geological Society of London, No. 8.

Smith, R A. 1980. The geology of the Dalradian rocks around Blair Atholl, central Perthshire, Scotland. Unpublished PhD thesis, University of Liverpool.

Smith, R A, and Harris, A L. 1976. The Ballachulish rocks of the Blair Atholl District. Scottish Journal of Geology, Vol. 12, 153–157.

Spencer, A M. 1971. Late Precambrian glaciation in Scotland. Memoir of the Geological Society of London, No. 6.

Spencer, A M. 1981. The late Precambrian Port Askaig Tilite in Scotland. 632–636 in Earth's pre-Pleistocene glacial record. Hambrey, M J, and Harland, W B (editors). (Cambridge: Cambridge University Press.)

Spencer, A M. 1985. Mechanisms and environments of deposition of late Precambrian geosynclinal ttillites: Scotland and East Greenland. Palaeogeography, Palaeoclimatology, Palaeoecology, Vol. 51, 143–157.

Stringer, P. 1957. Polyphase deformation in the Upper Dalradian rocks of the Southern Highlands of Scotland. Unpublished PhD thesis, University of Liverpool.

Sturt, B A. 1961. The geological structure of the area south of Loch Tummel. Quarterly Journal of the Geological Society, London, Vol. 117, 131–156.

Sturt, B A, and Harris, A L. 1961. The metamorphic history of the Loch Tummel area. Liverpool and Manchester Geological Journal, Vol. 2, 689–711.

Swainbank I G, and Fortey, N J. 1981. Lead isotope ratios of galena from stratabound mineralisation in the Scottish Dalradian. 20–23 in Caledonian–Appalachian Stratabound Sulphides, Scotland. Hall, A.J, and Gallagher, M.J. (editors). Extended Abstracts volume. University of Strathclyde/BGS publication.

Swainbank I G, Fortey, N J, and Boast, A.M. 1981. Lead isotope ratios of galena from stratabound mineralisation in the Scottish Dalradian. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied. Earth Science.), Vol. 90, B57.

Tanner, P W G. 1995. New evidence that the Lower Cambrian Leny Limestone at Callander, Perthshire, belongs to the Daladian Supergroup and a reassessment of the 'exotic' status of the Highland Border Complex. Geological Magazine, Vol. 132, 473–483.

Tanner, P W G. 1996. Significance of the early fabric in the contact metamorphic aureole of the 590 Ma Ben Vuirich Granite, Perthshire, Scotland. Geological Magazine, Vol. 133, 683–695.

Tanner, P W G, and Leslie, A G. 1994. A pre-D₂ age for the 590 Ma Ben Vuirich Granite from the Dalradian of Scotland. Journal of the Geological Society of London, Vol. 151, 209–212.

Thomas, P R. 1965. The structure and metamorphism of the Moinian rocks of Glen Garry, Glen Tilt, and adjacent areas of Scotland. Unpublished PhD thesis, University of Liverpool.

Thomas, P R. 1979. New evidence for a Central Highland Root Zone. 205–211 in The Caledonides of the British Isles —reviewed. Harris, A L, Holland C, and Leake, B E (editors). Special Publication of the Geological Society of London, No. 8.

Thomas, P R. 1980. The stratigraphy and structure of the Moine rocks north of the Schiehallion Complex, Scotland. Journal of the Geological Society of London, Vol. 137, 469–482.

Thomas, P R, and Treagus, J E. 1968. The stratigraphy and structure of the Glen Orchy area, Argyllshire, Scotland. Scottish Journal of Geology, Vol. 4, 121–134.

Thost, C H G. 1860. On the rocks, ores, and other minerals on the Property of the Marquess of Breadalbane in the Higlands of Scotland. Proceedings of the Geological Society of London, Vol. 16, 421–428.

Treagus, J E. 1987. The structural evolution of the Dalradian of the Central Highlands of Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol. 78, 1–15.

Treagus, J E. 1991. Fault displacements in the Dalradian of the Central Highlands. Scottish Journal of Geology, Vol. 27, 135–145.

Treagus, J E. 1999. A structural reinterpretation of the Tummel Belt and a transpressional model for the evolution of the Tay Nappe in the Central Highlands of Scotland. Geological Magazine, Vol. 136, 643–660.

