The Structure of the South-EasternPart of the Morar Antiform,

Inverness-shireby DEREK POWELL

Received 18 January 1965; taken as read 9 July 1965

CONTENTS

1. INTRODUCTION

2. PREVIOUS RESEARCH

3. STRATIGRAPHICAL CONSIDERATIONS

4. STRUCTURE

5. REGIONAL IMPLICATIONS

ACKNOWLEDGMENTS

REFERENCES

page 79808183959999

ABSTRACT: Four phases of folding are recognised within the Moine Schists of thesouth-eastern part of the Morar Antiform. The first recognisable fold phase has givenrise to a very large isoclinal fold. Few minor structures can be related to this fold. Thesecond phase offolding was intense on both a major and a minor scale, and has resultedin the formation of generally tight, similar type folds. This second phase was probablyassociated with the development of a tectonic break located at a pelitic horizon in theMoine succession. The third fold phase is characterised by generally small scale, openfolds and by a strong micro-folding of earlier planar elements which, in places, givesrise to a strain slip cleavage. The fourth phase is evidenced by major and minor folds ofvarying style. The minor folds are, more commonly, open and step-shaped.

The mutual interference of the various sets of fold structures is discussed briefly.The detailed structural analysis of this part of the Morar area, combined with

stratigraphical considerations (Powell, 1964), suggests that it is possible to regard themain outcrop of Lewisian rocks, within the core of the southern part of the MorarAntiform, as the core of a large, first phase, isoclinal fold (see also Ramsay & Spring,1962, and Lambert & Poole, 1964). It further suggests that the isoclinal fold, and thusits Lewisian core, may root in the west, perhaps on the western limb of the MorarAntiform.

1. INTRODUCTION

THE AREA CONSIDERED lies astride the main road, the A830, betweenArisaig (Grid ref. NM 664867), and Lochailort (Grid ref. NM 768826), inwestern Inverness-shire. It includes the south-eastern part of the MorarAntiform and ground lying to the east of this (Fig. 1).

With the exception of certain basic intrusions and occasional graniticveins, the rocks outcropping within the area consist almost entirely ofmedium grade psammitic, semi-pelitic and pelitic metamorphic rocksbelonging to the Moine Series. The stratigraphical succession of the rockgroups has been described elsewhere (Powell, 1964).

79

80 DEREK POWELL

STRATIGRAPHICAL SUCCESSION.Knoydarl & Soul h-EastNorth Morar. Morar.

!}~ 6PSAMMITE. ! :'.':.'.:.: :~~~~T~

5PElI TE. PELITIC GROU

~~'i~.'~IWt E . . PSAMMJnCGRi~AMMITE. .1SEM~PELITE PELITICGROUP

:\DLEWISIAN. DLEWIS,AN.

