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Geology of the Glen Shee district. Memoir for 1:50 000 geological sheet 56W together with adjacent parts of sheets 55E, 65W and 64E (Scotland)

By A Crane, S Goodman, M Krabbendam A G Leslie I B Paterson, S Robertson and K Rollin. Contributors: C A Auton, D F Ball, T P Fletcher

In memory of Steven Robertson 1960–2000

Bibliographical reference: Crane, A, Goodman, S, Krabbendam, M, Leslie, A C, Paterson, I B, Robertson, S, and Rollin, K E. 2002. Geology of the Glen Shee district. Memoir of the British Geological Survey, Sheet 56W with parts of sheets 55E, 65W and 64E (Scotland).

London: The Stationery Office 2002 NERC copyright 2002. First published 2002. ISBN 0 11 884546 2. Printed in the UK for the Stationery Office J1188000 C6 10/02

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 map, numbers 43, 44 and 53. © Crown copyright reserved. Ordnance Survey Licence No. GD272191/2002

(Front cover) Cover photograph: View south-south-eastwards along Glen Shee from Craig of Runavey [NO 1295 6966]. The rocks around the vantage point are calcareous schists (Ben Lawers Schist Formation) and amphibolites at the north-western margin of the late Silurian Glen Shee Pluton; dioritic and granodioritic rocks of the pluton occupy the poorly exposed, low-lying hummocky foreground. The rounded summit beyond (Creag na Bruaich) lies on the southern margin of the pluton, and comprises Ben Lui Schist Formation migmatites (Duchray Hill Gneiss Member). In the distance is Mount Blair where Mount Blair Psammite and Semipelite Formation crops out (D5136). Photographer T S Bain.

(Rear cover)

Acknowledgements

This memoir is the result of collaboration between BGS and a team from the University of Aberdeen and Queens University, Belfast. BFS surveyed the eastern and southeastern parts of the district with the remainder surveyed by the University team, all between 1990 and 1994. The university team also examined areas adjacent to but outwith the north-west corner of Sheet 56W. This allowed linkage with other recent work and facilitated a more comprehensive understanding of the way in which the Glen Shee district fits into the broader picture of the geology of the Southern Highlands. Effort was concentrated on the bedrock geology; Quaternary deposits were only resurveyed in the south-east of the district. Ground surveys of Total Field Magnetic Intensity and VLF–EM data were integrated with the mapping results in areas of poor exposure, principally in the central and southern parts of the Dairadian outcrop. These were particularly useful in tracing both mafic rocks and faults. The university effort was undertaken as part of NERC academic mapping contract F60/G2/34.

The chapters on Precambrian stratigraphy and igneous rocks, metamorphism, structure and Silurian intrusive rocks were written by A Crane and S Goodman from Aberdeen, A G Leslie and M Krabbendam from Belfast and S Robertson from BGS. D Stephenson commented on parts of the stratigraphy and structure based on his experience of the area immediately north-west of the district. The chapters on upper Silurian–Lower Devonian rocks and Carboniferous dykes were written by I B Paterson, and the chapter on Quaternary deposits was written by I B Paterson with additional contributions by C A Auton. The Applied Geology chapter was written by S Robertson and D F Ball with a contribution from T P Fletcher, whilst the Concealed Geology chapter was written by K Rollin and S Robertson. The memoir was compiled by the late S Robertson and edited by D I J Mallick and A D McAdam.

Notes

Throughout the memoir, the word 'district' refers to the area described in the memoir as detailed in Chapter 1. National Grid references are given in square brackets; all lie in 100 km2 NO unless specified otherwise.

All radiometric dates have been recalculated on the basis of the most recent decay constants at the time of publication.

Numbers preceded by the letter S refer to the Scottish BGS sliced rock/thin section collection held at BGS, Edinburgh. Numbers preceded by QY, VY, WY and ZY refer to specimens held at the University of Aberdeen. Numbers preceded by the letter D refer to the Scottish BGS collection of photographs.

Enquiries concerning geological data for the district should be addressed to the Manager, National Geosciences Record Centre, Murchison House, West Mains Road, Edinburgh EH9 3LA.

Preface

Understanding the geology of the UK is essential for both conservation and development. It is particularly relevant to exploration for resources, the avoidance of hazards and sensible land-use planning. In recognition of this, the British Geological Survey is funded by central Government through the Natural Environment Research Council to improve our understanding of the three-dimensional geology of Britain through a programme of data collection, interpretation, publication and archiving. One aim of this programme is to ensure coverage of the UK land area by modern 1:50 000 scale geological maps, mostly with explanatory descriptions, by the year 2005. This memoir on the Glen Shee district is part of the output from that programme. It is based on observations and interpretations made during the resurvey of the district between 1990 and 1994. It represents the first comprehensive account of the geology of the district since no memoir resulted from the primary survey conducted in the 1890s. The resurvey was a joint venture between BGS and a team from Queen's University, Belfast and Aberdeen University. The university effort was funded by a NERC University mapping contract. The BGS is pleased to foster co-operation between the Survey and the academic community and this Glen Shee study is a good example of how successful such arrangements can be.

The Glen Shee district straddles the boundary between the southern Scottish Highlands and the Midland Valley. In terms of the Highland geology, the district lies to the south-west of the areas recently resurveyed as part of the BGS multidisciplinary East Grampian Project district and to the north-east of the Pitlochry and Schiehallion districts of Perthshire. A number of publications have highlighted the stratigraphy and structure of the Perthshire areas. The Glen Shee district therefore provides an important link between BGS and university work in the Southern Highlands. Strati-graphical inconsistencies in the lower part of the Dalradian Supergroup between the present work and that of previous university work in the adjacent Braemar and Pitlochry districts have highlighted the need to re-examine some of the adjacent districts.

The tectonic setting of the Highlands in the late Precambrian is under debate at the present time, with one school of thought arguing that the region formed part of a rifting continental margin and another arguing that the region underwent orogenesis. Crucial to this argument is the structural setting of the Ben Vuirich Granite which was emplaced at that time. The north-eastern part of the granite is described in this memoir and provides useful evidence, albeit inconclusive, towards this debate. The structural development of the Glen Shee district is described in detail with a major section illustrating the response of the thick pile of Dalradian sedimentary rocks to low-angle crustal shearing during the Grampian orogeny.

The Midland Valley part of the district is underlain by Old Red Sandstone lavas and sedimentary rocks. It contrasts markedly in terms of land use and scenery from the Highland areas to the north, thus demonstrating the control on all aspects of the landscape exerted by the underlying geology. Resources in the district include sand and gravel, and water. The Old Red Sandstone and sand and gravel deposits are locally good aquifers. However, this feature also makes them vulnerable to groundwater pollution.

The Quaternary deposits of only the southern part of the district were resurveyed. However, a summary of the Quaternary history and deposits of the whole district have been produced using data from the partial resurvey together with original survey data and some recently published work and experience from adjacent districts.

David A Falvey, PhD Director, British Geological Survey Kingsley Dunham Centre Keyworth, Nottingham NGI2 5GG

Geology of the Glen Shee district—summary

The Glen Shee district straddles the boundary between the late Proterozoic Dalradian Supergroup of the Grampian Highlands and the Palaeozoic rocks of the Scottish Midland Valley. The Dalradian rocks comprise a thick succession of clastic and carbonate rocks with interspersed developments of basic igneous, metavolcanic and metavolcaniclastic rocks. Locally, thick developments of graphitic schist occur in the lower and middle parts of the succession. In overall terms, the lithostratigraphy of the Dalradian rocks can be correlated with adjacent districts. However, a major unit of quartzose and semipelitic rocks within the Blair Atholl Subgroup, the Tulaichean Schist Formation, has not been identified by previous workers.

The Dalradian rocks experienced four phases of regional deformation, at least in part attributed to the Ordovician Grampian orogeny. Upright D1 folds were subjected to subhorizontal shearing with a top to the south-east sense during D2; this resulted in regional inversion of the Dalradian rocks of the district. Major slides developed on the limbs of large-scale fold structures with resulting local attenuation or excision of parts of the stratigraphical succession. Lateral differential movement during the regional shearing event resulted in a transfer zone analogous to a lateral ramp system. D3 may have been a polystage event; strain was more localised and partly controlled by the pre-existing structural geometry. The D4 phase of deformation produced a major antiformal structure with a steep southern limb, adjacent and parallel to the Highland boundary.

The Ben Vuirich Granite was emplaced into the Dalradian succession before D2, and probably also before D1. Granite and pegmatite sheets were also emplaced, probably before and after D3, whereas basic igneous sheets were emplaced between D2 and D3. Regional metamorphism accompanied deformation with peak conditions attained during D2. Close to the Highland boundary, chlorite and biotite represent the peak metamorphic minerals, whereas in the north of the district kyanite developed in rocks of suitable composition. Migmatisation was extensive within the Duchray Hill Gneiss Member, with an early period of stromatic migmatisation followed by the development of granitoid rocks. Both types of migmatisation are attributed to in situ metamorphic segregation. Post-tectonic intrusive igneous rocks include the largely granodioritic Glen Shee Pluton and a number of minor intrusions including porphyries, microdiorites and monzonites.

Minor occurrences of probable Ordovician rocks of the Highland Border Complex crop out within the Highland Boundary Fault Zone. The fault zone comprises the Highland Boundary Fault, which in the Glen Shee district occurs entirely within Silurian to Devonian Old Red Sandstone rocks, and a number of splays. The Middleton Muir Fault, which separates Silurian to Devonian ('Old Red Sandstone') rocks from the Dalradian in the west of the district, is the major splay of the Highland Boundary Fault. Farther east, Silurian and Devonian rocks rest unconformably on the Dalradian succession. Regional gravity and aeromagnetic data suggest that a major unit of basic and ultrabasic rocks underlies the Highland Boundary Fault Zone at a very shallow depth and extends north to underlie the southern part of the Dalradian rocks.

The Silurian rocks comprise a dacitic ignimbrite, whereas the Devonian rocks are mostly andesitic to basaltic lavas and conglomerates. The conglomerates to the north of the Highland Boundary Fault contain mostly volcanic rock clasts and to the south metamorphic and plutonic igneous rock clasts. In the south of the district, the volcanic rocks pass gradually up into interbedded conglomerates and sandstones and then sandstones.

Basic dykes trending east-north-east were emplaced into both the Dalradian and Silurian to Devonian rocks during Carboniferous times.

During the Quaternary, thick ice sheets developed at various times in the district, although the main glacial features and deposits date from the last glaciation in the late Devensian. During deglaciation, ice retreated towards the west-south-west with meltwater issuing down the main north-west-trending valleys, depositing locally thick spreads of sand and gravel. Large kettleholes, some now infilled by lochs, and others with important occurrences of organic material, developed within these deposits.

(Table 1) Geological sequence in the Glen Shee district.

Geology of the Glen Shee district—(rear cover)

This memoir provides an account of the geology of the Glen Shee district that stretches from the rolling rich agricultural land around Blairgowrie to the mountainous upper parts of glens Isla, Shee and Fearnach. The district straddles the Highland Boundary Fault Zone which forms the regional boundary between the Late Proterozoic Dalradian Supergroup of the Southern Highlands and the Silurian–Devonian rocks of the Midland Valley. Here, the Palaeozoic rocks locally overlap north of the fault zone and rest unconformably on the Dalradian Succession.

The Dalradian rocks form a thick succession of originally elastic, carbonate and graphitic pelagic rocks with interspersed developments of basic igneous rocks. They were intruded by basic igneous and granitic rocks and experienced four phases of regional deformation, mainly attributable to the Grampian Orogeny. The first deformation is interpreted to have resulted in upright folds which were strongly modified by large-scale shearing during the second deformation.

Associated slides locally attenuated or excised parts of the succession. Deformation was more localised during the third and fourth episodes. Accompanying Barrovian metamorphism ranged from chlorite and biotite zone in the south to kyanite zone with local migmatisation in the north.

The Palaeozoic rocks comprise a Silurian dacitic ignimbrite overlain by Devonian conglomerates and andesitic to basaltic lavas. Southwards Devonian rocks pass up into interbedded conglomerates and sandstones and finally sandstones.

During the Quaternary, thick ice sheets developed periodically, although the main glacial features and deposits date from the latest glaciation in the late Devensian. During deglaciation, ice retreated overall to the west-south-west with meltwater issuing down the main north-west–south-east valleys and depositing locally thick spreads of sand and gravel. Large kettleholes developed within these deposits, and some of these are now infilled by lochs.

Chapter 1 Introduction

This memoir describes the geology of the Glen Shee district, situated in the southern part of the Scottish Highlands. The district encompasses 1:50 000 geological Sheet 56W (Glen Shee) together with those parts of Sheets 65W (Braemar), 64E (Ben Macdui) and 55E (Pitlochry) which lie adjacent to the north-west corner of Sheet 56W (Figure 2). The memoir is designed to be read in conjunction with the 1:50 000 geological map for Sheet 56W (British Geological Survey, 1996). Published 1:50 000 maps of the adjacent areas described in this memoir have not been revised. Revised information dating from the current survey (Figure 1), (Figure 2) is available at 1:10 000 scale. A directory of BGS information sources covering the district, including maps and reports, is given in Chapter 4.

Most of the district is underlain by Precambrian rocks which were deformed and metamorphosed during the Grampian orogeny. Granitic intrusions were emplaced both before and after the main episodes of deformation. In the south-east of the district, the Precambrian rocks are locally overlain unconformably by, and elsewhere faulted against, Silurian and Devonian sedimentary rocks and lavas. The faults make up the Highland Boundary Fault Zone, which forms the boundary between the disparate geological successions of the Southern Highlands and the Scottish Midland Valley.

Topography

The geology of the district exerts a strong control on topography on a variety of scales. On the broadest scale, the contrast between the Precambrian and Devonian rocks is reflected by the terrain of the Southern Highlands and the Midland Valley respectively. The Southern Highlands comprises upland heather-clad moorland which passes north into rugged mountainous terrain and culminates in the peaks of Glas Tulaichean (1050 m) and Carn an Righ (1029 m). The major valleys of Glen Shee, Strathardle, Gleann Fearnach and Gleann Taitneach, which developed into their present form during Quaternary glacial periods, drain south towards Blairgowrie, whereas in the east of the district, the upper part of Glen Isla drains south-east towards Alyth (Figure 2). The Midland Valley is largely composed of relatively low-lying agricultural land in the area to the north and west of Blairgowrie and Rattray. On a more local scale, lithologies which are resistant to weathering produce upstanding features. This is well demonstrated by the quartzite ridge of Carn an Righ and more widely by amphibolite units which commonly stand up as roches moutonnées. Similarly, conglomerates within the Devonian rocks produce upstanding features. This is particularly apparent north-east of Blairgowrie where a broad conglomerate ridge stands above the lowland plain of sandstones to the south-east. Igneous rocks vary widely in their response to weathering. For example, the Glen Shee Pluton forms a broad topographical depression between Glen Shee and Glen Isla, whereas some porphyries produce prominent hills. A good example is Knockton in the east of the district [NO 195 583].

Geological succession

The greater part of the district is underlain by deformed and metamorphosed Precambrian rocks which at least locally form a basement to the Silurian and Devonian sedimentary rocks in the south-east of the district (Figure 1). The Highland Boundary Fault Zone defines the southern edge of the Precambrian rocks and defines an upper crustal terrane boundary. Slivers of serpentinite and a diverse association of probably mainly Ordovician metamorphosed sedimentary and basic volcanic rocks referred to as the Highland Border Complex occur within the fault zone. These are poorly represented in the district. Devonian rocks straddle the fault zone, although there is a significant break in the Devonian succession at the fault. Geophysical evidence, referred to in Chapter 3, suggests that the terrane boundary at deeper levels in the crust may be up to 20 km north-west of the Highland Boundary Fault rather than mirroring the surface expression of the structure.

The Precambrian rocks comprise a succession of metamorphosed sandstones, mudstones and limestones, some of which are graphitic, and basic volcanic rocks, belonging to the Dalradian Supergroup. They were deposited in a range of environments which reflect a transition from shallow-water, intracratonic basins to continental margin turbidites. The oldest Dalradian rocks are assigned to the Appin Group. They were probably deposited about 750 million years ago, although there are few accurate constraints on the absolute age of deposition. They comprise a succession of psammites, quartzites, semipelites and pelites, some of which are graphitic or calcareous, and metacarbonate rocks. The protoliths of these rocks were probably deposited under stable marine conditions as indicated by the lateral continuity of lithological units over several hundred kilometres acrosss Scotland and Ireland. Thinly interbedded lithological units are interspersed with thicker developments, firstly of quartzite, succeeded by psammite and semipelite and finally by graphitic schist. Metamafic rocks occur in the upper parts of the group. Their protoliths are not easily determined because of subsequent deformation and metamorphism, although most appear to be intrusive.

The depositional environment became somewhat less stable during deposition of the Argyll Group. Lithological associations can be traced for long distances, although individual lithologies are more restricted in their occurrence. This has been attributed to progressive continental rifting and the development of second- and third-order depositional basins. The base of the group is marked by a tillite, which is poorly developed in the district. The absence of stratigraphical units recognised elsewhere in the Dalradian immediately below the tillite may indicate erosion and a break in deposition around the time of tillite formation. The tillite is succeeded by thick developments of quartzite. Relatively clean quartzites forming large lenses thought to reflect channel-fill deposits are succeeded by more extensive impure quartzites. These are succeeded in turn by thick developments of graphitic schist, the well-known Ben Eagach Schist, and calcareous schist with local metavolcanic developments. This basin deepening culminated in quite extensive eruption of basic volcanic rocks. These are succeeded by clastic rocks, dominated by semipelites and pelites which are thought to record turbidite deposition during a further phase of subsidence and basin deepening. The top of the Argyll Group comprises calcareous and metacarbonate rocks, also probably partly deposited by turbidity currents.

The succeeding Southern Highland Group was also largely deposited by turbidity currents, probably on what had now developed into a rifted continental margin. Interbedded psammites and semipelites in the lower part of the group pass upwards, especially in the east of the district, to more proximal deposits dominated by psammites and gritty psammites. The age of deposition of these highest exposed parts of the Dalradian is currently under debate. Field evidence elsewhere in the Southern Highlands suggests that at least part of the Southern Highland Group is of Early Palaeozoic age (Tanner, 1995). However, as the Ben Vuirich Granite was emplaced into the Dalradian at around 590 Ma, this indicates either that the whole succession is of Precambrian age or alternatively that the granite was emplaced at a high level within the evolving sedimentary basin.

The Dalradian succession was deformed soon after the end of deposition. Just as the minimum age of deposition is under debate, so is the age of earliest deformation. It has not yet been fully resolved whether the Ben Vuirich Granite was emplaced before or after the regional D1 deformation event. The former allows a single, multiphase, orogenic event, probably within the Ordovician, whereas the latter requires Precambrian deformation, which is at variance with field evidence that Dalradian deposition continued into the Palaeozoic. The main Ordovician deformation event, referred to as the Grampian orogeny, inverted the Dalradian succession of the Glen Shee district during a period of ductile crustal shearing. Large-scale D1 folds were strongly modified during this D2 shearing event with the development of slides and lateral transfers. Associated Barrovian metamorphism ranged from chlorite zone close to the Highland Boundary Fault to kyanite zone farther north in the district. Migmatisation associated with the meta morphism was locally significant in the north-east, and produced the Duchray Hill Gneiss Member. Minor intrusions of granite and pegmatite were emplaced during the later stages of deformation. Uplift following metamorphism was associated with the development of 'steep belts' both in the north-west and south-east of the Dalradian of the district. With continued uplift and cooling, brittle faults developed, the most significant of which is the Highland Boundary Fault. This structure has a history of both strike-slip and dip-slip movements. Ultimately the Dalradian was thrust southwards over the Highland Border Complex and the upper Silurian and Lower Devonian rocks. Numerous minor intrusions, including porphyries and microdiorites together with the largely granodioritic Glen Shee Pluton, were emplaced probably in the late Silurian, penecontemporaneous with faulting.

The history of the area south of the Highland Boundary Fault prior to the late Silurian is not known, and neither is the displacement across the fault. It may be that many tens or hundreds of kilometres once separated the two areas. However, from late Silurian times onwards, a limited separation of the two terranes is constrained by the Lintrathen Tuff Member which occurs north of the fault within the district and south of the fault elsewhere in the Midland Valley. These ignimbrites were succeeded by outpourings of Lower Devonian andesitic and basaltic andesitic lavas. A thin basal breccia is developed at least locally below the volcanic rocks. The lavas pass up into a more mixed association of lavas and massive conglomerates dominated by volcanic rock clasts. With time, volcanic activity waned and the 'volcanic' conglomerates were replaced by 'Highland' conglomerates with clasts largely of metamorphic rock types. These in turn pass up gradationally into sandstones and pebbly sandstones as the scale of intracontinental rifting and associated fault scarps progressively diminished.

The late Palaeozoic, Mesozoic and Cainozoic history of the area is not recorded save for a suite of late Carboniferous dolerite dykes which were intruded during regional crustal extension.

In common with the rest of northern Britain, the district experienced repeated alternations between glacial and interglacial conditions during the Quaternary. Thick ice sheets developed and these together with the associated meltwaters sculpted the landscape that had developed during Devonian to Pleistocene times. The main glacial features and deposits date from the latest (Dimlington Stadial) glaciation in the late Devensian which ended about 13 500 years ago. During the Dimlington Stadial, ice moved across the Glen Shee district, initially from the north-west and later from the west-south-west, plucking some hill tops, leaving bare rock surfaces and depositing blankets of till elsewhere. During deglaciation, ice retreated to the west-south-west with meltwater issuing down the main south-east-trending valleys leaving spreads of sand and gravel, particularly where these valleys open into the Midland Valley. In the postglacial period, peat developed in waterlogged basins and upland areas, and alluvium and river terrace deposits were laid down by the rivers.

Chapter 2 Applied geology

In this chapter the geological factors relevant to land-use planning and development within the Glen Shee district are reviewed. The key issues are listed and considered, and in some cases the reader is directed to sources where more information can be obtained.

Key issues

The Glen Shee district straddles the Highland Boundary Fault. In the south of the district, around Blairgowrie, agriculture is important to the local economy; to the north, in the thinly populated mountainous parts, tourism is of economic importance. Geological factors influence both, but also determine the mineral potential of the district, the exploitation of which may cause conflict with other uses.

Applied geological issues

Agriculture

Agricultural potential in the Glen Shee district is strongly controlled by geology. Upland areas with poor soils are generally underlain by the hard metamorphic rocks of the Dalradian Supergroup. Upland sheep farming and forestry are the main forms of agriculture; forestry is locally extensive, particularly in the Kindrogan area of Strathardle. Grouse moors cover substantial parts of the uplands, and deerstalking takes place over much of the district. In contrast, lowland areas around Blairgowrie, which are underlain by Devonian sandstones, provide good land for arable farming and the cultivation of soft fruit.

Tourism

The natural landscape provides the main attraction for tourists in the district, particularly within the upland areas. There are few foreseeable threats to the landscape. At present there is little likelihood of mineral workings having a significant impact on the landscape (see below). Infrastructure developments are likely to be limited and themselves linked to tourism, such as improved access to the Cairnwell ski area immediately north of the district.

Ground stability

Ground stability hazards have been considered only for 1:10 000 Sheet NO 14 NE, which covers the area around and to the north of Blairgowrie. Within this area, ground stability problems are restricted to landslips associated with oversteepening of stream or river banks undergoing fluvial erosion or dislocation of till, and more particularly head, on steep, water-bearing slopes. Significant landslips are recognised in the valley of the River Ericht. The precipitous scar of Heughs of Mause between [NO 1753 4725] and [NO 1710 4673] is a complex of individual failures of till, each associated with sporadic springs emanating from different levels in the scar. Slippage on the slopes east of Craighall between [NO 1739 4834] and [NO 1726 4783] has involved the A93 trunk road from Blairgowrie to Braemar where a lights-controlled bailey bridge crossing [NO 1710 4809] has been constructed. In this area, numerous springs have lubricated bouldery till on a steep slope above cliffs of conglomerate. Until the flow of water diminishes, the prospect of stabilising this slope is an unlikely one. Other less significant landslips occur elsewhere in the valley of the River Ericht. Landslips also occur in other parts of this area in the Lonny valley associated with spring activity [NO 1624 4629] and between [NO 1617 4640] and [NO 1662 4646].

Ground stability problems may be encountered locally on kame terraces where kettle holes have become infilled with peat, since peat has a high water content, a very low bearing-capacity and is highly compressible.

There are no problems with respect to undermining.

The subsidence risk is largely unknown; however, it is not expected to be high owing to the apparent absence of clay deposits. It is possible that glacial lacustrine deposits susceptible to shrinkage may be present, although no such deposits have been identified; however, systematic resurvey of the Drift deposits was only carried out in the Midland Valley areas.

Water resources

Groundwater occurs in all rock formations and most types of superficial deposits across the district. The amount of water available for exploitation from bedrock sources depends very largely on the rock type and the frequency of fissures and joints within 100 m depth of rockhead. The Highland Boundary Fault forms a broad boundary between poor Dalradian aquifers and higher yielding Devonian rocks to the south. In addition, there is a distinction between narrow superficial aquifers in valleys to the north and more widespread granular Drift to the south of the fault.

The rocks with the highest potential for the abstraction of groundwater are those of the Devonian Strathmore Group. To date, only limited exploitation of this aquifer has taken place and the few boreholes present were drifted to provide small volumes of potable water to houses and farms. The presence of many streams draining from Highland catchments, in addition to numerous lochs along the margins of the high ground, has allowed demands for irrigation to be met by surface water at most lower lying farms. Consequently, there has been less demand for groundwater sources capable of supplying at least 1000 m3/day to agricultural irrigation systems when compared with the drier areas of Strathmore farther east around Brechin.

The area to the north of the Highland Boundary Fault has recently been the subject of some renewed activity in the development of private groundwater supplies. For many years, isolated farms and dwellings in Strathardle and Glen Shee have relied on shallow dug wells, springs and burns for their water supply. The majority of these sources provide high quality water throughout the year. However, modern farming practices, combined with increases in the demand for water, have resulted in the deterioration of yield or water quality in several shallow wells or springs. Consequently, several of these have been replaced with boreholes drilled into Dalradian schists to depths averaging 35 m. Although not normally suitable for abstractions of more than 100 m3/day, boreholes such as these are a reliable source of good quality water if properly sited and constructed.

Large volumes of granular superficial deposits occur below the water table along the lower valleys of the Lunan Burn and rivers Ericht and Isla. To date, little development of this groundwater resource has taken place, but there exists the potential for the development of several high-yielding well fields in the area to the south and west of Blairgowrie for irrigation or fish farming purposes.

Within Strathardle and Glen Shee, alluvial sand and gravel of largely unknown thickness occurs beneath the floodplains. Particularly in Stathardle, these shallow aquifers may be suitable for providing groundwater either for fish farms or for alternative potable supplies. Flanking Strathardle on the eastern side of the valley, spreads of till contain irregular layers of more permeable granular deposits. These have been exploited as a source of groundwater through the construction of dug wells. Because these shallow aquifers are susceptible to pollution from agricultural fertilisers and other chemicals, the underlying Dalradian succession is a more reliable source of groundwater.

Vulnerability to pollution

The Lower Devonian sequence is regarded as being a highly permeable aquifer, with the capacity to transmit liquids rapidly from rockhead to the water table. As such, it is highly vulnerable to pollution from surface sources.

The covering of granular superficial deposits across much of the low-lying areas of land to the south of the Highland Boundary Fault has resulted in the accumulation of soils having high and intermediate rates of leaching, as shown in the Groundwater Vulnerability Map of Scotland, (British Geological Survey, 1995a). These areas are, therefore, extremely sensitive to land use activities that have the potential to pollute groundwater. These include the application of agricultural fertilisers, landfill sites and industrial units that process liquids.

North of Blairgowrie, the Dalradian, a weakly permeable aquifer, is less vulnerable to surface pollution except where highly fractured zones occur. Shallow sand and gravel aquifers are more susceptible to pollution, particularly from such sources as silage clamps and slurry pits.

Mineral resources

Baryte

The Ben Eagach Schist Formation outwith the Glen Shee district is characterised by local concentrations of stratabound baryte with associated zinc and lead sulphides (Coats et al., 1980, 1984). Most significant of these are the multi-million tonne Foss and Duntanlich deposits near Aberfeldy which currently produce up to 60 000 tonnes of direct shipping grade baryte per annum. Despite extensive geochemical and geophysical exploration in the wake of the Aberfeldy discoveries, no stratabound baryte was discovered in the Glen Shee district (Coats et al., 1987), although it is present in the Braemar district to the north (Gallagher et al., 1989). However, two occurrences of vein baryte within the district may have been remobilised from stratabound ore at depth. At Nether Craig [NO 169 612] a quarry in the Loch Tay Limestone Formation contains veins or sheets of baryte up to 30 cm thick. These locally show marginal development of coarsely crystalline baryte and adularia and they may be connected by thin veins. The thicker veins are parallel to the layering in the surrounding calcareous schist and metacarbonate rock. The baryte was described as stratabound by Gallagher (1991) but was interpreted as a syn- or post-metamorphic replacement of the host carbonate. In the Allt Coire Lanard [NO 137 648] coarsely crystalline baryte exposed in the stream occupies a west-north-west-trending fault in the Duchray Hill Gneiss Member of the Ben Lui Schist Formation. Minor base metal sulphides also occur in this vein.

Discordant baryte veins are also recorded in Devonian rocks in the vicinity of the Highland Boundary Fault on the east bank of the River Ericht 2 km north of Blairgowrie. At one locality [NO 1765 4723] several sinuous subparallel veins up to 1 cm thick strike 120°, whereas at another [NO 1769 4722] 2 cm-thick veins along joint surfaces strike 146°.

Base metal sulphides

Disseminated galena and sphalerite occur in iron-stained, fine-grained micaceous quartzite of the Ben Eagach Schist Formation in the Allt an Daimh [NO 1408 7162] (Coats et al., 1987). Traces of galena are also found in quartzite of comparable age on the southeast side of Carn an Daimh [NO 1365 7096]. Drill core analyses indicate that base metal contents are low with zinc generally below 0.8 per cent and lead less than 0.5 per cent. Anomalous values of zinc in stream sediments may indicate further base metal mineralisation in the Ben Eagach Schist Formation of Glen Lochsie.

Geochemical anomalies around the margins of the Creag Lamhaich Porphyritic Granodiorite occur in the vicinity of occurrences of base metal sulphides. Examples include the sulphide stockwork in the Allt Aulich [NO 085 748] and the mineralised enclave of Glen Taitneach Schist Member in Glen Lochsie [NO 058 726]. A mineralised sample from the latter locality contains high values of barium, (289 ppm) copper (2818 ppm) and molybdenum, (23 ppm) with only moderate concentrations of lead (30 ppm) and zinc (100 ppm). These values reflect the intrusive-related nature of the mineralisation (Rose et al., 1979, p.562). No mineralisation is visible in the porphyry itself and analysis of the petrologically similar Glen Shee Pluton shows no anomalous values of base metals (McCourt and Gallagher, 1980).

High levels of copper have been recorded in stream sediment downstream of the outcrop of the Loch Tay Limestone Formation (Coats et al., 1987), whereas stringers of pyrite and chalcopyrite are found in calcite veins in the metacarbonate rocks in the quarry at Wester Bleaton [NO 115 598]. These minerals may have been concentrated in the veins by fluids associated with an adjacent dolerite dyke.

Gold

Gold has been recorded in panned concentrates collected from streams on Devonian lavas at Bridge of Lally (Coats et al., 1993). The baryte occurrences at Nether Craig and in the Allt Loire Lanard are also considered to have potential for gold mineralisation, as they represent low temperature hydrothermal veins (Coats et al., 1993).

Bulk resources

Sand and gravel

Sand and gravel deposits in the form of extensive sheet-like spreads, kame terraces, eskers and alluvial deposits are widespread in the south-east of the district and along the major river valleys. Sand and gravel resources in kame terraces above the water table in Strathardle and Glen Shee have been estimated at 72 million tonnes and 20 million tonnes respectively (Paterson, 1977). Kame terraces pass downstream into outwash deposits yielding an estimated further 25 million tonnes in Glen Shee. Several tens of million tonnes are also estimated from kame terraces in Glen Isla, although not all these lie within the district. Thick deposits of hummocky and sheet-like spreads of sand and gravel occur in the Blairgowrie area. Large kettle holes are now water filled to the west of the town such as Loch of Drumellie [NO 140 445] and Loch of Clunie [NO 115 440]. The lochs are more than 15 m deep and it is inferred that the gravel thickness exceeds the depths of the lochs (Paterson, 1977). In 1993, there were no operational sand and gravel pits (Harris et al., 1994), although several smaller pits are undoubtedly operational periodically for local use. An example is the pit [NO 1724 4884] within a prominent esker beside the A93 north of Blairgowrie. In May 1983, only one major sand and gravel pit was operational within the district, namely at Cleaves [NO 161 434] near Blairgowrie (Aitken, 1983).

Peat

Commercial deposits of peat have been excavated on Muirton Moss [NO 150 490]. Other peat deposits such as those in the Forest of Alyth area have been worked for local use.

Constructional and industrial resources

Metacarbonate rocks in the Loch Tay Limestone Formation of the Dalradian Supergroup and minor occurrences in other formations have been extensively quarried for local agricultural use Small disused pits are abundant, for example in the Loch Tay Limestone Formation on the east slopes of Mount Blair [NO 17 64] and in Southern Highland Group rocks near Knockali [NO 162 594]. The large commercial quarries at Wester Bleaton [NO 115 598] (Plate 8) and An Dun [NO 110 592] were worked until the 1970s, with some of the limestone being used in cement manufacture.

Disused slate quarries in the Forneth area [NO 09 45] in the Dalradian Birnam Slate and Grit Formation provided roofing stone for the local market. They are minor workings compared with the large quarries in the same formation near Dunkeld to the south-west.

Two sandstone quarries, now disused, probably supplied local building stone. Both the larger quarry at Drumend [NO 1970 4595] and the smaller at Hillbarns [NO 1628 4520] worked Devonian maroon, well-jointed, planar-bedded pebbly sandstones.

Carboniferous dolerite dykes have been quarried for road metal in several parts of the district. According to local hearsay, dolerite quarried from the Dounie Burn [NO 0998 5854] was used to make millstones.

Conspicuously jointed Devonian volcanic rocks have been worked at Dykeside [NO 1606 4946] and near Glendams [NO 1903 4888]. Cobbly hardcore has been worked from weathered conglomerates in places as has till and some of the glaciofluvial gravels.

Man-made deposits

Household rubbish and backfilled quarry spoil infill former excavations around Blairgowrie [NO 1680 4610]; [NO 1788 4656]; [NO 1805 4656], and at Drumend [NO 1969 4597]. Other backfilled workings may occur elsewhere in the district but have not been recorded.

Little information is available on potential hazards arising from made ground. However, few problems are envisaged in the district, which is one of relatively low population density.

Chapter 3 Concealed geology

There are few sources of information on the concealed geology of the Glen Shee district. The depth to, and the nature of, the basement beneath both the Dalradian and Devonian rocks is only poorly understood. Interpretation of the LISPB seismic experiment (Bamford, 1979), part of which traversed the western part of the district, suggested that the upper boundary of crystalline basement with velocity of more than 6.4 km s−1 occurs at depths of 12 to 15 km beneath most of the Grampian Highlands and less than 10 km beneath the. Midland Valley. A marked step in the basement topography occurs some 15 to 20 km north-west of the Highland Boundary Fault. The Moho occurs at depths of 30 to 32 km throughout. Studies of the isotopic and geochemical characteristics of late Silurian granites suggested the juxtaposition, along a Mid Grampian Line, of contrasting types of basement beneath the Dalradian (Halliday et al., 1985). Basement to the north-west is isotopically similar to early Proterozoic basement which crops out on the Rhinns of Islay (Dickin and Bowes, 1991; Marcantonio et al., 1988; Muir et al., 1994). All that can be deduced about the basement to the south-east, beneath both the Southern Highlands and the Midland Valley, including the Glen Shee district, is that it is significantly younger than the early Proterozoic basement to the north-west (Halliday et al., 1985). It is not clear whether this contrast in type of basement matches the basement step recorded by LISPB. A new interpretation of LISPB (Barton, 1992) suggested a low velocity zone (5.8 km s−1 as opposed to 5.95 km s−1) in the upper crust (depths of 2 to 4 km) extending for up to 30 km north-west of the Highland Boundary Fault.

Geophysical data

Regional Bouguer gravity data have been collected at Ordnance Survey spot heights and benchmarks at a distribution of about one every 3 km2. The main feature of the district is a closed elongate positive Bouguer gravity anomaly of up to 10 mGal, which trends north-east and lies approximately 8 km north-west of Blairgowrie (Figure 3). Anomalies decrease gradually to the north-west across most of the Southern Highland Group and then sharply over the outcrops of the Argyll and Appin groups. They also decrease quite sharply to the south-east.

Total field aeromagnetic anomalies were collected in analogue form across the district in 1963 along east–west flight lines spaced approximately 2 km apart. The data have been digitised and sorted into flight lines. The main feature of the data is a prominent anomaly of about 800 nT above background, which trends north-east and lies approximately 5 km north-west of Blairgowrie (Figure 4). A secondary feature is a closed circular anomaly of about 400 nT over the Glen Shee Pluton.

Detailed ground surveys of Total Magnetic Intensity were acquired for 160 km2 of the southern parts of the Dalradian of the district. Additionally, in-phase and quadrature components of VLF–EM data were surveyed over 20 km2 in the vicinity of the Middleton Muir Fault. Detailed ground surveys using magnetic, self potential and VLF methods have been made in the Spittal of Glen Shee area as a follow up to an airborne geophysical survey of this part of the Southern Highlands. In the same area, detailed gravity profiling has been undertaken in the search for economic base metal and baryte deposits (Chapter 2). Ground magnetic data have also been collected in the Blairgowrie–Alyth area of the Highland Border, together with additional gravity measurements.

Interpretation of geophysical data

Lineament analysis

A lineament analysis based on shaded-relief images of the regional gravity and magnetic data, identified a series of lineaments resulting from geological structures crossing the district. The main features from the gravity data (Figure 3) include north-north-west-trending and northeast-trending lineaments. The former are evident through Dunkeld, immediately west of the district, the south-west part of the Ben Vuirich Granite and west of Duchray Hill. The geological significance of these lineaments is not clear. The lineament patterns of the magnetic data show similar trends (Figure 4), although the structures trending towards the north-west quadrant are rather less clear. East-north-east-trending magnetic lineaments extending through Kirkmichael probably result from Carboniferous dykes (Chapter 11). In contrast, the Highland Boundary Fault Zone (Chapter 12) is marked by both magnetic and gravity lineaments.

Blairgowrie anomaly

Local maxima in the aeromagnetic anomaly north of Blairgowrie are directly associated with minor outcrops of serpentinite within the Highland Border Complex (Chapter 8). Ground magnetic traverses to the west of Blairgowrie also indicate that similar rocks underlie Devonian rocks at very shallow depth. However, the width of the aeromagnetic anomaly suggests a deeper source. Farquharson and Thompson (1992) modelled a ground magnetic profile through Blairgowrie in terms of near-surface ultrabasic rocks and Devonian andesitic lavas above a wider basic or ultrabasic mass at a depth of about 2 km.

Deconvolution of profile (A–A′) based on the regional gravity and magnetic data (Figure 3), (Figure 4) supports this general model. Werner deconvolution of the aeromagnetic data suggests a source at a depth of approximately 1.5 to 2 km, while deconvolution of the gravity data suggests that the top of the source causing the 10 mGal anomaly may be at a depth of 1 to 2.5 km. These sources are considered to lie underneath the Dalradian and to represent basic and ultrabasic rocks which may be part of the Highland Border Complex.

This would imply that the Dalradian was thrust southeast over these sources.

In order to test this concept, part of a crustal scale section across the Grampian Highlands based on integrated modelling of the gravity and magnetic data is shown in (Figure 5). The section (B–B′) extends across the Highland Boundary Fault close to the maximum magnetic anomalies, and shows magnetic Highland Border Complex rocks beneath the Southern Highland Group Dalradian rocks and the Devonian Strata.

This is part of a whole-crust model to a depth of 40 km across the Grampian Highlands. Gravity and magnetic effects are calculated using the density and susceptibility parameters indicated in the model [Mg m−3/10−3 SI]. The Highland Boundary Fault anomalies are interpreted as ophiolitic Highland Border Complex rocks (HBX) beneath Devonian sandstones and lavas and extending north of the Highland Boundary Fault, beneath psammites and semipelites of the Southern Highland Group.

Glen Shee Pluton

The pluton is associated with discrete aeromagnetic and gravity anomalies and has observed volume susceptibilities of the order of 0.01 SI. No saturated density measurements are available, but the mean bulk oxide (Si, Ca, Mg) concentrations of 18 samples (65.31%, 2.88%, 2.56%) have been used to provide estimates of the bulk density of the intrusion (2.65 Mgm−3). This indicates a likely negative contrast of approximately −0.10 Mgm−3 compared with the host Dalradian rocks.

Gravity observations over the Glen Shee Pluton are sparse. The only observation made during the original survey in the central part of the intrusion was considered suspect because the height at the station was estimated from a contour line rather than a spot height or benchmark. However, there is no reason to believe that the observation was in error and the station has been reinstated and recalculated into the field in (Figure 3) to show a clear negative anomaly of less than −20 mGal over the intrusion. A regional gravity field across the intrusion is not easy to define because of difficulty in modelling the effects of the Argyll Group, the relatively low density Grampian Group and the Gleann Fearnach Fault. An approximate regional field was generated by elimination of 17 gravity stations on and around the intrusion and gridding the remaining data. This gives an estimated residual anomaly over the pluton of approximately −7 mGal. By applying a density contrast of -0.10 Mgm−3 and assuming a near vertical intrusion, this has been interpreted as indicating that the granite body is about 5 or 6 km thick. The aeromagnetic anomaly which would be expected to result from this gravity model, assuming an induced magnetisation of 0.75A/m, is approximately 400 nT. This compares well with the observed aeromagnetic anomaly. The three flight lines which cross the Glen Shee Pluton suggest that the maximum anomaly occurs over the south-east part of the intrusion. This may indicate an NRM component to the magnetisation. Turnell (1985) indicated a mean NRM direction of D = 75, I = 30 for the Comrie intrusion (Sheet 47W) which is probably of similar age.

Chapter 4 Precambrian: Dalradian Supergroup

The name 'Dalradian' was first assigned to the varied group of metamorphic rocks to the south-east of the Great Glen by Geikie (1891); see Stephenson and Gould (1995) for history of use of the name. Harris and Pitcher (1975) formerly defined the Dalradian Supergroup as comprising the Appin, Argyll and Southern Highland groups. The Grampian Group, which lies stratigraphically below the Appin Group, was subsequently incorporated into the supergroup (Harris et al., 1978). Remarkable features of the supergroup include the huge cumulative thickness which is of the order of 30 km (Soper, 1994b, and references therein) and the lateral persistence of lithological associations. The overall succession changes little along the whole strike length of over 700 km from the west of Ireland to the Banffshire coast of north-east Scotland (Harris et al., 1994; Stephenson and Gould, 1995).

The depositional environments of the Appin, Argyll and Southern Highland groups have been interpreted in terms of lithospheric stretching related to the break up of a Proterozoic supercontinent (Anderton, 1985). Progressive subsidence and lithospheric thinning resulted in shallow-shelf conditions during deposition of the Appin Group, which gave way to turbidite basins during deposition of the Argyll Group. The Southern Highland Group has been interpreted as indicating continental rupture with deposition of turbidity currents on the newly formed Laurentian continental margin (Anderton, 1985). Further details of the evolution of the Dalradian basin are given by Harris et al. (1978) and Anderton (1985).

The age of Dalradian deposition is the subject of ongoing work. The onset of deposition is likely to be at least 750 Ma; see discussion in Stephenson and Gould (1995). Continental break up is generally attributed to the development of the Iapetus Ocean around 600 Ma (Soper, 1994b). The minimum age for the Southern Highland Group is problematical, although present evidence suggests that there is no break between the Southern Highland Group and the Palaeozoic Highland Border Complex. However, isotopic evidence indicates that the Ben Vuirich Granite was emplaced into the Dalradian at 590 Ma, at a time when other lines of evidence suggest ongoing deposition. The structural age of the granite remains unresolved; see Chapter 5 and discussion by Tanner (1995, 1996).

The upper part of the Appin Group together with the Argyll and Southern Highland groups are represented in the Glen Shee district (Figure 6).

Appin Group

The Appin Group of the Dalradian Supergroup comprises three subgroups, namely the Lochaber, Ballachulish and Blair Atholl subgroups. Only the latter two are present within the Glen Shee district (Figure 6), (Figure 7).

Ballachulish Subgroup

Key elements of the lithostratigraphy of the Ballachulish Subgroup can be traced throughout the Dalradian of Scotland and Ireland (Harris et al., 1994; Stephenson and Gould, 1995). Regionally, the subgroup consists of a diverse succession of graphitic schists, semipelites, metacarbonate rocks and quartzites which were deposited under tectonically stable conditions on a slowly subsiding shelf (Anderton, 1985). In the Glen Shee district, only the upper part of the subgroup is present; this is represented by the Beinn a' Ghlo Transition Formation, the An Socach Quartzite Formation and the Glen Loch Phyllite and Limestone Formation. These are equivalent to the Appin Transition, the Appin Quartzite and the Appin Phyllite and Limestone of the type area at Ballachulish. The Beinn a' Ghlo Transition Formation grades up from the Glen Clunie Graphitic Schist Formation of the neighbouring Braemar district (Upton, 1986) and consists of interbedded quartzose and micaceous lithologies with some quartzites. The overlying An Socach Quartzite Formation is mainly composed of thick beds of quartzite. It is lithostratigraphically equivalent to both the An Socach–Cairnwell Quartzite in the Braemar district and the Beinn a' Ghlo Quartzite in the Pitlochry district. Lithologically, it is indistinguishable from the Appin Quartzite of the type section in Appin (Harris et al., 1994). The An Socach Quartzite Formation is overlain by the mixed clastic and carbonate succession of the Glen Loch Phyllite and Limestone Formation. This is equivalent to the upper Ballachulish Subgroup Gleann Baddoch White Limestone, the Gleann Beag Phyllite and the Gleann Beag Limestone of the Braemar district. It is also similar to the Monzie Limestone and Schist of the Pitlochry district.

Beinn A' Ghlo Transition Formation

This formation takes its name from the Beinn a' Ghlo range in the Ben Macdui district (Sheet 64E) where it is particularly well developed. In the Glen Shee district, it is exposed in the streams draining the north-west flanks of Màm nan Carn [NO 040 780], where it occurs in an anticlinal fold closure wedged between outcrops of the An Socach Quartzite Formation. It can be traced farther east through the col between Màm nan Carn and Beinn Iutharn Mhor [NO 047 784].

The formation comprises fine-grained phyllitic and schistose pelites and semipelites which are widely crenulated, and psammite These are interlaminated on a millimetre to centimetre scale, together with some thicker (metre scale) beds of quartzite which contain micaceous and graphitic impurities. The phyllitic rocks are graphitic in places and dark grey to black in colour, whereas the mica schist and psammite are light grey when fresh, but commonly reddish brown weathering. Graded bedding is present in centimetre-scale phyllitic beds, as in the Allt Beinn-Iutharn [NO 0410 7800]. In the same stream section, the contact with the stratigraphically higher An Socach Quartzite Formation is well exposed and, although sheared, is characterised by a marked increase in the proportion of quartzite. The thickness of the formation is not known since the lower boundary is not exposed in the district.

In thin section (S95887), the phyllites are fine grained and consist mainly of quartz and muscovite, with lesser amounts of biotite and chlorite. Centimetre-scale laminations may be present, with a perfect slaty cleavage (S1) subparallel to the laminations. Locally, the slaty cleavage is intensely crenulated with well-developed crenulation cleavage (S2) at a high angle to S0/S1. Garnet, plagioclase and rare K-feldspar porphyroblasts overgrow S1, but formed prior to the S2 crenulation cleavage. Accessory opaque minerals include fine-grained graphite; sphene, rutile, apatite and tourmaline have also been identified.

An Socach Quartzite Formation

The An Socach Quartzite Formation generally forms high hills and ridges characterised by large, conspicuous patches of grey scree. The quartzite is well exposed on the ridges and flanks of Stac na h-Iolair [NO 01 77], Carn an Righ [NO 02 77], Màm nan Carn [NO 04 77] and Beinn Iutham Mhor [NO 04 79] where it forms a 2 km-wide outcrop within a major F2 hinge zone. To the west, further exposures in the Allt a' Ghlinne Mhoir [NO 00 77] and the Allt a' Ghlinne Bhig [NO 01 78] are separated from the main outcrop on Carn an Righ by a major, north–south-trending porphyry intrusion.

The formation also occurs at a structurally higher level above the Baddoch Burn Slide (Chapter 6); it is exposed in Gleann Taitneach [NO 07 76] and on Creag Easgaidh [NO 07 76] and Creag Dallaig [NO 08 75]. The Beinn a' Ghlo Transition Formation does not occur above the Baddoch Burn Slide as was previously recorded on the published Sheet 65W.

The An Socach Quartzite Formation typically comprises rather fine-grained, greyish white, commonly feldspathic quartzite with sporadic arkosic units approximately 1 m thick. Pebble-sized clasts of quartz, plagioclase and alkali feldspar are common, with many feldspars reddened; in the least deformed rocks, angular feldspar pebbles are up to 1 cm in diameter. The stratigraphical thickness of the An Socach Quartzite Formation is not known as it is bounded by slides and thickened by folding.

Cross-bedding, generally defined by fine-grained micas and opaque minerals or dusty hematite-stained partings, is preserved in the quartzites where strain is low. The best examples are restricted to the southern slopes of Carn an Righ (Plate 1) where several exposures south and west of Stac na h-Iolair [NO 019 772] show well-preserved trough cross-bedding with individual foresets 20 cm high in beds which range up to 1 m or so thick. Graded bedding is also present on Stac na h-Iolair. Unequivocal evidence for stratigraphical younging in the quartzites on the northern flank of Carn an Righ is rare, and is restricted to cross-bedding at [NO 021 779] and graded bedding [NO 015 779]. However, many examples of 'pseudo cross-bedding' are seen in platy quartzite on Carn an Righ, particularly the crags on its northern flank and the section exposed in the Crom Allt [NO 037 780]. Here, hematite-stained partings are deformed into tight to isoclinal similar folds; commonly only one limb of the structures is preserved and the intersection of ?So and S2 appears superficially similar to planar cross-bedding. Such 'pseudo cross-bedding' argues for some caution in the interpretation of stratigraphical younging.

The top of the formation tends to be rather micaceous, as seen in outcrops west of the porphyry dyke [NO 011 782]. Layers of micaceous phyllite and pelite, 5 to 10 cm thick, within rusty-weathering quartzite occur close to quartz-rich, white metacarbonate rock at the base of the overlying Glen Loch Phyllite and Limestone Formation in a small stream [NO 0105 7810] and in the Allt a' Ghlinne Bhig [NO 010 785]. Micaceous quartzite on the crags south-west of Stac na h-Iolair [NO 013 772] are thought to occur near the upper stratigraphical contact of the formation which is here obscured by scree and till. Rusty-weathering, flaggy, rather micaceous quartzite and quartzose phyllite with massive units of purer quartzite up to 1 m thick occur in the Allt a' Ghlinne Mhoir [NO 009 772] west of the porphyry sheet.

In upper Gleann Taitneach, the Baddoch Bum Slide separates the An Socach Quartzite Formation from dark pelites of the Gleann Taitneach Schist Member. The quartzite is attenuated within 10 m of the slide and is locally platy with layering on a scale of up to a few tens of centimetres, for example on the slopes above Allt Easgaidh [NO 073 762] and [NO 075 769]. At the latter locality, alteration of fine-grained sulphides has produced rusty-weathering quartzites.

In thin section (S95884), (S95886), the quartzites are typically very pure, with only minor accessory minerals including rutile and opaque oxides. Common quartz textures include both mosaics of polygonal grains and a moderately well-developed shape fabric and crystallographic preferred orientation. Bedding is defined by minor variation in the amounts of micas, plagioclase and K-feldspar and is visible in outcrop as colour banding.

Glen Loch Phyllite and Limestone Formation

The Glen Loch Phyllite and Limestone Formation takes its name from the type section in the Allt Ruigh na Guile area of Glen Loch [NN 985 756] to [NN 987 755] (D Stephenson, written communication, 1991). It comprises mixed clastic and carbonate lithologies including phyllite, metacarbonate rocks and quartzite which generally occur in approximately 10 m-thick units. The Gleann Mor Limestone Member is a prominent strati-graphical marker at the top of the formation. The formation is well exposed in an incised section of the Allt a' Ghlinne Mhoir [NO 01 76]. The lower boundary, and that part of the formation below the Gleann Mor Limestone Member (approximately 300 m stratigraphical thickness), can be located in this area. Sparse exposure permits mapping of the formation upstream, and along its tributaries, as far east as Loch nan Eun [NO 06 78]. Folding and sliding have resulted in the repetition of the formation, so that on the northern slopes of Gleann Mor a limited section of the Glen Loch Phyllite and Limestone Formation is exposed in the Allt Coire an t-Sneachda [NO 02 76]. This occurrence is separated from the main outcrop on the valley floor by several hundred metres of An Socach Quartzite Formation.

North-west of Carn an Righ, the formation occurs around the confluence of the Allt a' Ghlinne Mhoir and the Allt a' Ghlinne Bhig, and farther upstream along the Allt a' Ghlinne Bhig. To the west and south-west the formation can be traced around the Meall Reamhar Synform via the Allt Feith Guithsachain [NN 99 76] and the crags east of Loch Loch [NN 98 74] to the Allt Glen Loch [NN 99 71]. The Glen Loch Phyllite and Limestone Formation also crops out at a structurally higher level above the An Socach Quartzite Formation east of Gleann Taitneach [NO 07 76].

The lowermost part of the formation can be seen in almost 100 m of section in a small stream [NO 0100 7810]. This contains colour-banded metacarbonate rock units (Plate 2); similar striped lithologies in the Appin area of the South-west Highlands have been descriptively designated 'Tiger Rock' on account of their pale cream or pink colour with dark stripes (Bailey and Maufe, 1916). In the Allt a' Ghlinne Bhig [NO 010 785], just downstream (west) of the porphyry intrusion, a white metacarbonate rock, probably equivalent to the Glen Baddoch White Limestone of the Braemar district of Upton (1986), occurs as the lowest unit, overlain by a series of dark slates and phyllites with calcareous units including typical Tiger Rock. Upstream from the intrusion in the same stream [NO 017 788], grey/green to dark grey phyllites, which locally show ripple lamination and grading, become darker and graphitic immediately below the Gleann Mor Limestone Member [NO 018 786].

An apparently continuous section through the Glen Loch Formation in the Allt a' Ghlinne Mhoir [NO 0093 7677] to [NO 0098 7656] (Figure 7) exposes white metacarbonate rock adjacent to the An Socach Quartzite Formation, which is succeeded by metasiltstones, metacarbonate rocks and quartzites, then laminated calcareous metasiltstones which fine up to graphitic pelites adjacent to the Gleann Mor Limestone Member. Textural and compositional grading indicates a south-younging sequence with the white metalimestone representing the oldest unit of the Glen Loch Phyllite and Limestone Formation. However, the Tiger Rock that appears in association with phyllites in the Allt Coire an t-Sneachda [NO 025 765] and in the upper reaches of the Allt a' Ghlinne Mhoir [NO 0405 7675] to [NO 0515 7750] is not seen in this section; this is probably the result of lateral facies variation, although structural excision cannot be excluded.

The formation in Glen Loch comprises grey to dark grey, quartzose or calcareous phyllites with thin, centimetre-scale, dark metacarbonate rock layers e.g. [NN 983 719]. Units adjacent to the Gleann Mor Limestone Member are typically dark, very fine-grained, biotite-rich semipelites with variable amounts of graphite.

In thin section the white metacarbonate rock at the base of the formation contains rounded grains of quartz in an irregular mosaic of calcite and dolomite. A poorly developed shape fabric, together with the preferred orientation of minor muscovite and phlogopite, defines a foliation. The Tiger Rock (S95883) contains interbedded impure metacarbonate rock and calcareous schist. The metacarbonate component comprises coarse-grained calcite with a strong shape fabric, commonly developed at a high angle to the bedding, together with some rounded grains of quartz and sericitised feldspar. The calcareous schist layers contain quartz, muscovite, biotite and calcite, with micas aligned parallel to the calcite shape fabric (S2). Accessory minerals include fine opaque oxides and zircon.

Some phyllitic lithologies (S95878)–(S95879) are exceptionally fine grained, with a mosaic of equidimensional quartz and plagioclase and stubby biotite grains. S2, defined by the preferred orientation of micas, is oblique to a lamination produced by a variation in mineral proportions. Rarely, S2 is a crenulation fabric, with folded S1 between S2 domains. Small sieved garnets are locally present in the phyllites; opaque grains and zircon are accessories. The phyllites grade into micaceous quartzites (S95877), with an increase in the proportion of quartz and feldspar; both K-feldspar and plagioclase may occur in such rocks. Graphitic schists (S95880) generally show fine compositional laminations, with a subparallel S1 fabric defined by the preferred orientation of muscovite and biotite. S1 may be crenulated, with S2 micas concentrated along S2 cleavage domains, and wrapping garnet porphyroblasts. Pyrite is commonly present in the graphitic schists, and may form up to 5 per cent of the mode.

Gleann Mor Limestone Member

This member, which comprises metacarbonate rocks and some calcareous phyllites, is typically about 50 m thick and can be traced from Sheet 64E (Ben Macdui) onto Sheet 55E (Pitlochry). It is well exposed in a 600 m-long section along strike in the Allt a' Ghlinne Mhoir [NO 010 765] to [NO 015 762] south of Stac na h-Iolair. East of this section in Gleann Mor it is not exposed but its position can be constrained by feature mapping; north of Glas Tulaichean [NO 050 770] to [NO 060 777] the outcrop is probably coincident with a line of linear topographical hollows and/or breaks in slope associated with spring lines. These features can be traced to the south-west corner of Loch nan Eun [NO 063 779] with the metacarbonate rocks reappearing in exposures in the source areas of the Allt Cac Dubh [NO 077 788] and the Baddoch Burn [NO 081 790]. The member can be traced around the Meall Reamhar Synform via the slopes east of Loch Loch and exposures west of Bothan Ruigh-chuilein [NN 9935 7177] to the Allt Ruigh-chuilein.

In the Allt a' Ghlinne Mhoir section the Gleann Mor Limestone Member succeeds approximately 5 m of graphitic schists. It is a relatively pure (compared with the stratigraphically lower, quartzose Glen Baddoch White Limestone) metacarbonate rock with interbanded fine- and coarse-grained layers (Plate 3). The fine-grained layers are dark grey and finely foliated with micaceous, quartzose and graphitic impurities defining the compositional laminae, whereas the coarse-grained layers are coarsely crystalline, decimetre-scale, metacarbonate rock layers; these are purer and appear beige when weathered.

In exposures west of Bothan Ruigh-chuilein, metacarbonate rocks are generally dark and fine grained with a tendency to form wide karstic surfaces with extensive solution cavities. The stratigraphical thickness of the member may be as little as 8 to 15 m in this area, although the effects of polyphase deformation hinder accurate determination. In the Allt Ruigh-chuilein [NN 987 716], metacarbonate rocks contain more micaceous impurities adjacent to pelitic rocks to the north-west. The carbonate is coarsely crystalline in places, with grain size of 2 to 3 mm, and it is commonly pale creamy brown and rather crumbly when weathered. The upper boundary to the member is sharply defined where metacarbonate rocks are succeeded by garnet mica schists which form the lowermost part of the Sron nan Dias Pelite and Limestone Formation.

In thin section (S95875)–(S95876), carbonate grains in fine-grained, dark grey metacarbonate rocks show a strong grain shape fabric; this, together with alignment of micas, defines the S2 foliation. A fine layering reflects variation in the proportion of quartzose and micaceous impurities. Accessory minerals include zoisite and opaque oxides (including pyrite). Coarser grained, buff-weathering metacarbonate rocks are composed of up to 95 per cent coarse carbonate grains, with either a grain shape fabric, or polygonal mosaics; the non-carbonate grains are mainly quartz and phlogopite. In some specimens, small lensoid domains of quartz and muscovite that are elongate parallel to the S2 grain shape fabric occur with some graphite.

Blair Atholl Subgroup

The Blair Atholl Subgroup shows more evidence of lateral facies variation than the preceding Ballachulish Subgroup (Figure 8), even though the lithologies present are similar, being the metamorphosed equivalents of shallow-water limestones, shales and sandstones. They succeed, apparently conformably, the topmost part of the Ballachulish Subgroup. In the Glen Shee district, the lower part of the Blair Atholl Subgroup is represented by the Sron nan Dias Pelite and Limestone Formation and the Tulaichean Schist Formation, while the Gleann Beag Schist Formation constitutes the upper part (Figure 7); this last formation is, however, the lower part of the subgroup in the Braemar district (Figure 6; Upton, 1986).

