J. geol. SOC. London, Vol. 137, 1980, pp. 469-482, 8 figs., 1 table. Printed in Northern Ireland.

The stratigraphy and structure of the Moine rocks N of the Schiehallion Complex,

P. R. Thomas

SUMMARY: Sedimentary structures are used to establish a ‘Younger’ Moine stratigraphical succession in Strathtummel. Polyphase re-folding of the Moine sequence between Schiehallion and Drumochter can be clearly subdivided into primary and secondary deformations. Primary NE-SW trending deformational events D, and D, include the formation of the south-eastward facing D, Atholl nappe re-folded by strongly overturned D, folds which become upright on the crest of theDrumochter dome. Secondary deformation resulted in D, (oblique)and D, (transverse) major folds, both of which plunge into the SE quadrant.

Since much of the published work on the tectonics of ferred to as the Struan or EildeFlags by most authors. theCentral Highlands has involved Dalradian sequ- It is composed largely of siliceous schists whose gen- ences, very little is known about the underlying Moine eral flaggy character varies with the proportion of the succession and structure. Considering the monotonyof constituentminerals and their grain size. The main the psammitic lithologies, the paucity of clean expos- minerals are quartz, plagioclase feldspars, biotite and ureon moorlands and the remoteness of manykey some muscovite, with very few porphyroblasts, other localities, this is not surprising. Except for the lengthy than microcline. paper by Barrow (1904), much of the published work Granularpsammitic schist is themost common on the maintract of Moine in the W Central High- lithologyin theGlen Garry-Strathtummel area, landsconsists of shortnotes on tunnel sections thoughwhite quartzites, pelitic and semi-pelitic (Robertson1939; Anderson 1951; Johnstone & schists, heavy mineral and calc-silicate bands are also Wright 1955) and isolated quarry exposures (McIntyre present. The term ‘schist’ has been adopted here since 1950). there are few of the discrete stromatic segregations of N of the Drumochter Pass theMoine and Lower quartzo-feldspathic material that may be found in the Dalradian rocks of the Monadhliath have been studied ‘gneisses’ of the ‘Central Highland Division’ (Piasecki byAnderson (1947, 1956), Whitten (1959), Smith & van Breemen 1976b). Unfortunately, most of the (1968) and Piasecki (1975), while to the S, the Dalra- useful non-psammitic bands are too thin to be traced dian of the Schiehallion Complex has been mapped in betweenriver sections. However, the obvious litho- detail by E. M. Anderson (1923), Bailey & McCallien stratigraphicaldisadvantages are partly alleviated by (1937) and Rast (1958). Treagus & King(1978) re- theabundance of sedimentarystructures and by the vised some of the stratigraphical detail in the Dalra- fact that these monotonous Moine lithologies react to dian around Strathfionan, while the continuing work stress more uniformly than the Dalradian succession, of Piasecki and van Breemen (1979 a, b) in the NE in which mineralogy and competence vary markedly. CentralHighlands is bringing to lightevidence of a Althoughmetamorphic recrystallization up to lower mucholder Moine stratigraphy at lower structural amphibolite facies has obliterated most of the original levels. Other publications relevant to the present studysedimentary grain boundaries, it is consideredthat are shown on Fig. 1. much of the lithological variation represents true bed- Thispaper is a summary of thework carried out ding. during 3 years of mappingand research from the Distinct ‘zones’ of banded schist, up to 4 km wide, Dalradian boundary NW to Loch Garry, an area of occur mainly in the proximity of the Boundary Slide over350 km2, together with observations in the (Fig.4) andDalnacardoch (NN 722 703) where the ground between Loch Garry and Loch Ericht. Subse- Moine contains thin regular platy units of psammitic quentwork by the author covering the important andpelitic material (each 20-300 mm inthickness) structuresfound more recently between Loch Ericht which may be equivalent to rocksof the ‘Moine Phase’ andLoch Treig is thesubject of a separatepaper referred to by King & Rast (1955). Within the ‘banded (Thomas 1979), which attempts to constructa continu- zones’ lithological changes lie sub-parallel to the schis- ous section across the W CentralHighlands without tosity, sedimentary structures are usually absent, and obliteration by plutonic masses. minor isoclinalfolds of highamplitude and low wavelength are a commonfeature. Away from the Moine lithologies ‘bandedzones’, semi-pelitic and psammitic Moine is often laminated with fine ribs of biotite. The Central Highland Moine succession has been re- There are few pelitic schist units greater than 30 m

0016-7649/80/0700-0469$02.00 @ 1980 The Geological Society

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FIG. 1. Location map indicating post-war research in the W Central Highlands.