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Solid geology of the Schiehallion district—summary (rear cover)

The district described in this memoir lies in the centre of the Scottish Grampian fold-belt (part of the Caledonide Orogen). The late Proterozoic and minor contemporary volcanic rocks of the Dalradian Supergroup were deformed and metamorphosed in the Grampian and Caledonian orogenies and cut by minor intrusions of late Silurian age.

The psammites of the Grampian Group, which underlie the northern half of the area, and the varied schists, quartzites and metamorphosed limestones of the Appin Group and Islay Subgroup, cropping out immediately to the south, were deposited in shallow water. The succeeding quartzites, garnet-bearing schists and hornblende-schists of the Easdale and Crinan subgroups represent deeper water, sedimentary and volcaniclastic rocks, and include a stratabound barite/sulphide ore body, of presumed exhalative origin. The overlaying Loch Tay Limestone, and the turbidites of the Southern Highland Group, were deposited at the foot of a submarine slope, and were intruded by numerous basic sheets.

These rocks were deformed and metamorphosed during the Lower Palaeozoic Grampian and Caledonian orogenies. Two major regional fold phases occurred in early Ordovician times. Ductile deformation caused extreme thinning of the limbs of the major folds, which in places has resulted in dislocation and in the formation of slides. A third regional phase and several locally developed phases occurred later in the Ordovician. The amphibolite-grade metamorphism bears witness to deep burial.

The Caledonian evolution of the area was completed by a period of fracturing, associated with minor intrusions and vein mineralisation. The Loch Tay Fault is a major fault trending north-north-east; it has a net sinistral displacement of 6 km and a net dipslip of 0.75 km down to the west. Some of the smaller faults contain gold-bearing sulphide veins. Minor sills and dykes, mostly microgranitic or microdioritic, were intruded during this fracture period.

In late Carboniferous times, a dolerite dyke, trending east-west, was intruded in the south-east of the district.

Figures, plates and tables

(Figure 1) Solid geological map of the Schiehallion district. Locations of some other figures are also shown.

(Figure 2) Topographical map of the Schiehallion district.

(Figure 3) Previous geological cross-sections through the Schiehallion district. a.   Area west of Loch Tay Fault b.Area east of Loch Tay Fault. Sections A, B and C were drawn along very similar lines and are drawn approximately to the same scale; section D was drawn to represent the general structure for the area of Sheet 55E immediately to the east. In section A, a different stratigraphical succession was used below the Ben Lawers Schist as shown in the key; sections B, C and D use the same succession as the present account. The axial trace in A is approximately equivalent to the Sron Mhor Syncline of section C . The heavy, upright axial trace in B is equivalent to the Loch Tummel Synform of the present account. In D the axial planes are those of the authors' major 'D₃' folds. b. Area east of Loch Tay Fault

(Figure 4) Lithostratigraphy of the Grampian, Appin and Argyll groups of the Strath Fionan area. (See p.11 for Key.) Transects A–A to D–D are discussed in Chapters 2 and 3 and the microdiorites I–X are discussed in Chapter 4.

(Figure 5) Closure of the Errochty Synform with severely attenuated Appin Group formations north of the Errochty Dam. The closure of the Errochty Synform is marked by the outcrop pattern of these formations as well as by the boundary with the the Grampian Group and by the conformable amphibolites. The microdiorite sheets post-date the fold.

(Figure 6) Comparison of the Appin and lower Argyll group succession in the Schiehallion district with other Dalradian successions.

(Figure 7a) Three extracts from the 1:10 000 maps of the district to illlustrate most of the principal, and some of the minor, lithologies that comprise the Easdale Subgroup. Key is common to a, b, c. a. The Killiecrankie Schist/Carn Mairg Quartzite transition on Meallanan Odhar, south-west of Schiehallion. The Killiecrankie Schist here contains, near its top, many units of coarse pebbly quartzite, which herald the overlying Carn Mairg Quartzite. The Carn Mairg Quartzite, near its base, contains a unit of graphitic schist identical to the overlying Ben Eagach Schist. The coarse pebbly units, as well as amphibolites, are folded about a major D₂ antiform, which is supported by the D₂ vergence; this fold is a component of the compound Allt Mor Synform. Other folds, not supported by the D₂ vergence, are of D₁ age. The area lies astride the hinge zone of the De Bohespic Antiform, reflected in the gentle curvature of the outcrop pattern.