A thick component of the Blair Atholl succession occurs on the southern slopes of Gleann Mor [NO 02 76]. Despite poor exposure over large tracts of this area, a relatively complete stratigraphical section has been compiled between the top of the Gleann Mor Limestone Member and the base of the Gleann Beag Schist Formation, based on evidence from small crags and stream sections. The maximum gap in the section is approximately 80 m and occurs a few metres above the Gleann Mor Limestone Member. The oldest Blair Atholl Subgroup rocks in this area comprise garnet mica schists and laminated phyllitic psammites assigned to the Tulaichean Schist Formation. The Sron nan Dias Pelite and Limestone Formation was not identified in this area.

Attempts to trace the Gleann Mor Limestone–Gleann Beag Schist stratigraphy along strike from the Gleann Mor area have highlighted problems that call for a reinterpretation of the stratigraphy and structure represented in the north-east corner of Sheet 55E and southwest corner of Sheet 65W (Figure 6). One important difficulty is that at least part of the Easdale Subgroup Killiecrankie Schist and its 'white saccharoidal quartzite' as mapped on Sheet 55E (Bradbury et al., 1979; Smith, 1980; Treagus, 2000) can be traced north-eastwards without major interruption or apparent change in lithology into the outcrop of the Tulaichean Schist Formation in the Gleann Mor area. This problem is compounded by the apparent absence of a lithostratigraphical equivalent to the Killiecrankie Schist in association with the Islay Subgroup quartzite in the Glen Shee district. A similar absence was also noted further northeast along strike in the area of Sheet 65W (Upton, 1986). These anomalies would be ameliorated if the Killiecrankie Schist contained both Blair Atholl and Easdale subgroup components with only the former present in the Glen Shee district.

The Tulaichean Schist Formation outcrop, identified by this study in the south-west corner of Sheet 65W, was originally mapped as undifferentiated schists (Geological Survey of Great Britain (Scotland), 1904). Bailey (1925) also delineated this outcrop accurately and assigned it to the Killiecrankie Schist. Bailey's interpretation is here rejected because the Tulaichean Schist Formation is succeeded in normal stratigraphical succession by the Gleann Beag Schist Formation as in grid square [NO 06 76]. On Sheet 65W (1989) the Tulaichean Schist outcrop of this study is represented partly as the Crinan Subgroup Caenlochan Schist and partly as Lochaber Subgroup Baddoch Burn Pelite. This is also rejected because there is no evidence of a large fault between the Crinan and Lochaber Subgroup outcrops along the western flank of Glen Taitneach as indicated on Sheet 65W. Furthermore, there is no need for a slide (the Baddoch Burn Slide) in the position envisaged by Bailey (1925, 1928), by Upton (1986) or as indicated on Sheet 65W (1989). The upper contact of the Tulaichean Schist Formation in Glen Taitneach is stratigraphical and the Baddoch Burn Slide is now mapped above the outcrop of the Blair Atholl Subgroup Gleann Beag Schist Formation (Figure 1) where the latter is overlain by older Ballachulish Subgroup formations. See Goodman et al. (1997) for more details on these matters.

Sron Nan Dias Pelite and Limestone Formation

Two approximately 10 m-thick units of metacarbonate rock occur within pelite and semipelite stratigraphically above the Gleann Mor Limestone Member north of Carn an Righ, in the Allt Choire na Moine [NO 018 785] and on the western slopes of Beinn Iutharn Mhor [NO 030 792]; they also occur in Glen Loch and the northern flank of Ben Vuirich [NN 99 71]. Although they show affinities with the Gleann Mor Limestone Member, the metacarbonate units, together with the intervening pelites, are assigned to the Sron nan Dias Pelite and Limestone Formation on the basis of the more pelitic background lithology. Pelitic lithologies are typical of the Blair Atholl Subgroup in contrast to the semipelitic or quartzose lithologies of the upper part of the Ballachulish Subgroup. The formation has not been recognised in the Allt a' Ghlinne Mhoir section, although some thin metacarbonate rock beds are observed close to the base of the Tulaichean Schist [NO 0412 7606]; [NO 0466 7647].

In Glen Loch the formation comprises dark biotitic, commonly fine-grained, schistose pelites and semipelites with some calcsilicate and metacarbonate rocks which occur above the Gleann Mor Limestone Member. The sequence contains at least two units of relatively pure metacarbonate rock, the stratigraphically higher of these being grey, 10 m thick and brown weathering with conspicuous orange-weathering layers [NN 999 718]. This unit can be traced high onto the northern shoulder of Ben Vuirich, from where it continues as a distinct unit for several kilometres to the south. From the Allt Glen Loch it extends northwards to the exposures of brown-weathering metacarbonate rock on Sron nan Dias [NN 994 730] which lie within schistose biotite pelite and semipelite. Boudinaged lenses of semipelite and pelite occur throughout the metacarbonate rock (D Stephenson, written communication, 1991). On the slopes immediately east of Sron nan Dias, conspicuous beds of brown-weathering metacarbonate rock occur within garnet mica schists. This association also occurs at [NN 999 719] close to the top of the formation.

In the Allt Choire na Moine [NO 019 786] and on the western slopes of Beinn lutharn Mor [NO 030 792], the Sron nan Dias Pelite and Limestone Formation comprises at least two units of dark graphitic metacarbonate rock within a background lithology of dark, variably graphitic, pelite. The upper unit is conspicuously coarse grained, granular, lacking in pelitic or other impurity and of the order of 10 to 15 m thick. Sinkholes and small caves occur in this unit on Beinn Iutharn Mor [NO 033 795].

In thin section, the muscovite-biotite semipelites locally contain some hornblende. Preferred orientation of the micas and amphibole defines a good foliation in a matrix of fine-grained, recrystallised quartz and plagioclase. Garnets are generally inclusion free and wrapped by the schistosity. The metacarbonate rocks resemble those of the Gleann Mor Limestone Member.

Tulaichean Schist Formation

The major part of the Tulaichean Schist Formation comprises schistose biotite-muscovite semipelite; subsidiary lithologies include laminated phyllitic semipelite and micaceous psammite, micaceous quartzite and minor developments of calcareous lithologies. Amphibolites are abundant and are described in Chapter 5.

The principal outcrop of the Tulaichean Schist Formation underlies more than 10 km2 in the Glas Tulaichean [NO 05 76]–Creag a' Chaise [NO 07 72]–Carn an t-Sionnaich [NO 01 75] area. The broad outcrop (up to 3.5 km wide) reflects thickening owing to polyphase deformation. To the north-east, the formation continues from Glas Tulaichean to the edge of the district on Carn a' Chlarsaich [NO 06 77]. Farther west, north of Carn an t-Sionnaich, the outcrop narrows to 1 km and can be traced around the Meall Rheamhar Synform (Figure 11) at the head of Gleann Fearnach [NO 00 75] and thence southwards towards Glen Loch and the northern part of the Ben Vuirich Granite [NO 00 70].

The formation is also exposed, at a structurally lower level, north of the An Socach Quartzite Formation on Carn an Righ and on the western slopes of Beinn lutharn Mhor [NO 02 78]; here, garnet mica schists of the Tulaichean Schist Formation lie between the Sron nan Dias Pelite and Limestone Formation and the Carn an Righ Slide, at the margin of the An Socach Quartzite Formation. Structural inliers of Tulaichean Schist Formation occur between Glen Lochsie [NO 06 72] and Beinn a' Chruachain [NO 04 70] and on the southern spur of Carn Dearg [NO 04 71], where they form arrays of small craggy knolls of generally massive aspect. Inliers also crop out in small isolated knolls, within otherwise poorly exposed ground, along the main Carn Dallaig–Carn Dearg watershed as well as in limited sections in the Glen Lochsie Burn and the Allt Ruigh nan Eas [NO 04 71]. These inliers occupy the anticlinal cores of a complex of folds which lie structurally above the Carn Dallaig Slide.

The formation is characterised by schistose semipelites which are mostly well foliated and abundantly garnetiferous, typically with up to 30 per cent fine-grained muscovite and biotite. The muscovites largely define the main S2 schistosity and impart the characteristically pale, lustrous appearance to the rocks. Muscovite-rich schists tend to be coarser grained than biotite-rich varieties. Garnet porphyroblasts are generally 1 to 3 mm in diameter but coarsely garnetiferous variants of the schists are common; at one locality [NO 078 729] garnets exceed 25 mm in diameter and, in a more pelitic variant [NO 0049 7483], they are up to 30 mm in diameter. Bedding laminae are usually discernible, especially in the more quartz-rich lithologies, where both grain size and compositional grading may indicate the younging direction (Plate 4). Bedding and a layer-parallel S1 schistosity, defined by alignment of biotite, are generally crenulated by the main S2 schistosity as shown by several exposures on the ridge north of Creag a' Chaise [NO 078 729]. The formation shows evidence of several phases of small-scale folds and a complex history of metamorphic fabric development. A unit of garnet-poor, finely laminated, phyllitic micaceous psammite and semipelite occurs within the schistose semipelites on the north-east limb of the Meall Reamhar Synform (Figure 11). It is well exposed on the ridge crest between Can an t-Sionnaich [NO 013 754] and Faire Ghlinne Mhoir [NO 034 754]. This outcrop extends east-south-east across poorly exposed ground south of Faire Ghlinne Mhoir, where it has an outcrop width of almost 1 km, towards Clais Bheag [NO 06 74] from where it extends north-west to the area around Glas Tulaichean. This unit is either free of garnet or contains rare, very small, pinhead-size crystals (Plate 4). The overall character of the laminated unit becomes more psammitic to the west. A sharp boundary with the schistose semipelites is exposed in the headwaters of the Allt Clais Mhor [NO 05 74] and the Allt Clais Bheag [NO 06 74].

Micaceous quartzite forms a large part of the Tulaichean Schist Formation on the south-west limb of the Meall Rheamhar Synform (Figure 11) and particularly in Glen Loch [NO 00 71]. This unit is designated as 'white saccharoidal quartzite' and included within the Easdale Subgroup Killiecrankie Schist Formation on published Sheet 55E. It is bounded, both to the west and east, by thin developments of schistose semipelite typical of the major part of the Tulaichean Schist Formation. Good exposures occur both along the Loch Burn [NO 000 718] and Daldhu [NO 022 706] and in abundant craggy exposure to the north-east of the burn. The micaceous quartzite is typically fine grained, grey and granoblastic and composed predominantly of quartz with only minor amounts of feldspar. No pebbles are recorded. Biotite occurs throughout and gives the quartzite a dusty, dirty appearance. Compositional layering is defined by subsidiary, millimetre-thick, biotite-rich foliae (some with garnets), and by concentrations of heavy minerals. Evidence for younging, in the form of 'dusty' hematite-stained partings which emphasise centimetre-scale trough cross-bedding, is preserved at one location [NN 9997 7290]. A crude schistosity of probable S1 age is developed subparallel to the compositional layering in the thin macaceous layers. Both bedding and S1 foliation are overprinted by the main S2 schistosity.

The western boundary of the micaceous quartzite is marked by a broad zone (approximately 100 to 200 m wide) of high D2 strain throughout the 2 km exposed in the district. This is characterised by finely laminated, platy, blastomylonitic quartzites, locally with well-developed down-dip rodding. This is one of a number of high-strain zones in this area which are described in detail in Chapter 6. The eastern contact with the overlying schistose semipelites is stratigraphical, and possibly transitional over a few metres, as indicated by a few outcrops of garnet mica schist with centimetre-thick layers of pale quartzite in the area west of Creag Uisge [NO 02 69]. To the south-west, the micaceous quartzite is intruded by the Ben Vuirich Granite [NO 00 70]; disorientated xenoliths of quartzite are common along the margin of the granite (Chapter 5), although no evidence of a contact metamorphic aureole is preserved here. The micaceous quartzite is thought to occupy a similar strati-graphical position within the Tulaichean Schist Formation to the laminated phyllitic micaceous psammites and semipelites, albeit on opposing limbs of the Meall Rheamhar Synform. This is supported by the westward increase in the psammite component of the phyllitic rocks.

Calcsilicate rock layers, mostly less than 1 m thick, occur sporadically throughout the Tulaichean Schist Formation. The main developments occur towards the boundary with the overlying Glen Lochsie Calcareous Schist Member such as in the Glen Lochsie Burn [NO 0623 7259], and in partly dislodged blocks on Meall Ruigh Mor Thearlaich [NO 0533 7184]. Here, calcareous schist, calcsilicate rocks and thin impure metacarbonate rocks are recorded. Calcsilicate rocks typically show trails of fine-grained epidote granules and coarse, actinolitic amphibole prisms up to 8 cm long arranged in garbenschiefer fashion on S2 crenulation planes.

In thin section, the schistose semipelites (S95868), (S95871) are relatively fine grained, and consist mainly of quartz, biotite and muscovite with some feldspar and pale amphibole. In many examples the penetrative S2 crenulation cleavage, defined by the orientation of micas, is parallel to a fine compositional bedding lamination (S95869). Traces of S1 may be visible between S2 crenulation planes. Euhedral garnets, up to 3 mm in diameter, contain sigmoidal S1 inclusion trails, continuous with the external foliation. Smaller garnets are generally more ragged, and may have occluded, inclusion-rich cores, rather than discrete inclusion trails. Accessory minerals include opaque oxides, apatite and zircon. Some of the laminated phyllitic lithologies (S95870) are very fine grained. Mineralogically, they are similar to the schistose rocks, albeit with only small amounts of garnet and a greater range in the proportions of quartz and micas. The micaceous quartzite (S95874) comprises fine-grained granoblastic quartz with rare feldspar. Biotite and muscovite flakes are randomly arranged along quartz grain boundaries. A poorly developed spaced fabric, defined by the distribution of biotite, is locally present, although it is generally more clearly visible in hand specimen. Calcareous rocks within the Tulaichean Schist Formation range from calcite-bearing variants of the schistose semipelite to calcsilicate rocks. These contain calcite in greater abundance than quartz, porphyroblasts of actinolitic amphibole and plagioclase. In a number of places, calcareous lithologies contain flakes of chlorite together with laminae enriched in epidote or zoisite with or without sphene.

Gleann Beag Schist Formation

The uppermost formation in the Blair Atholl Subgroup in the district, the Gleann Beag Schist Formation, takes its name from the broad valley running north-east from the Spittal of Glen Shee. In the Braemar district, Upton (1986) described the Gleann Beag Schist as 'interbedded graphitic schist and dark marble with psammite'. This equates with the Blair Atholl Dark Limestone and Dark Schist elsewhere (Harris et al., 1994) which forms the lower part of the subgroup (Figure 6). Representatives of the younger Blair Atholl Pale 'Group' which occur in the Braemar district (Carn Aig Mhala Limestone, Glen Callater Banded 'Group') do not occur in the Glen Shee district, probably as a result of original sedimentary facies variation. In this district, it has been possible to subdivide the Gleann Beag Schist Formation into two mappable units: a lower more calcareous member termed the Glen Lochsie Calcareous Schist Member, and an upper more graphitic member termed the Glen Taitneach Schist Member. The subdivision is one of degree, however, and both conform to Upton's original description of interbedded graphitic schist and dark marble with psammite. Locally, at the top of the formation, a succession of interbedded graphitic petite and quartzite has been distinguished as the Carn an Daimh Transitional Formation.

It should be noted that on the current Sheet 65W, the 'Glen Lochsie Schist' in Glen Lochsie and Glen Taitneach was attributed to the Easdale Subgroup and correlated with the Ben Lawers Schist Formation. These rocks are here assigned mainly to the Glen Lochsie Calcareous Schist Member in the Blair Atholl Subgroup.

Glen Lochsie Calcareous Schist Member

This member crops out extensively in the Glen Lochsie Burn [NO 06 72], on the south side of Glen Lochsie [NO 05 72] and on the west slopes of Glen Taitneach [NO 08 72]. It also underlies poorly exposed ground on the west slopes of Carn an Daimh [NO 12 71]. West of Glen Lochsie, it occurs both around the head of Gleann Fearnach [NO 01 74] and in complex fold hinges in the Beinn a' Chruachain–Carn Dallaig area [NO 04 69]. The member is of the order of 250 m thick where least affected by folding such as on the west side of Glen Taitneach. Elsewhere, it is thickened in fold hinges and attenuated on fold limbs.

In the type area in Glen Lochsie, the member comprises calcareous and calcsilicate schists, with interbedded pelites and semipelites, together with quartzose metacarbonate rocks commonly with brown- or buff-weathering surfaces. Fine-grained disseminated graphite occurs in all lithologies, imparting a dark tone to fresh surfaces. Exposures display a decimetre- to metre-scale lithological layering, which is emphasised by differential weathering; this results from the contrasting carbonate content of adjacent layers (Plate 5). The layering reflects bedding, albeit generally highly modified and transposed. Brown-weathering metacarbonate rock layers are commonly dolomitic, and range from 20 cm to over a metre thick [NO 074 713]. The metacarbonate rock layers form no more than 30 per cent of the member with apparent local variations in abundance reflecting differential exposure. For example, the Glen Lochsie Calcareous Schist Member can be traced along a grassy bench below the outcrop of the Tulaichean Schist Formation [NO 0210 7148] on the western side of Gleann Fearnach, although only quartzose calcareous rocks are seen at outcrop.

The lower boundary with the Tulaichean Schist Formation is commonly marked by a massive weathering, dark, garnet-biotite rock, with abundant garnet porphyroblasts up to 1 cm in diameter. The porphyroblasts may have inhibited the development of a penetrative schistosity.

Throughout much of the district the uppermost part of the Glen Lochsie Calcareous Schist Member is formed by a prominent blue-grey coarsely crystalline calcite metacarbonate rock, with only minor quartzose, micaceous and graphitic impurities. It can be traced around the folds on Beinn a' Chruachain [NO 0423 6957] via small exposures and sink holes to an isoclinal fold hinge [NO 0467 6920]. This metacarbonate rock is the only representative of the Glen Lochsie Calacreous Schist Member along the Carn Dallaig–Carn Dearg watershed where the stratigraphy is highly attenuated. Here, it is very streaky in appearance and thoroughly recrystallised, and occurs in solution pits and hollows just below the ridge [NO 0369 7278]. On Sron Charnach [NO 061 693] a 5 m-thick layer of grey, coarsely crystalline metacarbonate rock occurs in a comparable stratigraphical position at the boundary with the overlying Glen Taitneach Schist Member.

Around An Lairig [NO 088 681], beige-weathering meta-carbonate rocks and calcareous schists are over 10 m thick. They are succeeded to the south by calcareous and schistose pelites with thin psammites; these are thought to form part of the Glen Lochsie Calcareous Schist Member, although their affinity is uncertain.

In thin section, the calcite and calcite-dolomite metacarbonate rocks (S95866)–(S95867) generally comprise a mosaic of recrystallised carbonate grains of medium to coarse grain size. Bedding is marked by varying proportions of quartz, whereas a poorly developed foliation is defined by carbonate shape fabric and preferred orientation of some amphibole and mica flakes. Opaque minerals, graphite and sphene are the typical accessory minerals. The calcsilicate rocks contain quartz, plagioclase and carbonate, with needles of zoisite and clinozoisite along the biotite–muscovite foliation. Amphibole (hornblende and/or tremolite) either occurs within or cross-cuts the foliation in garbenschiefer fashion. With increasing proportion of micas, the calcsilicate rocks pass into calcareous schists (S95863)–(S95865) in which small amounts of garnet may be present.

Glen Taitneach Schist Member

The Glen Taitneach Schist Member crops out extensively on the east side of Gleann Beag at Creag a' Mhadaidh [NO 13 71], where it is some 500 m thick. It apparently pinches out to the west across Gleann Beag but reappears on the summit of Ben Gulabin [NO 10 72]. It is not clear whether these thickness changes are primary depositional or subsequent tectonic features. A spatial relationship between the schist and the similarly discontinuous, and structurally underlying, Carn Dubh Quartzite Member may indicate a tectonic control related to boudinage of the quartzite. The Glen Taitneach Schist Member is commonly intensely folded and attenuated, probably owing to enhanced ductility as a result of its graphitic nature. Changes in thickness over short distances make it difficult to generalise about the depositional thickness of the member; however, 200 to 300 m is thought to be a reasonable approximation.

The Glen Taitneach Schist Member is predominantly a black to silvery grey graphitic pelite, the steely lustre to the schistosity surface giving rise to its field name of 'gunmetal schist'. Minor calcareous and psammitic lithologies are associated with the graphitic pelite. All lithologies show a broad change from more graphitic rocks in the east to more quartzose rocks in the west.

In the east the member is formed dominantly of graphitic pelite, although thin micaceous, quartzose layers are quite common as on Creag a' Mhadaidh [NO 132 718]. The siliciclastic component increases westwards, so that in Gleann Fearnach graphitic pelite grades into laminated micaceous psammites and pelites, with some non-graphitic schistose semipelite. Carbonate layers are common throughout the member but particularly near the base. They range from brown-weathering stringers a few millimetres thick to more substantial metacarbonate rock units. In Glen Taitneach, fine-grained, black, calcite metacarbonate rocks are present as beds up to 1 m thick. The graphite content of the metacarbonate rocks decreases westwards, and in Gleann Fearnach [NO 0091 7480] metacarbonate rock layers are creamy white with only a few graphitic laminae

Distinctive dark chloritic pelite, with fine graphite and large hornblende porphyroblasts more than 5 mm long, occurs near the base of the member, as on Creag Dhearg in Glen Taitneach [NO 086 739]. Hornblende porphyroblasts are randomly arranged both on the schistosity surface and growing across the schistosity. This lithology is commonly garnetiferous; in Glen Taitneach [NO 0942 7312] the garnets are over 5 cm in diameter.

The proportion of psammite increases near the top of the member, culminating in the striped graphitic pelite and quartzite of the Carn an Daimh Transitional Member. On the north slopes of Meall Ruigh Mor Thearlaich [NO 05 72], graphitic pelites pass upwards into striped psammites and graphitic schists; some of the massive psammites are a metre or more thick. Here [NO 0575 7243], some of the micaceous and graphitic laminae contain significant heavy mineral concentrations, notably of zircon.

In thin section, the graphitic pelites (S95857), (S95858), (S95859) comprise fine-grained muscovite, biotite and graphite in anastomosing foliae which define the schistosity, separated predominantly by fine-grained recrystallised quartz. There may be a small amount of feldspar, much of which is sericitised, and small garnets, which are seldom visible in hand specimen. Opaque oxides, sulphides, apatite and zircon are common accessory minerals. Chlorite is locally developed, either as a retrogressive phase after biotite, or apparently as part of the peak metamorphic assemblage, and lies along the main schistosity.

Dark pelite with amphibole porphyroblasts (S95859) near the base of the member has a matrix which is predominantly composed of chlorite-quartz schist, without feldspar. Large, randomly arranged, blue-green hornblende porphyroblasts cut the foliation at a high angle. They have straight trails of quartz and opaque inclusions which have developed parallel to the planar external schistosity. Garnets are generally more than 10 mm in diameter, and may be 50 mm or more; they too have predominantly straight inclusion trails, with only a slight sigmoidal curve at the garnet edge, and are also concordant with the external fabric. These relations suggest that late porphyroblastesis occurred in this lithology, after the main fabric development (Chapter 7). The sedimentary protolith is unknown, although the high proportion of mafic minerals may indicate a volcanogenic component.

Both calcite- and dolomite-dominated metacarbonate rocks occur within the Glen Taitneach Schist Member (S95861)–(S95862), although the former are more common. The grain size is dependent on the amount of graphite present, as fine-grained graphite hinders recrystallisation and grain boundary migration. This point is well illustrated in very fine-grained graphite-rich metacarbonate rocks in Glen Taitneach [NO 090 728] which contrast with the medium- to coarse-grained less graphitic schists. One dolomitic metacarbonate rock in Gleann Fearnach contains grossular garnet in graphitic pelite laminae. Elsewhere, some of the metacarbonate rocks are composed almost entirely of carbonate, with only a minor amount of quartz, graphite and mica. Any foliation in these rocks is defined by subtle shape fabric in the carbonate grains, rather than by any mineral orientation fabric.

Carn an Daimh Transition Formation

This formation only occurs in the north-east of the district on Carn an Daimh [NO 136 718], where it attains a maximum thickness of 70 m. The Carn an Daimh Transition Formation and the Islay Subgroup 'Boulder Bed' are mutually exclusive; the 'Boulder Bed' is thus absent on Carn an Daimh where the transitional formation is best developed.

The formation consists of alternate beds of graphitic pelite and white quartzite, several centimetres thick. In some places, pelite–quartzite boundaries are quite sharp, whereas elsewhere the quartzite units fine upwards to graphitic pelite, giving good evidence of the younging direction of the succession. The transition may represent a transition between the graphitic pelites of the Gleann Beag Schist Formation and the quartzites of the overlying Creag Leacach Quartzite Formation. However, this requires the upper part of the Blair Atholl Subgroup above the Gleann Beag Schist Formation in the Braemar district–Glen Callater Banded Group and Carn Aig Mhala Pale Limestone of Upton (1986) — to be laterally equivalent to the Gleann Beag Schist Formation in the Glen Shee district.

In thin section (S95855), the quartzite beds comprise fine-grained quartz with minor amounts of muscovite and biotite; the micas are more abundant towards the top of the beds, even where grading is not apparent in hand specimen. Graphitic beds comprise a mosaic of very fine-grained quartz, muscovite and graphite. Accessory minerals, including opaque oxides, zircon and tourmaline, are concentrated in graphitic layers.

Argyll Group

The Argyll Group is composed of four subgroups: the quartzite-dominated Islay Subgroup which is succeeded by the calcareous and graphitic Easdale Subgroup, the mostly semipelitic Crinan Subgroup and the metacarbonate- and calcsilicate-dominated Tayvallich Subgroup. Lithological formations within the group are generally thicker and show greater lateral facies and thickness variation than the preceding Appin Group.

Islay Subgroup

The base of the Argyll Group is generally marked by the presence of a boulder bed or tillite, which Spencer (1969) identified as a marine till. This is only sporadically developed in the central and eastern parts of the Dalradian outcrop. It occurs at the boundary between the commonly graphitic upper parts of the Blair Atholl Subgroup and the dominantly quartzitic succession of the overlying Islay Subgroup. Where the 'Boulder Bed' is missing the base of the Islay Subgroup is placed at the first appearance of quartzites.

Creag Leacach Quartzite Formation

This formation consists of two main quartzite members: the clean quartzites of the older Carn Dubh Quartzite Member, and the relatively impure quartzites of the younger Bad an Loin Quartzite Member. The quartzites form an important marker, traceable throughout the district, although they are highly attenuated in places.

The formation is laterally continuous with the Creag Leacach Quartzite of the Braemar district to the northeast which also comprises a lower mature and an upper immature facies, which are correlated with the Islay Subgroup Schiehallion and Easdale Subgroup Carn Mairg quartzites of Perthshire (Upton, 1986). The Easdale Killiecrankie Schist, which separates the quartzites in Perthshire, is not present in either the Braemar (Upton, 1986) or Glen Shee districts. Unfortunately, these correlations involve two separate subgroups. However, they do imply that the upper part of the mainly quartzite Islay Subgroup and the mainly black schist Easdale Subgroup are in part contemporaneous. Consequently, it may be preferable to recognise an interdigitating relationship between the two subgroups, with the Carn Mairg Quartzite Formation belonging to the Islay Subgroup but intercalated within the Easdale Subgroup of most of Perthshire. This, however, is not an appropriate place to consider such regional correlations in any detail.

Boulder Bed

Only two occurrences of Boulder Bed are recorded in the district. On the southern part of the Carn am Daimh–Creag Leacach ridge [NO 136 714], are some loose blocks consisting of a fine-grained quartz-muscovite schist with rounded pebbles of quartz and rock fragments including carbonated dyke rocks and chloritised metasedimentary rocks. A similar lithology crops out at the base of clean quartzites on Carn Dubh [NO 112 721]. As the lower boundary of the Boulder Bed is not seen, no estimate of its thickness is possible Immediately north of the Carn Dubh area, the Boulder Bed is associated with a unit of striped metasedimentary rocks and dolomite, known as the Glen Callater Banded 'Group' and the Cairn Aig Mhala Limestone, respectively (Upton, 1986). Both units pinch out towards the south and do not occur in the Glen Shee district.

Carn Dubh Quartzite Member

The Carn Dubh Quartzite Member is the more mature of the Islay Subgroup quartzites, and is found as large lenses which probably reflect original channel-fill deposits. These are more common in the east of the district, with the quartzite member as a whole becoming more immature towards the west. The best exposures of the Carn Dubh Quartzite Member are on the Creag Leacach–Carn an Daimh ridge [NO 13 71], on Carn Dubh and the southern slopes of Ben Gulabin [NO 11 72]; [NO 11 71]. Smaller lenses occur elsewhere, such as on the south side of Glen Lochsie [NO 080 712]. In the largest lenses, the quartzite attains a thickness of 200 m, with bed thicknesses in the order of 0.5 to 1 m.

The quartzites are mature, relatively coarse grained and white on both fresh and weathered surfaces. Locally, they contain pebbly beds which may be graded with both quartz and feldspar pebbles a few millimetres in diameter. Rare examples of cross-bedding are recorded [NO 0800 7111]. In thin section (S95853)–(S95854), some quartzites consist of more than 95 per cent quartz; strained grains with undulose extinction and sutured boundaries suggest dynamic recrystallisation. The feldspar pebbles are generally polygonised perthite or microcline. Small muscovite flakes occur around quartz grain boundaries, and locally show a subtle preferred orientation parallel to a grain-shape fabric in the quartz. Accessory minerals are opaque oxides and zircon, which are commonly concentrated along bedding traces and possibly represent heavy-mineral layers.

Bad an Loin Quartzite Member

The most persistent member of the Creag Leacach Quartzite Formation is the Bad an Loin Quartzite Member. It lies stratigraphically above the Carn Dubh Quartzite Member on the Creag Leacach–Carn an Daimh ridge, and can be traced south-west from there, across Bad an Loin and Ben Gulabin, through poorly exposed ground in Glen Taitneach to Glen Lochsie. Farther west, the member is highly attenuated along the Carn Dallaig ridge and around the head of Gleann Fearnach; discontinuous lenses of quartzite occur to the west. The quartzite attains a thickness of 300 m on Bad an Loin, although it is no more than a few tens of metres thick where highly attenuated. A second occurrence of Bad an Loin Quartzite Member lies to the south of the main outcrop, in the Ben Earb Slide Zone (Chapter 6), where it is enclosed within the broad outcrop of Ben Lawers Schist Formation. It extends north-east from Beinn a' Chruachain [NO 04 69] across Creag an Dubh Shluic, and the watershed into Glen Shee, and thence north-eastwards to Black Hill [NO 16 71] at the northern margin of the district. North of this it may link with the quartzites mapped within the Glas Maol Schists in the Braemar district [NO 17 74].

The Bad an Loin Quartzite Member generally weathers white, but always has a buff or beige colour on a fresh surface. It is more thinly bedded than the underlying Carn Dubh Quartzite Member, and shows little internal structure, although rare pebbly horizons have been recorded. It may take on a glassy, striped appearance where highly strained, although it is invariably annealed. The quartzite becomes more immature with graphitic laminae a few millimetres thick towards its upper margin against the graphitic schists of the Ben Eagach Schist Formation. Around Bad an Loin and Carn an Daimh, graphitic schist laminae a few millimetres thick occur throughout the quartzite; in this area, the overlying graphitic schists reach their maximum development. To the west, graphitic laminae are restricted to the top few metres of the member and the overlying graphitic schists are less well developed. The quartzite with graphitic laminae is distinguished from the Appin Group Carn an Daimh Transition Member by both the finer scale of lamination and the dirty nature of the quartzites.

In thin section (S95851)–(S95852), the quartzites largely consist of fine- to medium-grained quartz, which is generally thoroughly recrystallised into an equigranular mosaic. Minor amounts of fine- to medium-grained plagioclase tend to be highly altered. The fabric of the rocks is defined by the orientation of fine muscovite and biotite in flakes, cords and anastomosing bundles; rarely, there are large biotite porphyroblasts, which appear to be coeval with the main fabric. Accessory opaque minerals, zircon and tourmaline are concentrated within the micaceous laminae. Towards the top of the quartzite, graphite is present in the laminae, although the nature of the quartzose portion of the rock is unchanged. Where the micaceous/graphitic laminae reach several millimetres thick, pinhead size garnets may be developed.

Easdale Subgroup

The Easdale Subgroup marks a return to deeper water sedimentation after the tidal shelf conditions of the Islay Subgroup (Anderton, 1985). The boundary between these subgroups is transitional, with quartzite containing graphitic laminae at the top of the Islay Subgroup Bad an Loin Quartzite Member, passing upwards into the graphitic schists of the Easdale Subgroup Ben Eagach Schist Formation. In the Glen Shee district, the base of the Easdale Subgroup is placed at the top of the highest mappable quartzite unit. The subgroup is divided into three formations. The predominantly graphitic Ben Eagach Schist Formation can be traced from the type area in the Pitlochry district through the Glen Shee district into the Braemar district (Bailey, 1925) where it is referred to as the Glas Maol Schist. The overlying calcareous Ben Lawers Schist Formation has locally interbedded volcanic rocks, whereas the Farragon Volcanic Formation is dominated by extrusive volcanic rocks. Both formations can be traced from their type areas in Perthshire, across the Pitlochry district into the Glen Shee district. The outcrop of Ben Lawers Schist Formation extends into the Braemar district where it was assigned partly to the Crinan Subgroup Caenlochan Schist and partly to the amphibolite on Sheet 65W (British Geological Survey, 1989).

Ben Eagach Schist Formation

This formation is characterised by fine-grained graphitic schists. These have acted as a locus for deformation (Chapter 6) with significant thickening in fold hinges and attenuation or excision on fold limbs and slides. In Gleann Fearnach [NO 0368 7064], for example, a gap in exposure, less than 10 m wide, separates exposures of strongly foliated rocks of the underlying Bad an Loin Quartzite and overlying Ben Lawers Schist formations. The major occurence in the district is around the Spittal of Glenshee, where an outcrop thickness of approximately 900 m is attained in a complex fold hinge. To the east, the Ben Eagach Schist Formation can be traced around the east side of the Creag Leacach ridge, where it is commonly some 100 to 200 m thick. To the west, on the south-west side of Glen Lochsie, it is highly attenuated in the Meall Ruigh Mor Thearlaich Slide Zone. Ben Eagach Schist Formation has not been identified in the Carn Tarmachain–Carn Dallaig area. It reappears in a region of lower strain in west Gleann Fearnach [NO 00 74], as a unit less than 100 m thick, from where it can be traced as far south as the Creag Uisge Slide [NO 02 70].

The Ben Eagach Schist Formation crops out south of the main outcrop within complex folds on Beinn a' Chruachain, along the Ben Earb Slide Zone from Lairig Charnach [NO 06 70] to Meall Uaine [NO 11 67]. It also occurs in the north-east of the district, on Black Hill [NO 165 715], where it is generally about 100 m thick.

The Ben Eagach Schist Formation is dominated by fine-grained, dull black, graphitic schists commonly with rusty weathering. They are distinguished from the Glen Taitneach Schist Member by their lack of metallic lustre and their higher magnetic susceptibilities. In the Spittal of Glenshee area [NO 10 69], psammitic and semipelitic rocks are a minor component of the formation. They are relatively more abundant where the outcrop of the formation is thinner, both to the west and east of the Spittal of Glenshee area. These changes in relative abundance of component lithologies may reflect preferential tectonic attenuation of the graphitic schist and/or primary depositional heterogeneity both in lithology and thickness of the formation. Near the base of the formation, thin beds and lenses of impure quartzite are locally developed, as at Sheanval [NO 0930 7146], and calcareous graphitic schists occur on Beinn a' Chruachain [NO 0422 6910]. Elsewhere, thin, fine-grained, micaceous psammites are interbedded with graphitic schists in places, as on Ben Earb [NO 077 688], whereas beds of gritty psammite up to 0.5 m thick occur at a locality to the west [NO 0410 6905]. Phyllitic semipelite units show grain size and compositional grading. Rusty weathering results from oxidation of abundant disseminated sulphide, particularly in the upper parts of the formation. Locally, the sulphide occurs within carbonate veins in Gleann Fearnach [NO 0230 7186]. Pyrite is the dominant sulphide, and locally occurs in the form of large, syntectonic porphyroblasts. Pyrrhotite and minor base metal sulphides have also been recorded, as in Glen Lochsie [NO 095 705]. However, no significant mineralisation comparable to the baryte and base metal deposits in the Aberfeldy area has been detected in the Glen Shee district (Chapter 2).

In thin section (S95848), (S95849), (S95850), the generally fine-grained and finely foliated graphitic schists are composed mainly of recrystallised quartz and graphite, with lesser amounts of calcic plagioclase, muscovite and biotite. Opaque accessory minerals are mainly pyrite, with some magnetite and minor base metal sulphides; other accessory minerals include stubby tourmaline and sphene. A fine penetrative schistosity (S1 or S2) defined by the preferred orientation of micas and graphite flakes is superimposed on a fine-scale compositional layering which reflects the proportion of quartz present. S2 is locally a closely spaced crenulation cleavage, marked by the concentration of graphite and micas; complex fabrics may result where crenulations have been folded subsequently.

Phyllitic and fine-grained psammitic beds (S95849) within the Ben Eagach Schist Formation also contain significant amounts of graphite, which is generally present as dusty inclusions within micas. The psammitic component is mostly composed of quartz, but both plagioclase and K-feldspar may occur in small amounts. Mica-rich laminae dominated by laminae-parallel muscovite flakes reflect bedding and are crenulated by F2 folding, although axial planar crenulation cleavage is rarely developed.

Ben Lawers Schist Formation

The dominantly calcareous Ben Lawers Schist Formation takes its name from the type area in Perthshire. It underlies a broad swathe of generally poorly exposed ground covered with grass rather than heather in the northern and western part of the district. The formation extends from the valley of the Allt an Daimh (the Green Glen) [NO 14 71] in the north, west across Glen Shee to underlie much of the watershed between the lower parts of Glen Lochsie and Gleann Fearnach and the eastern slopes of upper Gleann Fearnach. The formation extends south-west of the Gleann Fearnach Fault into the Pitlochry district (Sheet 55E). Its maximum thickness of approximately 700 m is developed in the Allt Doire nan Eun [NO 08 67]. Elsewhere, abundant small-scale folds preclude estimates of thickness.

The contact between the Ben Lawers Schist Formation and the underlying Ben Eagach Schist Formation is exposed in the Allt Ruigh nan Eas [NO 0392 6972]. Here, a layer of calcareous schist, 0.5 m thick and containing coarse tremolitic amphibole and garnet porphyroblasts, marks the base of the Ben Lawers Schist Formation. This is succeeded upwards by several metres of graphitic calcareous schists, then by a dark quartzose metacarbonate rock, 0.5 m thick, then by lithologies more typical of the Ben Lawers Schist Formation.

The formation is dominated by green calcareous schist with interbedded thin psammites, together with abundant amphibolites and metavolcanic rocks (Chapter 5). The green colour results from varying amounts of amphibole and chlorite. Psammite units, 1 to 2 cm thick, are locally abundant and occur throughout the calcareous schists. Many have pitted weathering surfaces resulting from dissolution of calcite. The psammite units are generally more resistant to weathering than the surrounding calcareous schists such that they reveal complex polyphase fold patterns on outcrop surfaces. Massive psammite units, 1 m or more thick, occur on the watershed north of lower Gleann Fearnach [NO 078 675]. A prominent psammite unit east of An Lairig [NO 095 686] is white weathering, but chlorite and epidote impart a green colour to fresh surfaces. A similar lithology forms much of the blocky debris on the summit of Creag an Dubh Shluic, and crops out on the western slopes [NO 089 687]. The formation also includes calcsilicate layers and rare metacarbonate rocks. North of Creag Loisgte [NO 1236 6783], pale, quartzose, calcareous schist containing 10 to 15 cm thick metacarbonate rock layers is exposed in a small quarry. On the west side of the ridge extending northwards from Creag an Dubh Shluic, decimetre-thick psammite units with interbedded calcareous schists are closely associated with both a 15 m-thick unit of relatively uniform schistose pelite and a 1 m-thick cream calcite metacarbonate rock unit [NO 087 688]. A similar association of lithologies occurs to the south-west [NO 082 686]. A lens of fine biotite-rich pelite with graphite, a few tens of metres thick, occurs in Coire nan Eich [NO 123 703].

Amphibole occurs in the calcareous schists, calcsilicates and some psammites, with local development of garbenschiefer texture. With increasing proportion of amphibole, the calcareous schists grade into schistose amphibolites.

A distinct facies comprising hornblende schists with thin psammites, within a background of calcareous and calcsilicate schists, is developed both in the lower part of the Allt Fearnach [NO 025 712] and in the upper part of the valley from Ruigh an Laoigh to Creag Beag [NO 01 73]. This facies is informally referred to as the laoigh metabasites'. On Creag Beag [NO 0113 7368], it contains a layer of cream metacarbonate rock, similar to that elsewhere within the calcareous schists. Geochemical analysis showed the Laoigh metabasites to be chemically similar to the stratigraphically overlying Farragon Volcanic Formation in the Gleann Fearnach area (Goodman and Winchester, 1993), with variations in Mg, Ni and Cr content likely to be due to differences in fractionation state. The Laoigh metabasites probably represent an early stage of the volcanic activity which culminated in the extensive eruption of the Farragon 'Beds'. Stable isotope studies (Scott et al., 1991) confirmed the importance of a volcanic and volcanogenic input into the Ben Lawers Schist Formation.

The main penetrative fabric within the Ben Lawers Schist Formation is generally considered to be Si, in contrast to the penetrative S2 in much of the district. S1 is deformed by abundant small D2 folds, with S2 crenulation cleavage developed in some fold hinges.

In thin section, the calcareous schists (S95841)–(S958442) are generally composed of fine-grained lenticular mosaics of sutured quartz and plagioclase, with elongate laths of muscovite, green-brown biotite and chlorite defining the S1 foliation. Some contain lenticular aggregates of calcite grains. Large poikiloblastic amphiboles are generally blue-green hornblende or actinolite. They contain quartz and feldspar inclusion trails contiguous with the external foliation. Trails of epidote grains lie parallel to the foliation within the micaceous domains; some of the epidotes have metamict cores which may have developed around small inclusions. Garnets are rare and rather poorly developed. Sphene, apatite and zircon are accessory minerals; where opaque grains are abundant, they are generally pyrite. Thin psammite units (S95842) are composed mainly of quartz, calcite and zoisite. Thicker psammite units with green fresh surfaces contain more abundant epidote than the thinner units, and may contain some amphibole and chlorite. Schistose pelites comprise up to 70 per cent phyllosilicate minerals, with muscovite dominating over biotite and chlorite. The micas are interleaved with aggregates of irregular quartz and feldspar grains, with some epidote granules and rare garnets. Accessory minerals in the pelites are zircon, apatite and opaque minerals.

Dark green hornblende in hornblende schists, including those of the Laoigh metabasites, are preferentially aligned in the dominant (S1 or S2) foliation. They are separated by fine quartz, plagioclase and epidote grains. Fine opaque and sphene grains lie in chains along the foliation, locally with some rutile. Amphibole related to development of an incipient S3 crenulation cleavage is blue-green in colour, in contrast to the earlier dark green hornblende. Fine-grained intrusive amphibolites are usually less well foliated, more homogeneous and leucocratic than the metavolcanic amphibolites, while coarser grained amphibolites have clear meta-igneous texture in thin section; both are described further in Chapter 5.

Farragon Volcanic Formation

In the district this formation, previously informally called the 'Farragon Beds', comprises exceptionally fine-grained hornblende schists with intercalated calcareous schists and psammites. Individual exposures of the formation may be difficult to distinguish from the hornblendic facies of the Ben Lawers Schist Formation. In the type area in the Pitlochry district, the formation has been described as a complex unit of green beds, quartzites, garnet mica schists and hornblendic rocks (Sturt, 1961).

The Farragon Volcanic Formation can be traced east across the Pitlochry district (Sheet 55W) from the type area around Farragon Hill [NN 84 55] to Creag Uisge [NO 02 69]. North-east of the Gleann Fearnach Fault in the Glen Shee district, it has a broad outcrop trending northwest–south-east on the north-eastern side of Gleann Fearnach. It extends across the watershed north of Meall Odhar [NO 11 66] into Glen Shee where it is truncated by the south-west margin of the Glen Shee Pluton. The Farragon Volcanic Formation does not occur north-east of the pluton; in this area the overlying Ben Lui Schist Formation rests directly on the Ben Lawers Schist Formation. The formation has a fairly uniform thickness of about 200 to 300 m, although it is significantly thicker in the Ben Vrackie area [NN 94 63] of the Pitlochry district and thinner east of Gleann Fearnach; adjacent to the Glen Shee Pluton it is only a few tens of metres thick.

The Farragon Volcanic Formation is considered to have an extrusive volcanic protolith on account of its fine grain size, lateral extent, concordance and intercalation with metasedimentary rocks. In the Ben Vrackie area, tuffs, lavas, agglomerates and volcaniclastic metasedimentary rocks have been distinguished (Institute of Geological Sciences, 1981). In the Glen Shee district, the Farragon Volcanic Formation was probably derived from basic lavas and tuffs that were intercalated with nonvolcanogenic sedimentary rocks. Geochemical evidence indicates that the volcanic rocks were probably erupted from a vent centred on Ben Vrackie (Goodman and Winchester, 1993). The variation in thickness is considered to be a primary effect, since the formation thins away from, and contains a progressively higher proportion of metasedimentary lithologies with increasing distance from Ben Vrackie.

Hornblende schists of the Farragon Volcanic Formation are very fine grained, green-black and with a silky appearance to schistosity surfaces. Interbedded psammites, which are abundant close to the base of the formation on Creag Uisge [NO 022 695], are commonly calcareous and form units up to a few decimetres thick. The proportion of psammite and schistose biotite semipelite increases towards the top of the formation on the southern slopes of Elrig [NO 07 66], adjacent to the Ben Lui Schist Formation. Lenses of calcsilicate rock are recorded from several localities, and discontinuous pink or cream, calcite-dolomite metacarbonate rocks, a few metres thick, are recorded in Gleann Fearnach [NO 0409 6831] and on Creagan Uaine [NO 0565 6837].

In thin section (S95839)–(S95840), the hornblende schists contain mainly fine-grained hornblende with some chlorite and biotite, small grains of epidote and zoisite and some large patches of calcite. A lamination is produced by a contrast in the abundance of fine granoblastic quartz and feldspar. The common accessory minerals are opaque oxides and sphene. Mafic minerals are strongly aligned along S1 and subparallel to the compositional banding. Hornblende, chlorite and biotite are also aligned along a locally developed S2 crenulation cleavage, whereas some blue-green amphibole overgrows S2. The calcsilicate rocks within the amphibolites comprise very fine quartz and zoisite, with larger granular patches of epidote and a few hornblende grains; some contain calcite. The metacarbonate rocks are fine grained and are composed of an interlocking mosaic of calcite and dolomite grains. They exhibit a weak grain-size layering but strong shape fabric. Quartz is interstitial to the carbonate grains. Chains and aggregates of rounded garnet and idocrase are studded with tiny grains of epidote. Muscovite flakes occur along some calcite grain boundaries. Fine opaque minerals are the only accessory mineral.

Crinan Subgroup

The Crinan Subgroup metasedimentary rocks mark a return to predominantly elastic deposition, and record a phase of rapid subsidence and basin deepening (Anderton, 1985). In the south-west Highlands, proximal turbidites referred to as the Crinan Grits (Borradaile, 1973), pass laterally north-eastwards into the more distal Ben Lui Schist Formation, named after the type area in Perthshire. In the Glen Shee district, the dominantly semipelitic to pelitic rocks of the Crinan Subgroup are considered to be equivalent to the Ben Lui Schist Formation.

Ben Lui Schist Formation

Most of this formation consists of schistose to gneissose semipelites and pelites assigned to the Duchray Hill Gneiss Member. A psammitic unit, the Corrydon Psammite Member, forms the lower part of the formation in the eastern part of the district, whereas the Golan Well Pebbly Psammite Member is interpreted to form the top of the formation in part of the east of the district south-east of the Glendoll Fault. Migmatitic textures are extensively developed in semipelitic lithologies in the eastern part of the district (Chapter 7).

On the current Sheet 65W (British Geological Survey, 1989), both the Corrydon Psammite and the Duchray Hill Gneiss together with part of the Ben Lawers Schist are recorded as Crinan Subgroup Caenlochan Schist. However, the limit of migmatisation in the Caenlochan

Schist accords well with the boundary between the Duchray Hill Gneiss Member and the Corrydon Psammite Member in the Glen Shee district. The boundary between the Duchray Hill Gneiss Member and the Ben Lawers Schist Formation at the northern margin of the district broadly coincides with that between 'granite, merging into muscovite-biotite gniess' and 'calc-sericite schist' shown on the original version of Sheet 56 (Blairgowrie) (1895) and between 'Caenlochan Schist' and 'Duchray Hill Gneiss' on Sheet 65 (Balmoral) (1904).

The Ben Lui Schist Formation crops out in a broad swathe in the northern part of the district. It extends south-westwards from the head of Glen Isla [NO 21 71] across Glen Shee and then north-westwards to the east of the Glen Fearnach Fault. It also occurs west of the fault at the western margin of the district [NO 030 692]. The Corrydon Psammite Member forms the lower part of the formation from the Meall Odhar area [NO 11 65] to the head of Glen Isla [NO 18 72]. To the west of the Meall Odhar area, the Duchray Hill Gneiss Member directly succeeds the Farragon Volcanic Formation. Between Meall Odhar and the western margin of the Glen Shee Pluton, the Corrydon Psammite Member stratigraphically overlies attenuated Farragon Volcanic Formation, whereas north-east of the pluton, in the absence of the Farragon Volcanic Formation, the Corrydon Psammite Member overlies the Ben Lawers Schist Formation. This spatial distribution, together with the high proportion of psammites and semipelites in the upper part of the Farragon Volcanic Formation on Elrig, may reflect a lateral equivalence of the Farragon Volcanic Formation and the Corrydon Psammite Member.

A small outlier of hornfelsed semipelite and pelite assigned to the Duchray Hill Gneiss Member, crops out within a synclinal fold closure, adjacent to the northern margin of the Glen Shee Pluton at Craig of Runavey [NO 143 700].

The Ben Lui Schist Formation passes stratigraphically upwards into the Loch Tay Limestone Formation (Harris and Pitcher, 1975). However, in this district stratigraphical continuity is only seen to the west of the Glen Fearnach Fault [NO 03 69], and inferred at Fergus in Glen Isla [NO 192 677]. Elsewhere, the Loch Tay Limestone Formation is excised along the sheared limb of a major F1 fold (Chapter 6) so that the Ben Lui Schist Formation is in tectonic contact with the Southern Highland Group. However, in the Auchintaple Loch area [NO 65 20] in the east of the district, the Golan Well Pebbly Psammite Member stratigraphically underlies the Loch Tay Limestone Formation. On this basis, it is assigned to the upper part of the Ben Lui Schist Formation and is interpreted as a local facies occurring in the hinge of the reclined Mount Blair Antiform.

Corrydon Psammite Member

This member, named after the settlement of Corrydon in Glen Shee, comprises fine-grained biotitic psammite and schistose semipelite, with minor amounts of impure brown quartzite. Bedding is well developed as a centimetre-thick dark and light coloured parallel lamination. Dark psammites, with 1 to 5 m-thick layers of impure brown quartzite, commonly bounded by dark brown quartzitic psammite, are well exposed in the lower part of the Allt a' Choire Dhomhain [NO 129 669]. Light grey-green calcsilicate beds, 2 to 3 m thick, occur in Glen Brighty Burn [NO 1812 7231]; [NO 1794 7248] to the west of Tulchan Lodge immediately north of the district.

In the west of the outcrop of the member, the boundary with the younger Duchray Hill Gneiss Member appears to be gradational, with an increase in the proportion of semipelite, together with the development of migmatitic semipelites, as the boundary is approached on Meall Dubh [NO 126 662]. In the east, white, fine-grained quartzite, seen only as scree, forms the top of the member north of Monamenach [NO 17 71].

In thin section (S95836), the psammites comprise over 60 per cent fine-grained quartz, with scattered oligoclase, muscovite and small, stubby biotites. Variation in the proportion of quartz and micas reflects bedding. Biotites are preferentially aligned parallel to bedding. Muscovite forms larger, more ragged flakes than biotite; some of these flakes are discordant to, and probably postdate, the biotite fabric. Accessory minerals include apatite, sulphides and epidote, with minor, small, sieved garnets present locally.

Duchray Hill Gneiss Member

This member takes its name from Duchray Hill [NO 16 67] which is situated between Glen Shee and Glen Isla. It is composed mainly of semipelites which range from schistose in the west to gneissose and migmatitic in the east (Chapter 7). Intercalated pelite and psammite units, no more than a few metres thick, account for less than 10 per cent of the member, although locally on Creag Feith nan Ceann [NO 13 65], psammite forms 50 per cent of the outcrop.

The degree of migmatisation within the semipelites decreases westwards, so that in Gleann Fearnach the member is represented by schistose semipelites with streaks and augen of quartzofeldspathic material a few centimetres thick that are concordant with S2 schistosity. In the schistose semipelites (S95832), biotite and muscovite have strong preferred orientation and form an anastomosing foliation, which is separated by a mosaic of fine- to medium-grained quartz and plagioclase. Large oligoclase porphyroblasts and rounded garnets are wrapped by the mica foliation; the apparent absence of kyanite may be a sampling effect, rather than the result of a decrease in metamorphic grade westwards (Chapter 7). Sphene, quartz and opaque minerals occur as inclusions in garnet, although external to the porphyroblasts the accessory suite consists of opaque minerals, apatite and zircon. The paucity of tourmaline in these rocks compared with the migmatitic rocks to the north-east may reflect development of migmatisation (Chapter 7).

Migmatitic semipelites (Plate 6) are well exposed on Cairn Derig, Duchray Hill and Ewe Crags [NO 15 65] to [NO 16 67], on the extensive crags on the western side of Glen Isla [NO 17 67] to [NO 18 69] and north-east of Fergus on the east side of Glen Isla [NO 196 691]. They are composed of quartz, plagioclase, biotite, muscovite and garnet. Tourmaline, in the form of schorl aggregates up to 2 cm long, and kyanite are locally developed. Generally the semipelites are texturally heterogeneous stromatic migmatites with 5 to 10 mm-thick quartzofeldspathic leucosome layers and lenses separated by quartz, plagioclase, biotite, muscovite and garnet mesosomes and mica and garnet-rich melanosomes (Plate 6). Leucosome layers define an S2 gneissosity. Oligoclase porphyroblasts occur in places. Lenses of coarse-grained muscovite-bearing pegmatite occur to the west of Bada na Bresoch [NO 2073 7141], whereas muscovite granite sheets, up to at least 3 m thick, occur in the Altvraigy Burn [NO 206 703]. Locally [NO 2127 7082], migmatitic leucosome grades into muscovite pegmatite which transgresses melanosomes. Tourmaline-bearing pegmatites concordant with the stromatic layering are developed adjacent to amphibolite near Duke's Lair [NO 1839 6797].

East of Glen Isla, with increasing proportion of neosome, heterogeneous stromatic migmatites locally grade into relatively homogeneous, medium- to coarse-grained granoblastic granitoid rocks (Plate 7). Mica-rich schlieren and inclusions of gneissose semipelite ranging from a few millimetres to tens of centimetres long, some of which preserve a stromatic texture, locally produce a 'ghost' layering which reflects a stromatic migmatite precursor. Rounded to tabular inclusions of psammite up to 1 m across are enclosed within the granitoid, and are commonly wrapped by mica-rich schlieren. Muscovite and subhedral oligoclase megacrysts up to 10 mm across are developed in places.

Thin psammite layers, less than 1 m and typically less than 30 cm thick, occur throughout the member. The psammites carry a spaced fabric defined by biotitemuscovite laminae which separate quartz-rich lithons. Thin (less than 5 cm thick) greenish pink calcsilicate lenses are recorded in the Glencally Burn [NO 1981 7024] and north-east of Fergus [NO 1965 6894]. Lenses of psammite, thought to represent boudins, are particularly abundant west of Cairn Derig [NO 1510 6631] and near Creagan Claise [NO 1780 6796]. Commonly, the psammites preserve early tectonic fabrics and folds not seen in the enclosing semipelite. Five metres of grey, fairly massive calcsilicate rock crop out in the Algeilly Burn [NO 2072 7096]. This is composed of coarse-grained poikiloblastic diopside, opaque minerals, tremolite, phlogopite, plagioclase, quartz and sphene (S92270)–(S92271). Rusty spots, 2 to 5 mm in diameter, are after sulphide.

A 600 m-wide zone of more micaceous migmatitic semipelites extends north-east across the lower part of the Algeilly Burn and Bada na Bresoch [NO 213 716]. This is characterised by abundant, coarse-grained kyanite and garnet. Kyanite grains are typically 5 mm and locally 10 mm long, and may make up to 20 per cent of the mode, whereas the garnets are typically 5 mm in diameter and may be concentrated in lenses composed of 50 per cent garnet as on Bada na Bresoch [NO 2073 7141]. Rusty weathering semipelites and pelites and 10 to 20 cm-thick quartz-biotite-kyanite layers with rusty spots crop out close to the south-east edge of the kyanite-rich rocks in the Algeilly Burn [NO 2062 7090].

In thin section, the schistose rocks, gneissose stromatic migmatites and granitoid rocks are only distinguished texturally. Mineralogically, they all contain quartz, plagioclase, biotite, muscovite and garnet with accessory tourmaline, opaque minerals, zircon, rutile and apatite. Kyanite is quite widely developed and locally very abundant (S92769) whereas orthoclase and fibrolite are rare (S92769). The stromatic migmatites (S94326), (S95833) have coarse-grained leucosome and melanosome layers, and medium-grained mesosome layers. Leucosomes are predominantly formed of coarse oligoclase and quartz, locally with granoblastic polygonal textures. Melanosomes are composed of muscovite, which is generally more abundant than biotite, together with garnet, which may show atoll forms, and kyanite. Minor staurolite has been identified near Auchavan [NO 1851 6964]. Mesosomes contain plagioclase, quartz, muscovite, biotite and garnet. Some large tourmalines are yellow in thin section, in contrast to the more typical blue-green colour of accessory tourmaline. The granitoid rocks (S92761), (S94331) are mineralogically similar to the stromatic lithologies, although they generally have less abundant kyanite and may contain oligoclase megacrysts.

Golan Well Pebbly Psammite Member

The Golan Well Pebbly Psammite derives its name from the only named locality in the vicinity of its outcrop. It underlies a 0.5 km2 area north-east of Auchintaple Loch [NO 202 655]; it is bounded to the north-west by the Glendoll Fault and on all other sides by the Loch Tay Limestone Formation and associated metamafic rocks. Although younging evidence is not preserved, the overall distribution of lithologies indicates that it stratigraphically underlies the Loch Tay Limestone Formation within the core of the Mount Blair Anticline. It probably represents a local facies development at the top of the Ben Lui Schist Formation.

The member crops out as small exposures and float composed of massive and thickly or very thickly bedded gritty and pebbly psammite and psammite with micaceous laminae. Pebbly psammite beds contain flattened quartz and subordinate feldspar pebbles up to 1 cm long with a groundmass of micaceous psammite. No evidence of any grading is seen. In thin section (S95731), flattened clasts of strained quartz occur within a fine-grained matrix of quartz, plagioclase, possible K-feldspar and biotite, together with some garnet and muscovite. Loose blocks of grey psammite around [NO 2135 6540] contain layers of white calcsilicate rock, 1 to 5 cm thick, composed of white feldspar, quartz, pink garnet and dark green amphibole.

Tayvallich Subgroup

The Tayvallich Subgroup is composed of metacarbonate and carbonate-bearing rocks, with minor intercalated clastic metasedimentary rocks and local basic volcanic rocks (Harris and Pitcher, 1975). Both the preceding Crinan Subgroup and the succeeding Southern Highland Group are characterised by turbidite sedimentation; much of the Tayvallich Subgroup carbonate sediment was similarly derived (Anderton, 1985), although some of the purer metacarbonate rocks may have developed in situ. The top of the Argyll Group is placed at the top of the highest metacarbonate rock unit. In the Glen Shee district, the Tayvallich Subgroup is represented by the Loch Tay Limestone Formation. This is correlated with the Loch Tay Limestone of the Pitlochry district and the Dounalt Limestone Formation of the Ballater district (British Geological Survey, 1995b).

Loch Tay Limestone Formation

The Loch Tay Limestone Formation in the district comprises a mixture of schistose calcareous semipelites and psammites, calcsilicate rocks and crystalline metacarbonate rocks (Plate 8). Thin units of concordant, fine-grained schistose amphibolite, interpreted to have a volcanic protolith, are common and locally intercalated with hornblendic psammite. Metacarbonate rocks range from a few centimetres to 15 m thick. Thick developments of coarse-grained amphibolite are invariably spatially associated with the formation. They are interpreted as intrusive sills and are described in Chapter 5.