in thickness,but several thin heavy mineral bands, Salmon Leap at Struan Gorge (NN804 656) and on similar to those in the Western Highlands (Ramsay & Craig nan Caisean (NN 776 607) (Fig. 2). Spring 1962; Powell 1964) have been found at a few These sedimentary structures are valuable in Moine localities in the Rivers Garry and Tummel. Magnetite, geology since they provide evidenceof younging direc- sphene and biotite are contained within the bands but tionsand the facing of folds. The preservation of no true hornblende schists have been mapped. Calc- cross-cuttingsedimentary dykes at highangles to silicate bands and lenses are recognized most readily slumped semi-pelitic beds indicates that at least some in the clean water-worn surfaces of the River Garry of the bedding in the Moine does not appear to have and in thenew cuttings for the A9 road. They are suffered from major tectonic dislocation parallel to the fairly widespread, but are less commonly encountered original sedimentary layering. Since many sedimentary in the upper parts of the succession. structuresare modified by internaldeformation and recrystallization, detailed sedimentological work is li- mited and interpretations of sedimentary environmen- Sedimentary shctores tal conditions necessarily vague. However, the general Sedimentarystructures are mainly preserved in absence of coarse-grained clasts, the uniform nature of laminated psammitic and semi-pelitic lithologies where the mineralogy and the abundance of dunes and rip- relativelyundeformed units occur outside banded ples are all worthy of note. Together with the presence zones. Trough cross-bedding forms the most reliable of calc-silicates, current scours and the soft sediment way-upindicator, as tabular truncations can be con- deformationstructures, they may indicatea shallow fused with sheared-outminor isoclinal folds. water marine or deltaic environment with a constant Moreover,what appear to be non-curved foreset source of fine- to medium-grained material which has laminaemay, in fact,be the result of metamorphic been influenced by variable conditions of current flow. differentiation parallel to cross-cutting schistosities (cf. In contrast,graded bedding has been reported by Bowes & Jones 1958). Dune bedding up to 0.4m in Piasecki (1980) from the lower part of the Grampian height is commonly observed in association with smal- Division in Strathspey. Although graded beds have not ler ripples, some of which display ‘flaser-like’ trough been observed in the uppermost 4 km of the Moine, laminations.More rarely the cross-bedded units are thinly-beddedunits withpossible traces of grading accompanied by scour and fill structures, convolutions, havebeen recognized in theunderlying rocks of slumpsand even sedimentary dykes (e.g. nearthe Drumochter and .

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0.1 m l 1

FIG. 2. Sedimentarystructures in theStrathtummel Succession: a, Troughcross-bedding; Craig nan Caisean. b, Dune bedding truncating trough cross laminations; Ches. c, Convolute bedding in laminated psammite; Craig nan Caisean. d, Sedimentary dykes cutting psammite between two slumped semi-pelitic beds; cross bedding and scour and fill structures below; River Garry, Struan.

Stratigraphy Successions within the Grampian Division Therehave been few attempts to subdivide the Theupper part of theGrampian Division, im- Struan or Eilde Flags stratigraphically. However, the mediatelyunderlying theDalradian of Strathfionan rocks in Strathtummelhave been found to young (Treagus & King 1978) can be subdivided on the basis consistently through nearly 4000 m into the Lochaber of quartzitic units of sufficiently consistent thickness to Subgroup of the Dalradian in Strathfionan (Treagus & be mapped over many km of strike. It is proposed to King 1978), and may be subdivided on a lithostratig- call this the ‘Strathtummel Succession’ as it has been raphical basis into a number of formations (Table 1). established on the uninverted SE limb of the Meal1 It has been suggested that these ‘younger’ Moines Reamhar foldin the hinge of thelater Bohespic shouldbe included ina ‘GrampianGroup’ of the Antiform. Here, the succession reaches its maximum Dalradian(Harris et al. 1978)since they appear to thickness of over 3000 m and contains cross-bedding havebeen affected by asimilar polyphase deforma- which consistently youngs upwards across the valley of tionalhistory as the Dalradian and may overlie the the Tummel towards Craig Kynachan and Strathfionan olderCentral Highland Division of theMoines (Fig. 3). (Piasecki & van Breemen 1979 a, b; Piasecki 1980). TheStrathtummel Succession(Table 1) is largely However, no definite unconformable or slide junction composed of alternating quartzite and psammitic schist has yet been found to separate the ‘younger’ Moines formations generally less than 800 m in thickness. The of this area from those of the Grampian Division, the lowest formation, the ‘Tummel Quartzites’ consists of base of which is thought to be of pre-Morarian age (c. up to 6 persistent bands of white, slightly feldspathic, 1000-800Ma) by Piasecki(1980). Furthermore, the quartzite, each no more than 20 m in thickness, sepa- existence of these older dates for foliated pegmatites rated by finely laminated psammitic schists rich in a at the base of the Grampian Division, which may be at greatvariety of sedimentarystructures. These can least 10 km inthickness, doesnot necessarilyimply most clearly be seen N of the forestry track on Craig thatthe uppermost parts have been affected by the nan Caisean (NN 776 607). same early events. In this paper, the ‘younger’ Moines The overlyingmassive Tummel Psammitic Schists of Strathtummel will be regarded as belonging to the separatethe Tummel Quartzites from the Kynachan upper part of the Grampian Division. Quartzite, which is much more feldspathic and thins