(Figure 7b). Three extracts from the 1:10 000 maps of the district to illlustrate most of the principal, and some of the minor, lithologies that comprise the Easdale Subgroup. Key is common to a, b, c.The Carn Mairg Quartzite, its transition to the Ben Eagach Schist and the two varieties of the Ben Lawers Schist, exposed in the Culdaremore area at the east end of Glen Lyon. The D₃ Culdaremore Antiform refolds a major D₂ closure; the two major D₁ folds, the Chesthill Syncline and the Meall Garbh Anticline, also pass through the area.

(Figure 7c). Three extracts from the 1:10 000 maps of the district to illlustrate most of the principal, and some of the minor, lithologies that comprise the Easdale Subgroup. Key is common to a, b, c. Outcrop patterns of the Ben Eagach Schist, Ben Lawers Schist, Farragon Volcanic and Ben Lui Schist formations around the D₂ Ruskich Antiform in the Slatich area of Glen Lyon. Most of the smaller-scale, tight, E–W-trending folds, north of the River Lyon, are of D₃ age.

(Figure 8) Spatial variations in the stratigraphy of the Farragon Volcanic Formation at selected localities east of the Loch Tay Fault, with actual stratigraphical thicknesses. Stratigraphical column left blank at levels where there is no exposure. Vertical scale = 1:2500.

(Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) Axial traces of principal folds. Lines of section are shown for the cross-sections in Glen Garry (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17).

(Figure 10) Plunge and vergence of selected D₂ minor folds and bedding/second cleavage intersections. Two bedding/first cleavage intersections are shown in the east of the map, with filled arrowheads.

(Figure 11). Plunge and vergence of selected D₃ minor folds and bedding/third cleavage intersections.

(Figure 12) a. Profile of De Bohespic Antiform and Errochty Synform, using plunges due south, varying from 20° in the north to 55° in the south. b. Cross-section of Grampian Group along Glen Garry; see (Figure 1) for localities (1–12) described in text and (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9) for axial-plane traces and line of section. Projection of Tummel Quartzite and axial-traces north-west of the trace of the Bohespic Antiform is speculative.

(Figure 13) Principal faults; named faults are discussed in text.

(Figure 14) Cross-section east of Loch Tay Fault, along the line shown on (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9).

(Figure 15)a 3D reconstruction of the interference between the D₃ Creag Chean Synform and the D₂ Meall Tairneachan Fold. Dashed lines represent D2 minor fold and intersection lineation attitudes. Figure 15b Reconstruction of pre-D₃ geometry of major D₂ folds, east of Loch Tay Fault. Dashes on upper surfaces of formations show the regional curvature of the minor D₂ folds and intersection lineations; axial planes of D₂ folds (horizontal) and subsequent D₃ folds (vertical) are shown as transparent planes. Figure 15c Reconstruction of pre-D₂ attitude of D₁ folds (abbreviations as above) on the south-east limb of the proto-Tay Nappe.

(Figure 16) Fold axial traces around Loch Tummel and conjectured pattern in the area covered by the loch.

(Figure 17) Cross-section [NN 691 522] to [NN 695 458] to illustrate structure of Glen Lyon area, assuming constant easterly plunge of D₂ and D₃ folds; for line of section see (Figure 12)b, across the area east of the Loch Tay Fault (Figure 14) and in Glen Lyon (Figure 17)." data-name="images/P999965.jpg">(Figure 9). Interference between D₂, DL and D₃ is well displayed in bottom left.

(Figure 18) Composite cross-section of the Schiehallion district drawn along trace of Loch Tay Fault (south-west from point Z on (Figure 1). Point A on (Figure 1) is shown in its displaced positions, A′ and Aʺ, in the sections on the east and west walls of the fault, respectively. The structure of the area west of the fault has been projected on to the fault plane, by using an easterly down-plunge projection of both D₁ and D₂ folds and intersections along the topographical profile on the west wall of the fault trace. The structure of the fault has similarly been projected, using the dominant westerly plunge of D₁ and D₂ as well as of the D₃ folds. The ground information is much more limited on this side of the fault and the section only extends as far south as the Loch Tay Antiform. Because D₁ plunge is variable, and not always co-axial with D₂ and D₃ folds, the trace of the closures of the D₁ Sron Mhor and Creag na h- Iolaire folds does not always coincide with that of their equivalents west of the fault. Information from upper Glen Lyon (Sheet 54E) is shown, bottom right and the Grampian/Appin boundary, bottom left, is projected from fold traces in the Glen Garry area.

(Table 3) and (Table 4))." data-name="images/P999975.jpg">(Figure 19) Glen Banvie Complex; see (Figure 1) for location. Numbers refer to analysed samples, (Table 3) and (Table 4)).