- - - Boundar y of areadescribed.

~~~$=f'...J MILESo I 2 J 4 KMS.

N

f

Fig. 1. Generalised geological map of the Morar-Knoydart-Lochailort areas. Basedlargely on Richey & Kennedy (1939), Ramsay & Spring (1962) and Powell (1964).Inset-map of NW. Scotland showing location of areas (black). A-A, B-B, pointsreferred to in the text

Despite the intense deformation and recrystallisation the psammiticrocks, in numerous localities, show delicate sedimentary structuresincluding cross, slumped and graded bedding. The determination ofyounging directions with the aid of these features, combined with struc­tural observations have allowed a new interpretation of this part of theMorar Antiform.

2. PREVIOUS RESEARCH

Since the initial, comprehensive account of the geology of the Morararea by Richey & Kennedy in 1939, the stratigraphical succession within,and the structure of this area have been subjects of considerable contro­versy. The work published before 1960, that of Richey & Kennedy (1939),

THE MORAR ANTIFORM, INVERNESS-SHIRE 81

Kennedy (1954), MacGregor (1948) and Lambert (1958 and 1959), is wellknown and has been summarised by Ramsay & Spring (1962). Morerecently Lambert & Poole (1964) have elaborated the suggestions made byRamsay & Spring. The various interpretations are shown in Fig. 2.

The work of Richey (1947) has not been summarised by Ramsay &Spring. Richey described certain 'miniature drag folds', lineations andcleavages which occurred in the Moine Schists of the western limb of theMorar fold. Such studies led him to suggest that two episodes of foldingaffected the Moine Schists (Richey, 1947, 216).

Little account has been given in the majority of the papers concernedspecifically with the Morar area, of the detailed structure of the rocks.Passing reference has been made to minor structures but no structuralsynthesis has yet been published. The conclusions reached by most of theauthors concerning the structure of the Morar Antiform have been basedessentially upon stratigraphical hypotheses.

In recent years the mapping of large parts of western Inverness-shireand Ross-shire, in particular the Glenelg and Knoydart areas (the latterincluding the northern tip of the Morar Antiform), has enabled Ramsay& Spring to establish a revised stratigraphical succession for the MoineSchists and this they have recognised in North Morar (Ramsay & Spring,1962, fig. 6). Its distribution in relation to the Lewisian rocks in bothNorth Morar and Knoydart has led to the proposal that the Lewisian rocksin these regions form the cores of anticlinal folds, and that the rocks lyingbeneath them are situated on the inverted limbs of such folds (Ramsay &Spring, 1962, 321).

The acceptance of Ramsay & Spring's conclusions concerning thestratigraphical succession of the Moine Schists, and work on the smalloutcrops of Lewisian rocks to the east and south-east of Mallaigmore inNW. Morar, have led Lambert & Poole to discard Lambert's previousinterpretation of the stratigraphy and structure of the Morar Antiform(Lambert & Poole, 1964). They reiterate the conclusions of Ramsay &Spring as to the siting of the Lewisian rocks in the cores of folds. Lambert& Poole, however, further postulate, on evidence from north-east Morar,that the Lewisian rocks constitute 'the nose of a recumbent fold whichcontains within itself an earlier recumbent isocline' (op. cit., 13), see Fig. 2d.

3. STRATIGRAPHICAL CONSIDERATIONS

Over the past four years a survey of the south-eastern part of the MorarAntiform and ground extending some five miles to the east of this, and ofselected areas within other parts of the Morar area has provided freshevidence.

In an earlier paper (Powell, 1964), it has been suggested that some of thestratigraphical conclusions reached by Richey & Kennedy and Kennedy

PROC. GEOL. ASS., YOLo 77, PART I, 1966 6

82 DEREK POWELL

were mistaken. In the south-eastern part of the Morar area there is noevidence for the existence of either an unconformity or a major tectonicbreak in the positions suggested by these authors. Furthermore the strati­graphical and structural evidence suggests the contrary: the rock groupslying between Glen Beasdale and Arnipol (Fig. 11) represent an unbrokenstratigraphical sequence (Powell , 1964, 242).

The evidence afforded by sedimentary structures establishes beyondreasonable doubt that the pelitic group, the Beasdale Pelitic Group, lyingimmediately to the west of the Lower Psammitic Group of the envelope(Kennedy, 1954)-equivalent to the author's Loch nan Uamh PsammiticGroup-is older than this Psammitic Group (Powell, 1964,231-3, and thispaper Fig. II), thus confirming Richey & Kennedy's earlier observations(Richey & Kennedy, 1939, 42-3) but extending the Moine Successiondownwards by a further pelitic group.

The revised succession established in the south-eastern part of theAntiform can be correlated with that given for the Glenelg and Knoydartareas by Ramsay & Spring (Powell , 1964, fig. 10). The correlation of the

b.......... .

' '''0' - r, --;-:-- ., ,

---r!?~:,.::':: : : : ::{::<~.l ...:-; ~ /~ &4'tJ c nc ~ ,. " : ,.O<.~ta -...... Psc ....... · , : "c..P

~ C. "'''OI '.,,-. 1< ; '"0,,"

I1lIIII s."ts c c", !r ,·~ .tS 7 0<."

.. "e-,. ~ c , 0"<1 ~ ' C c.r...n ,.l

a

c . ~

~;~~::,:

~'"'''' " ,,,.. ILo Ps O"- 1.'C Qr-o~11 L ."'f Ol \C

:.:.:.:. ~, - "" .r e rro..p f

:«';'.,.,,,.0 ...O......, IC Q' O<.P )

0 '. ",' I tJo ..n c f'C Stl .. » ....} , ( K IS . '"

d

Fig. 2. The various interpretations of the Morar Antiform.a. E-W. Section across south Morar after Richey & Kennedy, 1939.b. E.-W. Section across south Morar after Kennedy, 1954.c. E.-W. Section across south Morar based on Lambert, 1958.d. E.-W. Section across north Morar after Lambert & Poole, 1964.In a , band c the sections are drawn along approximately the same line. The successionsgiven in each key are supposed stratigraphical successions given in ascend ing order. Inb, however, there has been tectonic reduplication of the lower part of the MoineSuccession

THE MORAR ANTIFORM, INVERNESS-SHIRE 83

two successions indicates that the Beasdale Pelitic Group is equivalent tothe Basal Semi-pelite of the Glenelg/Knoydart Succession and gives riseto the strong possibility that the Beasdale Pelitic Group is the basal memberof the Moine Succession in this part of the Morar area. Both Ramsay &Spring and Lambert & Poole recognise the presence of the Basal Semi­pelite, lying adjacent to Lewisian rocks, in Knoydart and North Morar(Ramsay & Spring, 1962, 321 and fig. 6; Lambert & Poole, 1964, 4 andfig. I).