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GROUPS BENALDER

COlRECHEAP PELlTlC SCHIST IALRADIAN COIRE CHEAP BANDEOSCHIST

CHLAGGAN QUARTZITE ,UPERGROUP BEOILBANDED SCHIST MONADHLIATH SCHIST LOWER BEOILBANDED OU. ElLDEQUARTZITE

GEAL CHARN OUARTZITE KYNACHAN QUARTZITE STRATHTUMYEL COlRE SGOlR PSAMMITE GRAMPIAN TUMMEL PSAMMITE SUCCESSION 8 SEMI-PELITE TUMMELWARTZITES with HCOlRE SGOIR PSAMMITE DIVISION LAMINATED PSAMMITES OUARTZITESTHIN wllh

IMOlNEl PSAMMITES 8 SEMI-PELITES

BANDED SEMI-PELITES 8 BEN ALDER PELITESwith PSAMMlTlC

SUCCESSION UNITS 6 RARE OUARTZITES

laterally to die out in Glen Errochty. It has not been Structure positivelyidentified outside the Strathtummel-Glen Errochtyarea, though it maybe equivalent to the Detailed investigation of all available Moine outcrops CoireSgoir Quartzite of theBen Alder area. S of as far N as the Drumochter Pass has not only revealed CraigKynachan, the Beoil Quartzite reported by youngingevidence, but also fold orientations, styles Treagus & King (1978) occurs at the top of the Beoil and cleavage relationships which have enabled major Banded Schist and since the underlying banded white structures to be established in what has hitherto been quartziteforms a continuous unit, at least 150 m in regarded simply asan evenly-dipping sequence of thickness, from Lochan Beoil to the River Tummel, it flags. Cuttingsassociated with the newlyaligned A9 hasbeen named the Lower Beoil Quartzite. This roadbetween Calvine and Drumochter now expose quartzite may be equivalent to the Eilde Quartzite at further evidence, though the River Garry remains an the base of the Lochaber Subgroup of the Dalradian, outstandingcross-section of theMoine, providing so that the Beoil Banded Schist would represent the clean,natural water-worn channels and gorges. A Eilde Schist of Lochaber. At the culmination of sev- summary of the evidence of minor and major struc- eral antiforms S of Glen Errochty on Torr Dubh (NN turesresulting from sequencea of deformational 741 630) the succession may again reach the base of eventsdesignated D, to Ds isgiven below (Figs 4, the Lochaber Subgroupon the lower limb of the Meall 5, 6 and 7). Reamhar synform (Fig. 3). Belowthe Tummel Quartzites in the Forest of Primary deformation (Grampian Omgeny) Atholl and Drumochter areas the succession becomes First phase D,-Minor structures more monotonous with the general absence of mappa- ble units other than mixed flaggy psammites and semi- The character of all early structures in the Gram- pelites.It is proposedto refer to these rocks as the pian Division is one reflecting penetrative deformation Drumochter Succession, though little is known of their under ductile conditions. Beddinglschistosity relation- thickness. Themore peliticBen Alder Succession ships are very close in most exposures and the primary (Thomas 1979) is thought to underlie the Drumochter schistosity (S,) maybe regarded as sub-parallel to Succession W of LochEricht (Table l), butfurther bedding (S2), exceptat hinges of early folds. This work to the NW is required before the relationships penetrative fabric accentuates the bedding and band- with the older Grampian rocks can be established. ing to form the characteristic flagginess of the Moine,

