The Stratigraphical Succession of theMoine Schists around Lochailort(Inverness-shire) and its Regional

Significanceby DEREK POWELL

Received 30 August 1963; read 7 February 1964

CONTENTS

I. INTRODUCTION ... page 2232. DISTRIBUTION OFTHE MAIN LITHOLOGICAL GROUPS THROUGHOUT THE AREA 2253. THE MAIN STRUCTURAL FEATURES OFTHEWESTERN ASSEMBLAGE 2264. DESCRIPTION OF THE LITHOLOGICAL GROUPS COMPRISING THE WESTERN

ASSEMBLAGE 2285. THE EVIDENCE FOR THE STRATIGRAPHICAL SUCCESSION OF THE WESTERN

ASSEMBLAGE 2316. RELATIONSHIP OFTHE LOCHAILORT PELITIC GROUPTOTHESTRATIGRAPHICAL

SUCCESSION 2337. THE MAIN STRUCTURAL FEATURES OF THE EASTERN ASSEMBLAGE ... 2358. DESCRIPTION OF THE LITHOLOGICAL GROUPS COMPRISING THE EASTERN

ASSEMBLAGE 2359. COMPARISON OF THE ROCK GROUPS MAKING Up THE EASTERN ASSEMBLAGE

WITH THOSE OFTHE STRATIGRAPHICAL SUCCESSION IN THE WEST ... 24110. REVIEW OF THE PREVIOUS LITERATURE IN THE LIGHT OFTHE PRESENT WORK 24111. THE SIGNIFICANCE OFTHEAREA IN RELATION TO MORERECENT WORK 244

ACKNOWLEDGMENTS 246REFERENCES 246DISCUSSION 246

ABSTRACT: A revised stratigraphical succession for the Moine Series of the south­eastern part of the Morar antiform is proposed, and is applied to ground lying fartherto the east within the great Moine Injection Belt. The revised succession is comparedwith those proposed for the Morar antiform area by other authors and is correlated inpart with tbat given for the Glenelg and Knoydart regions by Ramsay & Spring (1962).The work casts doubt upon some previous interpretations of the Morar area.

1. INTRODUCTION

THE AREA UNDER CONSIDERATION covers some eighteen square miles ofcountry lying to the north and south of the Mallaig to Fort William road,the A830, between Loch nan Uamh (part of the Sound of Arisaig) andLoch Eilt (map, facing page 250). It includes the south-eastem part of theMorar antiform (Fig. 1), a structure which has been discussed at length inthe literature (Richey & Kennedy, 1939; MacGregor, 1948; Kennedy,1954; Lambert, 1958).

223

224 DEREK POWELL

Richey & Kennedy interpreted the Morar antiform as a folded uncon­formity with older, recumbently folded Sub-Moine rocks lying beneath theyounger, unconformable Moine Schists which were in true stratigraphicalorder. This early interpretation has since been modified and several mem­bers of the Sub-Moine succession have more recently been regarded asbelonging to the Moine (MacGregor, 1948; Kennedy, 1954; Lambert ,1958). Kennedy in 1954 regarded part of the Sub-Moine succession asbeing equivalent to part of the Moine succession seen on the flanks of thedome. He replaced the postulated unconformity by a tectonic slide. Therocks lying above this slide he termed the 'envelope' ; those below it, the'core' , The 'envelope' rocks, comprising relatively little deformed Moine

N

o PSAMMITIC ' ~~ ROCKS,

PELITIC & S~lo' l '?ELI ICt-'QINE ROCKS

LEWlSlAN (;I;~ISSE5

APPROX IMATE LIMITSOF lOCHAI ORr A'lEA

----...TwO MfLES .

Fig . 1. Simplified geological map of the Morar , Kno ydart and Lochailor t areas (afte rRichey & Kennedy, 1939; Kennedy, 1954; Lambert, 1958; Ram say & Spring. 1962,and the author), with inset map showing the geographical location of the Morar an dLochailort areas (black)

MOINE SCHISTS AROUND LOCHAILORT 225

Schists, he thought had travelled some considerable distance upon thisslide, over the comparatively highly deformed, autochtonous 'core' con­sisting of interfolded and thrust rocks of both Moine and Lewisian age.Lambert (1958), following MacGregor (1948), denied the existence ofeither a structural or stratigraphical break in the Moinian rocks of theantiform and suggested that the Moine succession could be extended fromthe 'envelope' into the 'core'.

The Lochailort area includes a part of the Morar antiform throughwhich the Moine-Sub-Moine and core-envelope boundaries are supposedto run.

With the exception of certain basic dykes and sills, and occasionalgranitic veins, the rocks outcropping within the area consist almost en­tirely of psammitic, semi-pelitic and pelitic metamorphic rocks belongingto the Moine Series. The metamorphic rocks within the area have beenfolded and metamorphosed several times.

The establishment of a stratigraphical succession within such meta­morphically and structurally complex terrain has been accomplishedprimarily by detailed mapping of minor lithological changes and by the useof cross-bedding as an indicator of sedimentary attitude (Vogt, 1930;Tanton, 1930; Bailey, 1930). In the mapping of these features, constantreference has been made to the major and minor structures affecting therocks.

The mapping has been carried out on published Ordnance Survey mapsof a scale of six inches to one mile, numbers CXXXV and CXXXVI (Inver­ness-shire), and in more geologically complex areas on enlargements ofthese maps to a scale of twelve inches to one mile. Aerial photographs havebeen used to supplement the lack of contours and topographical detail onthe six-inch maps.

The degree of rock exposure within the area is generally very good andhas allowed accurate geological mapping.

2. DISTRIBUTION OF THE MAIN LITHOLOGICAL GROUPSTHROUGHOUT THE AREA

The map facing page 250 illustrates the geographical distribution andstructural attitude of the main lithological groups. With the exception ofthat part of the area to the west of Lochailort where the rocks have a generaldip towards the south-east at moderate angles, the dip throughout the areais steep, often near vertical. Four predominantly psammitic and four pre­dominantly pelitic groups are recognised. These, in geographical orderfrom west to east are: the Beasdale Pelitic Group, the Loch nan UamhPsammitic Group, the Loch Mama Pelitic Group, the Ardnish PsammiticGroup, the Lochailort Pelitic Group, the Arieniskill Psammitic Group, the

226 DEREK POWELL

Arieniskill Pelitic Group and the Loch Eilt Psammitic Group. The groupscan, on the basis of broad textural differences, be divided into two assem­blages. Those lying to the east of the junction between the ArdnishPsammitic Group and the Lochailort Pelitic Group have suffered intenserecrystallisation and reconstitution so that the pelitic fractions of thevarious groups have developed gneissose textures and the psamrniticfractions have lost much of their sedimentary structure. In contrast thegroups lying to the west of this junction have suffered comparatively littlereconstitution and consequently, while the component pelites are schistoseand essentially metamorphic, they have not developed gneissose textures,and some of the constituent psammitic rocks have retained abundant,easily recognisable, sedimentary structures.

3. THE MAIN STRUCTURAL FEATURES OF THE WESTERNASSEMBLAGE

The rocks of the western assemblage are affected by several major foldswhose axial traces are indicated in the map facing page 250. The shapes ofthese folds and the attitudes of their axial planes are illustrated in Fig. 2.The folds are described in turn from west to east.

The Glen Mama antiform and the Arnipol synform (map, page 250)plunge generally towards the east and north-east at low angles but the axesof the folds appear to undulate beyond these limits, so that in places thefolds plunge at low angles to the west and south-west.