The main outcrop of the formation occurs within a major F1 anticlinal fold hinge, the Mount Blair Anticline, (Chapter 6) where it represents the base of the local succession and is enclosed by the younger Southern Highland Group (Figure 1). The formation underlies the east–west trough that connects Strathardle and Glen Shee in the central part of the district. The gently inclined attitude of the rocks produces a broad outcrop of metacarbonate rocks, locally in excess of 1 km wide, and spatially associated coarse-grained amphibolite. To the west, the formation is truncated by the Glen Fearnach Fault [NO 09 64], whereas, to the east, it can be traced around the eastern flank of Mount Blair [NO 17 62], where a grassy bench is developed on the calcareous lithologies. Here, where dips are steeper, the carbonate rocks form a unit less than 50 m wide, although the associated amphibolite may be more than 500 m thick. The metacarbonate rocks are truncated by the Glen Doll Fault near Forter [NO 17 64], although further outcrop occurs to the north-east near Auchintaple Loch [NO 20 65].

West of the Gleann Fearnach Fault, in the lower valley of the Allt Uisge [NO 03 68], the formation occurs in an unbroken stratigraphical succession between the Ben Lui Schist Formation to the north-west and the Southern Highland Group to the south-east. Elsewhere within the district, the boundary between the Crinan Subgroup and Southern Highland Group is marked by a zone of dislocation, with excision of the Loch Tay Limestone Formation, save for small outcrops of metacarbonate rocks around Fergus [NO 196 672] and immediately east of Gleann Fearnach [NO 06 65]. A small infold also occurs within the Ben Lui Schist Formation on Blar Achaidh [NO 066 668].

Natural exposure of the formation is generally poor, although good sections are revealed as a result of widespread quarrying. The formation can be traced in the intervening ground using numerous small workings.

It is typically marked by grassy hollows with metacarbonate rock debris and intervening low ridges and roches moutonnées on the outcrop of the associated coarse-grained amphibolite.

West of the Gleann Fearnach Fault in the area south of Meall Daimheidh [NO 0257 6807], there is an abrupt change from the schistose semipelites of the Ben Lui Schist Formation to calcareous psammites of the Loch Tay Limestone Formation. Calcareous psammites are succeeded by metacarbonate rocks and calcsilicate rocks which pass into fine-grained hornblende schists, calcareous and non-calcareous schistose psammites and semipelites in the Allt Uisge. Metacarbonate rock outcrop is marked by linear topographical hollows and sink holes. Lenses of dirty quartzite are developed locally, with a 2 m-thick lens at Milton Knowe [NO 0980 6016], and a unit 5 to 10 m thick farther east [NO 1058 5994] at the margin of a coarse-grained amphibolite.

North-east of Auchintaple Loch [NO 199 651], the Loch Tay Limestone Formation overlies the Golan Well Pebbly Psammite Member. Relationships between the metacarbonate rocks and the pebbly psammite are not exposed, although farther upslope [NO 204 657] a strip of metacarbonate rock is encircled on three sides by exposures of pebbly psammite. Nearby, metacarbonate rocks are well exposed in small quarries within a north–south-trending zone [NO 205 656] and in an old quarry [NO 1990 6516]. Farther north-east [NO 2050 6541], where 5 m of metacarbonate rock is exposed, units with biotite and muscovite partings defining a thin lamination are interbedded with thickly bedded massive grey metacarbonate rock, some of which is quite coarsely crystalline. A layering on a 1 to 2 cm scale is defined by more and less carbonate-rich layers. The metacarbonate rocks display brown-weathering surfaces, with some thin (5 to 25 cm) interbedded rusty-weathering, muscovite-bearing semipelite.

Elsewhere within the district, metacarbonate rocks are locally well exposed as a result of extensive quarrying. In Wester Bleaton Quarry [NO 115 597] in the central part of the district, calcareous schists are overlain successively by grey metacarbonate rocks, fine-grained amphibolite, grey metacarbonate rocks and coarse-grained amphibolite. The original thickness and way-up of individual units is obscured by folding. Metacarbonate rocks here, and in the adjacent An Dun Quarry [NO 110 592], are no more than 15 m thick. The fine-grained amphibolite contains fine-grained disseminated pyrite and chalcopyrite, as do the adjacent calcareous schists. Both quarries also contain exposure of a conglomeratic unit a metre or so thick, with rounded or ellipsoidal blocks of fine amphibolite within a matrix of metacarbonate rock. This is interpreted as a primary depositional feature and possibly a volcanic conglomerate. At broadly the same stratigraphical level, mainly massive, light grey, crystalline metacarbonate rocks, with minor, more flaggy, schistose metacarbonate rocks, are exposed in a 10 m-high disused quarry face near Soilzarie [NO 1285 5993]. On Creagan Beag [NO 1132 6060], alternate layers of hornblendic psammite and metacarbonate rock occur within a 4 m-thick unit. In the Allt Menach [NO 0978 6031], fine-grained cummingtonite amphibolite occurs in a concordant sheet which can be traced for 300 m along strike. In Glen Isla, a further disused quarry near Balloch Burn [NO 1765 6441] contains 10 m or so of metacarbonate rocks, within a calcareous unit 25 to 30 m thick. Metacarbonate rocks have been replaced by baryte in the quarry at Nether Craig [NO 1692 6108] (Chapter 2). Here, and elsewhere in thinly banded calcareous rocks, complex small-scale folds are well displayed.

In the Blair Achaidh area [NO 066 668], a structural outlier of the formation is encircled by the Duchray Hill Gneiss Member. Calcareous schists and metacarbonate rocks have a 'streaky', highly sheared appearance in outcrop. Contact relationships with the adjacent Duchray Hill Gneiss Member are not exposed. Metacarbonate and calcsilicate rocks assigned to the Loch Tay Limestone Formation also occur in the River Isla near Crandart [NO 1909 6761], where they lie south-east of the Duchray Hill Gneiss Member and immediately north of the inferred position of the Fergus Slide. Contact relationships with the adjacent lithologies are not exposed. Here, the formation includes 6 m of massive hornblendebiotite schist containing calcareous pods and ribs up to a few centimetres across and approximately 6 m of ribbed and 'knobbly' rock containing 3 to 10 cm-thick metacarbonate rock layers together with layers of calcsilicate and hornblendic rock. Sulphide is widely disseminated and abundant. Metacarbonate rocks contain mafic hornblende-biotite laminae and greenish grey calcsilicate rock layers. These are succeeded to the south by a unit with ribbing on a 5 to 50 mm scale, which is composed mostly of metacarbonate rock and calcsilicate rock with subordinate quartzose layers.

The boundary between the Loch Tay Limestone Formation and the overlying Southern Highland Group in this district is placed at the stratigraphical top of the highest calcareous unit. In many areas, this boundary is obscured by intrusive amphibolite. However, it is well exposed in a small crag on the eastern slopes of Creag Dubh-leitir [NO 067 654], where 3 m of beige-weathering quartzose metacarbonate rocks are overlain by schistose garnetiferous semipelites of the Southern Highland Group.

The presence of abundant intrusive amphibolite, folding and poor exposure preclude a detailed strati-graphical section through the entire formation. The number of metacarbonate rock units is not known, although some are inferred to grade laterally into the calcareous schists and calcsilicate rocks.

In thin section, the laminated calcareous schists (S95729) are composed of quartz, plagioclase, biotite and muscovite, with layers 1 to 4 mm thick which also contain carbonate, sphene and opaque minerals. Micas locally occur in thin laminae with muscovite also forming poikiloblastic plates several millimetres across. The meta-carbonate rocks (S94337), (S94340), (S95730,(S95828), (S95829) typically contain more than 80 per cent carbonate in a medium- to coarse-grained interlocking mosaic. They also contain quartz, plagioclase, phlogopitic biotite and small amounts of clinozoisite, sphene, microcline, muscovite, opaque minerals and diopside. Large (5 mm) idocrase porphyroblasts appear to have grown at the expense of diopside, whereas chlorite has replaced idocrase and biotite. In the Fergus area, calcsilicate rocks comprise the assemblages quartz, microcline, plagioclase, phlogopitic biotite, clinozoisite, sphene and opaque minerals together with accessory apatite (S94335) and quartz, plagioclase, poikiloblastic diopside, phlogopitic biotite and carbonate with accessory opaque minerals and sphene (S94336). Minor grossular and tremolite are also developed in places (S95829), whereas macroscopic dravite porphyroblasts are recorded from the volcanic conglomerate in Wester Bleaton Quarry [NO 1151 5982]. Fine-grained amphibolite (S95831) comprises a felted mass of actinolitic amphibole, studded with epidote together with some biotite. A poorly developed foliation wraps plagioclase porphyroblasts. Fine-grained quartz, plagioclase and calcite are present in small amounts, as are abundant fine-grained opaque minerals and accessory sphene and zircon.

Southern Highland Group

The Southern Highland Group comprises a succession of psammites, some of which are gritty and pebbly, and schistose semipelites together with intercalated volcaniclastic Green Beds, all considered to have been deposited by turbidity currents. The group occupies more than 50 per cent of the area of the district and more than 65 per cent of the Dalradian outcrop area. To the north, it has a normal stratigraphical contact with the Tayvallich Subgroup Loch Tay Limestone Formation on both limbs of the Mount Blair Anticline. It is juxtaposed tectonically with the Crinan Subgroup Duchray Hill Gneiss Member across both the Gleann Fearnach Fault and the Fergus Slide. In the south of the district, the group is overlain unconformably by Devonian rocks north-east of Strathardle, but farther west the group is juxtaposed with Devonian rocks across the Middleton Muir Fault. North of the Highland Border Downbend hinge line (Chapter 6), the Southern Highland Group is generally inverted and dips north-west, whereas to the south of the hinge, it is nearly vertically disposed. Therefore, the youngest part of the group is exposed in the hinge zone of this downward-facing antiformal structure.

There is a general southward increase in the proportion of psammitic lithologies which broadly represents a coarsening-upward sequence across the outcrop of the Southern Highland Group. In the east of the district, this is sufficiently well developed to allow division of the group into a lower unit of intercalated psammites and semipelites referred to as the Mount Blair Psammite and Semipelite Formation and an upper dominantly psammite and gritty psammite unit referred to as the Cairn Gibbs Psammite Formation. The boundary between the two formations is transitional and for convenience is placed at a major Green Bed unit. Subdivision of the group becomes increasingly difficult towards the west where the psammitic rocks are less dominant. West of Strathardle, intercalated psammites and semipelites occur throughout and any upward increase in the proportion of psammite is gradual so that no subdivision is possible. The boundary between the stratigraphically divided and undivided parts of the group is placed in the poorly exposed ground of Strathardle [NO 10 54] but this is not controlled by the Strathardle Fault. Lateral variation and correlation within the Southern Highland Group of the district is illustrated in (Figure 9).

A distinct succession composed of intercalated slates and gritty psammites, which is referred to as the Birnam Slate and Grit Formation, is recognised immediately north of the Middleton Muir Fault in the west of the district. The stratigraphical position of this formation remains unclear (Figure 9).

Elsewhere in the Southern Highlands, the lower parts of the Southern Highland Group such as the Pitlochry Schist and the Glen Effock Schist (Harris et al., 1994) are dominantly semipelitic, whereas the upper parts, such as the Ben Ledi Grit, Glen Lethnot Grit and Dunkeld Grit formations (Harris et al., 1994), are dominantly psammitic. The Mount Blair and Cairn Gibbs formations broadly reflect these regional changes in lithofacies. The Birnam Slate and Grit Formation is directly equivalent to the Birnam Grits and Birnam Slates in the Perth and Dundee district (Sheet 48W), but with more finely interbedded lithologies. The Dunkeld Grits of the Perth and Dundee district are probably at least partly equivalent to the stratigraphically unassigned Southern Highland Group rocks north of the Birnam Formation in the Glen Shee district.

Exposure of the Southern Highland Group on some higher hills and ridges in the south-west of the district is excellent, especially south-west of Strathardle. However, large tracts such as the Forest of Clunie [NO 08 50], Forest of Alyth [NO 19 55], Cochrage Muir [NO 13 49] and Strathardle are very poorly exposed. Ground magnetic surveys were invaluable in tracing Green Beds through these areas.

Southern Highland Group (unassigned)

The stratigraphically unassigned metasedimentary rocks of the Southern Highland Group are dominantly schistose psammites and semipelites, together with subsidiary pelites and gritty psammites. The proportion of semipelites and pelites gradually increases to the north with a concomitant decrease in the abundance, bed thickness and persistence of psammites and gritty psammites. In the south-west of the district, discrete psammite, gritty psammite and semipelite units up to 70 m thick can be traced for up to 3 km. Elsewhere, the group is lithologically undivided. Intercalated Green Beds are described below.

The schistose semipelites generally contain quartz, plagioclase, biotite, muscovite, garnet and accessory tourmaline. They are commonly laminated on a millimetre to centimetre scale with alternate micaceous and more quartz-rich layers with less than 30 per cent mica. More micaceous variants include notably lustrous schistose muscovite-biotite pelites with abundant 5 mm-diameter garnets on the north-eastern flanks of Meall Uamhaidh [NO 03 56], silvery green pelite [NO 056 514] and dark grey pelite [NO 091 529]. Steel grey pelite to the east of Lochan Oisinneach [NO 0454 5568] is interbedded on a 5 to 10 cm scale with semipelite. Interbedded decimetre-thick units of psammite are commonly present. Quartz veins are abundant, especially in the north of the outcrop.

The psammites are generally fine to medium grained, pale brown or grey. Locally, they display colour banding, typically on a centimetre to decimetre scale, which is caused by variation in the quartz–mica ratio. Many display a characteristic tectonic layering (Si) in which regularly spaced layers of quartz with subsidiary feldspar up to 10 mm thick are separated by thin (2 to 4 mm) biotitemuscovite laminae. The layers may be as little as 1 mm thick where subsequent deformation is intense. Small but locally relatively abundant 'pin-head' garnets are developed in places. Widespread, thin, pebbly layers and lenses contain matrix-supported, rounded to subangular quartz and feldspar clasts up to 3 mm in diameter. A prominent psammite bed, exposed on the eastern flanks of Creag Dubh-leitir [NO 064 657] in the north of the Southern Highland Group contains approximately 5 per cent calcite.

Gritty psammite units make up less than 10 per cent of the Southern Highland Group. The gritty psammites contain mostly quartz clasts with subsidiary plagioclase and alkali feldspar, within a matrix of fine quartz and feldspar with subordinate micas. They are generally less well sorted and less mature than the psammites. The clasts seldom exceed 10 mm in diameter and are matrix-supported. The characteristic tectonic striping of the psammites is not developed, although a grain shape fabric of oblate or prolate quartz clasts is commonly seen. Gritty psammite units are most abundant and thickest in the southern part of the Southern Highland Group. They form lenticular bodies, with a maximum thickness of about 10 m, although most are 2 to 5 m thick, with a lateral extent of only 200 to 500 m, and rarely 1 km. They are thought to represent channel fills. Farther north their abundance, thickness and lateral continuity decreases to the extent that they are no longer mappable as coherent units. A highly distinctive reddish gritty psammite with blue quartz and white feldspar clasts occurs in a lens only a few tens of metres long north of Loch Ordie [NO 0334 5042], whereas an exceptionally coarse gritty psammite, with grain-supported quartz and feldspar clasts of up to 20 mm, and showing good grading, is exposed at Craig Wood [NO 0521 5276].

Gritty psammites and pebbly psammites commonly show graded bedding (Plate 9) particularly on Deuchary Hill [NO 0305 4783], north-west of Loch Ordie [NO 0265 5152] and at Craig Wood [NO 0521 5276], whereas farther south close to the Middleton Muir Fault where the rocks are unaffected by D2, refraction of S1 is thought to reflect graded bedding. Grain-size grading in centimetre-scale units of psammite and semipelite may indicate younging directions on Creag an t-Sithein towards the east [NO 039 662] and west [NO 048 659]. Way-up may also be indicated by grading from a psammitic base to a semipelitic top over a metre or so, as seen northwest of Loch Ordie [NO 0274 5091] and in Kindrogan Wood [NO 0487 6156]. In the northern part of the Southern Highland Group outcrop, where semipelite prevails, such evidence is rare and always equivocal.

A thick succession of fine-grained, flaggy psammites lacking both gritty units and schistose semipelites crops out in the Drouthy Burn [NO 088 467] to [NO 083 470], and north of Laighwood [NO 072 462]. North and east of Ranageig [NO 106 494] and in the Baden Burn [NO 100 490], this succession appears to pass laterally into a distinctive unit of grey-green chloritic semipelitic phyllites with subsidiary bands of fine-grained, greenish grey, feldspathic psammites. The phyllites contain up to a few per cent steel-grey magnetite up to 1 mm across with the most magnetic rocks forming a lenticular body approximately 1500 m by 600 m. The psammites are generally a few centimetres thick and unusually rich in plagioclase with significant amounts of epidote and chlorite. This succession may have a volcaniclastic origin.

Mount Blair Psammite and Semipelite Formation

The Mount Blair Psammite and Semipelite Formation forms the stratigraphically lower part of the Southern Highland Group in the east of the district. It consists of more or less equally abundant interbedded semipelite and psammite Green Bed units ranging from 3 cm to 10 m thick crop out within the formation. The formation crops out both to the north and south of the underlying Loch Tay Limestone Formation, on opposing limbs of the Mount Blair Anticline. It is bounded to the north by the Fergus Slide which separates it from the Duchray Hill Gneiss member and to the south by the stratigraphically overlying Cairn Gibbs Psammite Formation. The formation is well exposed on the east-facing slopes of Mount Blair [NO 16 62], east of Glen Isla [NO 19 66], locally in the vicinity of Auchintaple Loch [NO 19 64] and, farther west, on Creag nam Brataichean [NO 11 61] and the low hills east of Clach Sgorach [NO 13 61].

Semipelite units are typically less than 50 cm thick, although some are several metres thick. They are mostly schistose with widespread quartz segregations up to 2 to 3 cm across. Quartzofeldspathic segregations occur at higher metamorphic grades in the north of the outcrop area close to the contact with the Duchray Hill Gneiss Member. Semipelites are typically muscovite rich with prominent dark, and in places, euhedral garnet, up to 4 to 5 mm across. With increasing mica, they pass into pelite [NO 2016 6391]. Staurolite is locally abundant to the south of Auchintaple Loch [NO 2016 6391], while coarse-grained kyanite up to 1 cm long occurs in The Banks area [NO 2242 6782].

Psammite units are less than 2 m and typically less than 0.5 m thick. Many display a strong, platy, planar fabric and in places are attenuated; this results in approximately 1 cm-thick quartz 'ribbons' within the semipelites. Gritty psammites are widespread with quartz and less abundant feldspar clasts typically up to 2 mm in diameter. East of Glenisla House [NO 2014 6379], a prominent, bedded, gritty psammite unit (clasts up to 3 mm in diameter), at least 1 m thick, can be traced for more than 60 m. Garnet is present in more micaceous laminae within psammites.

Laterally impersistent, impure, yellow to light brown, blocky weathering quartzite is developed in the Allt an Lair [NO 1393 6331] and west of Creag an Lair [NO 134 628]. These quartzites contain some feldspar and are gritty in places [NO 1379 6332], where a fabric of flattened grains is developed. On the east of Cnoc an Daimh [NO 098 628], quartz-rich psammite and dirty quartzite with diffuse bedding traces are prevalent. The quartzites are probably channel deposits.

Schistose coarse pebbly psammites occur both near the summit of Mount Blair [NO 1672 6269] and on its western flank [NO 1540 6271]; at the latter locality, graded bedding and channelling show the rocks to be inverted. Thin pebbly layers with flattened pebbles 10 mm across and 20 mm long occur within psammite at Lair [NO 1403 6330], whereas on the south flank of Creag nam Brataichean [NO 1116 6092], quartzofeldspathic gritty psammites occur with a grain size up to 5 mm Graded bedding north of Glenkilrie Lodge [NO 1397 6100] indicates that, at least at this locality, the rocks are the right way up. A rare, thin (300 mm) layer of calcsilicate rock occurs in Glen Beanie [NO 1803 6653], and a calcareous schist occurs adjacent to a large amphibolite sheet near Knockali [NO 15 58].

Cairn Gibbs Psammite Formation

The predominantly psammitic Cairn Gibbs Psammite Formation forms the upper part of the Southern Highland Group in the east of the Glen Shee district. It crops out between the Mount Blair Psammite and Semipelite Formation and the Devonian rocks of the Highland boundary. In the area north-east of Ballintuim [NO 12 56], the predominantly psammitic succession is inter-layered on a scale of hundreds of metres with the more varied stratigraphically undivided succession of the western part of the district. Elsewhere, relationships are obscured by poor exposure in Strathardle.

Large areas of the formation are poorly exposed. Few rock outcrops occur even outwith the extensive areas of drift in the mostly gently undulating ground in the Forest of Alyth. Moderate exposure occurs on the east slopes of Lindalla to the north of Kirkton of Glenisla [NO 210 622], around the Hill of Three Cairns [NO 194 552] and Whin Craigie north of Craighead [NO 197 558]. Locally good exposure occurs in the Alyth Burn [NO 191 544], the Burn of Watershiel [NO 209 552], and the Black Water [NO 145 519] to [NO 143 530], although parts of the latter section are inaccessible in deep gorges.

Psammites and gritty psammites make up 75 to 80 per cent of the formation with the remainder comprising thin semipelite and pelite units. A thick unit of Green Beds, extending south-west from Knockton through Corb occurs in the central part of the formation, stratigraphically above another, albeit thin, Green Bed unit in Glen Isla.

Striped and massive psammites are commonly interbedded. The former contain micaceous laminae which separate planar or lensoid quartz-rich or quartzofeldspathic lithons. Micaceous laminae are spaced 5 to 10 mm apart at the southern margin of the Dalradian outcrop, although more typically 1 to 3 mm apart further to the north where D2 deformation has produced significant attenuation (Chapter 6). Massive and gritty psammites are thick or very thickly bedded. The latter contain mostly subrounded clasts up to 8 mm in diameter. Approximately 75 to 80 per cent of these clasts are composed of grey to blue quartz and the remainder are white to pink feldspar. Rare quartzite units occur west of Loch Shandra [NO 21 62].

Semipelite and pelite units are less than 10 m thick and are rarely exposed outwith stream sections. In the south they are phyllitic and composed of chlorite, biotite, muscovite, quartz, plagioclase and tourmaline. Farther north, they are schistose and additionally contain garnet and locally staurolite (Chapter 7).

In thin sections of the gritty rocks, both perthite and plagioclase clasts mostly show patchy extinction with numerous included quartz blebs. A specimen from the south slopes of Hill of Fernyhirst [NO 2115 5556], 200 m from the southern edge of the Dalradian outcrop (S95767), contains plagioclase, K-feldspar (including microcline) and perthite feldspar clasts together with quartz within a quartz, plagioclase and microcline groundmass. Micaceous laminae are composed largely of muscovite and opaque minerals together with biotite, quartz, feldspar and tourmaline. Phyllites are composed essentially of muscovite, chlorite, opaque minerals, quartz and plagioclase together with tourmaline. Chlorite porphyroblasts are only recorded from Whin Craigie [NO 1971 5573]. In the area north of Corb, semipelites are schistose and composed of quartz, plagioclase, biotite, muscovite, garnet ± staurolite ± tourmaline. Chloritoid (S96412), (S96413) occurs in an approximately 20 m-thick semipelite/pelite unit 1.5 km north-east of Corb [NO 171 582].

Birnam Slate and Grit Formation

This formation comprises interbedded coarse- to very coarse-grained gritty psammite and slate, in contrast to the typical association of schistose psammite and semipelite prevalent elsewhere in the Southern Highland Group. Its stratigraphical position within the Southern Highland Group is unclear since it is only recognised south of the Highland Border Downbend; it is not clear whether its apparent absence north of the downbend is due to lateral facies changes, tectonic thinning, faulting or a change in appearance at higher metamorphic grade. It is not the youngest unit within the Southern Highland Group, since younger rocks occur between the formation and the downbend. The Middleton Muir Fault forms a tectonic base to the formation.

The Birnam Slate and Grit Formation is correlated with the Birnam Slates and Birnam Grits (Harris, 1972) which occur farther to the south-west at Newtyle and on Newtyle Hill, Birnam Hill and Craig Obney in the Perth and Dunkeld district, Sheet 48W, (Harris, 1972). This is despite a change from single discrete slate and gritty psammite units in the Perth and Dunkeld district to an interdigitating succession in the Glen Shee district.

The Birnam Slate and Grit Formation occupies a narrow (approximately 1 km wide) belt of generally low-lying, virtually unexposed ground immediately northwest of the Middleton Muir Fault. The main exposures in disused quarries and river sections are concentrated around Forneth [NO 09 45]. The most important quarries are Baldornoch Quarry [NO 0886 4498] and a system of quarries north of the A923 road around Quarry Hill [NO 092 457]. From here the formation can be traced by sparse outcrops farther to the south-west and by geophysical means towards the north-east. The gritty psammites appear to be associated with low-amplitude, impersistent anomalies on the magnetic anomaly maps while the pelitic and semipelitic rocks of the Southern Highland Group show no response at all. An isolated outcrop of green, phyllitic slate north of Lornty Burn [NO 1196 4831] provides the only exposure constraint north-east of Forneth. Towards the north-east the formation is progressively truncated by the Middleton Muir Fault.

Gritty psammites (S95812) are well exposed in a 150 m-long section in the Lunan Burn at Forneth [NO 0928 4525] to [NO 0940 4521]. They are pale green or blue-green with about 80 per cent quartz clasts and 20 per cent feldspar clasts in a muscovite-chlorite dominated matrix. Beds of matrix-supported gritty psammite, 100 mm to 1 m thick, with a grain size of 1 to 5 mm, alternate with beds of poorly sorted, coarse-grained (1 to 8 mm), grain-supported gritty psammite. Rare, thin metasiltstone lenses (20 mm thick) occur between gritty psammite beds. Graded beds are common, and fine towards the north-west in steeply north-west-dipping beds.

The slaty pelites (S95813) are very fine grained with a well-developed slaty cleavage. Dark green, pale-weathering slate and purple-weathering black slate are seen in Baldornoch Quarry. Bedding is distinguished by rare 10 to 40 cm-thick layers of coarser grained, pale, metasandstone as on the north-east slopes of Newtyle Hill [NO 0562 4299] and [NO 0559 4288]. These indicate that bedding is steep, whereas refraction of Sl in graded beds suggests younging to the north. At Baldornoch Quarry, the contact between slate and gritty psammite is essentially gradational, with a transition zone 5 to 10 m wide. This zone contains flaggy matrix-supported gritty psammite with a fine-grained muscovite-chlorite matrix. The amount of quartz and plagioclase clasts increases towards the gritty psammite, thus locally resulting in coarsening-upward sequences.

Units of pale, commonly yellowish green-weathering pelite, 5 to 10 cm thick, occur in a few places [NO 0905 4580], and are apparently intimately associated with pale grey, more quartzose slates with conspicuous chloritoid porphyroblasts (S95814). These consist almost entirely of a mixture of chlorite and white mica with little or no quartz. These may represent a volcanogenic association with the yellowish green beds being metamorphosed bentonitic clays or ash beds.

Green Beds

Heterogeneous units of medium or dark grey to green rocks, characteristically containing hornblende, epidote group minerals and sphene with quartz and feldspar clasts and devoid of muscovite, occur in the lower part of the Southern Highland Group. They have traditionally been referred to as Green Beds, and interpreted to have mafic volcaniclastic protoliths (van de Kamp, 1970).

In the Glen Shee district Green Beds generally produce positive topographical features and are locally well exposed. Two laterally continuous Green Bed units, both with outcrop widths locally in excess of 600 m, can be traced east from Strathardle to the eastern boundary of the district. The northern unit occurs mostly at the boundary between the Mount Blair Psammite and Semipelite and Cairn Gibbs Psammite formations, whereas the southern one occurs within the Cairn Gibbs Psammite Formation (Figure 10). Outcrops on Creag nam Brataichean [NO 11 61] probably represent fold repetition of the northern unit in a synformal keel related to the Mount Blair Anticline (Chapter 6). West of Strathardle, there is not the same degree of lateral continuity, although it is still possible to recognise two main units between Kirkmichael and Pitcarmick [NO 084 565]. The northern unit underlies a large area, as a result of the near horizontal attitude of the beds. It forms a broad ridge that extends from Strathardle to Creag na h-Iolaire [NO 05 57]. North of Kirkmichael the northern Green Beds have a complex outcrop pattern with several spatially discrete bodies of Green Beds, believed to be part of the same unit. The apparent discontinuity of the outcrop is due to the preservation of Green Beds in fold keels, as seen in Gleann Fearnach and on Kindrogan Hill. The southern unit can be traced west from Strathardle to the Capel Hill area [NO 03 51] where it occurs in the core of a major fold closure and is very well exposed on Spurn Hill, Craig Wood and Capel Hill. In areas of poor exposure, the magnetic characteristics of some lithologies allow the Green Beds to be traced readily (Figure 10).

Thinner discontinuous Green Bed units occur elsewhere; those within the Mount Blair Psammite and Semipelite Formation range from upstanding ribs, a few centimetres thick to units that are 10 m or so thick. Green Beds also occur within amphibolites which are spatially associated with the Loch Tay Limestone Formation (Chapter 5). The Craig Lair Hornblendic Gneiss at the type locality [NO 217 697] in the north-east of the district comprises a lithological association similar to the Green Beds, albeit migmatitic.

The boundary between the Green Beds and adjacent metasedimentary rocks is usually transitional, and is commonly marked by an increase in the proportion of biotite in the semipelites, over an interval of a few metres. Dark, biotite pelites, which may contain some hornblende, are commonly observed within the psammites and semipelites along the margins of the northern Green Beds unit.

The Green Beds encompass a significant range of lithologies, with assemblages composed of varying proportions of hornblende, plagioclase and quartz, commonly with biotite, epidote/clinozoisite, garnet, sulphides and chlorite porphyroblasts. This is well displayed at localities such as Corb [NO 164 568] and 2 km north-east of Auchintaple Loch [NO 217 660]. Quartz seams up to 1 cm thick are a prominent feature in many exposures. Layers of calcsilicate rock, mostly less than 5 cm thick, occur in several localities, such as at Corb [NO 1676 5710] and north-east of Auchintaple Loch [NO 2175 6677]. Some of these layers occur adjacent to quartz–carbonate veins suggesting that they are zones of hydrothermal alteration, whereas others are thought to represent primary layers of calcsilicate rock. Small calcsilicate boudins on Craig Loisgte [NO 0646 5316] and north-east of Capel Hill [NO 0564 5284] occur within fine-grained hornblende schists which typically have X:Y:Z aspect ratios of 1:3:20. The most widespread lithologies are fine- to medium-grained medium grey to green hornblende schists. In many areas the hornblende schists contain lenses and layers carrying rounded detrital quartz and tabular feldspar clasts; these commonly have rounded corners, may locally be up to 4 to 5 mm in diameter and may show graded bedding. Quartz is generally much more abundant than feldspar, although the relative proportion of the two minerals varies considerably. Some units contain only scattered large clasts, whereas others contain abundant small clasts within the green hornblendic matrix. With increasing proportion of clasts, the gritty layers grade into gritty semipelites. Quartz clasts in gritty layers may be rodded and up to 30 mm long with X:Z ratios up to 10:1. Rocks transitional between the gritty rocks and psammite and semipelite, containing hornblende and biotite and rare quartzite units, generally less than 1 m thick, are seen in several places, north-east of Auchintaple Loch [NO 2167 6596]. They essentially comprise the same overall assemblage as the more hornblendic rocks, albeit with more quartz. Elsewhere, such as at Blackhall Farm [NO 1434 5618] and south-west of Craighead [NO 2064 6326], thin (3 to 50 cm) psammite ribs occur within fine-grained hornblende schist.

A distinctive hornblende megacrystic Green Bed lithology occurs 500 m south-east of Auchintaple Loch [NO 201 643] and 4 km to the north-east near The Banks [NO 225 680], immediately east of the district. It has not been recognised in the poorly exposed intervening ground. This lithology also crops out in small exposures and loose blocks to the north of the Glen Doll Fault 500 m north-east of Badandun Hill [NO 2150 6849]. It comprises a fine- to medium-grained medium grey groundmass which locally contains smoky brown quartz as at The Banks [NO 2248 6796]. Large, randomly orientated and commonly subhedral to euhedral amphibole porphyroblasts, typically 6 to 7 mm and in places up to 35 mm long as east of Auchintaple Loch [NO 2022 6432], form up to 40 per cent of the rock.

Amphibolite crops out adjacent to this lithology at all localities. The thickness of the units is not known owing to poor exposure, although it is probable that there are several units within an approximately 250 m wide zone; one unit east of Auchintaple Loch [NO 2037 6420] is at least 3 m thick.

North of the Glen Doll Fault in the Mount Blair Psammite and Semipelite Formation, another distinctive unit of Green Beds to the north of Cuingard [NO 1956 6689] is up to 10 m thick. It is prominently striped, with adjacent 1 to 2 cm-thick layers containing contrasting abundances of hornblende and quartz. Adjacent to the Green Bed unit, psammites contain garnet-rich pods ranging in size from less than 1 to 5 cm, some of which contain epidote, quartz, feldspar and possibly hornblende, and thin (mostly less than 1 cm) discontinuous lenses containing coarse-grained hornblende.

On Conlan Hill [NO 04 47], a 7 m-thick unit of green, fine-grained hornblende schist with cross-cutting quartz veins can be traced for several hundred metres within psammite of the stratigraphically unassigned Southern Highland Group. Concordant, schistose amphibolite sheets (up to 20 m thick) occur at a few localities north of Rotmell Loch [NO 02 47]; these have some psammite intercalated at their margins and are, therefore, interpreted as a minor development of Green Beds. A 20 to 30 m-thick, unusually massive, grey, biotitic psammite (grain size 0.5 to 1 mm) occurs across the north-west flank of Benachally [NO 060 495]. Although lacking amphibole, this occurrence may also have some affinities with the volcaniclastic lithologies.

A gritty semipelite (S96426) from west of Corb [NO 1560 5674], derived from a bed approximately 1 m thick, contains approximately 60 per cent clasts in a matrix of biotite, hornblende, epidote, quartz, plagioclase and sphene. The quartz clasts comprise aggregates of grains, whereas the feldspars are generally single crystals. Those that can be readily identified optically are invariably plagioclase; however, many are indeterminate and marked by uneven extinction, so that it is not clear whether they are plagioclase or K-feldspar. Nearly all contain quartz 'blebs'.

In thin section, specimens of the hornblende megacrystic Green Beds (S94358), (S96488), (S94357) typically contain large hornblendes within a groundmass composed of hornblende-plagioclase-quartz-epidote with abundant accessory apatite and opaque minerals or sphene. In (S94357) a 1 cm-thick, zone of alteration is marked by the replacement of this assemblage by inter-growths of clinozoisite, carbonate and chlorite. Schistose semipelite (S95738) is composed of greenish brown biotite, quartz, plagioclase and chlorite in the form of cross-cutting porphyroblasts (predating D3), garnet, opaque minerals, fine-grained hornblende and rare staurolite crystals. Calcsilicate rocks (S95744), (S96418) are composed largely of quartz and epidote or clinozoisite with some carbonate, hornblende and garnet. Thin upstanding ribs in the Mount Blair Psammite and Semipelite Formation (S94555), comprise the assemblage hornblende-quartz-garnet-clinozoisite with some plagioclase, sphene and opaque minerals; muscovite is not present. The adjacent semipelite is typically fairly mafic and devoid of muscovite but with abundant garnet. They are interpreted as thin Green Bed units.

Craig Lair Hornblendic Gneiss

A well-exposed heterogeneous succession of quartz-rich, biotite-rich and hornblende-rich schists and gneisses, informally referred to as the Craig Lair Hornblendic Gneiss, underlies the high ground between Mid Hill [NO 221 710] and Craig Lair [NO 216 696] in the north-east part of the district. The gneiss is bounded to the north-west by the Duchray Hill Gneiss Member and to the south-east by intrusive amphibolite. The first boundary, which is marked by a gap in exposure at least 20 m wide, is interpreted as the Fergus Slide on the basis of the absence of the Loch Tay Limestone Formation in this area and the truncation of both the hornblendic gneiss and the overlying amphibolites towards the south-west. The other boundary is thought to be an intrusive contact. The stratigraphical position of the hornblendic gneiss is uncertain, as the overlying amphibolites are thought to be equivalent to amphibolites which are intrusive into the Loch Tay Limestone Formation south-east of the Glen Doll Fault in the Auchintaple Loch area (Chapter 5). However, lithologically they are akin to the Southern Highland Group Green Beds, albeit at a higher metamorphic grade. The Craig Lair Hornblendic Gneiss may therefore be either part of the Tayvallich Subgroup or the Southern Highland Group.

The absence of muscovite and presence of hornblende and sphene-bearing lithologies in the hornblendic gneiss serve to distinguish the Craig Lair Hornblendic Gneiss from the Duchray Hill Gneiss Member. The most abundant lithology is a leuco- to mesocratic quartzplagioclase-hornblende-biotite gneiss, although the spectrum of lithologies encompasses leucocratic quartzplagioclase-biotite psammite and mafic hornblende-rich amphibolite. They range from fine- to medium-grained massive rocks, especially in the Tarmach Cairn area, to coarse-grained and prominently migmatitic lithologies (Plate 10) with the more leucocratic lithologies typically coarser grained than the more mafic ones. In places, wholescale reconstitution and 'granitisation' of the rock has occurred, similar to that observed in the Duchray Hill Gneiss Member. Coarse biotite plates, up to 5 mm in size, are a feature of the melanosome in places, for example on Mid Hill [NO 2200 7030]. The granitoid lithologies contain mafic (biotite+hornblende?) schlieren and laminae and in places, as on Mid Hill [NO 2207 7060], coarse (1 cm or more), tabular feldspars and small garnets. Coarse granitoid segregations, up to 1 m in size, cross-cut the fabric on Mid Hill [NO 2247 7124] (Plate 10). These may relate to local cross-cutting granitoid and pegmatite veins and sheets up to approximately 1 m thick and feldspathic pegmatite veins. North of Craig Lair [NO 2200 7030] where pegmatite is seen to both cross-cut and interfinger with biotite gneiss, there is evidence of multiphase segregation/intrusion with a quartzofeldspathic pegmatite vein cross-cutting a biotitebearing granitoid.

Discontinuous units of psammite, mostly 5 to 50 cm and rarely up to a few metres thick, are typically grey and fine to medium grained, with locally developed discontinuous mafic laminae 1 to 2 cm apart.

Amphibolite/hornblende schist units composed predominantly of hornblende and plagioclase (as opposed to the hornblendic gneisses) are not abundant. However, mafic, or indeed very mafic, hornblendic units, up to 25 cm or so thick, occur to the south-west of Craig Lair [NO 2140 6940]; [NO 2145 6938]. At the latter locality, the mafic hornblendic rocks are in sharp contact with psammite, whereas the former locality reveals a transition between mesocratic quartz-plagioclase-biotite gneiss and amphibolite.

Chapter 5 Precambrian to Ordovician: pre- and synorogenic igneous rocks

Granitic and basic igneous rocks were emplaced both prior to D2, probably in the late Proterozoic, and after D2, in the Ordovician. Since the structural age of some of these igneous rocks cannot be constrained, they are grouped together as pre- and synorogenic igneous rocks and described in one section. Where relationships clearly indicate the structural age, this is detailed in the text.

Granites and pegmatites

Metamorphosed granitic rocks include the north-eastern part of the Ben Vuirich Granite, which occurs in the extreme north-west of the district, and pegmatite and granite sheets scattered across the northern part of the district. Many pegmatite and granite sheets are foliated, although unfoliated sheets which share characteristic features of the suite are described in this section.

Ben Vuirich Granite

The Ben Vuirich Granite (Figure 1), (Figure 20) was first distinguished as an older foliated granite by Barrow et al. (1905). For many years a U–Pb zircon age of 514 ± 7 Ma from the intrusion (Pankhurst and Pidgeon, 1976) provided a benchmark in the evolution of the Scottish Caledonides. However, the age of the granite was revised to 590 ± 2 Ma in the light of a more precise U–Pb zircon technique (Rogers et al., 1989), and recently confirmed by a SHRIMP ion microprobe zircon age of 597 ± 11 Ma (Pidgeon and Compston, 1992). Bradbury et al. (1976) and subsequently Rogers et al. (1989) considered that the granite was emplaced between regional D2 and D3 events. However, the results of the present study, together with the work of Tanner and Leslie (1994), demonstrate that the granite was emplaced before D2. Further work by Tanner (1996) suggested that the granite was intruded prior to D1. The Tayvallich mafic volcanic rocks are coeval (595 ± Ma, Halliday et al., 1989); the Ben Vuirich Granite may thus be part of a bimodal rifting event.

The granite is a sheet-like intrusion which has been substantially modified by deformation. Although it is typically poorly exposed, exceptions are crags on the eastern flank of Carn Dubh [NO 00 69] and approximately 1.5 km of the eastern contact below Carn Dubh [between 0079 6859] and [NO 0120 6995]. The contact forms a break of slope, with foliated, pinkish grey granite producing steep slopes standing above micaceous quartzite and minor semipelite of the Tulaichean Schist Formation. There is no evidence of either a chilled marginal facies to the granite or of a contact metamorphic aureole at this locality. The northern margin of the intrusion around [NN 99 70] can be located with reasonable accuracy, also at a prominent break in slope below outcrop of foliated, pinkish grey granite. There is only very poorly preserved evidence of contact metamorphism, although continuity of the original aureole has been greatly disrupted, possibly as a direct result of the D2 deformation. Tanner and Leslie (1994) and Tanner (1996) reported distinctive spotted slates of the Tulaichean Schist Formation immediately to the south-west [NN 992 703].

The granite is generally coarse grained (2 to 4 mm grain size) and granoblastic apart from K-feldspar megacrysts up to 5 to 6 mm across. It contains microcline, quartz, biotite, plagioclase, conspicuous regional metamorphic garnet and minor white mica with accessory zircon, sphene and apatite (S95893), (S95894), (S95895), (S95896). It is variably deformed with large areas showing a prominent S or L–S fabric, which is defined by biotite and white mica, and is co-planar with S2 in the host rocks. The mica fabric encloses elongate microcline porphyroclasts which were derived from the megacrysts and lenticular aggregates of quartz. This has resulted in asymmetric augen that are coaxial with a rodding lineation. Garnet porphyroblasts are common throughout and are usually inter-grown with biotite in S2.

The north-east contact of the granite is very irregular in detail with both apophyses of granite and the enveloping micaceous quartzites carrying an intense S2 foliation. S2 in the quartzites is defined by a preferred orientation of biotite and elongate aggregates of quartz crystals, with contemporaneous garnet growth. A few metres outside the granite contact [NO 0116 6919], granite apophyses crosscut an anastomosing spaced fabric which appears morphologically indistinguishable from S1 in Southern Highland Group rocks in the region of the Highland boundary (Chapter 6). S2 transects the granite contact [NO 0118 6970] and is the main penetrative fabric throughout the granite; it is nearly perpendicular to the boundary of the northern part of the granite. The contact relationships, therefore, demonstrate that the granite was emplaced before D2.

Abundant xenoliths of locally derived, micaceous quartzite occur in an approximately 50 m-wide zone adjacent to the contact. These are angular, centimetre to decimetre scale and mostly elongate parallel to an internal foliation which varies markedly in orientation from xenolith to xenolith. This pre-intrusion foliation in the xenoliths (Bradbury et al., 1976) is truncated by the enclosing granite. The foliation is defined by biotite-rich laminae a few millimetres to a centimetre apart, in which the micas show imperfect parallel orientation, with local concentrations of allanite, sphene, zircon and opaque grains. In some xenoliths the foliation resembles S1 preserved within D2 microlithons in the country rock quartzite, whereas in others it appears to have a sedimentary origin. This foliation is overprinted by a straight penetrative fabric which is interpreted as S2; it is defined by minute biotite flakes that are continuous with the fabric in the enclosing granite. No pre-intrusion folds have been seen in any xenoliths (Bradbury et al., 1976) but a number of poorly defined flexures of fabric in the quartzite along the margin of the granite [NO 0117 6977] are cut by granite apophyses. Evidence from the xenoliths, therefore, also demonstrates that the granite is pre-D2 and possibly pre-D1.

The micaceous quartzites which envelop much of the granite in the district show no discernible effects of contact metamorphism, even where they have become incorporated within the granite as xenoliths. Semipelites and pelites immediately adjacent to the north-west contact around [NN 997 708] characteristically show a blocky aspect despite clearly carrying an S2 fabric crenulated in D3 [NN 9950 7125]. The groundmass is more coarsely granoblastic compared with the spaced fabric and grain shape alignment normally seen in these lithologies. Garnet has grown on spherical aggregates of quartz, feldspar and mica which possibly pseudomorph original cordierite as indicated by microscopic yellowish (possibly pinitic) intergranular alteration. Pale muscovitic patches, which are possibly pseudomorphs after andalusite, have also been noted.

Pegmatite and granite sheets

Intensely foliated granite sheets, a few centimetres thick, occur in micaceous quartzite immediately north of the Ben Vuirich Granite [NO 0003 7178]. They are lithologically similar to the most foliated variants of the granite. West of Glen Taitneach [NO 0788 7288], a sheet-like body of foliated granite, approximately 20 m thick, is exposed within rocks of the Tulaichean Schist Formation. It is pale and fleshy pink with relict orthoclase, minor plagioclase and lenticular patches of quartz enclosed in a fine-grained groundmass of quartz and feldspar. The foliation is defined by trains of chloritised biotite approximately 1 mm apart, which are concordant with S2 in the surrounding schists. The contacts are not exposed.

Loose blocks of weakly foliated, medium-grained, biotite-muscovite granite occur within an approximately 100 m-wide north-east-trending zone to the south-west of Badandun Hill [NO 2014 6751]. This granite is somewhat porphyritic with feldspars up to 7 mm in size. Relationships with the host Mount Blair Psammite and Semipelite Formation and the regional S2 fabric are not seen. In thin section (S94351), the granite is composed of plagioclase, microcline, quartz, muscovite and biotite. Many of the feldspars form large (up to 4 mm) grains, which probably reflect the primary igneous texture. Plagioclase and rarely muscovite and biotite are included within microcline. Muscovite and biotite occur both as large plates and as small interstitial grains. Quartz occurs as recrystallised aggregates which are generally interstitial to the feldspars. Many feldspars are wrapped by zones of grain-size reduction; both small seams and larger plates of muscovite occur within these zones.

Foliated pegmatitic granite sheets and lenticular pods are preferentially hosted by the lower part of the Southern Highland Group and the Loch Tay Limestone Formation. The majority of pegmatites are concentrated in three main areas (Figure 22): lower Gleann Fearnach [NO 04 64], the Allt Menach [NO 09 61] and Mount Blair [NO 17 63]. Most of the pegmatite sheets have been intruded subconcordant with bedding and S2 but do not carry an S2 foliation; they either transect both the limbs and the axial plane of F2 folds [NO 0577 6568], or they have intruded preferentially along the well-foliated long limbs of large-scale F2 folds. This latter situation is well displayed on Creag Dubh-leitir [NO 05 65], where the major pegmatite sheets are hosted by gently inclined, strongly S2-foliated schistose semipelites that form the long limbs of west-verging F2 folds. A post-D2 age is thus inferred for the majority of the pegmatite sheets.

In lower Gleann Fearnach, numerous sheets and pods of coarse pegmatite, some exceeding 20 m in thickness, are conspicuous in an approximately 200 m-wide zone that extends from north of Straloch [NO 045 645] to the Allt Fearnach [NO 048 665]. A 500 m-wide zone with a similar concentration of pegmatites extends across the western crags and summit of Creag Dubh-leitir, thence southeastwards along the western side of Calamanach [NO 06 64] towards Dirnanean [NO 065 636]. S2 foliation is developed in a few thin pegmatite sheets; in one example in Glen Fearnach [NO 0375 6523] a 5 cm-thick sheet has been deformed in a west-verging F2 fold and has developed S2 axial planar foliation. Such pegmatite occurrences may be related to the Ben Vuirich Granite.

In the Allt Menach [NO 09 61], foliated pegmatites have intruded rocks of the Loch Tay Limestone Formation; here five pegmatite sheets up to 10 m thick crop out sub-parallel to bedding. Some extremely coarse-grained pegmatites, that form caps on the hilltops west of the Allt Menach [NO 089 611]; [NO 092 606], are hosted by Southern Highland Group rocks, and are also included in this grouping. These sheets lie south-eastwards along strike of (and probably represent the continuation of) the lower Gleann Fearnach pegmatite concentration.

Numerous pegmatitic granite sheets containing pinhead-size garnets also crop out on Mount Blair [NO 174 633], where they have intruded rocks of the Mount Blair Psammite and Semipelite Formation. They have a layering developed parallel to their margins, and concordant with S2. Another concentration extends between Cnoc an Daimh [NO 102 630] and Creag nam Brataichean [NO 112 612], where a flat-lying, 10 to 15 m-thick sheet of pegmatite forms a circular outcrop pattern around the hilltop. Tourmaline occurs in the pegmatites of this group, commonly at the contact with the country rock, [NO 1171 6181].

Most of the pegmatite bodies are composed of coarse-grained quartz, alkali feldspar and sodic plagioclase (< An28) crystals (S95892); many of these now occur as recrystallised monominerallic grain aggregates (5 to 15 cm across). The grain size of individual quartz and feldspar polyhedra within these aggregates typically ranges from 3 to 30 mm. Most of the pegmatite sheets contain small amounts of muscovite (generally less than 5 per cent of the mode) and many contain accessory proportions of garnet.

A non-uniform foliation is developed in most of the Mount Blair pegmatite sheets. Strain is generally partitioned in the thick, coarse-grained bodies, as indicated by a foliation defined by narrow, centimetre-scale anastomosing high-strain zones, enclosing lenticular low-strain domains. The high-strain domains have a reduced mean grain size of 2 to 3 mm; they comprise granoblastic, polygonised mosaics of quartz and feldspar in association with aligned flakes and sheaves of muscovite; pinhead-size, idiomorphic garnets are sporadically distributed. In the lenticular low strain domains, the primary coarse-grained quartz and feldspar show varied degrees of strain and polygonisation; microcline braid-perthites and chessboard albites are sometimes developed, whereas muscovite occurs as radiate sheaves and unfoliated coarse aggregates that may exceed 5 mm across. Many of the smaller bodies of pegmatite (generally those that are less than 1 m thick) are more homogeneously foliated, with a penetrative foliation developed parallel or sub-parallel to the sheet margins. The age of the foliation is unknown.

Several pegmatite sheets display well-developed pinch and swell structure as on Creag Dubh-Leitir ((Plate 11) [NO 0550 6524] ) or boudinage as on Cnoc an Daimh [NO 1012 6243]. Domino-style block rotation of disrupted pegmatite bodies has also been noted on Creag Dubh-Leitir [NO 0571 6508]). Each of these examples indicates the high viscosity of the pegmatite relative to its semipelitic host during ductile extension. The age of this deformational episode cannot be constrained tightly. An association with D3 is tentatively suggested for two reasons: because the patterns of ductile extension, vergence of accommodation structures and block rotations of the disrupted pegmatites are congruent with D3 fold geometries, and because the occurrence of garnet in foliated domains within the pegmatites is consistent with D3 metamorphic conditions.

Discordant muscovite ± garnet-bearing pegmatite sheets crop out on the south slopes of Cuingard [NO 1945 6664]. They are up to 2 m thick with muscovite-books up to 1 cm across. Scattered pegmatite sheets also occur within the metamafic rocks to the south-west of Auchintaple Loch [NO 1943 6462], where a 15 cm-thick, sharp-margined, parallel-sided sheet is subconcordant and steeply dipping. Abundant blocks of muscovite pegmatite indicate that pegmatite sheets are also widespread to the north-east of Auchintaple Loch.

Pegmatite is abundant at the eastern margin of the district, 1.5 km east of Badandun Hill [NO 223 678]. The distribution of exposures and loose blocks suggests that the pegmatite forms approximately 5 m-thick sheets dipping gently west.

Abundant blocks and scree of fine-grained, leucocratic, aplitic microgranite with a saccharoidal texture, occur to the north-west of Bodnasparet [NO 2214 6969]. The microgranite, which is not seen in situ, contains quartz phenocrysts up to 1 mm in diameter together with sporadic pegmatite segregations up to a few centimetres across.

Metamafic rocks

The Dalradian metasedimentary and metavolcanic rocks contain mostly concordant, sheet-like bodies of metamafic rock. These range from 1 m to over 200 m thick and can be traced for up to several kilometres along strike. Two suites have been distinguished, although no cross-cutting relationships have been observed. Members of the older suite predate D2 and have S2 as their dominant metamorphic fabric whereas members of the younger suite are significantly less deformed and are thought to carry only post-D2 fabrics. These younger metamafic rocks may occupy a similar position in the relative chronology of Dalradian events to the Younger Basic suite of north-east Scotland (Fettes, 1970). It is not possible to assign all metamafic rock bodies to either 'older' or 'younger' suites owing to inadequate exposure of critical contact relationships.

Pre-D2 metamafic rocks

At outcrop, metamafic rocks of the older suite have a massive aspect and range from foliated, coarse-grained amphibolites to fine-grained hornblende schists. They have moderate magnetic susceptibilities and the extent of some bodies has been traced with the aid of the ground magnetic survey. Some are derived from intrusive protoliths as indicated by both discordance and the presence of xenoliths, whereas some fine-grained, hornblende schists may have volcanic protoliths, based on their present close interleaving with metasedimentary rocks. Good examples of the latter type are found in the Ben Lawers Schist Formation, Farragon Volcanic Formation and Green Beds, particularly on Creag Uisge [NO 022 695] and in Gleann Fearnach [NO 012 734]. A significant number of metamafic rocks have undefined protoliths.

Many coarse-grained amphibolites have finer grained, more highly deformed, schistose margins. S2 foliation and L2 lineation within the coarse-grained domains are defined by lensoid aggregates of amphibole and plagioclase that typically range between 5 and 10 mm in diameter and 1 and 10 cm in length. The distribution pattern of these aggregates reflects relict igneous (gabbroic) texture, which is best viewed on surfaces normal to L2.

The greatest development of pre-D2 amphibolites is hosted by the upper parts of the Argyll Group and lower parts of the Southern Highland Group, although small sheet-like bodies are widely distributed throughout the Dalradian of the district. The largest body of amphibolite forms a sill complex intrusive within the Loch Tay Limestone Formation; its irregular, arcuate outcrop attains a width of several hundred metres and can be traced for over 10 km from near Auchintaple Loch [NO 19 65] south-westwards to Wester Bleaton [NO 11 59].

The southern portion of the sill complex between Whitehouse [NO 15 60] and An Dun [NO 11 59] transgresses the boundary between the Loch Tay Limestone Formation and semipelites of the Southern Highland Group. Similar relationships occur near Whitefield Hill [NO 08 62]. The contact between metacarbonate rocks and amphibolite is well displayed on Nether Craig [NO 169 611]. In the Allt Menach [NO 09 62], an amphibolite unit approximately 50 m thick is in direct contact with the Loch Tay Limestone Formation, but on Whitefield Hill the amphibolite lies within a complex F3 fold closure that is enclosed within Southern Highland Group rocks. This amphibolite can be traced southwards via Mains of Dounie [NO 091 588], and thence eastwards beyond Knockali [NO 15 58]; it is hosted entirely by the Southern Highland Group.

A small occurrence is also associated with the Loch Tay Limestone Formation at Fergus [NO 19 67] and the Craig Lair Hornblendic Gneiss in the extreme north-east of the district. The sill complex generally has a positive topographical expression and forms prominent crags, including Cairn Craig [NO 178 642] and Creag a Phris [NO 181 638]. In the Fergus area several exposures display fine-scale (less than 1 mm) leucocratic–mafic layering resulting from contrasting abundance of quartz + plagioclase and hornblende.

Feldspar-rich aggregates with or without quartz and up to 4 mm across are also present in places. In the Auchintaple Loch area, metamafic rocks are generally well exposed and result in the prominent upstanding strike-parallel ridge in the area of The Knaps [NO 194 653] as well as roches moutonnées immediately south of Auchintaple Loch [NO 197 644]. They range from foliated, medium-grained amphibolite with numerous small garnets, particularly north-west of Auchintaple Loch, to massive medium- to coarse-grained amphibolite to the south-east of Auchintaple Loch. Quartz-filled tension gashes occur in the massive amphibolite at the latter locality and 10 cm lenses of pegmatite and hornblende-bearing pegmatite occur near Fergus [NO 1970 6785].

Amphibolites crop out to the south-east of the Craig Lair Hornblendic Gneiss to the east of Glen Isla. They are well exposed in the Sluggan area [NO 216 689]. In the Bodnasparet area [NO 222 695], they have an outcrop width of 1 km and are bounded to the south-east by the Glen Doll Fault. To the north-west of Badandun Hill, the unit is truncated to the west by the Fergus Slide. The amphibolite is lithologically very similar to that emplaced into the Loch Tay Limestone Formation with the exception of the local development of a gneissose banding with 1 to 2 mm-thick leucocratic and mafic bands. However, in the Sluggan area, somewhat coarser grained discordant quartzofeldspathic leucosome lenses up to 2 cm wide occur together with the fine concordant gneissose banding. In places the leucosome occurs within small shear zones as between Badandun Hill and Craig Lair [NO 2135 6880]. One centimetre-thick discordant pegmatite vein is recorded on Craig Lair [NO 2174 6912].

Subconcordant sheets of coarse-grained amphibolite also crop out within the Mount Blair Psammite and Semipelite Formation to the north of the Loch Tay Limestone Formation. They form conspicuous hills and ridges on Craigies [NO 122 623] and to the south-east around [NO 13 62]. One unit of an en échelon array of amphibolites which occurs around Carn an Fhidhleir [NO 17 65] and Presnerb [NO 18 66] intrudes the Duchray Hill Gneiss Member on Creagan Caise [NO 18 68].

West of Lower Gleann Fearnach [NO 024 670], concordant units of schistose metamafic rock and interlayered psammites with fine-grained garnetiferous hornblende schist occur within the Loch Tay Limestone Formation; these probably have a volcanic protolith. However, the largest sheets of metamafic rock in this area have coarse metagabbroic textures and intrude semipelites of the Southern Highland Group, notably on Druim Cul [NO 03 68], and on Sgurr Gael [NO 0448 6653] where relict igneous layering is preserved in an approximately 50 m-thick coarse, schistose metamafic rock unit that extends southwards to Straloch [NO 0445 6435]. At least six approximately 20 m-thick elongate pods of coarse-grained amphibolite occur stratigraphically below the Loch Tay Limestone Formation on the northern flank of Creag Uisge [NO 019 695]; these may represent D2 boudinage of a single intrusive body emplaced into the lower part of the Farragon Volcanic Formation. Examples of folded and pod-like coarse-grained amphibolite masses also outcrop within the Ben Lawers Schist Formation, notably the folded bodies at Creag Dhubh [NO 03 71], on the prominent spur of Sron Breacach [NO 05 70] and the lensoid masses near Craig of Runavey [NO 13 69]. In this last example, the margins of the amphibolite units are locally discordant to foliation in the enclosing schists. All of these bodies were probably part of more continuous sheets prior to D2 folding and boudinage.

Other occurrences of pre-D2 metamafic rocks include the small outlying, S2-foliated, coarse-grained metagabbroic bodies that occupy low ground south-east of Meall Dubh [NO 07 53] and occur within the hinge zone of the F1 Capel Hill syncline near Creag Loisgte [NO 0697 5287]. Flat-lying, well-foliated sheets of medium- to coarse-grained garnet amphibolite form transgressive sills within the Southern Highland Group psammites and semipelites on Meall Reamhar [NO 03 56]. Numerous amphibolite sheets have intruded the Tulaichean Schist Formation, as on Carn an t-Sionnaich [NO 01 75] and Creag Chreumh [NO 066 720]; these are seldom more than a few metres thick and can be traced for a few tens of metres. Locally, these bodies are discordant to bedding, but tend to have a margin-parallel foliation that is parallel to S2 in the host rock schists.

Discordant relationships between layering in gneissose semipelite and metamafic rocks are seen in loose blocks near Fergus [NO 1950 6764] whereas an inclusion of schistose semipelite, 4 by 1 m in size with alternate tourmaline-rich and garnet-rich layers, within amphibolite to the south-west of Auchintaple Loch [NO 1937 6418], is thought to be a xenolith.

Many exposures of metamafic rocks in the area southeast of Fergus contain lenses or ribs of pink calcsilicate rock interpreted as modified xenoliths of the Loch Tay Limestone Formation. Lenses are typically 5 to 10 mm across and ribs are up to a few centimetres thick; locally they ' contain abundant garnet, as near Fergus [NO 1951 6749], together with lenses of pale pyrite up to 10 by 2 mm in size. The calcsilicate lenses weather preferentially producing 'knobbly' and/or ribbed outcrop surfaces which locally contain cavities with brown-weathering surfaces. Similar rocks crop out 1 km east of Auchintaple Loch [NO 2089 6493] close to the south-east edge of the metamafic rocks in this area; loose blocks with a similar surface weathering occur at several points up to 380 m to the north-north-west of this locality.