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STRATHTUMMEL

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I

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even.outside the banded zones. The intersection of outcrop by thepresence of amuscovite-rich schist S,/S, is rarely seen and cannot usually be measured as band withsmall quartz blebs, known as the Beoil a linear structure. schist, which reaches a maximum thickness of 95 m SE Few lineations of any type are associated with the of Lochan Beoil Chathaiche (NN 750 564). Detailed D, folds,except for a stretching fibre in themicas mapping between and Glen Tilt shows whichconstantly plunges SE to SSE independent of that this band transgresses both Moine and Dalradian modifications by primaryfolding. Apartfrom the stratigraphy. It was referred to as a tectonic schist by characteristic sub-parallel schistosity, the earliest de- Rast(1958), but Treagus & King (1978)prefer to formation is detected in numerous small-scale isoclinal retain its stratigraphical identity in Strathfionan. The folds which affect the banding. They often acquire an author believes the break in Upper Strathfionan to be amplitudetoo large to be measured in thefield, deceptively small, due to the coincidental juxtaposition whereastheir wavelength may only be a few cmin of thealmost complete Ballachulish and Lochaber magnitude. The S, schistosity is axial planar in charac- Subgroups over a small area. Towards Foss the appar- ter at the closures, so that no cleavage can be traced ent stratigraphic continuity is soon lost (cf. Roberts & parallelto the bedding around the hinges. Since the Treagus 1979). Generally the sliding involves the tec- foldsare contained within the flagginess they are tonizedremnants of thetransitional rocks lying be- difficult to observe, and may only be visible on water- tween the Moine and Dalradian sequences. The break eroded surfaces, or where they have been etched out probably developed during the evolution of the prim- by weathering. Few plunges can be recorded from the ary(DJ nappes and has been reactivated in places isoclinal D, folds since they are rarely seenin 3-D, but during later deformation phases. those observed at Struan are approximatelyNE-SW in trend. In some places folds plunging SE share an S, D, major folds schistosity, and are regarded as D, hinges. Because of thepresence of horizonswith slump The Meall Reamhar fold closure occurs N of Loch folding in the meta-sedimentary sequence the positive Tummel, where the axial trace separates an uninverted recognition of earlyminor isoclinal folds can be steeplimb to the SE froman inverted SE-dipping difficult. Slumping can be recognized in zones of well gentle limb. The deceptively large interlimb angle of preserved sedimentary structures, but where deforma- the Meall Reamhar synformal anticline is due to the tionhas destroyed much of thecross-bedding, it is fact that its axial plane steepens rapidly towards the more difficult to separate deformed slump folds from Boundary Slide and it has been modified by later Dz early tectonic folds. Both have S, penetrative schistos- co-axial structures. The exposures Wof the surge shaft ity imprintedupon them and neither consists of the on Craig non Caisean indicate the D1 age of the fold, irregular disharmonic folding characteristic of tectoni- since here S, is parallel to Sz and perpendicular to S, cally undeformed slumps. (Fig. 5). The fold is considered to be the closure of a Althoughthe occurrence of tectonicallyformed much larger nappe structure (the Atholl nappe), whose isoclinal folds rarely coincides with the preservation of lowerinverted limb is exposedover much of the goodsedimentary structures, several examples of Forest of Atholl.The nappe is recumbentnear south-easterlyfacing on minor D, foldshave been Drumochter but becomes downward facing to the SE found in the area. The main facing evidence associated below the Dalradian, and is comparable with the early with majorD1 folds is foundN of LochTummel recumbentfolds of GlenOrchy and Glen Lochy wherea penetrative S, schistosity on thesouthern (Thomas & Treagus1968) and S of LochRannoch slopes of Craig nan Caisean (at NN 771 600) intersects(Roberts & Treagus1979). Although poor exposure steeplydipping beds at a high angle.There is no doesnot allow the axial trace of theclosure to be schistosity subparallel to the stratification, which con- tracedwith much confidence elsewhere, it is consi- tains cross-bedding younging consistently SE along the dered to be wrapped around the D3 major folds and high angle schistosity. The complete alignment of all occurs in a much tighter isoclinal form in Craiganour micas parallel to the schistosity is characteristic of S, Forest W of Beinn a’chuillaich. so that it is consideredthat D, folds face SE and downwards (Figs 4, 5). Primary deformation: Second phase D2 The Boundary Slide Dz minor structures Within the banded zones sedimentary structures are generally lacking and the strong sub-parallel schistos- The second schistosity Sz also has a NE-SW strike ity is composite. The Moine rocks become increasingly althoughbut penetrative in morepsammitic morefinely-banded towards the Dalradian, where a lithologies,italso develops acrenulation cleavage structuralbreak has resulted in theBoundary Slide. wherethe rock is morepelitic. S2 is across-cutting Thismay be recognized almost everywhere along its axial planar structure in D, fold hinges, but where S,

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SSE. N NW.