The Ardnish synform has a variable plunge. In the northern part of itsoutcrop it plunges at angles of between 50 and 80° to the east and south­east. Traced southwards the direction of plunge swings gradually rounduntil it is towards the south-east at angles of between 70 and 800

• To theeast of the Ardnish synform but still contained by the outcrop of theArdnish Psammitic Group, lie three large folds. Of these the Arnabol hillantiform plunges towards the south-south-west at a low angle. The remain­ing folds have axes which undulate about the horizontal, plunging in someplaces towards the south-south-west, in others towards the north-north­east.

The outcrop of the Loch Mama Pelitic Group is remarkable in that itthins rapidly across the area from north to south (map, page 250). Thethinning is thought to be the result of movements both within and alongthe junction between the pelitic group and the Ardnish Psammitic Group,movements which took place during the folding. The evidence for themovements is not the concern of this communication but they are givenbriefly here as the effectsof the movements are significant to the argument:

(0 Rocks within that part of the Ardnish Psammitic Group adjacent tothe Loch Mama Pelitic Group in the area to the west of Beinn Chaorach

MOINE SCHISTS AROUND LOCHAILORT 227

and Loch Dubh (map, page 250) strike obliquely into the Pelitic Group,indicating that there is a discontinuity along the southern part of thejunction between the two groups.

(ii) The Loch Mama Pelitic Group changes its character from north tosouth along its outcrop.

(iii) Structures belonging to an early phase of folding have differentorientations on either side of the Loch Mama Pelitic Group while laterstructures have a constant orientation across it.

The tectonic discontinuity is obviously important in a consideration ofthe stratigraphy of the area but it must be pointed out that the featureslisted above become less apparent northwards, and, to the north of Loch

... \.::................. ....\

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b

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d : ii

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SECTIONS A and B.

SECTION C.

o Ektasdalo Polit ic Group. 0 loch nan lJamh Psarrmtoc GI"cup

® loch Mama R>hlic Group,

® as above. 0 Ardnish Psammil ic Groupo loc hailorl R>lilic (;roup.

a.b.c ole. - Sub. groups r<forrod to in tho t..1.

" - Fold axial plan..\

1- Fault.

Fig. 2. Geological sections across the rocks of the western assemblage. (For location ofsection lines see map facing page 250)

228 DEREK POWELL

Mama there appears to be no traceable discontinuity. Thus while themovements have caused considerable re-orientation and thinning of rockgroups in the south of this area they have not precluded the stud y of thestratigraphy.

4. DESCRIPTION OF THE LITHOLOGICAL GROUPS

COMPRISING THE WESTERN ASSEMBLAGE

(a) The Beasdale Pelitic Group

The whole thickness of this group is not exposed within the limits of thearea under consideration. That part exposed consists of a banded series ofpredominantly pelitic and semi-pelitic units each of which is composed offinely interbanded pelites, semi-pelites and psammites in various propor­tions. Mineralogically the constituent rock types are made up of dark greenbiotite, muscovite, epidote, clino-zoisite, plagioclase felspar, microcline,quartz, rare calcite and various accessories. Cross-bedding- is uncommon.The nature of its junction with the Loch nan Uamh Psammi tic Groupvaries from north-east to south-west across the area. To the north-east, andwhere it runs across Druim Fiaclach, the junction is sharp and apparentlyplanar. To the south-west, however, between Druim Fiaclach and themouth of Beasdale Burn, the junction becomes transitional.

(b) The Loch nan Uamh Psammitic Group

On lithological grounds the group (Figs. 2 and 3) is divisible into threesub-groups :

(i) Sub-group a. This sub-group lies against the Beasdale Pelitic Group atthe south-western end of its outcrop and consists of a series of thinly­interbanded, occasionally cross-bedded, psammites, semi-pelites and peliteswhich differ from rocks of the Beasdale Pelitic Group only in that theycontain a higher percentage of psammitic material. The sub-group islaterally impersistent. It is exposed on the shore at the mouth of BeasdaleBurn and on the south-western end of the Druim Fiaclach ridge. OnDruim Fiaclach the sub-group passes laterally into thickly interbandedcross-bedded psammites and semi-pelites.

(ii) Sub-group b. This consists of a thick series of cross-bedded psam­mites which are interbanded on a large scale with semi-pelitic and peliticunits. These units are themselves composed of thinly interbanded psam­mites, semi-pelites and pelites in which the psammitic fraction is generally

1 Cross-bedding is found in most of the psammitic rocks within the area. It ranges in scale. withoutrefe rence to the major stratigraphical groups, from units where the cross-bedded bed s a re approxi­mately three inches thick to those where the beds are approximately twelve inches thic k . No det ailedwork on the nature of the cross-bedding has yet been attempted.

• Thinly and thickly are used to define banded rocks where the ind ividual band s are betweenapproximately two or three inches thick and less than a tenth of an inch thick, and between approxi­mately three inches and three feet respectively.

MOINE SCHISTS AROUND LOCHAILORT 229

high. Pebbly horizons and calcareous bands are present in some of thepsammites. Calc-silicate bands" are found but are very rare. The sub-groupgrades into sub-group a and its lateral equivalent.

The sub-group becomes extremely pelitic in many of its higher horizonsand towards the south-east it grades rapidly into sub-group c.

(iii) Sub-group c. The remaining division of the Loch nan UamhPsammitic Group comprises a series of thinly interbanded psammites andsemi-pelites which are often laminated. The psammitic bands are oftenmicaceous and occasionally cross-bedded.

Mineralogically the psammites, semi-petites and petites making up theLoch nan Uamh Psammitic Group are essentially similar to those of theBeasdale Pelitic Group except that garnet is occasionally present andcalcite more common.

(c) The Loch Mama Pelitic Group

The thinning of the Loch Mama Pelitic Group has been mentioned in apreceding section. Associated with this thinning there is a marked changein the character of the rocks making up the group. To the north of LochMama (map, page 250), the group consists of thinly interbanded pelites andsemi-pelites with occasional ribs of psammite in its lower portions. Calc­silicate bands and lenses are common throughout the thickness of thegroup. In thin section the constituent pelites and semi-pelites consist ofvarying proportions of garnet, brown biotite, muscovite, plagioclasefelspar, quartz and accessory minerals.

Southwards the thin interbanding is gradually lost and the rocks arereplaced by more massive but strongly schistose petites. Psammitic ribsand calc-silicate bands and lenses are uncommon. The pelites are essen­tially similar in their mineralogy to the equivalent rock types to the north,but there is a marked increase in grain size.

Cross-bedding has not been found within the rocks of the Loch MamaPelitic Group.

The junction of the Loch Mama Pelitic Group and the Loch nan UamhPsammitic Group is not well exposed but the available evidence suggeststhat it is undisturbed and rapidly gradational. Its junction with theArdnish Psammitic Group is, on the other hand, sharp and planar and tothe south-west appears to be a plane of tectonic discontinuity.

(d) The Ardnish Psammitic Group

The outcrop of this group (Figs. 2 and 4) is complicated by several folds(map, page 250), which make it difficult to unravel the full succession of

a Calc-silicate bands are thin, measuring usually less than two inches in width, laterally impersist­ent, pink or white lenticles and ribs containing quartz, plagioclase felspar, and various combinationsof biotite, garnet, clino-zoisite, epidote, hornblende. calcite and sphene.

HALF' "'ll [

b

._. 'l~t. _.' r ll~ Fault .- ' - '

Basal s{idq of Uppqr Moincz Napp¢,at ter K2rmdy 1954 _ - -

Basal MaiN LnConformi ty alte rRICh¢y& K~rYl~dy 1939 .

..... .-.......

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BEASOAlE PELITIC GROUP.o LOCH NAN UAMH PSAMMITIC GROUp' (a.b &c-Sub·groups S~~ I~xt )

Q]) LOCH MAMA PELITIC GROlJ'.

o ALLUVIUM.t

LOCH NAN UAMH

Fig. 3. Lithological map of the Loch nan Uamh Psammitic Group with posit ions of the supposed Moine- Sub-M oine unconformity (afte rRichey & Kennedy, 1939) and the supposed basal slide of the Moine nappe (after Kennedy, 1954)

Half

o ALLUVIUM.