In the Fergus and Auchintaple Loch areas, small exposures of homogeneous fine- to medium-grained psammite, micaceous psammite and mesocratic to melanocratic biotite schist with thin (5 cm) more quartzose ribs occur within the metamafic rocks. Mesocratic hornblende-biotite schists also occur within amphibolite immediately north of Bridge of Forter [NO 1871 6498]. Metamafic rocks also locally contain quartz and feldspar clasts. Close to the southern edge of the metamafic rocks in the Auchintaple Loch area [NO 2022 6452], mafic medium-grained amphibolite is in sharp contact with more leucocratic amphibolite which contains ovoid quartz and large feldspar clasts, mostly 2 mm in size, but locally up to 5 mm. Garnet is more abundant in the latter amphibolite.

In thin section (S94332), (S94339), (S95728), (S92775), (S92782), (S92783) the amphibolites are composed of hornblende, plagioclase and quartz, locally with garnet and with aggregates of sphene, some of which enclose ilmenite. Local aggregates of both hornblende and plagioclase with quartz may represent strongly recrystallised relics of ophitic texture. A few biotites cross-cut a schistosity defined by the preferred orientation of hornblende.

The xenoliths of metasedimentary rock south-west of Auchintaple Loch (S94549), (S94550) contain quartz, plagioclase, biotite, muscovite, tourmaline, chlorite and garnet with layers of quartz, plagioclase and garnet rock. Biotite forms porphyroblasts and tourmaline locally comprises 25 per cent of the rock. Garnet is more abundant in the amphibolite close to the contact with the semipelite. The calcsilicate rocks south-east of Fergus (S94333) show pink skeletal to poikiloblastic garnet which is commonly enclosed by pale green pleochroic pyroxene Clinozoisite is also abundant with rather less abundant hornblende, carbonate, quartz and plagioclase. Sphene, opaque minerals and microcline are minor phases. Micaceous psammite xenoliths in the Auchintaple Loch area (S94348) are composed of quartz with plagioclase and biotite and a little muscovite. Aggregates of quartz and rare rounded perthites up to 2 mm across are thought to be relict detrital grains indicative of a gritty protolith. Melanocratic biotite schist (S94345) is composed of biotite and hornblende, which make up 60 to 70 per cent of the rock with the remainder compsed of quartz, plagioclase and opaque minerals.

Age constraints

Whereas the presence of S2 foliation provides definitive evidence of the pre-D2 age of some amphibolite sheets, there is no direct evidence of their relationship with Si. Elsewhere in the Dalradian, metabasic sheets were intruded prior to D1 (Mendum, 1987; Graham and Borradaile, 1984). The major amphibolite sheets in the Glen Shee district are concentrated within a relatively restricted stratigraphical interval and they do not appear to transgress stratigraphy across major F1 closures. These two factors offer some circumstantial support for a pre-D1 age for intrusion. The small outlying outcrops of amphibolite within the hinge zone of the F1 Capel Hill synform add more weight to this interpretation. Those amphibolite sheets within or below the Tayvallich Subgroup may be related to the Tayvallich volcanic rocks, which are dated at 595 ± 4 Ma (Halliday et al., 1989). However, those sheets that occur in the Southern Highland Group must be younger.

Post-D2 metamafic rocks

The apparently younger suite of metamafic rocks is composed mainly of massive, coarse-grained amphibolites, intruded as concordant or subconcordant sheets. Generally, they are only weakly foliated with the exception of marginal zones and narrow, anastomosing zones of high strain which separate lensoid, low-strain domains as seen near Loch Granach [NO 057 681] and An Lairig [NO 0882 6831]. This strain is attributed to D3 on the basis of both F3 folding of amphibolite outcrop patterns as seen near the Spittal of Glenshee [NO 10 69] and Creag an Dubh Shluich [NO 08 68] and D3 deformation of sheet margins and internal compositional banding. There is no evidence of a tectonic foliation predating the folds.

In some bodies, notably that north-east of Creag Bhreac [NO 088 684], igneous texture is well preserved. The rocks contain fresh, coarse prisms of tremolite and labradorite laths, which commonly exceed 5 mm in length; the amphiboles are zoned and have darker actinolitic rims about tremolitic cores. The proportion of amphibole and plagioclase varies; both constituents may modally comprise 45 to 55 per cent, and centimetre-scale compositional layering is developed at several localities. This layering is generally developed parallel to the margins of the amphibolite as at Allt Daire nan Eun [NO 083 676] and [NO 084 689], but, in the sheet that forms the crags at the summit of Elrig [NO 076 669], the compositional banding is at a high angle to the margins of the unit.

If the tremolite-labradorite assemblages represent original igneous textures, their presence suggest that H2O and CO2 activities were high during crystallisation of this younger suite of basic magmas, with resultant loss of pyroxene. Subsequent metamorphism (during D3) caused the replacement of tremolite by aggregates of fine-grained actinolite, chlorite ± biotite; plagioclase was polygonised and partially replaced by clinozoisite and epidote. Lensoid aggregates of coarse-grained calcite are common in the more highly D3-strained domains and may exceed 30 per cent of the mode. Irregular sphenes are also common and may exceed 5 per cent. Opaque minerals occur in accessory proportions, and are commonly associated with sphene. Some amphibolites are extensively chloritised and carbonated such that they are now amphibole free [NO 081 690].

Most of the coarse-grained amphibolites within the Meall Uaine [NO 11 67] to Ben Earb [NO 07 69] area have the weakly foliated, chloritised and carbonated characteristics of the post-D2 suite. Other coarse amphibolites, which crop out within the Ben Lawers Schist Formation near Lairig Charnach [NO 06 70] and on Sron Charnach [NO 06 69], may represent along-strike equivalents of this latter group. Their location, either along or close to the Ben Earb Slide, is thought to indicate control by this D2 structure upon the emplacement of these younger amphibolites.

In many areas, well-developed S2 foliation is associated with transposition of stratigraphical boundaries, so that So and S2 have subparallel attitudes; this situation gives the impression of a stratigraphical control upon the geometry and location of many of the amphibolite bodies that have been tentatively assigned to the post-D2 suite. A large subconcordant intrusive sheet, at least 50 m thick, of weakly foliated, coarse-grained amphibolite extends from Creag Mholach [NO 07 57] to Creag na h-Iolaire [NO 05 67] and lies entirely within the Green Beds outcrop. Another post-D2 amphibolite body, which contains large porphyroblasts of epidote set in a chloritic matrix, has intruded along the contact between the Farragon Volcanic Formation and the Ben Lawers Schist Formation north of Meall Odhar [NO 11 66]. However, the major amphibolite unit that separates the Ben Lawers and Ben Eagach schist formations in Glen Shee [NO 09 70], displays intrusive, cross-cutting relationships on Creag Beag [NO 098 700], where an irregular apophysis with massive, meta-igneous texture cuts S2 foliated rocks of the Ben Lawers Schist Formation.

Differentiation of the two suites of amphibolites is difficult within the Southern Highland Group. North of Cnoc Eirionnaich, the lower margin of a coarse-grained amphibolite sheet is well exposed, and cuts probable S2 in schistose semipelite [NO 1143 5798]. Grain-size reduction within the arnphibolite, and loss of schistosity within the host metasedimentary rocks probably indicate respective chilling and hornfelsing. Clearer evidence of post-D2 amphibolites is provided by a suite of west–east-trending, subconcordant sheets near Craighead. These bodies preserve relict gabbroic textures with margin parallel foliation most intense in finer grained contact zones. At two localities north of Craighead [NO 1289 5531] and [NO 1238 5554], the amphibolite sheets branch at metre-scale F2 closures defined in massive psammite units, but do not themselves appear to be folded by F2.

Age constraints

Clear evidence demonstrating the age relationships of the post-D2 amphibolite suite is not common. Contacts between amphibolites and their hosts are only rarely exposed; many of the amphibolite sheets have a foliation parallel to their margins, and subconcordant with both stratigraphical boundaries and S2 foliation of the country rocks.

Reliable designation of the amphibolites into pre-D2 and post-D2 categories is thus not always possible, but the following criteria summarise the main differences and may assist in their discrimination. Firstly, the post-D2 amphibolites tend to be less deformed and have better preservation of their original igneous mineralogy and fabrics, as noted above. It is thus considered most likely that the olivine (of igneous provenance) that survives in an ultrabasic pod within amphibolite south-east of Creagan Caise [NO 1818 6793] postdates the pervasive D2 event. Secondly, whereas F3 folding has clearly affected the post-D2 amphibolites, these bodies show no evidence of S2 foliation, as noted above. Thirdly, none of the post-D2 amphibolites contains garnet. The absence of garnet could reflect a bulk compositional control, but probably indicates that amphibolite emplacement postdated D2 and peak metamorphic conditions. Fourthly, apparently only post-D2 amphibolites contain tremolite and actinolite.

Thus it is tentatively concluded that amphibolites were emplaced within Dalradian rocks in the Glen Shee district both prior to, and postdating the D2 event and peak metamorphic conditions. The post-D2, pre-D3 amphibolites occupy a position within the structural chronology equivalent to that of the Younger Basic Suite of north-east Scotland (Read, 1919; Fettes, 1970), and a suite of gabbros in Connemara in the Irish Dalradian (Wellings, 1998). Both these suites have been dated in the range of 475 to 456 Ma (Friedrich et al., 1999; Rogers et al., 1995), and provide good constraints for the age of the Grampian orogeny (Soper et al., 1999). The magmas represented in Perthshire appear to have been rich in H2O and CO2 and crystallised with amphibolitic mineralogy, whereas the amphibolites in north-east Scotland are closely associated with flaser gabbros and norites and have clearly recrystallised from pyroxene-bearing igneous rocks (Munro, 1986). However, in this study no pyroxene was observed, even in amphibolites with well-preserved igneous textures.

Chapter 6 Structure of Dalradian rocks

In broad terms, Dalradian formations within the Glen Shee district are disposed in stratigraphical order and are younger farther to the south or south-east. The oldest rocks, assigned to the Appin Group, are exposed on Carn an Righ and Beinn Iutharn Mhor in the north of the district [NO 04 78]. The youngest Dalradian strata occur within the Southern Highland Group outcrop; this group is faulted against the unmetamorphosed Upper Silurian to Lower Devonian strata in the south-east of the district. The Dalradian has gentle or moderate dips to the northwest over most of the district; notable exceptions occur in Gleann Mor where dips are steep to the south or southeast, and in Gleann Fearnach where dips range from being gentle to steep towards the north-east. The prevalent direction of dip is thus opposed to the overall younging direction, implying regional inversion of the Dalradian strata. In detail, the pattern is more complex; large-scale folding has produced some reversals in younging direction and stratigraphical facing, whereas there are several examples of stratigraphical attenuation and excision associated with tectonic slides. On a regional scale, the zone of north-east dips in the area of Gleann Fearnach represents a structural anomaly, not least because it is the site of a prominent (approximately 15 km) offset in the north-east-trending regional alignment of the Loch Tay Limestone Formation (uppermost Argyll Group) outcrop that occurs between Gleann Fearnach [NO 03 68] and Blacklunans [NO 14 60].

The present structural geometry of the Dalradian metasedimentary rocks in the district is a product of the interaction of four main phases of ductile deformation (Table 2), (Figure 11). D1 and D2 structures are widely developed. Most of the Dalradian rocks contain evidence of the D1 event, while all, except for those at the highest structural levels (now exposed close to the Middleton Muir Fault), were deformed in D2. D2 was the most important event in terms of fabric development; the S2 fabric forms the dominant regional schistosity seen in most outcrops, whereas the mineral assemblages that represent the peak of the regional metamorphism were broadly coeval with its development (Chapter 7). S2 typically has gentle or moderate inclination in areas that are least affected by post-D2 deformation; the large-scale D3 and D4 folds that fold S2 are mainly inclined or upright structures with gentle or moderate plunge. It is thus envisaged that S2 was originally gently inclined, and that domains of steeply dipping S2 were developed by reorientation during the D3 and D4 events.

Much of the district lacks extensive continuous exposures; all the large-scale structures have been identified on the basis of the mapped outcrop patterns of lithostratigraphical units and of syntheses of small-scale structural data. Exposures of D2 slides are rare and their exact location is in some cases conjectural. In the majority of cases the recognition of slides is based on evidence of stratigraphical attenuation or omission that cannot be more satisfactorily explained by sedimentary facies variation.

Correlation of structures

Wherever possible, structural elements within the Glen Shee district have been correlated with regional deformation events (Table 3), on the basis both of descriptive similarities and re-examination of previously described sections. There is general agreement in correlating structures across the D4 antiformal Highland Border Downbend (Harris et al., 1976; Harte et al., 1984; Mendum and Fettes, 1985; Robertson, 1994). However, the relationship between the downbend and the D4 event in the Schiehallion area (Treagus, 1987) is not clear. In the Braemar district, Upton (1986) did not recognise a specific D4 event, but stated that the axial plane of his F3 Devil's Elbow Synform is refolded 'by a late Caledonoid trending monoform'.

Problems in correlating D3 structures have been ameliorated following reassignment of the dominant foliation in the Ben Vuirich Granite from S3 (Bradbury et al., 1976, 1979; Rogers et al., 1989) to S2 (Tanner and Leslie, 1994; Chapter 5).

Recent studies recognise the widespread nature of the D1 and D2 events, and that the development of the main regional schistosity (S2) was broadly coeval with the regional metamorphic peak (Goodman and Winchester, 1991; Robertson, 1991, 1994). The effects of the D3 and D4 events are more domainal in their distribution.

Structural domains

For convenience of description, the district has been divided into several domains (Figure 12) based upon the orientation of the dominant planar and linear structures. Generally, the D2 structural elements are dominant; these comprise S2 schistosity or pressure solution foliation together with L2 mineralogical rodding and small-scale F2 fold hinge lines. D1 structures are preeminent in a 1 to 2 km-wide zone along the south-east margin of the Dalradian outcrop. Variation in the orientation of these dominant foliations is illustrated by the representative stereographic planar data sets in (Figure 12). This variety in attitude is attributed to several phases of folding and faulting as described in detail in the appropriate sections; a summary of the major structures represented in the district is given in (Table 4).

Highland Border Steep Belt

This comprises an approximately 4 km-wide north-east-trending zone lying between the outcrop of the Middleton Muir Fault and the axial trace of the Highland Border Downbend (Figure 12). Within this zone So, S1 and S2 are vertical or have steep north-west dips (Figure 13) although dips are less steep in the west of the district. F2 folds typically plunge 5° to 20° to the northeast. The steep attitudes of the planar structures within this belt are attributed to rotation from their Flat Belt orientation about the gently north-east-plunging hinge line of the D4 Highland Border Downbend.

Flat Belt

The Flat Belt extends from the axial trace of the Highland Border Downbend northwards to the outcrop of the Duchray Hill Gneiss Member, the Gleann Fearnach Fault and the area of Kindrogan Wood [NO 04 61] (Figure 12). Across most of this belt, S0, S1 and S2 form gently inclined sets of subparallel surfaces; however, it should be noted that their attitudes are not strictly subhorizontal, as previously depicted by several authors (Bradbury et al., 1979; Harte et al., 1984). For a significant proportion of this belt, but especially in an approximately 6 km-wide swathe that lies immediately north of the Highland Border Downbend, the planar structures dip gently to moderately to the north or north-west. A structural thickness amounting to several kilometres of Southern Highland Group rocks is thus exposed (Figure 13).

F2 folds are generally reclined throughout the Flat Belt. The L2 mineral lineations plunge consistently north, whereas F2 axes have more variable plunge azimuths, generally between north-north-west and north-east; plunge inclinations are typically from 5° to 25° (Figure 11). Dips and plunges become subhorizontal in the area between Pitcarmick [NO 08 56] and Kirkmichael [NO 08 60] which marks the location of a plunge depression of probable D4 age.

In the northern part of the Flat Belt, dips and plunges are more variable owing to the effects of D3 and D4 deformation upon pre-D3 structures. This greater variability in structural attitude is particularly noticeable over the outcrop of the northern unit of Green Beds, but is also a feature of the outcrop of the Mount Blair Psammite and Semipelite Formation and the main outcrop of Loch Tay Limestone Formation. The outcrops of these latter two formations define a shallow bowl-shaped structure with gentle inward dips from its western, southern and eastern flanks, although there are departures from this general trend.

Tummel Steep Belt

The Tummel Steep Belt (Bradbury et al., 1979) is recognised south-west of the Gleann Fearnach Fault in the area north and west of Kindrogan Wood. S2 shows a progressive steepening towards the north-west with northwest dips of more than 50° in the north-west of the belt. L2 and F2 both plunge moderately to the north-east within this zone.

Cairnwell Steep Belt

In the north-east of the district, the Flat Belt is succeeded north-westwards by a broad zone (Figure 12), across which the dip of the dominant foliation, S2, gradually increases towards the north-west; between Auchintaple Loch [NO 19 64] and Black Hill [NO 16 71] dips increase from approximately 30° to 70°, whereas F2 plunges are generally gentle to the north-east throughout this belt. These attitudes prevail in the south-east corner of the adjacent Braemar district. This steep zone, the Cairnwell Steep Belt, lies along strike from the Tummel Steep Belt and appears to represent an analogous structure. They are separated by an intervening zone, the Caul Dallaig Transfer Zone, in which steep north-eastwards dips predominate. Both the Tummel and Cairnwell steep belts developed after D2, although there is no more precise evidence for the age of development.

Carn Dallaig Transfer Zone

This north-west-trending zone is approximately 5 km wide and forms a conspicuous interruption to the regional north-east–southwest 'Caledonide' trend. It is associated with an approximately 15 km offset in the outcrop of the Loch Tay Limestone Formation. The zone is bounded by the outcrop of the Gleann Fearnach Fault in the south-west, and by Gleann Taitneach to the north-east. Regional strike is north-west within this zone with mostly steep (typically 50° to 90°) dips, both to the south-west and north-east (Figure 12). Fold hinge lines and mineralogical rodding have a range of attitudes which reflect both original variance and the effects of subsequent folding; however, plunges of 20° to 40° to the south-east tend to prevail (Figure 12).

D1 Deformation phase

Evidence of a strong tectonic fabric (S1) is widely preserved within the Dalradian rocks, although across most of the district S1 has been strongly modified and overprinted by the dominant S2 schistosity. A southern limit to the effects of the

D2 deformation has been mapped within the Highland Border Steep Belt (Figure 11). To the south-east of this limit, S1 fabrics unaffected by D2 deformation can be observed, although they are not well exposed in the district. Unmodified D1 structures crop out in the Drouthy Burn and Lunan Burn [NO 0940 4521] and nearby in quarries around Forneth. Much better exposures of comparable D1 structures can be found farther south-west along strike, around Birnam and Obney Hill in the Perth district (Harris, 1972; Harris et al., 1976).

S1 represents the most consistently recognised D1 structure throughout the district; this, together with a relatively small number of minor F1 folds is described below. Larger-scale F1 folds recognised from facing changes on S2 are also considered below.

D1 structures unmodified by D2

These structures are only preserved in metamorphosed greywackes and shales of the Southern Highland Group within the Highland Border Steep Belt, including those of the Birnam Slate and Grit Formation.

S1 fabric

In psammitic rocks, S1 is a conspicuous spaced pressure solution cleavage (S1p) defined by thin (0.5 to 1 mm), micaceous (muscovite and chlorite) laminae that delimit 7 to 30 mm-thick quartz-rich microlithons. In gritty psammites, this pressure solution cleavage is progressively replaced, with increasing grain size, by a grain shape fabric of flattened quartz clasts (S1G) wrapped by aligned chlorite and muscovite flakes. Some gritty psammites with calcareous matrix components have developed S1 grain shape fabrics of flattened clasts set in a matrix of quartzofeldspathic and calcsilicate minerals. In slaty pelites, S1 is a well-developed penetrative slaty cleavage (S1s) defined by the preferred orientation of muscovite and chlorite.

F1 folds

In the Lunan Burn and Drouthy Burn, and near Green-crook [NO 088 460], angular relationships between bedding (So) and S1G in steeply north-west-dipping gritty psammites indicate northerly vergence. Graded units, commonly recognised by the refraction of S1p towards Sts in the finer grained upper parts of the beds, indicate northwards younging and imply that the F1 folds in the Highland Border Steep Belt are downward facing. These observations are consistent with those of Harris (1972), Harris et al. (1976) and Rose and Harris (2000), who reported downward-facing F1 folds and fanning S1p in psammitic layers on Obney Hill [NO 02 38] and Newtyle Hill [NO 04 41], to the south-west in the Perth district.

D1 structures modified by D2

North of the Highland Border Downbend all D1 structures were overprinted to some degree during the pervasive D2 event (Plate 12), (Plate 13). S1 can still be clearly identified in many places, albeit attenuated and tightly folded. Mesoscopic F1 folds are rarely observed, and So–S1 relationships are generally obscured by D2 effects; therefore, attention is focused on S1–S2 relationships.

Modified S1 fabrics

S1 is commonly seen to be transected by S2 planar fabrics (Plate 12), particularly in the hinge zones of small-scale F2 folds, where S1 and S2 can be clearly distinguished. This distinction is less clear on strongly attenuated limbs without the aid of thin sections. Here, S1p, the most readily preserved of the S1 fabrics and best seen in psammites, is strongly attenuated with the thickness of D1 microlithons reduced to 1 to 5 mm or less. In domains where there is clear angular discordance between S1 and S2 (see (Plate 12)), S2 is generally defined by the preferred orientation of micas within the S1 micaceous laminae.

In strongly deformed domains where S2 is subparallel to S1, the two fabrics are commonly indistinguishable, even in thin section. Examples include psammites that crop out on Mount Blair and those within the Corrydon Psammite Member. S2 generally dominates these highly modified fabrics leaving relicts of S1 visible only within S2 microlithons, and/or as internal trails (S1) within porphyroblasts (Plate 14)c, d.

S1 is more commonly defined by muscovite and chlorite than by biotite, with the exception of micaceous quartzites and semipelites of the Tulaichean Schist Formation where biotite is more common than muscovite in S1. Relict S1 in the micaceous quartzites in Glen Loch comprises an imperfect parallel alignment of biotite, which is coplanar with So compositional layers; a similar fabric in quartzite xenoliths within the Ben Vuirich Granite is parallel to concentrations of allanite, sphene, zircon and opaque minerals.

S1 is comparatively unmodified in some semipelites and comprises either a fine-grained continuous foliation, or a spaced cleavage defined by micaceous laminae set 1 mm or less apart. In thin section, small rutile needles are commonly seen along the S1 laminae. Within semipelites of the Tulaichean Schist Formation, as on Creag a' Chaise [NO 078 729], S1 comprises a fine millimetre-scale striping expressed by elongate quartz grains and the parallel alignment of biotite, muscovite and opaque minerals.

S2 is the dominant planar fabric in most semipelites, although the extent to which this represents a composite fabric of S1 and S2 elements is difficult to assess. Some examples of relatively unmodified S1 mica subfabrics defined by stout but fine-grained flakes of muscovite and biotite are preserved within S2 lithons, and small-scale F2 fold hinge zones (Plate 15)b. Such relationships are seen in some of the semipelites of the Gleann Beag Schist Formation on Beinn a' Chruachain [NO 0415 6984].

S1 is also preserved as internal straight or curved trails of elongate quartz inclusions within garnet and staurolite porphyroblasts. In many of the garnet porphyroblasts, the internal fabric within the garnet cores makes high angles with the external S2 fabric, but internal and external fabrics are usually continuous from the garnet rims and across the grain boundaries. It thus seems likely that both the external fabric and the contiguous internal fabric of the garnet rim zones have a composite S1–S2 nature ((Plate 14)c, d).

Most of the relatively pure carbonate lithologies, together with many of the calcsilicate and calcareous schist units, preserve little evidence of S1 fabrics owing to the strong D2 deformation and recrystallisation. However, although D2 structures are paramount over the outcrop of the Ben Lawers Schist Formation, modified S1 foliation is generally the most conspicuous planar metamorphic fabric element seen in the finely interlayered calcareous and calcsilicate schists and psammites. Typically this S1 foliation is defined by aligned micaceous foliae, trails of amphibole and/or epidote granules and lenticular aggregates of granoblastic quartzofeldspathic and carbonate components. It mostly appears to be developed subparallel to bedding, with mean grain size typically 0.1 to 0.5 mm ((Plate 15)e).

Modification of minor F1 folds

Only a few small-scale F1 folds have been identified. They are generally recognised where S2 schistosity transects both limbs of a fold with no accompanying change of F2 vergence; examples may be seen in the Green Beds in Glen Brerachan [NO 0300 6409], and in an isoclinal fold of compositional banding in the Tulaichean Schist Formation in upper Gleann Fearnach [NO 0056 7482]. Minor F1 folds can also be identified by changes in younging and stratigraphical facing on S2, as in a folded gritty psammite south-west of Deuchary Hill [NO 030 478], where an upright, upward-facing F1 fold was refolded in D2. In the absence of transection or interference between D1 and D2 structures, the recognition of D1 and D2 elements may be difficult; for example at Clach Sgorach [NO 1390 6156], a tight metre-scale north-west-vergent recumbent fold with axial planar spaced cleavage is thought to be of D1 age, although this cannot be proven.

D2 Deformation phase

The second phase of deformation (D2) was of paramount importance throughout most of the Dalradian of the district, with S2 forming the dominant regional foliation. The only area to escape its effects is a 2 km-wide zone at the south-eastern edge of the Dalradian outcrop immediately north-west of the Middleton Muir Fault (Figure 11). Development of peak metamorphic mineral assemblages was broadly coeval with D2. F2 folds are seen in many exposures; the majority are close or tight structures that range from microscopic crenulations to major folds exceeding 1 km in amplitude. Many have relatively unmodified hinge zones and highly strained limbs. In several large-scale structures, the highly strained limbs are replaced by tectonic slides. These D2 syn-metamorphic structures (Hutton, 1979) have been mapped on the basis of the stratigraphical attenuation and/or excision associated with the slide zone. Within the Flat Belt, the majority of F2 minor folds show west or north-west vergence (Bell, 1981) on axial planes which now dip to both south-east and north-west.

F2 fold development

The onset of D2 deformation, with its consequent overprinting of D1 structures, can be readily examined in the Southern Highland Group rocks that occur in the Highland Border Steep Belt. In psammites, S1p is generally more conspicuously affected by small-scale F2 folds than S0. F2 fold axial planes and S2 foliation probably developed in gently inclined or flat-lying attitudes (see introduction to this chapter), whereas the overall orientation of S1 generally dips more steeply than S2 (Figure 13). Thus the flat-lying F2 fold structures are interpreted as being superimposed upon more upright D1 folds, as observed north-west of Loch Ordie [NO 028 510] and on Craig Loisgte [NO 0673 5304].

Typical F2 folds are close to tight, strongly asymmetrical structures with rather angular hinge zones and similar fold geometry. In the Flat Belt, short F2 fold limbs are steeply inclined, and usually show S1P affected by subsidiary F2 folds of buckle type, whereas long F2 fold limbs are more gently dipping with planar, rather attenuated S1P (Plate 13). The predominant vergence of these folds is towards the west or north-west. Regular, decimetre-scale alternations of short and long limb domains are commonly developed, resulting in 'herring-bone' structure (Mendum and Fettes, 1985). Comparison with areas unaffected by D2 deformation indicates that the spacing of S1p is reduced on F2 fold limbs, rather than increased in the adjacent hinge zones. This suggests that F2 folds formed in response to oblique shear, rather than buckling processes. The shear displacements generally have a top to the south-east sense and are, therefore, opposed to the direction of F2 fold vergence (Krabbendam and Leslie, 1996). S1milar patterns of south-east-directed shear are indicated by centimetre- to decimetre-scale shear bands that have developed as local modifications of F2 folds within some zones of high D2 strain (Plate 13). Fabric analysis of orientated thin sections, obtained from shear bands on the long limb of an F2 fold, resulted in girdle patterns of quartz c-axis measurements, arranged clockwise from the normal to S2 (Figure 14). This quartz fabric also indicates south-east-directed shear (Law, 1990, and references therein) by means of dynamic recrystallisation (Urai et al., 1986). The similar nature of these F2 folds likewise accords with an origin by simple shear (Ramsay, 1967; Harris et al., 1976).

It is thus considered that the F2 folds within the Flat Belt originated within a deformational regime characterised by a strong component of simple shear with a top to the south-east sense; this caused thinning and rotation to gentler dips of formerly more upright D1 structures. The short steep limbs of the F2 folds that are preserved in the Flat Belt thus appear to be unrotated remnants of steeply inclined S1, and represent zones of relatively low D2 strain. These findings are consistent with Harris et al. (1976) and Treagus (1987), but contradict those of Mendum and Fettes (1985).

Within the Flat Belt, there appears to be a general tendency for the frequency and wavelength of F2 folds to decrease with increasing D2 strain; this is due to progressive consumption of the steep limbs by subhorizontal shearing. Close to the Highland Border Downbend short limbs account for 30 to 50 per cent of the exposed rocks. Farther north the short limbs are both narrower and less common, reflecting the more pervasive development of S2. This indicates a decrease in D2 strain partitioning as total D2 strain increases (Plate 13) (Krabbendam et al., 1997).

Despite the predominance of westerly vergent F2 folds, there are some zones where easterly vergence predominates, notably around Creag nam Mial [NO 05 54] and Creag Loisgte [NO 06 53]. Commonly, easterly vergent F2 folds have more rounded hinge zones than westerly vergent examples, with Class 1C geometries (Ramsay, 1967) of folded S1p implying a component of near-vertical shortening associated with buckle folding.

North of the Flat Belt, there are many examples of both large- and small-scale F2 folds that affect the varied lithologies of the Argyll and Appin groups. In most of these folds, bed thickness is greater in fold hinge zones than on adjacent fold limbs.

Stratigraphical attenuation and/or excision on F2 fold limbs occurs on several scales. Many small-scale F2 folds are either 'rootless', or show evidence of boudinage.

Some large-scale F2 folds demonstrate pronounced attenuation and excision of mapped lithostratigraphical units on their limbs such that their limbs are replaced by tectonic slides. An example occurs in Gleann Fearnach [NO 03 70] where the Ben Eagach Schist and Creag Leacach Quartzite formations thin and become excised on the D2 Allt Creag Dubh Slide. The locations of several slides, notably the Cant Dallaig Slide in Upper Gleann Fearnach [NO 01 74] and the Baddoch Burn Slide in Gleann Taitneach [NO 07 75], are closely associated with the graphitic lithologies of the Gleann Beag Schist and Ben Eagach Schist formations. These and the other slides form high-strain zones that are associated with considerable offset and translation of lithostratigraphical units; they are thus considered to represent sites of substantial simple shear displacement. The tendency of some small-scale F2 folds within the slide zones to develop strongly curvilinear hinges approaching sheath-like form, as in Gleann Fearnach [NO 045 687], is also consistent with a deformation style that involves intense heterogeneous simple shear displacements.

Disharmonic folding is recognised in many examples of F2 folds that affect heterogeneous successions within the Appin and Argyll groups. Incompetent layers show more evidence of layer-parallel shortening, with more apparent thickening of fold hinge zones relative to limbs, when compared with competent layers. The latter generally have geometries that conform with those of Class 1C folds (Ramsay, 1967), from which the operation of active buckling processes during the D2 deformation is inferred.

It therefore appears that throughout the Dalradian outcrop of the district, and in common with other parts of the Central Highlands (Treagus, 1987), the D2 episode involved elements of both near-vertical shortening and subhorizontal simple shear deformation. The high-strain zones of simple shear that form such a conspicuous feature of the long limbs of F2 folds within the Southern Highland Group rocks of the Flat Belt have their counterparts in the tectonic slides that have been recognised across the outcrops of Appin and Argyll group strata.

D2 fabric development

S2 foliation

Within psammites in the Flat Belt, S2 is defined by the alignment of micas, mainly biotites with stubby habit.

Those small-scale F2 folds with an interlimb angle of greater than 15° generally show a small angular discordance between S1 and S2, even on the attenuated long limbs ((Plate 15)a). However, over most of the Flat Belt, the typical spacing of S1p is 1 to 5 mm (compared with 7 to 30 mm in areas unaffected by D2) and discordance between S1 and S2 is hard to detect. A composite S1/S2 fabric is all that can be recognised on long limbs, and reliable distinction between S1 and S2 can only be made within the hinge zones and on the steep short limbs of F2 folds. The spacing of fabrics in psammites is also controlled by composition, with S1 more closely spaced and S2 more intensely developed where the mica content is higher.

S2 mainly occurs as an intense crenulation cleavage in schistose semipelites within the Flat Belt ((Plate 15)b); thin sections reveal that the S, domains contain a higher proportion of biotite than the distinct crenulated S1 domains. In many cases ((Plate 15)a), the S2 crenulation cleavage takes the form of an S/C fabric (Berthe et al., 1979). This is usually developed on a coarse scale in relatively psammitic facies within the semipelites, as at Butterstone [NO 0595 4615], with S1p as the S fabric spaced at 2 to 3 mm, while S2 is a 2 to 3 cm-spaced C fabric. In typical semipelites, both S1 and S2 are spaced at 1 mm or less, whereas in schistose pelites the dominant element of the rock fabric is usually a penetrative S2 schistosity. In many pelites, evidence of a pre-D2 structural history is only shown by F2 folds of early quartz veins.

In the northern parts of the Flat Belt, the S2 schistosity is developed with even greater intensity. The spacing of the composite 51/S2 fabric in Southern Highland Group psammites is only approximately 1 mm on Creag an t-S1thein [NO 034 657], and decreases to less than 1 mm on Mount Blair [NO 165 630], where S1 and S2 are parallel and indistinguishable, except in rare F2 hinge zones. A similar finely spaced S1/S2 fabric can also be seen in the Corrydon Psammite Member, which is lithologically similar to the Southern Highland Group psammites.

In the semipelites, the lithons that contain relict S1 become progressively thinner across the northern part of the Flat Belt; here S2 commonly appears as a continuous foliation that is partly composed of transposed and thoroughly recrystallised S1 elements, and partly defined by preferred shape and crystallographical orientation fabric components. In many places, however, an anastomosing domainal schistosity occurs in which micaceous laminae envelop elongate quartzofeldspathic lithons. This fabric comprises both S1 and S2 elements that cannot be distinguished separately.

S2 generally forms a continuous foliation in the Green Beds. It is defined mainly by the preferred shape orientation of hornblende, biotite and quartzofeldspathic components. Where the Green Beds are sufficiently micaceous, S2 is also developed as a crenulation cleavage with lithons typically spaced 1 to 2 mm apart.

North of the Flat Belt, the nature of S2 is controlled by the lithology. In most exposures of the An Socach Quartzite Formation on Carn an Righ, S2 is a strongly imposed platy structure defined by a well-developed quartz grain-shape fabric; in many cases this is associated with a down-dip rodding of recrystallised detrital feldspar clasts which have an X:Z ratio of up to 10:1. In pelites and semipelites, S2 is generally a penetrative schistosity defined by mica alignment. However, S2 crenulation fabric development seems to have been relatively inhibited in some graphitic pelites and fine-grained semipelites, as in the Ben Eagach Schist Formation in Gleann Fearnach [NO 0443 6871] and south of Carn Dearg [NO 0479 7074] ((Plate 15)c). The abundant finely disseminated particles of graphite appear to have inhibited grain-boundary migration during the D2 recrystallisation, so that, despite being intensely crenulated during D2, these rocks have preserved some of their very fine-grained So and S1 fabrics ((Plate 14)a).

The full range of fabrics S0, S1 and S2 is commonly seen in the semipelitic Tulaichean Schist Formation. Generally, where S1 and S2 can be distinguished, S2 is a finely spaced crenulation foliation, formed by the crenulation of a penetrative S1 that generally transects So at low angles. Even where S2 appears penetrative in hand specimen, it is almost always seen to be a crenulation foliation in thin section.

The main foliation in the Duchray Hill Gneiss Member is a pervasive, commonly anastomosing, mica fabric defined by quite coarse (0.5 to 2 mm) recrystallized flakes of muscovite and biotite. This mica fabric is generally parallel to the stromatic layering (Chapter 7), but is also seen to be axial planar to tight to isoclinal, rootless intrafolial folds which deform the stromatic layering, as seen on Meall Easganan [NO 1174 6446]. S2 seems to be particularly strongly developed in outcrops of the less migmatised and more schistose Ben Lui Schist Formation in the Allt a' Bhuirich [NO 097 644]; recrystallised stringers of quartz are conspicuous in thin section and are enveloped in an anastomosing mica-defined schistosity. This fabric has no apparent consistent sense of asymmetry, but appears to have developed by extreme attenuation.

S2 in calcareous schists of the Loch Tay Limestone Formation forms a penetrative foliation defined by micas, predominantly biotite. These, together with intercalated crystalline metacarbonate rock layers, impart a colour layering that is almost parallel to a weakly developed S2 shape fabric defined by coarse grains of carbonate. Some of the metacarbonate rocks in Wester Bleaton Quarry [NO 115 595] preserve a layering which looks superficially like bedding. However, this is a recrystallised tectonic fabric, parallel to S2 in adjacent calcareous schists, and coplanar with the axial planes of rare fold closures, which have deformed early calcite veinlets within the metacarbonate rock. Within some of these calcareous schists, the D2 deformation was locally so intense that bedding traces are entirely disrupted by strong boudinage; this may give the rock a chaotic appearance, as on Creagan Beag [NO 113 606].

In contrast to the calcareous rocks of the Loch Tay Limestone Formation those of the Ben Lawers Schist Formation do not readily develop S2 crenulation fabrics, so that F2-folded S1 fabrics are commonly preserved. So is usually defined by elongate aggregates of amphibole, epidote and clinozoisite granules, or by concentrations of fine-grained micas and chlorites, which are aligned parallel to S1 ((Plate 15)e). The F2 folds occur as both small- and large-scale tight to isoclinal folds of So, with associated crenulation of S1. These crenulations are conspicuous in mica-rich beds; S2 crenulation foliation, where developed, comprises irregular, lensoid, micaceous foliae that are coarser grained than their S1 counterparts. Interbedded thin 'pitted' psammite units, conspicuous in most exposures of the Ben Lawers Schist Formation, have developed convolute F2 buckle fold forms; these folds generally reveal a continuous S2 axial planar foliation under microscopic examination that is not very conspicuous in the field. This foliation is typically defined by the alignment of dispersed, stubby, fine-grained (0.1 to 0.2 mm) micas within the psammite layers, and by changes in the habit of small amphibole granules. The amphibole shapes change progressively from oblate (transposed S1 shapes) on F2 fold limbs to prolate shapes in F2 hinge zones. Coarse-grained, subidioblastic porphyroblasts of actinolitic amphibole are common and occur widely across the Ben Lawers Schist Formation outcrop; they have grown parallel to transposed So/S1 or continuous S2, but have no preferred linear orientation. They were developed during a grain-coarsening event that was associated with the syn-post D2 peak metamorphic conditions (Chapter 7). S1milar S2 fabrics with folded So and S1 elements have developed in some fine-grained amphibolites of the Farragon Volcanic Formation ((Plate 15)d).

A wavy, anastomosing spaced cleavage occurs within coarse-grained intrusive amphibolite, and is defined by a compositional layering of hornblende and plagioclase aggregates which contains a shape fabric of hornblende. This planar fabric commonly merges into a more pronounced linear fabric with prolate aspect.

Lineations

Several different types of lineations in addition to F2 fold axes are associated with D2 deformation. Although varied in character and origin, they are generally coaxial, but angular variance of up to 20° to 30° between L2 and F2 axes is common. A quartzose or quartzofeldspathic rodding, seen in many of the psammites and semipelites, is the most common and widely developed type of D2 lineation. Quartz rodding is also common both in some of the the coarse amphibolites, as on Whitefield Hill [NO 08 62], and in pelitic lithologies that contain quartz veins deformed in D2. North-west of Dalnoid [NO 1347 6260] quartz mullions with an aspect ratio of 1:3:10 define small-scale cuspate–lobate F2 fold-hinge zones.

The most conspicuous lineation, seen in the Green Beds, is probably that defined by fine hornblende needles; a well-developed quartz rodding occurs in the more quartzose Green Beds. Calcsilicate boudins up to 30 cm long, which have a typical aspect ratio of 1:3:20, occur rarely within the Green Beds as at Meall Dubh [NO 0646 5316] and Craig Wood [NO 0564 5284]. In coarser grained intrusive amphibolites, a lineation is defined by coarse (millimetre-scale) prolate aggregates of hornblende that represent recrystallised relicts of pre-D2 igneous textures.

Porphyroblasts with internal fabric

Garnet, amphibole and some staurolite porphyroblasts contain an internal fabric of inclusion trails. Garnets are particularly abundant in the schistose semipelites and pelites of the Southern Highland Group and the Tulaichean Schist Formation. The internal (S1) fabrics within garnets are generally defined by trails of elongated quartz and/or opaque inclusions. In the southern part of the Southern Highland Group, the internal fabric is commonly straight, significantly finer in grain size and has rotated through large angles relative to the external S2 foliation. It is, therefore, concluded that the internal trails are relicts of S1 (Plate 14)b. This and other relationships between garnet growth and the D2 deformation are outlined in (Table 5).

Garnet growth occurred before, during and after D2, with several phases of growth apparent in some places. In most cases, garnets are round to subidioblastic; they are wrapped by the mica fabric, and commonly have associated pressure shadows, both features indicating either pre-D2 or syn-D2 growth. Locally, some garnets have formed an idioblastic rim that has partly overgrown the S2 schistosity and thus indicates a phase of post-D2 growth (Plate 14)e. The internal S1 fabric is usually continuous with the external S2 fabric at the rim of the porphyroblasts and attests the composite (S1/S2) nature of the external foliation, even where no other evidence of S1 is preserved in the external fabric.

Four main types of S1 inclusion pattern have been observed, namely straight, slightly sigmoidal, strongly curved and crenulated; the last contains both S1 and S2 relicts in the internal fabric (Plate 14)f. These four types respectively represent pre-D2, early-D2, syn-D2 and post-late D2 growth of the porphyroblasts (Plate 14)b to f. Some garnets with straight or slightly sigmoidal S1 inclusion trails, have the internal fabric of several garnets parallel (Plate 14)c, but inclined to the external S2. This indicates uniform conditions of deformation, in which the developing S2 foliation rotated relative to static porphyroblasts (Bell, 1985, 1986). In the cases of those garnet porphyroblasts that have curved S1 inclusion trails, it is still a matter of debate as to whether these indicate rotation of the porphyroblasts during growth (Bell et al., 1992; Passchier et al., 1992). There are, however, no implications arising out of this debate that change the relative timing of deformation and porphyroblast growth as originally interpreted by Zwart (1962).

As noted above (Table 5), garnets with straight and slightly sigmoidal S1 inclusion trails are most abundant in the southern part of the Southern Highland Group outcrop; garnet porphyroblasts with strongly curved inclusion trails are most common in the northern part of the Southern Highland Group and the Tulaichean Schist Formation outcrop (see (Plate 14)d, e). There thus appears to be a zonation in the distribution of garnets based upon the timing of their growth relative to the D2 event. This may indicate a diachronous relationship between D2 and metamorphic conditions (Chapter 7).

Amphibole porphyroblasts with internal fabrics are most common in the calcareous schists of the Gleann Beag Schist and Ben Lawers Schist formations; most of these porphyroblasts are hornblendic or actinolitic amphibole arranged in garbenschiefer or random orientation textures. The S1 trails are composed mainly of elongate quartz and opaque grains, in some places accompanied by fine-grained granular epidotes. Most of the inclusion trails are straight as on Crag Lamhaich [NO 0972 7300], and are normally aligned parallel to, and continuous with external foliation; they thus show little evidence of rotation during S2 fabric development. Relicts of crenulated S1 and S2 are preserved as S1 trails within amphibole porphyroblasts in Glen Lochsie [NO 0751 7220] and the Allt Linne a' Bhuirein [NO 0543 6939] respectively, whereas in the Allt Ruigh nan Eas [NO 0392 6972] tremolite porphyroblasts have cores with straight S1 trails that become F2 crenulated towards the margins of the porphyroblasts. Growth of the amphibole porphyroblasts thus appears to be in part coeval with F2 and S2 fabric development, but mainly occurred during the D2–D3 interval.

Large-scale D1 and D2 structures

The geometry of large-scale D1 and D2 structures has been considerably modified by the effects of D3 and D4 deformations (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15). The largest post-D2 fold structures plunge gently north-east and appear to have rotated S2 into the relatively steep attitudes of the Tummel–Cairnwell and Highland Border Steep belts. The original inclination of S2 is inferred to have been gentle but it cannot be tightly constrained. Wide belts of either gently south-east-dipping S2 (Bradbury et al., 1979; Upton, 1986), or gently north-west-dipping S2, on the respective north-west and south-east flanks of the Tummel–Cairnwell Steep belts, may indicate that the original regional attitude of S2 was subhorizontal.

Other large-scale post-D2 structures that have reorientated S2 occur within the Carn Dallaig Transfer Zone (Figure 12), (Figure 16). A complex, south-east-plunging polyphase synformal fold, situated at the head of Gleann Fearnach, and known as the Meall Reamhar Synform is the largest of these structures. It lies between the Tummel and Cairnwell Steep belts, in an area where there are marked swings in the trends of L2 lineations and F2 fold axes (Figure 11). Simple unfolding of the post-D2 folds in this area does not remove these swings in lineation trend, because many of the small-scale D2 folds and lineations plunge south-east, (i.e. they are coaxial with this major synform). Some of the variation in attitude of small-scale D2 folds and lineations thus appears to have a D2 origin, related to curvilinear large-scale F2 folds. Small-scale examples of curvilinear and sheath-like F2 folds thus have their large-scale counterparts in this Carn Dallaig Transfer Zone.

Most of the large-scale D1 structures have been modified by D2 shearing and sliding. The presence of large-scale F1 folds can, in most cases be demonstrated by reference to changes in stratigraphical facing on S2, or by the same (usually north-west) vergence of small-scale F2 folds on both limbs of an obvious large-scale F1 fold. The contribution of the D1 deformation cannot be tightly constrained, partly because of inadequate exposure in some areas, and partly because of strong overprinting by post D1 structures. Some D1 structures are, therefore, interpreted from their effects upon outcrop patterns, by analogy with those where no doubt exists as to the nature of their D1 origin.

Summarised descriptions (from south to north) of the large-scale D1 and D2 structures are given in (Table 4). Reference should also be made to (Figure 11) for the location of these structures and to (Figure 13), and (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15), (Figure 16), (Figure 17), to (Figure 18) for appropriate cross-sections.

Capel Hill Syncline

The most southerly major D1 structures recognised north of the Highland Border Downbend are the east-facing Capel Hill Syncline (Figure 13) and a complementary anticline to the south. The syncline is responsible for a wide outcrop of Green Beds around the closure on Capel Hill [NO 03 51] (Figure 10). The northern limb, which forms the upper long limb of the syncline, is inverted whereas the southern short limb is the right way up (Figure 13). Both limbs have been refolded by smaller scale asymmetric F2 folds. These consistently verge northwestwards on both limbs of the syncline; however, the stratigraphical facing on S2 changes across the closure, indicating the D1 age of the syncline.

Medium-scale F2 folds are spectacularly developed along the margins of the Green Beds unit north-east of Loch Ordie [NO 043 510] and on Capel Hill [NO 036 517]; they are recumbent, with gently north-dipping axial planes, and low north to north-east plunges. Large scale F2 folds also affect the Green Beds east of Craig Wood [NO 061 531] and [NO 063 521], whereas the tongue-like outcrop of Green Beds in the core of the Capel Hill hinge zone between Craig Wood and Capel Hill [NO 049 523] is caused by D1/D2 interference (Figure 13). The interruption of the Green Beds outcrop on Craig Wood [NO 05 52] may have been caused by shearing along an F2 fold limb

The complementary anticline occurs on Riemore Hill to the east of Loch Ordie, as indicated by the change from right-way-up strata at [NO 053 503] to inverted strata at [NO 058 502]. The position of this anticline is poorly constrained due to the absence of Green Beds here (Figure 11); south of Riemore Hill most of the rocks are inverted.

Folding of the northern Green Beds unit

In the western half of the district, the Green Beds north of Pitcarmick Burn are poorly exposed but appear to represent a single stratigraphical unit. The manifestly complex outcrop pattern of this northern (and strati-graphically older) unit of Green Beds is due to the interference between four sets of folds and subsequent displacement along the Gleann Fearnach Fault (Figure 11), (Figure 17).

The westernmost portion of the Green Beds outcrop in this area is the body that extends from An t-Sron [NO 03 59] and Meall Uaine [NO 03 60] to the Gleann Fearnach Fault [NO 03 68]. This body dips steeply to the west, but youngs to the east (i.e. is inverted). F2 folds within and adjacent to this body are westerly vergent and face upwards to the east on gently north-plunging axes. Farther east, the outcrops of Green Beds in the Allt Fearnach [NO 04 66] and on Kindrogan Hill [NO 04 62] also dip moderately to steeply to the west, and have westerly vergent F2 folds with gently north- or south-plunging fold axes. Younging evidence, based on graded bedding in Gleann Fearnach [NO 049 663], shows that at least part of this body youngs to the west and is the right way up. Since vergence on F2 is consistently to the west in both bodies, an upward-facing D1 synclinal closure linking these bodies of Green Beds, lies between the Allt Fearnach and Creag na Cuinneige [NO 03 64]. The westernmost parts of the Green Beds outcrop thus form the western, structurally higher limb of an upward-facing F1 syncline (Figure 11), (Figure 17).

The Gleann Fearnach Fault truncates the northern end of both bodies described above. The northern Green Beds unit reappears further south and east near Enochdhu [NO 06 62] and Menachban [NO 08 64] as a gently east-dipping F2-folded sheet on the eastern limb of a large-scale upright north–south-trending F3 antiform, the Creag Dubh-leitir antiform (Figure 11), (Figure 17). On Creag Dubh-leitir [NO 05 65], mainly inverted Southern Highland Group rocks young westwards from exposures of Loch Tay Limestone Formation at [NO 0669 6544]. Thus, the attenuated Green Beds unit at [NO 058 656] is considered to be part of the same synclinal F1 fold limb as the An t-Sron–Meall Uaine body. A structurally lower upward-facing F1 anticlinal keel is shown (Figure 17) between Glenfernate Lodge [NO 047 649] and Creag Dubhleitir. This folds the northern Green Beds unit back towards outcrops that occur farther west on Sheet 55E. Poor exposure does not permit the geometry of this anticline to be determined and its hinge zone is not seen.

The complex outcrop pattern of Green Beds within the triangle Craig Dubh-leitir–Menachban–Enochdhu is mainly caused by upright north–south-trending F3 folds that are parasitic on the large F3 antiform. A type 3 (coaxial) F2/F3 gently south-plunging fold interference pattern (Ramsay, 1967) has thus developed. A major anticlinal (probably synformal) D2 hinge zone of this interference structure is present at the lower end of the Allt Doire nan Eun [NO 063 630], although it is possible that the closure extends as far as the outcrop of Green Beds in the River Ardle at [NO 0572 6274]. The complementary synclinal (antiformal) D2 closure is probably located north of Menachban around [NO 081 646], although its position is disguised by intense D3 folding and by the fault along the Allt Doire nan Eun. The southwards continuation of the Green Beds towards Kirkmichael has been traced using ground magnetic data.

South of Kirkmichael the Green Beds outcrop covers a large triangular area stretching between Kirkmichael [NO 081 601], Creag na h-Iolaire [NO 054 570] and Dalvey [NO 083 574]. This broad outcrop defines a gentle plunge depression of probable D4 age.

Mount Blair Anticline and Fergus Slide

These two structures are responsible for three important features of the geology (Figure 1). The first of these is the occurrence of Southern Highland Group lithologies on either side of the main outcrop of the Loch Tay Limestone Formation. Secondly, the Loch Tay Limestone Formation is absent along the southern margin of the Duchray Hill Gneiss Member (Crinan Subgroup) between Badandun Hill [NO 210 686] and Lamh Dearg [NO 120 634]. Finally, there is a small occurrence of rocks of the Loch Tay Limestone Formation, apparently within the Duchray Hill Gneiss Member outcrop at Blar Achaidh [NO 06 66].

The main outcrop of the Loch Tay Limestone Formation includes both limbs and the hinge zone of a major isoclinal fold, the Mount Blair Anticline (Figure 11), (Figure 18), which closes near Auchintaple Loch [NO 199 650]. This fold appears to face upwards and to the east, based on the evidence of rare right-way-up sedimentary structures preserved in psammites near Glenkilrie Lodge [NO 1397 6100] north of the limestone outcrop, together with evidence of inverted graded beds and channels in the Southern Highland Group rocks that lie south of the limestone outcrop. The Mount Blair Anticline is considered to be a major F1 fold, since most of the minor F2 folds on both of its limbs are westerly vergent. Within the anticline, the overall dip of So and S1 is generally steeper than S2; so it appears that the now gently inclined limbs of the Mount Blair Anticline were rotated from originally steeper attitudes by subhorizontal shearing during D2. Numerous F2 folds on a 10 to 100 m scale are defined by the form of the amphibolite outcrops that lie adjacent to the Loch Tay Limestone Formation, such as around Menachmore [NO 09 63], Whitefield Hill [NO 08 62], Bleaton Hill [NO 12 60], Milton Knowe [NO 10 60] and Wester Bleaton Quarry [NO 11 59].

South of Fergus, a small occurrence of metacarbonate and calcsilicate rocks is exposed in the River Isla [NO 1906 6761] at the boundary between the Mount Blair Psammite and Semipelite Formation (Southern Highland Group) and Duchray Hill Gneiss Member (Crinan Subgroup). No metacarbonate or calcareous rocks are found farther south-west along this boundary despite good exposure south of Cairn Derig [NO 158 655] and on Creagan Caise [NO 1802 6760]. Strongly D2-sheared psammite occurs at the latter contact, where a gently north-west dipping S/C fabric, indicating top to the south-east sense of shear, is developed. The absence of Loch Tay Limestone Formation along most of the contact, together with the occurrence of the strongly sheared rocks, is attributed to the D2 Fergus Slide. The main outcrop of the slide, as seen north-eastwards from Dalnaglar Castle [NO 14 64], developed on the upper limb of an east-facing syncline, structurally above the Mount Blair Anticline (Figure 18). The closure of this syncline may lie within the small occurrence of Loch Tay Limestone Formation at Blar Achaidh [NO 06 66], now detached or 'rootless', whereas its lower limb is represented by the part of the Fergus Slide outcrop that lies between Lamh Dearg [NO 12 63] and the Gleann Fearnach Fault. Alternatively, it is possible that the high strain zone represented by the Fergus Slide developed only on the upper limb of the major east-facing syncline. In this case, the Blar Achaidh structure may be parasitic on a larger synclinal closure that lies concealed beneath the Gleann Fearnach Fault. Satisfactory testing of these alternative hypotheses is not possible, owing to poor exposure in the ground between Blar Achaidh and Cnoc Meadhon [NO 10 64] that prevents definitive description of the structural geometry. However, although some structural detail remains obscure, it is concluded that the Mount Blair Psammite and Semipelite Formation north of the Mount Blair Anticline lies on the right-way-up limb of an upward and east-facing F1 fold pair, the uppermost limb of which became intensely sheared in the D2 Fergus Slide zone.

The axial trace of the Mount Blair Anticline and the outcrop of the Loch Tay Limestone Formation define arcuate and crescentic forms respectively (Figure 1), (Figure 11); So sheet dips and S2 foliation form a shallow bowl-like structure, attributed to later D3 and D4 folding. Small-scale F2 fold axes and L2 rodding lineations are generally either subhorizontal and strike-parallel, or plunge gently either to the west/north-west or to the south-east. However, within the bodies of hornblende schist that occur in the areas of Bleaton Hill [NO 12 60] and Milton Knowe [NO 10 60], both F2 hinge lines and a strongly developed L2 rodding are strongly curvilinear and approach sheath-like forms that cannot be attributed to the effects of post-F2 folding.

Ben Earb Slide

Slivers of Creag Leacach Quartzite Formation and Ben Eagach Schist Formation occur within the main outcrop of Ben Lawers Schist Formation in a narrow zone that extends from the Allt Linne a' Bhuirein [NO 05 69] to Meall Uaine [NO 11 67]. Thin, tightly folded units of calcareous schist and metacarbonate rock, reminiscent of the Glen Lochsie Calcareous Schist Member, also occur on Sron Charnach [NO 06 69] and near An Lairig [NO 09 68]. An analogous situation, without the calcareous rocks, occurs farther north-east on Black Hill [NO 163 715] (Figure 1).

The D2 Ben Earb Slide (Figure 11) has been invoked to account for the presence of these narrow but persistent outcrops of quartzite and graphitic schist, set between two wide outcrops of the Ben Lawers Schist Formation, although the slide itself is not exposed. These outcrop of Ben Lawers Schist Formation are anomalous, in that their combined structural thickness is approximately 2 to 3 times greater in the the Ben Earb area [NO 07 69] than in adjacent areas of the Pitlochry district. Large-scale tight F1 folding is presumed to be responsible for this anomalous thickness by repetition of the Ben Lawers Schist Formation outcrop in the hinge zones and limbs of a major east-facing asymmetric anticline–syncline fold pair. The Ben Earb Slide developed on the short common limb of this fold pair, and the apparently discontinuous, lenticular nature of some formations within the slide zone is considered to reflect the effects of D2 boudinage and extensional failure.

In the absence of observed evidence of the age of these major folds, three considerations concerning their age appear to be relevant. Firstly, no F2 folds of comparable magnitude have been mapped elsewhere within the district; secondly, these inferred F1 major folds bear striking resemblance to the Mount Blair Anticline and Fergus Slide structures; thirdly, changes in stratigraphical facing on S2 are known to occur along the D2 Carn Dallaig Slide zone (Figure 11) as it transects the hinge zone of a major F1 fold. Thus, a scenario that views the major fold pair as F1 folds, modified by D2 sliding, is preferred over one that attributes these major folds solely to the D2 event.

The trace of the Ben Earb Slide follows an east–west trend (although refolded by F3 folds) from Sron Charnach via Ben Earb and An Lairig to Meall Uaine. Along this section, S2 dips are usually steep to the north or north-east, whereas typical F2 plunges are gentle towards the north-east. Around Meall Uaine the slide trace changes to a south-west–north-east orientation; S2 dips along this section are moderate to the north near Meall Uaine, becoming moderate to the north-west farther north-east. Between Slochnacraig [NO 126 687] and Craig of Runavey [NO 136 697] the slide is cut by the Glen Shee Pluton. Farther north-east, the occurrences of Creag Leacach Quartzite Formation and the Ben Eagach Schist Formation on Carn Dearg and Black Hill [NO 16 71] are also considered to lie within the Ben Earb Slide zone, which may thus extend as far as Little Glas Maol [NO 17 75] to the north of the Glen Shee district.

Beinn a' Chruachain Complex

This is a complex of folds that appears to have an overall anticlinal form. Its core comprises rocks of the Blair Atholl and Islay subgroups that are almost completely enveloped by D2 slides and by younger rocks of the Ben Lawers Schist Formation (Figure 16), (Figure 19). The Beinn a' Chruachain Complex may thus represent a structurally deeper analogue of the large-scale F1 anticline that has been inferred to lie immediately below the Ben Earb Slide.

The north-east flank of the complex is delimited by the Carn Dearg Slide. Here, although structural attenuation and incomplete exposure make the unequivocal recognition of some formations difficult, rocks of the Tulaichean Schist Formation appear to have tectonic contact relationships with the Ben Lawers Schist Formation east of Beinn a' Chruachain [NO 05 69], with the Ben Eagach Schist Formation north of Beinn a' Chruachain [NO 04 70] and finally with the main outcrop of Islay Subgroup quartzite farther north (Figure 16). Adjacent to the Carn Dearg Slide, a highly attenuated and tightly F2-folded section through almost the entire Blair Atholl and Islay subgroup stratigraphy is exposed within the narrow col on the eastern flanks of Beinn a' Chruachain [NO 05 69]. The Ben Earb Slide has been mapped as the continuation of this zone of attenuation eastwards and south-eastwards from the col. The Ben Earb and Carn Dearg slides are thus considered to form linked sections of a D2 high-strain zone that extends along the entire eastern margin of the Beinn a' Chruachain Complex.

Within the complex itself, rocks of the Ben Eagach Schist Formation and Islay to Blair Atholl subgroups form an upward facing gently south-south-east-plunging F2 asymmetric anticline–syncline pair, which closes around Beinn a' Chruachain (Figure 16), (Figure 19). This fold pair has been highly modified by close to tight F3 folds with steep north-north-west-trending axial surfaces, such that S2 has generally moderate to steep north-east or south-west dips.

A complete stratigraphy, albeit attenuated, from the Gleann Beag Schist Formation up to the Ben Lawers Schist Formation occurs in the Allt Ruigh nan Eas [NO 040 697] on the western side of the complex. Traced north-westwards from this section, both the Ben Eagach Schist Formation and Islay Subgroup quartzites are progressively excised along the Allt Creag Dubh Slide, such that the Gleann Beag Schist Formation rests in tectonic contact with the Ben Lawers Schist Formation east of Gleann Fearnach [NO 036 705].

The south-east part of the complex appears to comprise several small subsidiary slides and F2 folds. The Islay subgroup quartzites are absent in the Allt Linne a' Bhuirein [NO 045 687], where small-scale F2 sheath folds occur within a slide zone that juxtaposes the Ben Eagach and Gleann Beag schist formations. Nearby, the uppermost metacarbonate rock of the Glen Lochsie Calcareous Schist Member occurs in the closure of a large rootless intrafolial fold on Beinn a' Chruachain [NO 0467 6920].