Meall Reamhar OF S1 Il'Sp Anticline S1 01 _--- S2 0 Kilomet res 3 4 Younging Direction

FIG. 5. Diagrammatic cross-section of the closure of the Meall Reamhar D, anticline indicating: (i) the zone on Craig nan Caisean where S, schistosity parallels S, overprinting, with S, at a high angle. (ii) the consistent way-up of cross-bedding on the steep limb of the fold, which establishes its downward facing to the SE. (iii) the main quartzitic units in the Strathtummel succession. TQ = Tunnel Quartzites; KQ = Kynachan Quartzite; LBQ = Lower Beoil Quartzite.

was moderately inclined prior to the development of D, major folds latercleavages, S, andeven S, may be difficult to distinguishfrom S, since thecomposite schistosity Although lithostratigraphical markers are difficult to appears to be a single planar fabric. Various forms of find, by combiningthe evidence of D, minorfold S, schistosity may be observed in the crags on the N vergences,the attitude of SJS, and S, planesand side of GlenErrochty between BlairFetty way-up evidence, a number of major overturned folds (NN 756 649) and Kinaldy (NN 765 646). may beattributed to the second phase. The most D, minorfolds form the most conspicuous struc- important of these structures are the Clunes antiform tures in theMoine of GlenGarry. Typical folds are andsynform. They can be traced through the River overturned towards the NW with interlimb angles of Garryat Clunes to thehinge of thelater Bohespic 40-50" andmoderately inclined axial planes dipping antiform at Torr Dubh, where a major swing of strike SE. Plunges vary from 0-30" towards the NE exceptin diverts all structuresnorthwards. It is believedthat the W where plunges are generally SW, at similar low these major D, folds reappear to the NW beyond the angles. Overturned limbs are usually rich in sedimen- bandedzone of thelater Errochty synform as the tarystructures, whereas the more attenuated gentle Craig a'Mhadaidh antiform and Garry synform which limbs rarely display way-up evidence. The downward thoughre-folded by later D, foldshave agentle facing nature of most of the folds confirms a pre-D, Caledonoid plunge as well as a style and wavelength structural inversion. similar to the Clunes folds. Younging evidence in the Themost strongly developed D, linearstructures, River Garry and on Craig a'Mhadaidh indicates that with low plunges parallel to D, fold axes, are at S,& these folds, like those at Clunes, are downward facing intersections and crenulations associated with S2 strain on the lower limb of the Atholl nappe. slip cleavage. Mullion structures are well developed at Other overturned D, folds occur on a smaller scale severallocalities, notably onthe River Garry at atStruan (NN 804655), Dailnamine (NN 751695) Clunes (NN 786 668). They appear to be governed by and on the upper limb of the Meall Reamhar synfor- D, rodding, cleavage and fold hinges which form the malanticline atCraig nan Caisean (NN 776 607) 'organpipe structures' first reported by Barrow where S, and S, are almost parallel. (1 904). To the NW of the Garry synform a gradual change