@) SUB-GROUP E.

o SUB-GROUP D.

SUB-GROUP C.

@ SUB-GROUP B.

~ SUB- GROUP A.

Other symbols as in Figure 6.

mile

ARDNISH PSAMMITIC GROUP

Fig. 4. Lithological map of the Ardnish Psamrnitic Group [To face p, 230

MOINE SCHISTS AROUND LOCHAILORT 231

lithological sub-groups within it. However, there appear to be five litho­logical divisions (Fig. 4).

(i) Sub-group a. Thinly interbanded psammites and semi-pelites withoccasional ribs of very pure psammite. Calc-silicates are common as thinlayers and lenticles. The sub-group has been attenuated by tectonic move­ments associated with the thinning of the Loch Mama Pelitic Group.

(ii) Sub-group b. Generally poorly bedded interbanded psammites andsemi-pelites alternating with bands of predominantly semi-pelitic andpelitic material. Cross-bedding and slumped bedding- are commonlocally within the psammitic bands. Calc-silicate rocks are uncommon.

(iii) Sub-group c. Thinly interbanded semi-pelites and micaceouspsammites with abundant calc-silicate ribs. The psammitic bands areoccasionally cross-bedded.

(iv) Sub-group d. Interbanded massively bedded, often coarse-grainedpsammites, thin pelites and semi-pelites, and thinly banded psammites andsemi-pelites, Cross-bedding and slumped bedding are extremely commonin the massive psammites and common in the thinly banded psammites.Calc-silicate ribs and lenticles are common throughout the sub-group.

(v) Sub-group e. Thinly interbanded, occasionally laminated, psammitesand semi-pelites with occasional calc-silicate rocks. Cross-bedding isuncommon.

The psammitic, semi-pelitic and pelitic rocks of this group are made upof gamet, brown biotite, muscovite, plagioclase felspar, quartz and variousaccessory minerals.

5. THE EVIDENCE FOR THE STRATIGRAPIDCAL SUCCESSIONOF THE WESTERN ASSEMBLAGE

The rocks to the west of the axial trace of the Ardnish synform (map,page 250) lie with a predominantly south-easterly dip of between 50 and 89°.Within this region the four major lithological groups lie upon each otherwith no major discordance of strike or dip. Sedimentary structures arecommon within the two psammitic members of this succession and areoccasionally found within the Beasdale Pelitic Group. Cross-bedding,slumped bedding and occasionally graded bedding offer evidence for theoriginal order of superposition of the rocks (Fig. 5). Care has been exercisedin the interpretation of sedimentary structures, particularly cross-bedding,in view of the occurrence of tectonic structures which have a superficiallysimilar appearance. The map facing p. 250 illustrates directions of young­ing, as indicated by cross-bedding and slumped bedding, of the rockswithin this western area. It is apparent on this evidence, combined with a

• Slumped bedding is relatively uncommon throughout the area and is normally slumped cross­bedding of a type illustrated in Fig. 5.

232

.A.

DEREK POWELL

6 INe Hl S

.c . .. _.. - :.. ::. .:.,." - -Y~--;::;-- - ::;:

»:> <« ; ,~"" /' .. ,. .'

I roor

t-;• -< <

I root

Fig. 5. Sedimentary structures in the Moine Schists of the Lochailort area

For explanation ofdiagrams see opposite

MOINE SCHISTS AROUND LOCHAILORT 233

consideration of the major structures affecting the rocks, that the rockgroups can be placed in stratigraphical order:

4. Ardnish Psammitic Group ... youngest.3. Loch Mama Pelitic Group.2. Loch nan Uamh Psammitic Group.I. Beasdale Pelitic Group (base not seen) ... oldest.

In addition to the evidence afforded by sedimentary structures, the fourgroups have very different characteristics (see Table I) which lend supportto the establishment of the fourfold stratigraphical succession above.

6. RELATIONSHIP OF THE LOCHAILORT PELITIC GROUPTO THE STRATIGRAPHICAL SUCCESSION

Between the axial trace of the Ardnish synform and the junction of theLochailort Pelitic and the Ardnish Psarnmitic Groups the outcrop of theArdnish Psammitic Group is complicated by several major folds. In spite ofthis, sedimentary evidence is sufficientlyabundant to ascertain the originalorder of deposition of the rocks in most of this area (map, page 250). Sub­group e of the Ardnish Psammitic Group unfortunately lacks the wealth ofsedimentary structures characteristic of the greater part of this group butthe available sedimentary evidence, combined with the fact that there is nomappable fold closure contained by this sub-group, strongly suggests thatsub-group e is the topmost member of the Ardnish Psammitic Group andthat it youngs towards the east-that is, towards the Lochailort PeliticGroup. The junction of the Ardnish Psammitic and Lochailort PeliticGroups is sharp and planar and there is no major discordance of strike ordip across it. It therefore appears that the Lochailort Pelitic Group isyounger than the Ardnish Psarnmitic Group and forms a fifth group in thestratigraphical succession.

A. Complex cross-bedded unit in banded psammite. The stippled areas are coarse­grained, often pebbly psammite. The large fold towards the centre of the diagramwhich affects the whole unit is tectonic. Ardnish Psammitic Group, Bheinn Chaorach.

B. Slumped, cross-bedded psammite. It is possible that some of the folds in thisexample, particularly those at the top of the diagram, are tectonic but the overall sedi­mentary nature of the structure leaves no doubt as to the direction of younging.Ardnish Psammitic Group, Bheinn Chaorach.

C. Cross-bedded psammite. The foreset and bottomset beds are poorly preserved.Loch na Uamh Psammitic Group, 200 yards east of the mouth of Beasdale Burn.

D. Slumped cross-bedding. Ardnish Psammitic Group, Arnabol hill.E. Load structure. Psammite (black), has sunk down into semi-pelite (dashed).

Ardnish Psammitic Group, roadside north of Loch Dubh, Ardnish.

In each diagram the arrow indicates the direction of younging deduced from thestructure. The plane of the paper represents a plane approximately normal to bedding.The symbols 1-- --I indicate the plane of the major bedding

PROC. GEOL. ASSOC., YOLo 75, PART 2,1964 16

TABLE I. The Main Characteristics of the Rock Groups of the Western Assemblage

GROUP SUB-GROUPS LITHOLOGICAL NATURE CALC-SILICATES

Ardnish Psammitic Group 5 sub-groups including a well- Psammites, semi-pelites and Calc-silicate rocks presentdefined, thick semi-pelitic pelites comparatively well throughout whole thicknesshorizon (sub-group c) differentiated

Loch Mama Pelitic Group None recognisable Component rock types Present throughout wholecomparatively well thicknessdifferentiated

Loch nan Uamh Psammitic Divisible into 3 sub-groups Component psammites, semi- Calc-silicate rocks found onlyGroup pelites and pelites relatively very rarely

poorly differentiated

Beasdale Pelitic Group Whole thickness not exposed Rocks poorly differentiated None foundwithin the area

~~

t:itTl::ctTl~

"Co:stTlr­r-

MOINE SCHISTS AROUND LOCHAILORT 235

7. THE MAIN STRUCTURAL FEATURES OF THE EASTERNASSEMBLAGE

Four major folds occur within the eastern assemblage, two of which, theGlenshian synform and the Loch Eilt antiform (map, page 250), havecaused considerable re-orientation of the rocks. The former fold is more orless parallel-limbed and plunges to the north-north-east at a low angle. TheLoch EUt antiform has a similarly tight shape but plunges to the south­west at a moderate angle. The Creag Bhan folds (map, page 250) are moreopen in shape and plunge towards the south-east at approximately 60°;they effect no major re-orientation of the rock groups.