Meall Ruigh Mor Thearlaich Slide and Glen Lochsie Slide

Large-scale F2 folds with attenuated limbs, similar to those of the Beinn a' Chruachain Complex, occur north and east of the trace of the Can Dearg Slide (Figure 11), (Figure 16), (Figure 19). The axial trace of an east- to north-east-trending, upward-facing F2 anticline is located on Carn Dearg [NO 044 720] and Meall Ruigh Mor Thearlaich [NO 053 720]; this anticline has rocks of the Tulaichean Schist Formation in its core. More than 50 m of metacarbonate rock of the Glen Lochsie Calcareous Schist Member of the Gleann Beag Formation crops out in the hinge zone of a subordinate F2 anticline near Coire Domhain [NO 052 709]; but just to the north-west [NO 046 713] the entire Gleann Beag Schist Formation is reduced to approximately 30 m on the southern limb of the main Can Dearg–Meall Ruigh Mor Thearlaich Anticline.

On the northern limb of this anticline, an outlier of Islay Subgroup quartzite represents the hinge zone of a small synclinal closure on Meall Ruigh Mor Thearlaich [NO 050 723]. A zone of attenuation in which the Gleann Beag Schist Formation is less than 50 m thick occurs on the common limb of this anticline–syncline couplet at [NO 048 722]. This is referred to as the Meall Ruigh Mor Thearlaich Slide. The slide is not exposed, although its position on the Glen Lochsie–Gleann Fearnach watershed [NO 043 724] is marked by litter of schist with a strong blastomylonitic fabric.

Farther north-west along this watershed, an inlier of the Tulaichean Schist Formation that forms a whaleback hill [NO 03 73] is interpreted as another east-facing, eastsouth-east-plunging F2 anticlinal core. On the northern limb of this anticline, litter derived from the striped psammites and graphitic pelites of the Gleann Taitneach Schist Member of the Gleann Beag Formation occurs under the peat cover in the upper part of Glen Lochsie [NO 035 734], whereas exposure of Gleann Taitneach graphitic pelites close to the watershed to the south [NO 0371 7295] is considered to form part of the southern limb. No exposure of the Glen Lochsie Calcareous Schist Member is seen within this structure just to the north of the district west of [NO 047 728], so it is concluded that the limbs of this anticline are strongly attenuated, and that along the Glen Lochsie Burn northward-younging rocks of the Gleann Taitneach Schist Member and southwardyounging rocks of the Tulaichean Schist Formation are juxtaposed along the Glen Lochsie Slide.

A small inlier of the Tulaichean Schist Formation, exposed along the Glen Lochsie Burn near the ruined Glenlochsie Lodge [NO 063 725], is considered to represent the hinge zone of the same anticline, whereas higher on the southern slopes of Glen Lochsie, a larger inlier of Tulaichean Schist Formation is considered to represent the eastwards continuation of the Carn Dearg–Meall Ruigh Mor Thearlaich Anticline. The closure of this fold is well exposed on Creag Chreumh [NO 065 719].

These F2 anticlines generally plunge moderately to the east-south-east, but show considerable plunge variation and are, therefore, similar to F2 structures within the Beinn a' Chruachain Complex. Some of the plunge variation is due to F3 refolding; thus an F2 plunge depression, roughly coincident with the position of the porphyry sheet between Meall a' Choire Bhuide [NO 06 70] and the Glen Lochsie Burn, correlates with an F3 synformal hinge zone. However, all the F2 plunge variation cannot be explained by F3 refolding; it seems either that the F2 folds developed with curvilinear hinge zones, or that they were superimposed on F1-folded So surfaces. Given the absence of observed F1 folds in this area, and the strongly curvilinear F2 hinges exposed west of Creag Bhreac [NO 0647 7347], the former alternative is preferred. Ductile graphitic and calcareous units appear to have been most susceptible to sliding in association with the sheath-like folding.

The anticlines described in this section, together with those within the Beinn a' Chruachain Complex, form a structure that resembles a positive flower structure or large-scale anticlinal stack system. The axial traces of the F2 folds and their associated slides all converge in upper Gleann Fearnach [NO 02 73], when traced north-westwards into the Carn Dallaig Slide zone.

Carn Dallaig Slide

The Carn Dallaig Slide zone is marked by an attenuated Glen Lochsie Calcareous Schist Member which intervenes between the Tulaichean Schist and Ben Lawers Schist formations in upper Gleann Fearnach [NO 02 73]. The position of the slide is accurately located by the occurrence of lines of sinkholes over the Glen Lochsie Calcareous Schist Member. Metacarbonate rocks with a very streaky mylonitic but strongly annealed appearance crop out in a sinkhole near the watershed between Glen Lochsie and Gleann Fearnach [NO 0369 7278] and are also seen nearby [NO 0364 7284] (S95867).

The Carn Dallaig Slide is an important D2 slide that links with the Glen Lochsie, Carn Dearg and Allt Creag Dubh slides at higher structural levels (Figure 16). At lower structural levels, a high-strain zone, marked by attenuated Blair Atholl–Easdale Subgroup formations in west Gleann Fearnach [NO 00 74], links with the Carn Dallaig Slide and Creag Uisge Slide. The Carn Dallaig Slide, together with S2 in the vicinity of the slide, has a north-west strike and dips steeply north-east. F2 plunges are moderate to the south-east, and are thus approximately perpendicular to the regional trend (Figure 11). This anomalous trend is evident even allowing for post-F2 folds, and is therefore thought to be a D2 feature. Hence, the slide is interpreted as a lateral ramp or transfer that links D2 sliding on the lower limb of the F1 Gleann Fearnach Syncline, across the hinge zone, and onto the upper limb of this syncline. The complex of F2 folds that occurs between the Glen Lochsie and Allt Creag Dubh slides (Figure 16) forms a positive flower structure that is considered to indicate the transpressive nature of the Carn Dallaig transfer.

Killiekrankie–Glen Loch and Creag Uisge slide systems

Parts of the Blair Atholl to Easdale subgroup stratigraphy that occurs in the Carn Dallaig Slide zone can be traced around the D3 Meall Reamhar Synform towards the west side of Gleann Fearnach (Figure 11). A complete, albeit attenuated, succession from the Tulaichean Schist to Ben Lawers Schist formations can be seen between the ridge and eastern slopes of Braigh Feith Ghuibhsachain [NO 005 736] and the hinge of the Meall Reamhar Synform. When traced farther south-south-east towards Daldhu [NO 02 70], there is progressive excision of formations within this succession, beginning with the Creag Leacach Quartzite Formation, then the Gleann Beag Schist Formation and finally the Ben Eagach Schist Formation and Gleann Taitneach Schist Member (Figure 1), (Figure 11). At Daldhu, the Ben Lawers Schist and the Tulaichean Schist formations are juxtaposed along the Creag Uisge Slide, which thus effects excision similar to that of the Carn Dallaig Slide. South-west of the district on Carn Liath, a prominent zone of mylonitic rocks that can be traced for about one kilometre near the south-eastern contact of the Ben Vuirich Granite [NN 987 665 to NN 996 668] is also considered to mark the outcrop of the Creag Uisge Slide.

On the west side of the Ben Vuirich Granite the Killiecrankie Slide was previously thought to have excised the succession between the Blair Atholl Subgroup 'Dark Group' and the Easdale Subgroup Killiecrankie Schist (Bradbury et al., 1979). The stratigraphical reinterpretation placed upon parts of the 'Killiecrankie Schist' (Chapter 4), now requires that the Killiecrankie Slide (as mapped on Sheet 55E) worked within the Blair Atholl Subgroup in the area west of Ben Vuirich. There is no evidence that the slide is cut by the Ben Vuirich Granite (Tanner and Leslie, 1994). It is therefore suggested that the segment of the slide system that lies north of the Ben Vuirich Granite (Bradbury et al., 1979) be renamed the Glen Loch Slide.

The Ben Vuirich Granite is thus considered to lie between the D2 Killiecrankie–Glen Loch and Creag Uisge slide zones; and, although strongly deformed in D2, the granite does not contain discrete large-scale D2 slides. Instead, the Killiecrankie and Creag Uisge slides have developed in the country rocks along the respective western and eastern sides of the intrusion. North of the Ben Vuirich Granite in Glen Loch [NN 999 720], the Glen Loch Slide forms the tectonic contact between the quartzitic and semipelitic facies of the Tulaichean Schist Formation. The outcrop of this slide farther north is poorly constrained.

Thus both the Ben Vuirich Granite and the quartzitic facies of the Tulaichean Schist Formation are virtually enveloped by two slides that possibly converge south-west of the Ben Vuirich Granite [around NN 96 66]. This is considered to represent further evidence of a strong lithological control on the position of the major slides; these tend to branch or step around massive competent units, and are mainly confined to finely bedded graphitic pelites and calcareous units.

Baddoch Burn Slide

In Gleann Taimeach between Creag Easgaidh [NO 07 76] and Glenlochsie Farm [NO 087 716], the D2 Baddoch Burn Slide (Figure 11) juxtaposes the upper margin of the Glen Taitneach Schist Member to the west against progressively younger rocks to the east; these range from the An Socach Quartzite Formation in the north to the Ben Eagach Schist Formation in the south.

The current Sheet 65W attributes this to the post-D3 Glen Taitneach Fault which is also shown as terminating against the Baddoch Burn Slide in Gleann Mor. In Gleann Mor, to the south-west of Loch nan Eun [NO 06 78], there is no evidence of stratigraphical excision coincident with the trace of the Baddoch Burn Slide, as represented on Sheet 65W. Instead, there is an apparently unbroken right-way-up succession from the An Socach Quartzite Formation to the Gleann Beag Schist Formation, between the summit of Màm nan Carn [NO 04 77] and the hinge zone of a major F3 synform on Glas Choire Bheag [NO 06 76]. The outcrops of the Baddoch Burn Pelite, the Baddoch Burn Dolomite and the Glen Clunie Schist in this area (Upton, 1986) are reinterpreted as the lateral equivalents of the Tulaichean Schist Formation, the Glen Lochsie Calcareous Schist Member and the Glen Taitneach Schist Member respectively.

The Baddoch Burn Slide, as mapped in this study, is exposed on Creag Easgaidh [NO 075 770], where inverted An Socach Quartzite Formation in the hanging wall rests upon southward-younging, right-way-up Gleann Taitneach Schist Member of the footwall. Near the slide [NO 0750 7695], metre-scale beds of An Socach Quartzite Formation are reduced to platy, intensely schistose rocks with centimetre-or millimetre-scale lamination or foliation over a zone some 10 m wide. There is little direct evidence of high strain in thin sections of the most schistose quartzites as a result of thorough post-D2 annealing. The Gleann Taitneach Schist Member likewise shows no outward sign of high strain along the slide zone. Elsewhere, in the absence of exposures, its location has been inferred from formation mapping.

Localised zones of intense D2 strain are also present deeper within the footwall of the Baddoch Burn Slide. On the rim of Glas Choire Mhor [NO 0585 7540]; [NO 0520 7620], psammites and semipelites of the Tulaichean Schist Formation show attenuated ribbons of quartz, recrystallised into very fine-grained (20 to 50 mm) mosaics, and asymmetric augen of quartz and micas. In thin section these show a consistent sense of shear, and are associated with garnet porphyroblasts that have either curved or rotated inclusion trails (see (Plate 15)d).

The Glen Taitneach Schist Member forms the footwall to the Baddoch Burn Slide and continues westwards from Glenlochsie Farm to the ridge between Meall Ruigh Mor Thearlaich and Carn Tarmachain [NO 056 717]. The slide cannot, however, be identified as a discrete planar feature associated with stratigraphical excision in this area. Instead, the displacements are partitioned between the Glen Lochsie, Ruigh Mor Thearlaich and Carn Dearg slides that form the attenuated limbs of several large-scale F2 folds. These folds face upwards to the north-east; they and their associated slides form part of a positive flower structure developed above the Carn Dallaig Slide. Thus the Baddoch Burn and Killiecrankie slide systems are considered to be linked via complex oblique or lateral slides in the Carn Dallaig and Glen Lochsie areas. These lateral/oblique slides transfer D2 displacements from the lower limb of the Fl Gleann Fearnach Syncline to the common upper limb of this syncline and the overlying F1 Cairnwell Anticline (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15), (Figure 20). The Baddoch Burn Slide thus has the effect of repeating the south-east-younging footwall succession of An Socach Quartzite Formation to Glen Taitneach Schist Member in its hanging wall.

The prominent bends that occur in the outcrop of the Baddoch Burn Slide at the upper and lower ends of Glen Taitneach (Figure 11) are attributed to the effects of folding around a large F3 antiformal anticline near Glenlochsie Farm and an F3 synformal syncline on Glas Tulaichean. As a result of these F3 folds the plunge azimuth of minor F2 folds within the slide zone changes from east-south-east-plunging in Gleann Taitneach to west-north-west-plunging in Glen Lochsie.

Carn an Righ folds and associated slides

The An Socach Quartzite Formation on Carn an Righ [NO 025 773] and Stac na h-Iolair is arranged in a very large, tight synformal F2 anticline, the Carn an Righ Anticline (Figure 11), (Figure 16). This fold faces down to the south and west and has moderate to steep plunge towards the east-south-east. The bedding fabric (S0) is relatively well preserved on the southern, south-younging limb where the level of D2 strain is low (Plate 1); So/S2 relationships here imply the presence, farther north, of an F2 synform. On the northern limb of this fold the level of strain is more intense, and convincing evidence of facing is rare; in the few examples seen, S2 lies clockwise of more gently inclined, northwardyounging, relict So and is, therefore, consistent with an east-south-east-plunging synform to the south.

A narrow high-strain zone with up to approximately 10 m of extremely platy quartzite coincides with the boundary between the low- and high-strain fold limbs. The high-strain zone can be traced from Stac na h-Iolair across the summit of Carn an Righ, and it probably represents a slide that may also extend eastwards through the Beinn Iutharn Mhor–Màm nan Carn col [NO 04 78] to form the south-east margin of the Benin a' Ghlo Transition outcrop. The Beinn a' Ghlo Transition here lies within the hinge zone of the Carn an Righ Anticline.

Small-scale downward-facing folds are observed in the col at [NO 0410 7799], where graded and laminated phyllitic semipelites are intensely D2 crenulated. The major closure forms a complex of satellite F2 folds; vergence changes occur over a distance of 30 to 50 m. The northwest margin of the transitional member is exposed nearby [NO 0409 7799] and appears to be sheared against more massive quartzite units. The south-east contact is not exposed but may be a zone of attenuation and sliding as noted above.

On the north-facing slopes of Carn an Righ, northward-younging rocks of the An Socach Quartzite Formation lie against southward-younging rocks of the Tulaichean Schist Formation that are underlain by metacarbonate rocks. This change of younging direction and the associated stratigraphical hiatus is considered to indicate the location of a major zone of D2 attenuation, here named the Carn an Righ Slide. The slide excises the Gleann Mor Limestone Member and the Sron nan Dias Pelite and Limestone Formation on the northern limb of the Carn an Righ Anticline. The Tulaichean Schist, exposed in the footwall of the Carn an Righ Slide, is considered to form part of the northern, southward-younging limb of a downward-facing D2 syncline that lies adjacent to the synformal Carn an Righ Anticline.

Other slides, each associated with large-scale downward- and westward-facing F2 folds, occur on the southern limb of the Carn an Righ Anticline. The Glen Loch Phyllite and Limestone Formation in the Allt Coire an t-Sneachda [NO 02 76] is situated in a downward- and westward-facing asymmetrical antiformal syncline. A complementary westward-facing synformal anticline is inferred, but not exposed, within the outcrop of An Socach Quartzite Formation that lies south of the Allt Coire an t-Sneachda. A 0.5 m-wide zone of friable, finely foliated, thoroughly annealed, quartzofeldspathic schist occurs in the Allt Coire an t-Sneachda [NO 0231 7638]. This is considered to represent a high-strain D2 slide zone that has effectively excised the short common limb of the fold couplet, and juxtaposes two southward-younging limbs.

The hinge line of another downward- and westwardfacing-antiformal F2 syncline lies within a complex of metre-scale F2 folds exposed in the Allt a' Ghlinne Mhoir [NO 0097 7738]. The southern limb of this syncline is associated with stratigraphical attenuation and excision of units of the Glen Loch Phyllite and Limestone Formation in a D2 slide zone that marks the northern contact of strongly rodded quartzites of the An Socach Quartzite Formation, here presumed to be southward younging. This latter slide may thus be structurally equivalent to that seen in the Allt Coire an t-Sneachda.

The small-scale F2 folds in the Carn an Righ area are typically reclined structures that plunge east or south-east at moderate angles. F2 axial planes are generally steep, and the L2 lineations, defined by stretched clasts, F2 crenulations, So/S2 intersections and well-developed mineralogical rodding, are broadly co-axial with the F2 folds. Steep down-dip L2 attitudes are apparent in the vicinity of the slide zone in Gleann Mor [NO 0097 7734] and in the platy quartzites on the eastern flanks of Carn an Righ [NO 03 76].

D3 Deformation phase

The present outcrop patterns of Dalradian lithostratigraphical units and D2 slides reflect the profound effects of the D3 deformation on the structural development of the district. The D3 deformation was superimposed upon a geometrically complex terrane that had developed large-scale Fl/F2 interference structures and curvilinear

F2 folds. The changes in orientation of L2 and L3 linear structures strongly reflect the heterogeneity of both the D2 and D3 strain patterns. D3 is considered to be at least partly responsible for the steep attitude of S2 in both the Tummel Steep Belt (Bradbury et al., 1979; Treagus, 1999) and the Cairnwell Steep Belt (Figure 12); and F3 folds are common and widely developed in the northern and western parts of the district. Structures attributed to the D3 deformation do not, however, affect rocks within the Flat Belt or Highland Border Steep Belt domains, although D3 structures have been recognised within the Flat Belt in the Loch Lomond and Ben Ledi areas (Mendum and Fettes, 1985). Generally, throughout the Southern Central Highlands, detailed correlation of D3 structures is difficult (Nell and Treagus, 1994) because the intensity and style of deformation varies greatly over a few kilometres.

F3 folds vary greatly in their style, orientation and amplitude within the Glen Shee district. Many are coaxial with F2, although they are easily distinguished by the fact that S2 is folded by F3 folds. In contrast with the majority of F2 folds, many F3 folds have no associated axial planar fabric. Where developed, S3 is generally a relatively widely spaced (5 to 10 mm) crenulation foliation that is nowhere associated with widespread transposition and obliteration of pre-D3 fabric elements.

The D3 structures considered here are probably not all strictly associated with the same deformation phase. Two stages of 133' folding have been identified around Beinn a' Chruachain [NO 044 694]. The relationship between these folds and the Tummel–Cairnwell Steep Belt is uncertain, so it is possible that three different stages of deformation have been grouped together here as 133'. The merits of this collective approach lie in the restricted geographical occurrence of some F3 fold sets, and the now deeply entrenched association of the Highland Border Downbend with the regional D4 event (Harris et al., 1976; Harte et al., 1984; Robertson, 1994). The D3 event of this account is thus viewed as comprising several sub-phases or stages of folding that were developed between the regional D2 and D4 events.

F3 fold style and S3 fabric

Small-scale F3 folds are common throughout the northern part of the district. Typically these form close to tight, asymmetrical upright folds. Many F3 folds have a Class 1C geometry (Ramsay, 1967), congruent with the suggestion that they have developed from 'flattened' buckle folds. Disharmonic folds in which wavelength and amplitude relationships reflect competent layer thickness variation and viscosity contrast (common in buckle folds) are widely developed. Thickened hinge zones are best developed in incompetent pelite and carbonate-rich lithologies, but are less pronounced in competent psammitic units.

F3 folds do not generally have a strong axial planar schistosity, although an axial planar crenulation fabric is widely developed in susceptible pelitic and semipelitic lithologies and in some schistose amphibolites. A notable exception is seen in the areas of Ben Earb [NO 08 69] and Creag an Dubh Shluic [NO 08 68], where steeply east-dipping S3 crenulation foliation is developed in pelites of the Ben Lawers Schist Formation. This fabric cannot always be reliably distinguished from the S2 crenulation fabrics of the Ben Lawers Schist Formation. The typical development of S3 comprises a widely spaced (5 mm or more) crenulation foliation that is far less intense than the S2 crenulations of SI. There is little evidence of extensive mica recystallisation associated with the D3 events, although limited recrystallisation of muscovite has occurred along some S3 crenulation planes; irregular aggregates of chlorite are common in crenulation hinge zones. Some garnets are mantled by chlorite; otherwise there is little evidence of the metamorphic response to D3 deformation.

The D3 event is considered to be responsible for the heterogeneous development of foliation within post-D2 amphibolites (Chapter 5). The foliation is generally sub-parallel to, and most intense at the margins of these bodies and is defined by fine-grained chlorite and actinolitic amphibole. However, locally developed foliation within some thick amphibolite sheets is highly oblique to the intrusive margins, as on An Lairig [NO 0887 6835]. Deformed post-D2 pegmatite sheets (Chapter 5) also have a foliation, attributed to D3, that is generally more intensely developed at their intrusive margins.

Reference has already been made to two stages of F3 folds that are present in the Beinn a' Chruachain area; these folds also occur within the Ben Lawers Schist Formation on Creag an Dubh Shluic [NO 08 68] and An Lairig [NO 09 68]. On Beinn a' Chruachain [NO 0423 6921] the limbs of upright north-plunging metre-scale 'F3a' folds show evidence of superimposed 'F3b' crenulations with east-west axial traces. Most of the later 'D3b' structures are reclined crenulations and folds up to several metres in wavelength with axial planes that dip northwards at moderate angles. Typically these folds are close structures that tend to symmetrical forms; they are widely developed and particularly prominent on the southern slopes of Beinn a' Chruachain where formation boundaries have been rotated towards the attitude of F3b axial planes. No new foliation has developed. F3b plunges are generally northwards, and down-dip on the F3b axial planes; this geometry largely reflects the superimposition of east-west trending F3b fold sets on previously steep north–southstriking surfaces.

On a somewhat larger scale, it is considered that the swing in strike that affects the boundary between the

Gleann Beag Schist and the Ben Lawers Schist formations near Daldhu [NO 022 715], may also be due to the D3b folding that resulted in a large reclined open fold pair on north-dipping axial planes.

Large- and medium-scale F3 folds

Most of the large-scale F3 structures on Sheet 56W occur in the northern and western parts of the district, where they typically form north-plunging inclined or upright folds with moderately or steeply dipping axial planes. These plunge beneath the large-scale, upright to inclined, moderately south-east-plunging F3 folds in the south-east corner of Sheet 64E and south-west corner of Sheet 65W.

Creag Dubh-Leitir Antiform

The hinge zone of a large-scale, gently north-plunging F3 antiform (synclinal) is situated in lower Gleann Fearnach (Figure 11), (Figure 17). This structure folds the Loch Tay Limestone Formation over Southern Highland Group rocks which occur within the antiformal hinge zone. This F3 fold is, therefore, partly responsible for the westward steepening of strata into the Tummel Steep Belt. A culmination is evident on this structure in the vicinity of Creag Dubh-leitir [NO 05 65] where F3 plunges become either horizontal or gently south plunging; similarly, on Kindrogan Hill, subsidiary F3 folds that are well exposed along the tracks [south of 045 620] have either subhorizontal or gentle south-plunging hinge lines.

On Whitefield Hill [NO 085 625] and Menachban [NO 085 640] other subsidiary F3 folds lying close to the F3 plunge culmination are defined by the sinuous outcrop pattern of Green Beds and an amphibolite sheet. The regional S2 foliation is folded by these tight upright folds which plunge gently towards either north to north-west or south to south-east. S3 planar fabrics are only rarely associated with these folds as south of Whitefield Hill [NO 0844 6177] where S3 is thought to be represented by a steeply dipping, poorly developed fracture cleavage.

The hinge zone and eastern limb of the large-scale F3 Creag Dubh-leitir Antiform is transected obliquely by the Gleann Fearnach Fault. The outcrop of the Farragon Volcanic Formation, also faulted in Gleann Fearnach around Creag Loisk [NO 04 68], can be mapped across Creagan Uaine [NO 05 68] and Elrig [NO 07 66] around a series of close to tight, west-verging upright F3 folds that plunge gently northwards on the eastern limb of the major antiform.

Sron Bhreacach Plunge Depression

Large-scale west-vergent and gently north-plunging F3 folds are considered to be responsible for the folded form of the outcrop pattern of the D2 Ben Earb Slide around Lairig Charnach [NO 06 70] and An Lairig [NO 09 68]. S2 and transposed S0 in exposures close to the slide trace dip steeply westwards on Sron Charnach [NO 0632 6977] and steeply north-eastwards along the Ben Earb–Creag Bhreac ridge [NO 07 68]; these divergent dips are considered to be representative of locations on opposed limbs of the F3 Lairig Charnach Antiform. The axial trace of this antiform extends towards Carn Tarmachain [NO 057 713], while a complementary synform farther west is responsible for the re-emergence of the Ben Earb Slide, here [NO 05 70] named the Carn Dearg Slide. This synform is a complex of F3 folds in which several changes of F3 vergence can be identified over approximately 100 m intervals along the prominent ridge of rocky knolls that extends from Sron Bhreacach [NO 061 707] towards Beinn a' Chruachain. On the north-west and south-east flanks of this ridge, the majority of F3 folds in the Ben Lawers Schist Formation are gently plunging, respectively either to the south-south-east or to the north-north-west towards the ridge axis. Although the small-scale folds in this area show considerable variation in plunge angle and azimuth, this ridge feature is broadly coincident with the large-scale F3 Sron Bhreacach plunge depression.

In the Beinn a' Chruachain Complex west of the Carn Dearg Slide, the majority of small-scale F3 folds plunge gently northwards and the Sron Bhreacach plunge depression is not identified. It is thus considered likely that the observed variation in F3 fold plunge reflects their superimposition on pre-S3 surfaces with a range of dips, and that the plunge depression coincides with a large-scale F1 or F2 closure within the Ben Lawers Schist Formation. North-plunging and south-east-plunging F3 folds within this structure have therefore developed on the respective north-dipping and south-east-dipping limbs of the main F1/F2 fold.

Glas Tulatchean Synform

The overall distribution of lithologies on Glas Tulaichean [NO 051 760] is largely the result of a major eastsouth-east-plunging F3 synform with superimposed north-north-east-trending folds of subsidiary importance. These later folds may belong to a late-D3 subset or be of possible D4 age. The hinge zone of the main F3 synform has been identified from vergence changes in F3 minor folds; its location coincides with exposures of Glen Lochsie Calcareous Schist Member on the rim of Glas Choire Bheag [NO 054 766]. From here the axial trace extends eastwards, folding the Baddoch Burn Slide in upper Gleann Taitneach [NO 068 765], then south-eastwards into Gleann Taitneach. Upright, gently north-east-plunging close (possibly F4) folds have reorientated small-scale F3 folds on Glas Tulaichean [NO 053 768]; these possible F4 folds, also seen on the north-east flank of Creag Bhreac [NO 074 743], are considered to be responsible for the reorientation of the axial trace of the F3 synform to its east–west trend north of Glas Choire Mhor [NO 06 76]. Other members of the possible F4 fold set occur in the Carn an t-Sionnaich area [NO 01 75], where folds with steeply south-east-dipping axial planes vary from close chevron-style crenulations to open folds with wavelengths of several metres. They plunge steeply north-eastwards since they are superimposed on steeply dipping S2, but otherwise appear to be similar in style and orientation to their along-strike possible D4 equivalents on Glas Tulaichean.

On the southern limb of the Glas Tulaichean Synform, reorientated small-scale F2 folds plunge steeply to the south-west. Close, small-scale F3 folds folds plunge east or slightly south of east at 30°, verge south or south-west and have axial planes which dip steeply to the south-west.

Creag Bhreac Antiform and Gleann Beag folds

The D3 Creag Bhreac Antiform plunges moderately south-east and is complementary to the Glas Tulaichean Synform. Its axial trace extends south-east from Creag Bhreac [NO 070 736] via Creag a' Chaise towards Glenlochsie Farm [NO 085 715] and possibly farther southeastwards towards the Spittal of Glenshee. It is recognised by both the strike of S2 and the distribution of lithostratigraphical units. S2 strikes north to the northeast of the axial trace in Glen Taitneach, whereas on the opposing limb in Glen Lochsie the strike is north-west.

A few small-scale F3 folds occur in lower Glen Lochsie [NO 08 71]; they are close to tight upright structures that are broadly co-axial with F2. However, many small-scale F2/F3 interference structures are present in the Gleann Beag Schist, exposed in the Glen Lochsie Burn [NO 065 725] and one of its tributary streams [NO 052 727]; in these areas the superimposed F3 folds vary greatly in plunge as they are traced across F2 fold closures.

Farther east in the area around Bad an Loin [NO 120 708], outcrop-scale D3 folds are open to tight structures, generally with steeply dipping axial planes and fold axes that plunge gently or moderately towards the north-east.

Meall Reamhar Synform

The synclinal Meall Reamhar Synform has a complex D1 to D3 history of development, and incorporates a major F1 fold referred to as the Gleann Fearnach Syncline (Figure 19), (Figure 20). The closure of the synform is located at the head of Gleann Fearnach (Figure 11). The Ben Lawers Schist Formation crops out in the fold core, enveloped by the Tulaichean Schist and older formations. The synform is upright to inclined, tight and plunges gently towards the south-east.

The folded trace of both the S2 foliation and the D2 high-strain zones (Figure 19) attests to F3 development of the synform. The existence of the pre-D3 Gleann Fearnach Syncline is demonstrated by the absence of overall change in F3 vergence patterns across the synclinal synform, shown in (Figure 19). Minor F3 folds verge south-west on both limbs of the syncline. Most of these minor F3 folds form close or tight upright structures that plunge at moderate inclinations to the north or north-east, reflecting their superimposition upon steeply north-east-dipping S2 surfaces. An F1 age is assigned to the Gleann Fearnach Syncline on the basis of F2 vergence and facing relationships (Figure 11), (Figure 16). On the south-west limb of the synform, the stratigraphy faces upwards to the south-east on the axial planes of southwest-vergent F2 folds, whereas on the north-east limb of the synform, south-west-vergent F2 folds in the Gleann Mor–Carn Dallaig areas face downwards to the west on the south side of Gleann Mor [NO 02 75]. These relationships are compatible with F3 reorientation about southeast-plunging axes of F2 folds, provided the F2 folds had consistent younging and facing prior to F3.

On the north-east limb of the Meall Reamhar Synform, the F2 folds in the Meall Ruigh Mor Thearlaich area [NO 05 71], that is above the Glen Lochsie–Carn Dallaig slides in (Figure 16), face upwards to the east or north-east, whereas F2 folds of the Beinn a' Chruachain Complex face up to the east or south-east. In the absence of F2 facing and vergence changes, it is considered that the opposed limbs of a large-scale F1 anticline, parasitic upon the major Gleann Fearnach Syncline, are juxtaposed along the Carn Dearg Slide. Although the evidence for this parasitic fold is based on mapping of poorly exposed ground between Carn Dearg [NO 04 71] and Beinn a' Chruachain, it concurs with the suggestion that the D2 Carn Dearg–Ben Earb Slide system has greatly modified the upper limb of a large F1 anticline.

The axial trace of the Gleann Fearnach Syncline is difficult to identify within the Ben Lawers Schist Formation outcrop, partly owing to insufficient exposure in Gleann Fearnach, but also because there is no way-up evidence that allows determination of the stratigraphical facing of minor folds within the Ben Lawers Schist Formation. Satisfactory representation of the highest structural levels represented on (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15) is also difficult. The section, looking down the F2 plunge, has been projected across the Sron Breacach F3 plunge depression, and grossly distorts the structures represented at this level. Thus, the Ben Earb Slide and Carn Dearg Slide occur, not as depicted in this diagram, but as equivalent along-strike structures on either side of the plunge depression. A regional D2 down-plunge projection (Figure 16), normal to the strike of the Carn Dallaig Slide, is also unsatisfactory because of distortions introduced by large-scale F3 folds in the Gleann Fearnach area. However, upon removal of the steepening associated with F3, the large-scale F2 folds of the Ben Lawers Schist Formation and Farragon Volcanic Formation on Creag Uisge [NO 02 69] face north-west, whereas the F2 folds on Beinn a' Chruachain face southeast. Because of their opposed facing, these latter F2 folds are considered to have been superimposed upon different limbs of the major F1 Gleann Fearnach Syncline. (Figure 20) offers a diagrammatic representation of (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15) with F3 unfolded. It illustrates the interpretation adopted here, and shows the D2 Carn Dallaig Slide and its associated flower structure as developed across subordinate possibly F1 folds on the upper limb of the F1 Gleann Fearnach Syncline, while the Creag Uisge Slide and folds are thought to occur on the lower limb of the syncline.

F3 Folds: Creag Nam Brataichean to Glen Isla

In the central and eastern parts of district, F3 folds are of lesser importance than in the adjacent areas to the north and west; however, they still affect the 1:50 000 scale mapped outcrop patterns. Amphibolite sheets within the Southern Highland Group around Clach Sgorach [NO 13 61], Craigies [NO 12 62] and Creag nam Brataichean [NO 11 61] are folded by north-east-vergent, subhorizontal or gently south-east-plunging F3 folds which have wavelengths of 100 to 300 m. These form asymmetric, close to tight folds, with gently north-east-dipping axial planes and with crenulation foliation locally developed in the hinge zones. It is commonly difficult to distinguish the tight F3 folds from F2 folds, since both may develop a crenulation foliation and many are coaxial, notably on the west side of Glen Shee north of Glenkilrie Lodge [NO 13 61] where the F3 folds are best exposed. Intersection between the topographical features, here comprising strike-parallel ridges and hollows, and these subhorizontal plunging folds has resulted in the distinctive outcrop pattern of the amphibolite sheets in this area.

North of Dalvanie [NO 186 661] in Glen Isla, abundant small-scale F3 folds, which fold S2 throughout the Duchray Hill Gneiss Member, are coaxial with F2. Typically, these F3 folds form asymmetrical, open to tight structures that range in wavelength from less than 10 mm to about 1 m; a millimetre-spaced crenulation cleavage is developed in the hinge zones of some folds. They are parasitic on medium-scale, south-east-verging F3 folds which have north-west-dipping axial planes and gently north-east- or south-east-plunging fold axes. The larger scale F3 folds are particularly clear at two localities, east of Duke's Lair [NO 1810 6808] to [NO 1794 6800] and southeast of the summit of Creagan Caise [NO 1833 6884]. F3 folds are also reflected in the folded outcrop at 1:50 000 scale of amphibolite sheets and the Fergus Slide in the Glen Beanie to Glen Isla area.

D4 Deformation phase

Highland Border Downbend

The main manifestation of the D4 event is a major northeast-trending antiformal fold, the Highland Border Downbend, which affects rocks of the Southern Highland Group. Gently or moderately (20° to 35°) north-west-dipping S1, S2 and mostly inverted bedding surfaces within the Flat Belt are folded by the downbend into the steep (70 to 90°) north-west dips and generally right way-up succession typical of the Highland Border Steep Belt (Figure 12), (Figure 13). The axial plane of the downbend strikes approximately parallel to the Middleton Muir Fault and dips 50 to 60° north-west; the hinge zone is marked by both the change from gentle to steep regional dips, and by changes in vergence of numerous small-scale F4 folds. The F4 Highland Border Downbend has rotated gently upward facing F1 and F2 folds of the Flat Belt into steep downward-facing attitudes within the Highland Border Steep Belt.

Geometrically, the downbend hinge zone comprises a broad closure, over several hundred metres across. Measurement of So, S1 and S2 within a 2 to 3 km-wide zone north-west of the axial trace of the downbend such as on Benachally [NO 06 49], indicates that the downbend is a close fold with an interlimb angle of 40 to 70° (Figure 13). This is tighter than previously documented elsewhere (Harris et al., 1976; Bradbury et al., 1979; Harte et al., 1984), where the Highland Border Downbend is generally depicted as a 90° fold, with a horizontal north-west limb.

The axial trace has been mapped across the district from East Point in the south-west [NO 022 442], and northeast across Craig More [NO 042 462], where its position is very well constrained by good exposure. The axial trace is also accurately placed in the Buckny Burn [NO 0640 4698]. Several offsets of the axial trace are the result of later north-west-trending faults (Figure 11).

Several smaller monoformal fold pairs that on the basis of their style, orientation and vergence are considered to be coeval with the Highland Border Downbend occur at some distance from the major F4 axial trace. For example, one of these monoformal fold pairs with a wavelength of approximately 150 m occurs at Knock of Findowie [NO 039 468], some 800 m north-west of the axial trace of the downbend. S1milar scale minor southvergent monoformal folds near Dalnabreck [NO 088 550], have steep and flat limbs which step down to the southeast.

On a regional scale the Highland Border Downbend is a major fold which can be mapped across Scotland from Rothesay on the west coast to Stonehaven on the east coast. Its axial trace is approximately parallel to the outcrop of the Highland Boundary Fault; these two features lie between 2 and 10 km apart for a distance of over 200 km. Both structures produce several kilometres of downthrow to the south-east; it thus seems likely that the Highland Border Downbend and the Highland Boundary Fault express responses to movement on the same steep structure, deep within the basement. The Highland Border Downbend may represent an earlier ductile phase of activity, while later reactivation under brittle conditions gave rise to the Highland Boundary Fault (Harte et al., 1984).

Minor structures associated with the Highland Border Downbend

Minor F4 folds are common in a 1 to 2 km-wide zone that straddles the downbend hinge zone; these folds become progressively more abundant towards the axial trace itself. The incidence of minor F4 folding dies away rapidly beyond approximately 1 km north-west of the axial trace, although narrow, discrete zones of D4-steepened earlier structures and small-scale D4 folds occur farther northwest, as on Benachally [NO 0658 4890].

The geometry of minor F4 folds mimics the large-scale structure; they have a gentle plunge, approximately 10° to 15° towards the north-east, with axial planes dipping north-west at approximately 45°. Small-scale F2 folds and L2 rodding lineations are refolded and reorientated by F4 folds; this is well displayed on the Hill of Gaily [NO 1299 5189] where north-west-vergent F4 folds, with wavelengths of several metres, have rotated north-west-plunging L2 rodding lineations into south-west-plunging attitudes on their steep south-east limbs In psammitic rocks, the minor F4 folds have parallel fold-style with rounded hinge zones; wavelengths of 10 to 200 cm are typical, and interlimb angles vary between 70° and 120°. Small-scale F4 hinge zones are more angular in semipelitic rocks; interlimb angles of approximately 60° to 80° are somewhat tighter, and wavelengths of 1 to 40 cm are shorter than in the psammites (Plate 16). A 1 to 3 mm-spaced crenulation cleavage is locally developed in the hinge zones of some F4 folds, as at Butterstone House [NO 0595 4629], Mill Dam [NO 030 464] and Steps of Cally [NO 1299 5189]; this is commonly intensely developed in semipelites as at [NO 0749 4865]. S4 has also developed as a 5 to 10 cm-spaced fracture cleavage in some psammites, while an extensional crenulation cleavage that deforms S2 micaceous felts occurs on the limbs of small-scale folds.

D4 structures north of the Highland Border Downbend

Some folds that affect Dalradian rocks north of the Highland Border Downbend may have developed during the D4 phase of folding, but cannot be directly associated with the downbend itself. These structures include kilometre-wavelength broad open warps and more restricted domains of close F4 folding.

The broad, open structures have exerted an important control on the outcrop patterns in the Flat Belt. The wide area of outcrop of the Green Beds south of Kirkmichael occupies a shallow S2 plunge depression (Figure 11) that forms the hinge zone of an open north-east-trending D4 synform in the.Milton Knowe–Kirkmichael area. The same synform is also partly responsible for the swing in strike and the wide outcrop of Loch Tay Limestone Formation around Milton Knowe [NO 10 60]. Other northeast-trending, open folds of the outcrop of Green Beds between Cnoc an Daimh [NO 10 62] and Creag nam Brataichean [NO 11 61] are subsidiary to this large synform.

On Creagan Beag [NO 114 607], a localised domain of close, inclined F4 folds has steepened older structures considerably. S1 and S2 dips are steepened to over 40° and in some places 70°, on the limbs of these folds. The outcrop pattern of the Green Beds and psammites of the Mount Blair Psammite and Semipelite Formation on Creagan Beag is the result of an approximately 200 m-wavelength F4 antiform–synform pair that intersects this small but pronounced hill. Just north of Creagan Beag, [NO 1110 6111]; [NO 1116 6112], D4 has produced a differentiated layering as an axial planar fabric; interference of D4 with earlier folds can be seen at these localities. Such fabric development is rather rare, but the schistose psammite at these localities appears to be especially susceptible to development of this axial-planar fabric. This localised S4 fabric development and the tighter geometry of the F4 folding in the Creagan Beag area are thought to reflect its location close to the hinge line of the major F4 Milton Knowe–Kirkmichael fold.

Another example of a broad open north-east-trending synform occurs near the Glen Shee Pluton. This synform causes a change in the orientation of S1, S2 and the axial planes of the F3 folds from east dipping in the Ben Lawers Schist Formation to north-west dipping in the Ben Lui Schist Formation on the opposed limbs of the synform. The Ben Earb Slide is also folded by this synform (Figure 17). On Craig of Runavey [NO 1263 6955] and Creag na Bruaich [NO 1472 6715], minor upright possible F4 folds have open buckle-type geometry and no associated fabric development.

Farther west, open folding that effects a swing in the strike of S2 in lower Glen Loch [NN 99 71] may be part of the D4 event. In this area the dip-azimuth of moderately inclined S2 changes from north-east to south-west, respectively from north to south across the Allt Glen Loch. No minor folds associated with this structure have been recognised. The axial trace of this open fold extends east–west, and becomes coaxial with one of the large reclined folds in Gleann Fearnach [NO 02 71] that were designated 'Dab' in the section describing D3 structures. Generally, the Dab folds have been distinguished from the D4 folds on the basis of the difference in strike of their axial planes. However, given the absence of direct evidence of the age relationships between Fab and F4 folds, the late-D3 and D4 folds may be coeval.

Synthesis of structural history

The deformational history of the Dalradian rocks in the Glen Shee district has been interpreted in terms of four main episodes of ductile deformation (D1 to D4). Large-and small-scale fold structures were developed during each of these events, but the metamorphic recrystallisation that accompanied the D3 and D4 events was much less intense, and at lower grades than that associated with the syn- to post-D2 metamorphic 'climax'. Thus, over most of the district, S2 forms the main regional foliation or schistosity. Most of the large-scale F3 and F4 folds are gently plunging inclined or upright structures that fold S2 and pre-S2 surfaces. Since there is no evidence of post-D4 rotation, it is considered that D3 and D4 structures are mainly responsible for the steepening of originally flat-lying S2 and recumbent F2 folds into their present attitude.

Figure 15 illustrates a section looking down the regional F2 plunge of the Glen Shee district and adjacent areas viewed looking north-east, while (Figure 20) shows a schematic version of this section with the effects of the D3 and D4 deformations removed. Several F1 folds can be positively identified either from changes in stratigraphical facing on S2, or by the observation of consistently north-west-verging F2 folds across observed younging reversals. These F1 folds now have a horizontal spacing of 5 km or less, and probably developed as relatively upright structures. Metamorphism associated with the D1 event was probably in greenschist facies and produced S1 foliations similar to those in the Dunkeld area of the Highland Border Steep Belt (Harris, 1972).

Over much of the district, the D2 folds and fabrics are considered to have developed in a regime that involved a strong simple shear component of deformation; this shearing was subhorizontally directed with a top to the south-east sense of shear. The F2 shear folds produced are predominantly north-west-vergent, and inclined/upright

F1 folds were rotated into a recumbent position (Figure 20) by the D2 shearing. Most D1 folds thus face up to the south-east, on axial planes that dip more steeply to the north-west than the presumed horizontal attitude of S2 surfaces. Facing changes on S2 occur across the F1 fold axial planes. Most of the Dalradian rocks within the district lie on the inverted common limb of two major F1 folds, the Cairnwell Anticline (Upton, 1986) and the Gleann Fearnach Syncline (Figure 20).

D2 slide zones have developed on the long limbs of many of the F2 folds; most of the main D2 slides also coincide with incompetent graphitic schist and calcareous units on F1 fold limbs, as is the case with the Ben Earb Slide and the Baddoch Burn Slide. Owing to inadequate exposure it is not always possible to determine whether the slides have caused excision or merely extreme attenuation of stratigraphical units. However, considerable stratigraphical excision is associated with two major D2 slides, the Creag Uisge Slide and the Baddoch Burn Slide of the Killiecrankie Slide–Baddoch Burn Slide systems. These slides have developed on respective lower and upper limbs of the Gleann Fearnach Syncline, and are linked via a complex transfer zone, the Carn Dallaig Slide. Along much of their outcrop, the Killicrankie and Baddoch Burn slides are parallel to stratigraphical units, but a complex hanging wall structure has developed where the Carn Dallaig transfer system transects the hinge zone of the Gleann Fearnach Syncline (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15), (Figure 20).

The Carn Dallaig Slide and related structures in the transfer zone strike at right angles to the regional trend (Figure 11), (Figure 12) and are thought to have developed in response to lateral differential movement during the D2 shearing event. They are, thus, analogous to lateral ramps in thrust systems (McClay, 1992). The north-west-striking transfer zone was not developed by post-F2 rotation as F2 folds within the transfer system still plunge south-east, normal to the regional plunge, after the post-

F2 folds in this area have been unrolled. The D2 structure of the Beinn a' Chruachain Complex resembles that of an anticlinal stack, and is analogous to the positive flower structures seen in transpressive fault zones (McClay, 1992). It therefore seems likely that movements along the Carn Dallaig transfer zone involved dextral transpressive rather than purely strike-slip elements. It also seems reasonable to suggest that the relatively large mass of unstratified competent rocks represented by the Ben Vuirich Granite may have functioned as a rheological obstacle, and thus promoted differential movement along strike.

A conspicuous change in the facing of F2 folds occurs across the hinge zone of the F1 Gleann Fearnach Syncline, and it is largely on the basis of these changes that the existence of this syncline was recognised. On the lower limb of the Gleann Feamach Syncline, structurally below the Killiecrankie–Carn Dallaig–Baddoch Burn slide system, the F2 folds face westwards. Thus in the Gleann Mor [NO 01 76] –Carn an Righ [NO 02 77] area the F2 fold structures verge south-westwards and appear to be downward facing on S2; this impression is reinforced by the evidence of south-west younging of formations on Can Dallaig [NO 01 74] and grading observed nearby [NO 0246 7513] (Figure 16). The west-facing pattern is probably maintained as far as the large closure of Beinn a' Ghlo Quartzite (= An Socach Quartzite Formation) on the south-west flanks of Beinn a' Ghlo [around NN 95 71]; this major possibly F2/F3 anticline (Bradbury et al., 1979) plunges and closes south-westwards, and is, thus, upward facing on the western limb of the Meall Reamhar Synform.

In the Gleann Mor area the vergence of the large-scale F2 folds (Figure 16) is opposed to the typically northwest-vergent F2 folds on the upper limb of the Gleann Fearnach Syncline. Owing to inadequate exposures in Gleann Mor, the geometry of these F2 folds has been determined by mapping of formation boundaries, rather than by systematic mapping of F2 minor structures. However, it is noted that the D2 slide, which affects the Allt Coire an t-Sneachda [NO 02 76] fold pair, is anomalous and has developed on the short common fold limb, rather than on the longer more attenuated limbs. The sense of shear displacement on this short common limb is congruent with that interpreted for the majority of F2 folds on the upper limb of the Gleann Fearnach Syncline, so it is tentatively suggested that a consistent sense of D2 simple shear has affected both limbs of this syncline, and that the south-west-vergent F2 folds in Gleann Mor may represent D2-modified F1 folds parasitic on the lower limb of the Gleann Fearnach Syncline.

The interpretation of the facing change that occurs along the Killiecrankie–Carn Dallaig–Baddoch Burn slide system as being developed on upper and lower limbs of the Gleann Fearnach Syncline, has also led to a reinterpretation of the Devil's Elbow Synform (D3 according to Upton, 1986). Upton (1986) explained the changes from downward-facing to upward-facing F2 folds as being due to their rotation about the hinge of the D3 Devil's Elbow fold. His interpretation is not accepted here firstly because the predicted folding of the axial plane of the F1 Cairnwell Anticline about the Devil's Elbow fold does not occur in the area that lies south-west of the limits of Upton's (1986) mapping; secondly his section (Upton, 1986, fig. 3, p.17) implies the reappearance of older formations on the south-east limb of the Devil's Elbow fold, rather than the south-east-younging sequence that has been mapped. In the alternative interpretation adopted here, the north-west and south-east limbs of the Devil's Elbow fold are considered to represent respective lower and upper limbs of the D2-sheared F1 Cairnwell Anticline

Overall, the interpretation of D2 deformation in terms of a regime that was dominated by subhorizontal southeast-directed simple shear (Figure 20) is considered to provide a satisfactory model for the Dalradian rocks of the district. This model also appears to have a wider relevance, as it is consistent with the interpretation of D2 by Harris et al. (1976) and the D1/D2 relationships envisaged by Treagus (1987, 2000). However, it should be noted that, although the D2 deformation was dominated by simple shear, there is ample evidence of domains in which irrotational strain components are considered to be predominant. Much of this evidence is provided by the buckle folds that are developed on the short limbs of F2 folds, quartz c-axis analysis (Krabbendam and Leslie, 1996) and rare south-east-vergent F2 folds (Treagus, 1987).

The recognition within the district of a D2 strain regime that was dominated by subhorizontal south-east-directed simple shear has important implications for interpretation of the geometry of the Tay Nappe Complex. Although several large-scale D1 folds have been recognised in the Glen Shee district, none of these (either collectively or individually) represents the closure of a single large D1 nappe as proposed by earlier workers (Roberts, 1974). The same is true of the Aberfoyle–Ben Ledi–Ben Vane system of F1 folds (Mendum and Fettes, 1985) which form part of a whole series of F1 folds within the Tay Nappe complex. In the interpretation adopted here, the lower limb of the Tay Nappe developed as a D2 structure, and subhorizontal D2 simple shear rotated originally more upright F1 folds towards recumbent attitudes. This is, thus, in agreement with the view that the Tay Nappe, or the regional inversion attributed to it, is essentially a result of D2 shear (Harris et al., 1976), but is not a D1 structure (Mendum and Fettes, 1985). Consequently, there is no need to look for a root zone to the Tay Nappe (Thomas, 1979; Shackleton, 1979).

Section B of (Figure 21) illustrates the effects of a horizontally directed simple shear deformation (ψ = 80°) applied to Section A of the same diagram. These diagrams should also be compared with (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15) and (Figure 20), so that the essential similarities between the modelled and observed F1 structures may be assessed. One particularly striking feature of the model is the tight gently inclined Mount Blair Anticline that is developed after shearing a relatively open upright fold. Shear strain values of 5 to 10 are considered reasonable for modelling purposes, based on a strain analysis of Southern Highland Group rocks within the Flat Belt (Krabbendam et al., 1997); so the modelled ψ = 80° which gives a shear strain of approximately 6 falls at the lower conservative end of measured shear strains.

The D3 event produced both large- and small-scale folds and may have been a poly-stage event. The majority of F3 folds affect Dalradian rocks in ground that lies north of the Flat Belt. In the Gleann Fearnach–Gleann Taitneach area, that is within and adjacent to the Carn Dallaig Transfer Zone, the F3 folds are mainly upright structures with north or north-west-striking axial planes; whereas in Glen Isla and Glen Shee, the F3 folds have more gently inclined north-east-striking and generally north-west-dipping axial planes. Other structures that are probably attributable to the D3 event include the Cairnwell and Tummel steep belts which have a regional north-east strike. It thus appears that the D3 event involved more localised and differentiated responses to imposed stresses than either of the two preceding deformational events. The present distribution of D3 structures also appears to be related to, and probably therefore guided by, the pre-D3 structural geometry. These features, thus, further enhance the impression of the domainal nature of the D3 events within the district.

Speculative consideration of the broader significance of the D4 event has already been presented. Within the Glen Shee district, the major D4 structures are the Highland Border Downbend and the Kirkmichael–Milton Knowe plunge depression.

The current interpretation of the geochronological framework for the structural development of the Da'radian rocks, is summarised in (Table 6). It is now generally accepted that the Ben Vuirich Granite is wholly preorogenic and probably rift-related and therefore its age does not provide a direct constraint on the timing of the Grampian orogeny (e.g. Soper et al., 1999). The onset of the Grampian orogeny is now regarded as better constrained by the age of the youngest Da'radian sediments: the Lower Carnbrian Leny Limestone (Cowie et al., 1972; Tanner, 1995; Tanner and Pringle, 1999) and the Lower Ordovician Macduff Slate. The fact that deposition of the Carnbro-Ordovician Durness succession in north-west Scotland ceased during the mid-Ordovician (Arenig–Llanvirn) supports a wholly Ordovician age for the Grampian orogeny. The D2 deformation phase is thought to have occurred prior to the intrusion of the Younger Basics, which intruded at 468 ± 8 Ma (Rogers et al., 1996), whereas D3 is thought to be broadly coincident with the intrusion of the Aberdeen Granite at 470 ± 1 Ma (Kneller and Aftallion, 1987). Robertson (1994) concluded that D2 and D3 were closely related in time. The main regional metamorphism started before or during the early stages of D2, but continued throughout the D2 event. The main migmatisation event of the Duchray Hill Gneiss Member was probably late D2, but definitely pre-D3 (Chapter 7). Metamorphic cooling ages at about 460 Ma (Dempster, 1985) and the intrusion of the post-tectonic Kennethmont Granite at 458 ± 1 Ma (Oliver et al., 1998) heralded the end of the Grampian orogeny in Scotland, although the D4 deformation phase may be related to a stage of uplift around that time. The Grampian orogeny was therefore a fairly short-lived event, with the D1 and D3 deformation phases and their related metamorphism following each other in quick succession (e.g. Robertson, 1994).

There timing constraints are consistent with the timing of the Grampian orogeny in Ireland, which has recently been very tightly constrained to between 470 and 460 Ma (Soper et al., 1999; Friedrich et al., 1999; Dewey and Mange, 2000).

Chapter 7 Regional metamorphism of Precambrian rocks

Regional metamorphism of Precambrian rocks in the Glen Shee district occurred during the Grampian orogeny. Barrovian chlorite to kyanite zones are represented; these zones are well defined in the east of the district but become progressively broader and less well defined farther to the west (Figure 22) with increasing distance from the type Barrovian area (Barrow, 1893) in the Glen Clova to Glen Esk area. In the highest grade areas in the north-east of the district, at least two periods of migmatisation accompanied metamorphism. Metamorphic zone index minerals are mostly restricted to aluminous pelitic or semipelitic rocks, which are found in the Tulaichean Schist and Ben Lui Schist formations and throughout the Southern Highland Group. Metamorphic grade has also been determined from amphibolite mineral assemblages, although this is complicated by the occurrence of two suites of amphibolites (Chapter 5) which were emplaced at different stages in the metamorphic history of the district. Qualitative observations of metamorphic grade based on mineral assemblages are supported by thermobarometric calculations of peak pressure and temperature. Metamorphic mineral assemblages in each Barrovian zone and migmatisation are described for psammitic, semipelitic and pelitic rocks; mineral assemblages in metamafic and calcareous rocks are described separately. Estimates of metamorphic temperatures and pressures have been calculated from analyses of mineral chemistry. The following abbreviations are used in this chapter: Chi = chlorite, Ms = muscovite, Bt = biotite, Qtz = quartz, Grt = garnet, Kfs = K-feldspar, Prg = paragonite, Ab = Albite, Ctd = chloritoid, St = staurolite, Ky = kyanite.

Metamorphism of pelites, semipelites and psammites

Chlorite zone

Chlorite zone rocks are only recognised in the River Ericht south-west of Milton of Drimmie [NO 162 511]. In this area, gritty psammites are composed of quartz, perthitic K-feldspar and plagioclase clasts within a fine-grained matrix of similar minerals together with chlorite, white mica and opaque minerals.

Biotite zone

The biotite zone extends for some 2.5 km north of the Middleton Muir Fault but is restricted to the area south of the Highland Border Downbend (Figure 22). The typical mineral assemblage is: quartz + muscovite + chlorite + biotite + feldspar ± calcite together with accessory opaque minerals, apatite, zircon and tourmaline. Chlorite porphyroblasts are developed in places (S95775). Rocks showing the first appearance of biotite occur in the east of the district close to the southern limit of outcrop of Dalradian rocks (e.g.(S96496). Biotite first develops within chlorite porphyroblasts or within matrix chlorite and white mica as irregular, poorly formed flakes. Pelitic rocks are mostly fine-grained phyllites and schists, with muscovite and chlorite more abundant than biotite.

In the south of the biotite zone, S1, where unmodified by S2 (Chapter 6), is generally a well-developed slaty cleavage containing muscovite, biotite and chlorite. S2 varies from a crenulation cleavage to a penetrative schistosity depending on the intensity of F2 folding. S2 is also defined by the preferred orientation of muscovite, chlorite and biotite (Chapter 6).

Garnet zone

The first appearance of pinhead garnets in semipelitic and pelitic rocks is only 2.5 km north-west of the Middleton Muir Fault. A kilometre farther north-west, garnets are common even in psammitic lithologies. The garnet zone ranges from less than 2 km wide in the east of the district to about 5 km wide in the west.

A typical mineral assemblage in pelitic rocks is: quartz + plagioclase + muscovite + biotite + chlorite + garnet, with accessory opaque minerals, apatite, tourmaline and rutile. Quartz and feldspar may retain a detrital form as scattered small pebbles; no primary calcite is present. The abundance of chlorite decreases across the garnet zone from south to north, with a concomitant increase in biotite, suggesting a reaction such as:

Msl + Chl → Bt + Ms2 + Qtz + H2O Reaction 1

though garnet-forming reactions such as reactions 2 and 3 will also consume chlorite (Yardley, 1989, p.65; Winkler, 1979, p.220).

Chl + Ms → Grt + Bt + Qtz + H2O Reaction 2

Chl + Bt1 + Qtz → Grt + Bt2 + H2O Reaction 3

Biotite porphyroblasts are not common in the garnet zone; there is no direct replacement of biotite porphyroblasts by garnet. Garnets tend to be small and rather ragged, with poorly developed inclusion trails composed mainly of quartz and opaque oxides. The inclusion trails are straight to slightly sigmoidal, and continuous with the external foliation. The garnets thus grew over the S1 slaty cleavage prior to extensive F2 crenulation, and are wrapped by the S2 foliation, indicating post-D1 and pre-to early D2 growth.

Psammites in the garnet zone contain detrital quartz, plagioclase and alkali feldspar grains. Muscovite, chlorite and biotite with strong preferred orientation are concentrated in micaceous laminae which represent the spaced S1 fabric. The quartz and feldspar clasts commonly have an elongate shape fabric parallel to the S1 foliation. The presence of detrital alkali feldspar in such rocks allows the development of biotite by reactions such as:

Kfs + Chl → Bt + Ms + Qtz + H2O Reaction 4.

S2 is generally a crenulation cleavage with symmetry and spacing of the cleavage planes dependent on the degree of strain imposed (Chapter 6). Garnets in psammites appear as pinhead grains concentrated in the S1 micaceous domains. They lack clear inclusion trails; however, their distribution in S1 laminae argues for post-SI development.

Chloritoid-bearing assemblages

In aluminous pelites, chloritoid occurs in the biotite and garnet zones (Yardley, 1989, p.67) and developed by reactions such as:

Prg + Chl + Qtz → Ab + Ctd + H2O Reaction 5.

However, its occurrence is strongly compositionally controlled and in the Scottish Dalradian suitable compositions occur only rarely. Chloritoid occurs in aluminous rocks with compositions that plot above chlorite compositions on Thompson's AFM projection (Thompson, 1976; (Figure 23)) in assemblages such as: chloritoid + chlorite + muscovite + plagioclase + quartz in the biotite zone. Near Forneth [NO 0917 4586]; [NO 0969 4561], pale grey slates with abundant chloritoid porphyroblasts (Chapter 4) occur in the Birnam Slate and Grit Formation. These rocks have the assemblage: quartz + muscovite + chlorite + opaque minerals + chloritoid. The chloritoid porphyroblasts have overgrown the S1 schistosity, but have been deformed during D4.

In the Barrovian garnet zone, the occurrence of chloritoid can be predicted on an AFM projection in aluminous rocks with compositions that plot above the garnet–chlorite tie line. This indicates that chloritoid and biotite should not occur together, and the typical assemblage will be: chloritoid + chlorite + muscovite + plagioclase + quartz. The association of chloritoid and biotite occurs commonly at lower pressure than the Barrovian zonal sequence, as in the Buchan area (Leslie, 1988), and its occurrence within the Dalradian near Stonehaven (Chinner, 1967) has been used to argue for a low pressure episode in the metamorphic history of that area (Harte, 1975). There are, however, isolated occurrences elsewhere in the Dalradian where biotite and chloritoid occur together within the garnet zone (Atherton and Smith, 1979). Chloritoid and garnet are apparently coeval and syn- to post-S2, and there is no evidence for low-pressure metamorphism in the prograde sequence.