(Figure 20) Distribution of Geal Charn breccias near Kinloch Rannoch; see (Figure 1) for location.

(Figure 21) Microdiorite sheets (I–VIII) in Clunes road-cut on A9 and in adjacent River Garry. a. Vertical section showing road-cut on north side of road; geometry of D₂ folds indicated. b. Plan of road and River Garry location close to 7 on (Figure 1) and (Figure 12).

(Figure 22) AFM diagram for Caledonian and later intrusive rocks of the Schiehallion district

(Figure 23) Sketches of structural/metamorphic textures drawn from thin sections of pelites and semipelites in the district. Column A illustrates the development of S₂ in the matrix minerals; columns B, C and D illustrate the inclusion trail patterns preserved in garnet porphyroblasts, according to the time of growth with respect to the D₂ deformation; in column D the garnets are assumed to have grown slowly during D₂. The effect of porphyroblast growth on the matrix is also illustrated. After Nell (1984).

(Figure 24) Aberfeldy baryte-sulphide horizon, showing the full extent of the horizon in the Ben Eagach Schist (modified from Coats et al., 1981 and Beveridge et al., 1991).

(Figure 25) Stream sediment anomalies associated with the Aberfeldy baryte-sulphide horizon showing the barium anomaly in streams draining the outcrop of the Ben Eagach Schist on the ridge between the Tay and Tummel valleys (from Coats et al., 1981).

(Figure 26) Aberfeldy baryte-sulphide horizon in the Foss area (for location see (Figure 1)). The barium-enriched lithologies are amalgamated and represent the lithologies between the topmost and lowermost baryte quartz-celsian horizon. Based on data from: Moles, 1985a; Dresser Minerals (especially from the work of mine geologists A R Burns and N J Butcher); Coats et al., 1980; Coats et al., 1981.

(Figure 27) Creag an Chanaich and the opencast workings at Foss Mine, showing the surface distribution of the Lower and Upper Mineralised horizons. Mapping to the west of easting [145] based on Moles (1985a) and this study, mapping to the east after J M Maclachlan and N J Butcher; compilation and structural observations by J E Treagus.

(Figure 28) General profile reconstruction of structure of mineralised horizon in Foss opencast pits and in track sections on east slope of Creag an Chanaich. For location see (Figure 27). A plunge of 25° towards 255° is assumed. Fault movement is restored.

(Figure 29) Geology of the area south of Dericambus, Glen Lyon, showing the position of the sphalerite-galena-pyrrhotite mineralisation (part based on mapping by P A R Nell (1984); Coats and Pease, 1984). For location see (Figure 1).

(Figure 30) Calliachar–Urlar area showing mineralised veins. (Mapping by J M Maclachlan and Colby Gold plc.) For location see (Figure 1).

(Figure 31) Tombuie area, showing veins and trenches. For location see (Figure 1).

Plates

(Front cover) Cover photograph: View of Schiehallion from the north shore of Loch Rannoch [NN 6040 5895]; main ridge is made of Schiehallion Quartzite, which dips steeply to the south. The ridge in front is crossed by the Boundary Slide (centre to top right) (D 2717). Photographer: T S Bain

(Rear cover)

(Plate 1a). Meall Dubh Quartzite Formation, Ballachulish Subgroup, Appin Group; quartzite showing cross-bedding younging to the south. Meall Dubh [NN 7285 5667] (Z00035).

Plate 1b. Blair Atholl Dark Limestone and Dark Schist Formation, Blair Atholl Subgroup, Appin Group; graphitic limestone with bedding marked by laminae of quartz, graphite and opaque minerals. Isoclinal D₂ minor folds verge to the north-west. Strath Fionan [NN 721 561](Z00036).

(Plate 1c). Killiecrankie Schist Formation, Easdale Subgroup, Argyll Group; thinly bedded psammite and semipelite. The D₂ minor folds verging to the north-west are folded by upright D₃ minor folds and cross-cut by S3. Dubh Chnocan [NN 7984 5694] (Z00037).

(Figure 2) Schiehallion Boulder Bed Formation, Islay Subgroup, Argyll Group; cut specimen of Boulder Bed showing smaller clast of microgranite and larger of grey granite in semipelitic matrix. Clasts are elongated in the S₂ cleavage. Scale in centimetres. Allt na Moine Buidhe [NN 703 611] (Z00038).