8. DESCRIPTION OF THE LITHOLOGICAL GROUPSCOMPRISING THE EASTERN ASSEMBLAGE

It is shown later that the lithological groups present in the eastern partof the area can be correlated with the stratigraphical succession establishedin the west. In view of this the lithological groups are described and theirsub-groups lettered in a manner which presumes these correlations.

(a) The Lochailort Pelitic Group

The group comprises an interbanded series of pelitic and semi-peliticgneisses, and predominantly psammitic rocks. Calc-silicate ribs and lensesare common throughout and amphibolite pods consisting essentially ofquartz, plagioclase, hornblende, biotite and garnet occur in several places.It is difficult to divide the group into sub-groups because of the complexoutcrop pattern but two laterally persistent horizons are present which arediagnostic of the group as a whole. The group is bounded at both itseastern and western margins by a thin but persistent band of massivegneissose pelite, marked a in Fig. 6. Adjacent to this pelite is another thinbut laterally persistent horizon, marked b in Fig. 6. This horizon consists ofthinly banded psammite with micaceous partings.

The remainder of the group can be divided into lithological units invarious parts of its outcrop, but these are laterally impersistent and are nottherefore considered to be important with respect to correlation.

In thin sections the main constituent rock types consist of garnet, red­brown biotite, muscovite, plagioclase felspar, quartz and other minerals inaccessory amounts.

(b) The Arieniskill Psammitic Group

The junction of this group with the Lochailort Pelitic Group is con­cordant, sharp and planar along most of its length. However, to the northof Arieniskill the junction is complicated by the occurrence of a thrust

236

N

tDEREK POWELL

FAULTS / ' (mostly inferr~d).

LITHOLOGICAL JUNCTIONS:_--- exposed and inferred.

'" '- ''-'' ' approx imat~. ~~ transitiona l.~.:.:.,

Pr~dominantly PELITIC rocks ~Pr~dominantly SEMI-PELITIC rocks .:.:.:.:

Pr~dominantly PSAMMITIC rocks

ONE MILE

Fig. 6. Lithological map of the Lochailor tPelitic Group.

Fig. 6

fault (Fig. 7). The group is divisible throughout most of its outcrop intofive sub-groups (Fig. 7).

(i) Sub-group e. Thinly banded psammites with micaceous partings andoccasional thin semi-pelitic bands. Calc-silicate layers are common. Thesub-group has not been observed along the outcrop of the PsammiticGroup north of the thrust fault.

(ii) Sub-group d. Interbanded, banded psammites and semi-pelites withcalc-silicate layers.

MOINE SCHISTS AROUND LOCHAILORT 237

(iii) Sub-group c. Poorly banded, occasionally schistose, psammiteswhich locally contain cross- and slumped bedding. To the north the sub­group contains, and is partly replaced by, a thin band of massive pelite.Calc-silicate bands are uncommon.

(iv) Sub-group b. Generally thinly interbanded psammites and semi-

@o0"' . '

o®

a,b,c,d and e

Other symbols as in Figure 6.

ONE MILE

Fig. 7 Lithological map of theAr ieniskill Psammitic Group.

• a

Fig. 7

238 DEREK POWELL

pelites which are occasionally cross-bedded. Calc-silicate rocks are un­common.

(v) Sub-group a. Thinly interbanded and often laminated psammites andsemi-pelites with abundant calc-silicate rocks.

In thin section the constituent psammitic, semi-pelitic and pelitic rocksof the group consist of garnet, brown biotite, muscovite, plagioclasefelspar, quartz and various accessory minerals.

(c) The Arieniskill Pelitic Group

The junction of this group with the Arieniskill Psammitic Group is sharpand planar and has suffered no apparent tectonic disruption. The disposi­tion of the various rock types within the group is somewhat complex. Itappears, however, that the group can be divided into six sub-groups all buttwo of which are laterally persistent (Fig. 8).

(i) Sub-group f. Interlaminated semi-pelites and pelites with calc-silicateribs and occasional thin psammitic bands. The sub-group is present alongall but the extreme southern end of the outcrop where it grades into morepelitic rock (Fig. 8).

(ii) Sub-group e. Thinly banded, often gneissose, pelites, with calc-silicaterocks and occasional thin psammitic bands.

(iii) Sub-group d. Thinly interbanded psammites and semi-pelites withcalc-silicate bands. Locally the sub-group becomes strongly psammitic.The sub-group is laterally impersistent (Fig. 8).

(iv) Sub-group c. Generally massive pelites with calc-silicate bands.(v) Sub-group b. Thinly interbanded pelites, semi-pelites and psammites

with calc-silicate bands.(vi) Sub-group a. Thinly banded pelites with occasional calc-silicate

rocks. A thin impersistent band of thinly interbanded semi-pelitic andpsammitic rocks occurs along the south-eastern margin of this sub-groupin the northern part of the outcrop.

The psammitic, semi-pelitic and pelitic rocks of this group are made upof garnet, brown biotite, muscovite, plagioclase felspar, quartz and variousother minerals in minor amounts.

(d) The Loch Ellt Psammitic Group

The group has a marked and undisturbed junction with the ArieniskillPelitic Group. The whole thickness of this group is not exposed within thearea. The exposed part is, however, divisible into two sub-groups (Fig. 9)which in viewof the correlations proposed later are referred to as band c tocorrespond with sub-groups band c of the Loch nan Uamh PsammiticGroup. It is thought that the eastern equivalent of sub-group a of the Lochnan Uarnh Psammitic group is not exposed here.

MOINE SCHISTS AROUND LOCHAILORT 239

• Predominantly PEUTIC rocks.

o Predominantly SEMI- PELITIC rocks.

o Predominantly PSAMMITiC rocks.

Other symbols as in Fig ure 6 .

a-I sub groups refer red 10in text,

ARIENISKlllo

ONE MILE

Fig.8. Lithological map of theAr ieniskill Pelitic Group.

Fig. 8

(i) Sub-group c. Interbanded banded psammites, psammitic schists andsemi-pelites. Calc-silicate rocks are uncommon. Thin lenses of magnetite­rich rock occur locally.

(ii) Sub-group b. Banded, often gneissose, psammitic schists and semi­pelites with occasional bands of psammite and pelite. Calc-silicate bandsare rare. The upper portions of the sub-group are characteristically semi­pelitic in nature and contain thin bands of pelite (Fig. 9).

In thin section the main constituent rock types consist of garnet, darkgreen and brown biotite, muscovite, plagioclase felspar, potash felspar,quartz, and various other minerals.

SUb-group b with pelitic bandsornamented black.

o Sub-group c.