In garnet zone pelites in the Loch Ordie area [NO 03 50], there are several occurrences of dark biotite pelites with retrogressed chloritoid porphyroblasts, north of the loch [NO 0287 5101] to [NO 0262 5085]; south of the loch chloritoid occurs as fresh porphyroblasts up to 4 mm in length [NO 0362 4919] (Plate 14)g. The restricted occurrence of chloritoid also argues for compositional control; it is possible that these rocks represent the repetition of the chloritoid-bearing schists of Forneth to the north of the Highland Border Downbend. Fresh chloritoid porphyroblasts occur in a matrix of very fine-grained muscovite and biotite with some fine-grained quartz, and large (3 to 4 mm) cracked garnets; accessory minerals comprise opaque oxides, rutile and tourmaline. Muscovite and biotite occur within both a strong S1 foliation and a poor S2 crenulation cleavage associated with F2 microfolds. The chloritoid porphyroblasts occur in random arrangement with respect to the foliation, and neither the chloritoid nor the garnet porphyroblasts have clear inclusion trails. Both, however, contain inclusions of biotite and rutile, possibly indicating that they grew as part of the same metamorphic episode; by analogy with garnets in other samples this could span the D2 deformation episode. There is no doubt that the chloritoid is part of the prograde assemblage, nor that chloritoid and biotite are stable together, as indicated by the included biotite (Plate 14)g. Chlorite is, however, present only as a retrogressive phase. The composition of the rock may therefore fall within the chloritoid-garnet-biotite field on the AFM diagram (Figure 23), chloritoid being stabilised by the high iron content, rather than a high aluminium content which would allow only the chloritoid + garnet + chlorite assemblage.

Chloritoid has been found at only one location north of these localities, in the staurolite zone 1.5 km north-east of the Corb [NO 171 582]. Within the staurolite zone, chloritoid should be consumed by reactions such as:

Ctd + Qtz → St + Grt + H2O Reaction 6.

Staurolite zone

Staurolite porphyroblasts appear quite abruptly in pelites and semipelites north of a line from Loch Ordie [NO 03 50] to Loch Shandra [NO 21 62]. South of this line no staurolite is present, whereas to the north the porphyroblasts are large and well formed. This is perhaps the clearest example of an isograd in the district, and represents the formation of staurolite by reactions such as:

Chl + Ms → St + Bt + Qtz + H2O Reaction 7

and

Chl + Ms + Grt → St + Bt + Qtz + H2O Reaction 8.

Chlorite and, to a lesser extent, muscovite are less abundant as prograde minerals in pelitic rocks of staurolite grade, but there is little evidence for the consumption of garnet in staurolite-bearing rocks, suggesting that reaction 7 may be more common than reaction 8. The typical pelite assemblage in the staurolite zone is: quartz + muscovite + biotite + feldspar + garnet + staurolite, with accessory opaque minerals, tourmaline, rutile and zircon. The muscovitebiotite-quartz matrix of these rocks is somewhat coarser grained than that of lower grade rocks, and quartz and feldspar no longer form recognisably detrital grains. Structurally, the staurolite zone occurs in the Flat Belt which is dominated by intense F2 folding (Chapter 6), and while earlier fabric elements are still visible in psammitic and semipelitic lithologies, at outcrop the pelites show only the S2 schistosity. This may be a penetrative schistosity, with S1 represented only by early quartz veinlets and porphyroblast inclusion trails. However, in some thin sections, S2 is an intense crenulation cleavage developed on a finely spaced S1. The crenulation is symmetrical in sections from F2 fold hinges; in sections from fold limbs there is marked asymmetry, and the S2 foliation appears more as a shear or S/C type fabric (Chapter 6). The S1 and S2 foliations are defined by strong preferred orientation of biotite and muscovite, with biotite being more abundant along the S2 schistosity. Biotite also occurs as porphyroblasts, which are folded by F2 microfolds, thus indicating pre-D2 growth. Garnet is present as large euhedral porphyroblasts, which commonly contain inclusion trails of quartz, opaque minerals and rutile, which represent the S1 schistosity. These trails may be straight, though they are more commonly sigmoidal, and garnets with crenulated inclusion trails are not uncommon at this grade; there are even examples of garnets which have overgrown the S2 crenulation cleavage (Plate 14)f. The garnet growth thus postdates D1 and occurred before, during and after D2. It, therefore, continued later with respect to the structural chronology in staurolite zone rocks compared with lower grade zones. There is also some suggestion that the more complex inclusion trails occur in garnets in assemblages containing staurolite, which suggests later initiation of garnet growth in such rocks, or possible reaction of early garnet during staurolite formation (reaction 8), followed by formation of new garnet during D2. Inclusion trails in staurolite are not generally well developed, but where visible they tend to be crenulated and comprise quartz, opaque minerals and rutile, indicating syn-D2 growth of staurolite. In many cases the external S2 foliation is approximately axial planar to the microfolds preserved in S1 in the garnet and staurolite inclusion trails. The internal crenulations may be oblique to external S2, however, and the degree of obliquity may vary between porphyroblasts in the same section, although the sense of asymmetry is the same. Such cases are found on F2 fold limbs, and the inclusion trails presumably represent the original orientation of S2, with apparent relative rotation of the external S2 foliation caused by increasing tightening and development of fold structures. Implicit in this model is a component of extension along the S2 foliation, as well as flattening across it. This is supported by the S/C appearance of the S1/S2 foliations, and by the micro-boudinage of elongate minerals lying in S2, for example tourmaline, rutile or staurolite (Plate 14)h.

Psammites which are dominated by quartz and plagioclase do not contain staurolite at this grade, though small garnets may be relatively abundant. A spaced S1 fabric is commonly preserved, with micaceous domains containing biotite and muscovite, and very little prograde chlorite. A shape fabric parallel to the S1 spaced cleavage may be visible in quartzofeldspathic domains, even where the rocks have been affected by F2 folding. S2 in psammites is defined by the orientation of mica laths which lie axial planar to F2 microfolds, in both micaceous and quartzofeldspathic domains; the S2 fabric is a very subtle fabric in outcrop in these rocks. Coarse-grained gritty psammites, however, commonly show a good S2 shape fabric in elongate quartz and feldspar clasts. Quartz clasts show strain bands and recrystallisation, whereas feldspars have undergone strain twinning and polygonisation. The lack of any sign of an earlier S1 fabric suggests that plastic deformation undergone by the clasts during D1 was largely recoverable during D2.

Kyanite zone

The kyanite zone is well defined in the east of the district, east of Glen Isla where kyanite occurs in Southern Highland Group rocks up to 2 km south-east of the Glen Doll Fault as well as in the Southern Highland Group and Duchray Hill Gneiss Member north-west of the fault (Figure 22). Farther west the kyanite zone is poorly defined and restricted to rocks of suitable composition, mostly within the Duchray Hill Gneiss Member and some semipelitic and pelitic units within the Southern Highland Group (Figure 22). Rare kyanite also occurs in the Glen Taitneach Schist Member on Glas Choire Mhor [NO 0690 7637] and the Ben Eagach Schist Formation on Beinn a'Chruachain [NO 0394 6942]. Published reactions for the generation of aluminosilicate minerals (kyanite or sillimanite) involve the consumption of staurolite by reactions such as:

St + Ms + Qtz → Ky + Bt H2O Reaction 9

(Winkler, 1979, p.228; Yardley, 1989, p.68). In the Tulaichean Schist Formation, where no aluminosilicates are present, staurolite was probably consumed by reactions such as:

St + Ms1 → Ms2 + Bt + Qtz + H2O Reaction 10

or

St + Ms1 → Ms2 + Bt + Qtz + Grt + H2O Reaction 11

Rocks of the kyanite zone in this district only rarely retain evidence of the D1 deformation event. They are dominated by D2 structures, and may also show moderately intense D3 deformation. Local D4 folds are uncommon (Chapter 6).

Rocks containing staurolite and kyanite are restricted to a few localities in the Southern Highland Group outcrop, where suitable compositions and metamorphic conditions occur. The rocks are muscovite-biotite semipelites and pelites, with the assemblage: quartz + biotite + muscovite + plagioclase + garnet + staurolite + kyanite, with accessory opaque minerals, tourmaline and zircon. Rutile, which is a common accessory mineral in Southern Highland Group semipelites and pelites in the lower grade zones, is less commonly found in the kyanite zone. This is probably due to the increasing proportion of biotite in the rocks, and the increase in the Ti content of biotite with increasing metamorphic grade (Deer et al., 1966, p.214). The garnets are large and euhedral, and may contain straight S1 inclusion trails; some have inclusion-free rims which apparently grew over the S2 foliation. Kyanite tends to be small and rather ragged, while staurolite may be large and euhedral; in places it is present as interpenetrant twins. Straight inclusion trails in some staurolite porphyroblasts infer that they, like the garnets, grew largely between D1 and D2; others, however, contain crenulated internal trails which are continuous at grain margins with the external foliations, indicating growth during D2. A third group appears to postdate development of the S2 foliation. Timing of kyanite growth with respect to foliation development is not clear in these rocks; although kyanite porphyroblasts are generally wrapped by S2, some kyanite growth apparently occurred after S2. Kyanite porphyroblasts up to 2 cm long occur in quartzofeldspathic segregations north-east of Badandun Hill [NO 2111 6804]. These segregations probably developed during D2. Kyanite may have been produced by the prograde breakdown of staurolite by reaction 9 for example. Where a biotite-muscovite S3 foliation is developed, this wraps garnet, staurolite and kyanite. This suggests that D3 occurred after the peak of regional metamorphism in these rocks.

Kyanite without staurolite is found most commonly within the migmatitic rocks of the Duchray Hill Gneiss Member, although it also occurs in two samples of migmatitic pelite from the Southern Highland Group in the east of the area. Its abundance in the Duchray Hill Gneiss Member reflects the relatively aluminous nature of the migmatites. Other occurrences of kyanite are recorded in a quartz sweat in the Glen Taitneach Schist Member on Glas Choire Mhor [NO 0690 7637], and in the Ben Eagach Schist Formation on Beinn a' Chruachain [NO 0394 6942].

Kyanite-bearing migmatites comprise quartz + oligoclase + minor alkali feldspar in the leucosome and biotite + muscovite + kyanite + garnet in the melanosome, with accessory apatite, opaque minerals and tourmaline. Both the stromatic layering and mica fabric in the melanosome represent the S2 foliation. Biotite and muscovite dominate the melanosome, and kyanite porphyroblasts are orientated parallel to the micas along S2, suggesting syn-S2 growth. The kyanites are generally a few millimetres in length, although crystals up to 1 cm in length occur locally at the edge of quartz segregations. Garnets are large, rounded and wrapped by S2, and may have quartzose pressure shadows, indicating pre to synD2 growth. They do not have inclusion trails, and the internal parts are commonly replaced by biotite, muscovite and feldspar to give an atoll form. Where such rocks have undergone D3 deformation, a biotitemuscovite S3 cleavage may be developed, but no syn-D3 growth of garnet or kyanite has been recognised.

To the north of the outcrop of Ben Lui Schist Formation, there are only isolated occurrences of rocks with suitable composition for the development of aluminosilicate index minerals. Micaceous rocks of the Ben Lawers Schist Formation are calcareous, those in the Ben Eagach and Gleann Beag schist formations are graphitic, and the Tulaichean Schist Formation is dominantly semipelitic. At this grade, semipelites comprise the assemblage: quartz + biotite + muscovite + plagioclase + alkali feldspar + garnet, with accessory opaque minerals, apatite and zircon. Semipelites in the Tulaichean Schist Formation retain a spaced S1 foliation parallel to bedding, with S1 defined by biotite-muscovite foliae; this is crenulated and cut by the dominant fabric, S2, which also contains biotite and muscovite. Quartzofeldspathic lithons are dominated by quartz with minor plagioclase and alkali feldspar, all of which are present in a medium-grained strain-free mosaic between the micas. Garnets are commonly abundant, and may be inclusion-free or have sigmoidal inclusion trails in the internal part of the porphyroblast, with an inclusion-free rim. Rarely, they have a core of fine inclusions in random orientation, surrounded by a zone with trails of inclusions, and an inclusion free rim, suggesting three stages of growth. Garnets are wrapped by the S2 foliation, and apparently grew syn to late D2 based on inclusion trail evidence, and before development of the S3 crenulation cleavage.

Psammites at kyanite grade contain the same assemblage as the semipelitic rocks, although quartz, plagioclase and alkali feldspar are more abundant, and micas and garnet are less abundant. They tend to be dominated by the S2 foliation, which is defined both by orientation of the micas and by a strong shape fabric in recrystallised quartz and feldspar grains. Garnets are restricted to micaceous domains; they are small and are generally sieved with fine quartz inclusions, which gives them an opaque pink appearance in hand specimen. There is no direct evidence as to the timing of their development with respect to the structural chronology, although they are assumed to have grown approximately synchronously with those in more pelitic lithologies.

Retrogression in pelitic to psammitic lithologies

Retrogression of peak metamorphic assemblages in pelitic to psammitic lithologies is most common where the rocks have been affected by D4 deformation, particularly close to the Highland Border Downbend. Where an S4 crenulation cleavage is developed, this typically contains muscovite and may contain chlorite. Chlorite replaces both biotite and garnet in strongly retrogressed rocks.

Migmatisation

Migmatitic rocks are defined here as those containing macroscopic quartzofeldspathic segregations, in line with the definitions of Ashworth (1985) and Mehnert (1968). The granitoid rocks of the Duchray Hill Gneiss Member are also included, owing to their mode of origin, although they are strictly not migmatites, being homogeneous rather than segregated. Those rocks which contain only quartzose segregations are not considered to be migmatites.

Migmatisation is restricted to the north-east of the district, principally within the Duchray Hill Gneiss Member and adjacent parts of the Mount Blair Psammite and Semipelite Formation. The Duchray Hill Gneiss Member was first described by Barrow (1893) as an intrusive 'Muscovite–Biotite Gneiss' of an 'Older Granite' suite. Barrow et al. (1905, p.101) described 'a melange (sic) of igneous and metamorphic material' occurring at Carn Dubh in the Pitlochry district (Sheet 55E), which indicates that he did recognise the mixed nature of the rock. Williamson (1935) envisaged the Duchray Hill Gneiss Member as a metasedimentary rock injected with magmatic material, but he also noticed that 'Duchray Hill Gneiss often resembles coarsened Ben Lui Schist'. Subsequent workers recognised the metasedimentary origin of the gneiss, assigning it to the Crinan Subgroup as a lateral equivalent of the Ben Lui Schist (Harris and Pitcher, 1975). In the Glen Shee district, the migmatitic nature of the Duchray Hill Gneiss Member is expressed by both stromatic textures and the development of a granitoid rock.

Stromatic migmatitic rocks

Texture

The Duchray Hill Gneiss Member consists mainly of migmatitic garnetiferous semipelite. Psammite layers and boudins are locally abundant, notably in the Ewe Crags, west of Cairn Derig [NO 1510 6631], and south and southwest of Creagan Caise [NO 1780 6796 to1750 6830]; the psammite is not migmatitic. Quartz-feldspar-muscovitetourmaline- bearing pegmatites occur locally throughout the migmatitic rocks, as sheets disposed at low angles to the migmatitic layering. The stromatic texture is prevalent in the area from Meall Easganan [NO 12 65] to Monamenach [NO 176 707]. West of Meall Easganan, quartz–feldspar segregations are largely replaced by quartz segregations within an otherwise schistose rock, although isolated patches of stromatic migmatitic rock occur throughout this area.

The stromatic layering comprises leucosome segregations, 0.5 to 2 cm thick, separated from mesosome layers by thin melanosome layers (Plate 6). Many leucosome segregations can be followed for tens of centimetres, although on a larger scale they show an anastomosing pattern. The leucosomes are leucotonalitic in composition and consist of coarse-grained polygonal quartz and plagioclase (oligoclase–andesine), generally some biotite and rarely K-feldspar. Melanosome layers are characterized by an enrichment of biotite, muscovite and garnet; they are 0.2 to 1 mm thick and are commonly poorly developed. Where kyanite occurs it generally does so in the melanosome layers. The mesosome layers are mineralogically similar to the combined leucosome and melanosome; they vary in thickness from 1 to 10 cm. The grain size of the leucosome and the melanosome is coarser than the mesosome, in places by a factor of 2 to 5. Nonetheless, the mesosome is coarser grained than schistose semipelites in other parts of the Dalradian succession. In many cases the leucosome layers thicken into augen, 10 to 20 cm across, thus giving the rock a pseudoopthalmitic texture (Mehnert, 1968). It should be noted that the stromatic appearance is not everywhere so clearly defined. In many places, mesosome and leucosome have rather gradual and diffuse boundaries, especially where the melanosome is poorly developed. A well-developed mica fabric (muscovite and biotite) is subparallel to the stromatic layering.

Timing of stromatic migmatisation

The stromatic texture developed before D3, since the stromatic layering and the mica fabric is folded by D3 folds (Plate 6). These folds can be studied in many places, notably on the southern end of Creagan Caise. The timing of development of the stromatic layering with respect to D2 is more complex.

The S2 mica fabric is commonly well developed despite the coarseness of the mica and is generally parallel to the stromatic layering. Psammite boudins carry a folded S1 foliation, with an axial planar S2 mica fabric parallel to that in the surrounding migmatitic rocks. On the basis of such field evidence, the stromatic texture would seem to have developed during D2. The S2 mica foliation is, however, axial planar to rare intrafolial F2 folds which deform the stromatic layering, as on Meall Easganan [NO 1174 6446]. Thin leucosome segregations within the Craig Lair Hornblendic Gneiss north-east of Mid Hill [NO 2240 7119] and south of Craig Lair [NO 2144 6927], both in the north-east of the district, are also folded by tight F2 structures, whereas at the latter locality, more pervasive segregations cross-cut similar structures. These relationships suggest that migmatisation predated or occurred during the early stages of D2 (Plate 6).

Textures in thin section demonstrate that the leucosomes commonly underwent ductile deformation with dynamic recrystallisation. These include serrate or lobate quartz boundaries, undulose extinction (subgrain boundaries of quartz) and, in places, well-developed crystallographic preferred orientation. Some subsequent annealing or static recrystallisation has partly destroyed these textures. Plagioclase within the leucosome and mesosome may also show a strained appearance, and locally the micas may show an S/C fabric, notably in the melanosome. Leucosomes may show pinch-and-swell structures and psammite layers show boudinage on an outcrop scale. These deformation features may be due to the main regional D2 event, thus indicating that migmatisation predated or occurred during the early stages of D2. Well-developed crystallographic preferred orientation of quartz within psammite boudins indicates that the psammite was deformed prior to migmatisation of the semipelite. Evidence of this deformation in the semipelite was obliterated during migmatisation. The balance of evidence, therefore, suggests that the stromatic migmatites developed at an early stage during the D2 event, rather than pre-D2.

Locally, within the Craig Lair Hornblendic Gneiss south-west of Craig Lair [NO 2151 6933] and on the Calls of Finlet [NO 2206 7020], leucosome segregation occurs along F3 axial surfaces thus indicating further local segregation during D3.

Gneiss of stromatic migmatites

Four methods of migmatite generation have been suggested (Ashworth, 1985; Yardley, 1978), namely magmatic injection, in situ anatexis, metasomatism and metamorphic differentiation/segregation. The first two require the presence of a melt phase, while the second two are melt-absent methods. Anatexis and metamorphic segregation both assume a chemically closed system, while magmatic injection and metasomatism require an open system.

The isochemical character of the migmatite can be tested by means of mass balance (Olsen, 1985). In the Duchray Hill Gneiss Member the mesosome layers contain the same mineral assemblage as the combined leucosome and melanosome. Two representative SEM-backscatter traverses across mesosome, melanosome and leucosome layers were carried out. These distinguished between quartz, plagioclase and micas together with other mafic minerals. Image processing of SEM-backscatter images led to estimation of surface-percentage (equivalent to the volume-percentage) of each of the three mineral groups. In addition an EDAX X-ray spectrum of all feldspar grains showed that no alkali feldspar was present. The ratio of mafic and non-mafic constituents in mesosome versus leucosome plus melanosome is similar (Figure 24)a, thus suggesting a more or less closed system; no significant loss or gain of specific minerals occurred during migmatisation (Olsen, 1985). There is no evidence to suggest open-system behaviour; the thin discontinuous leucosomes are unlikely to represent magmatic injection, and wholesale metasomatism is regarded as an unlikely mechanism to produce large masses of migmatitic rock (Ashworth, 1985).

Closed-system migmatite formation may be either by anatexis or metamorphic segregation. Anatectic leucosomes are likely to have a composition which plots close to the eutectic point or along a cotectic line in the granite ternary system (Yardley, 1978). They are likely to show strong plagioclase fractionation; plagioclase in anatectic leucosome is expected to be 10 to 40 per cent more albitic than plagioclase in the mesosome (Yardley, 1978, but see Mehnert, 1968, Johannes and Gupta, 1982). Microprobe analyses of plagioclase in both leucosome and mesosome are shown in (Figure 24)b. All plagioclase is oligoclase to andesine in composition. Plagioclase fractionation between the leucosome and the mesosome is confined to 3 per cent albite or less with the range of plagioclase compositions of both leucosome and mesosome greater than the difference of the averages.

The absence of alkali feldspar means that the composition of the migmatitic rocks as a whole and the leucosome in particular does not represent a minimum melt composition (Figure 24c). The peak metamorphic temperature is a consideration too; temperatures between 610 and 690°C were established by Baker (1985), for samples taken from Creagan Caise. Bleser (1989) reported temperatures between 550 and 640°C for samples taken farther north at the head of Caenlochan [NO 21 76] in the Braemar district, at the edge of the Duchray Hill Gneiss Member, and, although results from the Glen Shee district are slightly higher, these temperatures do not seem sufficient to induce in situ anatexis. Therefore, metamorphic segregation is favoured as the mode of genesis of the stromatic layering of the Duchray Hill Gneiss Member. This is consistent with both the findings of McLellan (1989) and preliminary whole rock geochemical analyses.

Plausible mechanisms for metamorphic segregation were provided by Robin (1979), and refined by Lindh and Wahlgren (1985). The model of Robin (1979) requires a syn-orogenic origin, deviatoric stress being the driving force for segregation, which takes place by (wet) diffusion. Lindh and Wahlgren (1985) concluded that metamorphic segregation is a very likely mechanism at temperatures between 575 and 675°C. It is possible that either original bedding, or (more probably) a regularly spaced S1 or early S2 pressure-solution cleavage influenced the process of metamorphic segregation, and that the present layering is in effect a strongly modified S1 fabric. The slight enrichment of quartz with respect to plagioclase in the leucosome as compared to the mesosome ( (Figure 24))a, b might indicate a stronger segregation of quartz, which is compatible with the observation of abundant quartz sweats outside the zone of migmatisation in both Southern Highland Group and Crinan Subgroup rocks.

The question of why the Ben Lui Schist Formation was more susceptible to migmatisation than other formations is relevant to both the stromatic gneiss and granitoid rocks. The importance of bulk chemistry has been suggested (Goodman, 1991). For example, it has been suggested that boron from abundant accessory tourmaline may aid the segregation process (Manning and Pichavant, 1983).

Granitoid rocks

Texture

In the north-east of the district, particularly east of Glen Isla, the Duchray Hill Gneiss Member ranges from a stromatic gneiss to a homogeneous medium- to coarse-grained (1 to 4 mm) granitoid rock comprising the assemblage: quartz + plagioclase + biotite + muscovite + garnet. Muscovite and subhedral plagioclase megacrysts are developed in places. Lenses of coarse-grained muscovite-bearing pegmatite occur to the west of Bada na Bresoch [NO 2073 7141], although the main developments of apparently intrusive granitic rocks occur in the Altvraigy Burn [NO 206 703] where medium-grained muscovite granite sheets cut the granitoid rock. Nearby [NO 2127 7082], stromatic gneissose garnet semipelite with abundant but variable proportions of leucosome grades into a 0.5 to 1.0 m-thick layer of white unfoliated granitoid rock containing some mafic schlieren. This transgresses the melanosome in the stromatic gneiss. The granitoid rock contains a spectrum of domains which can be identified as components of the stromatic gneiss. Variation in the abundance and size of these domains produces alternations between stromatic and granitoid lithologies over distances ranging from several centimetres to tens of metres. Semipelitic domains range from mica-rich schlieren to lenses of stromatic gneissose semipelite (Plate 7), some of which are many metres in size, and many of which have diffuse margins. The granitoid rock grades into stromatic gneiss as the proportion of the stromatic inclusions increases. Psammite domains are mostly less than 1 m in size and are particularly well seen south-west of Fore Brae [NO 2019 6917]. They typically have an S1 internal foliation, which may be a very strongly developed platy fabric, and which is truncated at the margin of the psammitic domain. This internal fabric may also be deformed by tight folds. Mafic schlieren in the surrounding granitoid rock commonly wrap the stromatic domains. Much of the granitoid rock is not foliated. Additionally, at a few localities such as in the area south-west of Fore Brae [NO 1962 6908], quartz and feldspar are aligned within what is interpreted as an S2 fabric, which is defined by mafic schlieren and psammite domains.

Timing of granitoid development

Temporal relationships between development of the stromatic gneiss and the granitoid rock are provided by the inclusions of the former in the latter. The absence of an S2 fabric suggests that the development of the granitoid rock took place largely after D2. However, in places a layering defined by variation in the abundance of micas is coplanar with S2 in adjacent rocks. This is considered to be a 'ghost' S2 fabric. The mafic schlieren and relict layering in the granitoid rocks are crenulated by F3 folds to the south-west of Fore Brae around [NO 197 691]. A weak S3 foliation is developed in places, together with an intersection lineation. The granitoid rocks therefore developed between D2 and D3.

Genesis of granitoid rocks

Since the granitoid rock is, by its nature, not segregated, the question of mass balance is irrelevant. However, preliminary whole-rock geochemical analyses indicate that the overall composition of the granitoid rocks is similar to that of the stromatic migmatitic rocks and that there has not been a significant change in volume. The pegmatite veins might represent escaped melt, but this does not explain the development of the homogeneous texture. McLellan (1989) suggested an anatectic origin for the granitoid rocks. However, the rocks are of tonalitic composition with no alkali feldspar. Their composition is not that of a minimum melt and, therefore, they are unlikely to have an anatectic origin, unless they represent restite compositions, rather than melt (Bea, 1989). Again, metamorphic reconstitution is a likely process, as suggested for similar rocks at the same stratigraphical level in the Ballater district (Goodman, 1991). The absence of deviatoric stress during the reconstitution might have been responsible for the absence of migmatitic layering.

Metamorphism of metamafic rocks

Throughout the district there are numerous bodies of metamafic rock, some of volcanic origin (e.g. Farragon Volcanic Formation), some tuffaceous (e.g. Southern Highland Group Green Beds), and some intrusive. All are now present as amphibolites, with mineral assemblages which are a function of both metamorphic grade and original composition. Change in metamorphic grade from south to north is indicated by the change from epidote amphibolite to garnet amphibolite, but this change takes place over a broad transitional zone, and is highly dependent on the chemistry of the rock. The picture is further complicated by the presence of two suites of intrusive amphibolites (Chapter 5), which may record different metamorphic peak conditions.

Pre-D2 metamafic rocks

These include intrusive sheets, metavolcanic rocks of the Farragon Volcanic Formation and the Laoigh metabasites within the Ben Lawers Schist Formation and the Green Beds within the Southern Highland Group (Chapter 4).

Epidote amphibolites

No amphibolites occur in the southernmost part of the Dalradian of the district, the most southern exposures being within the Green Beds of the Southern Highland Group. The southern unit of Green Beds, which extends from Capel Hill [NO 03 52] to Hill of Persie [NO 12 55], typically comprises the assemblage: hornblende + actinolite + biotite + plagioclase + quartz + epidote, with accessory opaque minerals, zircon and rutile. Note that epidote refers to the epidote group of minerals rather than to epidote sensu stricto. Rare intrusive epidote amphibolite sheets in this area are similarly garnet free. The S2 foliation is defined by strong preferred orientation of fine- to medium-grained amphibole and biotite, separated by small grains of quartz and plagioclase Clinozoisite is more common than epidote, with both typically occurring as small granules studding the mafic minerals. Large-scale compositional layering which reflects bedding is commonly visible in outcrop of the Green Beds. In thin section the amphibolites show only S2.

Epidote–garnet amphibolites

The northern unit of the Green Beds is lithologically indistinguishable from the southern unit, apart from the occurrence of garnet in the amphibolites. The incoming of garnet is considered to be due to a northwards increase in metamorphic grade, and occurs within the middle of the Barrovian staurolite zone. Garnet also appears in amphibolite sheets derived from intrusive protoliths at about the same position. To the north, Green Beds and intrusive amphibolites typically comprise the assemblage: amphibole + plagioclase + quartz + garnet + epidote, with accessory opaque minerals, sphene, rutile and zircon. Biotite is less common as part of the prograde assemblage at this grade, although its abundance in the Green Beds is controlled by both the proportion of detrital input and metamorphic grade. The composition of the amphibole varies from south to north, with actinolitic compositions being progressively replaced by hornblende. The change from dominantly blue-green to dark green amphibole reflects this compositional change. Typically this might be achieved by reactions such as:

Ab + Act → Hbl + Qtz Reaction 13

(Cooper, 1972). This reaction also produces a more calcic plagioclase as metamorphic grade increases. Garnet forming reactions may enhance this trend, for example:

Pl + Act → Grt + Hbl + Qtz Reaction 14.

The S2 foliation in epidote–garnet amphibolites is also defined by the orientation of the amphiboles, locally together with a shape fabric in quartz and feldspar grains in leucocratic domains. Epidote and clinozoisite grains tend to be small, either studding the amphiboles or occurring in chains along the S2 foliation. Garnets may be up to 5 mm diameter, and are generally wrapped by the S2 foliation, although some have euhedral rims which overgrow S2. In rare examples garnets contain straight inclusion trails at a high angle to S2. Where biotite is absent in the rock matrix, quartz and biotite may occur as inclusions in garnet, reflecting earlier assemblages which developed on S1. External to the porphyroblasts, no evidence of S1 remains. Garnet growth, therefore, mainly took place between D1 and D2, with some local growth after D2. It should be noted that garnets with S1 inclusion trails have only been observed in amphibolite from the Green Beds, and not in the amphibolites with intrusive protoliths. Therefore, no conclusion can be made as to whether the basic sheets were emplaced before or after D1.

Whereas Green Beds and intrusive-derived amphibolites typically contain both garnet and epidote, volcanic-derived amphibolites in the Farragon Volcanic and Ben Lawers Schist formations only rarely contain garnet. The typical mineral assemblage in these metavolcanic rocks is: hornblende + plagioclase + quartz + epidote, with accessory opaque minerals, zircon and sphene. These rocks are generally very fine grained and highly schistose, with a strong planar orientation fabric of the amphibole defining S2. Calcite is not unusual in these rocks, and it may be that a more calcic bulk composition delays the appearance of garnet, by stabilising epidote + hornblende to higher grade. Where garnets do occur, they show the same textural relationships with S2 as in the amphibolites discussed above; they have straight S1 inclusion trails, and are wrapped by S2. In one rock where S2 has been crenulated by F3 folding, there is incipient development of an S3 fabric. Calcite pressure shadows around garnet are elongate along the limbs of F3 microfolds, and some new hornblende has developed axial planar to the crenulations. This is apparently more actinolitic in composition than that along S2, being pleochroic in shades of blue green–mid green–straw yellow, rather than the typical dark green–olive green–straw yellow of hornblende in S2.

Garnet amphibolites

Amphibolites which contain garnet and no epidote occur only in the far north-west of the district, in Gleann Mor and Gleann Beag (Figure 22). The typical assemblage here is: hornblende + plagioclase + quartz + garnet, with accessory opaque minerals and sphene; biotite is probably retrogressive. Sigmoidal inclusion trails within the garnets contain epidote, calcite and sphene, and represent a lower grade section of the prograde metamorphic path. These trails are continuous with S2 outside the porphyroblasts, and indicate that the garnet developed during D2. This area of largely epidote-free garnet amphibolites may indicate either a northwestwards increase in metamorphic grade, or a variation in bulk composition.

Post-D2 and pre-D3 metamafic rocks

Coarse-grained intrusive sheets of amphibolite which carry only the S3 foliation occur throughout the district (Chapter 5), although they are concentrated in areas of

D2 sliding and attenuation (Figure 22). They commonly preserve a relic gabbroic texture in hand specimen, the D3 event being represented by a crude S3 foliation or a rodding that deforms the mafic and felsic domains. Their assemblage is: actinolitic amphibole + plagioclase + quartz + biotite + epidote, with accessory opaque minerals and sphene. In thin section, a relict ophitic texture may be preserved, with pseudomorphs after pyroxene formed of opaque cores, probably of magnetite, which are progressively rimmed by biotite and blue-green amphibole. There may be some preferred orientation of amphibole parallel to the S3 fabric. The original coarse igneous plagioclase is commonly replaced by finer grained plagioclase, quartz and clinozoisite; there may be myrmekitic intergrowth of feldspar and quartz. Macroscopic euhedral sphene forms up to 5 per cent of the mode. The lack of garnet and the actinolitic nature of the amphibole are indicative of greenschist facies metamorphism, and suggest that metamorphism associated with the D3 event was of a lower grade than the earlier peak regional metamorphism.

Metamorphism of calcareous rocks

Calcareous rocks in the Glen Shee district vary in composition from relatively pure metacarbonate rocks through calcsilicate lithologies to calcareous schists. Such rocks form major parts of the Glen Loch Phyllite and Limestone and Gleann Beag Schist formations in the Appin Group, and of the Ben Lawers Schist and Loch Tay Limestone formations in the Argyll Group. Minor amounts of calcareous rocks are found within the Tulaichean Schist and Ben Lui Schist formations and the Southern Highland Group. Even the graphitic schists of the Ben Eagach Formation and the metavolcanic rocks of the Farragon Volcanic Formation and Green Beds have associated metacarbonate or calcsilicate rocks. Only the relatively pure quartzite units are devoid of any calcareous lithologies. The wide geographical spread of calcareous rocks should allow any systematic change in metamorphic assemblage to be readily identified across the district. The variation in original composition and the dependence of metamorphic assemblage on fluid composition, however, exert a great influence on the assemblages developed, and it is difficult to distinguish differences in assemblage which are due simply to variation in grade, rather than such additional factors. Although local zonal schemes have been described using calcsilicate minerals (Kennedy, 1949; Ferry, 1983), no such systematic trend has been identified in the Glen Shee district.

Metacarbonate rocks

Relatively pure metacarbonate rocks may be mainly calcite or calcite + dolomite, in an assemblage also containing minor phlogopite + muscovite + quartz, with fine accessory opaque minerals and sphene. A small amount of tremolite may be present and some rocks contain plagioclase and/or perthitic feldspar. Scapolite has been identified as a regional metamorphic mineral in one rock. Tourmaline occurs locally as part of the accessory suite, with porphyroblasts of schorl and of dravite within the Loch Tay Limestone Formation at Wester Bleaton [NO 1151 5982]. The dominant foliation (generally S2) is marked by a weak to moderate shape fabric in the carbonate grains, and alignment of micas.

Calcsilicate rocks

Calcsilicate lithologies contain very little free carbonate, and are commonly chiefly composed of quartz, calcic plagioclase and biotite, with significant amounts of calcsilicate minerals. The typical assemblage is: quartz + plagioclase + biotite + epidote + garnet + amphibole, with accessory opaque minerals, sphene and zircon. Zoisite is the most common of the epidote group minerals, although clinozoisite and epidote may also occur. Garnets are grossular rich, and in places associated with shapeless masses of vesuvianite. Amphibole may be tremolite or hornblende, and is commonly studded with grains of epidote. Small amounts of diopside have been identified from only one rock. Calcsilicate rocks are generally poorly foliated, with a weak preferred orientation of the micas. They provide no evidence on the timing of mineral growth with respect to the structural chronology.

Calcareous schists

Calcareous schists contain a similar assemblage to the calcsilicate rocks, but a greater proportion of micas and amphiboles has caused the development of a schistose fabric. The most typical assemblage is: quartz + muscovite + biotite + chlorite + amphibole + feldspar + epidote, with possible accessory opaque minerals, sphene, rutile, apatite and zircon. Biotite is commonly pale and is probably phlogopitic, and, although the amphibole is generally hornblende, some actinolite or tremolite may be present. Plagioclase feldspar is mostly of calcic composition, and is more common than alkali feldspar, which is generally perthitic and may be quite altered. The Glen Taitneach Schist Member from Allt Linne a' Bhuirein [NO 048 691] contains oligoclase porphyroblasts which grew before F2 crenulations in the matrix. Zoisite is more common than either epidote or clinozoisite, and, although the epidote minerals may be abundant in the rock, they generally form small granular masses. Calcareous schists of the Ben Lawers Schist Formation only locally develop garnet, whereas those within the dominantly semipelitic rocks of the Tulaichean Schist Formation are generally garnetiferous, and may contain garnets up to 2 cm in diameter.

A strong preferred orientation of micas and amphiboles with fine-grained strain-free quartz and feldspar between the mafic minerals imparts a good schistosity to these rocks. Epidote and garnet are better developed in mafic domains in the rocks. In the Ben Lawers Schist Formation, the main penetrative schistosity is commonly the S1 foliation, which is present at a low angle to bedding; S2 occurs as a crenulation cleavage in these rocks. Both S1 and S2 carry the same assemblage of micas and amphiboles, although this may be due to mimetic growth, rather than S1 and S2 developing under similar metamorphic conditions. Garnets in such rocks generally have straight S1 inclusion trails, and are wrapped by the S2 crenulation cleavage, although the inclusion trails may be sigmoidal close to the garnet margin; in a few cases marginal parts of the garnet have overgrown F2 crenulation microfolds. Garnet growth, therefore, chiefly occurred after D1 and before or during D2 but with some overgrowths after D2. Garnets within calcareous schists in the Tulaichean Schist Formation show similar textural relations; their inclusion trails commonly contain calcite and sphene, even when these minerals are not present outwith the porphyroblasts. They probably represent an earlier, lower grade metamorphic assemblage that was present in S1 at the time of garnet growth. A rock from the Glen Taitneach Schist Member in the area of complex fold geometry on Beinn a' Chruachain [NO 0479 6909] contains garnets of 5 mm diameter, with very complex inclusion trail geometry. The cores of the garnets contain inclusions of quartz and calcite in trails with the form of isoclinal folds. These are succeeded outwards by trails with the form of the F2 crenulations external to the garnet. The garnet rim is inclusion free, and apparently grew after D2.

In addition to their presence as matrix minerals, hornblende and/or tremolite may also be present as large porphyroblasts, which are commonly arranged in random garbenschiefer texture on the schistosity surface. Such textures are most common in the Ben Lawers Schist, Gleann Beag Schist and Tulaichean Schist formations, but are found wherever calcareous schists occur. They may become more prevalent from south to north across the district, but this may be compositionally rather than metamorphic grade controlled. The garbenschiefer porphyroblasts are present in random arrangement, but where they contain inclusion trails, these are often parallel throughout a section. Typically the inclusion trails mimic the foliation in the surrounding rock, whether this is straight S1, crenulated S1, or S2 crenulation cleavage, although, where an S2 foliation is present, this commonly wraps the margin of the porphyroblasts, indicating that they grew during D2. F3 microfolds in the Ben Lawers Schist Formation north-west of Sron Charnach [NO 060 700] postdate the development of garbenschiefer hornblende porphyroblasts. In the Glen Lochsie Calcareous Schist Member, an S3 crenulation cleavage associated with such microfolds contains the retrograde assemblage muscovite + chlorite + biotite.

Mineral chemistry

Mineral chemistry, principally for thermobarometric estimates, has been determined for semipelites, pelites and amphibolites. The minerals analysed are those whose composition can be expected to vary with grade; nonetheless certain generalisations can be made. Typical compositions of all the minerals discussed below are given in (Table 7).

Semipelitic to pelitic rocks

Biotite, garnet, muscovite, feldspar and opaque minerals have been analysed from samples of thf Southern Highland Group, the Ben Lui Schist Formation and the Tulaichean Schist Formation.

Biotite compositions are typically K2(Mg2.2Fetot2.6 Al0.7Ti0.2)[Si5.5Al2.5O20](OH)4.Those within the Tulaichean Schist Formation tend to be more Mg rich, although the variation is very small. Mn is present in only minor amounts. There is negligible substitution of Na and Ca for K. Cl has been identified in several biotites from the Tulaichean Schist Formation, substituting for OH up to 0.1 ion per formula unit; no analyses for F have been undertaken. No consistent compositional zoning is present in the biotites, such that rim compositions are no different to core compositions.

Garnets from all formations typically comprise over 70 per cent almandine. Microprobe analysis does not allow separation of Fe into Fe2+ and Fe3+. Stoichiometric calculation of Fe3+ (Droop, 1987) indicates that only a small number of garnets contain any Fe3+ and these have only a negligible amount of up to 0.1 ion per formula unit. Garnets from the Tulaichean Schist Formation and the Southern Highland Group are typically Alm70Grs15Prp10Sps5)

S5, although some from the Tulaichean Schist Formation are relatively calcic, and contain over 20 per cent of the grossular molecule. Almandine garnets from the migmatitic part of the Duchray Hill Gneiss Member in the Ben Lui Schist Formation contain equal amounts of grossular and pyrope, but little spessartine. In contrast, those within non-migmatitic Ben Lui Schist Formation are mainly almandine-spessartine garnets (Alm28Sps14Prp6Grs2)with little or no grossular, and small amounts of pyrope. Zoning in garnets is not pronounced, with Fe, Ca and Mg generally showing flat profiles from core to rim (Figure 25). Manganese is the only element to show consistent variation, typically being more abundant in the core; however, as the Mn ion is generally present only in minor amounts, such zoning is not particularly significant.

Muscovites from all formations come close to the model composition K2Al4[Si6Al2O20](OH)4, with only minor substitution of Na for K up to 0.4 ions per formula unit, and up to 0.4 ions of Fe + Mg substituting for octahedral Al.

Plagioclase compositions are in the oligoclase to andesine range, with low K contents.

Accessory pyrite in semipelitic to pelitic lithologies locally contains trace amounts of copper, whereas tourmaline contains approximately equal amounts of Fe and Mg, and therefore lies between schorl and dravite end-member compositions.

Metamafic rocks

Amphibole, garnet, epidote, plagioclase and accessory opaque minerals have been analysed from a range of metamafic rocks. These include the suite of pre-D2 intrusive sheets of garnet amphibolite, hornblende schist of probable metavolcanic origin which contains epidote but no garnet, and garnet-bearing schistose amphibolite from the Southern Highland Group Green Beds.

Analysed amphiboles all have compositions within the tschermakitic hornblende range (Leake, 1978). Recalculation of probe analyses stoichiometrically suggests that Fe3+ occurs in negligible amounts in only a few hornblende grains, and that Fetot can in general be taken as equivalent to Fe2+. Typically the composition is (Na0.3K0.1)(Ca1.7Na0.3)(Fe2Mg2Al)[Si6.3Al1.7O22](OH)2.The main variation in composition is in the amount of Fe and Mg present, with the Fe/Mg ratio varying from 0.82 to 1.35, reflecting the bulk chemistry of the rock (Leake, 1965) as well as grade-controlled cation partitioning (Ghent and Stout, 1986). There is no obvious difference in composition of hornblendes in epidote- and garnet-bearing assemblages.

Garnets have relatively calcic almandine compositions, which typically range from Alm55Grs23Prp10Sps10 to Alm60Grs30Prp8Sps2. The most iron-rich garnets with composition Alm70Grs20Prp5Sps5 are those within the schistose amphibolites of the Green Beds. Very few garnet analyses require the presence of Fe3+ to be stoichiometrically correct, and in those that do, the Fe3+ content is minimal. Zoning is not significant in the garnets, with only small variations in the amount of Mg and Mn; Mg tends to decrease in concentration toward the centre of the garnet, while Mn increases. However, these variations are very slight (Figure 25).

Epidote from one garnet-free sample has the composition Ca2Fe3+0.5Al1.5O.Al2O.OH[Si2O7][SiO4],and thus lies midway between clinozoisite and epidote end-member compositions.

Plagioclase compositions range from oligoclase to andesine, with plagioclase in epidote-bearing amphibolite being more albitic (Ana)) than that in garnet-bearing rocks (An25 to An30).

Accessory minerals from amphibolites include ilmenite and pyrite, the latter commonly containing trace amounts of Cu and Co.

Thermobarometric estimate of peak metamorphic conditions

In a metamorphic assemblage in chemical equilibrium, the chemical potential (p) is zero, giving the general equation

Δμ = 0 = a + bT + cP + RT1nK Equation 1

where T is temperature, P is pressure, R is the gas constant (8.314JK−1), K is the equilibrium constant, and a, b and c are constants. Mutually stable mineral compositions are related to K, and will vary with temperature and pressure. In theory, the composition of co-existing minerals can, therefore, be used to deduce the possible pressure and temperature at which the metamorphic assemblage formed, with the value of K giving a straight line on a graph of P against T. Suitable equilibria may involve cation exchange, primarily the exchange of Fe2+ and Mg2+ between co-existing silicate minerals; such cation exchange involves no change in volume and hence K varies little with pressure, making cation exchange equilibria good geothermometers. Net transfer equilibria based on metamorphic reactions involve large volume changes and so make good geobarometers. Calibration of the thermobarometers (i.e. determination of the constants a, b and c in Equation 1) may be either experimental or empirical. Accurate calibration is hampered by many factors including the complexity of the geochemical system and non-ideality in mineral solid solutions, so that there are large potential errors associated with the absolute pressure and temperature values calculated (Hodges and McKenna, 1987). However, if a single set of thermobarometers is applied to many different samples, any systematic difference in PT conditions should be apparent, irrespective of the absolute values obtained.

There are further problems in applying thermobarometry, however. The first relates to the choice of samples for analysis. The metamorphic assemblage should be in chemical equilibrium, and, while textural equilibrium can be identified, this is no guarantee of chemical equilibrium. Ideally, several analyses from the same sample, and several samples from the same area, should give similar results for K, although the compositions of the individual minerals may vary considerably. Any samples showing evidence of retrogression should be avoided (Duebendorfer and Frost, 1988), and prograde reaction history should be considered, as this may also have a significant effect on the composition of co-existing phases (Florence and Spear, 1993). Taking these factors into account, samples for thermobarometric analysis were chosen as shown in (Figure 22). Their distribution is not ideal, as the area with the most dramatic change in mineral assemblages in the south-east is poorly represented by thermobarometric estimates. The rocks in this area have been affected by folding related to the Highland Border Downbend, however, and retrogression is pervasive.

One further problem exists in the determination of the mineral compositions by electron microprobe, as Fe2+ and Fe3+ are not distinguished by the probe and many equilibria involve Fe2+. It is possible to recalculate the analyses stoichiometrically for some minerals (Droop, 1987), and this has been done where it is possible for all minerals involved in the equilibrium (e.g. Fe2+/Mg2+ partitioning between hornblende and garnet). Where no unique solution exists for one of the minerals involved (as is the case for biotite), the recalculation has not been undertaken for any of the minerals (e.g. for Fe2+/Mg2+ partitioning between biotite and garnet), as this is considered to give more accurate results than recalculation for only some of the minerals (Schumacher, 1991). It should, however, be stressed that, in the absence of full analysis of Fe2+, Fe3+ and OH in the minerals, thermobarometry gives an indication of differences in temperature and pressure between samples, rather than an absolute measure of conditions (Cosca et al., 1991).

Compositional zoning of minerals can be problematical, as different zones will give different PT estimates, and to check for such effects analysed pairs of points were chosen both near the rim and towards the centre of minerals. Within a single sample, the standard deviation of rim analyses is less than that for the more central points, but the latter give estimates that are closer between samples from a small geographical area. The central analyses are, therefore, considered to be closer to equilibrium compositions, with rim analyses probably affected by re-equlibration (Duebendorfer and Frost, 1988).

Results from thermobarometric estimates are given in (Table 8) and illustrated in (Figure 22).

Pelite and semipelite thermobarometry

The Ferry and Spear (1978) and Hodges and Spear (1982) calibrations for Fe2+/Mg2+ partitioning between biotite and garnet were used to estimate temperature at 6 kb pressure (Dempster, 1985); variation of the pressure by +/−2 kb makes very little difference to the temperature estimates. The Hodges and Spear (1982) calibration consistently gives results some 50°C higher than the Ferry and Spear (1978) calibration, as has been found elsewhere (Beddoe-Stephens, 1990), but both show similar variations. Over much of the district, peak metamorphic temperatures are estimated at 450 to 550°C, which corresponds to kyanite grade at 6 kb on the KFMASH petrogenetic grid of Spear and Cheney (1989). This confirms the deduction from the petrological studies that much of the area must have reached kyanitegrade conditions, and the lack of kyanite is compositionally rather than grade controlled. Temperature declines gradually southwards, with the Capel Hill area giving estimates of around 350 to 400°C, which again confirm the qualitative petrological observations.

The highest temperature estimates come from migmatitic rocks of the Duchray Hill Gneiss Member; non-migmatitic parts of the member give results similar to those from other lithologies. The high temperatures from the Duchray Hill Gneiss Member, in the region of 700°C, have been noted previously (Baker, 1985), and may reflect a later thermal event, rather than peak metamorphic conditions (Goodman, 1991). Some migmatitic rocks (although not those chosen for analysis) show evidence of disequilibrium such as atoll garnets. It may be that the high calculated temperatures are spurious, and reflect re-equilibration and the distinctive cooling histories of these rocks (Crowley, 1991).

Estimates of peak pressure were made from three samples, using the calibrations of Hoisch (1990) for assemblages containing quartz + plagioclase + muscovite + biotite + garnet. The average result for each sample for the six calibrations given by Hoisch (1990) is shown in (Figure 22). The non-migmatitic Duchray Hill Gneiss Member gives an estimate of 6.1 kb (at temperatures around 500°C), which is in good agreement with previous work (Dempster, 1985) and qualitative grade determination (Spear and Cheney, 1989). Migmatitic rocks of the Duchray Hill Gneiss Member give a much lower estimate of 5.1 kb, the possible cause of this variation being the same as that for the anomalous temperature estimate discussed above. The Tulaichean Schist Formation gives an estimate of 13 kb, which is unfeasibly high, and presumably reflects the high calcium content of the garnets in the rock, which leads to values of K which are outside the ranges specified in the calibrations.

Amphibolite thermobarometry

Samples analysed are three garnet amphibolites from the pre-D2 intrusive suite, and one sample of garnet-free amphibolite from a unit of probable metavolcanic origin within the Ben Lawers Schist Formation. Temperature estimates were made using the Graham and Powell (1983) calibration of the Fe2+/Mg2+ exchange reaction between garnet and hornblende, and the amphibole-plagioclase thermometer of Blundy and Holland (1990) and Holland and Blundy (1994). As with the pelite thermo-metry, the pressure was assumed to be 6 kb for the purpose of calculation, although varying the pressure by ± 2 kb has negligible effect on the calculated temperatures. The hornblende-garnet thermometry suggests temperatures of 570 to 600°C, slightly higher than those from the pelite thermometry in the same area, although within the same range of facies conditions. The hornblende-plagioclase thermometer (Holland and Blundy, 1994), taking account of non-ideality in hornblende, gives results which are in good agreement with those from hornblende-garnet thermometry. A garnet-free metavolcanic amphibolite within the Ben Lawers Schist Formation in Gleann Fearnach gives a temperature estimate of 578°C, very close to that from an adjacent garnet-bearing intrusive amphibolite at 566°C (Table 8). This suggests that the lack of garnet in the metavolcanic amphibolites is related to bulk composition, rather than grade.

Estimates of pressure were made using the calibrations of Kohn and Spear (1989, 1990) for the exchange reaction between amphibole + plagioclase and garnet + quartz. For temperatures in the range 500 to 600°C, the results are around 8 kb, which seems somewhat high, although there is no obvious cause of error, with mineral compositions lying within the ranges specified for the calibration (Kohn and Spear, 1990).

Interpretation and implications of regional metamorphic distribution

Barrow (1893, 1912) recognised the significance of the metamorphic zones as an indication of varying metamorphic grade. His original interpretation was that the metamorphism was due to the presence of an intrusion of muscovite-biotite gneiss, partly represented by the Duchray Hill Gneiss Member. It is now recognised that the variation in grade is due to regional rather than contact metamorphism, and that the Duchray Hill Gneiss Member itself is a result of the regional metamorphism. Regional metamorphism in the Caledonide orogen is now considered as broadly collision related (Brown, 1993), in which case peak metamorphic temperature and pressure would be expected to increase steadily from the margin to the core of the orogen. Local variations in thermal gradient, structural history, uplift and exhumation will affect this simple pattern, and give a more complex arrangement of metamorphic zones as seen at the present erosion level.

In the Glen Shee district, the growth of the minerals in the peak assemblages took place before, during and after the D2 deformation event, which was the dominant event in terms of large-scale deformation and fabric development. Any low-grade metamorphism that may have been related to early D1 deformation (M1) was overprinted by syn-D2 metamorphism (M2), while both D3 and D4 deformation took place under declining temperature and pressure conditions (M3 and M4), as indicated by retrogressive assemblages and S4 fabrics (Table 6). Limited evidence from S3 foliations suggests that D3 also took place after peak metamorphism. The distribution of peak metamorphic facies, therefore, has implications for the post-D2 structural geometry of the district; any major D3 or D4 structural features would be expected to deform the metamorphic pattern.

The change in metamorphic grade across the district, and the results from the thermobarometric calculations are shown in (Figure 22). The figure illustrates the three metamorphic regimes that occur in the district. Regime 1 includes the gradual increase in metamorphic grade across the Flat Belt northwards into the zone of steeper orientation at the northern edge of the district. Regime 2 is the rapid change in metamorphic grade adjacent to the Highland Boundary Fault in the south of the Dalradian, where grade increases from biotite zone to staurolite zone within 6 km. Regime 3 involves the migmatisation in the Ben Lui Schist Formation, and the westwards decrease in the degree of migmatisation away from the east of the district and the type area of Duchray Hill.

The gradual change in metamorphic conditions described under Regime 1 is evident from the northward change from staurolite- to kyanite-bearing assemblages in pelitic rocks, and from garnet-epidote-bearing to garnet-bearing, epidote-free amphibolites. As indicated on (Figure 22), the geothermometry indicates an increase in peak metamorphic temperature across this area, which would agree with qualitative analysis of the assemblages. Although it cannot be substantiated, there may also be a concomitant increase in pressure across the same area, as indicated by the progressive dominance of garnet over epidote in amphibolites (Will et al., 1990). This change in grade is of the scale to be expected in a traverse across a section of an orogen of this size (Brown, 1993). If it is accepted that the major mineral growth spans D2 deformation, then the dominant top to the south-east sense of movement on D2 structures (Chapter 6) will tend to exhume rocks from deeper crustal levels in the north-west, an effect emphasised by the sense of displacement during D3 folding (Chapter 6). The metamorphic gradient across this area is low, and probably approximates to the PT profile of the normal geotherm (Harte and Hudson, 1979).

The rapid change in metamorphic grade adjacent to the Highland Boundary Fault requires further explanation, as the metamorphic gradient is so steep. It has been suggested that the Highland Boundary Fault lies close to the location of a major tectonic discontinuity that was active during metamorphism (Harte and Hudson, 1979). A comparatively cold crustal terrane to the south of this boundary would have had a chilling effect on the adjacent area, causing a high lateral thermal gradient, now visible as closely spaced metamorphic zones (Harte and Dempster, 1987). This assumes that the metamorphic zones are purely temperature related, and that metamorphism was isobaric over this area. It should also be noted that the lowest grade areas are those which show the least effects of D2 deformation; these areas must always have been at a relatively high level in the orogen, and did not undergo the major ductile D2-D3 deformations. Taking Regimes 1 and 2 together, metamorphism was caused by crustal thickening during orogenesis; the close spacing of the lower grade zones was probably due to a thermal boundary effect at the margin of the orogen, and enhanced by the steep attitude of the rocks - the Highland Border Steep Belt. The lower grade rocks probably record earlier thermal conditions than those at higher grade (Burton and O'Nions, 1992), as peak conditions were reached at progressively later times at deeper crustal levels.

Under Regime 3, rocks of the Ben Lui Schist Formation are migmatitic, as clearly displayed in the area around Duchray Hill. Read (1927) was the first to comment on the susceptibility to migmatisation of the Ben Lui Schist Formation and its equivalents; more recent studies have confirmed this compositional control (Goodman, 1991). However, the degree of migmatisation and, in particular, development of the granitoid rocks decreases westwards in the Ben Lui Schist Formation; stromatic migmatites are still locally present in the Pitlochry district, as far south as Carn Dubh [NN 98 62]. In the Glen Shee district, granitoid rocks developed after the stromatic migmatites. To the northeast, in the Braemar and Ballater districts, there is good evidence that the formation of granitoid rocks was significantly later than the stromatic migmatites, and was part of a later thermal event, which was also responsible for a sillimanite overprint on regional metamorphic assemblages (Chinner, 1961). This thermal event may have been due to high heat flow related to the intrusion of basic magmas (Goodman, 1991). This late thermal event could also have been responsible for the development of granitoid rocks in the Glen Shee district and the anomalous results from the thermobarometric calculations. Overall, therefore, the Glen Shee district, together with adjacent areas, provides a section through a collision orogen, with a high lateral metamorphic gradient in the south-east at the margin of the orogen, followed by a more normal gradient through the staurolite and kyanite zones towards the northwest. Superimposed on this pattern in the east is a later thermal event which was responsible for the formation of the granitoid rocks in the Ben Lui Schist Formation.

Chapter 8 Ordovician

Highland Border Complex

The Highland Border Complex, which occurs in fault slices along the Highland Boundary Fault Zone, comprises a number of rock bodies of disparate origin and affinities assembled as a result of tectonic movements (Curry et al., 1984). The assemblages include igneous rocks as well as rocks of sedimentary origin. Fossils recovered at a number of localities are mostly of Ordovician age. It is presumed that the emplacement of the complex took place during transpressive movements, mainly of Silurian age, within the Highland Boundary Fault Zone.

Within the Glen Shee district, rocks of the Highland Border Complex occur at two localities. At the first, to the south of Loch of Clunie, the rocks exposed in Limestonebank Quarry [NO 114 435] include limestone and calcareous lithic sandstone, the latter containing grains of volcanic rock. The main quarry was worked for agricultural lime as long ago as the mid-1700s (MacCulloch, 1824). All the rocks are broken and veined with calcite. Etched residues of the limestone have yielded a small assemblage of chitinozoans considered most probably to be of early to mid-Ordovician age (Curry et al., 1984).

The second locality is in a drain [NO 2165 5004] which flows south to join the main stream in Den of Welton. The rock consists of calcite-veined serpentinite (S96884). Its relations with the rocks on either side are not seen but the serpentinite body is presumed to be contained in a fault slice. It is possible that the outcrop is a part of a major body of serpentinite, considered by Farquharson and Thompson (1992), on the basis of geomagnetic evidence, to extend at depth along the Highland boundary from Dunkeld to Glen Clova (see also Chapter 3). The northern boundary of the serpentinite against the Dalradian may be at the Middleton Muir Fault or may extend farther north beneath Southern Highland Group rocks (Figure 5).

Chapter 9 Silurian

Late Caledonian post-orogenic intrusive igneous rocks

Late Caledonian post-orogenic intrusions occur throughout much of the northern half of the Glen Shee district (Figure 1), (Figure 22). The largest bodies are the Creag Lamhaich Porphyritic Granodiorite, representing the southern extension of the Carn Mor Granite of the Braemar district (Sheet 65W), and the Glen Shee Pluton. The remainder are minor intrusions, a few metres wide and up to a kilometre or more long, which range in composition from felsites and quartz-feldspar porphyries to microdiorites and monzonites. They are mostly confined to areas of steeply dipping host rocks where many are locally concordant but generally discordant on a regional scale. The majority of dykes trend north-east–south-west with a subsidiary set trending north-west–south-east.

The post-orogenic intrusions are assigned to the 'Newer Granite' suite (Pankhurst and Sutherland, 1982), and are probably late Silurian in age, although no radiometric dating has been carried out in the district. A thermal aureole developed around the Glen Shee Pluton indicates that a significant time interval separated peak regional metamorphism and intrusion.

Glen Shee Pluton

The Glen Shee Pluton underlies a large, poorly exposed, boggy hollow, over 2 km from north to south and over 4 km from east to west, on the eastern side of Glen Shee. Interpretation of geophysical data indicates that all of this area is underlain by the pluton. The margins of the pluton and the surrounding thermal aureole are locally well exposed with extensive crags, especially in the west and north (Craig of Runavey) and in the east (Ewe Craigs). Elsewhere, the margins of the pluton are mapped at subdued topographical features. Faults radial to the pluton cut the western and northern contacts.