(Plate 3a) Tummel Psammite Formation, Strathtummel Subgroup, Grampian Group; psammites and semipelites. The D₂ folds, with south-east to neutral vergence, are on the north-west limb of the D₂ Clunes Antiform. Clunes road cutting on A9 road, west of dyke VI of (Figure 21), height about 20 m [NN 7867 6685] (Z00039).

(Plate 3b) Carn Mairg Quartzite Formation, Easdale Subgroup, Argyll Group; massive pebbly psammite. The beds dip north-west near the hinge of the D1 Creag na h- Iolaire Anticline; the S₁ cleavage is parallel to the hammer shaft. Creag na h- Iolaire [NN 826 556] (Z00040).

(Plate 3c) Ben Lawers Schist Formation, Easdale Subgroup, Argyll Group; a rotated boudin of calcareous quartzite in chloritic calc-schist containing quartz segregations. South-west of Creag Chean [NN 789 529] (Z00041).

(Plate 4a). Intermediate-scale D₂ folds, verging north-west, in psammites and semipelites of the Killiecrankie Schist Formation. Dubh Chnocan [NN 799 571] (Z00042).

(Plate 4b). D₂ fold-pair, verging to the south-east, refolding tight D₁ synform (to right of D₂ synform): the flat-lying cleavage parallel to the hammer shaft is S_c. Thinly bedded Beoil Quartzite. North of Lochan an Daim [NN 7209 5755] (Z00043).

(Plate 4c). D₃ minor folds and cleavage, verging south-east, on south-east limb of Creag Chean Synform. Chloritic calc-schist of Ben Lawers Schist Formation. Meall Dubh Mor [NN 7840 5276] (Z00044).

(Figure 5) Cut specimen of Farragon Volcanic Formation, Easdale Subgroup, Argyll Group. The thin quartz- and amphibole-rich beds are affected by isoclinal D₁ folds (arrowed) and refolded by tight D₂ folds. Slatich, Glen Lyon [NN 6407 4778] (Z00045). Scale in centimetres.

(Plate 6) Photomicrograph of late amphibolite sheet. The weak orientation of the hornblende prisms (top left to bottom right) is typical of the interior of these sheets. The white matrix minerals are quartz (dominant) and feldspar; the dark minerals are sphene. The large, slightly turbid area right of centre is a cluster of large plagioclase crystals mostly altered to clinozoisite, muscovite and carbonate; such areas are considered to be original plagioclase phenocrysts. Lassintullich trackside [NN 6987 5733]. Slide 2954, plane polarised light, x 20 magnification (Z00046).

(Plate 7) Photomicrographs of Caledonian intrusive igneous rocks. All plane polarised light, x 25 magnification. a. Appinitic diorite, Ruighe nan Saorach, south-west of Loch Errochty [NN 6683 6445] (Z00047). Abundant euhedral crystals of hornblende, associated with turbid plagioclase and minor biotite. Secondary muscovite and colourless amphibole are also present. b. Melamicrodiorite, A9 road cutting, Glen Garry [NN 6743 7172] (Z00048). Abundant prismatic hornblende crystals, showing slight preferred orientation. The turbid matrix comprises heavily altered feldspar (mostly plagioclase), chlorite and carbonate. c. Melamicrodiorite, Achloa, River Lyon [NN 766 485] (Z00049). Augite phenocryst in moderately mafic microdiorite. Prismatic hornblende crystals (showing flow orientation) are associated with turbid plagioclase and opaque oxides. d. Microdiorite, Geal Charn [NN 6839 5481] (Z00050). Average microdiorite, dominated by interlocking euhedral-subhedral plagioclase crystals, with minor amounts of interstital quartz and secondary chlorite. e. Porphyritic felsite, Black Tank, A9 road [NN 7742 6791] (Z00051). Euhedral (turbid) feldspar and subhedral quartz phenocrysts in very fine-grained felsitic groundmass, consisting of feldspar, quartz and muscovite. f. Quartz-dolerite, Allt a' Bhealaich near Tombuie Cottage [NN 787 450] (Z00052). Typical doleritic texture of interlocking plagioclase, augite and ilmenite crystals. Some of the augite crystals have cores of pigeonite (with more prominent fractures and well-defined edges).

(Plate 8) Garnet porphyroblasts in amphibolite of Farragon Volcanic Formation, Easdale Subgroup, Argyll Group. S2 dips steeply to the north-west and the vertical surface is approximately parallel to amphibole and pressure shadow lineation. Quartz mantles to garnets are asymmetric, suggesting a component of simple shear top-down to the north-east. Creag nan Cop, south of Creag Chean [NN 7874 5230] (Z00053). Coin is 23 mm in diameter.