Other symbols as intigure6 .

o T

N

tHALF ~. ,

~

t:lrn~tTl~

'"tlo~tTlr'r-

Fig. 9. Lithologicalmap of the Loch Eilt Psammitic Group

TABLE II. Comparison of the Rock Groups making up the Western Assemblage with those of the Eastern Assemblage

WESTERN ASSEMBLAGE (Stratigraphical Succession) EASTERN ASSEMBLAGE

SUB- SUB-GROUP

GROUPSLITHOLOGICAL CHARACTERISTICS

GROUPSGROUP

Lochailort Pelitic Group Well-differentiated gneisses and psammites Lochailort Pelitic Groupcontaining red-brown biotite, with calc-silicatesthroughout and occasional lenticular bodies ofamphibolite

Ardnish Psammitic e Component rock types Component rock types e Arieniskill PsammiticGroup d well differentiated; well differentiated; d Group

c containing brown biotite. containing brown biotite. cb Calc-silicates present Calc-silicates present ba throughout throughout a

Loch Mama Pelitic Rock types relatively Rock types well differen- f Arieniskill Pelitic GroupGroup well differentiated; con- tiated; containing brown e

taining brown biotite. biotite. Calc-silicates dCalc-silicates present present throughout cthroughout b

a

Loch nan Uamh c Component rock types Component rock types c Loch Bilt PsammiticPsammitic Group b comparatively poorly comparatively poorly b Group

a differentiated; contain- differentiated; contain- Q

ing green biotite. Calc- ing green biotite. Calc-silicates very rare silicates uncommon

Beasdale Pelitic Group Component rock typespoorly differentiated;containing green biotite.Calc-silicates have notbeen found

[To face p. 240

MOINE SCHISTS AROUND LOCHAILORT 241

9. COMPARISON OF THE ROCK GROUPS MAKING UP THEEASTERN ASSEMBLAGE WITH THOSE OF THESTRATIGRAPIllCAL SUCCESSION IN THE WEST

The general lack of well-preservedsedimentary structures in the rocks ofthe eastern assemblage makes the positive correlation of the rock groups ofthis assemblage with the stratigraphical succession difficult. It is possible,however, to suggest such correlations on the basis of a comparison of thedetailed lithological characteristics of some of the groups. The correlationsare summarised in Table II.

These correlations are supported by other features:(i) Dark-green biotite is found within the rocks of the Beasdale Pelitic

and the Loch nan Uamh Psammitic Groups, in contrast to the red-brownbiotite of the other groups of the western assemblage. A similar contrast inthe colour of biotite is seen in some of the groups of the eastern assemblage;that of rocks within the Loch Eilt Psammitic Group is almost always darkgreen whereas that in rocks of the other groups is red-brown.

(ii) The occurrence of a few examples of current bedding which affordway-up evidence within the Arieniskill Psammitic Group (map, page 250)indicates that the group is younger than the Arieniskill Pelitic Group, olderthan the Lochailort Pelitic Group, and the equivalent of the ArdnishPsammitic Group on the eastern limb of the Glenshian synform.

It seems probable therefore that the rock groups of the eastern assem­blage can be correlated with the western stratigraphical succession in themanner shown in Fig. 10.

10. REVIEW OF THE PREVIOUS LITERATURE IN THE LIGHT OFTHE PRESENT WORK

The Morar area has been a subject of research and controversy for someyears and while the Lochailort area includes only a small part of the Morardome the findings put forward here do have a bearing on the interpretationof this structure. No detailed work has been published relating to theeastern part of the area and the author believes this to be the first time thatcorrelations of the highly recrystallised Moinian rocks of the 'MoineInjection Belt' (Phemister, 1948) with the comparatively lowly meta­morphosed rocks of the western seaboard of Inverness-shire have beenattempted.

A detailed discussion of the previous literature would of necessity belengthy in view of the many different interpretations of the Morar region.Consequently the reader is referred to the excellent summary of thepertinent past research which is given by Ramsay & Spring (1962). A fewcomments are, however, necessary.

242

, .

DEREK POWELL

Fig. 10. Comparisons of the stratigraphical successions established for the MoineSeries in Glenelg, Knoydart and the Lochailort areas

The stratigraphical succession established for the Moine Schists of thewestern flank of the Morar antiform by Richey (1935) is comparable withthat found by the author on the eastern limb of this fold.

youngest-Upper Psammitic Group = Ardnish Psammitic Group-youngestStriped? and Pelitic Group = Loch Mama Pelitic GroupLower Psammitic Group = Loch nan Uamh Psammitic Group

Though, in detail, these correlations may prove to be more complex thanindicated above, the author believes them to be in broad terms correct.

Richey & Kennedy (1939, see reference to Richey & Kennedy inRamsay & Spring 1962, 301-3) interpreted the Morar antiform as a foldedunconformity with older Sub-Moine rocks lying beneath the younger,unconformable Moine Schists. Their evidence for this proposal was that theSub-Moine and Moine rocks had apparently different lithological, tex­tural and structural characteristics. The author has not found any evidencefor the existence of such an unconformity in that part of the Morar regionincluded within the Lochailort area (Fig. 3 and map, page 250).

MOINE SCHISTS AROUND LOCHAILORT 243

MacGregor, in 1948(see Ramsay & Spring, 1962,303), reported that hecould find no evidence for the unconformity proposed by Richey &Kennedy (1939). He also stated that he thought that there was a more orless continuous stratigraphical succession of Moine rocks facing south­eastwards from the head of Loch nan Uamh to Ardnish. The author's ownwork substantiates these observations.

In 1954 Kennedy reconsidered the problems of the Morar antiform andpublished a revised interpretation of the structure (see Ramsay & Spring,1962,303-7). His re-interpretation was based essentially upon the recogni­tion of two tectonic facies: a structurally lower, core facies which consistedof interfolded and interthrust Lewisian hornblendic rocks and MoineSchists; and a structurally higher, envelope facies of less highly deformedMoine Schists which had ridden some considerable distance over theunderlying core rocks. The features which were said in 1939 to distinguishthe Moine rocks from the Sub-Moine were now thought to have resultedlargely from differences in tectonic environment and from the lateraltelescoping of the Moine Schists. The psamrnitic and pelitic rocks pre­viously regarded as Sub-Moine Kennedy here regarded as tectonic re­duplications of the Moine Schists seen in the envelope. They were in factequivalent to the Lower Psammitic Group and part of the Lower StripedGroup (Kennedy, 1954, 362). This hypothesis necessitated the occurrenceof a structural break between the core and the envelope; a break whichKennedy placed in more or less the same position as the 1939unconformity.

Kennedy's re-interpretation of the Morar structure is incompatible withthe evidence advanced by the author. The rocks lying immediately withinthe 'core' of the Morar dome (Fig. 3) are older members of the Moinesuccession seen in the 'envelope', and are in continuous stratigraphicalsequence with them. There is no mappable discontinuity within the outcropof the Loch nan Uamh Psamrnitic Group (=Lower Psamrnitic Group,Kennedy) in the south-eastern part of the Morar dome (Fig. 3), nor is thetectonic disruption evident along, and in association with, the Loch MamaPelitic Group of the magnitude envisaged by Kennedy for the transport ofhis 'Upper Moine Nappe' over the underlying Moine-Lewisian complex(Kennedy, 1954, 373).

Lambert has more recently (Lambert, 1958)considered the succession ofMoine rocks in the Morar region to be a simple right-way-up stratigraphicalsequence complicated only by the interthrusting of Lewisian gneisses atseveral horizons. Lambert recognised no structural or stratigraphical breakin the positions postulated by Richey and by Kennedy. Instead he givesevidence to show that the core-envelope boundary represents a transitionbetween two metamorphic states. His succession for the Moine Schistsconsists of five members and he suggests that the base of the succession isnowhere to be seen:

244 DEREK POWELL

Youngest-Upper PsammiticGroupUpper PeliticGroupLower PsammiticGroup

-metamorphic boundary­Lower PsammiticGroupLower PeliticGroupCentral PsammiticGroupLewisian (thrust bounded)

Lambert, however, cites no sedimentary evidence for his succession.The present author is in agreement with Lambert in that he considers

there are no fundamental differences between the psammitic and peliticfractions of the 'Sub-Moine' or core rocks and those of the 'Moine' orenvelope. All the metamorphic rocks of the area, except the Lewisian, havesuffered the same structural processes and may be regarded as Moine inage.

