• • • • • • • •

• • •

• • • •

• •

Report 83/6 INSTITUTE OF GEOLOGICA L SCIENCES

The geology of t he Minches, Inner Sound and Sound of Raasay

• •

• • • • •

• /I •• •

• •

•• •

•

Report 83/6

© Crown copyright 1983

ISBN 0 " 884268 4

INSTITUTE OF GEOLOGICAL SCIENCES

Natural Environment Research Council

The geology of the Minches, Inner Sound and Sound of Raasay

J. A. Chesher, D. K. Smythe and P. Bishop

London Her Majesty's Stationery Office 1983

CONTENTS

Introduction The sea areas Offshore investigations Aims and methods of interpretation

Bathymetry 3

Solid geology 5 Onshore geology 5 Offshore geology 9

Quaternary geology 11 Glacial and post-glacial sediments 11 Surface sediments 19

References 28

FIGURES 1 Survey coverage 2 2 Bathymetry In pocket 3 Solid geology In pocket 4 Pte-glacial drainage 3 5 Deep seismic profiles 4 6 Shallow seismic profiles 6- 7 7 Bouguer anomaly map In pocket 8 Aeromagnetic anomaly map In pocket 9 Quaternary sediment thickness In pocket 10 Depth to rockhead map In pocket 11 Location of Quaternary seismic profiles 12 12 Line drawings of Quaternary seismic profiles

13-16 13 Possible late glacial shorelines 19 14 Surface sediment map In pocket 15 Location of sediment sample traverses 20 16 Variation of surface sediments along traverses

21-23 17 Genetic classification of surface sediment 25 18 Surface sediment C-M diagram 26

11

Bibliographical riference CHESHER, J. A., SMYTHE, D. K. and BISHOP, P. 1981. The geology of the Miches, Inner Sound and Sound of Raasay. Rep. Inst. Geol. Sci., No. 83/6.

Authors J. A. Chesher, BSc, PhD, D. K. Smythe, BSc, FRAS Institute of Geological Sciences, Murchison House, West Mains Road, Edinburgh EH93LA P. Bishop, BSc, PhD University College, London, WCl

The geology of the Minches, Inner Sound and Sound of Raasay

j. A. Chesher, D. K. Smythe and P. Bishop

INTRODUCTION

THE SEA AREAS The Minches, for the pUlposes of this report, comprise the sea area from Skye and the Scottish mainland to the Outer Hebrides and bounded to the north and south by latitudes 58° 40'N and 57° 30'N. The Little Minch lies between Skye and the Outer Hebrides islands of Harris and North Uist, whereas the North Minch is the larger sea area east of Lewis. The narrow strips of water on either side of the islands of Raasay and Rona are the Sound of Raasay to the west, and the Inner Sound to the east.

OFFSHORE INVESTICA TIONS Prior to the 1960's, understanding of the offshore geology of the region advanced very little, and it was believed that the Minches were simply under-lain by a gentle NNE-SSW-trending shallow basin of Mesozoic sediments, perhaps up to 800 m deep (compare, for example, MacCulloch, 1819, vol. 1, p. 443; vol. 3, pI. 32, fig. 3; MacKinder, 1907, pp.74-75; Hallam, 1965, pp. 410-411; and Donovan, 1968, p. 9). George (1966, p. 16), however, alluded to the necessity of offshore geo-physical exploration, which began with the region-al aeromagnetic surveys flown for the Institute of Geological Sciences (IGS) in 1964 and 1965.

The marine survey work on which this report is based began in 1968 with a reconnaissance cruise by the RRS John Murray carrying .out geophysical traverses on a NW-SE and NE-SW grid, at a spacing of about 10 km (Figure 1). The equipment consisted of a 1000-joule sparker, side-scan sonar, echo sounder, gravity meter and magnetometer (McQuillin and others, 1968). A marine deep seismic survey of the Hebrides was shot in 1969 by Seismograph Service Ltd for IGS, and five of those lines fall into the area covered by this report (McQuillin and Bacon, 1974). This was followed in 1970 by a shallow sampling programme and further geophysical profiling, using the RV Vz'ckers Venturer (Kenolty, 1971; Eden and others, 1971). Drilling, together with additional sampling and geo-physical work, was undertaken from MV White-thorn between 1970 and 1973 (Chesher and others, 1972; Institute of Geological Sciences, 1974), and a limited programme of further drilling has since been carried out (Institute of Geological Sciences, 1977; Evans and others, 1981).

Observations from the Vickers manned sub-mersible Pisces were made in 1970 and 1971 (Eden and others, 1971; 1973) and detail geo-physical investigations of the Little Minch were made by Glasgow University in 1971 on board the RRS John Murray (Smythe and others, 1972), and in 1972 aboard the MV Calanus.

The results of some other commercial and academic surveys mentioned below have not been explicitly incorporated into the present account for reasons of confidentiality and/or duplication of data. During the early 1970's, several commercial deep seismic surveys were shot in the Minches and Sea of the Hebrides, but as the area has proved to be 'non-prospective' for hydrocarbons, no such work has been undertaken recently. A provisional Bouguer anomaly map of the North Minch was compiled from the results of Durham University cruises in 1967 and 1968 (Allerton, 1968; Bott and Watts, 1970), and some airgun reflection records were obtained west of Skye during a University College, London, cruise in 1972 (E. J. W. Jones, personal communication).

AIMS AND METHODS OF INTERPRETA TION The aim of the pre-Quaternary section of the report is to present the solid geology. The deeper (pre-Permian) structure below the Minches is mentioned only briefly, as this would require detailed discussion of the reflection and refraction surveys on and around Skye by IGS and Glasgow University (Smythe and others, 1972), and the presently confidential reflection surveys, mainly in the North Minch, made by several oil companies. However, the interpretation of the solid geology, undertaken jointly by Chesher and Smythe, is compatible with our unpublished interpretations of the deeper geology; and the Quaternary section of the report by Bishop is based on work done during a NERC studentship based jointly at University College, London and IGS.

The techniques used to compile an offshore geology map from geophysical and rock-sample data are described in McQuillin and Ardus (1977). Examples of problems of interpretation in the Hebrides-Minches region are described further in McQuillin and others (1979, pp. 104-109). The particular characteristics of the interpretation of the Minches region presented in this report are: i excellent geological control is provided by the land outcrops;

7°

Geophysical traverse line

Sample station

o I.G.S. borehole

58° 30' 0 10 20km , ! ,

UTM PROJECTION

58°N

"

Figure 1 Survey coverage

ii the unusually complex bathymetry is a considerable aid, both in geological intrepretation, and in correcting navigational errors (see be la w) ;

iii the Decca Main Chain hyperbolic radio-navigation system, used for most of the surveys, is here very poor. Systematic errors of up to 1 km in line positioning have been corrected where possible

2

58°N

WESTER ROSS

57° 30'

N

5° 30'

by re-locating profiles to match the bathymetry, which was surveyed by the Admiralty using the much more accurate Decca Hifix system (McQuillin and Ardus, 1977, p. 21); iv the density of profile and sample coverage is very high, relative to most other areas on the UK continental shelf, but the quality of the seismic records is relatively low by present standards.

THE SUBMERGED PRE-GLACIAL DRAINAGE

OF THE MINCHES

o 10 20 km Probable former river Courses

L' -----'-,-----', Butt of Lewis

58°N

........ Major pre-glacial watershed

N

Sea-bed deeper than 100 m

(Slightly modified after Godard 1965)

Figure 4 Pre-glacial drainage

BATHYMETRY

A bathymetric contour map of the Minches (Figure 2) has been prepared from Hydrographic Office data, supplemented by echo sounder traces obtained during the IGS surveys.

The North Minch consists of a fairly shallow basin averaging 120 m in depth below sea level. The absence of faults controlling the margins has

3

BOW

58"N

WESTER ROSS

57° lI'

led to a gently shelving coastline, which along the eastern margin is marked by large embayments. The Shiant East Bank, which reaches 40 m below sea level, separates the North Minch Basin from the Little Minch Basin to the south.

The bathymetry of the Little Minch is more complex, but is essentially basinal with an average depth of 100-140 m below sea level. The bathy-metric variability is largely due to the resistant

.j:>.

!is'' 4O'N urw OOOW !is'' 4O'N I I I I

o 10 20km I I I

58°N

57° 10'N t· .. ', !.L. -[~,

Figure 5 Deep seismic profiles. Diagrammatic representation of seismic lines 1 and 2 British Petroleum and line 3 by Shell UK

S Line 1

11J "--"'--~ ___ J_ur_a= ... :::.c:-_________ _ ~ I Torridonktn ~ Permo~Triass\c -----.5 . ~ 2

o 5 10km I I I 13

f-

Borehole

Borehole

72/33 N

o W Line 3 E 0 0 W +76/55 Line2 E r -:: ?B~~B'S.~ I!:::= ~ /'! '.= --t'!!'rno-Tri.ssi~ BaseJU~-l .5 ~2 ." >-

~3 ~

T orridonian

Minch Faul! o 5 10km

I

2 2 Torridonian

o 5 10km 3 3 I I I

Minch Fault

.-L ________________________________________ ~I 4 4~----------------------------------~

S Line 4 N

1 ------ lase Jurassic Reflect()(s not ~=:o~to

.5

~ 2 .'" >-~ ~ 3 f-

-----? Base P",mo-~ ? Torridonian

o 5 10 km 1 I ,

courtesy of

......

~

E o W Line 5 se. ' ..... ~ • Basalt outcrop

~ No reflectors __ visib4e

... ~~"-~ .. _beneath ~basalts ? T orridonian

o 5 10 km ! I I

.... d j

2 ~ . :r

3 i ~

d j

2 ~. :i"

31 -c. ..

I d'

s·

~ ~

basic sills of early Tertiary age which intrude the Mesozoic sediments to give rise to numerous banks, knolls and arcuate scarp and dip ridges. The north western margin of the basin south of Kebock Head is marked by a steep bathymetric scarp following the south-east coastline of the Outer Hebrides along the line of the Minch Fault which here throws down the less resistant Mesozoic rocks against Lewisian basement.