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takes place in the style of D2 folds. They become more Second phase D, minor structures upright, open and almost symmetrical in places as the The existence of a deformationphase post-dating sheet dip flattens north-westwards and undulates over the broad crest of the Drumochter dome. S, schistosity the thirdwas confirmed when open folds, which is steep almost everywhere on the dome, but begins to plunge down the dip of the banding, were foundin the dip in theopposite direction beyond the crest, and RiverGarry and its tributaries. They are often ob- continues to dip north-westwards throughout the Ben served in associationwith fracturing on verticaldis- Alder Forest (Thomas 1979). placement zones in banded psammites, and affectall earlier structures. Throughout the Dalnacardoch area, open foldsplunge down the moderately dipping S, Secondary deformation (CaledonianOrogeny) surfaces and consistently trend 130-145". A poor frac- ture cleavage is sometimes associated with steep faults striking sub-parallel to the fold axis. These symmetri- First phase D3 cal minor folds rarely have an interlimb angle less than Laterre-folding of all NE-SW trendingfolds re- 90" and are indicative of deformation under less duc- sulted in largescale reorientation of previousstruc- tile conditions than previously existed. tures on the N-S steep limb of a major foldpair Monoformal folds with a similar trend but a lower previouslynamed the Errochty synform and the plunge occur together with the open structures in the Bohespicantiform by Rast(1958), who considered bandedzone W of FeurLochan (NN708 684). A them to be F, folds. Minor folds associated with these banded vertical belt trending at about 140" is picked major D3 structures plunge steeply towards the south- out by a line of ridges, including the hill N of Loch ern half of the SE quadrantwith a strain-slipand Errochty called Meall Dail Chealaich (NN 710 670). It occasionallypenetrative schistosity only slightlyobli- was marked on Sheet 55 of the Geological Survey of que to the Caledonoid trend. Thus the folds are almostScotland, and described as 'aline of disturbance' in reclined with overturning to the WSW. Strong mineral the memoir. lineationsare recognizably D3 where S,& strike is Faulting is associated with the belt, but minor folds Caledonoid,but on the steep limb of theBohespic found there and S of Loch Con (NN 685 680) indicate antiform reorientated D2 folds and lineations can eas- that the zone is the steep limb of a monoformal fold ily be confused with D, structures. Near the hinge of which dies out downwards on Meall a-Bhiord, where theBohespic antiform earlier lineations and schis- the closure is exposed. In contrast to the D3 folds, the tosities are wrapped around the D, fold axis, but it is monoform has a moderate axial plane dip to the W onthe limbsthat flexural slip formedmost of the andplunges constantly at a lowangle towards 145". minor overturned folds. Associatedminor folds resemble large scale kink InGlen Garry, cross-cutting S, schistosities are bands,often in conjugatesets, plunging 140-150" rarely seen except at Black Tank (NN 769 693) and except in the W where a greater diversity of trend is Dailna Fhraoich (NN 739 698) where they are as- encountered.Nothing more than a poorfracture sociated with tight major D, closures. In the Dalnacar- cleavage is developed, and the folds are accompanied doch banded zone S, may form a regularly dipping 35" by retrogressionand the formation of diaphthoritic composite schistosity parallel to S, and S2, and few D3 chlorite. foldsdevelop. D, slidingparallel tothe banding is SE of Trinafour, D, maybe represented by small more likely tohave contributed to the very regular scale crenulations of the earlier schistosities, although nature of the banded zone. SE-plungingfolds of possible D, agebecome abun- The two major folds mentioned aboveaffect most of dantnear the Boundary ,Slide on An Catachan the area under consideration. The large rounded hinge (NN720575) and Craig nan Caisean (NN 780607). of the Bohespic antiform which re-folds the Boundary Slidebecomes progressively more acute towards the NE, and can be observed as a tight overturned steeply D, majo. folds plungingfold in the A9 cuttingat Black Tank (NN The majority of important D, folds affect the lower 769 693). Similarly, the Errochty Synform tightens in limb of the Errochty synform and soon die out on the the Moine to become almost isoclinal in Glen Garry steeperlimb (Fig. 6).Several major open antiforms (Fig 4, 6). re-fold earlier structures. The Allt Chulaibh antiform A further major D, fold, the Sithean Beag antiform, plunges gently SE from Creag Stalcair (NN 684 724) trends SSW towards Loch Rannoch in the W, but is andtightens down plunge from a gentlefold with a not strongly overturned or reclined. It re-folds the D2 wavelength of 8 km to a sharp antiform dying out on Craig a'Mhadaidh antiform E of Craiganour Burn, and the hillside above Auchleeks House. appears to open out towards Loch Rannoch. This fold The AlltSleibh antiform which trendsESE down maybe equivalent to the Cam Chreag antiform of AlltSleibh (NN 665 662) is smallerand opens out Treagus (1964). upwards towards Loch Errochty. The open Meall na

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/ a. D- IObliquelStructuresMajor

b. D4 ITransversel Major Structures

FIG. 6. Secondary major fold profiles. a, E-W profile of major D, folds. b, NE-SW profile of major D, folds.

Leitreachsynform also tightens downwards towards complementaryD, folds of SronCon plunge almost Loch Garry, but becomes a gentle flexure 6 km SE on southerlyinassociation withsub-parallel fracture Sron Con (NN 690 663). Thereis not enough evidence zonesrunning along the line of steepdips. One of to show that this fold is a continuation of the Croft- theselines passes through Loch Meall na Leitreach nagowan synform which folds both Moine and Dalra- (NN 638 689) E of Loch Garry, and can be traced to dian,since it is accompanied by theSSE trending Allt Poll Dubh-Ghlas (NN 647 670) over 2.5 km to complementary monoformal folds of Sron Con. the SE. The most important D, monoformal structure is the Trinafourmonoform which re-folds not only Moine Late faulting butalso the Dalradian in theErrochty synform (cf. D5 Rast 1958, fig. 4). Thesteep limbis almost vertical The late Caledonian dykes which traverse the area andthe amplitude increases to the SE fromMeall with a NE-SW trendare cut by evenlater faults. a’Bhiord (NN 688 702) to reach maximum develop- These are dominated by the NE-SW to ENE-WSW ment on Meall Dail Chealaich (NN 710 670) before wrenchmovements which formthe and dying out at a higher structural level SE of Trinafour Bridge of Balgie-Loch Garry fault systems. Numerous (NN 726 645), 7 km down plunge. The axial plane is small scale faults with similar trends occur elsewhere moderately inclined to the SW, so that the steep limb andare especially noticeable where they affect the outcrops progressively further NE at higher elevations. BoundarySlide. An associated set of secondorder Reference to the logs of the Garry-Errochty Tunnel wrench faults trend NNE-SSW and are recognized as made by the Geological Survey of Scotland indicates crushzones which become easily weathered and that the steep limb of the monoform does not extend eroded so thatthey characteristically form river verticallyfor morethan about 250 m. Some of the gorges.