In south-east Morar a group of psammitic rocks intervenes between theoutcrop of the Beasdale Pelitic Group and that of the Lewisian. Lambert,as well as Kennedy (1954, 362), regarded these psammites as older thanthe overlying Pelitic Group (Lambert 1958, fig. I). Richey & Kennedyconsidered these psammites to be younger. The author's recent work in thisarea confirms this latter view. Within the psammitic rocks adjacent to thePelitic Group cross and graded bedding are preserved which indicate thatthe psammites young towards the north-west, away from the Pelitic Group(Fig. 11). Since there is no evidence to suggest that the contact between thetwo sets of rocks is disturbed the psammites must be stratigraphically theyounger and therefore equivalent to the Loch nan Uamh PsammiticGroup. These relationships are further indicated by the occurrence ofsedimentary structures within the Beasdale Pelitic Group which youngtowards the north-west (Fig. 11), and by the lithological similarity of thepsammitic rocks occurring on either side of, and adjacent to, the PeliticGroup. The psammites, in both cases, are white to grey, massive rocks, withlittle conspicuous micaceous material save at occasional thin pelitic part­ings. The main characteristic of the psammites is that they contain thin,often graded, bands of heavy minerals being predominantly iron ore. Thebands are not found elsewhere in the Succession in this part of the Morararea but occur in an analogous stratigraphical position in the Glenelg/Knoydart Succession (Ramsay & Spring, 1962, 314), and also in the so­called Sub-Moine psammites (Richey & Kennedy, 1939, 39).

4. STRUCTURE

(a) General Remarks

In the south-eastern part of the Morar Antiform the Moine Schists havesuffered at least four phases of folding. The folds developed during thevarious phases are distinct in general style, geometry and the nature ofminor structures developed in conjunction with them.

The mutual interference of folds, linear and planar elements has beenused in interpretation. The fold phases have been placed in chronologicalorder on the evidence afforded by refolding. The first phase structures arethe earliest recognisable tectonic structures in the Moine Schists of this

84 DEREK POWELL

area. They are probably analogous with the isoclinal structures recognisedby Ramsay & Spring (1962, 310).

(b) Minor Structures

Phase 1. Small folds with long and parallel limbs. No linear or planarstructures have been found which can be related to these folds (Figs. 3and 4). The folds apparently deform only the bedding. These characteristicsare, however, of necessity incomplete as examples of this group of foldsare extremely rare.

Phase 2. Generally tight, asymmetrical folds of varying size, whichoccasionally have long and parallel limbs. The axial planes of the foldsgenerally intersect the bedding at angles of less than 20°. The foldsinvariably develop a strong axial planar cleavage and are consequentlyassociated with a cleavage/bedding intersection lineation. The folds areof 'similar' type, they often extend for large distances in the direction oftheir axial planes and exhibit characteristic apparent thickening of the bedsin the fold hinges and thinning on the fold limbs-the thickness beingmeasured normal to the bedding plane (Fig. 3). Folds of this phase deformthe bedding and Phase I folds (Figs. 3 and 4).

The axial planar cleavage which is characteristic of these folds is particu­larly well developed where the folds affect the Beasdale Pelitic Group andpelitic horizons in the Loch nan Uamh Psammitic Group. This cleavage isprobably identical to the 'oblique foliation' regarded by Richey & Ken­nedy as being characteristic of the Sub-Moine rocks (Richey & Kennedy,1939,41). While this cleavage is certainly more strongly developed in somerocks previously assigned to the Sub-Moine, it is by no means restrictedto these rocks and can be found wherever second phase folding is strongand where the folded rocks are pelitic.

Fig. 3. Minor folds of the various fold phases.a. Relatively open second folds with axial planar cleavage deforming earlier foldstructures. Arrows indicate plunge of fold axes. The earlier folds may belong to the firstfold phase but they may be slump folds. Loch nan Uamh shore section. BeasdalePelitic Group.b. Typical second folds in interbanded psammite and semi-pelite. Loch nan Uamhshore section. Beasdale Pelitic Group.c. Unusually tight third folds in thinly interbanded semi-pelites and pelites. DruimFiaclach. Loch nan Uamh Psammitic Group.d. Typical third folds in banded semi-pelites. Glen Beasdale. Beasdale Pelitic Group.e. Small third folds in pelite. Druim Fialach. Loch nan Uamh Psammitic Group.f. Large fourth phase 'step' fold in banded psammites (white) and semi-petites (stipple).Arnabol Hill. Ardnish Psammitic Group.g. Unusually tight fourth folds in micaceous psammite. Axial region of ArdnishSynform. Arnabol Hill. Ardnish Psammitic Group.With the exception of diagram a the plane of the paper represents a plane approxim­ately normal to the fold axes

~ ~ AXiAl PLANE FIRSTFOLO. ./, / : XIAL PL'-E 5ECONDFOLD.

~- -- - , ._.....~

%'---_/ -

- F> : .- .- -,~ - - - - - ---,..... _ ... _ ... 1I,

d. a INCHES

f. SF££r.

Fig. 3

6 INCHES. g.

86 DEREK POWELL

Phase 3. Generally small, variably tight to open, asymmetrical to sym­metrical, short limbed folds whose axial planes intersect the bedding atbetween 30 and 900

• The folds are almost always associated with micro­folding and thus a crinkle lineation. Not infrequently when folds of thisphase are tight and affect pelitic rocks, a strain-slip cleavage is developedparallel to the axial planes of the microfolds. Where rapidly alternatingbands of psammitic and pelitic rock are involved in the folding, the strain­slip cleavage is refracted.

A.

~=---- - .- ._~~

-~:.."~/ """' "

9 INCHES 1

D.o INCHES 4