It should be noted that the new bathymetric map presented in this report provides no support for Dearnley's (1962) conjecture that a supposed line of 200 m-deep troughs might be the bathy-metric expression of a NW-SE-trending sinistral transcurrent fault running up the centre of the Little Minch.

The major influence on the bathymetry, aside from the primary control of the solid geology (Figure 3), has been the pre-glacial drainage pattern as mapped by Ting (1937) and Godard (1965) (Figure 4). The major watershed which then ran from Trotternish to Harris, some 25 km south-west of the Shiant East Bank basement ridge is still expressed as a bathymetric high, although it was glacially breached in the Sound of Shiant.

During the Quaternary glaciation, Ice preferentially followed the fluvial valleys and overdeepened them, especially at the more constricted points, for example at the north of the Inner Sound, and to the east and west of the Shiant East Bank. Although the overdeepened valley of the Inner Sound has been partially infilled by glacial sediments, it is nevertheless, at 320 m, the deepest water on the UK Continental Shelf. Ice also scoured Mesozoic sediments from the down-thrown sides of the major faults, for example in the Inner Sound, and along the south-eastern coast of the Outer Hebrides, south of Kebock Head.

SOLID GEOLOGY

ONSHORE GEOLOGY The sedimentary rocks of north-west Scotland, resting on the metamorphic Lewisian gneisses, span the time interval from the Late Precambrian, about 950 Ma ago, to the present. An important phase of Palaeogene volcanic activity is represented by the igneous rocks of Skye and its neighbouring islands. Some aspects of the onshore geology are summarised below.

Lewisian These high-grade gneisses, derived dominantly from pre-2600 Ma igneous intrusions with minor metasediments and meta-volcanics, were last reworked during the Laxfordian orogeny, about 1800 Ma ago. They crop out in two major areas trending NNE-SSW, one on the mainland of north-west Scotland and the other forming most of the islands of the Outer Hebrides. On the main-land, NW-SE structures predominate; these

5

include foliation, fold axes, shear zones and dyke swarms (Bowes, 1969). It is possible that this base-ment trend has influenced structures in the sedi-mentary cover. The granulite-grade Lewisian gneisses of the Loch Laxford Scourian separate two regions of dominantly quartzo-feldspathic gneisses and schists to the north and south. The granulites are characteristically denser, more highly magnetic (Powell, 1970) and of significant-ly higher P-wave velocity (Hall and AI-Haddad, 1976) than the gneisses and schists which were strongly reworked during Laxfordian deformation and metamorphism. The Lewisian geology of the Outer Hebrides shown in Figure 3 is simplified from recently published IGS maps. The main structural grain is again NW-SE, although in north Lewis an older Scourian (2600-2800 Ma) NNE-SSW trend predominates. The eastern coastline of the Outer Hebrides closely parallels the Outer Isles Thrust zone, which in Pre-Cambrian and again in late Caledonian times thrust dense granulites westwards over less dense amphibolite gneisses and granites (McQuillin and Watson, 1973). The thrust zone is made up of mylonites and other cataclastic rocks, and normally dips eastwards at about 25°.

Torridonian These essentially unmetamorphosed Precambrian sediments rest on the Lewisian basement of the mainland and are divided into an older Stoer Group, and a younger Torridon Group (Stewart, 1975). The ages of the two groups, determined by Moorbath (1969) using the Rb-Sr method on shales, are about 995 Ma and 810 Ma respectively (Brook and others, 1977, p. 494). Maximum thicknesses are 2.3 km for the Stoer Group and 7 km for the Torridon Group, with an intervening angular unconformity of about 25°. Stewart (1969, fig. 2) suggested that thicknesses would have been even greater to the west, and it is likely, therefore, that several kilometres of Torridonian (sensu la to ) are still preserved below the Minches.

The low-grade metamorphosed Sleat Group in south-east Skye occurring within the Kishorn nappe may be in part the lateral equivalent of the Torridon Group.

Moinian The Moine Succession of dominantly arenaceous and argillaceous meta-sediments reaches a considerable thickness which IS difficult to estimate due to structural complexity, and occurs above the gently eastward-dipping Moine Thrust on the mainland (Figure 3). These metamorphic rocks, which are not found farther west than the outcrop of the thrust, may possibly be the stratigraphical equivalents of the Torridonian (Soper and Barber, 1979).

Cambro-Ordovician Up to 1600 m of Cambro-Ordovician rocks lie on the low angle unconformity above the Torridonian

______ ~~~----------------------~OOTO~W~----------------------_,~~N

Figure 6 Shallow seismic profiles

6

-g 0.O~"Nc;;sw ____ -----------__ u_n? J " "

-; 0.4 ! 0.5-L--________________________________________ ~

SW Une' NE

loo~e. ti'l 0.1 " .E 0.2 ".",,'" ~ pt ,~ ,. 0.3 j ~ 0.4 l O.5-L----------------------------_______________________ ~

-g W line 10 E ~ W

0.0 " 0 Sea T g ~ 0.1 L I!

.£ 0.2 .r: .. .. g 0.3 .g >- ,.

1 0.4 ~ 0.5 0 ,!

7

Une11 E

Intrusion

Qu Quaternary

Jurassic

pt Perm"" Triasslc

pt·j Permo-Triassic·Jurassic

T T otridonian base_nt

L Lawislan basement

F Fault

Horizontal scale

km

(Swett, 1969), but the post-Caledonian tilt to the ESE of 50 _200 makes it unlikely that they are now preserved beneath the Minches. Much of the underlying Torridonian was probably also eroded from the Minches during the same period.

Permo-Triassic The red beds at Stornoway, first described as of 'Primary' age by Macculloch (1819, vo!. 1, p. 195; vo!. 3, 'General map') were subsequently assigned to the Torridon Sandstone or Old Red Sandstone (Morris on , 1888; Steavenson, 1928; Jehu and Craig, 1934; Kursten, 1957). Modern sedimentological techniques (Steel, 1971; 1974a; 1974b; Steel and Wilson, 1975) were used recently to support the suggestion originally put forward by Stevens (1914; see also Peach and Horne, 1930, pp. 84-85) that the beds are more probably of New ~ed Sandstone age. ;Palaeomagnetic pole vectors (Storetvedt and Steel, 1977) also suggest a Triassic age, and do not support the older dates previously put forward. Furthermore, the discovery of mixed polarity appears to rule out deposition during the Kiaman interval of reversed geomagnetic polarity, which lasted from the Upper Carboniferous (Stephanian) until about the end of the Permian (about 300-230Ma), with the exception of a short period of normal polarity at about the Carbon-iferous-Permian boundary (McElhinny, 1973).

The Stornoway Formation consists mainly of conglomerates deposited in alluvial fans, and has an apparent stratigraphic thickness of about 4 km. Modelling of the Bouguer anomaly suggests, however, that only about 1 km is preserved in a westward-tilted wedge down thrown against the Lewisian (R. McQuillin, personal communication). The discrepancy between these two thickness \ estimates can be accounted for by the strong I diachronism of the sequence (see also Steel and , Wilson, 1975, fig. 5), as the locus of the fault-generated sedimentation shifted westwards with time. The Stornoway Formation displays the best evidence in the Minches of syn-depositional fault-ing in the Permo-Triassic, a process which is believed to control the infill of major basins both in the Minches and in other areas off western Scotland (Evans and others, 1981).

Other Triassic outcrops comprise minor outliers on the mainland (Richey, 1961), and minor pockets and patches in Skye and Raasay (Steel and others, 1975), where they underlie Jurassic rocks, and are preserved as infills of hollows in the Triassic topography.

Jurassic J urassic rocks are extensively exposed in Skye and Raasay, where the cumulative thickness is over 900 m. The Lower Jurassic (Lias) is up to 400 m thick and consists of a dominantly marine fades of shales and calcarenites deposited in a shallow sea bordering a low-lying landmass in roughly the present position of the Scottish mainland (Hallam, 1965; MacCallum, 1971).

8

The overlying Bearreraig Sandstone of the Middle Jurassic (Bajodan) (Morton and Hudson, 1964) comprises massive crossbedded marine sandstones with clays and shales. It shows great variation in thickness and lithology in the south of Skye and Raasay, near to a contemporary shore-line (Morton, 1965), whereas in northern Skye, farther from the source of sediments, more uniform conditions of deposition prevailed.

The Great Estuarine Series, also of Middle Jurassic (Bathonian) age, overlies the Bearreraig Sandstone, and is a mainly non-marine sequence of sandstones, shale and limestones, attaining a maxi-mum thickness of about 200 m in the north Skye (Hudson, 1962). Faunal and petrological studies (Hudson, 1963; 1964) indicate an environment of deltas built into brackish-water lagoons, in what was probably an early Minch basin, since there appears to have been a small landmass to the west, in the area of the Outer Hebrides, as well as a larger one to the east. As Hudson (1964. p. 524) stated, the Middle Jurassic palaeogeography 'looks remarkably like present-day Scotland without the Tertiary volcanics'.

Upper Jurassic rocks, mainly in the form of blue-grey clays and shales of Oxfordian to Kimmeridgian age, crop out on land mainly in northern Skye, and are up to 100 m thick. These rocks present a return to fully marine conditions. Other occurrences of J urassic rocks are present at Applecross and Gruinard Bay, where Lower Lias sediments unconformably overlie the Triassic, and on the Shiant Islands where Upper Lias strata crop out.

Cretaceous The widespread absence of Upper J urassic and complete absence of Lower Cretaceous throughout the Hebrides is due to their non-deposition or erosion prior to the Cenomaman transgression (Upper Cretaceous), which laid down greensands now found as small outliers throughout the Hebrides, including Raasay, Scalpay and central and southern Skye. Additionally in Skye, a 5 m-thick limestone is considered to be altered chalk (Richey, 1961, p. 39).