11. THE SIGNIFICANCE OF THE AREA IN RELATION TO MORERECENT WORK

Ramsay & Spring (1962) have proposed a revised stratigraphical suc­cession for the Moine Schists of the Glenelg and Knoydart areas (Fig. I).Using the stratigraphical range of calc-silicate bands, lithological differ­ences, and the evidence afforded by cross-bedding, combined with adetailed knowledge of the tectonic structures affecting the rocks, they haveerected the stratigraphical succession which is summarised in Fig. 10.

They have attempted to correlate this succession with the rocks of theMorar area. They accept MacGregor's and Lambert's proposals that thereis neither an unconformity nor tectonic discontinuity in the Morar regionas suggested by Richey & Kennedy (1939), and by Kennedy (1954). How­ever, they differ from Lambert in regarding all the psammitic rocks of thecore as a single stratigraphical unit equivalent to their groups 2, 3 and 4.Lambert's Lower Pelitic Group they correlate with their group 1 (the basalmember of the Moine succession) (see Ramsay & Spring, 1962, figs. 5and 6).

Part of the stratigraphical succession established by the author in theLochailort area can apparently be correlated in detail with the successionof Ramsay & Spring (Fig. 10), and so serves to confirm their succession andto extend its application more definitely to this southerly part of the westernMoine outcrop. There are, however, a number of points which requirediscussion.

The youngest group of rocks recognised by Ramsay & Spring is theAonach Sgoilte Psammite. Within the Lochailort area, however, the lateralequivalent of this group appears to be the Ardnish Psammitic Group(Fig. to), and as this is apparently older than the Lochailort Pelitic Group,

MOINE SCHISTS AROUND LOCHAILORT 245

the author suggests that the Moine succession can be extended upward toinclude a further major predominantly pelitic group. The presence of thisthird pelitic group within the Moine succession may help in clarifying someof the difficulties of correlation of the Moine stratigraphy established in theGlenelg, Knoydart, Morar and Lochailort areas with the Moine rocksfarther to the north in Ross-shire.

The various groups comprising the Lochailort succession have beendivided into component stratigraphical sub-groups. These have been usefulin correlating the lithological groups within this area and may perhaps be ofuse in much wider correlations, particularly where sedimentary evidence ispoor or lacking. In this respect it is noteworthy that the Rubha Ruadhsemi-pelite is probably equivalent to the upper portions of sub-group b ofthe Loch nan Uamh Psammitic Group= Loch Eilt Psammitic Group,which are distinctly pelitic in nature.

The stratigraphical range of the calc-silicate bands is strictly defined byRamsay & Spring (Fig. 10), and their range within the rocks of theLochailort area is similar in that they are characteristic of the upper por­tions of the Moine succession. In detail, however, there are several differ­ences between the two areas. These are summarised in Fig. 10. Of particularnote are: the extension of the range of calc-silicate bands to the base of theLoch Mama Pelitic Group (:==Ladhar Bheinn Pelite); their presencethroughout the thickness of the remaining younger part of the succession;and their occurrence within sub-group b ofthe Loch nan Uamh PsammiticGroup and within sub-groups band c of the Loch Eilt Psammitic Group.The stratigraphical range of calc-silicate bands does not therefore appearto be a reliable marker horizon in any but very local correlations (see theauthor in discussion of Ramsay & Spring, 1962).

The stratigraphical succession suggested by the author for the westernassemblage can be compared with the findings of Richey & Kennedy(1939).

Subsidiary Striped Group (g) }Main Striped Group (gj =:= Beasdale Pelitic Group

Outer Psammitic Group (x") }L P iti G (1) =:= Loch nan Uamh Psammitic Groupower sammi IC roup rn

Striped? and Pelitic Group (m'') =:= Loch Mama Pelitic Group

Upper Psarnrnitic Group (m") =:= Ardnish Psarnrnitic Group

Not present =:= Lochailort Pelitic Group

These correlations agree in essence with the correlations of the Glenelg­Knoydart succession with the Morar region (Ramsay & Spring, 1962,321),and indicate that the revised succession is applicable to a large area of theMoine outcrop..

246 DEREK POWELL

ACKNOWLEDGMENTS

The author expresses his thanks to Dr. W. J. Phillips and Dr. N. Rastfor their criticism of the manuscript.

The work presented here was carried out during the tenure of a D.S.I.R.Research Studentship, and more recently while in receipt of aUniversity ofWales Fellowship.

REFERENCESBAILEY, E. B. 1930. New Light on Sedimentation and Tectonics. Geol. Mag., 67,77-92.KENNEDY, W. Q. 1954. The Tectonics of the Morar Anticline and the Problem of the

North-West Caledonian Front. Quart. J. geol. Soc. Lond., 110, 357-90.LAMBERT, R. ST. J. 1958. A Metamorphic Boundary in the Moine Schists of the Morar

and Knoydart Districts of Inverness-shire. Geol. Mag., 95, 177-94.----. 1959. The Mineralogy and Metamorphism of the Moine SchisIs of the

Morar and Knoydart Districts ofInverness-shire. Trans. roy. Soc. Edinb., 63,553-88.

MACGREGOR, A. G. 1948. Resemblances between Moine and 'Sub-Moine' Meta­morphic Sediments in the Western Highlands of Scotland. Geol. Mag., 85,265-75.

PHEMISTER, J. 1948. The Northern Highlands (2nd ed.). Brit. reg. Geol.POWELL, D. 1962. In discussion of Ramsay, J. G. & J. Spring., Proc. Geol. Ass., Lond.,

73,325-6.RAMSAY, J. G. & J. SPRING. 1962. Moine Stratigraphy in the Western Highlands of

Scotland. Proc. Geo/. Ass., Lond., 73, 295-322.RICHEY, J. E. & J. B. SIMPSON. 1935. Summ. Progr. geo/. Surv., Lond. (1934).---- & W. Q. KENNEDY. 1939. The Moine and Sub-Moine Series of Morar,

Inverness-shire. Bull. geol, Surv. G.B., 2, 26-45.TANTON, T. L. 1930. Determination of Age Relations in Folded Rocks. Geol. Mag., 67,

73-6.VOGT, T. V. 1930. On the Chronological Order of Deposition of the Highland Schists.

Geo/. Mag., 67, 68-73.

DISCUSSIONPROFESSOR SUTTON congratulated the author on the succession he had been able toerect. The distinctions Dr. Powell had been able to establish between the uppermembers of his succession and the underlying groups should prove useful in workingout the regional structure. Whether these uppermost groups represented stratigraphicaldivisions which extended the classic Morar succession, as Dr. Powell had suggested, orwhether they turned out to be the equivalents of part of the Morar succession tectoni­cally transported to their present position, the new information would be equallyuseful. If the second alternative proved to be correct, it might tum out that Dr. Powellhad recognised some ofthe sedimentary characters which would enable us to distinguishbetween the rocks of the Morar succession of Western Inverness-shire and theirstratigraphical equivalents, originally deposited farther east, and now transported onmajor slides into positions where they appear to overlie the type Morar succession.

MR. P. W. G. TANNER, referring to Dr. Powell's remark that in several respects theLochailort succession differed from that established in Knoydart (Ramsay & Spring,1962), wished to put on record an analogous situation in the area which he had mappedto the north-east of Knoydart,

(i) The lower boundary of the calc-silicate bearing zone in the Ladhar Bheinn (No.5)pelite transgresses into the upper portion of the Barrisdale (No.4) psammite. Inconjunction with this change the pelite becomes more semi-pelitic and interdigitateswith the underlying psammite.

MOINE SCHISTS AROUND LOCHAILORT 247

(ii) The No. 4 psammite contains heavy mineral bands which in Knoydart areconfined to the lower psammitic horizon, the Amisdale psammite (No.2).

(iii) The Rubha Ruadh (No.3) pelite thins towards the north and finally disappears.The Striped and Pelitic Group of Morar and Knoydart (No.5) has been correlated

with the main pelitic belt of Central Ross-shire (T. N. Clifford, 1957). However, thefollowing evidence from the Kinloch Hourn area indicates that they may not be at thesame stratigraphic level.