The bulk of the pluton is composed of granodiorite, with diorite underlying approximately 1 km2 close to the northern margin. The central part of the pluton around [NO 1382 6903] ranges from fine-grained, pink porphyritic granodiorite with phenocrysts of zoned albite-oligoclase, orthoclase and quartz, to medium-grained, grey, equigranular quartz-monzonite, composed mainly of plagioclase and orthoclase, with some interstitial quartz. Both the granodiorite and the quartz-monzonite contain a small amount of biotite and hornblende. Sphene, apatite, opaque minerals and zircon are typical accessory minerals. Hornblende- and pyroxene-bearing cognate inclusions containing little quartz generally have sharp margins with the surrounding rock. Marginal parts of the granodiorite show narrow xenolithic chilled zones. On

Ewe Craigs [NO 1661 6771], psammite xenoliths retain sharp outlines whereas migmatitic semipelite xenoliths (both lithologies are part of the Ben Lui Schist Formation) show signs of partial melting. Abundant apophyses and minor dykes of granodiorite occur in the host rocks. These are finer grained and more prominently porphyritic than the main pluton. On Craigenloch Hill [NO 165 692], both the absence of xenoliths in medium-grained grey hornblende-biotite granodiorite and veining in the contact metamorphosed host rocks, are used to infer a faulted contact to the pluton in this area. On the west side of Glen Shee at Dalhenzean [NO 127 679], medium-grained grey granodiorite sheets contain xenoliths of hornfels. Similar sheets, a few metres thick, parallel the pluton margin just to the south [NO 126 680].

The northern margin of the intrusion, at the base of Craig of Runavey, comprises medium-grained grey diorite. The boundary between granodiorite and diorite is not exposed but is interpreted to be gradational on the basis of evidence from abundant boulders. The diorite exhibits a porphyritic chilled margin, several centimetres wide, against the Dalradian host rocks. This chilled margin comprises andesine, biotite and hornblende phenocrysts within a groundmass composed of hornblende, biotite, minor interstitial quartz, and accessory opaque minerals, occluded apatite, zircon and sphene. A marginal zone, several metres wide, contains hornfels xenoliths; both the diorite and hornfels are cut by aplitic microgranitic veins, a few centimetres wide. The adjacent country rock on Craig of Runavey [NO 1374 6983] is cut by sheeted microdiorite dykes which carry a margin-parallel flow foliation, defined by the alignment of groundmass feldspar and biotite. The northern contact of the pluton is highly irregular in detail. Some radial faults are occupied by quartz–feldspar porphyries, which extend laterally in thin sheets into the surrounding hornfels. These locally cut microdiorite sheets in the aureole. The porphyries are orange weathering and highly altered, with phenocrysts of quartz and feldspar, up to a centimetre across, in a fine-grained quartzofeldspathic ground-mass. Porphyries are also found in several larger masses at the northern margin of the intrusion, such as at the eastern end of Craig of Runavey [NO 139 697] and the head of Coire nan Eich [NO 136 702]. A small occurrence of explosion breccia at the former [NO 1390 6994] contains fragments of porphyry and other igneous facies as well as hornfels, all within a matrix of crushed rock.

Eighteen samples of the pluton were analysed (Table 9) as part of the BGS Mineral Reconnaissance Programme (McCourt and Gallagher, unpublished report). SiO2 ranges from 57.9 per cent to 75 per cent. Aluminium, iron, magnesium, calcium, titanium and manganese show slight negative correlation with silica, whereas sodium and potassium show a positive correlation (Figure 26a). With increasing silica content, strontium, barium and zirconium decrease slightly (Figure 26b), whereas the transition metals (e.g. Cr, Cu, Ni, Pb, V, Zn) decrease markedly (Figure 26c). These trends are typical of those of the Caledonian calc-alkaline intrusive suite (Pankhurst and Sutherland, 1982), and are probably due to crystal fractionation within a parental magma.

The pluton is encircled by a thermal aureole about 500 m wide, in which the rocks show varying degrees of induration and recrystallisation. Hornfelses developed in the Ben Lawers Schist Formation are commonly spectacular blue- and green-banded rocks, which reflect original pelitic and calcareous layers respectively. Semipelites of the Duchray Hill Gneiss Member show signs of partial melting adjacent to the contact, as at east of Craig of Runavey [NO 143 700]. Williamson (1935), in a detailed study of the thermal aureole, described cordierite-andalusite-corundum, oligoclase-biotite-andalusite-corundum-spinel-sericite (possibly after cordierite), andesine-biotite-hornblende-hypersthene, and biotite-labradorite-diopsidic pyroxene assemblages from the Ben Lawers Schist Formation. Additionally, calcsilicate assemblages observed during this study include plagioclase-epidote-grossular-wollastonite-idocrase, scapolite-diopside-quartz-plagioclase, and diopside-wollastonite-quartz-sphene. Williamson (1935) described the development of hornfelsed semipelite in the Duchray Hill Gneiss Member as involving the destruction of biotite, muscovite and garnet and the formation of cordierite This resulted in a cordierite hornfels with green spinel, andalusite, corundum, sillimanite and alkali feldspar. Such assemblages are typical of those around relatively basic intrusions of the Newer Granite suite, such as the Glen Doll Diorite (Jarvis, 1987) and the more basic phases of the Lochnagar Complex (Chinner, 1962). The aureole of the Glen Shee Pluton is similar to facies series lb of Pattison and Tracy (1991), thus suggesting a pressure of around 3 kb at the time of emplacement. This suggests an exhumation of around 6 to 10 km of the surrounding rocks after peak metamorphism but prior to emplacement of the pluton. Contact melt rocks developed in the Duchray Hill Gneiss Member east of Craig of Runavey [NO 1432 6995] comprise a recrystallised matrix of quartz, albite and biotite with abundant apatite needles; centimetre-scale restite inclusions of red biotite, fine quartz and feldspar, cordierite and fibrous sillimanite may have grown topotactically on the biotite. The development of thermal sillimanite in the hornfelsed semipelite suggests that the temperature at the contact may have been as high as 600°C at 3 kb (Bohlen et al., 1991), although fibrolite can develop metastably within the andalusite stability field at temperatures below 550°C at 3 kb (Kerrick, 1987).

Creag Lamhaich Porphyritic Granodiorite

An irregular sheet of porphyritic granodiorite can be traced from the head of Coire Charnach [NO 069 699], obliquely across the watershed at Meall a' Choire Buidhe, and thence, via the prominent shoulder of Creag Chreumh, to a 750 m-long section in Glen Lochsie Burn. This major intrusion extends through Glen

Lochsie and Glen Taitneach, and then broadens eastwards to form the hills of Can Mor and Carn nan Sac in the Braemar district (Sheet 65W). On Carn Mor [NO 11 75], the porphyritic granodiorite is a marginal facies to granite, although westwards from Glen Taitneach the porphyritic facies dominates, with a central equigranular variant becoming less well developed. The textural variation is thought to reflect differences in the cooling history rather than be the result of multiple intrusion.

The central facies exposed on Creag Lamhaich in Glen Taitneach is a pink-grey, medium-grained granodiorite, comprising phenocrysts of albite-oligoclase, orthoclase and quartz in a medium-grained matrix of feldspar and quartz with some biotite and hornblende. The porphyritic variant has a similar phenocryst suite within a much finer grained groundmass. Both variants contain accessory opaque minerals and apatite. The central granodiorite is very similar to parts of the Glen Shee Pluton, while the porphyritic facies is similar in field appearance and composition to the porphyritic felsite sheet on Sron Chrion a' Bhacain in Gleann Fearnach [NO 01 72].

Contacts are generally steep, based on evidence from exposures in the Allt Clais Beag [NO 063 728], and aeromagnetic surveys (A D Evans, oral communication, 1993). A positive aeromagnetic anomaly over the intrusion closely mirrors the mapped boundaries, except in the area to the west of Glen Taitneach, where the data suggest that the Dalradian rocks mapped on the Creag a' Chaise spur [NO 076 735] may form a thin veneer on top of the porphyry sheet. On Creag Lamhaich, the contact between porphyry and schists is particularly well exposed. For the most part it is steep, although towards the top of the crags both the north and south contacts become subconcordant with the moderate easterly dip of the Dalradian schistosity. Here, the intrusion has a chilled margin a metre or so wide. At the contact itself, the porphyry is very fine grained, verging on glassy, with fine flow-banding picked out in layers of grey and pink. In thin section (S95803); (Plate 15)f) the rock shows phenocrysts of subhedral quartz and orthoclase with chloritised biotite in a glassy or microcrystalline matrix. The layering due to variation in grain size in the groundmass wraps around the phenocrysts, with local asymmetrical wrapping suggesting phenocryst rotation. The grain size of the groundmass gradually increases away from the contact; a grey quartz–feldspar porphyry with prominent flow banding, many country rock xenoliths and calcite vugs passes gradually into typical porphyritic granodiorite (S95800). Contact metamorphic effects in the country rocks are restricted to minor recrystallisation. However, hydrothermal activity associated with the intrusion has had a more marked effect, causing local base metal mineralisation (S95804) (Chapter 2). A dioritic sheet exposed in Allt Aulich [NO 086 748], considered to be a more basic apophysis from the main porphyritic sheet, is mantled by a sulphide stockwork in surrounding schists.

Minor intrusions

Five suites of dykes are recognised in the district: felsites and quartz–felspar porphyries, microgranodiorites, microdiorites, lamprophyres and monzonites. A close spatial relationship between porphyries and microdiorites is recognised with a sheet in Faire Ghlinne Mhoir containing both lithologies. Age relationships between the minor intrusions are only locally seen. A porphyritic felsite cuts a microgranodiorite on Glas Choire Mhor [NO 0570 7536], whereas in the Allt Coire a Ghearraig [NO 0880 7052], a microdiorite with a chilled margin cuts a felsite.

Quartz–feldspar porphyries and felsites

Felsites and porphyries are widely distributed in the Dalradian rocks, particularly in the north of the district. The majority trend towards the north-east-quadrant, although north-north-east- and north-west-trending dykes are also common. Many are less than 2 m thick and exposed for no more than 100 m. Smaller dykes may be aphyric felsites whereas larger dykes are mostly quartz-and feldspar-phyric, locally with chilled margins on Carn an t-Sionnaich [NO 0191 7507] and west of Beinn Iutharn Mhor [NO 0325 7918].

East of Glen Isla, swarms of east-north-east- to northeast-trending porphyries occur in discrete zones. Two suites are distinguished on the basis of colour and the relative abundance of megacrysts. Porphyries to the north of Badandun Hill are pink to buff and contain more megacrysts than the grey to lilac dykes to the south. The more northerly suite occurs in zones around The Call [NO 200 720] and Mid Hill [NO 215 708]. Dykes are up to 30 m thick and contain megacrysts of rounded quartz, tabular white or red feldspar and biotite, partly replaced by chlorite, up to 5 mm across. Biotite and chlorite may show a preferential alignment parallel to dyke margins. The more southerly suite occurs in two zones; one zone extends through Knockton [NO 195 583] to the eastern margin of the district, and the other parallels and occurs immediately north of the Glen Doll Fault (Chapter 12). The dykes comprise a very fine-grained microgranitic groundmass with scattered smoky grey quartz, subangular to subrounded white to orange-weathering feldspar megacrysts and a few composite quartz–feldspar aggregates which make no more than 10 per cent of the rock.

Around Knockton, the dykes are generally up to 60 m thick, although a fault-bounded segment of a dyke which forms the hill of Knockton is 180 m thick. Much of this broad dyke is closely jointed (less than 1 to 3 cm spacing) with joint surfaces and cavities stained reddish brown. This is particularly well seen in brecciated and broken cliff exposures above the River Isla [NO 2220 5996]. Near the southern margin of the dyke [NO 1957 5829], closely jointed porphyry contains an area of breccia 50 cm by 100 cm in size with angular porphyry clasts up to 1 cm across. Both clasts and matrix are similar in composition. The margin of the breccia is sharp but lobate into host porphyry. North of the Glen Doll Fault, dykes are up to 15 m thick. In thin section (S95727), one specimen comprises a quartzofeldspathic groundmass with radiating sheaves of white mica. The megacrysts are subhedral to euhedral quartz, K-feldspar generally in the form of very fine-grained mosaics and composite quartz + K-feldspar, commonly in graphic intergrowths. Some quartz megacrysts are rimmed by a narrow halo of K-feldspar.

A strikingly different porphyry is exposed in a small quarry beside the Glen Isla to Kilry road [NO 2094 5847]. The dyke is about 6 m wide and dips moderately to steeply towards 325°. It contains subhedral to euhedral megacrysts of feldspar up to 5 mm in size, biotite up to 1 mm across which in thin section is smoky-greyish green in colour, composite feldspar and biotite aggregates, opaque minerals and rare quartz (S95766). The composition of the feldspar cannot be determined because of alteration to very fine-grained mosaics, intergrown with quartz. The matrix is fine grained and brown at outcrop; in thin section it is fine grained and feldspathic with numerous fragments of the megacryst phases. A foliation in part of the specimen is defined by the preferred orientation both of opaque minerals in the matrix and of the biotite and feldspar megacrysts. The rock has similar characteristics to the Lintrathen Tuff Member (Chapter 10).

Elsewhere in the district, a spatial relationship between the porphyries and faults is common. A large northnorth-east-trending porphyry dyke on the west flank of Carn an Righ [NO 0084 7693] intrudes brecciated quartzite, and was clearly emplaced into a fault zone across Cnapan Liath [NO 00 75] and northwards to Allt a' Ghlinne Bhig [NO 01 78]. Parallel trails of felsite debris amongst the quartzite litter south-west of Stac na h-Iolair [NO 014 772] indicate the presence of small intrusions parallel to the main dyke. A discontinuous north-west-trending quartz–feldspar porphyry dyke, up to 40 m thick, crops out along the Glen Fearnach Fault [NO 03 68] for more than 1 km. A marginal intrusion breccia against amphibolite [NO 0387 6843], contains Dalradian lithic fragments and xenocrysts of garnet, hornblende and strained quartz. Acid dykes in the vicinity of the Glen Shee Pluton [NO 11 67], some of which may be traced on a northeast–south-west trend for a kilometre or more, were emplaced along radial fractures around the pluton.

Felsite breccia is exposed in a trackside quarry [NO 2152 6254] immediately north-west of Loch Shandra. Sub-rounded clasts of grey to pink felsite up to at least 6 cm in size occur within a felsite matrix. Some parts of the rock are composed of over 50 per cent clasts, although other areas have very few clasts and rusty spots. In thin section (S95754), the rock is essentially composed of quartz and feldspar. Scattered megacrysts of quartz and feldspar suggest an affinity with the porphyries. Clasts are slightly coarser grained than the matrix with both sharp and gradational boundaries. Some clasts are coherent, whereas others comprise disaggregated large grains within the fine-grained matrix.

Porphyritic microgranodiorites

A few porphyritic microgranodiorite dykes occur in the north-west of the district. On Sron Chrion a' Bhacain [NO 01 72], a dyke has intruded a north-west-trending fault zone and towards Daldhu [NO 022 711], a smaller (20 to 30 m thick) dyke occurs en échelon with the larger intrusion. The larger dyke has 2 to 3 mm-size phenocrysts of fresh quartz, altered albite and more-calcic plagioclase (approximately An25) as well as minor hornblende, in an altered, almost opaque, brown, very fine-grained ground-mass of quartz and feldspar. A narrow (centimetre scale) dark grey to black chilled margin is locally developed and the country rocks are locally indurated by the intrusion. The contact is steep on the crags at [NO 0134 7216]; strongly developed jointing at this location suggests some reactivation of the pre-existing fault along the margins of the intrusion. Farther south-east, the intrusion may become more sheet-like with the topography causing the apparent thickening along the ridge towards Daldhu, before it thins down again to become a dyke a few metres thick exposed in the Allt Fearnach [NO 0256 7087].

A similar, generally north-east- or east-north-easttrending intrusion which crosses the Allt Glen Loch [NO 016 710] has steep contacts against the host quartzite which is reddened and somewhat glassy adjacent to the contact. The dyke is cross-cut by a very fine-grained dark grey feldspar-phyric felsite dyke. Abundant secondary calcite throughout the intrusion may be related to a north-west-trending fault which disrupts this intrusion along the Allt Glen Loch.

An east-trending microgranodiorite containing phenocrysts of quartz, plagioclase and chloritised biotite traverses the southern rim of Glas Choire Mhor. It has a thin chilled margin adjacent to indurated country rocks. The contact is steep and broadly concordant with S2 on the crags at [NO 057 754]. The intrusion appears to divide eastwards towards Gleann Taitneach where three separate dykes are seen in the crags around [NO 0725 7585]. The most southerly of these, which contains abundant brown hornblende and might more aptly be termed quartz-diorite, has an aureole of thermally metamorphosed, cordierite-bearing Gleann Taitneach Schist Member some 30 m wide. The dyke is emplaced into a zone of intense D2 strain in the Tulaichean Schist Formation on Glas Choire Mhor, where there is also a zone 30 m wide of thermally metamorphosed rocks.

A north-north-east-trending, fine- to medium-grained, almost equigranular microgranodiorite sheet with tabular plagioclase and reddish pink alkali feldspar cuts across the Gleann Mor and Tulaichean Schist formations west of Beinn Iutharn Mhór [NO 03 78].

Microdiorites

Microdiorite dykes occur widely in the north of the district, although they are generally poorly exposed and some are decomposed. They are typically 1.5 to 4 m thick and mostly trend between 035° and 060°. However, a swarm of dykes is more extensively exposed between Bad an Loin [NO 114 707] and Carn an Daimh [NO 137 718]; the largest of these dykes is 10 to 20 m wide, and is recognised in outcrop and boulder trails for 2 km. These dykes lie within 2 km of the Glen Shee Pluton. They are typically grey weathering and comprise dark, chloritised mega-trysts (up to 2 mm size) of originally subidiomorphic or idiomorphic biotite, amphibole, possible clinopyroxene and saussuritised plagioclase in a feldspathic, commonly reddened, pilotaxitic matrix with minor interstitial quartz (S95810). Replacive carbonate patches and epidotic alteration products are common. Accessory minerals include opaque minerals with some zircon, hematite and sphene. Three dykes are exposed in the

Glen Cally Burn to the east of Linns [NO 198 703]. They dip very steeply (70° to 90°) towards the north-west, parallel to joints in the host metasedimentary rocks. They are fine to medium grained with some feldspar phenocrysts together with sulphide up to 0.5 mm in size. One dyke contains 3 to 4 per cent sulphide in pale cubes. The contact of one dyke is well exposed and displays a 5 mm-wide chilled margin which is rather more mafic than the interior of the dyke.

A leucocratic to mesocratic, fine- to medium-grained microdioritic dyke with faint pink weathering, at least 2 m thick, is exposed south-east of Fergus [NO 1977 6698]. The dyke contains rounded quartz and quartz–feldspar xenocrysts up to 1 cm in size. In thin section (S95724), feldspar is strongly altered to very fine-grained carbonate and/or sericite. The xenocrysts occur within an ophitic texture of plagioclase laths and chlorite with widely disseminated opaque minerals and some interstitial quartz. The rock has a magnetic susceptibility of 16 X 10−3 SI.

An unusual hornblende porphyry with 2 to 3 mm-scale green/brown oscillatory zoned hornblende phenocrysts in a saussuritised, feldspathic groundmass occurs on Glas Tulaichean [NO 0434 7506]; the small exposure permits no estimate of scale or trend but it is probably subconcordant with the prevailing east–west attitude of S2.

Monzonites

A group of monzonitic dykes, ranging from 1 to approximately 80 m thick occurs in the area around and to the west of Mid Hill. The thickest dyke can be traced northeast, from a point 1 km south of Mid Hill [NO 216 701] for 4 km to the head of Glen Prosen R Mendum, oral communication, 1991). A microdioritic or micromonzonitic dyke to the west of Badandun Hill [NO 2028 6854] may be the same dyke 2 km farther to the south-west. It is composed of coarse-grained amphibole in the form of blades 5 to 7 mm long and white and pink feldspar, together with some quartz, typically in subhedral to euhedral crystals which partly infill drusy cavities (S92764), (S94366). Rusty or dark brown spots are spatially associated with the cavities. These spots are at least in part after sulphide; pale coloured sulphide is quite abundant and disseminated throughout the rock. Spectacular graphic textures with intergrown plagioclase and K-feldspar are developed around tabular plagioclase. An intrusion breccia occurs at the north-west contact of the dyke (S94365). This contains angular clasts of felsitic and/or fine-grained igneous rock up to several centimetres across within a fine-grained felsitic matrix. The matrix displays a range of colours related to iron-staining from orange to deep reddish purple. Brick red cataclasite veins within the breccia contain feldspar fragments.

A 30 m-wide zone north-west of the dyke is underlain by a mixture of intrusive igneous rocks which are separated by screens of variably altered metasedimentary rocks. The abundance of intrusive material in the form of dykes which range from 1 m to several metres thick, increases towards the main dyke margin, as does the degree of decomposition, ochreous coloration and brecciation in the metasedimentary rocks (S94364). The intrusive material ranges from fine-grained orange-yellow, decomposed rock with more deeply coloured orange spots (S94362) to a purple, fine-grained lithology with rusty orange spots, quartz blebs up to 4 mm and bladed amphiboles.

Elsewhere, the monzonitic dykes are typically fine to medium grained, greyish brown in colour, but commonly with pink feldspars accounting for up to 60 to 70 per cent and mafic minerals (possibly amphibole) 20 to 25 per cent of the rock. Cavities are ubiquitous; some are rusty with relicts of sulphide, whereas the majority are at least partially infilled by well-formed quartz crystals. A dyke in the Algeilly Burn [NO 204 708] contains up to 25 per cent quartz in aggregates, smoky-coloured in part, up to 2 cm in size (S92772). Quartz appears to occur only in these cavities. The dykes typically trend north-north-east to north-east, although one dyke in the Algeilly Burn [NO 2076 7095] apparently trends north-north-west.

Quartz-monzonite porphyry sheets comprising idiomorphic oligoclase, biotite and blue-green hornblende in a matrix of polygonal quartz, K-feldspar and oligoclase, crop out in the Allt Creag Dhubh [NO 0310 7090].

Lamprophyres

Fine-grained, small lamprophyre dykes, 1 to 2 m thick, occur in the Tullochcurran Burn [NO 0623 6090] and in the Glenbrighty Burn [NO 1784 7248]. Loose blocks of granular, pinkish grey, calcite-bearing lamprophyre occur north of Cnoc an Daimh [NO 1008 6336]. These lamprophyres consist of feldspar, calcite, chlorite, hornblende and quartz with minor muscovite, biotite and opaque minerals. Calcite and chlorite may have replaced calcamphiboles. The primary survey in 1884 to 1885 recorded limestone here, although no metasedimentary calcareous rocks have been found nearby.

Chapter 10 Upper Silurian-Lower Devonian

In the Glen Shee district, the upper Silurian to Lower Devonian supracrustal rocks are entirely of terrestrial origin, and consist mainly of fluvial sandstones and conglomerates with subordinate developments of lavas and associated volcaniclastic sedimentary rocks (Figure 27). Long referred to as Lower Old Red Sandstone, they form part of a large outcrop that extends along the northern side of the Midland Valley of Scotland. Generally the outcrop is bounded on the north by the Highland Boundary Fault but, in the area around Blairgowrie, Lower Devonian rocks extend north of the fault and rest unconformably on Dalradian rocks.

Armstrong and Paterson (1970) reconciled earlier stratigraphical classifications of the Lower Old Red Sandstone in Angus and Kincardineshire, and set up a scheme that consisted of six lithostratigraphical groups. Browne et al. (2002) have rationalised this scheme by combining some of the groups. Continuity with the previous scheme is retained in the new hyphenated group names which are, in ascending order, Stonehaven, Dunnottar–Crawton, Arbuthnott–Garvock and Strathmore groups. The oldest fossils diagnostic of a Devonian age consist of fish and arthropods and assemblages of macrospores from strata low in the Arbuthnott–Garvock Group (Westoll, 1951; Westoll in House et al., 1977; Richardson et al., 1984). It is proposed that the Devonian base be placed at the top of the highest litho-logically distinctive formation below the oldest fossiliferous strata. Accordingly, the base of the Lower Devonian is placed at the base of the Arbuthnott–Garvock Group (Figure 27), which, in much of the Highland Border area, means at the top of the distinctive body of quartzbiotite-feldspar-bearing vitric tuff of the Lintrathen Tuff Member at the top of the Dunnottar–Crawton Group. Locally, however, Arbuthnott–Garvock Group rocks rest directly upon Dalradian rocks.

The above decision means that the oldest two of the original groups constituting the Lower Old Red Sandstone are referred to the Silurian, and the Lower Devonian now comprises only the Arbuthnott–Garvock and Strathmore groups. The Dunnottar–Crawton Group and the two Lower Devonian groups occur within the district (Figure 1), (Figure 27).

Upper Silurian

Dunnottar–Crawton Group

Crawton Volcanic Formation

Lintrathen Tuff Member

Of the various formations assigned to the Dunnottar–Crawton Group elsewhere (Browne et al., 2002), only the Lintrathen Tuff Member of the Crawton Volcanic Formation occurs within the Glen Shee district. It consists of quartz-feldspar-biotite-bearing vitric tuff of dacitic composition which at localities near Dunkeld is partially welded (Paterson and Harris, 1969). The tuff has been radiometrically dated at 415 ± 5.8 Ma (Thirlwall, 1988). Correlation of outcrops of the tuff on either side of the Highland Boundary Fault, supported by Trench and Haughton (1990), means that post-late Silurian transcurrent movements on the fault have a net displacement of the order of tens of kilometres.

The most extensive outcrop of the tuffs is at Craighead [NO 195 542] where they may rest directly upon the Dalradian. In thin section (S95778), (S95779), (S95780), the tuffs show large cracked and corroded crystals of quartz, fresh crystals of greenish yellow and yellowish brown biotite and altered feldspar laths in a microcrystalline ground-mass containing flattened shards of pumice. A planar structure within the groundmass is in places wrapped around the crystal grains.

The Lintrathen Tuff Member occurs also in a narrow outcrop within the Highland Boundary Fault Zone in Den of Welton [NO 217 501]. The rock is much broken but appears relatively fresh in thin section (S96872)–(S96873). The well-developed planar structure seen in the rocks at Craighead is, however, lacking. On its south side, the tuff appears to be faulted against serpentinite of the Highland Border Complex.

Lower Devonian

Arbuthnott–Garvock Group

Craighall Conglomerate Formation

Armstrong and Paterson (1970) allocated the sedimentary rocks, mainly sandstones and siltstones, and the volcanic rocks of their Arbuthnott Group in the Dundee area to the Dundee Formation and the Ochil Volcanic Formation respectively. In the Glen Shee district, however, the rocks are poorly exposed and much affected by faulting with the result that the stratigraphical relations of the sedimentary rocks, mainly conglomerate, and the lava bodies cannot be determined The conglomerate and lava bodies are, therefore, regarded as members of a single formation, here termed the Craighall Conglomerate Formation (Figure 27).

The formation rests unconformably upon Dalradian rocks, or locally upon the Lintrathen Tuff Member at the top of the Dunnottar–Crawton Group, and passes up gradationally into the Scone Formation. It consists mainly of very coarse-grained conglomerate, characteristically with clasts of lava, and volcanic members, mainly of andesite lava. Because the conglomerates are generally massive and the outcrop is much affected by faulting, the thickness of the formation is difficult to assess, but it is estimated at about 2000 m. Volcanic members account for less than 10 per cent by volume.

Conglomerates

The conglomerates of this formation are grey or purplish grey, clast supported and commonly very coarse grained, with subangular to well-rounded clasts up to 0.7 m long. Characteristically the clasts consist predominantly of various types of andesitic lava, with smaller amounts of rocks of Highland origin, mainly quartzite and psammite. However, exposures south-west of Welton of Creuchies [NO 1995 4920] and east of Mains of Creuchies [NO 210 509] are notable for the occurrence of well-rounded granite clasts up to 0.7 m in diameter. The matrix consists of coarse-grained sandstone with angular grains, mainly of lava, loosely cemented by silica or calcite (S7435). The conglomerate is generally massive, to the extent that bedding can be recognised at only a few localities, even in large exposures. However, at a few places, thin beds of coarse-grained volcaniclastic sandstones are intercalated with the conglomerates. In the Morganston Burn [NO 163 495], maroon siltstones and mudstones occur in association with andesite lavas.

At the base of the sequence, where the conglomerates rest upon the Dalradian, there is commonly a thin basal breccia containing angular clasts up to 0.2 m long of low-grade metasedimentary rocks of local origin. The basal breccia is seen in contact with the Dalradian on the right bank of the River Ericht [NO 161 511], south-south-west of Milton of Drimmie. Nearby, on the left bank of the river, 3 to 4 m of coarse volcanic conglomerate are exposed. The conglomerate overlies Dalradian rocks and is overlain in turn by purple aphyric andesite lava.

Angular breccia with clasts of local Dalradian rocks is exposed on the south bank of the River Ardle [NO 146 512], a little upstream of its confluence with the Black Water. The actual contact with the Dalradian is not seen. The breccia is overlain by purplish brown silty mudstone and, to the east, is faulted against andesite lava. The basal breccia is also seen farther upstream on the south bank of the River Ardle [NO 1345 5125], west of Bridge of Cally, but here it contains rounded clasts of vitric tuff of the Lintrathen Tuff Member as well as Dalradian fragments. These occurrences of Devonian rocks are at a level about 200 m lower than exposures of Dalradian rocks to the north on Hill of Cally [NO 12 53], thus indicating the existence of considerable relief on the Dalradian land surface prior to deposition of the Devonian rocks. An angular breccia with Dalradian clasts is exposed also in a tributary of the River Ericht [NO 1707 5095], south-southeast of Rannagulzion Lodge, but no Dalradian rocks are seen. A contact of the basal breccia on cleaved Dalradian psammite is exposed in the Burn of Watersheal [NO 2089 5518], west of Nether Drumhead.

The best exposures of the typical volcanic conglomerates are in the mainly inaccessible walls of the gorges cut by the River Ericht upstream of Samuel's Pool [NO 1750 4765]. Just west of the pool, the conglomerates contain a few thin beds of sandstone. Farther downstream, a section extending either side of the bridge carrying the Glen Shee road shows steeply dipping, well-bedded relatively fine-grained volcanic conglomerates alternating with conglomerates and sandstones in which clasts of Highland rock types predominate. The Highland conglomerates are assigned to the Hatton Conglomerate Member of the Scone Formation. The rocks in this section are believed to lie to the south of the Highland Boundary Fault.

Lavas

The Craighall Conglomerate Formation contains numerous lava members, which range from single flows, as at Gormack House [NO 144 463], to bodies composed of many flows of several different compositions or textural variants. They form part of a major volcanic suite of calcalkaline affinity which developed widely in central and southern Scotland during late Silurian and early Devonian times. It has been suggested (Thirlwall, 1981) that the volcanism in the Midland Valley was related to active westnorth-west-directed subduction and was most probably of Silurian age. However, spore assemblages found at horizons near the base of the Arbuthnott–Garvock Group volcanic sequence in the area adjacent to the Firth of Tay (Richardson et al., 1984) confirm that the main volcanicity took place during the Devonian. Thirlwall (1981) noted that the lavas close to the Highland Boundary Fault are generally richer in strontium than rocks farther south in the Sidlaw and Ochil hills.

The lavas are mostly andesites of various types. They are usually purplish grey but in some cases are strongly hematised, as in the stream section [NO 164 495] north-west of Morganston. Most commonly the lavas are basaltic andesite, and contain phenocrysts of olivine, pseudomorphed in chloritic minerals and iron ores (S55200), (S55204), and/or clinopyroxene (S55198), (S55202). Rhombic pyroxene is present as a phenocryst phase in a few examples, partly or wholly replaced by chloritic minerals (S55213). Plagioclase feldspars are generally present as small laths which form the bulk of the ground-mass and also, sporadically or in some abundance, occur as phenocrysts less than 2 mm long (S55197). In rocks showing platy structure, the laths of the groundmass are strongly flow-aligned (S55201), (S56267).

The lavas are commonly very fine grained and have only a few phenocrysts. In some cases, xenocrysts of sedimentary rocks are present, enclosed by well-developed reaction rims consisting mainly of granules of clinopyroxene. Sedimentary infilling of fissures, a feature common in the lavas of the Sidlaw Hills (Armstrong et al., 1985), is very rare in the lavas of the Blairgowrie area, the only known example being in the Ochrie Burn [NO 2125 5440], south-west of Nether Drumhead.

The detailed stratigraphy of the lava members within the Arbuthnott–Garvock Group is not known. However, the oldest and thickest development, which is composed mainly of basaltic andesite lavas, fluxioned in places, extends north-east from Strone House [NO 144 517] to beyond Nether Drumhead [NO 217 551]. Lavas considered to lie at a similar stratigraphical level have been traced north-east from Kincairney House [NO 086 442] to Cochrage Muir [NO 148 501].

In the small outlier at Bridge of Cally, a flow of basalt or basaltic andesite is exposed on the north bank of the River Ardle below the Corriefodly Hotel. The lava, of which about 6 m is seen, is fine grained and vesicular with spheroidal weathering. It rests upon about 0.3 m of purplish brown medium- to coarse-grained volcanic sandstone, which rests in turn upon coarse volcanic conglomerate.

Lavas somewhat higher in the sequence are exposed in the stream bed and banks of Morganston Burn between [NO 1569 4948] and [NO 1626 4948]. The oldest lavas, exposed upstream of the Old Military Road, are deeply weathered, well-jointed, maroon, basaltic andesites. Just upstream of the road bridge [NO 1591 4952], hematite is developed within closely jointed lava. Downstream, relatively fresh grey lava alternates with decomposed hematitic lava. Fine-grained basaltic andesite (S55199) is separated from the main body by a thin sequence of maroon silty mudstones with turquoise reduction spots [NO 1636 4951] and conglomerate [NO 1650 4945]. The sequence is bounded on the south by a major fault.

Lavas which are possibly at a similar stratigraphical level, are exposed at a section in the Ochrie Burn west of Derryhill [NO 2125 5418], where parts of at least five distinct flows, from 10 to 15 m thick, are visible. In this area, the lower part of the lava sequence consists of feldspar-phyric basaltic andesite, the upper part of fluxioned, aphyric andesite.

Volcanic members occur at higher levels in the succession but their stratigraphical relations are not known. They consist mainly of olivine-phyric basaltic andesite, as in the faulted outcrop in the area around Loch of Clunie [NO 115 443] and the single flows traceable for distances of up to 2 km in the area around Gormack House [NO 143 464]. In the case of the lava member exposed in the vicinity of Welton of Creuchies [NO 211 502], thin flows of aphyric basaltic andesite alternate with markedly feldspar-phyric flows.

Lavas, which may be at the highest stratigraphical levels in the formation, occur in the area north and south-west of Mains of Creuchies [NO 208 509]. They are commonly well fluxioned and contain small phenocrysts of clinopyroxene (S55198), (S55212).

Scone Formation

The Scone Formation in the district comprises a lower conglomerate, the Hatton Conglomerate Member, overlain by the largely arenaceous Tannadice Sandstone Member (Figure 27). In the conglomerate member, clasts of Highland origin typically predominate, in marked contrast to the underlying conglomerates which are characterised by abundant volcanic material. The Ashbank Sandstone Member is an arenaceous facies within the Hatton Member. The Scone Formation crops out only on the south-east side of the Highland Boundary Fault (Figure 1).

Hatton Conglomerate Member

The member consists of reddish brown, generally well-bedded, fine- to coarse-grained, clast-supported conglomerate with subordinate amounts of gritty sandstone and a few beds of reddish brown siltstone. The clasts are subangular to well rounded, up to 0.3 m in diameter, and composed mainly of rocks of Highland origin, predominantly quartzite, vein-quartz and schistose psammite. There are variable amounts of volcanic rock types, mostly andesitic lava but also a range of intrusive rocks which are presumed to have originated in bodies emplaced in the Dalradian. The matrix consists of medium- to coarse-grained sandstone. Cross-bedding, imbricate structure and channel forms are present. In the basal part of the member there are some beds which contain a preponderance of lava clasts. The base of the member is taken at the base of the lowest bed in which Highland clasts predominate.

The best exposures of the member are on the banks and in the bed of the River Ericht upstream from Blairgowrie, where the strata generally have dips in excess of 50° and an estimated thickness of about 800 m. At Cargill's Leap [NO 178 460], typical coarse-grained conglomerate with only thin beds of sandstone form a picturesque gorge. At one point [NO 1779 4603], the conglomerates are intruded by a thin sandstone dyke. Imbrication of the clasts indicates transport from the north or northwest. Pebbly lithic sandstone forms the upper part of upward-fining beds in the sequence upstream of Cargill's Leap and in the area close to the confluence with the Lornty Burn. Upstream of the confluence, as far as Craighall Bridge [NO 176 473], beds with a high proportion of lava clasts are intercalated with the Highland conglomerates.

Ashbank Sandstone Member

Thinly bedded sandstones, maroon siltstones and silty mudstones are exposed in the mill lade excavated below Ashbank House [NO 1759 4641]. This development of fine-grained rocks, which can be traced north-east along strike in the Rattray Burn upstream of Hatton [NO 1898 4718] to [NO 1943 4755], has been referred to the Ashbank Sandstone Member.

The proportion of sandstone increases in the upper part of the member and an arbitrary boundary is drawn at the top of the highest conglomerate bed beneath a sequence of cross-bedded channel-fill pebbly sandstones exposed in the River Ericht [NO 1787 4577], upstream of the mouth of Cuttle Burn Den. Typical conglomerates and sandstones of the formation, which lie adjacent to the Highland Boundary Fault, are exposed on the hillslopes on the south side of Mill Burn [NO 190 480] and in the Den of Welton [NO 205 495]. The fine-grained conglomerates and sandstones that form the high ground east of Kinloch [NO 149 448] have been worked as building stone in a number of small quarries close to the village.

Tannadice Sandstone Member

The Hatton Conglomerate Member is overlain by a sequence of reddish brown, micaceous, cross-bedded, pebbly, channel-fill sandstones that have previously been referred to the Tannadice Formation (Armstrong and Paterson, 1970) but is here renamed the Tannadice Sandstone Member. The outcrop of the member is largely covered by superficial deposits and there are few good exposures. The main section is in the River Ericht just upstream of the mouth of Cuttle Burn Den [NO 1790 4568], where sandstones conformably succeed the conglomerates. Pebbly sandstones at a higher level in the member, which are exposed in the disused Drumend Quarry [NO 1970 4597], are more planar bedded and maroon in colour. The sandstones were formerly worked at the now largely infilled Blairgowrie Quarry [NO 182 444].

Because of the cover of superficial deposits, it is difficult to estimate the thickness of the Tannadice Sandstone Member in the district. In the River Ericht section, the sandstones have dips ranging from 7° to 17°, and it is likely that only some 400 m of sequence is present. In the south, the Tannadice Sandstone Member is believed to be faulted against strata of the Teith Sandstone Formation.

Strathmore Group

In Strathmore as a whole, the Strathmore Group occupies the axial part of the Strathmore Syncline (Armstrong and Paterson, 1970). Generally the group comprises the Cromlix Mudstone Formation, which consists predominantly of reddish brown, blocky silty mudstone, and the overlying Teith Sandstone Formation, which is composed of reddish brown or grey sandstones with beds of reddish brown silty mudstone arranged in upward-fining fluvial cycles. It is believed that a major north-east-trending fault close to the axial surface of the Strathmore Syncline crosses the district and cuts out the Cromlix Mudstone Formation. As a result of the extensive Drift cover, there are no exposures of the Teith Sandstone Formation within the district.

Conditions of deposition

The Lower Devonian rocks were deposited in a major, rapidly subsiding basin that was aligned northeast–south-west along the Midland Valley of Scotland. Movements on the Highland Boundary Fault Zone during deposition probably helped to maintain the relief of the Highlands to the north. It is considered that in the area close to the Highland boundary, coarse detritus from the Highlands was laid down in large alluvial fans. Closer to the axis of the basin, this material was redistributed by a river system which flowed towards the south-west, parallel to the basin axis.

The very coarse-grained and massive volcanic conglomerates of the Craighall Conglomerate Formation have yielded no evidence relating to palaeocurrent flow directions. It is not known, therefore, whether the volcanic centres lay south or north of the Highland Boundary Fault Zone. The little evidence available from imbrication in the Hatton Conglomerate Member is consistent with the north-westerly origin indicated by the clast composition.

Chapter 11 Carboniferous

Late Carboniferous dolerite dykes

The Glen Shee district contains representatives of the swarm of east- or east-north-east-trending quartz-dolerite dykes which extends across the Southern Highlands and Central Scotland (Walker, 1935; MacDonald et al., 1981). These dykes are a conspicuous feature of Strathmore (Armstrong et al., 1985, fig. 11). Individual members of the dyke swarm can be traced for considerable distances; some pass from the Midland Valley into the Highlands, and also eastward into the North Sea, and may reach at least as far as the Viking Graben (Smythe et al., 1995). Dykes of the swarm are considered to be of late Carboniferous age and have yielded K–Ar radiometric ages of 302 to 297 Ma (Fitch et al., 1970; de Souza, 1979).

In general, the dykes of the swarm are aligned 065°, but individual dykes vary in orientation between east–west and north-east as is the case with the 2 to 3 km-wide swathe that extends from the west edge of the district at Lochan Oisinneach Mor [NO 029 553] via Wester Bleaton [NO 114 598] to Auchintaple [NO 195 653]. However, in the Highland boundary area, the dykes have trends between 050° and 065° that are approximately parallel to the Highland Boundary Fault.

Exposure is poor and, where no geophysical investigation has been carried out, along-strike continuity of the dykes commonly has been inferred rather than proved. In general, however, the dykes appear not to be continuous at surface but to occur as a series of linear bodies, arranged en echelon and typically 10 to 20 m wide and several hundred metres to a few kilometres in length. Most exposures appear to indicate that the bodies have vertical margins but, at Dell's Briggs [NO 1758 4642] and just west of the district near Rotmell Loch [NO 023 472], a more sill-like disposition is apparent. Narrow chilled margins are developed and the adjacent country rocks are hardened and bleached in zones up to 0.3 m wide.

In the Highland boundary area, there are two major dykes. The more extensive can be traced for approximately 5 km from near Kinloch House Hotel [NO 133 453] to Craighall. The dyke has a south-west-trending branch to the south of Glasclune Castle [NO 154 467] and, northeast of Mains of Drumlochy [NO 1620 4712], it seems to bifurcate around a mass of conglomerate. Quarries in the dyke near Easter Mause [NO 1652 4745] and at Craighall [NO 1740 4788], show the dolerite is closely jointed. There is a good exposure of the dyke in the River Ericht [NO 1729 4780]. Narrow dykes striking 030° in the River Ericht [NO 1737 4769] and on the trackside [NO 1708 4787] are possibly offshoots of the main dyke.

The second major dyke can be traced between Deil's Mills Waterfall [NO 1505 4537] and Dell's Briggs [NO 1758 4642]. This one is less complex and maintains a fairly constant width of about 10 m throughout. In the southwest, it is more or less vertical, where exposed in a quarry [NO 1535 4552] near South Gormack. In the now infilled Knocky Quarry [NO 1688 4612], Barrow (1893) measured its margin as dipping 70° towards the north, but, as noted above, at Dell's Briggs it is inclined southwards in a sub-concordant sheet. Farther north-east, a 5 m-wide dyke, striking 095°, has been quarried between [NO 1790 4656] and [NO 1817 4654] just west of Berrybrae. An outcrop of dolerite farther east [NO 1828 4650] may be a continuation of the Berrybrae body. To the south-west, two north-east-trending dykes form the north and south walls of the disused quarry south of Loch of Clunie [NO 115 435]. A small dolerite dyke of unusual trend (008°) cuts conglomerate in a quarry [NO 1619 4515] on the hillside north of Hillbarns.

The northern set of dykes extending east-north-east across the district through Wester Bleaton [NO 114 598] intrudes Dalradian rocks (Plate 8). Two main dykes are recognised in the Auchintaple Loch area. The more northerly is approximately 20 m thick and exposed sporadically in the area north-west of Auchintaple Loch [NO 1935 6513] as well as in the River Isla at Little Forter [NO 187 649]. The central parts of the dyke are medium to coarse grained but decomposed whereas fine- to medium-grained chilled margins are fresh (S94346), (S94347). The more southerly dyke is approximately 15 m thick and exposed north-west of Craighead [NO 2095 6382]. Amethystine quartz xenocrysts occur in the dyke in Wester Bleaton Quarry [NO 1151 5983], which also displays a narrow chilled margin against the Loch Tay Limestone Formation. The bodies were traced partly on the basis of their high magnetic susceptibilities. There is a prominent negative anomaly on the south side of the dyke which suggests that an element of reverse polarity has been retained.

Two doleritic dykes [NO 0092 7482] and [NO 0093 7496] near the Allt Fearnach watershed have an unusual north-west trend which may be related to the stress regime around a major fold closure in the area.

In thin section, the most abundant mineral is plagioclase feldspar as elongate lath-shaped crystals, mostly of labradorite, but Walker (1935) noted the presence of phenocrysts of a more basic plagioclase (anorthite) in the lornty dike one mile north of Blairgowrie'. The plagioclase is in some cases strongly zoned from labradorite to oligoclase. Fresh pale brown clinopyroxene occurs in some abundance, in some cases having a subophitic relationship with the feldspar laths. In the dykes exposed at Craigie [NO 1150 4335], south-east (S56237) and north-east [NO 1425 4613] of Thorngreen (S56260), a few small phenocrysts of olivine, pseudomorphed by chloritic minerals are present. Rods and octahedra of opaque minerals are common, and presumably include magnetite to account for the high magnetic susceptibilities. In some cases, only a small amount of intersertal material is present, which may include obvious quartz, as in thin section (S57368), or may consist of a fine-grained, partially chloritised, quartzofeldspathic mesostasis. In section(S56260), a relatively abundant groundmass includes glassy or cryptocrystalline material.

Chapter 12 Faults

Both north-east- and north-west-trending faults are widely distributed throughout the Glen Shee district. Many of the structures have prominent topographical expression but few can be either traced for great distances or shown to have produced substantial displacement. Exceptions include structures related to the Highland Boundary Fault Zone which traverse the south-east part of the district (Figure 1). North-west-trending faults are apparently more numerous than north-east-trending ones, particularly within the Dalradian outcrop. The largest north-west-trending fault is the Gleann Fearnach Fault; a number of smaller structures occur in the central part of the district. They are nearly perpendicular to the regional strike and can be readily located by the offset of lithological units. This may partly explain their apparent greater abundance. Faulting may have been initiated in the later stages of the Grampian orogeny as indicated by the relationship between the Highland Border Downbend and the Highland Boundary Fault (Chapter 6). Movement continued until after the Early Devonian since both main orientations of faults displace Devonian rocks. Carboniferous dykes, however, appear to have been emplaced after fault movements had ceased. Emplacement of many late Silurian minor intrusions may have been fault controlled, and radial fractures around the Glen Shee Pluton suggest faulting during emplacement. The relative age of many faults is not clear. Many north-east-trending faults, are truncated against north-west-trending ones, although the fault pattern within the Devonian rocks suggests that they formed a conjugate set at that time.

North-east-trending faults

Highland Boundary Fault Zone

This major structure can be traced across Scotland from Stonehaven to Arran and thence into Ireland. In Scotland, it forms the southern boundary of the Late Proterozoic Dalradian Supergroup of the Highlands. Along most of its length, a single major fracture separates the Dalradian from the Devonian successions of the Midland Valley but, in places, the fault zone is more complex with several splay faults and Silurian–Devonian outliers. These outliers are unconformable on the Dalradian and extend north of the main fracture. In the Glen Shee district, the fault zone comprises at least two major faults and numerous lesser fractures (Figure 1). One major fault lies within the Lower Devonian outcrop, intersecting the River Ericht a little upstream of Craighall Bridge [NO 177 476] and lying along the line of Den of Welton. It is proposed that this structure be regarded as the Highland Boundary Fault. A little east of Welton of Creuchies, this fault divides into separate fractures which enclose a fault slice consisting of serpentinite of the Highland Border Complex on the south and Lintrathen Tuff Member on the north. Being older than the Devonian rocks on either side, these bodies of Lower Palaeozoic rocks constitute a horst-like structure.

The second major element within the Highland Boundary Fault Zone is the north-east-trending Middleton Muir Fault which forms the south-eastern boundary of the Dalradian in the Forneth to Cochrage Muir areas. North-east of the River Ericht, however, the continuation of this fault lies within the outcrop of the Devonian rocks. In the area west of the River Ericht, the Middleton Muir Fault is marked by a strong contrast between low amplitude, minor magnetic responses over the Dalradian rocks on the north and extreme, high-frequency positive anomalies over the Devonian rocks. The Middleton Muir Fault coincides with the northern margin of a very pronounced positive Bouguer anomaly, which appears to be bounded on the south by the Highland Boundary Fault sensu stricto. Farquharson and Thompson (1992) considered that the large magnetic anomaly was caused by the presence of a vertical rectangular body, 3 km wide and 13 km deep, with a top surface at a depth of about 2 km below ground surface. The composition of the body was thought most likely to be that of highly altered ultrabasic rocks, perhaps similar to serpentinites such as those that crop out at Ballantrae.

Both the Highland Boundary Fault sensu stricto and the Middleton Muir Fault were active as strike-slip faults during collision of the Grampian and Midland Valley terranes in Silurian times. However, at least part of the displacement on both faults must postdate deposition of the Arbuthnott–Garvock Group. Elsewhere in Strathmore, the Highland Boundary Fault sensu stricto cuts strata of the Strathmore Group and it may have acted as a high-angle reverse fault during Mid-Devonian compressive earth movements. The numerous smaller faults of north-east trend cutting Arbuthnott–Garvock Group rocks in the area north of the Highland Boundary Fault sensu stricto presumably formed during the same Mid-Devonian tectonic episode. The large Spittalfield Fault probably also formed at this time and is believed to be a high-angle thrust which, in the Blairgowrie area, juxtaposes the Scone Formation of the Arbuthnott–Garvock Group and the Teith Sandstone Formation of the Strathmore Group. Movement on the Highland Boundary Fault Zone may have ceased by the end of the Carboniferous as quartz-dolerite dykes of late Carboniferous or early Permian age, which were intruded along fractures in the zone, do not appear to have been affected.

Glen Doll Fault

The Glen Doll Fault extends south-west from its type area in the vicinity of the Glen Doll Intrusion (Smith et al., 2002) into the north-east corner of the district (Figure 1). The fault forms a subdued feature in the area north-east of Auchintaple Loch [NO 20 66] and skirts the lower slopes of Badandun Hill. In this area, at least three quartz–feldspar porphyry dykes occur immediately north-west of, and parallel to the structure (Chapter 9). The fault is exposed in the River Isla [NO 1892 6546] and is marked by 25 m of cataclasite. The most southerly 10 m consist of massive siliceous cataclasite with orange-weathering surfaces and bleached white fresh surfaces. In thin section the rock (S95733) is composed predominantly of quartz with a little muscovite. Flattened quartz grains with carbonate laminae attest to a period of ductile deformation, although this is overprinted by brittle fracturing with quartz fragments in a fine-grained matrix of quartz and opaque minerals. The cataclasite is intruded by both an orange-weathering microgranite and a quartz dolerite dyke (S95735), (S95736). Neither are brecciated, although both show extensive sericitisation. The cataclasite is succeeded to the north by 15 m of less intensely deformed, although still cataclastic, garnet-bearing semipelite and pelite (S95737). This is intruded by a quartz–feldspar porphyry dyke which appears unaffected by deformation. Evidence for the fault is only present for 3 km west of the River Isla; it is interpreted to terminate in the poorly exposed Balloch Burn valley north of Mount Blair.

The displacement across the fault is difficult to determine without knowledge as to the relative importance of strike-slip and dip-slip movements. Displacements of the order of several kilometres are inferred on the basis of the mismatch in stratigraphy across the structure. Solely dip-slip movement would require downthrow to the south-east, whereas sense of strike-slip displacement cannot be determined since the structure is broadly strike-parallel.

Minor changes in the stratigraphical succession across the fault may indicate that the structure originated early in the history of the area. These changes include more abundant Green Beds within the Mount Blair Psammite and Semipelite Formation south-east of the fault, the occurrence of the Golan Well Pebbly Psammite Member only south-east of the faults, and the presence of the Craig Lair Hornblendic Gneiss only north-west of the fault. The minimum age of faulting is constrained by the presence within the fault rocks of the undeformed microgranite and porphyry dykes, which are assumed to be part of the late Silurian suite. These do, however, show evidence of extensive fluid movement, in the form of sericitisation.

Other faults

Other north-east-trending faults are developed in the Southern Highland Group rocks in the Flat Belt and Highland Border Steep Belt but are difficult to detect since they are mainly strike-parallel. The apparent mismatch of lithologies and metamorphic grade across the hinge zone of the downbend together with a particularly sharp south-eastern limit of D2 structures in the Bridge of Cally area [NO 13 51] may mask a strike-parallel dip-slip fault with a southerly downthrow.

North-east-trending faults are common in the area north of Kirkmichael and at the southern end of Gleann Fearnach. Many are truncated by the north-west-trending Gleann Fearnach Fault. Felsite and/or dolerite has intruded along some of these faults, indicating that emplacement was controlled by pre-existing fractures. North-east of the Gleann Fearnach Fault, north-east-trending faults are indicated by topographical features, rare breccias and the offset of distinctive lithological units. An example of the last occurs on Cnoc na Cuinneige [NO 09 69] where prominent gullies are associated with faults which displace quartzite layers within the Ben Lawers Schist Formation.

In the extreme north of the district on Sheet 65W, the margin of the Creag Lamhaich Porphyritic Granodiorite is displaced by a fault which forms a prominent feature on Craig Dhearg [NO 076 742] and can be traced for nearly 3 km north-east of Glen Lochsie. On Ben Gulabin, a north-east-trending fault cuts the Creag Leacach Quartzite Formation [NO 10 71]; this fault has a large effect on the outcrop pattern despite its modest throw, since units are gently dipping in this area. Faulting on Carn an Daimh is marked in its northern section by an ochreous fault breccia [NO 135 713]; a small fault parallel to the southern part of this structure cuts the Ben Lawers Schist Formation and contains a fault gouge of pulverised graphitic material with a stockwork of calcite veinlets. There are no large displacements apparent on any of these structures, although many faults can be traced for several hundred metres and may form prominent air photo lineaments.

A large north-north-east-trending porphyry dyke west of Carn an Righ was emplaced into a large-scale discontinuity with downthrow to the east; brecciated quartzite occurs against the margin of the intrusion [NO 0084 7693]. An apparent variation in displacement along the trace of the structure is a function of gently dipping units in the north and steeply dipping units in the south. A number of parallel fractures are indicated by displacement of the Gleann Mor Limestone. Thin felsite dykes with the same orientation in the adjacent quartzites may have made use of pre-existing fractures.

An east-north-east-trending lineament extends from the northern flanks of Meall Lochan Oisinneach [NO 02 55] towards Pitcarmick Loch [NO 05 56]. Farther east, a fault on the eastern flanks of Creag na h-Iolaire can be recognised by offset of lithological boundaries. This extends north-north-east towards Glen Derby, where it apparently marks the edge of the Green Beds outcrop. This fault is thought to be steeply inclined with downthrow to the south-east. Dolerite was emplaced into the western part of the lineament. Dolerite blocks in a prominent gully [NO 046 552] may also represent intrusion along a fault or joint plane.

North-west-trending faults

Gleann Fearnach Fault

The Gleann Fearnach Fault is the largest of the northwest-trending faults. It extends south-east from the head of Glen Loch along the length of Gleann Fearnach, parallel to earlier local ductile structures (Figure 1), (Figure 11), (Figure 19). The fault occupies the broad peaty valley north-east of Menachban [NO 08 64], is located in the pass [NO 100 638] between Cnoc an Daimh and Cnoc a' Chaorainn and follows the course of the Allt Coire a' Bhaille [NO 11 62]. The Gleann Fearnach Fault is not recognised between Craigies and Glenkilrie Lodge [NO 140 600], where fault displacement of the Loch Tay Limestone Formation is insignificant. Farther south-east, displacement may be partitioned across a number of north-west-trending predominatly dextral faults in the Forest of Alyth [NO 17 58]. These are about 1 km apart and have apparent displacement of up to 500 m. North-west of Daldhu [NO 02 70], in Glen Loch, the fault zone comprises several north-west- or west-north-west-trending fractures. Displacements on these structures are no more than a few tens of metres, although the faults can be traced for up to 3 km. It is considered that displacement on the Gleann Fearnach Fault, therefore, probably dies out towards the north-west.

Brecciated fault rock is exposed just west of the district at Creag Uisge [NO 027 696], while eastwards in Gleann Fearnach [NO 034 690] sheets of porphyry intrude the shatter zone, with intrusive breccia farther to the south-east [NO 038 684]. The direction and scale of fault displacement cannot be accurately determined, but a downthrow component of at least 500 m to the north-east is inferred from offset of the Loch Tay Limestone Formation.

Several smaller faults parallel the Gleann Fearnach Fault within a 5 km-wide zone that straddles the major structure. These have little displacement and none can be traced for more than a few hundred metres.

Other faults

North-west-trending faults are widespread in the Southern Highland Group rocks of the Flat Belt. They are particularly abundant in the Forest of Clunie area [NO 07 52] in the south-west of the district where locally they are only 200 m apart. They produce deep gullies or steep slopes but most appear to have displacements only of the order of 10 to 15 m. However, a fault which traverses Deuchary Hill [NO 03 48] producing a deep gully, results in an apparent dextral offset of the axial trace of the Highland Border Downbend of some 300 m. Farther east, apparent displacements of 250 m dextral and 700 m sinistral are recorded across structures of the same trend [NO 062 470] and [NO 0755 4800].

A mismatch in the exposure patterns of Green Beds across poorly exposed ground in Strath Ardle is used to invoke the existence of a significant north-north-west-trending fault. The Middleton Muir Fault is sinistrally offset by 500 m farther south-east along this conjectural fault trace.

Elsewhere, north-west-trending topographical lineaments are well developed as in the Auchintaple Loch–Craig I air area [NO 21 65] in the north-east of the district, although there is little evidence of significant displacements. An 8 m-wide fracture zone on the line of a north-west-trending fault on crags south of Creag na Cuinneige [NO 0308 6415] contains centimetre-scale lensoid domains of brecciated rock; the structure has a sense of downthrow to the north-east.

Chapter 13 Quaternary

Evidence from ocean floor sediments indicates that at least 16 cold events have occurred during the last 1.6 million years (Bowen, 1978; Price, 1983), many of them causing a thick ice sheet to develop over northern Britain. Most of the glacial features and deposits in the Glen Shee district date from the last of these major arctic episodes — the Dimlington Stadial (late Devensian) which lasted from about 28 000 to about 13 500 years before present (BP). It is probable, however, that the major ice-moulded features owe their form to the accumulated effects of more than one glaciation and that some of the highest ground was affected by periglacial processes during cold periods, such as the Loch Lomond Stadial (between about 11 000 and 10 000 BP).

The most extensive superficial deposits, which consist of till and glaciofluvial sands and gravels, are developed in the low ground of Strathmore in the south. In the north, till occurs on the lower hillslopes with narrow spreads of glaciofluvial sand and gravel within the valleys. Peat occurs widely, infilling basins and blanketing hill-slopes. Fluvial deposits, mainly sands, gravels and clays, form river terraces and floodplains in the valleys throughout the district.

Outline of Quaternary history

It is generally assumed that the onset of colder climatic conditions caused ice caps to develop first in the western Highlands — an area of high precipitation facing the prevailing winds — and that for a time ice flow from this centre dominated the successive glacial episodes that affected the Glen Shee district. Glacial striae tend to occur on the highest ground in the district and, in the adjacent area to the south and east, they have a general south-east orientation (Figure 28). They are presumed to have formed during the Dimlington Stadial, while ice flow from the western Highlands prevailed. Subsequently this western Highlands ice mass coalesced with ice caps which had developed over the Southern Uplands, the Lake District and Northern Ireland and the main ice centre migrated towards the south (Paterson et al., 1998). Flow directions towards the east or east-north-east, indicated by glacial striae and the long axes of drumlins in central Scotland, are considered to relate to ice flow from this enlarged ice mass. Typically, in the vicinity of the Glen Shee district, striae of this east-north-east trend occur at lower levels and are associated with drumlins of similar orientation, which suggests that they were formed during the waning stages of glaciation.

When late-Devensian deglaciation commenced, in the period following about 18 000 BP, climatic conditions initially continued to be arctic, as inferred from the tem perature tolerance attributed to marine faunas from contemporaneous deposits laid down in the coastal waters of eastern Scotland (Armstrong et al., 1985). Retreat of the ice-sheet margin was accompanied by downwasting of its surface, with the result that the highest ground, in the northern part of the Glen Shee district, was the first to become ice free. Numerous breaches (cols) were cut in the interfluves which separate the catchments of the chief present-day river systems. These breaches carried overflowing meltwater across the interfluves during the deglaciation of the upland. A temporary position held by the ice front as the watershed between the Ericht and Alyth Burn catchments emerged from beneath the ice is marked by an arcuate terminal moraine near Glendams [NO 20 48].