Tertz"ary Hebridean Palaeogene igneous actIVIty is well represented in Skye, beginning with basal tuffs and followed by effusion of flood basalts over a mature, base-levelled landscape. On north Skye, five separate groups of lavas are recognised, preserved by later faulting. Anderson and Dunham (1966) argued that the 1300 m thus preserved may approach the maximum thickness of the pile, and it is also clear that the two lowest groups formerly extended much farther than the present area of north Skye. Towards the end of the period of basalt eruption the basic Cuillin centre was intruded. The age of these events is believed to be mid-Palaeocene, about 59 Ma ago (MacIntyre and others, 1975; Brown and Mussett, 1976). The

basalts and Cuillin mass are reversely magnetised, in good agreement with the reversed geomagnetic polarity interval preceding anomaly 26, and which lasted from 61-58Ma (Hailwood and others, 1979).

The emplacement of the Cuillin centre and cessation of basalt eruption were followed by intrusion of a basic sill complex into the Jurassic sediments in northern Skye. The aggregate vertical thickness of the leaves of the complex is about 250 m, and the individual sills frequently transgress the sediment bedding-planes, so as to maintain a roughly constant depth below the base of the lavas. A period of injection of basic dykes, trending NW-SE followed, and last came the intrusion of the Western and Eastern Red Hills granitic centres, perhaps as late as the early Eocene, about 52Ma ago (Brown and Mussett, 1976).

No Tertiary sediments, other than thin inter-basaltic horizons, are recognised onshore.

OFFSHORE GEOLOGY The Minches are underlain by two separate post-Carboniferous sedimentary basins about 2-2Y2 km deep occupying the North Minch and Little Minch respectively (Figure 3). These are separated by the Shiant East Bank structural high. The basins form half grabens bounded to the west by the Minch Fault.

North Minch The line-drawing interpretations of representative seismic reflection lines in the North Minch (Figure 5, lines 1-3; Figure 6, lines 1-3) show that the basin is asymmetric. It is faulted along its western margin only, and plunges gently NNE towards the basin outlined by gravity low 'C' of Bott and Watts (1970) west of the Orkneys.

South of about 58° 20'N the base Jurassic reflector marks the bottom of an acoustically transparent sequence up to 0.7 seconds of two-way travel time (TWT) thick. Below is a sequence of strong reflectors with no well-defined base, but possibly more than 0.5 s TWT thick. These seismic characters are well illustrated by the right-hand section reproduced in McQuillin and others (1979, fig. 7/22). The two acoustic sequences are typical of shales overlying sandstones, and this interpreta-tion is corroborated by the lithologies of the bore-hole sample sites in the area. The dates assigned to the samples suggest that the base of the Jurassic coincides with the base of the acoustically trans-parent sequence, on the assumptions that i), that there is little or no marine Rhaetian; and ii), that the red sandstones are Triassic or older (perhaps including rocks of Rhaetian age).

If the identification of the base Jurassic horizon is approximately correct, there must be a great thickness of Lower Lias in the southern part of the North Minch Basin. Borehole 76/55 encountered shales and mudstones of Lower Lias

9

(Upper Hettangian to Lower Sinemurian) age, on line 2 (Figure 5), about 0.7 s TWT above the base of the Jurassic. This implies that around 1 km of Lower Lias is preserved here, a figure comparable with the 900 m drilled in the Mochras borehole (Woodland,1971).

Between 58° 20'N and 58° 30'N there is a gentle ridge trending approximately E-W, mappable on deep seismic profiles (Figure 5, line 1), sparker traverses (Figure 6, line 1) and discernible from the Bouguer anomaly map (Figure 7). On this ridge Borehole 72/33 cored black mud-stones dated as Pleinsbachian-Toarcian (Upper Lias), about 0.5 s TWT above the base Jurassic reflector. The total thickness of Lias here is thus of the order of 700 m.

To the north of the ridge, the character of the Lower Jurassic changes from acoustically transparent to a series of good reflectors, suggest-ing a lithological change from shales and mud-stones to a more arenaceous or calcareous sequence. The top of this Jurassic 'well-bedded' acoustic sequence can be mapped as an approximate base mid-Jurassic horizon, by correlation with its out-crop near Borehole 72/33 (Figure 3).

The strongly reflective sequence below the Jurassic is interpreted as Permo-Triassic. Its base, generally about 0.5 s TWT below the base Jurassic horizon (Figure 5), is poorly defined on the deep seismic reflection sections and has been extrapolat-ed downwards from outcrops determined by sparker sections (Figure 6) calibrated by boreholes. It is distinguished from the Torridonian by the generally rugged topography of outcrops formed by the latter and by the absence of reflectors on sparker sections (Figure 6). In the centre of the basin, infrequent, steeply dipping poor reflectors on deep seismic records can be seen below the flat-lying reflector picked as the base of the Permo-Triassic; these suggest an angular unconformity with the underlying Torridonian. The isolated coastal outliers of Permo-Triassic on the mainland show that the unconformity is highly irregular, and that the overlying beds comprise locally-derived material. Even in the more distal environment of the basin centre such an unconformity is unlikely to make a good reflector, so the reflector picked as the base may well be somewhat above the actual base of the Permo-Triassic, perhaps by 0.1-0.2 s TWf.

Shiant East Bank Ridge The existence of an apparently continuous well-defined basement ridge, trending NW-SE, cropping out on the seabed and connecting Rubha Reidh on the mainland to Kebock Head on Lewis, as shown on earlier published maps (Eden and others, 1971; IGS, 1979), does not agree with the current re-interpretation. The ridge is not continu-ous at surface to Kebock Head, as shown by the well bedded, westerly-dipping acoustic sequence at the western ends of sparker lines 4 and 5 (Figure 6)

which are now interpreted as J urassic and correlat-ed with the smooth bathymetry east of Kebock Head. This does not preclude the existence of a basement ridge at depth.

The Shiant East Bank Ridge consists of an inlier of Precambrian rocks that form irregular out-crops on the seabed making the gentle shoal of Shiant East Bank. The shape and trend of this structural high are poorly defined, but it appears to be roughly dome-shaped, with younger sedi-ments encircling the inlier and dipping radially out-wards away from it. The inlier is mainly Torridonian and the two boreholes (71/13 and 78/3) which provide the main evidence for this can be tentatively ascribed to the Stoer and Torridon Groups on their lithology (G. Blackbourn, personal communication). However, the discovery of supposedly tOn situ acid gneiss at the southern margin of the bank during Vickers Pisces sub-mersible dive P71/7B (Eden and others, 1973) remains a problem (Figure 3). It is possible that the Loch Maree Fault runs through the basement high on a NW-SE trend, bounding an area of relatively shallow Lewisian to the south-west, as suggested by the NW-SE trending gravity high of around 30 mGal (Figure 7).

Inner Sound and Sound of Raasay The Lewisian of north Raasay and Rona is fault-bounded to the east by the offshore continuation of the Raasay Fault northwards up the Inner Sound. Lewisian crops out on the seabed as an inlier over an area 5 to 10 km north of Rona (Figure 6, line 10) and, on the evidence of the 35 mGal gravity high north-east of Trotternish, probably forms a buried ridge trending north-west from there. The westerly-dipping post-Torridonian sediments below the Inner Sound cannot be divided into discrete Permo-Triassic and Jurassic sequences. The horizontal orange and yellow band-ing shown on the solid geology map (Figure 3) indicates schematically that the lowest part of the succession here is expected to be Permo-Triassic. The outcrop of Permo-Triassic to Jurassic between Broadford Bay and Loch Kishorn has been delineated using bat1}ymetry, so its extent north-eastwards into Loch Kishorn is speculative.

Between Trotternish and Raasay, the apparent-ly unfaulted westerly-dipping sequence seen in the Sound of Raasay can be interpreted as lower to middle Jurassic by extrapolation from the eastern coastline of north Skye. Jurassic rocks here have overstepped the fault-bounded Permo-Triassic of northern Skye, to rest on Torridonian (Smythe and others, 1972), and north of Rona the Jurassic rests directly on Lewisian. West of the 6°W meridian the Jurassic sequence is heavily intruded by Tertiary sills.

Little Minch The sedimentary basin below the Little Minch and northern Skye is the northerly extension of the Sea

10

of the Hebrides Trough (McQuillin and Binns, 1973; 1975; Binns and others, 1974). At outcrop on and around northern Skye the J urassic rocks form a saucer shaped depression centred on Loch Snizort, . and largely covered by early Tertiary basalts. The sediments are heavily intruded by sills, probably the leaves of one major sill complex, which can readily be mapped offshore using sparker and shipborne magnetometer traverses in combination with the bathymetry, since they form prominent features on the seabed, often with a pronounced dip and scarp slope. The intrusions are probably continuous at shallow depth over the whole area, resulting in very poor quality multichannel seismic reflection sections (such as, Figure 5, line 4). South of about 57° 30 ' N the sills are less frequent, and an attempt can be made to map the base Jurassic and base Permo-Triassic, employing similar criteria to those used for mapping the North Minch. A tentative two-way time map for the lower horizon has been published (McQuillin and Binns, 1975, figure 5), and parts of deep seismic line 5 (Figure 5) have been reproduced previously (Smythe and others, 1972, figure 2; McQuillin and Bacon, 1974, figure 2) to show reflection quality.

The Permo-Triassic infill below northern Skye is up to 1.5 km thick, and is probably bounded by a fault or faults, trending NW-SE, below Trotternish (Smythe and others, 1972; Hall and Smythe, 1973. figure 1). Thus at Staffin Bay, Jurassic rests directly on Torridonian. West of north-west Skye, in the area of 57° 30/N, 7°W, a structural high, trending NNE-SSW, may bring pre-Jurassic rocks to outcrop at the seabed (Figure 6, lines 7 and 8). North-west of this ill-defmed anticline the sediments dip consistently WNW towards the Minch Fault. Recent drilling, (Bore-hole 80/14, 57° 42.9/N, 6° 54.6/W) undertaken since the completion of the geology map (Figure 3), has shown that these sediments consist of clays and lignites of probable Oligocene age, and may equate to those of the Canna Basin (Smythe and Kenolty, 1975).