Fig. 4. Examples of refolding:a. A second fold (axial plane dash-dot), refolds a first fold (axial plane dash). GlenMama. Loch nan Uamh Psammitic Group.b. A third fold (plunge indicated by open arrow) refolds second folds (black arrows).Arnabol hill. Ardnish Psammitic Group.c. A second fold (black arrow indicates plunge) is refolded by a fourth fold (openarrow). Ardnish, Ardnish Psammitic Group.d. Second phase cleavage (AP.2) oblique to the bedding (intersection shown by blackarrow), is deformed by third folds (axial plane-AP3). The plunge of the third foldsis determined by the attitude of both the second cleavage and the bedding (openarrows). Interbanded psammite (white) and pelite (lined). Roadside west of GlenBeasdale. Beasdale Pelitic Group

THE MORAR ANTIFORM, INVERNESS-SHIRE 87

In as much as the thicknesses of beds are often constant around minorfold closures, and the folds can almost always be seen to die out rapidly atdepth (Fig. 3d), the folds are of parallel type. The minor folds are rarelydeveloped in conjugate pairs. They deform both bedding, and planar,linear and fold structures belonging to the second fold phase (Fig. 4):Their geometry is often dependent on the pre-existing second fold phasestructural elements (Fig. 4d).

It seems highly likely that the third fold phase is responsible for thedevelopment of the 'herring-bone' structure and the 'subdued foliation'oblique to bedding, described by Richey (1947, 209 and 213). It can also beequated with the 'crinkling' mentioned by Lambert (1958,186-7).

Phase 4. Open, generally asymmetric, often step-shaped folds which varyconsiderably in size (Fig. 3). An axial planar cleavage is rarely and weaklydeveloped. In many examples quartz-filled fractures occur, lying parallelto the axial planes. The beds involved generally do not thicken or thin atfold closures. Folds of this phase are restricted in occurrence. They aremore commonly found in the psammites of the Ardnish Psammitic Group(Fig. 8). The folds have been seen to deform second phase linear and planarstructures (Fig. 4c) and on a larger scale, third phase linear structures(Fig. 5).

This sequence of structural events affects not only the Moine Schists ofthe south-eastern part of the Morar Antiform but also those of the Morararea in general. The occurrence of analogous fold sequences have beennoted in many parts of the Morar area.

(c) Major Structures

The superimposition of several fold phases has resulted in the develop­ment of complex structural patterns within which it is often difficult torecognise folds belonging to the earlier phases. Consequently the majorstructures are described in turn, beginning with those last to form andhence the generally more simple structures-those of the fourth phase, andending with those which are more difficult to recognise-the first phasestructures.

With the exception of first phase structures, the minor structures of thevarious phases of folding can be directly related to major folds.

The attitudes of the major fold axes have been determined largely byreference to the plunge of associated minor structures and by reference tothe geological map. Wherever possible, however, statistical analysis of theattitudes of bedding planes have been used to supplement and check theseobservations.

(i) Major Folds of the Fourth Phase. The outcrop of the Ardnish Psam­mitic Group is complicated by the occurrence of four large folds whoseaxial plane traces, on the ground surface, trend between east-north-east

00

00 ~ tI1 ~ tI1 ~ '"d o ~ tI1 e­ r-

Fig.

5.T

here

lati

onsh

ips

betw

een

seco

ndan

dth

ird

min

orfo

lds

and

the

Ard

­ni

shS

ynfo

rm.

Inth

est

ereo

grap

hic

plot

sth

eax

iso

fth

eA

rdni

shSy

n­fo

rmha

sbe

enro

tate

dto

the

vert

ical

.Pl

ots

of

the

seco

ndan

dth

ird

fold

axes

tend

tofo

rmsm

all

circ

les

arou

ndth

eax

isof

the

Ard

nish

Syn

form

SE

CO

ND

PH

AS

EMINO~

FO

LD

S.

N -+:<

;j(@~ o

NN +

(;~'

~~

~ SO

rHfR

DP

HA

SE

MIN

OR

FO

LD

S.N +

-:00.:

::::::.:."

;

,,:t.S

OI

I>6r

7/

~46k""""

57

1

<':..s2 \ ~6

8

js~

/63K"

~~l

67~S6

:'66

\'~

:~r

\

~P

LU

NG

EO

FT

1fIR

DP

HA

SEM

INO

RF

OL

D,

--~

".

SECO

ND

'I"

..;t

I/A

XIA

LP

lAN

ET

RA

CE

OF

AR

DN

ISH

SYN

FO

RM

.

MA

JO

RU

1H)L

OG

ICA

LJU

NC

rlO

N.

MIN

OR

THE MORAR ANTIFORM, INVERNESS-SHIRE 89

and north-east (Fig. 8). Of these the Arnabol Hill Antiform and the CamasDriseach Folds have axes which undulate about the horizontal but have amore persistent plunge towards the west-south-west at 1 to 45°. The shapesof these folds vary. The Arnabol Hill antiform is consistently tight andalmost parallel limbed along the whole of its outcrop (Fig. 6), whereas the

'~""

\.\.0.•. 11" If ..\\ .... ·.f. :{L\\IImII+:Y:''::>

L~~ . ~ .:.-: .. } ..".~. 'T' .,., I ',-/,::: I········:···:D.

ARDNISH SYNFORM1

'~.~

. \.'

r

ARNABOL HillANTIFORM 2

CAMAS DRlSEACH FOLDS3 •: :

1000fl

E.

600f1

F.

; ....

~ 4

l,

T .: ~.{: ./....I :. .. './. .1. ..

I .....

/i\\\~;¥/ r(.

G. ...--l--:

"\:,::{0)\'.\

MILES

o KMs' U

Fig. 6. Semi-diagrammatic vertical sections across the Ardnish Psammitic Group. Forlocation of section lines see Fig. 11

90 DEREK POWELL

Camas Driseach folds are relatively open structures in the southern partof their outcrop, becoming increasingly tight northwards (Fig. 6). Thisvariation in shape is in part at least a reflection of a change in shapeof the folds in a vertical sense since the folds are exposed in the north atapproximately 1200 feet above sea-level but in the south at only a few feetabove sea-level. The attitudes of the axial planes of the folds are indicatedin Figs. 6 and 9.

The Ardnish synform which lies to the west of the Arnabol Hill antiformis a complex fold. Its shape at the ground surface changes from asymmetricand open in the south-west, to almost symmetric and tight in the north(Figs. 5 and 11). Unlike the Camas Driseach folds this variation in shape isin a horizontal sense, the topographical differences from north to south­west along the fold outcrop being only slight.

In detail the fold has several peculiarities. To the south-west of LochDubh on Ardnish the fold consists of three units: the northern limb, thetrough and the south-eastern limb (Figs. 6 and 11). Statistical analysis of theattitude of bedding planes in this region shows that the northern limb dipsat 69° towards the south, the trough at 80° towards the south-west, and thesouth-eastern limb at 65° towards the west. At the ground surface theintersections of the limbs with the trough are angular (Fig. 1I), but thereis no apparent evidence for the presence of dislocations. To the north­north-east of Loch Dubh the trough unit of the fold becomes progressivelyless well developed until finally it disappears.