(i) They are lithologically distinct-the No.5 pelite has much semi-pelitic material;the Sgurr Beag pelite (equivalent to the Central Ross-shire pelite) is a well-differentiatedgroup of pelites and 'clean' psammites.

(ii) There is no continuity of outcrop from one pelite to the other; they are alwaysseparated by a thin, mainly psammitic, horizon. The Officers of the Geological Surveyfirst noted this fact in the region to the south of Kinloch Hourn and they suggested thatthere might be a tectonic break between the pelites (Summ. Progr. geol. Surv, for 1963,in press).

(iii) The beds between the pelites are disposed in an asymmetric manner and thegeometry of minor structures precludes the presence of a fold closure.

(iv) Lewisian rocks of Eastern Lewisian facies are found at the base of Sgurr Beagpelite, The underlying psammite faces toward this Lewisian slice.

It therefore appears that the Lewisian slice and the rocks above it represent the basalportion of a nappe resting on the parautochtonous Knoydart succession, the formerbeing correlated with the Scardroy Lewisian and its overlying succession (Sutton &Watson, 1962).

In view of the obvious comparisons that can be drawn between the Kinloch Hournand Lochailort successions, Mr. Tanner asked the author whether he had evidence of aslide at the junction of the Loch Mama pelite and the overlying rocks.

REFERENCES

CLIFFORD, T. N. 1957. The Stratigraphy and Structure of Part of the Kintail District ofSouthern Ross-shire. Quart. J. geol. Soc. Lond., 113, 68.

RAMSAY, J. & J. SPRING, 1962. Moine Stratigraphy in the Western Highlands ofScotland. Proc, Geol. Ass., Lond., 73, 295-326.

SUTTON, J. & J. WATSON. 1962. An Interpretation of Moine-Lewisian Relations inCentral Ross-shire. Geol. Mag., 99, 527--41.

DR.M.J. FLEUTYSuggested that the terms 'younging' and 'to young' as used by the authormight well be replaced by 'facing' and 'to face' respectively. Bailey apologised for 'thebarbaric verb "to young" , when he introduced it (1934, Quart. J. geol Soc. Lond., 90,469), and it must be admitted that 'younging' is not very euphonious. The synonym'facing' has been defined in a standard work (Shrock, Sequence in Layered Rocks, 1948,16-17) and there seems no obvious reason why this should not be employed. Its usagehas been extended from beds to folds by Shackleton (1958, Quart. J. geol. Soc. Lond.,113, 363), but the meaning remains as defined by Shrock.

DR. J. G. RAMSAY commended the author on the excellent illustrations of deformedsedimentation structures in the Moine rocks which had been shown at the meeting.There was no doubt of the way up of the strata even though strongly folded anddistorted. Professor Sutton had previously asked if it were possible to determine theoriginal orientation of these structures and hence the palaeocurrent directions in theseMoinian strata. It was apparent from Dr. Powell's illustrations that the internaldeformations in these rocks were very severe; for example, the angle between theforeset beds and the truncating surface at the top of the false bedded unit was some­times more than 60°; it is generally agreed that the maximum angle of repose of sand inforeset strata cannot exceed 35°. As the deformation is so intense and as several phasesof deformation have been recognised in these rocks the speaker thought that theproblem of restoring the beds to their original orientation was an almost insuperableone.

248 DEREK POWELL

Had Dr. Powell found any sedimentation structures in the Moine rocks exposed inthe core of the Morar antiform to the north-west of the area shown in his maps? Somepublished accounts of this region had stated that these rocks are inverted, while otherswere equally emphatic that the rocks are mostly in normal order. This was a criticalregion to determine whether the sheets of Lewisian basement rocks interleaved in theMoine sediments represented decollement nappes (or strip sheets), or whether theyoccupied the cores of relatively complete isoclinal folds as seen in the Glenelg regionto the north.

MR. R. L. BROWN said that the revised stratigraphical succession for the Moine Series ofthe south-eastern part of the Morar antiform, proposed by Dr. Powell, agreed well withthe findings of recent workers to the south of this area. Howkins (1961) found apelitic unit within the Upper Psammite Group just to the SSW. of Loch Ailort that isstructurely above the Upper Psammitic Group. On the basis of current-beddingdirections seen in the west of this pelitic unit he concluded that the structures are'right way up' and that the pelite is younger than the Upper Psammitic Group.

Clark (1961) found current-bedding in the Upper Psammitic Group to the east ofLoch Ailort that suggested to him that the pelitic rocks immediately east are youngerthan the Upper Psammite. This pelitic unit extends north of Loch Ailort and is thesame unit which Dr. Powell has also found to be younger than the Upper PsammiticGroup.

I have been working south of Loch Eilt to the north and south of Loch Shiel. A beltof complexly folded pelitic and semi-pelitic rocks, as seen in Glen Moidart, can becorrelated with the pelitic rocks discussed above. If the succession proposed by Howkins,Clark and Powell is correct then the pelitic rocks of Glen Moidart are also youngerthan the Upper Psammitic Group.

Dr. Dalziel (1963) has worked in the Glen Finnan area to the south and east of LochShiel. He found numerous cross-bedding directions, and was able to show that thepelitic rocks in this area are older than the psammitic rocks which he correlates withthe Upper Psammitic Group.

I cannot correlate the pelitic rocks of Glen Moidart directly with those mapped byDalziel to the east but the structural data suggests that the pelite of Glen Moidart is ata higher structural level.

No evidence for the existence of a major slide has been found in the Loch Shiel area,and only further work will answer the question as to whether or not a continuoussuccession exists.

REFERENCESHOWKINS, J. B. 1961. The Structural and Metamorphic History of the Moidart area,SW. Inverness-shire. Unpublished Ph.D. Thesis, Edinburgh University.

SW. Inverness-shire. Unpublished Ph.D. Thesis, Edinburgh University.CLARK, G. C. 1961. The Structural and Metamorphic History of the Lochailort

(Moidart) Area. Unpublished Ph.D. Thesis, Edinburgh University.DALZIEL, I. W. D. 1963. A Structural Study of the Granitic Gneiss and Associated

Moinian Rocks between Loch Shiel and Loch Eil, Argyll and Inverness-shire.Unpublished Ph.D. Thesis, Edinburgh University.

In reply THE AUTHOR said that he felt that both Professor Sutton's and Mr. Tanner'scomments could be dealt with together.

The occurrence of a major tectonic break within the Morar area was postulated in1954 by Professor Kennedy. The break was thought to occur within the LowerPsammitic Group of the Morar succession which appears to be in part equivalent to theLoch nan Uamh Psammitic Group of Lochailort. No evidence has been found tosupport the existence of a break in the position postulated by Kennedy, but a tectonicbreak has been found in association with the outcrop of the Loch Mama Pelitic Group(equivalent to the Garnetiferous Group of the Morar succession).