Pisces dive P71/6A in the same area of WNW-dipping sediments recovered a sample of mudstone with miospores assigned to the Upper Carboniferous (Eden and others, 1973, pp. 19-21) (figure 3). This fauna is probably derived, as are the Carbon-iferous miospores found in sample SH 207 (Binns and others, 1974, pp. 19, 36) of Zechstein age, obtained at a similar location near to the Minch Fault east of Benbecula. These Carboniferous miospores, together with the erratics of Carbon-iferous limestone found on the Outer Hebrides Uehu and Craig, 1925, p. 639), imply that Carboniferous rocks may still be preserved below the Little Minch; however, no in situ Carbonifer-ous sediments have yet been proved along the western seaboard of Scotland between the Rona Ridge, west of the Shetlands (Ridd, 1980), and Inninmore Bay, Morvern (Richey, 1961).

The Mz'nch Fault This major normal fault system forms the western margin of the Permian to Tertiary sedimentary basins of the Sea of the Hebrides and Minches. A most remarkable feature of the fault complex is its parallelism and close proximity to the Outer Isles Thrust, which dips eastwards at around 30°. The thrust is believed to have been active during both the Proterozoic and the early Palaeozoic (Mendum, 1979). It has clearly acted as a locus for the late Palaeozoic and younger normal movements on the Minch Fault, which presumably intersects the thrust a few kilometres below sea-level.

The Minch Fault is a single fracture, clearly defined bathymetrically and magnetically from Barra Head (at the southern tip of the Outer Hebrides) to Kebock Head (Figure 2) as a result of the physical contrast between Lewisian gneiss and J urassic mudstones. The fault is difficult to identify only where Tertiary sills intrude the J urassic adjacent to the fault or, locally, the Lewisian to the west (Mackinnon, 1974). From Kebock Head, the fault runs northwards to Stornoway, where the down throw of the Stornoway Formation is taken up along two faults trending NE-SW. South of the Eye Peninsula, Borehole 78/4 cored Jurassic black shales with ammonite fragments. The structural relations between this J urassic outcrop and the Triassic Stornoway Formation are not entirely clear, but as is shown schematic ally on Figure 3, the two main segments of the Minch Fault described above may be linked at depth (or at outcrop) across the mouth of Stornoway Bay. The Jurassic proved by Borehole 7S/4 just north of the inferred fault dips south to south-east.

The main branch of the Minch Fault continues en echelon NNE from the eastern coastline of the Eye peninsula, but its character in this area is very different. Instead of the consistent dip of the downthrown sediments westwards towards the fault, the beds on the eastern, downthrown side dip basinwards (Figure 5, line 2; Figure 6, lines 1, 2). It is noteworthy that this section of the fault has no close link with the Outer Isles thrust, unlike the section farther south. The fault in this region has been mapped from the change in structural patterns on sparker sections such as those of Figure 6, lines 1 and 2, and by fault diffraction patterns on multichannel seismic data (not shown here). These observations are corroborated by the ages of the sediments on either side of the fault. To the north of the area shown in Figure 3 (Borehole 77/S, 5So 50.3'N, 6° 6.3'W), to the west of the fault, cored Lower Jurassic, which when related to seismic sections, constrains the outcrop west of the Minch Fault to be Lower Jurassic or older. By constrast, the out-crop east of the Fault in the northern North Minch is no older than Middle Jurassic, and may even include Cretaceous sediments.

11

Tertiary Intrusive Igneous Rocks The high density of dolerite sills in the Little Minch has been mentioned above. The basement rocks of the Shiant East Bank may have formed a northward limit to the intrusion of the sills from the Skye centre.

The aeromagnetic map (Figure 8) shows a number of linear negative anomalies, trending NNW-SSE, of which the most prominent is that running from Loch Ewe across the North Minch. This anomaly is attributed to the Loch Ewe dyke which modelling suggests is about 500 m wide with its top at a depth of around 1 km. Two seismic sections (reproduced in McQuillin and others, 1979, fig. 7/22) show diffraction patterns and minor faulting due to this major dyke. It appears to terminate in the south at the Loch Maree Fault, and can be traced northwards to the top of the continental slope at 59° 25'N, 6° 30'W on Sheet 1 of the aeromagnetic map of Great Britain.

A swarm of large, reversely magnetised dykes can also be inferred from the aeromagnetic map. These run through central Lewis at depth, and into the Little Minch in the area of the Shiant Islands. This swarm passes through the Outer Roag Tertiary igneous complex postulated by Bullerwell (1972) from inspection of the same map.

QUATERNARY GEOLOGY

GLACIAL AND POST GLACIAL SEDIMENTS

Introduction Quaternary sediments occur mainly in the North Minch, the Inner Sound and the Sound of Raasay, and the Little Minch. These areas are separated by regions with considerable rock exposure. In places, the Quaternary is as thick as 150 m (Figure 9). This thickness has been calculated from shallow seismic profiles using an assumed propagation velocity of sound through drift of 1.S km/so On most seismic records, the bedrock surface can be distinguished as a major reflector at a level of stratigraphic discordance at the base of the Quaternary. The depth to bedrock is greatest in tlae glacially over-deepened troughs, such as the Inner Sound. Figure 10 shows contours on the base of the Quaternary in metres below OD.

Study of sparker profiles gives some indication of the character of the Quaternary sediments. Different sediment types tend to display character-istic seismic textures on the sparker records, a property which Binns and others (1974) used in an attempt to describe sediment types in the Sea of the Hebrides. Boreholes which cored Quaternary sediments have been used to correlate lithology with geophysical characteristics. Unfortunately only four boreholes in the Minches provide

7'

N

Scale t L-____ ~ ______ ~ ______ ~ ____ ~ ____ ~~km • I.G.S. Borehole 58" 30'

\ S

\

8a

8b

~Q,.

SHIANT ISLES

T rotternish

-

Figure 11 Location of quaternary seismic profiles

s"W

la 72/32 ..... 3b

Tolsts Head

5a V 4b 4aA 5b

76/56

~6b

118

~

Rubha Reidh

1~ L

~ ~

~ 14b

1 Inner Cl') Sound

RAASAY V D

~LP~Y

12

la

X2/33 • 2b 2a

1b

~ Rhu COlgach

"'9b 5S'N

+Gairloch

WESTER ROSS

57' 30'

.....

\.>:)

lin

e l-N

ort

h M

inch

1

a

100m

s

.~

200

ms

Ho

rizo

nta

l re

flect

ors

mer

ge

gra

du

allv

in

to ju

mb

led

and

h

um

mo

cky

area

I.G

.S.

BO

RE

HO

LE

72

/33

1

km t

v N

E

\ ~\

-----~ ... ~~

'r~-?

----~~> -..

...7

Up

pe

r la

ver

of

stro

ng

and

co

ntin

uo

us

ho

rizo

nta

l re

flect

ors

-S

ea

flo

or

I

Str

on

g a

nd

co

ntin

uo

us

hori

zont

al

refl

ecto

rs,

slig

htly

dip

ping

an

d

risin

g, a

nd w

fth

min

or

hu

mm

ock

s 7

~

Ro

ckb

ea

d

Geo

logi

cal

stru

ctur

es b

eio

w r

ockh

ead

~

Jum

bfe

d a

nd h

um

mo

cky

refle

cto

rs

Lin

e 2

-No

rth

Min

ch

21i1 .

. 100

rns

Up

pe

r la

ver'

of

:~~~

t~~:

izon

tal

r::~O~

f,~~~e

rC:~;e

~t M

idd

le la

yer

of

jum

ble

d

an

d h

um

mo

cky

refl

ect

ors

Lin

e 4

-No

rth

Min

ch

~ ..

Sea

flo

or

- 4b

2b

Jum

ble

d a

nd h

um

mo

cky

refl

ecto

rs t

hrou

ghou

t

Lin

e 3

-No

rth

Min

ch

3a

I.G.S

. B

OR

EH

OLE

7

2/3

2

Jum

ble

d a

nd

hu

mm

ock

y re

flect

ors

/ ---

-----

Ro

okb

ea

d

200

rns

lin

e 5

-No

rth

Min

ch

5a

lG.5

. B~1%~OLE

-2"!

/100

ms

S8.~\or

J SUb

'ho

rrnt

al h

umm

ocky

'ef

l.ct

o",

100

ms

Sea~loor

I Sub·

hor

izon

tal

hum

moc

ky r

efle

ctor

s

~ ~ -=

----

.:;_:

::::::

-£

OO

ms

100m

s

---~

--.

200

ms

'\ Ro

ckh

ea

d

Lin

e 6

-No

rth

Min

ch

Ro

ckh

ea

d

Up

pe

r la

yer

is .... a

cou

stic

ally

tra

nsp

are

nt

apar

t fr

om

a f

ew

wea

k, h

ori

zon

tal

bu

t co

ntin

uo

us

refle

ctor

s

Fig

ure

12

Lin

e dr

awin

gs o

f Q

uat

ern

ary

sei

smic

pro

file

s

\ I

-r --.~

1 2

00

ms-

--:

::::

:-~

.. -

=

Ro

ckh

ea

d

Sea

flo

or

g~;:~iO~k~

lb

1illr

"ms

20

0..

,.

3b

100

ms

--::

20

0m

s

5b

lOO

ms

200--;;

;.1

.- ..po.

lin

e 8

-No

rth

Min

ch

8a

120

rn.

lOO

ms

Lin

e 7

-No

rth

Min

ch

7a~

Jum

ble

d a

nd

hu

mm

ock

y re

flect

ors

Lin

e 9

-No

rth

Min

ch

9a

hoo m

s --.

.. " ....

.. -~-

'Sea

flo

or

\

Lay

ers

abov

e an

d be

low

maj

or r

efle

ctor

ar

e la

rgel

y st

ruct

urel

ess,

alm

ost

acou

stic

ally

!fi=

~~u~

U~:r

;i;~

n~r~

!lt:

t~'

Jum

ble

d a

nd

hu

mm

ock

y re

flect

ors

I

~-:::--

-~.

~~ ~-

~ -~

~

r--.

.

20

0m

s

7b

12

0m

s 8

b

Str

ong

refl

ecto

r

160m

s

lOO

ms

--:;

::

240

Jum

ble

d a

nd

hu"

mm

ocky

re

flect

ors

2

80

ms

9b

lOO

ms

-'"'~

:::---..

..

r--.

_~---..

1"

7"/2

"00"

" R

..lh

&ad

!