The changes in the shape of the Ardnish Synform are accompanied bychanges in the direction and amount of plunge of the fold axis. In thesouth-western part of the fold outcrop the overall plunge is towards thesouth-west at 65°, but north-east of Loch Dubh the fold axis graduallyswings so that on the south-western slopes of Arnabol Hill it plunges to­wards the south-east at between 75 and 80°. The attitude of the axialplane(s) of this fold and its changes in shape are illustrated in Figs. 6 and 9.

Two other peculiarities are notable. The western limb of the synform isthinner than the eastern limb except at the extreme southern end of thearea. The axial trace and thus the axial plane of the synform lies at somedistance from the outcrop of the Loch Mama Pelitic Group on Ardnishbut traced north-eastwards it becomes progressively closer until at thenorthern edge of the area it lies only 160 yards away from this pelitic group(Fig. 8). These phenomena are referred to later.

The major fold described above deforms minor structures of both thesecond and third fold phases (Fig. 5).

(ii) Major Folds of the Third Phase. Two large open folds can be relatedto the third fold phase. Both affect the outcrops of the Beasdale PeliticGroup and the Loch nan Uamh Psammitic Group. They are the GlacRuadh Antiform and the Loch nan Uamh Synform. Both are almost

THE MORAR ANTIFORM, INVERNESS-SHIRE 91

symmetrical structures with axial plane traces trending north-north-westto south-south-east. They plunge towards the south-south-east and theiraxial planes lie close to the vertical. The folds deform major, second phasestructures (Figs. 8 and 9).

(iii) Major Folds of the Second Phase. Two major folds and a major foldcomplex affect the rocks between Glen Beasdale and the outcrop of theLoch Mama Pelitic Group (Fig. 8). These are, from west to east: theDruim Fiaclach Fold Complex, the Glen Mama Antiform and the ArnipolSynform (Fig. 7). The folds have several features in common. The ir axialplane traces trend from north-east to south-west, their axial planes areinclined at varying angles towards the south-east, and they are generallytight structures (Figs. 7 and 9).

The Druim Fiaclach Fold Complex comprises several fold closureswhich can be traced from the north-eastern end of Glen Beasdale, acrossDruim Fiaclach, to the coast west of the mouth of Beasdale Burn (Fig. 8).In the north the folds are almost paralIel limbed and their axes plungetowards the east-north-east at between 40 and 60° (Figs. 7 and 8). Tracedsouthwards the attitude of the fold axes changes. The amount of plungedecreases so that where the folds run against the Loch Mama Fault theyplunge to the north-east at 5 to 30° (Fig. 8). To the south-west of the fault,for approximately half a mile, the north-easterly plunge persists but be­comes less steep . Farther to the south-west the second folds begin to plungetowards the south-west until at the coast these structures have a consistent,moderate plunge in this direction. These changes in fold attitude may beattributable to the interference of the third phase major folds, the GlacRuadh Antiform and the Loch nan Uamh Synform.

To the south of the Loch nan Uamh Fold the major second folds notonly change their attitude but also their shape. Where the Druim FiaclachRidge is cut by the railway line the folds are much more open structures,and along the shore section to the west of Beasdale Burn the folds havedecreased considerably in amplitude save for the more easterly synform(Fig. 7).

The Glen Mama Antiform and the Arnipol Synform are relatively tightstructures along the whole of their length. The western limb of the GlenMama Antiform and the eastern limb of the Arnipol Synform dip towardsthe south-east at 50 to 800 whereas their shared limb dips in the samedirection at 20 to 500 (Fig. 7). The axes of the two folds lie close to thehorizontal plunging in places towards the south-west, in others towards thenorth-east at low angles (Fig. 8).

An important feature of the second phase folds in this area is that theevidence afforded by cross and graded bedding demonstrates that the foldsaffecting the rocks close to the north-western psammitic groupjBeasdalePel itic Group junction face upwards, whereas those affecting rocks close

A

B

c

,, ,, ', ', ', ' .

reer 1lXJO

KILOMeTERS

LOCH " A" A PELITIC GROUP.

LOCH NAN UA"H PSA.... ITlC GROUP.

BEASDALE PELITIC GROUP.

.....<, A.X IAl PLANE PHt.SE 2 FOLD.. .....

'..... AXIAL PLANE PHASE 1 FOLD.

~ DIRECTION OF FACING OF PHASE 1 FOLOS.

Fig. 7. Semi-diagrammatic vertical sections across the Beasdale Pelitic and the Lochnan Uamh Psammitic Groups. For location of section lines see Fig. 9.

71 72 73I

74 75 76 77

mJ PREDOMINANTLY PELITIC ROCKS.

---FAULT.

----AXIAL PLANE TRACE OF MAJOR SECOND FOlD.__ - - THIRD FOLD.

FOURTH FOLD.

-23 PLUNGE OF SECOND MINOR FOLDS AND ' LINEATIONS.--117 .. THIRD--.45 FOURTH

Structure of S.E. Morar.

Fig. 8. Structural map of south-east Morar

76

N

0 V2! I

100011.KMS.

I

~0 MILES

77

85

84

83

[To la ce p . 92

THE MORAR ANTIFORM, INVERNESS-SHIRE 93

to the Beasdale Pelitic Group/Loch nan Uamh Psammitic Group junctionface downwards (Fig. 7). The implications of this are discussed in thefollowing section. ('Facing' is used here in the sense of Shackleton, 1958).

(iv) Major Folds of the First Phase. The general lack of minor structureswhich can be positively identified as belonging to the first phase of foldingmakes the recognition of related major folds extremely difficult. Fortun­ately there is sufficient evidence for the sedimentary attitude of the rocksto allow the recognition of pre-second phase folds.