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Allt Culaibh Anti form

DRUYOCHTER ?ASS\ \ Trinafwr \ a:MhadaidhCreag N. E.- Antrform Cluncs ’ D7 r*. Antlform Meall Reamhar Anticline Meall no Lcitre

- BLAl R ATHOLL

/ S.W. Errochty &Boh,espic Synform Antlform D3 03

FIG. 7. Simplified structural block diagram of rocks N of the Boundary Slide in relation to the A9 road in Glen Garry. The road line is 30 km in length. C = Calvine; DL= Dalnacardoch.

Structural implications withhigh angle cleavage relationships which are not compatible with the formation of D2 recumbent nap- The relationship between the earliest visible schistosity pes (Thomas 1979). (S,) andbedding (S,) atthe hinge of theMeall In agreement with Roberts & Treagus (1975, 1977, Reamhar synformal fold near Craig nan Caisean indi- 1979) it is believed that much of the Dalradian above catesthe primary anticlinal nature of thisfold. It the Boundary Slide is dominated by D2 re-folding and provides evidence for a SE-facing Atholl nappe in the that the zone of divergence from to Ben Moine rocks below the Boundary Slide, despite strong Lui (Rast 1963) is the result of differential movement overprinting by D, structures and to a lesser extent by between a Moine core and a Dalradian envelope. minor secondary structures. In this paper then, D1 and D2 are equivalent to the SE-facingfolds of asimilar nature have been re- ubiquitousprimary phases found throughout the corded in Glen Orchy (Thomas & Treagus 1968) and Moine/Dalradianoutcrop of the SW and W Central upper Glen Lyon (Roberts & Treagus 1979), though Highlands(Roberts & Treagus1964, 1977, 1979; Roberts & Treagus (1975) re-interpreted the isoclinal Hickman1978). These early fold phases are closely Beinn Chuirn anticline as a D2 nappe, and applied the linkedand it is tentativelysuggested that they may concept of D, nappe formation to the rest of the SW representthe reaction of the‘younger’ Moine and Highlands (Roberts & Treagus 1977). Dalradianmetasediments to under-thrusting (D,) by Mapping in the Ben Alder, Ossian and Stob Garbh older Moine and Lewisian crust prior to a ‘Grampian’ areashas provided evidence that D1 nappes in the continental collision (DJ (Fig. 8). Moine root to the SE of the Ossian-Geal Charn steep TheDalradian rocks of theTay nappe may have beltlying tothe NW of theDrumochter-Dochard initially formed an envelope to the Grampian Division dome,and that D2 foldsverging symmetrically on in the core of the Atholl nappe, but dislocation along eitherside of thedome become upright in itscrest the developing Boundary Slide(s) may have separated

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S.E. N.W.

a. DEPOSITION

GRENVILLIAN AND/OR GRAMPIANMOINE [m] DALRADIAN - MAJOR SLIDES LEWlSlAN BASEMENTLEWlSlAN ::I:: SUPERGROUP OS OSSIANSTEEPBELT

FIG. 8. Tentative model for the Grampian Orogeny in the Central Highlands of Scotland. a. Late Precambrian and Cambrian ensialic basin. b. Early Ordovician primary deformation D, involving underthrusting and nappe formation in the deeply buried Grampian Moine. c. Later Ordovician deformation D, involving collision between Precambrian basement blocks, the final emplacement of the Tay Nappe and widespread re-folding. b & c incorporate the work of Longman et al. (1979).

the hinges, so that the Tay nappe went on under the developed and may be regarded as late Grampian (or influence of deep-seated (Dz) simple shear (Harris et Caledonian)events. In the Moine rocks N of the d. 1976;Shackleton 1979), whilst the rising Schiehallion complex, D3 resulted in folds of variable Drumochter-Docharddome (Thomas 1979) con- tightnessplunging towards the southern hemisphere, tinued to influence the orientation of Dz folds in the butelsewhere in theCentral Highlands Caledonoid underlying Moine. trendsare known. Again, it is tentativelysuggested The secondary deformation phases are more locally that this phase may have involved the prolonged and