Dip

ping

str

ata

be

low

roc

khea

d

Un

e 1

0-N

ort

h M

inch

, R

ub

ha

Re

idh

to

Lo

ch E

we

10a ms

Ro

ckh

&a

d

Str

ong

honz

onim

ref

lect

ors

(Roc

kh&

&d ~ i

n m

ulti

ple)

Up

pe

r 1a

yer o

f st

ron

g

ho

rizo

nta

l re

flec1

0rs

'""'

\

, ........

... --.

;Om

s

Lo

we

r la

yer

of

jum

bl&

d an

d hu

mm

ocky

ref

lect

ors

u.

lin

e 1

1-l

nn

er

So

un

d a

nd

So

un

d o

f R

aasa

y,

Ru

bh

a R

eid

h t

o G

airl

och

11

a160

ms

Ve

ry s

tro

ng

re

fle

cto

r

ms

'40

ms

28

0m

s

Laye

rs a

bove

and

be

low

ma

ior

refle

cto

r at

e al

mos

t st

ruct

urel

ess.

ap

art

from

WM

k an

d di

scon

tinu

ous horizon~ r

efle

ctor

s

160

Sea

flo

or

40

m~

:::::.::

:::;:::

280

Ro

clih

aa

d

160m

o \6

0ms1

1b

\ S

ea

flo

or

---\

1200

ms

'''''-

--2

80

ms

lin

e 1

2-G

air

loch

to

Ro

na

l~a

160 ms~ilirUc,w,;~;,;p.;TaY~----~

20

0m

s Fa

irly

con

tinu

ous

hori

zont

a.l

refl

ecto

rs

I~'

24

0m

s

280~~-~~

/--

~.~ st

ron

g r

efl

ect

or

/

\ -

-....:..-.~~~

~~ -r

Up

pe

r an

d lo

we

r la

yers

hav

e .,

,---

-.,-

.. -.

.. -h

ori

zon

tal

refle

ctor

s

l'l

~

20

0m

s

~

240

...

Ro

ckh

ea

d

28

0 m

\60m

s

200m

s S

ea f

loo

r

L 2

40

""

28

0m

s

-===

=----

0;;;;

;;;;..

____

_ ~ ~

____

_ 2

0

Str

on

g in

tern

al ref

l~tQt'

s in

tow

er

laye

r R

ock

h$

8d

Ju

mb

ied

and

hu

mm

ock

y re

flect

ors

lin

e 1

3-S

ou

nd

of

Raa

say

13a'

60m

s

200"

", ..

~

L R

ockh

....

d

Ver

y st

,o~,

elle

ctor

S

ea f

loo

r

/ --

: 16

0ms

~~~ ~~

~:~Ies~I~~~~::k

240

ms;

an

d d

isco

ntin

uo

us

hori

zont

al r

efle

cto

rs

Larg

ely

stru

ctur

elss

s, b

ut

wit

h w

ea

k an

d d

isco

ntin

uo

us

ho

rizo

nta

l re

flect

ors

lOO

ms

t lo

wer

~fayer'structlireless-:apaii-ff-Om

dis

con

tinu

ou

s, h

ori

zon

tal

:tec.t

ors

16

0m

s

Sea

flo

or

~3b

:=::

;;;:

;;;;

;::;

;;;;

~=~~

~~~

:,

/ "" ""

--

-/'

240

msl

1-----

>-....

....---

------

-=--..

...-....

.....--.

........

.-. ~"'\c:::::?

,240

ms

Ro

ckh

ead

240

28

0 m

s

32

0

-0'1

Lin

e 1

4-l

nn

er

So

un

d P

artia

lly e

xpos

ed r

ock

sill

of

Inn

er

So

un

d

14

a 1

60 m

.

200m

s

:40m

s

2~~~

32

0m

s

,Om

s

Lin

e 1

5-L

ittl

e M

inch

Jum

ble

d a

nd h

um

mo

cky

refle

cto

rs

24() m.~~

lin

e 1

6-l

ittl

e M

inch

,Lo

ch

Sn

izo

rt

68

40

ms

ms

_2' _

_

"..

..--

"''''

1

60

ms

Jum

ble

d a

nd h

um

mo

cky

refle

ctor

s,

mer

ging

bel

ow w

ith

the

rock

.hea

d.

20

0

whi

ch is

oft

en i

ndis

tinc

t

lin

e 1

7-L

ittl

e M

inch

. Lo

ch D

un

veg

an

1

78

16

0m

s --

---

Th

in ~irnoAnt

rnV

4'!

r

24()

1'M

280m

s

320

ms

/'

Sea f

loo

r

Om

s15

b

12

0m

ms

Th

ick

succ

essi

on o

f la

rge

ly

stru

ctur

eles

s se

dim

en

ts w

ith

nu

me

rou

s d

isco

ntin

uo

us

ho

rizo

ota

l re

flect

orS

(Ap

pro

xim

ate

ly 2

00

m o

f se

dim

ent

he

re)

20

0m

s

~ms

i 280

ms

Exp

osed

ro

ck s

ill o

f Lo

ch S

niz

ort

----

Sea

flo

or

Sea

flo

or

Ro

ckh

ea

d

~

----

---

Co

nti

nu

ou

s h

ori

zon

tal

refle

cto

rs

0'"

:::=

:--:

indi

stin

ct

Jum

ble

d a

nd h

um

mo

cky

refle

cto

rs in

lo

we

r la

yer

10

0 m

s

Roc

khea

d m

ore

cle

arl

y di

scer

nabl

e

~~~~

rn~S

V::Z

i~~r

refle

cto

rs

lOO

ms

200m

s

24()

ms

280

ms!

36

0m

s

17

b

32

0m

s

Quaternary sequences sufficiently thick for calibra-tion of seismic profiles so that interpretation of seismic records over a wide area is necessarily tentative. Selected seismic profiles across the Minches are shown in Figures 11 and 12.

In the Sea of the Hebrides, Binns and others (1974) recognised four formations within the Quaternary, each possessing its own seismic 'texture'. At the base, Formation 1 was observed to rest directly upon bedrock. It was identified in boreholes as till, with a characteristic texture of jumbled and hummocky reflectors. Above it, Formation 2 consisted of firm, poorly sorted sandy muds with scattered pebbles, recognisable by a generally structureless seismic texture with a few widely spaced horizontal reflectors. This formation is best developed along the courses of major ice flows, and although micropalaeontological assemblages show it to be of marine origin, it appears to have been deposited in close association with ice. Formation 3 was recognised on the seismic records by closely spaced horizontal reflectors which fade laterally and cannot be correlated with any lithological feature in the boreholes. Samples show it to consist of poorly sorted sandy muds, somewhat similar to Formation 2, but with fewer pebbles. Micropalaeontological evidence and a radio-carbon dating of 8680 ± 250 years BP for Formation 3 deposits suggest it to be of late to post glacial age and of periglacial origin, perhaps laid down in part a short distance from a retreating ice front. Formation 4 consists of a thin layer of modem sediment, a few metres or less in thickness.

North Minch This is by far the largest area of Quaternary sedi-ments, extending northwards from the margins of the Shiant East Bank. The maximum thickness of sediment is present in two overdeepened troughs which run to the east and west of the Shiant East Bank, possibly along the paths of pre-glacial drain-age courses. North of Greenstone Point (Figure 2), the bathymetric map shows a ridge, trending N-S, which has no expression in the rockhead contours. It appears to be a medial moraine deposited between coalescing ice flows emerging from the Inner Sound and the Loch Broom-Gruinard Bay area.

Seismic line 1 (Figure 12) in the North Minch passes within 1 km of Borehole 72/33 which penetrated 37 m of Quaternary sediment before reaching rockhead. The nearest point on the line to the borehole indicates 32 m of sediment. The upper 11 m of the seismic section shows strong and continuous horizontal reflectors of Formation 3 type, while below, a mass of jumbled and hummocky reflectors can be tentatively identified as Formation 1. The borehole log and micro-palaeontology report can be summarised as follows:

17

Lithology Depth Thick- Micro-below ness palaeontology seabed (m) (m)

Fine grey sand 0 25 Present day boreal with shell frag- fauna. ments, becoming more muddy with increasing depth

Stiff grey-brown 25 5 Fewer cold water ele-clay with shell ments than below. fragments and Climatic amelioration -small pebbles more temperate forms

Sandy clay with 30 7 Upper part: Increased cobbles near base number of cold water

forms Lower part: Late glacial fauna of mixed cold and warm water forms

Rockhead 37

With allowance made for the distance between the borehole and the seismic line, there is quite a reasonable correlation between the two. The late glacial shelly clays of the lower part of the sequence which are tentatively interpreted as boulder clays agree with the Formation 1 identifi-cation from the geophysical data, while the upper muddy sands appear to be Formation 3 deposits, passing upwards into Recent sands at the sea floor. Line 2 also shows lateral changes in this area with the Formation 1 sequence developing reflectors of Formation 3 type at the base of the sequence towards the south-west.

Borehole 72/32, near Tolsta Head (Figure 11), penetrated more than 27 m of grey-brown clay, probably till, beneath a thin veneer of Recent sand, which correlates well with reflectors of Formation 1 type of seismic line 3. Borehole 76/55 was located at the junction of seismic lines 4 and 5, both of which display a pattern of fine-scale hummocky but sub-horizontal reflectors throughout the whole Quaternary sequence. At this site, the borehole penetrated over 65 m of dark grey, tenacious sandy and pebbly clay above bedrock. Here, the lithology is of Formation 2 type, but the seismic profIle seems to be between Formations 1 and 2 in character. To the south of Borehole 76/55, line 6 shows only a thin layer of deposits of Formation 2-type above thick tills of Formation I-type.

In the southern part of the North Minch, to the west of the Shiant East Bank (lines 7 and 8), thick Formation 2 deposits are present, with only thin underlying patches of Formation 1 deposits.

Line 9 crosses the morainic ridge north of Greenstone Point and shows well developed Formation 1 sediments. Line 10, from the minor sediment body around Rubha Reidh, reveals Formation 1 tills and Formation 3 deposits, the tills being best developed opposite the mouth of Loch Ewe.

Inner Sound and Sound of Raasay The second largest area of Quaternary sediment in the Minches occupies the Inner Sound and the Sound of Raasay. Although glacially eroded to a depth of 410 m below sea level and partly infilled by 90 m of sediment, the Inner Sound has a rock sill at its mouth with a sediment cover of less than 20 m. The neighbouring Sound of Raasay possesses a rock sill with 30 to 40 m of sediment cover, behind which up to 100 m of sediment have accumulated in the trough.

Although not proved lithologically by borehole data, the deep glacial troughs in the Inner Sound and Sound of Raasay are considered to be filled by thick Formation 2 deposits (lines 11, 12, 13, 14), with underlying patches of Formation 1 overlying bedrock. A strong, generally horizontal reflector is developed within Formation 2 sediments in this area, but the significance of this horizon is at present uncertain.

Little Minch Quaternary sediments are, in general, not as thick in the Little Minch as they are in the North Minch the Inner Sound, or the Sound of Raasay. Never-theless an overdeepened channel with thicker sediments exists north of Vaternish Point, representing a major ice path from Loch Snizort. The rock sill of Loch Snizort is only thinly buried and is partly exposed. It separates the thicker Quaternary sediments of the loch from those of the Little Minch where the thinner Quaternary sequence is due to the absence of Formation 2 deposits, even within the deep troughs, as seen on line 15.

Borehole 71/7, at the mouth of Loch Snizort, located at the northern end of seismic line 16 (Figure 11), proved 34 m of dark olive grey boulder clay over rockhead. Line 16 shows jumbled and hummocky reflectors of Formation 1, with only thin overlying horizontal reflectors of Formation 3. Some development of Formation 3 occurs round the coast of Skye at Loch Dunvegan (line 17), but thick Formation 2 only occurs within the rock sill in Loch Snizort.

Correlation The seismic profiles from the northern North Minch are generally more difficult to interpret than are those from farther south, and give rise to problems apparently not encountered by Binns and others in the Sea of the Hebrides. There, it was possible to arrange the four formations in chronological order. Line 1 from the North Minch, however, apparently reveals Formation 1 tills passing laterally to the north-west into Formation 3 sediments. Line 2 also shows lateral changes in this area, this time towards the south-west, with a Formation 1 sequence developing seismic character-istics of Formation 3 reflectors at the base. Although the lithological reality of these 'seismic facies' cannot be confirmed without further bore-hole evidence, it appears that, in the North Minch

18

at least, the four formations cannot be regarded as chronostratigraphic units.

A possible explanation for fades variation within the Quaternary sediments of the North Minch comes from a consideration of sea levels in late-glacial times. Figure 13 shows a proposed late-glacial/early post-glacial shore-line for north-west Scotland, dated at 12 000 years BP, the date of withdrawal of the main ice sheet. The shoreline is based upon a 120-m submergence of the Uists (see Ritchie, 1966; Valentin, 1953), and isostatic rebound occurs within a dome centred on the southern-western Highlands, with a curved hinge-line running through the Little and North Minches, to cross the mainland south of the Durness area. The difficulties of estimating the inter-relation-ships of isostatic and eustatic changes in an area like north-west Scotland were discussed by Bishop (1977), but even this sketched interpreta-tion demonstrates that a 120-m submergence of the Uists, when interpolated around the margin of the dome of rebound into the North Minch, would put its floor (with an average depth today of about 120 m) very close to sea level in late-glacial-early post-glacial times. With such even topography, the location of the coastline in the North Minch is difficult to define; farther south, greater relief makes the palaeogeography easier to determine. The deeply eroded glacial troughs, such as those in the Little Minch and the Inner Sound, formed a large system of salt-water lochs opening southwards into the Sea of the Hebrides. In the deeper parts of the North Minch, seawater may have penetrated around or below a waning ice sheet in late-glacial times. On lines 1 and 2, morainic sediments of Formation 1 are found on higher ground, and pass with deepening water into Formation 3 deposits, laid down contemp-oraneously with the tills in shallow water close to the ice front. The possible existence of till. above Formation 3 deposits on line 2 may be the result of slight fluctuations in the complex balance of land and sea levels in late-glacial times causing relatively large regressions or advances of the coastline over the flat North Minch.

To the south, interpretations of the seismic records show greater similarities to the Sea of the Hebrides. Although not proved lithologically by borehole data in the Minches, the deep glacial troughs have thick Formation 2 deposits (lines 7, 8, 11-14) with underlying patches of Formation 1 over rockhead. As deep seawater may have pene-trated under the ice in these troughs during late-glacial times, Binns' conclusion that in the Sea of the Hebrides Formation 2 represents waterlain glacial deposits, is also compatible with the evidence from the Minches. On seismic profiles, Formation 2 deposits can be seen as far north as the Eye Peninsula in line 6, where they form a thin layer above a much thicker lower till. However, geophysical records with characteristics similar to Formation 1 correlate with Formation 2 lithology

7'W

Land

in Borehole 76/55 a short distance farther north. One feature of Formation 2 sediments in the Minches, not mentioned by Binns, is the develop-ment of a strong reflector within this formation. Various relationships between the upper and lower parts of the formation are shown on lines 7, 8, 12 and 13. A major reflector associated with a lithological change is also present on line 2 in the North Minch. This can be traced from the North Minch into the mouth of the Inner Sound, where it disappears (line 14). Without lithological or palaeontological evidence, the significance of this feature is not known.

The relatively thin Quaternary sediments of the Little Minch are explained by the absence of Formation 2 deposits even in the deep troughs (line 15). Constriction of the ice flow in the narrow Little Minch channel may not have favoured deposition, for sediment thicknesses only start to increase to the south, in the broader Sea of the Hebrides (Binns and others, 1974).

19

S'W

~ ........•.... -i&>

5B"N

=

SURFACE SEDIMENTS

Introduction

The surface sediments are those deposits present at the seabed and formed under the Recent sediment-ary environment. The surface sediment throughout most of the Minches consists of a thin layer of material derived from deposits of glacial age, re-sorted to varying degrees under post-glacial and Recent marine conditions, during which only the finer fractions have been subjected to a significant amount of transport. Direct detrital contributions do not appear to be important except very near to the coasts, but quite large areas are covered by local biogenic sediment. Some influx of sand-grade material from outside the Minches is indicated in the coastal areas of the extreme north of the North Minch. The fundamental textural characteristics of the surface sediments are summarised ip Figure 14, where the five sediment categories are simple

Scale

10 30

58" 30'

r

N

'm t Bull 01 lewis

NORTH MINCH

TRAVERSE A

M 282 M296

M 311 M 315

NS 2()2

STORNOWAY BASIN

'it;, SHIANT ISLES

TRAVERSE C M

105

TRAVERSE B M 251

M 146

+ Gairloch

WESTER ROSS

RONA

i 08 J~ i~ Inner I:) Sound

('/)

RAASAY V \)

~LP~

Figure 15 Location of sediment sample traverses

20

Cape Wrath

Lochinver

5S"N

S1" 30'

DEPTH (m)

STORNOWAY BASIN (SOUTH)

M247 M248 M33

SHIANT EAST BANK

M147 NS180 M250 M251 M146 o ,.---r-.---.----~------__r-.__---__,,_-----_,_-------_._-_r Sea-level

40

80

120

140~~~~~~~~~~~~~~~~~~~~~ __ ~~,.~~~~~~~~~~~~~~~~

M 247 M248 M33 M147 NS180 M250 M251 M145

GRAB

o

Rockhead

50

100

150

Gravel

200 i~\V{/l Sand

Mud

250

300

Centimetres

Figure 16 Variation of surface sediments along traverses

21

tv

tv

TO L

ST

A

TIU

MP

AN

H

EA

D

HE

AD

B

RO

AD

BA

Y

DE

PT

H (

m)

M2

73

M2

73

BI';:':;:;;::"

GR

AB

-:~

:.:.::

}: :.::

:: ... ':'.

:

o

200

Cen

tim

etre

s

NS

18

9

NS

18

9

1~g;}

ST

OR

NO

WA

Y B

AS

IN

NS

18

8

M2

22

NS

18

8

M2

22

RI

M2

24

NS

20

2

NO

RT

H M

INC

H

M3

17

M2

24

N

S20

2 M

31

7

~

[iJ

....

.... .

: u

: l1li

G

rave

l

0

0"

<>

0

San

d

Mu

d

M2

82

M2

96

M2

82

M

29

6

~~~)

; ",

-0-'

.0

' O.a.·~

•. "

,oe .

. :--

_··~O

...

M3

15

M3

15

EN

AR

D B

AY

M2

78

M2

78

Sea

-leve

l

t-..:l

I.>

:>

LIT

TLE

MIN

CH

S

KY

E!S

HIA

NT

PLA

TF

OR

M

DE

PT

H (

m) O

+--

-.--

----

----

----

----

---.

----

----

---r

---_

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

40

8

0

M24

5 ,-

--

SH

IAN

T E

AS

T B

AN

K M

92

M96

M

94 M

202

RU

BH

A

LOC

H E

WE

R

EID

H

NS

182

M89

I

M~2 :71

S

ea

-le

vel

120~===Jb~~G[21~±1[ili~iiiGllfSill[[dilit~22E2~2K2221i212IDillt~JJiE[2[t=£i2~::::::IIdtlI2ki3Z2GITE:~~~~~§S2J2211JjSWdSdlli£iilif1S~

18

0 :I

:

M75

M

S1

M10

4 M

105

M23

7 M

245

M96

M

94

M 2

02

M92

M

91

M89

N

S18

2 M

292

[hl

~

[ill]

~

~

m

.......

GR

AB

""

;': .. ;

o:··~

:~ .. ::~

: ' ..

... -

-~-

~':'

i-'

=-.,

. -

.

01 g

i I

~

ij

[I ---

.": .. '

.. ·.·0

0:;

=0

~-'-!-

.~

I ~"".~

50 100

Gra

vel

150

[~t}(/l

Sa

nd

Mu

d

200

250

Cen

tim

etre

s

amalgamations of fifteen types defined by Folk (1965). Comparison of sediment distribution with bathymetry shows that gravels are found in shallower water, muds in the deepest hollows and sheltered lochs, while sands occur at intermediate depths.