It has already been mentioned that the second folds near to the easternmargin of the Beasdale Pelitic Group face downwards. This attitude is trueof these folds as far east as the Loch Mama Pelitic Group (Fig. 7). Secondfolds both near to the north-western margin of the Beasdale Pelitic Group,and within the psammitic rocks lying to the north-west, as far as they havebeen mapped in detail, face upwards (Fig. 7). The change in the facingdirection of the folds takes place towards the centre of the outcrop of theBeasdale Pelitic Group. These changes in the structural-sedimentaryattitude of the second structures imply the presence of a fold closure whichis earlier than the second folds, and has its axial plane contained by theoutcrop of the Pelitic Group (Fig. 7). This closure, termed the Beasdalefold, almost certainly belongs to the first phase of folding. It is isoclinalbut neither its plunge nor its direction of closure can be directly observed.

No other major first phase folds have been found within the area.

(d) Tectonic Breaks

In a previous paper (Powell, 1964) evidence was brought forward insupport of MacGregor's and Lambert's contention that no stratigraphicalor structural break exists in the positions postulated by Richey & Kennedyand Kennedy respectively, at least in the south-eastern part of the Morararea.

The evidence in support of this conclusion can be summarised:The rocks on either side of the positions postulated for the breaks are

conformable and disturbed only by major and minor folds (Figs. 8, 9,and 11).

There are no fundamental differences in lithology, texture or minera­logy between the rocks on either side of the proposed breaks (Mac­Gregor, 1948; Lambert, 1958).

There are no structural or metamorphic differences which wouldsupport the existence of an unconformity. The rocks have all sufferedthe same phases of folding and metamorphism.

There are no differences in the character or attitudes of folds of thevarious fold phases on either side of the proposed breaks (Figs. 8 and 9).The author also described a tectonic break which affected rocks lying

structurally and stratigraphically above the breaks postulated by the earlier

94 DEREK POWELL

workers (Powell, 1964, 226-8). The new break is indicated by severalfeatures:

(i) The Loch Mama Pelitic Group thins rapidly from north to south(Fig. 11). Associated with this thinning is a change in the lithological natureof the group. In the north it consists throughout its thickness of thinlyinterbedded pelites and semi-pelites, with occasional concordant ribs andlenses of psammitic and calc-silicate rock. Traced southwards the groupbecomes more uniform in lithology. It becomes more pelitic and the regularpelite/semi-pelite interbedding is largely lost. In many places in the southof this outcrop there are no indications of bedding, the only visible planarelement is a strong schistosity which is parallel to the external contacts ofthe group and to bedding planes.

(ii) To the south of Arnipol the strike of the Ardnish Psammitic Group,adjacent to the Loch Mama Pelitic Group, is markedly oblique to thecontact between the two groups (Fig. 11). The contact must thereforerepresent a plane of discontinuity.

(iii) The succession of rocks within the Ardnish Psammitic Group on thewestern limb of the Ardnish Synform is much thinner than the samesuccession preserved on the eastern limb (Fig. 6).

(iv) Second phase fold structures on either side of the Loch MamaPelitic Group have different orientations. Those to the west generally havesub-horizontal axes, whereas those to the east, between the eastern marginof the Loch Mama Pelitic Group and the axial trace of the ArdnishSynform, have axes which plunge towards the south-east at moderate tosteep angles (Fig. 8). Second phase folds are rarely found within the peliticgroup.

The nature of this break is indicated by the unusual changes in attitudeof second minor folds. Within the Loch nan Uamh Psammitic Group,between the axial trace of the Arnipol Synform and the Loch Mama PeliticGroup, the folds of this phase change their orientation so that, as thepelitic group is approached,the fold axes swing round from an attitudewhich is typical of other parts of the Loch nan Uamh Psammitic Group,to one approaching that found within the Ardnish Psammitic Group (Fig.8). Such a swing is most apparent around Arnipol and to the south-west. Itdoes not occur at the northern end of the area where there is a moreabrupt change in the plunge of the fold axes across the Loch nan Uamhpsammitic/Pelitic Group contact (Fig. 8).

It appears therefore that the movements responsible for the featuresoutlined above did not take place on any single plane of movement butwere dispersed throughout a considerable thickness of rocks. The changesin the orientation of second folds, the thinning and change in lithologicalcharacter of the Loch Mama Pelitic Group and the thinning of the ArdnishPsammitic Group, suggest that preferential distortion occurred within the

LOCH NAN

- Axial plane trace of fourth fold.- -- .. .. th ird fold.._ .- .. .. second fold.

- .,- - Dip and strike of fourth fold axial plane.third ..

.. second ' .Trend of fourth axial planes.

-- - - .. third- - .. second ..- - - Fau It (minor faults ornrnited)

-- Lithological junction.

T,··--:"': ":" ·· ·· ···I 1 . • '

//1I /

Fig. 9. Map illustrating the axial plane trends of folds in south-east Morar

1 KMS.

[To face p. 94

THE MORAR ANTIFORM, INVERNESS-SHIRE 95

Pelitic Group and those parts of the two adjacent Psammitic Groups whichlie close to the Pelitic Group. There is also evidence that movements wereconcentrated along planes within this zone, for example the northern Lochnan Uamh Psammitic Group/Loch Mama Pelitic Group contact.

Structures belonging to the third fold phase, both minor folds andlineations, have a consistent attitude throughout this area. They show nochange in orientation across, or on either side of, the Loch Mama PeliticGroup (Fig. 8). Thus the movements which gave rise to the break must haveoccurred before the formation of the third folds. The reorientation ofsecondstructures suggests that the bulk of the movement took place either atsome late stage in the formation of these folds or succeeded it. In view ofthe characteristic 'similar' style of second folds and the location of thetectonic break on the limb of a large second phase major fold (the ArnipolSynform) the break appears to have been generated during the later stagesof the growth of these folds.

The stratigraphical implications of this break have been discussedelsewhere (Powell, 1964). The structural implications, in terms of thegeneral structure of the area, do not appear great. It seems very unlikelythat this break could constitute the basal slide of a nappe, such as Kennedyenvisaged (Kennedy, 1954, 248-9).