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complex stresses associated with the oblique closing of ACKNOWLEDGMENTS.More recent workcompleted on the theIapetus Ocean (Phillips et al. 1976; Lambert & A9 road cuttings has been supported by Paisley College of McKerrow (1976). The more brittle deformation, re- Technologyand encouraged by Professor 1. A. MacLeo4 suiting inthe clearly developed major and minor Head of the CivilEngineering Department. Much of the transversefolds (D4),affects the banded limbs of the Original study was during the tenure Of a DSIR studentship (1962-1965) at Liverpool University and I would Errochty ’Wforrn and has not been reported like to thank Dr N. Rastfor his helpful supervision at that elsewhere. time. References ANDERSON,E. M. 1923. The geology of the schists of the - & WRIGHT,J. E. 1955.A section through the Iltay Schiehallion District, . Q. J. geol. Soc. London, Boundary Slide in Errochty hydro-electric tunnel. Bull. 79, 423-45. geol. Sum. G.B. 7, 101-2. ANDERSON,J. G. C. 1947. The KinlochlagganSyncline, KINGB. C. & RAST,N. 1955. Tectonic styles in the Dalra- Southern Inverness-shire. Trans. geol. Soc. Glasgow, 21, dianand Moineof partsof the Central Highlands of 97-1 IS. Scotland. Proc. Geol. Assoc. London, 66, 243. - 1951. Geology of the Glen Tromie hydro-electric tun- LAMBERT,R. ST. J. & MCKERROW, W.S. 1976. The Gram- nels, Inverness-shire. Geol. Mag. 88, 133-9. pian Orogeny. Scoff.J. Geol. 12, 271-92. - 1956. The Moinian and Dalradian rocks between Glen LITHERLAND,M. 197% The stratigraphyand structure of the Royand the Monadhliath Mountains, Inverness-shire. Dalradian ‘rocksaround LochCreran, Argyll. Thesis, Trans. R. Soc.Edinburgh, 63, 15-36. PhD, Univ. Liverpool (unpubl.). BAILEY,E. B. & MCCALLIEN,W. J. 1937. PerthshireTec- LONGMAN,C. D., BLUCK, B. J. & VAN BREEMEN,0. 1979. tonics. Schiehallion to Glen Lyon. Trans. R. Soc. Edin- Ordovician conglomerates and the evolution of the Mid- burgh, 59, 79-117. land Valley. Nature,London, 280, 578-81. BARROW,G. 1904. On the MoineGneisses of theEast MCINTYRE,D. B. 1950. Lineation, boudinage and recumbent Central Highlands and theirposition in theHighland folding at Dalnacardoch. Geol. Mag. 87, 427-32. sequence. Q. J. geol. Soc.London, 60, 400-49. - 1951. The tectonics of the area between Grantown and BORRADAILE,G. J. 1973. Dalradian structure and stratig- Tomintoul(Mid-Strathspey). Q. J. geol.Soc. London, raphy of the northern LochAwe district,Argyllshire. 107,1-22. Trans. R. Soc.Edinburgh, 69, 1-21. PANTIN,H. M. 1961. The stratigraphy and structure of the BOWES,D. R. & JONES,K. A. 1958. Sedimentaryfeatures BlairAtholl-Ben a’Gloe Area, Perthshire,Scotland. andtectonics in theDalradian of WesternPerthshire. Trans. R. Soc. N.Z. 88, 597-622. Trans.geol. Soc. Edinburgh, 17, 133-40. PHILLIPS,W. E. A., STILLMAN, C.J. & MURPHY,T. 1976. A CHADWICK,P. K. 1971. Theoretical, experimentaland field Caledonianplate tectonic model. J. geol.Soc. London, studies on folds and cleavage. Thesis, PhD, Univ. Liver- 132, 579-609. pool (unpubl.). PIASECKI,M. A. J. 1975. Tectonic and metamorphic history CUMMINS,W. A. & SHACKLETON,R. M. 1955. The of the Upper Findhorn, Inverness-shire, Scotland. Scott. recumbent syncline. Geol. Mag. 92, 353-63. J. Geol. 11, 87-115.

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~ 1972. The Dalradian rocks at Dunkeld, Perthshire. Bull. geol. Soc. London 8, 139-44. geol. Suru. G.B. 38, 1-10. POWELL,D. 1964. Thestratigraphical succession of the - & FETTES, D. J. 1972. Stratigraphy and structure of MoineSchists around Lochailort (Inverness-shire) and Upper Dalradian rocks at the Highland Border. Scoff.J. its regional significance. Proc. Geol. Assoc. London, 75, Geol. 8, 253-64. 223-46. -, BRADBURY,H. J. & MCGONIGAL,M. H. 1976. The RAMSAY,J. G. & SPRING.J. 1962. Moinianstratigraphy in evolution and transport of the Tay Nappe. Scoff.J. Geol. the WesternHighlands of Scotland. Proc. Geol. Assoc. 12, 103-13. London, 73, 295-326. -, BALDWIN,C. T., BRADBURY,H. J., JOHNSON,H. D. & RAST, N. 1958. Tectonics of the Schiehallion Complex. Q. J. SMITH,R. A. 1978. Ensialicbasin sedimentation: The geol. Soc. London, 114, 25-44.

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Received 25 July 1979; read 12 December 1979; revised typescript received 22 December 1979. PETERR. THOMAS, Department of CivilEngineering, Paisley College, High Street, Paisley, PA1 2BE.

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