Figure 15 shows the location of sediment sample traverses across the Minches, and Figure 16 summarises the variability of the surface sediment layer. Many core samples penetrate the surface layer to reach the sediment below. Where variation occurs, coarser surface sediments generally pass downwards into finer material. The contrast is most dramatic on the shallow banks north of Skye and off the coastal headlands. In deeper water the muddy sands, which cover much of the central North Minch and Little Minch, generally prove to be no more than 0.8-1.0 m thick, and pass gradually downwards into more muddy sediment. The soft muds of the deep hollows differ from most other samples from the Minches. No textural contrasts with depth were encountered, and only occasional slight colour changes were present.

Palaeomagnetic investigation of muddy cores from the Inner Sound (Bishop, 1975) reveal a maximum sedimentation rate within this material of about 0.60 m per 1000 years, with the rate decreasing slightly towards the present. Such a rate of accumulation suggests that the muds may be only 7-10 m or less in thickness. Probably the Recent sediments in the Minches are rarely more than 10 m thick.

Sands and muds related to tills Figure 17 is a classification of surface sediment types according to their mode or origin. A very large area is covered by sand and mud with textural properties similar to those of tills. In Figure 18 the graphic mean grain sizes of grab samples have been plotted against their coarsest grain sizes (the 1% value on the cumulative curve). The resultant C-M diagram has genetic fields which have been taken, with some modifications, from Schlee (1973). The degree of sorting tends to increase towards the C-M line. In Zone A, farthest from the line, Schlee has plotted known tills, the sediment type with most disparity between mean and coarsest sizes. Numerous Minches surface sediment samples fall within this same area. Zones D, E and B are transitional zones, not used by Schlee, between the till-field and the suspension- and traction-fields. All three zones are filled by dense spreads of data points which lead from the till-field into the suspension mud, traction sand and gravel fields, zones G, F and C. All till-like sediments, from zones A, D, E and B, have been mapped together on Figure 17. The spread of data points on Figure 18 displays greater continuity between the till-field and the sands, muds and gravels than exists among these three sediment end members themselves, suggesting that although they are all related to a till-like

24

parent material, they are not closely inter-related to each other. Till-like sediments survive in the Minches to intermediate depths, where they are neither altered by the strong bottom currents of shallower water nor masked by the mud deposition of the deep hollows. Nor are they to be found on the more exposed floors of the northern North Minch or southern Little Minch.

Suspension deposited muds On the C-M diagram, samples from the mud-floored hollows and lochs fall into the suspension deposit field. Visual observations from a manned submersible in a deep hollow in the Little Minch (Eden and others, 1973) suggest that net sedi-mentation is in progress, a dusting of fine sediment in filling and covering shell debris on the sea floor. Indication that the muds are derived from exposed glacial clay or till comes from the study of their mineralogy by X-ray diffraction (Bishop, 1977). Both the glacial clays on the banks and also clay fractions from deeper mud deposits consist of a similarly proportioned mixture of chlorite, illite and calcite. The older sediment displays a slightly greater chlorite content, as is consistent with clays of direct glacial origin, while mud from semi-enclosed bodies like the Inner Sound of Loch Broom, has a little more illite, possibly due to some modern input. Bathymetry alone is not sufficient to explain fully the distribution of suspension-deposited mud, because some mud-floored areas, especially Loch Snizort and the Sound of Raasay, are no deeper than the sand covered floor of the north of the Little Minches. In these cases, mud deposition is partly explained by the shelter of the surrounding land, and partly by the net circulation pattern of these broad and deep mouthed lochs (Craig, 1959; Milne, 1972), with an inshore-flowing bottom current of denser saline water in which mud may be suspended and carried in from offshore sources. Bishop (1975) has shown how much a current could be responsible for falling sedimentation rates south-wards along the Inner Sound. The muds of the hollows and lochs contain minor quantities of fine sand-sized quartz and mica, and foraminifera are abundant.

Lithic sand In the more exposed southern and northern extremities of the Minches, till-like muddy sands pass gradually into cleaner fine-grained sand, possessing textural characteristics that plot into Zone F of the C-M diagram, which has been associated with sediments laid down after transport by bottom traction. However, there is no evidence that the sands have undergone any significant transport. They may represent either a less muddy, more current-agitated derivative of till-like sedi-ments, or are possibly derived from periglaciaI deposits. The textural and mineralogical properties of these dominantly lithic sands do not suggest

58· 30'

[IIIll]]]

• ~ L5 ~ EEJ §

r

Sand and gravel size shell debris of local origin

Dominantly lithic gravel, derived from till or exposed bedrock.

Sands and muds, closely related to tills

Re-sorted lithic sand with relatively little gravel or shell. Uttle transport

Well sorted shell sand influx

Suspension deposited muCls

Scale

N

Figure 17 Genetic classification of surface sediment

6·W

25

/XI/N.r;H..:::: : ., ......... , .... . ................. .................

........... , . ..............

ill!!!!!!!'! .............. Lochinver

WESTER ROSS

57"

-... B ... -9 !

_5+-__________ ~~A

-4 .. .. -3 . . -2 , I:. e •• -1

e ......... ;.:. ----. .---;'"'"' . • 0 0;--------+---••

. . .. 2 '. . . . . 3

G

4 '. '" . e-,. .: •. 5

6

7

8

9

9 8 7 6

. .

5

. .

4

Figure 18 Surface sediment C-M diagram

3

o • -.. .\

2

that sediment movement is taking place, and side-scan sonar records display no evidence of sand transport.

Shell sand Towards the coast of Lewis, the lithic sands of the North Minch gradually give way to a distinctive body of shell sand, consisting of well sorted and very worn shell fragments of medium sand grade. This sand is similar to the shell sands that can be seen on the Atlantic coast of Lewis, and which has accumulated in wide beaches near Tolsta Head and in Broad Bay. In contrast to the biogenic sand found elsewhere in the Minches, there is no evidence that this deposit is locally derived; on the contrary, it bears every indication of having been

26

B

o -1 -2 -3 -4 -5 -6 -7 -8 -9

Graphic mean grain size

transported for some distance. The most likely source of the shell sand is from the shelf west of the Outer Hebrides, which, however, has not yet been mapped in detail. Shell debris generated in this area could be transported along the Lewis coast towards the Butt of Lewis and be drawn into the Minches by strong tidal currents around the headland caused by the unusual tidal gradient. Arrival of high water on the Minches side is 15-20 minutes later than that on the Atlantic coast. A combination of tidal and coastal currents near the shore (Ministry of Defence, 1958; 1973) carries the sand as far south as Broad Bay, where it is impounded. Tidal currents are less strong immediately south of the Eye Peninsula.

Lithic gravels Lithic gravels form the surface sediment on much of the Shiant East Bank and around coastal head-lands; no gravel deposits occur at greater depths. Lithic (as opposed to biogenic) gravel is more abundant around the coasts, and the lithology of the pebbles matches that of the adjacent coasts. Lewisian metamorphic rocks dominate along the Hebridean coasts, Torridonian sandstones and Lewisian along the mainland coast, and Mesozoic sediments and basic igneous rocks occur north of Skye. The Shiant East Bank similarly displays a close relationship between geological outcrop and pebble lithology; Mesozoic, Tertiary, igneous and Torridonian pebbles are present. To a certain extent, details of the distribution of rock types among the pebble gravels of the banks reflects probable glacial transport, although this can be more clearly seen in the pebble component of the till-like sediments from greater depths, where lithologies indicate the source areas of the ice sheets which once covered the Minches. On the banks, a thin veneer of pebble gravel often passes downwards into pebbly clay, and not directly onto bedrock, and the surface deposit probably represents the winnowed residue of a once thicker sediment layer. The bottom currents of these shallow areas are strong enough to be responsible for the removal of the finer fractions, much of which, it is thought, has come to rest in the neighbouring mud-floored hollows. The slowing down of the rate of sedimentation of the muds over the last few thousand years, as revealed by the palaeomagnetic properties of the Inner Sound cores, may be partly related to the increasing protection afforded to finer sediment in shallow-er areas by a growing cap of gravel and sand.

Locally derived shell sands and gravels In shallow areas away from the coasts, pebble gravels are replaced by a sediment consisting of sand and gravel-sized biogenic debris, in which the contribution of lithic material often falls below 5% (by volume). This sediment, like the pebble gravel, forms a thin veneer over rockhead or glacial clay and although it is best developed on Mesozoic and Tertiary igneous ground in the Little Minch and southern East Bank, it also occurs on the sea-ward side of the Cape Wrath Bank. A variety of biogenic debris is present, from unbroken shells to comminuted and unrecognisable fragments, with juvenile forms common. The sediment represents the debris of a diverse living community of modem benthonic species on banks which are productive fishing grounds. Bivalve shells make the greatest contribution to the sediment together with gastro-pods, echinoderms, worm tubes and crustaceans. Solitary corals were found at two stations in the Little Minch. For the most part, this sediment is poorly sorted, but immediately south of the Shiant Isles reworking has produced a small area of well-sorted shell sand. The distribution of biogenic

27

debris in the Minches corresponds with the general pattern of carbonate distribution on Inner Shelf areas (Milliman, 1974), elaborate biogenic assem-blages being common near rock exposures swept by strong currents and away from major river mouths.

REFERENCES

Allerton, H. A. 1968. An interpretation of the gravity of the North Minch, Scotland. MSc Thesis, University of Durham [Unpublished] .

Anderson, F. W. and Dunham, K. C. 1966. The geology of Northern Skye. Mem. Geol. Surv., 216 pp.

Baden-PoweU, D. F. W. 1938. On the glacial and inter-glacial marine beds of northern Lewis. Geol. Mag., Vol. 75,395-409.

Binns, P. E., McQuillin, R. and Kenolty, N. 1974. The geology of the Sea of the Hebrides. Rep. Inst. Geol. Sci., No. 73/14,43 pp.

Bishop,P.-1975. Thepalaeomagnetismofmuddy sedi-ment cores from the Inner Sound, North West Scotland, and the glacial and post-glacial history of sedimentation in the area. Earth Planet Sci. Lett., Vol. 27,51-56. 1977. Glacial and post-glacial sedimentation in the Minches, north west Scotland. PhD thesis, Univ. College, University of London. [Unpublished.]