As the ice withdrew towards the south and west, its front extended in a north-west–south-east direction, approximately at right angles to the axis of Strathmore. However, this means that the ice front was oblique to the mainly north–south-trending river valleys and intervening ridges which occur in the north of the district. Consequently, meltwaters from each catchment were diverted eastwards into the adjacent catchment through breaches in the intervening watershed. Glaciers that occupied valleys such as Strathardle and Glen Shee were deprived of their ice supply and stagnated when the ridge which formed the western margin of their catchments emerged above the surface of the downwasting ice sheet. Meltwater passing around and through the dead ice from sources farther to the north and west laid down sand and gravel as kettled kame terraces, on both sides of the valleys (Paterson, 1977). In the valley of the River Ericht, to the north of Blairgowrie, the ice-contact glaciofluvial deposits also form esker ridges at lower elevations closer to the valley axis.

At a late stage in the late-Devensian deglaciation of the district, ice was mainly confined to the low ground of Strathmore. Successive positions of the retreating ice margin are marked by the prominent series of meltwater channels incised on south-facing slopes east and west of Blairgowrie (Figure 28). At this time, thick ice in the Perth area blocked the lower Tay valley and constrained meltwaters that entered western Strathmore to flow eastward and escape by way of a spillway, incised in the watershed near Forfar. The spillway now lies at a height of about 60 m above OD (Paterson, 1974; Armstrong et al., 1985).

At an even later stage in the deglaciation of the area, the supply of ice from the west diminished to the point where the ice occupying western Strathmore became largely stagnant. This occurred while ice in the Perth area continued to block the lower Tay valley. Meltwaters that flowed eastwards laid down extensive ice-contact sand and gravel deposits, including esker ridges to the east of Rattray [NO 210 454], which are now largely quarried away, and also the esker complex at Links [NO 17 38], a little to the south in the Perth district (Sheet 48W; Paterson, 1974; Armstrong et al., 1985). A lake might have been expected to occupy western Strathmore while the lower Tay valley was blocked by ice, but the presence of dead ice in this area would have inhibited the deposition of fine-grained lacustrine sediments and obscured any shoreline features.

Withdrawal of the ice front in the Perth area eventually allowed the meltwater from western Strathmore to reach the sea by way of the lower Tay valley. A major outwash deposit, which forms the Meikleour Terrace (Paterson, 1974), was laid down in the area south of Blairgowrie, mainly in the Perth district, and buried or enclosed blocks of dead ice. The upper surface of the terrace descends southwards and appears to grade to a base level of about 48 m above OD at Cargill [NO 15 37], which may represent the contemporaneous sea level (Armstrong et al., 1975; Browne, 1980). A sea level standing at about 48 m above OD at Cargill is close to the height of the projected continuation of East Fife shoreline EF-6 of Cullingford and Smith (1966), for which a tentative age of 14 750 BP has been suggested (Sissons, 1974). This age is consistent with the postulated ages of the oldest pollen assemblages obtained from deposits that partly infill Stormont Loch [NO 190 420] in the Perth district. This is one of a series of large kettleholes in the surface of the Meikleour Terrace. The kettleholes formed by the melting of buried or enclosed blocks of ice after the terrace deposits had been laid down.

The oldest pollen assemblages at Stormont Loch have been correlated by Caseldine (1980) with Lateglacial assemblages, recovered by Walker (1977), from peats and organic muds that infill a large kettlehole at Corrydon [NO 132 674] in Glen Shee. Caseldine also correlated the Stormont Loch assemblages with similar material dated to about 13 800–13 000 BP at Windermere (Pennington, 1977). In fact, the Scottish material may be somewhat younger as deglaciation in the Lake District probably took place earlier than in Glen Shee. The infills of the Stormont Loch and Corrydon kettle-holes also include younger sediments that are thought to have been deposited during the cold periods associated with the Older Dryas chronozone (12 000–11 800 BP), the Lateglacial Interstadial (11 800–11 000 BP) and the Loch Lomond Stadial (about 11 000–10 000 BP). There has been some debate regarding the extent to which ice masses developed in the Grampian Highlands during the cold periods. No glaciers appear to have formed in the Glen Shee district at these times and the Corrydon pollen profile indicates that, after the retreat of the Dimlington Stadial ice, the upland area was colonised by open-habitat herbaceous plants and later by shrub-heath communities. The heath was replaced, for a time, by open grassland, before a renewed expansion of dwarf-shrub vegetation occurred. These vegetational changes were probably controlled by minor fluctuations in climate but, in the absence of radiometric (C14) dating of the Corrydon sequence, it is not possible to establish, in detail, whether these are equivalent to climatic oscillations that have been recognised at several other sites in northern Britain.

During the final stages of the deglaciation, meltwater rivers and streams became incised as the drainage adjusted to a lowered base level. This lowering took place when the rate of isostatic uplift of the land (formerly depressed beneath the load of the ice sheet) exceeded the rate of eustatic sea-level rise. The downcutting of the rivers resulted in the preservation of fragments of the earlier glaciofluvial outwash deposits as terraces on the sides of the valleys. Melting of blocks of ice within these outwash terraces produced large kettleholes, such as Marlee Loch [NO 140 445], which must postdate the rejuvenation of the drainage which formed the terraces. Falling temperatures at the beginning of the Loch Lomond Stadial led to the destruction of soils formed during the Lateglacial Interstadial and the establishment of a tundra landscape.

During the Flandrian, alluvial deposits were laid down by streams and rivers, and river terraces formed as the drainage adjusted to changes of sea level. Most of the surface peat deposits, mainly in upland areas, date from this period. Man has had little impact on the geology of the district, but there are small deposits of made ground in the built-up areas of Blairgowrie and Rattray and a number of worked out quarries and gravel pits.

Glacial erosional features

Large-scale features

In the Highland parts of the district, large-scale features formed by glacial erosion include valleys, such as Glen Shee, that have been widened, straightened and deepened by glaciers, as well as corries and glacial breaches.

Small-scale features

There are isolated glacial striae, trending north-west, that occur above 200 m above OD to the north and west of Blairgowrie (Figure 28) and also a few examples on the poorly exposed Lower Devonian outcrop. One of the best examples occurs on a rounded knob of andesite [NO 204 511], 500 m north-west of Mains of Creuchies. Miniature crag-and-tail structures leave little doubt that the ice flowed towards the east.

Glacial deposits

Till

The Dimlington Stadial ice sheet laid down a blanket of till over much of the lower lying ground of the district. The deposit is a diamicton of variable composition, generally a stiff to hard silty or sandy clay, which contains mainly angular to subangular clasts. In much of the area it is grey in colour but weathers to yellowish brown. It contains a preponderance of clasts of resistant metamorphic rock types. This is the case even over much of the Lower Devonian outcrop, although the till becomes sandier and browner in colour towards the south-east.

The till is generally from 2 m to 5 m thick, but the deposit may reach 20 m or more in thickness where it infills pre-existing hollows in the bedrock surface. Deposits of till are at least 15 m thick on the hillslopes to the west of Blairgowrie and east of Rattray, where they have been deeply incised by meltwater and form the flanks of spectacular groups of glacial drainage channels.

Hummocky glacial deposits

Hummocky glacial deposits identified as being of possible morainic origin occur as an arcuate ridge in the south-east of the district. The ridge stands at a height of about 220 m above OD in the valley of the Mill Burn (Figure 29), downstream of Glendams [NO 190 483]. The deposits, which are composed of sand and gravel, at least in part, were regarded by Watson (1939) as forming both a lateral and terminal moraine of a glacier that occupied the Ericht valley. The direction of ice retreat during deglaciation postulated by Watson differs, however, from that proposed here. He considered that the glacier that occupied the Ericht valley retreated upstream (i.e. northwestwards) and was distinct from the 'Strathmore Ice' which withdrew towards the south-west. In any event, it is apparent that a glacial lake was impounded at Glendams, by ice that reached a level of at least 254 m above OD. This ice dam caused water that overflowed from the lake to pass over the watershed into the catchment of the Alyth Burn.

Glaciofluvial erosional features

Drainage channels

By far the most conspicuous glaciofluvial erosional features in the south-eastern part of the district are the extensive systems of meltwater channels, which mainly occur on south-facing hillslopes (Figure 29), (Figure 30). Many of the channels formed along the margins of the ice sheet and were cut, at oblique angles to the contours of the present land surface, by meltwater flowing at the contact between the top of impermeable basal ice and the hillslope. The most spectacular channel systems are those on the hillsides between Cochrage Muir and Lornty Burn [NO 17 47] (Watson, 1939), where individual channels can exceed 20 m in depth. Similar channels occur north of Loch of Clunie [NO 11 46] and east of Blairgowrie [NO 20 46].

Meltwater channels provide much information regarding positions held by the retreating ice front. Particularly instructive is the disposition of the channels in the area west and north of Mains of Creuchies (Figure 29)a. The group of ice-marginal channels north-east of Ollies Burn mark the earliest positions of the ice front as it withdrew south-westwards across the area. Meltwaters escaped along the valley now occupied by the Myth Burn, which must have been open at this time. During this early stage of retreat the ice front lay against the ridge which extends north-eastwards from Hill of Drimmie [NO 185 501]. This enabled meltwater originating in the Burn of Drimmie to escape over the ridge and cut the drainage channels which join the present course of the Myth Burn in the neighbourhood of Mains of Creuchies [NO 207 509]. Subsequently, the ice front retreated south-westwards and further ice-marginal channels developed on the hillslope to the east of Burnside of Drimmie [NO 177 517].

An analogous situation existed in the ground to the east of Glendams (Figure 29)b where a number of meltwater channels originate close to the crest of a ridge which extends north-eastwards from Broad Moss. The most northerly group of channels, with intakes ranging in height from above 270 m above OD to below 265 m above OD, were the first to operate and carried meltwater eastwards from ice that lay on the northern side of the Broad Moss ridge. As south-westward retreat of the ice front continued, cols at lower levels on the Broad Moss ridge were exhumed and the meltwater that escaped from the Ericht valley cut the more southerly series of channels which originate at heights from about 259 to 253 m above OD. The lowest and southernmost of these acted as a spillway for the ice-dammed lake at Glendams.

Glaciofluvial meltwater deposits

Sediment, derived both from till and bedrock, was eroded and transported by meltwater and laid down to form extensive deposits, mainly of sand and gravel, in contact with decaying ice and also as outwash plains in front of the retreating ice margin. Although several forms of glaciofluvial deposit have been recognised (mounds, ridges and flat-topped spreads), there is a gradation between each type and they are shown on (Figure 30) as Glaciofluvial Deposits (undivided). Thus the large sheetlike spread of sand and gravel to the south of Blairgowrie, which was laid down in contact with stagnant Strathmore ice by meltwater issuing from the Ericht valley, contained many buried or enclosed blocks of ice. When these subsequently melted, large kettle-holes such as Stormont Loch formed in the surface of the outwash spread giving it, in places, a moundy appearance.

Most of the ice-contact deposits have the form of moundy kame-and-kettle spreads or kame terraces but eskers occur in the valley of the River Ericht, to the north of Blairgowrie, and in the low ground of Strathmore, east of Rattray.

Kame terraces

Sand and gravel deposits forming kame terraces occur along much of the length of the principal Highland valleys such as Strathardle and Glen Shee (Paterson, 1977). The terraces are up to 200 m in width and extend to heights of up to 30 m above the level of the present rivers. The upper surfaces of the kame terraces are commonly pitted by kettleholes. At exposures near Milton [NO 074 614] and Kirkmichael [NO 081 603] in Strathardle, the deposits forming the kame terrace consist of poorly sorted coarse gravel with boulders up to 0.6 m long, mostly of quartz, quartzite and psammite, with subordinate amounts of various schists and igneous rock types.

Kame terraces, 50 to 200 m in width and standing up to 15 m above floodplain level, flank Glen Shee to the north of Tigh-na-Coille [NO 142 652]. Farther south, the glaciofluvial deposits are hummocky, with individual mounds up to 25 m high. More extensive sand and gravel deposits forming kame terraces occur where tributary valleys join Glen Shee, as at Lair [NO 142 633] and Soilzarie [NO 135 595]. These deposits are poorly sorted, with boulders up to 0.25 m long, mainly of resistant metamorphic rock types such as psammite and quartzite.

Eskers

A number of esker ridges up to 150 m long are present in the valley of the Ericht east-south-east of Rochallie [NO 160 508]. All the ridges lie below 150 m above OD, but nearby terraced spreads of sand and gravel reach 170 m above OD on both sides of the valley. It is, therefore, apparent that the terraced deposits must have been laid down against ice as kame terraces and that the eskers were laid down by meltwaters flowing through the stagnant ice.

A more prominent esker system is present in the Ericht valley, in the area east of Middle Mause [NO 167 485]. Here, an esker ridge up to 15 m high, trending north–south, is composed of coarse sand and gravel. It is joined from the west by a second, more sinuous ridge.

A large esker at Castle of Rattray [NO 207 453], now almost completely removed by sand and gravel workings, is one of a series of eskers that extends for about two kilometres along the north side of western Strathmore. The eskers were laid down in stagnant ice that occupied western Strathmore, by glacial meltwaters which were compelled to flow eastwards over the watershed at Forfar while the lower Tay valley was blocked by ice.

Sheet deposits

Although the deposits of sand and gravel in the southeast of the district have the general form of outwash spreads, many were laid down in contact with stagnant ice that partially occupied western Strathmore. Numerous large kettleholes, such as Marlee Loch, Fingask Loch, Rae Loch and Stormont Loch occur close to areas south of the Lunan Burn and north of Marlee Mill [NO 156 431] where kame and kettle topography predominates. All indicate the downwasting of stagnant ice.

Groups of deposits of two ages have been identified (Paterson, 1974). The upper surfaces of the older group stand at somewhat higher elevations and have been more affected by ice-collapse. These older deposits, which mostly occur either side of Marlee Loch, may have been laid down while the lower Tay valley was blocked by ice. The younger deposits of sand and gravel form the Meikleour Outwash Terrace (Paterson, 1974). The upper surface of the terrace is generally planar, except where it is pitted by large kettleholes, and falls southwards with a diminishing gradient and appears to grade to a base level at about 48 m above OD at Cargill (Armstrong et al., 1975). This base level is considerably lower than the present-day watershed at Forfar. This difference in elevation indicates that the terrace was laid down after the melting of the ice that had blocked the lower Tay valley in the Perth area. The 48 m base level may represent sea level (Browne, 1980). If so, it is at a height close to that obtained by projection of Shoreline EF-6 of the East Fife suite of raised shorelines of Cullingford and Smith (1966), for which an age of about 14 750 BP has been suggested (Sissons, 1974). From evidence in the area around the Firth of Tay, Armstrong et al. (1985) suggested that formation of shoreline EF-6 was preceded by a rise of sea level. This may have caused accelerated ice retreat by calving of the ice front in the Firth of Tay at Perth, thereby unblocking the lower Tay valley.

There are few sections of any note in the glaciofluvial sheet deposits. About 8 m of gravel with well-rounded pebbles, mainly of Highland rocks, alternating with thin beds of medium to coarse pebbly sand, were exposed in a pit [NO 158 436] north-north-east of Marlee Mill. Cross-bedding indicates deposition from meltwaters which flowed south-westwards. An upward-coarsening sequence of sand and gravel overlying fine-grained, ripple-laminated sand was seen in a small pit [NO 161 446], to the east of Rae Loch.

Peat

Peat bogs developed widely in the district after deglaciation, both within hollows as thick basin peat (which may be interbedded with, or overlie, organic silts, sands and clays) or as extensive, thinner surficial spreads of blanket peat on hillsides. The peat deposits probably date from various periods in Flandrian times, that is, after 10 000 BP. The oldest known organic sediment was discovered by Walker (1977), concealed beneath basin peat at a depth of almost 11 m, at Corrydon [NO 13 66] in Glen Shee. The peat and the underlying organic muds infill a large kettlehole within undulating, ice contact, sand and gravel deposits that were laid down by meltwaters from the decaying Dimlington Stadial ice sheet. A similar sequence of organic sediments was recorded by Caseldine (1980) at a depth of about 6 m within the infill of the kettlehole at Stormont Loch [NO 19 42], a little to the south in the Perth district. Pollen recovered from these sediments shows affinities with assemblages dated at about 13 800 to 13 000 BP at Windermere. It is probable that sediments of comparable age also occur as part of the kettlehole infills at Rae Loch and near Ardblair Castle [NO 163 445].

Landslips

Landslips are not common in the district but they usually occur where rivers have cut into thick Quaternary deposits. The most prominent slips occur within the deeply incised valley of the River Ericht, to the north of Blairgowrie [NO 17 46] to [NO 17 48] and in the valley of its tributary, the Lornty Burn [NO 16 46].

Alluvium and river terrace deposits

Alluvium was deposited towards the end of the late Devensian and throughout the Flandrian period, to form floodplains along the valleys of the principal rivers and burns that drain the district. River terraces, which were formed in the fluvial deposits as the drainage adjusted to changes of base level, now stand above the present flood-plains. The alluvium is composed of clay, silt, sand and gravel in different proportions. In general, there is a higher proportion of gravel in higher (earlier) river terraces and in deposits forming the floodplains of the steep headwater valleys.

Man-made deposits

As there is little large-scale industrial development within the Glen Shee district and, apart from the spreads of sand and gravel, there are almost no known mineral deposits of economic value, Human activity has had relatively little impact on its geology. The former workings for sand and gravel at Castle of Rattray contain the largest area of back-filled worked ground. Small workings, now overgrown, are present in the esker ridges east of Middle Mause [NO 167 485].

Information sources

Further geological information held by the British Geological Survey relevant to the Glen Shee district is listed below. It includes published material in the form of maps, memoirs and reports. Also included are other sources of data held by BGS in a number of collections, including borehole records, rock samples, thin sections and photographs. Enquiries concerning geological data for the district should be addressed to the National Geological Records Centre, BGS, Edinburgh.

Searches of indexes to some of these collections can be made on the Geosciences Index System in BGS libraries and on the web site at http://bgs.ac.uk.

Maps

The original geological survey was carried out at 1:10 560 scale by G Barrow and J Geikie and was published as part of 1:63 360-scale Sheet 56 (Blairgowrie) in 1895. The solid geology was resurveyed at 1:10 000 scale by A Crane and S Goodman from Aberdeen University, M Krabbendam and A G Leslie from Queens University, Belfast and T P Fletcher, I B Paterson and S Robertson from BGS. 1:10 000-scale maps covering the Glen Shee district, together with the initials of the surveyors and the dates of survey, are listed below. The list indicates whether the maps show both the solid and drift or solid geology only. Other surveyors of adjacent districts covered by these maps include A L Harris.

The maps are available for consultation in the Library, British Geological Survey, Murchison House, Edinburgh, EH9 3LA. Dyeline copies may be purchased from the Sales Desk.

NN 97 SE

Glen Loch

S

1990–1992

AGL

NO 04 NW

Loch Ordie

S

1990–1993

SG, AC

NO 04 NE

Butterstone

S

1993

AGL, MK

NO 04 SW

Dunkeld

S

1993

SG

NO 04 SE

Loch of Lowes

S&D

1967–1970, 1990

MK, ALH, IBP

NO 05 NW

Glen Derby

S

1990–1993

SCG, AC, AGL

NO 05 NE

Kirkmichael

S

1992–1993

SG, AC, MK, AGL

NO 05 SW

Capel Hill

S

1991–1993

SG, MK, AGL

NO 05 SE

Loch Benachally

S

1993

MK, AGL

NO 06 NW

Gleann Fearnach

S

1990–1993

AC, SG, AGL

NO 06 NE

Elrig

S

1990–1993

AC, SG, AGL, MK

NO 06 SW

Kindrogan

S

1991–1993

AGL

NO 06 SE

Enochdhu

S

1991–1993

MK, AGL, AC

NO 07 NW

Carn an Righ

S

1990–1992

AGL, AC, SG, MK

NO 07 NE

Glas Tulaichean

S

1990–1992

AGL, AC, SG

NO 07 SW

Daldhu

S

1990–1992

SG, AGL, AC

NO 07 SE

Glen Lochsie

S

1990–1992

SG, AC, AGL

NO 14 NW

Lornty Burn

S&D

1993–1994

AGL, IBP

NO 14 NE

Blairgowrie and Rattray

S&D

1990, 1994

TPF, IBP

NO 14 SW

Loch of Clunie

S&D

1969, 1994

IBP

NO 14 SE

Rosemount

S&D

1969, 1994

IBP

NO 15 NW

Dalrulzion

S

1991–1993

AC, SG, MK, SR

NO 15 NE

Cairn Gibbs

S

1992–1993

SR, SG

NO 15 SW

Bridge of Cally

S

1993–1994

AC, SG, MK, AGL, SR, IBP

NO 15 SE

Milton of Drimmie

S&D

1990–1994

SR, IBP

NO 16 NW

Spittal of Glenshee

S

1991–1993

SG, MK, AGL

NO 16 NE

Duchray Hill

S

1991–1992

MK, SG, SR

NO 16 SW

Creag nam Brataichean

S

1991–1992

MK, AGL, SG

NO 16 SE

Mount Blair

S

1990–1992

SG, MK, SR

NO 17 SW

Ben Gulabin

S

1990–1992

SG, MK

NO 17 SE

Tulchan

S

1991–1992

MK, SG, SR

NO 24 NW

Alyth

S&D

1994

IBP

NO 24 SW

Coupar Grange

S&D

1968, 1994

IBP

NO 25 NW

Scruschloch

S

1994

SR

NO 25 SW

Mains of Cheadries

S&D

1990–1994

SR, IBP

NO 26 NW

Badandun Hill

S

1990–1993

SR

NO 26 SW

Loch Shandra

S

1991–1992

SR

NO 27 SW

Bawhelps

S

1990–1991

SR

Books

Memoirs and other books, reports relevant to the Glen Shee district arranged by topic. Most are not widely available, but may be consulted at BGS and other libraries.

General geology

British regional geology

STEPHENSON, D, and Gouu), D. 1995. British regional geology: the Grampian Highlands (4th edition). (London: HMSO for the British Geological Survey.)

CAMERON, I B, and STEPHENSON, D. 1985. British regional geology: the Midland Valley of Scotland (3rd edition). (London: HMSO for the British Geological Survey.)

Memoirs

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 and 49 (Scotland).

SMITH, C G, GOODMAN, S, and ROBERTSON, S. In press. Geology of the Ballater district. Memoir of the British Geological Survey, Sheet 65E (Scotland).

Reports

General geology

ARMSTRONG, M, and PATERSON, I B. 1970. The Lower Old Red Sandstone of the Strathmore region. Report of the Institute of Geological Sciences, Vol. 70, No. 12.

CRANE, A, GOODMAN, S, and KRABBENDAM, M. 1994. The Solid Geology of 1:10 000 Sheet NO 15 NW (part sheet) (Dalrulzion). British Geological Survey Technical Report, WA/94/43.

CRANE, A, GOODMAN, S, KRABBENDAM, M, and LESLIE, A G. 1994. The Solid Geology of 1:10 000 Sheet NO 06 NE (Elrig) and part Sheet NO 06 NW (Gleann Fearnach). British Geological Survey Technical Report, WA/94/38.

CRANE, A, GOODMAN, S, KRABBENDAM, M, and LESLIE, A G. 1994. The Solid Geology of part of 1:10 000 Sheet NO 15 SW (Bridge of Cally) and 1:10 000 part Sheet NO 14 NW (Cochrage). British Geological Survey Technical Report, WA/94/44.

GOODMAN, S, and KRABBENDAM, M. 1994. The Solid Geology of 1:10 000 Sheet NO 16 SE (Mount Blair) and part Sheet NO 15 NE (Cairn Gibbs). British Geological Survey Technical Report, WA/94/40.

GOODMAN, S, CRANE, A, and LESLIE, A G. 1994. The Solid Geology of 1:10 000 Sheet NO 07 SW (Daldhu). British Geological Survey Technical Report, WA/94/34.

GOODMAN, S, CRANE, A, KRABBENDAM, M, and LESLIE, A G. 1994. The Solid Geology of 1:10 000 Sheet NO 05 NE (Kirkmichael) and part Sheet NO 05 NW (Glen Derby). British Geological Survey Technical Report, WA/94/41.

GOODMAN, S, CRANE, A, KRABBENDAM, M, and LESLIE, A G. 1994. The Solid Geology of 1:10 000 Sheet NO 07 SE (Glen Lochsie) and part Sheet NO 17 SW (Ben Gulabin). British Geological Survey Technical Report, WA/94/33.

KRABBENDAM, M, and GOODMAN, S. 1994. The Solid Geology of 1:10 000 Sheet NO 16 NE (Duchray Hill) and part Sheet NO 17 SE (Tulchan). British Geological Survey Technical Report, WA/94/35.

KRABBENDAM, M, and GOODMAN, S. 1994. The Solid Geology of 1:10 000 Sheet NO 16 SW (Creag nam Brataichean). British Geological Survey Technical Report, WA/94/37.

KRABBENDAM, M, GOODMAN, S, and LESLIE, A G. 1994. The Solid Geology of 1:10 000 Sheet NO 05 SE (Loch Benachally) and 1:10 000 part Sheet NO 05 SW (Capel Hill). British Geological Survey Technical Report, WA/94/42.

KRABBENDAM, M, GOODMAN, S, and LESLIE, A G. 1994. The Solid Geology of 1:10 000 Sheet NO 16 NW (Spittal of Glenshee). British Geological Survey Technical Report, WA/94/36.

KRABBENDAM, M, LESLIE, A G, and CRANE, A. 1994. The Solid Geology of 1:10 000 Sheet NO 06 SE (Enochdhu) and part Sheet NO 06 SW (Kindrogan). British Geological Survey Technical Report, WA/94/39.

LESLIE, A G. 1994. The Solid Geology of part of 1:10 000 Sheet NN97SE (Glen Loch). Part of 1:50 000 Geological Sheet 55E (Pitlochry) and Sheet 64E (Ben Macdui). British Geological Survey Technical Report, WA/94/31.

LESLIE, A G, GOODMAN, S and KRABBENDAM, M. 1994. The Solid Geology of 1:10 000 Sheet NO 04 NE (Butterstone) and part sheets NO 04 NW (Loch Ordie), NOO4SW (Dunkeld) and NO 04 SE (Loch of Lowes). British Geological Survey Technical Report, WA/94/45.

LESLIE, A G, CRANE, A, GOODMAN, S, and KRABBENDAM, M. 1994. The Solid Geology of part of 1:10 000 Sheet NO 07 NW (Carn an Righ) and part Sheet NO 07 NE (Glas Tulaichean). British Geological Survey Technical Report, WA/94/32.

PATERSON, I B and HARRIS, A L. 1969. Lower Old Red Sandstone ignimbrites from Dunkeld, Perthshire. Report of the Institute of Geological Sciences, Vol. 69, No. 7.

Bulk minerals

AITKEN, A M. 1983. A preliminary study of the sand and gravel deposits of Strathmore. Open-file report of the Institute of Geological Sciences, Edinburgh.

PATERSON, I B. 1977. Sand and gravel resources of the Tayside Region. Report of the Institute of Geological Sciences, No. 77/6.

Metalliferous minerals

COATS, J S, FORTEY, N J, GALLAGHER, M.J, GREENWOOD, P G, and PEASE, S F. 1987. Mineral exploration for zinc, lead and baryte in Middle Dalradian rocks of the Glenshee area. British Geological Survey Mineral Reconnaissance Programme Report, No. 88.

COATS, J S, SHAW, M H, ROLLIN, K E, ROBERTSON, S, and REDWOOD, S D. 1993. Mineral exploration in the Pitlochry to Glen Clova area, Tayside Region, Scotland. British Geological Survey Mineral Reconnaissance Programme Report, No. 126.

GALLAGHER, M J, SMITH, C G, COATS, J S, GREENWOOD, P G, CHACKSFIELD, B, C, FORTEY, N J, and NANCARROW, P.H.A. 1989. Stratabound barium and base-metal mineralisation in Middle Dalradian metasediments near Braemar, Scotland. British Geological Survey Mineral Reconnaissance Programme Report, No. 104.

MCCOURT, W J, and GALLAGHER, M J. 1980. Geological reconnaissance and assessment of certain Highland diorites with special reference to porphyry-style mineralisation. Unpublished British Geological Survey Report.

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 Glen Shee district the collection consists of only three boreholes in the Blairgowrie area.

Material collections

Geological survey photographs

Photographs (31) illustrating aspects of the geology of the Glen Shee district, taken in 1995, are deposited for reference in the libraries at BGS, Murchison House, West Mains Road, Edinburgh EH9 3LA and BGS, Keyworth, Nottingham NG12 5GG; and in the BGS Information Office, Natural History Museum Earth Galleries, Exhibition Road, London SW7 2DE.

A list of tides 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.

Petrological collections

The petrological collections for the Glen Shee district consist of approximately 330 hand specimens and thin sections, predominantly of the metamorphic and igneous rocks in the area north of the Highland Boundary Fault. The sedimentary rocks in the south-east of the district are less well represented.

References

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

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Figures, plate and tables

Figures

(Figure 1) Generalised Solid geology of the Glen Shee district.

(Figure 2) Physical features and location of the Glen Shee district.

(Figure 3) Bouguer gravity anomaly map of the Glen Shee district. Data reduced to OD at a density of 2.75 Mg m−3. Contour values in mGal. Lineaments shown in brown are taken from the images of the gravity data over northern Britain.

(Figure 4) Aeromagnetic anomaly map of the Glen Shee district. Total field anomaly in nT above a linear regional field for the UK. Lineaments shown in brown are taken from the images of the aeromagnetic data over northern Britain.

(Figure 5) 2.5D model along section BB′ ((Figure 3)) across the Blairgowrie anomaly.

(Figure 6) Stratigraphical correlation between the Pitlochry, Glen Shee and Braemar districts (not to scale).

(Figure 7) Stratigraphy of the Appin Group in the Glen Shee district (not to scale).

(Figure 8) Schematic palinspastic reconstruction (not to scale) of the Blair Atholl Subgroup succession in a south-west to north-east transect across the Pitlochry to Braemar area in Islay Subgroup times. The section is constructed using key sequence stratigraphical datum lines. Note the upward areal expansion of the Tulaichean Schist Formation (interpreted as a transgressive delta and associated shoreline). The formation becomes more semipelitic from south-west to north-east.

(Figure 9) Stratigraphy of the Southern Highland Group in the Glen Shee district (not to scale).

(Figure 10) Total field magnetic intensity map of the Capel Hill area. Prominent anomalies over Green Beds show the D1 Capel Hill Syncline.

(Figure 11) Structural map illustrating main slide zones and fold axial traces in the Glen Shee district.

(Figure 12) Map showing structural domains in the Glen Shee district with stereograms showing poles to S2.

(Figure 13) Cross-section extending north-west from the Middleton Muir Fault [NO 0973 4500] to Capel Hill [NO 0385 5390].

(Figure 14) Quartz c-axis fabric (N = 111) in Southern Highland Group psammite [NO 0555 5296]; section cut parallel to lineation and normal to S2 (strike 090°, dip 19°N).

(Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15) Composite regional section of Dalradian rocks between Pitlochry and Glen Shee viewed parallel to the plunge of F2 folds. The section is drawn assuming a constant north-east plunge azimuth. Average plunge inclinations of 15° are used for most of the construction, but south-west of Gleann Fearnach, these decrease to zero near Pitlochry. Post-F2 igneous intrusions are included in the section for geographical reference where they cut the structure in the present topography. The folded appearance of the major slides is due to the effects of F3 folding about axes that lie close to the plane of this section (see ((Figure 16)) and text for further details).

(Figure 16) Composite section of the Gleann Mor and Gleann Fearnach areas viewed parallel to local F2 plunge. The part of the diagram that includes Carn an Righ and Gleann Mor (lower left) has been constructed on the basis of a mean F2 plunge of 45° towards a 120° azimuth; the part of the diagram that includes the Glen Lochsie–Carn Dallaig–Ben Vuirich area (upper and right areas of section) has been constructed using a 135° plunge azimuth with plunge inclination reducing from 45° near Carn Dallaig to 10° near Beinn a' Chruachain. A roughly constant section thickness is maintained for the main outcrop of the Tulaichean Schist Formation in this composite section. Note: (i) the main F2 anticline (Carn Dearg–Meall Ruigh Mor Thearlaich anticline) located between the Glen Lochsie and Carn Dearg slides plunges in and out of the section plane, (ii) the Ben Earb Slide lies south-east of a F3 plunge depression which is responsible for the prominent 'cusp' in the Ben Earb–Carn Dearg Slide trace.

(Figure 17) Cross-section extending from Gleann Fearnach [NO 0275 6521] and Strath Ardle to Whitefield Hill [NO 1143 6285].

(Figure 18) Cross-section extending south-east from Meall Uaine [NO 1108 6885] to Cairn Gibbs [NO 1890 5700].

(Figure 19) Geological map of Beinn a' Chruachain and Upper Gleann Fearnach (inset area of (Figure 11)).

(Figure 20) Schematic section of D2 structure, after removal of post-D2 structures (compare with (Figure 16)) and text for further details)." data-name="images/P1000032.jpg">(Figure 15)), not to scale.

(Figure 21) Simple shear model illustrating the effects upon upright F1 folds (A) of a horizontal simple shear deformation (B) of an angular shear of 80°.

(Figure 22) Map of metamorphic mineral zones, migmatitic rocks, peak metamorphic thermobarometric estimates and important intrusive igneous bodies.

(Figure 23) AFM diagrams of chloritoid-bearing assemblages. a with chloritoid + chlorite b with chloritoid + biotite

((Figure 24)) Composition of stromatic migmatitic rocks of the Duchray Hill Gneiss Member: a Mica content b Plagioclase composition c Quartz: plagioclase: orthoclase cotectic lines with PH20 = 5 kb and eutectic point at 650°C after Wyllie (1976) d Quartz: plagioclase: mica Data for a and d obtained by SEM-backscatter and image processing, and for b by microprobe analysis. Data points represent composition of leucosomes (filled brown circles), mesosomes (open circles) and melanosomes (crosses) in domains of 1.8 X 2 mm.

(Figure 25) Compositional zoning profiles in garnets. a Amphibolite QY546 [NN 9910 7580] (Garnet 1) b Pelite QY547 [NN 9910 7580] (Garnet 2)

(Figure 26) Chemical analyses of 18 samples of the Glen Shee Pluton. a Major elements v silica b Sr-Ba-Zr v silica c Transition metals v silica

(Figure 27) Generalised vertical section of upper Silurian to lower Devonian sedimentary and extrusive igneous rocks.

(Figure 28) Selected geological features formed during the glaciation and deglaciation of the area around Strathmore (and the Sidlaw Hills).

(Figure 29) Glacial drainage systems. a Mains of Creuchies area b Glendams area

(Figure 30) Glacial and glaciofluvial landforms and deposits in the south-east part of the Glen Shee district.

Plates

(Front cover)

(Rear cover)

(Plate 1) Cross-bedding in the An Socach Quartzite Formation, Carn an Righ [NO 0219 7689] (D 5137).

(Plate 2) Tiger Rock: striped metacarbonate rock of the Glen Loch Phyllite and Limestone Formation in the Allt a' Ghlinne Bhig [NO 0120 7863] (Photograph:D Stephenson).

(Plate 3) Domino-style boudinage and extension structures in metacarbonate rocks of the Gleann Mor Limestone Member, Gleann Mor [NO 0137 7634] (D 5138).

(Plate 4) Laminated garnet-poor metasiltstones of the Tulaichean Schist Formation, near the watershed on the Carn Dallaig–Faire a' Ghlinne Mhoir ridge [NO 0246 7513] (D5139).

(Plate 5) Interference between F2 and F3 folds in the Glen Lochsie Calcareous Schist Member of the Gleann Beag Schist Formation, near Glenlochsie Lodge [NO 0632 7268] (D5140).

(Plate 6) Migmatitic rocks of the Duchray Hill Gneiss Member with stromatic layering deformed by small-scale close F3 folds, which refold tight F2 folds (above coin), Craigan Caise, west of Glen Isla [NO 1780 6791] (D5141).

(Plate 7) Reconstituted Duchray Hill Gneiss Member with domains of stromatic gneiss within more homogeneous granitoid, 1 km northeast of Fergus, Glen Isla [NO 1990 6916] (Photograph:S Robertson).

(Plate 8) Wester Bleaton Quarry in Loch Tay Limestone Formation [NO 115 597]. Quarry wall 20 m high. The pale grey metacarbonate rock has been quarried back to the vertical margin of a Carboniferous dolerite dyke, which forms the black north wall of the quarry at the right side of the photo. The coarse amphibolite which is everywhere associated with the Loch Tay Limestone Formation can be seen overlying the metacarbonate rock at the centre of the photo (dark grey) (D5142).

(Plate 9) Inverted coarse-grained gritty psammite unit in the Southern Highland Group showing grain-size grading and scour at the base of the bed, south-west of Deuchary Hill [NO 0294 4799]. The main foliation is S2 (D5143).

(Plate 10) Craig Lair Hornblendic Gneiss with granitoid segregation cross-cutting stromatic layering, Mid Hill [NO 2230 7075]. Lens cap is 5 cm across (Photograph: S Robertson).

(Plate 11) Pegmatite in Southern Highland Group showing pinch and swell structure, north-east of Glen Fearnate Lodge [NO 0550 6524]; an earlier F2 fold lies to the left of the hammer head (D5144).

(Plate 12) S2 crenulation cleavage transecting F2-folded S1 pressure solution cleavage at a high angle, Southern Highland Group psammites, Riemicke [NO 0722 5185] (Photograph:M Krabbendam).

(Plate 13) Spaced S1 pressure-solution cleavage folded by angular, close F2 folds in Southern Highland Group psammite, Conlan Hill [NO 0435 4791]. View to the northeast (Photograph: M Krabbendam).

(Plate 14) Photomicrographs. Longer edge of each photomicrograph is 3 mm. PPL plane polarised light; XPL cross polarised light. a. Calcareous graphitic schist with S2 foliation developed parallel to the longer edges of the photomicrograph. The S2 foliation in part comprises transposed, isoclinally folded S1 muscovites that contain much included graphite. Granoblastic aggregates of quartz and calcite (high relief) polyhedra have developed preferred shape-orientation parallel to S2, Specimen WY352, Ben Eagach Schist Formation, Gleann Fearnach [NO 0432 6862], PPL. b. Laminated graphitic schist with So bedding traces subparallel to S1 foliation defined by aligned muscovite and biotite. F2 crenulations of S1 and S2 crenulation foliation are developed in the matrix but not in the internal fabric of the irregular garnet porphyroblast in the centre of the photomicrograph. Specimen WY317, Glen Loch Phyllite and Limestone Formation, north slopes of Carn Bhinnein [NO 0092 7752], PPLL. c. Subidioblastic garnet porphyroblast in schistose semipelite. The internal fabric within the garnet comprises relict quartzofeldspathic components of the S1 fabric. This is slightly sigmoidal in form and is continuous with the external S1/S2 composite fabric. Most of the garnet porphyroblasts adjacent to this sample have their internal fabric orientated at a similar high angle to the external foliation. Specimen ZY187, Southern Highland Group, Meall Dubh [NO 0778 5422], XPL. d. Schistose-muscovite-biotite-garnet semipelite containing ribbons of very fine-grained (<50 pm) sheared psammite separating asymmetric quartz and K-feldspar-rich lenticular domains. The idioblastic garnet porphyroblast contains a sigmoidal internal fabric composed of relict S1, continuous with the external ribbon-like domains of strongly D2-sheared psammite. Specimen VY044, Tulaichean Schist Formation, Glas Tulaichean [NO 0590 7547], PPL. e. Garnet porphyroblast with strongly curved inclusion trails of fine-grained graphite in this graphitic schistose pelite. The internal trails within the garnet are continuous with the external S2 foliation which shows differential development of post-F2 crenulations adjacent to the garnet porphyroblasts. Specimen WY606, Ben Eagach Schist Formation, west slopes of Beinn a' Chruachain [NO 0399 6972], PPL. f. Part of a large garnet porphyroblast with inclusion trails showing S1 crenulated by F2. Garnet that is relatively inclusion-free has developed along S2 crenulation foliation domains, and crenulated S1 elements are preserved as sigmoidal elongate quartzofeldspathic aggregates. In several of the garnet porphyroblasts in this sample, the internal foliation is continuous with the external S2 foliation; such a relationship has been slightly modified by post-D2 strain in the garnet figured. Specimen WY796, Southern Highland Group, Creag Dubh-leitir [NO 0539 6570], XPL. g. Very fine-grained dark pelitic schistose matrix composed of muscovite, biotite and chlorite with a few quartz grains. A large idiomorphic porphyroblast of garnet (right) is cracked and contains small inclusions of rutile. The large subidioblastic porphyroblast (left of centre) is chloritoid that has inclusions of randomly orientated biotite (dark flakes) and also contains included rutile. Specimen(S95822), Southern Highland Group, Loch Ordie [NO 0362 4919], PPL. h. Schistose pelite with S2 defined by elongate quartz and feldspar aggregates and aligned muscovite and biotite. A large porphyroblast of staurolite (top) has been stretched parallel to S2 and now contains cracks filled with quartz. The large subidioblastic garnet porphyroblast (bottom) contains crenulated S1 inclusion trails which now make high angles with S2. Specimen(S95820), Southern Highland Group, 1 km north-west of Capel Hill [NO 0275 5320], PPL.

(Plate 15) Photomicrographs. Longer edge of each photomicrograph is 3 mm. PPL plane polarised light; XPL cross polarised light.  a. Micaceous psammite with well-developed S–C fabric. S2 biotite and muscovite fabric (aligned parallel to longer edges of photomicrograph) has modified S1 spaced cleavage. Modified S1 quartz lithons, enveloped by micaceous laminae are now lozenge-shaped and show dynamic recrystallisation textures with lobate grain and sub-grain boundaries. Specimen ZY196, Southern Highland Group, Craig Wood [NO 0555 5296], XPL. b. Semipelite in F2 hinge zone. F2 folds of bedding (S0) are indicated by the relatively quartz-rich layer (about 1 mm thick) of this photomicrograph. S1 slaty cleavage (defined by micas formed subparallel to S0) has also been crenulated by F2, and S2 crenulation foliation has developed axial planar to F2 crenulation folds. Specimen ZY197, Southern Highland Group, 1 km north-east of Deuchary Hill [NO 0448 4813], XPL. c. Interbedded psammite and graphitic pelite with relict detrital texture in the psammite. So is transected obliquely by S1 spaced cleavage, defined by concentrations of aligned muscovites in the psammite, and by concentrations of graphite in the pelite. A penetrative foliation of aligned muscovites is also developed in the pelite and has been deformed by F3 crenulations. Specimen WY445, Ben Eagach Schist Formation, Beinn a' Chruachain [NO 0409 6921], PPL. d. So compositional layers and subparallel S1 foliation in fine-grained hornblende schist are folded about tight F2 folds. The folded fabric is defined by very fine-grained hornblende epidote and biotite in a quartzofeldspathic matrix. These have recrystallised parallel to S2 in the hinge zones of the F2 folds. Subidioblastic poikiloblastic porphyroblasts of hornblende and biotite have also developed parallel to S2. Specimen QY737, Farragon Volcanic Formation, 1 km west of Elrig [NO 0695 6731], PPL. e. Tight F2 fold closure in calcareous psammite. So compositional layers and S1 foliation defined by aligned biotites and fine-grained hornblende prisms occur in two main orientations that represent the limbs of this angular fold. Continuous axial planar S2 foliation is defined by the shape orientation of fine-grained granoblastic aggregates of calcite and quartzofeldspathic grains, and by the alignment of some biotite flakes. Specimen WY682, Ben Lawers Schist Formation, Gleann Fearnach [NO 0356 6967], PPL. f. Flow banding in the marginal facies of the Creag Lamhaich Porphyry. The subhedral quartz megacryst (centre) is set in a flow banded glassy-microcrystalline felsitic groundmass that wraps the megacryst. Specimen QY756, Quartz–feldspar porphyry, Creag Lamhaich [NO 0905 7441], XPL.

(Plate 16) F4 fold related to the Highland Border Downbend in Southern Highland Group schistose semipelites with minor intrafolial F2 folds, Birkenburn [NO 0253 4509] (D 5145).

Tables

(Table 1) Geological sequence in the Glen Shee district.

(Table 2) Summary of structural history of the Glen Shee district.

(Table 3) Correlation of deformation phases in the Glen Shee district with those recognised in adjacent districts.

(Table 4) Major structures within the Glen Shee district described from south to north ((Figure 11)).

(Table 5) Relationship between porphyroblast growth and deformation.

(Table 6) Timing of deformation and metamorphism.

(Table 7) Representative mineral compositions for samples of pelite and amphibolite.

(Table 8) Results of thermobarometric calculations.

(Table 9) Whole rock analyses of the Glen Shee Pluton.

Tables

(Table 2) Summary of structural history of the Glen Shee district.

D1

D1 produced a spaced cleavage in psammitic rocks and a penetrative cleavage in pelitic rocks. The folds were originally close to tight and probably upright structures.

D2

F2 folds are close to isoclinal folds, predominantly NW-vergent, and are associated with the dominant schistosity, S2. S1 became intensely crenulated during D2, and is strongly attenuated on F2 fold limbs. D2 slides have developed on the attenuated limbs of some markedly asymmetric F2 folds; these are mostly associated with top to the SE-directed shear. F2 folds are widespread over the Dalradian outcrop, except in a narrow zone north of the Middleton Muir Fault.

D3

F3 folds are probably the result of several subphases rather than a single coherent phase but are referred to as F3 for convenience. Mostly they are open to tight, upright or inclined folds with moderately/steeply dipping axial planes. D3 produced crenulation cleavage (s) of variable intensity. The F3 folds have various orientation and vergence; they are commonly coaxial with F2 folds. F3 folds have an uneven distribution but are common north of gridline 58.

D4

F4 folds are open to close folds with gentle south-west or north-east plunge, and with moderate to steeply NW-dipping axial planes. Locally, crenulation cleavage is poorly developed in pelitic rocks. The F4 folds are mainly associated with the large-scale D4 Highland Border Downbend; some open warps north of the downbend are probably also associated with the D4 event.

(Table 3) Correlation of deformation phases in the Glen Shee district with those recognised in adjacent districts

District

Deformation phase

Sheet 56W, this memoir Harte et al., 1984

1

2

3

4

Mendum and Fettes, 1985

Robertson, 1994 (Southern Highlands)

1

2

3

4

Harris et al., 1976 (Dunkeld area)

1

2/?3

?

4

Upton, 1986 (Sheet 65W, Braemar district)

?1

1/2

?3

?

Bradbury et al., 1976; 1979 (Sheet 55E, Pitlochry district)

1

2/?3

?3

4

Treagus, 1987 (Schiehallion area)

1

2

3

(Table 4) Major structures within the Glen Shee district described from south to north (Figure 11)

Middleton Muir Fault

This forms part of the boundary between Dalradian and Devonian rocks in the district and is a splay off the Highland Boundary Fault.

Highland Border Downbend

A major D4 antiform that has rotated gently NW-dipping rocks of the Flat Belt into their near-vertical attitudes in the Highland Border Steep Belt.

Capel Hill Syncline

A SE-facing D1 syncline, modified by D2, expressed by folding of the southern Green Bed unit.

Mount Blair Anticline

A (now) inclined east-facing D1 anticline, with Loch Tay Limestone Formation in its hinge zone; this structure was highly modified by D2 deformation.

Fergus Slide

D2 tectonic contact between the Mount Blair Psammite and Semipelite Formation (Southern Highland Group) and the Ben Lui Schist Formation. The Loch Tay Limestone Formation has been attenuated or excised along the Fergus Slide, which now forms the upper limb of a major D1 syncline, the structurally higher complement of the Mount Blair Anticline.

Gleann Fearnach Fault

A major NW-trending fault that juxtaposes rocks of the Tayvallich Subgroup and those of Crinan to Easdale subgroups.

Ben Earb Slide

A D2 slide, marked by slivers of Ben Eagach Schist and Creag Leacach Quartzite formations occurring within the main outcrop of the Ben Lawers Schist Formation. This slide has developed on the attenuated common limb of a D1 anticline–syncline fold pair that produces a wide outcrop of Ben Lawers Schist Formation. The Ben Earb Slide is linked via the Beinn a' Chruachain Complex into the Carn Dallaig Slide zone.

Beinn a' Chruachain Complex

A SE-plunging stack of F2 folds refolded by F3 folds. The complex is anticlinal, being more or less enveloped by Easdale Subgroup rocks, and with F2 anticlinal cores mostly occupied by Tulaichean Schist Formation. The north-east limb of the complex is strongly attenuated along the D2 Carn Dearg Slide (equivalent to the Ben Earb Slide). North of the Beinn a' Chruachain Complex sensu stricto, analogous upward-facing F2 fold pairs have likewise developed slides on their attenuated limbs (the Glen Lochsie Slide and the Meal! Ruigh Mor Thearlaich Slide).

Carn Dallaig Slide

A steep, NW-trending D2 slide that juxtaposes rocks of the Ben Lawers Schist (south-west side) and Tulaichean Schist (north-east side) formations; it appears as a 'root zone' for the Beinn a' Chruachain Complex, the Glen Lochsie Slide and the Meall Ruigh Mor Thearlaich Slide. North-east of the Carn Dallaig Slide, F2 folds are downward facing, south-west of the slide they are upward facing.

Killiecrankie–Glen Loch Slide and Creag Uisge Slide

These form two subparallel, roughly NE-trending D2 slide systems on either side of the Ben Vuirich Granite. The Creag Uisge Slide juxtaposes Ben Lawers Schist Formation (on east) against Tulaichean Schist Formation (on west); it links with the Carn Dallaig Slide via a structurally attenuated Islay–Easdale subgroup succession north of Daldhu. The Killiecrankie–Glen Loch Slide system, in the north-east corner of Sheet 55E, occurs entirely within Blair Atholl Subgroup rocks; it probably converges with the Creag Uisge Slide south-west of the Ben Vuirich Granite.

Baddoch Burn Slide

This D2 Slide juxtaposes An Socach Quartzite Formation (on south-east) and Glen Taitneach Schist Member (on north-west) along the valley of the Baddoch Burn. Southwards along Gleann Taitneach, it places progressively younger rocks in the hanging wall (on east) against the Glen Taitneach Schist Member of the footwall. This slide has complex linkages with the Carn Dallaig Slide and the Beinn a' Chruachain Complex.

Carn an Righ Slide

The outcrop of this slide is situated on the northern flanks of Carn an Righ; it represents the structurally lowest slide in the district, and juxtaposes An Socach Quartzite Formation (on south) and a north-younging Gleann Mor Limestone Member–Tulaichean Schist Formation succession. The Carn an Righ Slide is associated with two subsidiary slides on Carn an Righ itself; one of these occurs on the limb of an anticline with rocks of the Beinn a' Ghlo Transition Formation in its hinge zone.

(Table 5) Relationship between porphyroblast growth and deformation

Pre-D2

straight S1garnets wrapped by S2 schistosity common in southern Southern Highland Group

Early D2

S1lightly sigmoidal garnets wrapped by S2 schistosity common throughout the district

Syn-D2

S1strongly sigmoidal or curved garnets wrapped by S2 schistosity common in north of district

Late post-D2

S1crenulated euhedral rims, overprinting external S2 on syn-D2 garnets, rare, confined to specific areas

(Table 6) Timing of deformation and metamorphism

Intrusion of Kennethmont Granite (458 ± 1 Ma)9

F4

inclined folds

c.460 Ma onwards8 (metamorphic cooling ages)

S4

very localised crenulation foliation

M4

greenschist facies

Intrusion of Aberdeen Granite (470 ± 1 Ma) 7

D3

F3

upright/inclined folds (domainal sub-sets)

S3

localised crenulation foliation/shear fabrics

M3

greenschist-amphibolite facies transition

Intrusion of Younger Basics (468 ± 8 Ma)6

D2

F2

recumbent folds/strong simple shear component

S1

transposed

S2

crenulation/spaced/continuous foliation (main regional foliation)

M2

amphibolite facies/local migmatisation

D1

F1

upright/inclined folds

S1

continuous/spaced foliation

M1

greenschist facies

Younger Dalradian sedimentary rocks

Leny Limestone: Early Carnbrian (c.520 Ma)3,4

MacDuff Slate: Early Ordovician (490–480 Ma) 5

Eruption of Tayvallich Volcanic Formation (595 ± 4 Ma)1 and intrusion of Ben Vuirich Granite (590 ± 2 Ma) 2
  • 1 Halliday et al., 1989
  • 2 Rogers et al., 1989
  • 3 Cowie et al., 1972
  • 4 Tanner, 1995
  • 5 Molyneux, 1998
  • 6 Rogers et al., 1995
  • 7 Kneller and Aftalion, 1987
  • 8 Dempster, 1985
  • 9 Oliver et al., 2000
  • Note: It is not clear at the present time whether the Ben Vuirich Granite was emplaced before, during or after D1.

(Table 7) Representative mineral compositions for samples of pelite and amphibolite

Weight per cent oxide

a. Pelite S 95832

b. Amphibolite QY722

Ms

Bt

Grt

Pl

Hbl

Grt

Pl

SiO2

46.211

36.806

36.882

59.628

43.234

39.006

60.047

TiO2

0.97

2.328

0.261

0

0.626

0.199

0

A12O3

32.837

18.679

21.073

25.34

16.186

21.046

24.862

Cr2O3

0

0

0

0

0

0.008

0

FeO

1.841

18.284

31.881

0.177

14.963

24.274

0.171

MnO

0

0.095

1.521

0

0.233

3.647

0

MgO

1.242

10.617

2.78

0

9.701

2.629

0

CaO

0.033

0.006

5.198

1.281

10.62

9.178

1.137

Na2O

0.692

0.464

0

2.652

1.832

0

2.66

K2O

10.079

9.16

0

0.002

0.536

0

0.054

Formula

Ms

Bt

Grt

Pl

Hbl

Grt

Pl

Si

6.258

5.485

2.969

10.673

6.346

3.066

10.791

Ti

0.099

0.261

0.016

0

0.069

0.012

0

Al

5.241

3.281

1.999

5.346

2.8

1.95

5.266

Cr

0

0

0

0

0

0.001

0

Fe

0.209

2.279

2.146

0.027

1.837

1.596

0.026

Mn

0

0.012

0.104

0

0.019

0.243

1.137

Mg

0.251

2.358

0.334

0

2.123

0.308

2.66

Ca

0.005

0.001

0.448

1.281

1.67

0.773

0.054

Na

0.182

0.134

0

2.652

0.521

0

0

K

1.741

1.741

0

0.002

0.1

0

0
  • a. pelite(S95832), Ben Lui Schist Formation, east side of Gleann Fearnach [NO 0559 6718].
  • b. amphibolite QY722, Allt Fearnach [NO 0236 7167].
  • Determined by microprobe; all Fe as FeO.
  • Mineral abbreviations as in (Table 8).

(Table 8) Results of thermobarometric calculations

Pelite thermobarometry

Temperature

Pressure Kbar

Sample

Grid reference

Assemblage

Ferry and Spear, 1978

Hodges and Spear, 1982

Hoisch, 1990

QY502

[NO 0324 5234]

Ms, Bt, Qtz, Pl, Grt, St

320

366

QY503

[NO 0377 5218]

Ms, Bt, Qtz, Pl, Grt, Chl

350

409

QY530

[NO 0275 5302]

Ms, Qtz, Bt, Pl, Grt, St

287

339

QY547

NN 9910 7580]

Bt, Qtz, Pl, Grt, Chl

481

548

QY570

[NO 0615 7633]

Ms, Bt, Qtz, Grt, Chl

520

546

QY572

[NO 0788 7235]

Ms, Bt, Qtz, Grt

424

477

QY585

[NO 0780 7157]

Ms, Bt, Qtz, Pl, Grt, Chl

419

471

QY699

[NO 1729 7061]

Ms, Bt, Qtz, Pl, Kfs, Grt

724

747

5.1

QY713

[NO 0141 7241]

Ms, Bt, Qtz, Pl, Grt, Hbl

539

612

13

QY715

[NO 0021 7285]

Ms, Bt, Qtz, Grt, Chl

451

514

QY731

[NO 0418 6638]

Ms, Bt, Qtz, Pl, Grt, St, Chl

425

492

QY735

[NO 0559 6718]

Ms, Bt, Qtz, Pl, Grt

522

573

6.1

Ms = muscovite, Bt = biotite, Grt = garnet, Pl= plagioclase, Hbl = hornblende, Qtz = quartz St = staurolite, Chl = chlorite, Kfs = K-feldspar, Ep = epidote, Spn = sphene and Cal = calcite.

Amphibolite thermobarometry

Temperature

Pressure Kbar

Sample

Grid reference

Assemblage

Graham and Powell,

1983

Holland and Blundy,

1994

Kohn and Spear,

1989

QY546

[NN 9910 7580]

Hbl, Pl, Qtz, Grt, Bt

590

540

8.6

QY708

[NO 0248 7107]

Hbl, Qtz, Pl, Ep, Spn

578

QY722

[NO 0236 7167]

Hbl, Qtz, Pl, Cal, Grt

571

566

7.7

QY733

[NO 0446 6648]

Hbl, Qtz, Pl, Grt, Spn

610

540

8.1

Ms = muscovite, Bt = biotite, Grt = garnet, Pl= plagioclase, Hbl = hornblende, Qtz = quartz St = staurolite, Chl = chlorite, Kfs = K-feldspar, Ep = epidote, Spn = sphene and Cal = calcite.

(Table 9) Whole rock analyses of the Glen Shee Pluton

MXR 143

MXR 144

MXR 147

MXR 149

MXR 151

MXR 152

MXR 153

MXR 154

MXR 155

MXR 156

MXR 158

MXR 159

MXR 160

MXR 163

MXR 164

MXR 167

MXR 169

MXR 171

NGR

[NO 1373 6900]

[NO 1365 6965]

[NO 1267 6815]

[NO 1375 6969]

[NO 1426 6971]

[NO 1405 6975]

[NO 1525 7045]

[NO 1344 6972]

[NO 1342 6972]

[NO 1670 6923]

[NO 1670 6923]

[NO 1663 6793]

[NO 1667 6782]

[NO 1420 6765]

SiO2

68.11

61.41

61.36

60.4

57.94

74.46

68.73

68.95

68.54

69.15

75.02

65.07

65.22

62.7

62.58

60.17

59.22

64.98

TiO2

0.45

0.68

0.77

0.84

0.96

0.13

0.51

0.47

0.4

0.38

0.22

0.62

0.64

0.75

0.72

0.87

0.9

0.64

Al2O3

15.44

16.11

16.41

16.77

16.31

13.5

14.71

15.45

16.01

15.88

13.26

15.91

15.96

15.77

16.19

16.08

16.32

15.64

Fe (total)

2.73

3.88

4.81

5.12

5.66

1.33

2.98

2.72

2.89

2.89

1.55

3.94

3.96

4.54

4.3

5.11

5.46

3.78

MnO

0.04

0.04

0.08

0.08

0.09

0.07

0.06

0.04

0.04

0.02

0.04

0.05

0.06

0.07

0.07

0.07

0.09

0.05

MgO

1.68

2.82

3.38

3.59

4.99

0.21

1.83

1.55

1.53

1.28

0.43

2.51

2.7

3.27

3.22

4.2

4.26

2.67

CaO

2.55

3.27

3.76

4.05

4.71

0.37

2.05

2.16

2.09

1.75

0.38

3.07

3.26

3.53

3.65

4.02

4.28

2.94

Na2O

4.56

4.17

4.44

4.51

3.92

3.75

3.9

4.62

4.68

4.62

3.9

4.28

4.31

4.19

4.23

3.94

3.83

4.43

H2O

2.98

2.63

2.69

2.22

2.66

5.62

3.79

2.68

2.46

2.72

4.62

2.66

2.92

2.97

2.88

2.99

3.06

2.95

P2O5

0.18

0.27

0.3

0.34

0.35

0.02

0.17

0.16

0.15

0.16

0.04

0.25

0.25

0.3

0.27

0.34

0.36

0.25

TOTAL

98.72

98.28

98

97.92

97.59

99.46

98.73

98.8

98.79

98.85

99.46

98.36

98.28

98.09

98.11

97.79

97.78

98.33

Ba

772

589

808

719

764

163

679

657

807

599

325

976

775

916

1978

1012

980

720

Cr

21

39

96

60

138

5

31

18

17

15

7

32

53

80

72

114

108

41

Cu

9

14

48

26

27

20

9

57

59

10

10

23

13

28

20

19

26

11

La

34

77

42

48

37

15

40

48

17

35

45

24

28

112

39

53

51

35

Ni

17

26

61

38

91

7

22

15

14

14

6

22

38

48

63

82

76

31

Pb

10

15

7

9

10

35

11

5

7

8

49

7

4

19

18

11

15

10

Rb

93

106

68

74

71

372

131

137

74

77

184

70

62

80

62

100

84

86

Sr

670

684

830

855

919

62

435

619

837

796

111

809

739

866

759

816

829

714

V

51

59

91

89

153

5

54

44

52

49

3

62

79

104

98

138

101

89

Y

13

18

24

22

23

45

18

16

11

8

22

21

16

22

22

25

28

23

Zn

42

54

57

34

46

0

57

29

22

64

57

28

28

62

90

61

62

35

Zr

147

130

250

240

184

29

180

155

121

150

71

294

259

223

253

351

365

136

Numbers prefixed by MXR refer to specimens in the BGS collection, Edinburgh.