5. REGIONAL IMPLICATIONS

The remapping of the south-eastern part of the Morar area has demons­trated several factors which are important in a consideration of the structureof the Morar Antiform:

(0 The Beasdale Pelitic Group is older than the Loch nan UamhPsammitic Group (=Lower Psammitic Group; Kennedy, 1954) and isalmost certainly equivalent to the Basal Semi-pelite of the Glenelg/Knoydart Succession.

(ii) The psammitic rocks lying to the north-west of the Beasdale PeliticGroup are younger than this group and are stratigraphically equivalent tothe Loch nan Uamh Psamittic Group.

(iii) The outcrop of the Beasdale Pelitic Group contains the axial planeof a first fold phase isocline. This is refolded by tight second phase folds(Fig. 7). The present interpretation of the structure in this area differsfrom those of Richey & Kennedy, Kennedy, and Lambert (Fig. 2). Noneof these authors recorded the presence offolds ofmore than one generation.Each considered that the Beasdale Pelitic Group formed an easterlydipping sheet which was folded by several major folds whose axial planeslay at a steeper angle than the bedding (Fig. 2a, b and c). The presentobservations demonstrate that the apparent easterly inclination of thePelitic Group is attributable to the attitude of the second fold limbs; the

96 DEREK POWELL

group has in fact an overall westerly inclination. The Pelitic Group iswithin itself a fold core and has been refolded by four second folds withaxial planes dipping towards the east at a shallower angle than the bedding(Fig. 7).

(iv) The Druim Fiaclach Fold Complex together with the Glen MamaAntiform form a large antiformal structure (Fig. 7).

(v) The structure of the rocks between the Beasdale Pelitic Group andthe Loch Mama Pelitic Group is such that the Beasdale Fold does notreappear at the ground surface to the east. Indeed no major first phase foldis found for some considerable distance in this direction. The second phasefolds within the Loch nan Uamh Psammitic Group face downwardsconsistently (Fig. 7).

The stratigraphical identity of the majority of the major rock groups inthe Morar area is now known with some certainty. The Moine rocks lyingabove the Lewisian in north and south-east Morar (Fig. 1), can be directlycorrelated (Powell, 1964). There is however no definite evidence for thestratigraphical position of the Moine rocks lying beneath the Lewisian.Their correlation with the No. 1 semi-pelite and No.2 psammite in Knoy­dart and North Morar (Ramsay & Spring, 1962, and Lambert & Poole,1964) is based upon their structural position, lithological similarities andparticularly the presence in the psammites of heavy mineral bands. Thereseems little reason to doubt these correlations, and continuity of outcropinto South Morar suggests that the similar rocks lying in the same struc­tural position in South Morar belong to these basal groups of the MoineSuccession. Such a correlation, as Ramsay & Spring suggest, implies thatthese rocks are inverted.

Above the Lewisian in south-east Morar the relationships between therock groups are complex. The recognition of the psammitic rocks lyingimmediately above the Lewisian, and the pelitic rocks above these, asGroup 2 and Group I respectively, produces an anomalous situation.Although Group 1 occurs at a higher structural level it does not separateGroup 2 from the Lewisian as it does in many other parts of the Morararea (Fig. 1). There are three possible explanations for this. Either Group 1was never present, or it was deposited but has since been removed byerosion or by some tectonic process. Ramsay & Spring have commentedupon the remarkable lateral persistence of Group I throughout the Glenelg/Knoydart area (Ramsay & Spring, 1962, 313). In the Morar area it isexceptionally well developed (Fig. 1) and consequently the author holdsthat its local absence is more readily explained by assuming that it wasdeposited and that it has since been removed tectonically (see also Ramsay& Spring, 1962, 313).

Preliminary observations in the psammites lying close to the Moine/Lewisian junction in south-east Morar lend additional support to this

THE MORAR ANTIFORM, INVERNESS-SHIRE 97

conclusion. Current bedding within the psammites indicates that theyyoung towards the south-east, that is away from the Lewisian and in theopposite direction to that in the same psammitic rocks where they areadjacent to the Beasdale Pelitic Group.

If the Moine Succession does young outwards from the Lewisian insouth-east Morar, the occurrence of older rocks farther to the south-eastsuggests that the Group 2 psammites which lie between the Lewisian andthe Beasdale Pelitic Group contain the axial plane of a large tight fold.

It now appears likely that the Lewisian rocks of south-east Morar areone-sided, in as much as the Moine Succession is repeated, though locallypartly reduced, on either side of the Lewisian outcrop. The Lewisian rocksmust therefore be sited in the core of a large isoclinal fold, which is pre­sumably the same fold as that containing the larger outcrops of Lewisianrocks in Knoydart (Fig. 1).

The isoclinal fold containing the Lewisian rocks of Knoydart belongs tothe first of the three fold phases found by Ramsay & Spring (op. cit., 310).This is almost certainly analogous with the first fold phase found by thepresent author. The relationship between the Beasdale Fold, which belongsto the first fold phase, and the fold which contains the Lewisian, isimportant.

In this connection the author would draw attention to the followingobservations.

The most westerly of the folds making up the Druim Fiaclach FoldComplex, which belong to the second fold phase, is an antiform (Fig. 7).

The southern part of the Morar Antiform is essentially a second phasestructure. The small open folds with north-westerly trending axial traces,which affect both limbs of the Antiform (Fig. 1, pts. A-A, B-B) belongto the third phase of folding. They refold the Morar Antiform. As theAntiform is not isoclinal and all the recognised first folds in the Moineare, it is unlikely that it could belong to the first phase of folding.