(Plate 9a). Photomicrographs of metamorphic rocks.. Ben Lui Schist Formation, Crinan Subgroup, Argyll Group. Garnet preserves bedding (top left to bottom right) as well as oblique S₁ (near vertical as elongate inclusions of quartz and opaque minerals). External S2 fabric, which augens the syntectonic garnet, comprises coarser quartz grains together with biotite and muscovite. Allt Coire Pheiginn, Inchgarth [NN 7625 5040] (Z00054). Slide 2795, x 35; plane polarised light.

(Plate 9b). Photomicrographs of metamorphic rocks.. Pitlochry Schist Formation, Southern Highland Group. Penetrative S₂ subhorizontal schistosity of muscovite, bioitite and quartz at the top and bottom of the garnet and concentrated in the pressure solution tails to the sides. The garnet typically has curved inclusion trails of quartz parallel to S₁, which was syntectonically deformed during D₂. Glen Quaich, west of Tombuie Cottage [NN 7828 4474] (Z00055). Slide 2777. x 35; crossed polarisers.

(Plate 9c). Photomicrographs of metamorphic rocks.. Ben Lawers Schist Formation, Easdale Subgroup, Argyll Group. S₂ fabric of fine muscovite and biotite and coarse quartz grains has been corrugated by upright D₃ microfolds. The three minerals have been recrystallised along the S₃ planes (dark), together with fine-grained opaque minerals. A partly chloritised garnet (bottom right) shows typical pattern of quartz inclusions (S₁), indicating syn-D₂ garnet growth. Frenich Burn [NN 8261 5602]. Slide 2817 (x 25, crossed polarisers) (Z00056).

(Plate 9d). Photomicrographs of metamorphic rocks.. Pitlochry Schist Formation, Southern Highland Group. An S₁ fabric, of fine muscovite and lesser biotite, chlorite and quartz, has been strongly affected by D₂ microfolds with subhorizontal axial surfaces. New biotite (e.g. dark area, bottom left) and muscovite have grown along S2. Long ilmenite crystals (black) parallel to S₁ have been rotated and, in one instance here, folded by D₂. Drummond Hill [NN 7640 4674]. Slide 2811, (x 90, plane polarised light) (Z00057).

(Plate 10a). Opencast workings at Foss East (looking west) with massive white baryte of the Upper Mineralised Horizon exposed in the foreground. The underground mine entrance is at the base of the steep slope, the access track (and hairpins) can be seen cutting into the slope (see Figure 27) and the ridge of Creag an Chanaich. (Z00058).

(Plate 10b). Vein 6, Calliachar suite exposed in trench. The 20 cm-wide oxidised vein (right) is hosted by psammite (see Figure 30) (Z00059).

(Plate 11a). Bladed porphyroblasts of celsian/hyalophane in foliated quartz muscovite schist from BGS borehole 9, Creag an Chanaich (from Fortey and Beddoe-Stephens, 1982). Field of view 10 mm wide (Z00060).

(Plate 11b). Reflected light photomicrograph showing electrum (E, white) in fractures in pyrite (P, light grey) with galena (G, mid grey). Field of view 250 µm wide (Z00061).

Tables

(Table 1) Summary of the geological sequence in the Schiehallion district.

(Table 2) Age relationships of igneous rocks.

(Table 3) Whole rock analyses of igneous rocks from the Schiehallion district.

(Table 4) Representative electron microprobe analyses [EPMA] of pyroxenes, amphiboles and biotites from analysed rocks.

(Table 5) EPMA of silicates in the Aberfeldy mineralisation and calculated atomic proportions.

(Table 6) EPMA of electrum from the Calliachar and Urlar Burn veins (wt%). a) Calliachar veins. Analyses C1, C6, primary ore in drill core; P3 in Au-rich pod; C3S in gossan.

(Table 2) Age relationships of igneous rocks.

(Table 3) Whole rock analyses of igneous rocks from the Schiehallion district

(Table 4) Representative electron microprobe analyses [EPMA] of pyroxenes, amphiboles and biotites from analysed rocks

(Table 5) EPMA of silicates in the Aberfeldy mineralisation and calculated atomic proportions

(Table 6) EPMA of electrum from the Calliachar and Urlar Burn veins (wt%).