Bott, M. H. P. and Watts, A. B. 1970. Deep sedimentary basins proved in the Shetland-Hebridean continental shelf and margin. Nature, London, Vo!. 225, 265-268.

Bowes, D. R. 1969. The Lewisian of the north-west highlands of Scotland. Pp. 575-594 in Kay, M. (Ed.). North Atlantic - geology and continental drift. (Tulsa, Oklahoma: American Association of Petroleum Geologists.)

Brook, M., PoweD, D. and Brewer, M. S. 1977. Grenville events in Moine rocks of the Northern Highlands,

Geikie,J. 1878. On the glacial phenomena of the Long Island, or Outer Hebrides. Quart. J. Geol. Soc. London, Vol. 29,532-570.

-". George, T. N. 1966. Geomorphic evolution in Hebridean Scotland. Scott.]. Geol., Vol. 2, 1-34.

Godard, A. 1965. Recherches de geomorphologie en Ecosse du Nord-Ouest. (Paris: Societe d'Editions les Belles Lettres.) 701 pp.

Hailwood, E. A., Borck, W., Costa, L .• Dupeuble, P. A., Muller, C. and Schnitker, D. 1979. Chronology and biostratigraphy of northeast Atlantic sediments, DSDP leg 48. In Montadert, L. and others, Initial Rep. Deep Sea Drill. Proj., Vo!. 48, 1119-1141.

Hall, J. and AI-Haddad, F. M. 1976. Seismic velocities in the Lewisian metamorphic complex - 'in situ' measurements. Scott. J. Geol., Vo!. 12,305-314. and Smythe, D. K. 1973. Discussion of the relation of Palaeogene Ridge and basin structures of Britain to the North Atlantic. Earth Planet Sci. Lett., Vo!. 19,54-60.

Hallam, A. 1965. Jurassic, Cretaceous and Tertiary sedi-ments. Pp. 401-419 in Craig, G. Y. {Ed.}. The geology of Scotland. (Edinburgh: Oliver and Boyd.)

Hudson,J. D. 1962. The stratigraphy of the Great Estuarine Series (Middle Jurassic) of the Inner Hebrides. Trans. Edinburgh Geol. Soc., Vo!. 19, 139-165. 1964. The recognition of salinity-controlled mollusc assemblages in the Great Estuarine Series (Middle Jurassic) of the Inner Hebrides. Palaeontology, Vo!. 6, 318-326.

Scotland. J. Geol. Soc. London, Vol. 133, 489-496. -~.--Brown, G. C. and Mussett, A. E. 1976. Evidence for two

1964. The petrology of the sandstones of the Great Estuarine Series, and the Jurassic palaeogeography of Scotland. Proc. Geol. Assoc., Vo!. 75,499-528. discrete centres in Skye. Nature, London, Vol. 261,

218-220. BuUerweU, W. 1972. Geophysical studies relating to the

Tertiary volcanic structure of the British Isles. Phil. Trans. R. Soc. London, Vol. 271, 209-215.

Chesher, J. A., Deegan, C. E., Ardus, D. A., Binns, P. E. and Fannin, N. G. T. 1972. IGS marine drilling with mv Whitethorn in Scottish waters 1970-71. Rep. Inst. Geol. SCI:, No. 72/10,25 pp.

Craig, R. E. 1959. Hydrography of Scottish coastal waters. Mar. Res. Ser. Scott. Home Dept., Vo!. 2.

Dearnley, R. 1962. An outline of the Lewisian complex of the Outer Hebrides in relation to that of the Scottish mainland. Q. J. Geol. Soc. London, Vot 112 143-176. '

Donovan, D. T. 1968. Geology of shelf seas. (Edinburgh: Oliver and Boyd.) 160 pp.

Eden, R. A .• Ardus, D. A., Binns, P. E., McQuillin, R. and Wilson, J. B. 1971. Geological investigations with a manned submersible off the west coast of Scotland 1969-1970. Rep. Inst. Geol. Sci., No. 71/16. 49 pp.

__ - - Deegan, C. E., Rhys, G. H., Wright,J. E. and Dobson,'~~M. R. 1973. Geological investigations with a manned submersible in the Irish Sea and off western Scotland 1971. Rep. Inst. Geol. Sci., No. 73/2. 27 pp. Wright,J. E. and BulIerwell, W. 1971. The solid geology of the East Atlantic continental margin adjacent to the British Isles. Rep. Inst. Geol. Sci., No. 70/14. 115-128.

--- Evans, D., Chesher, J. A., Deegan, C. E. and Fannin, N. G. T. 1981. The offshore geology of Scotland in relation to the IGS shallow drilling programme, 1970-1978. Rep. Inst. Geol. Sci., No. 81/12. Wilkinson, G. C. and Craig, D. C. 1979. The Tertiary sediments of the Canna Basin, Sea of the Hebrides. Scott. J. Geol., Vo!. 15,329-332.

Folk, R. L. 1965. Petrology of sedimentary rocks. (Austin, Texas: Hemphili.) 159 pp. --

28

Institute of Geological Sciences. 1974. IGS boreholes 1973. Rep. Inst. Geol. Sci., No. 74/7. 23 pp. 1977. IGS boreholes 1976. Rep. Inst. Geol. Sci., No. 77/10. 46 pp. 1978. IGS boreholes 1977. Rep. Inst. Geol. Sci., No. 78/21. 24 pp. 1979. IGS boreholes 1978. Rep. Inst. Geol. Sci., No. 79/12. 1979. Sub-Pleistocene geology of the British Isles and the adjacent Continental Shelf. 2nd Edition. Scale 1 :250 000.

Jehu, T. J. and Craig, R. M. 1925. Geology of the Outer Hebrides. Pt 2. South Dist and Eriskay. Trans. R. Soc. Edinburgh, Vol. 57,615-641.

1934. Geology of the Outer Hebrides. Part 5. North Harris and Lewis. Trans. R. Soc. Edinburgh, Vo!. 57,839-874.

Kenolty, N. 1971. Cruise report - geophysical work in Scottish waters on rv Vickers Venturer in 1970. Rep. Mar. Geophys. Unit, Inst. Geol. Sci., No. 16, 4 pp. '

Kursten, M. 1957. The metamorphic and tectonic history of parts of the Outer Hebrides. Trans. Edinburgh Geol. Soc., Vol. 17, 1-31.

MacCaUum, J. A. A. 1971. Sedimentation of the Middle Liassic rocks in the Inner Hebrides of Scotland. PhD Thesis, University of Glasgow. [Unpublished.]

MacCulloch, J. 1819. A description of the western islands of Scotland, including the Isle of Man. 3 vols. (London: Constable.)

McElhinny, M. W. 1973. Palaeomagnetism and plate tectonics. (Cambridge: Cambridge University Press.) 358 pp.

Maclntyre, R. M., McMenamin, T. and Preston,J. 1975. K-Arresults from Western Ireland and their bearing on the timing and siting of Thulean magmatism. Scott. J. Geol., Vo!. 11,227-249.

MacKinder, H.J. 1907. Britain and the British Seas. 2nd edn. (Oxford: Clarendon Press.)

MacKinnon, A. 1974. The Madadh Rocks: a Tertiary olivine dolerite sill in the Outer Hebrides. Scott. I. Geol., VoI. 10,67-70.

McQuillin, R. and Ardus, D. A. 1977. Exploring the geology of shelf seas. (London: Graham and Trotman.) 234 pp. and Bacon, M. 1974. Preliminary report on seismic reflection surveys in sea areas around Scotland, 1969-, 1973. Rep. Inst. Geol. Sci., No. 74/12, 7 pp. - andBarclay,W. 1979. An introduction to seismic interpretation. (London: Graham and Trotman.) 199 pp. and Binns, P. E. 1973. Geological structure in the Sea of the Hebrides. Nature, Phys. Set'., Vol. 241,2-4.

1975. Geological structure in the Minches, the Sea of the Hebrides and in the adjacent northwest -British continental shelf. In Yorath, C. J., Parker, E. R. and Glass, D. J. (Eds.), Canada's continental margins and offshore petroleum exploration. Can. Soc. Pet. GeoL, Mem. 4, 283-293. Eden, R. A. and TuUy, M. C. 1968. Project 68/5. Geophysical and geological investigations in the Sea of the Hebrides and Minches. Rep. Mar. Geophys. Unit, Inst. Geol. Set'., No. 28, 16 pp. and Watson,J. 1973. Large·scale basement structures of the Outer Hebrides in the light of geophysical evidence. Nature, Phys. Sci., VoI. 245, 1-3.

Mendum, J . R. 1979. Caledonian thrusting in NW Scotland. Pp. 291-297 in Harris, A. L., Holland, C. H. and Leake, B. E. (Eds.). The Caledonides of the British Isles - Reviewed. (Edinburgh: Scottish Academic Press.)

Milliman,J. D. 1974. Marine carbonates. (Berlin: Springer.) 375 pp.

Milne, P. H. 1972. Hydrography of Scottish west coast sea lochs. Mar. Res. Ser. Dep. Agric. Fish Scotl., Vol. 3, 50 pp.

Ministry of Defence, Hydrographic Dep. 1958. West coast of Scotland pitut (corrected to 1971). (London: HMSO.) 1973. Tidal stream atlas: west coast of Scotland and north coast of Ireland. (London: HMSO.)

Moorbath, S. 1969. Evidence for the age of deposition of the Torridonian sediments of north·west Scotland. Scott. I. Geol., Vol. 5(2), 154-170.

Morrison, W. 1888. Precambrian conglomerate of Lewis. Trans. Edinburgh Geol. Soc., Vol. 5, 235-242.

Morton, N. 1965. The Bearreraig Sandstone series (Middle Jurassic) of Skye and Raasay. Scott. I. Geol., Vol. 1,189-216. and Hudson,J. D. 1966. The stratigraphical nomen· clature of the Lower and Middle J urassic rocks of the Hebrides. Geol. Mag., Vol. 101,531-534.

Pea