The apparent juxtaposition of two second phase antiforms, the Morarfold and the Glen Mama Antiform, produces a peculiar structuralsituation: there is apparently no intervening major second phasesynform. This phenomenon is most readily explained by presuming thatthe fold whose axial plane may be contained by the outcrop of the Group 2psammite which lies immediately above the Lewisian, is a second phasesynform (Fig. lOA). Alternatively this fold could be regarded as a firstphase fold, or a tectonic break could exist somewhere between theLewisian and the outcrop of the Beasdale Pelitic Group. In either of thelatter two cases a pre-second fold phase difference in the attitude of therocks, to east and west of the first fold or break, is necessary to explainthe juxtaposition of two antiforms of the same age.Whether the outcrop of the Group 2 psammites lying above the Lewisian

PROC. GEOL. ASS., VOL. 77, PART I, 1966 7

98 DEREK POWELL

in south-east Morar contain the axial plane of a second phase synform, afirst phase fold or a tectonic break of considerable importance, it appearsthat the Beasdale fold roots on the south-eastern limb of the Morar Anti­form. Since there are no structural repetitions of the stratigraphy, as fareast as the author has mapped, which could suggest that the Beasdalefold has its roots farther to the east than its present outcrop (pt. V., p. 20),it appears that the fold is an anticline closing upwards towards the east.Consequently the author suggests that the root zone for the first formedfolds probably lies no farther east than the area of the south-eastern limbof the Morar antiform. It may in fact lie on the western limb of this fold(Fig. lOA). This latter suggestion is however incompatible with the existing

A.

NW

Morar Antiform,, Glen MamaAntiform\

\

S.E.4

\\

B.

NW

\\

?

S.E.34

2\\

~ Direction of younging of Moine Succession.

Fig. 10. Interpretative sections across the Lochailort and south Morar areas.For explanation see text

83

85

0·5M i t e s

..,; '/' : ../ ;' 't' :•• Of 'I .

; /,: : ../; : : : r,::. :: ,,:' -':- + ~_ :..~~- --;-

• .• '#o "' j-7 --'} . '2h· : :' :' 'j_.;.7.....;·'.:...."':.;--

77

o Kms . 0 ·5

!I,.

.'

• ... ...;.. ,.iJ ·1

...1~.~tpR~..· ..' ..,"0. : >:(. ' .' 0

NM76

SEMI-PELITE.

No.6 PSAMMITE.

No.5 PELITE.

Nos.2.3 &4. PSAMMITES WITH INTERVENING

No.1 BASAL SEMI- PELITE .

LEWISIAN.

GLENELG/KNOYDART SUCCESSION.(RAMSAYand SPRING).

ALLUVIUM.

ARDNISH PSAMMITIC GROUP

LOCH MAMA PELIT IC GROUP

LOCH NAN UAMH PSAMMITIC GROUP

BEASDALE PELITIC GROUP

DIRECTION OF YOUNGING OF THE ROCKS(within and adjacent to Beasdale Pr!/itic Grp

FAULT.

POSI TION OF UNCONFORMITY AFTER RICHEY and KENNEDY 1939.

POSITION (y: SLIDE AFTER KENNEDY 1954

LINES OF SECTIONS .

SPOT HEIGHTS IN FEET.

STRATIGRAPHICAL SUCCESSION.

T-:-::-'~

-;;- DIP AND STRIKE OFBEDDING.

__ LITHOLOGICAL JUNCTION

._. -.-. APPROXIMATE.

---.-1£'' - 51.""

-+ ~A

71 72 73 74 75 76 77

86 86

0969 E Un :N~ J .1011

t

D A~ I'92 '

N 8 E A 5G L E /

85

Fig. 11. Geological map of south-east Morar [To fa ce p , 98

THE MORAR ANTIFORM, INVERNESS-SHIRE 99

map of the south-western part of the Morar Antiform (Fig. 1). As the map ofthis area stands the Beasdale Pelitic Group is shown to be continuousaround the nose of the Morar 'Anticline', with a thick group of peliticand semi-pelitic rocks. Consequently the axial plane of the Beasdale Foldmust, if this part of the map is correct, be contained by this western outcropof pelitic rocks and cannot without envisaging unlikely structural com­plexities be continuous with the proposed isoclinal fold containing theLewisian rocks.

Some of the structural conclusions derived from a study of the south­eastern part of the Antiform are apparently not at present applicable to theremainder of South Morar, and before any weight can be given to theidea of the first folds rooting on the western limb of the Morar Antiformfurther detailed mapping is obviously necessary. It is interesting to note,however, that the hypothesis of a western origin for the earliest structuresseen in the Moine Schists has been put forward by Spring (1961), and byRamsay (1963, 166), to explain the occurrence of Lewisian sheets in theMoine Schists of Glen Orrin and Monar (Ross-shire).

ACKNOWLEDGMENTS

I am grateful to Dr. N. Rast for his advice and encouragement at allstages in the preparation of this work. I am also indebted to Professor J.Sutton, Professor A. Wood, Dr. N. H. Kirk and Dr. W. J. Phillips for theirhelp with an earlier draft of this paper. My thanks are due to ProfessorW. S. Pitcher for his criticism of the final manuscript.

The earlier work towards this paper was carried out in the Departmentof Geology at Aberystwyth while I was in receipt of a University of WalesFellowship. The work was completed at the University of Liverpool underthe tenure of a D.S.I.R. Fellowship.

REFERENCESKENNEDY, W. 1954. The Tectonics of the Morar Anticline and the Problem of the

North-West Caledonian Front. Q.JI 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 Schists of the

Morar and Knoydart Districts of Inverness-shire. Trans. R. Soc. Edinb., 63,553-88.

--- & A. B. POOLE. 1964. The Relationship of the Moine Schists and LewisianGneisses near Mallaigrnore, Inverness-shire. Proc. Geol. Ass., 75, 1-14.

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

PHEMISTER, J. 1948. The Northern Highlands. Br. reg. Geol.POWELL, D. 1964. The Stratigraphical Succession of the Moine Schists around Lochai­

lort (Inverness-shire) and its Regional Significance. Proc. Geol. Ass., 75,223-50.

!OO DEREK POWELL

RAMSAY, J. G. 1960. The Deformation of Early Linear Structures in areas of RepeatedFolding. J. geol., 68, 75-93.

---& J. SPRING. 1962. Moine Stratigraphy in the Western Highlands ofScotiand.Proc. Geol. Ass., 73, 295-322.

---. 1963. In British Caledonides, Edinburgh.RICHEY, J. E. & W. Q. KENNEDY. 1939. The Moine and Sub-Moine Series of Morar,

Inverness-shire. Bull. geol. Surv. Gt, Br., 2, 26-45.---.1947. Pre-metamorphism Cleavage in the Moine Schists of Morar, Inverness­

shire. Trans. Edinb. geol, Soc., 14,200-19.SHACKLETON, R. M. 1958. Downward-Pacing Structures of the Highland Border.

Q. Jl Soc. Lond., 113, 361-92.SPRING, J. S. 1961. In discussion of Fleuty, M. J., The Three Fold-Systems in the

Metamorphic Rocks of Upper Glen Orrin, Ross-shire and Inverness-shire.Q. JI geol. Soc. Lond., 117, 477.

VaLL, G. 1960. New Work on Petrofabrics. Lpool. Manchr geol. J., 2, 503-67.

D. PowellDepartment of GeologyBedford CollegeLondon N.W.!