The break appeared to be the result of movements within some considerable thick­ness of rocks including the top of the Loch nan Uamh Psarnmitic Group, the Loch

MINERAL BANDING IN GUERNSEY 249

Mama Pelitic Group, and that part of the Ardnish Psammitic Group adjacent to thispelitic group. The author said that he had found no evidence to suggest that there is asingle plane of discontinuity but there appear to be zones in which intense movementhad taken place. These are (i) the southerly part of the Loch Mama Pelitic Group/Ardnish Psammitic Group contact, and (ii) the northern part of the Loch MamaPelitic Group/Loch nan Uamh Psammitic Group contact. It is across these zones that arap id change in the orientation of minor folds occurs. To the north and south, however,the first and second contacts respect ively appear to be relatively undisturbed junctionsand no rapid change in the orientation of the early folds can be observed. Thus thejunctions can be regarded as essentially stratigraphic and a stratigraphical successionof rock groups up to and including the base of the Loch Mama Pelitic Group, and fromthe Upper part of the Loch Mama Pelitic Group up to and including the LochailortPelit ic Group can be establ ished. The full stratigraphical succession within the LochMama Pelitic Group is, however, uncertain. In the south of its outcrop the group isthin and markedly schistose, but traced northwards it becomes thicker, and alternatingbands of pelite and semi-pelite with subs idiary bands of psammite and calc-silicaterock appear. This compositional banding represents sedimentary banding, and itsdestruction in the south, together with the th inning of the group and the developmentof a schistose character, are attributed to the intensification of movements southwards.It seems likely that the succession within the pelitic group in the north is almost the fullstratigraphical sequence.

In view of the evidence within the Lochailort area the author holds that the classicMorar succession can be extended both upwards and downwards. With respect to theposition of the tectonic break the author would like to bring attention to the fact thatif the correlations of the rock groups from west to east across the southern part of theMorar antiform which have been suggested by Richey & Kennedy, and accepted byKennedy, MacGregor, Lambert and the author, are correct, then the tectonic breakfound in the Lochailort area would probably occur in the middle of the classic Morarsuccession .

These observations and deductions do not preclude the greater development andmore marked effect of the tectonic break outside the Lochailort area but the closesimilarities between the Glenelg /Knoydart, the Lochailort, and the Morar successions,and the observed nature of the break in the Lochailort area, suggest that at least inthese areas the upper part of the Moine succession is essentially unbroken.

The lithological distinctions which Mr. Tanner has drawn between the Sgurr Beagpelitic group and the No .5 pelite of the Glenelg/Knoydart succession (thought by thepresent author to be equivalent to the Loch Mama Pelitic Group), are dist inctionswhich are paralleled within the Lochailort area between the Lochailort Pelitic Groupand the Loch Mama Pelitic Group. Further dist inctions can be drawn in this area . TheLochailort Pelitic Group contains numerous amphibolite bodies ; these have not beenobserved within the Loch Mama Pelit ic Group. The author would suggest that theSgurr Beag Pelite is possibly equ ivalent to the Lochailort Pelitic Group and that thepsammite which separates it from the No .5 pelite is equivalent to the Ardnish Psam­mitic Group. If this is the case, and if the Lochailort succession represents an essent iallyunbroken succession, it then follows that the Sgurr Beag Pelite is not far out of its truestr atigraphical position.

In repl y to Dr. Fleuty the author said that he had used the terms 'younging' and'young' when referring to the stratigra phical sequence of rocks. The terms areunambiguous and immediately und erstood. Consequently the author prefers to use theterms rather than 'facing' and 'face'.

The author thanked Dr. Ramsay for his observations and said that he agreedentirely with his comments concerning the difficulties which were apparent in attempt­ing to determine palaeocurrent directions in such highly deformed rocks.

In reply to Dr. Ramsay's quest ion concerning the core rocks of the Morar antiformthe author said that he agreed that th is was a critical region. Unfortunately the authorhas not looked at these rocks in detail but the ground has recently been mapped byMr. J. F. Lewry who is at present writing up his results. The author would, however,like to bring attention to the follow ing . Th e Beasdale Pelitic Group (=Main StripedGroup, Richey & Kennedy, 1939), has been shown to be older than the Loch nan

250 DEREK POWELL

Uamh Psammitic Group (== Outer Psammitic Group and Lower Psammitic Group,op, cit.). The author has also shown that there is no evidence for the existence of eitheran unconformity or a slide of major importance in the positions proposed by earlierauthors. This means, according to both Richey & Kennedy's and Kennedy's maps, thatthe psammitic rocks of the core which lie structurally above the hornblendic gneisses,are continuous in outcrop with the Loch nan Uamh Psamrnitic Group. Thus Richey &Kennedy's statement that the Intermediate Psammitic Group is younger than the MainStriped Group (Richey & Kennedy, 1939,43) is probably correct, and Kennedy's laterstatement that the reverse relationship is apparent (Kennedy, 1954, 362) is untrue.

Richey & Kennedy, Kennedy, and Lambert have postulated that the pelitic rocks inthis south-eastern part of the core overlie the psammitic rocks, and, consequently, ifthis is the case, the psammitic and pelitic rocks must be inverted. Whether or not thisinversion is true of the remaining rocks within the core of the antiform is, until furtherevidence is available, a matter for conjecture. Sedimentary structures have not beenreported from the rocks lying structurally beneath the Lewisian. Nevertheless, on thebasis of the succession of rock groups and lithological similarities the author thinks ithighly likely that the Lewisian rocks can be regarded as essentially one-sided, and thatthe structural succession seen beneath the Lewisian is an inverted stratigraphicalsuccession which is repeated, though complicated by folding, above the hornblendicgneisses at a higher structural level, where it is, unfortunately, not seen in contact withthe Lewisian gneisses.

The most likely explanation of the Morar antiform appears to be, as Ramsay &Spring have suggested, that the Lewisian rocks form the cores of anticlinal folds(Ramsay & Spring, 1962, 321), and that the rocks below the Lewisian are on theinverted limb of such a fold (op. cit., 321). It now appears that the rocks lying directlyabove the Lewisian on the south-eastern limb of the Morar antiform are in fact inter­folded members of the lower Moine succession equivalent to the Beasdale Pelitic andLoch nan Uamh Psammitic Groups. The inversion of these rocks and the juxtapositionof Lewisian gneisses in places against the lower psammitic group of the Moine succes­sion with the absence of an intervening pelitic group, can probably be explained byregarding the upper contact of the Lewisian and Moine rocks as tectonic, and ajunction along which shearing has removed much if not all of a fold limb.

The author said that he was interested to hear of the work which had been completedin areas to the south and south-east of the area he had discussed. Some of the areaswhich Mr. Brown had mentioned were continuous with the Lochailort area in as muchas many of the rock groups described by the author crossed them. The LochailortPelitic Group had been termed the Roiss Bheinn Pelite by Clark and the author isgratified to hear that the tentative conclusions reached by Clark as to the age of thisgroup, with reference to the psammitic group lying to the west of it, and the author'sobservations, appear to be substantiated in ground farther south, in Moidart.

It would be significant if the repetition of the succession to the east of the LochailortPelitic Group, which the author had proposed, could be matched in these more southerlyareas.

D. PowellDepartment of GeologyUniversity College of WalesAberystwyth

L.M.P.G. LOCH MAMA PELITIC GROUP.L.P.G. LOCHAILDRT PELITIC GROUP. Ar.Pe.G. ARIENISKILL PELITIC GROUP.Ar.P.G. ARIEN I S~~LL PSAMMtTlC GROUP. L.E.P.G. LOCH EILT PSAMMITIC GROUP.

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B.P.G. BEASDALE PELITIC GROUP.L.u.P.G LOCH NAN UAMH PSAMMITIC GROUP.A.P.G

OZ1ARDNISH PSAMMITIC GROUP.

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GEOLOGICAL MAP OF THE LOCHAILORT AREA. INVERNESS-SHI RE.

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81

[To fa ce p , 2SO

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E / L

ONE MILE100"o

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NM 77 National Grid.

LINES OF SECTIONS~SEE FIGURE 2.

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o Predominantly PELITIC ROCKS.

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AX IAL TRAC E OF ANTIFORM.

• SYNFORM

SIDEWAYS CLOSING FOLD. 'i

DIP AND STRIKE OF BEDDING AND FOLIATION.

FAULT.

ALLUVIUM. 0 Predominantly PSAMMITIC ROCKS.C\V:2Y

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