The Geology of Southern Raasay

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The Geology Of Southern Raasay
Christopher Kelly
Durham University, Department of Earth Sciences
January 2017
This dissertation is submitted to Durham University in partial fulfilment of the
requirements for the degree of Geology F600 (BSc)

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1.0 ABSTRACT
The island of Raasay lies east of the Isle of Skye forming part of the Inner Hebrides. This
dissertation records the geology and structure of 16sq km of southern Raasay which was mapped
over a period of 6 weeks between the 25th of June and 2nd of August 2016. In total 9 sedimentary
formations and 3 igneous formations were mapped and are described here. Precambrian, Permo-
Triassic and Jurassic sedimentary deposits, each bounded by paraconformities, were recognised
with the latter dominating the stratigraphic sequence. Paleogene igneous formations include the
Raasay Granite, Little Minch Sill Complex and North Britain Paleogene Dyke suite which were
intruded with the opening of the North Atlantic Little Minch Basin. Raasay displays an extensive
faulting system dominated by normal and dextral strike-slip faulting of Paleogene age with some
quaternary faulting to the east. Quaternary glacial erosion has exposed the current topography. The
economic value of Raasay’s resources are devalued by the additional cost of ferrying material.
Granite and basalt for aggregate, water and peat could all be viably used by the local population.
2.0 ACKNOWLEDGEMENTS
This project could not have been completed with the help and support of many people. I would like
thank Professor Mark Allen for his support and motivation through the span of this dissertation, as
well as Durham University’s Department of Earth Sciences which provided funding for this
mapping dissertation. Most importantly I give thanks to Elizabeth and Darryl Simpson, whose
company and hospitality at the Oystercatcher B&B made every cold and rainy day a joy to come
back from.

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3.0 TABLE OF CONTENTS
1.0 Abstract p.2
2.0 Acknowledgements p.2
3.0 Table of Contents p.3
3.1 List of Figures p.6
3.2 List of Plates p.6
3.3 List of Models p.8
4.0 Methodology p.8
5.0 Introduction p.9
6.0 Stratigraphy p.10
7.0 Sedimentary Lithologies p.10
7.2 Eyre Formation p.12
7.3 Gleann Formation p.14
7.4 Rail Formation p.15
7.5 F.I.M. Formation p.18
7.6 Boulder Rock Formation p.21
7.7 Leabaidh Formation p.23

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7.8 Iarann Formation p.24
7.9 Bhonaid Formation p.27
7.10 Churchton Formation p.28
8.0 Igneous Formations p.31
8.2 Snow Granite Formation p.31
8.3 Battery Formation p.32
8.4 Dyke Formation p.34
9.0 Structure p.35
9.2 Brittle Deformation p.36
9.2.1 Eyre Fault p.38
9.2.2 Beinn Na’ Leac Fault p.38
9.2.3 Other Faults p.38
9.2.4 Fault Magnitudes p.40
9.3 Jointing Within The Snow Granite Formation p.40
9.4 Dyke Formation Intrusion p.40
9.5 Regional Tilt p.41

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(Discuss) 9.6 Interpretations and Discussion p.41
10.0 Economic Potential p.42
10.2 Water p.42
10.3 Peat p.43
10.4 Energy p.43
10.5 Churchton Formation p.43
10.6 Iarann Formation p.43
10.7 Rail Formation p.44
10.8 Snow Granite Formation p.44
10.9 Battery Formation p.45
11.0 Geological History p.45
12.0 Conclusions p.46
13.0 References p.47
13.1 Dissertation References p.47
13.2 Additional Map & Software References p.50
14.0 Appendices p.50

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3.1 List of Figures
Figure.1 – Log of the Eyre Formation p.13
Figure.2 – Log of the Gleann formation p.14
Figure.3 – Sketch of columnar jointing within the Battery formation p.33
Figure.4 – A sketch map of the relationship between the Battery formation, Snow
Granite formation and the Churchton formation (looking south from a birds-eye view) p.36
Figure.5 – Stereonet plot of jointing within the Snow Granite formation p.40
Figure.6 – Stereonet plot of Dyke planes mapped across Raasay p.41
Figure.7 – Stereonet plot of average bedding across Raasay p.41
3.2 List of Plates
Plate.1
Plate.1.1 – Karst weathering exploits cleavages within the Wackestone of the
Rail formation p.16
Plate.1.2 – Convolute bedding and bioturbation visible in the Rail formation p.16
Plate.1.3 – Gryphaea found within the formation were oriented in a
life-mode position p.16
Plate.2
Plate.2.1 – Ammonoid cast fossils p.19

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Plate.2.2 – Deformation of bedding around an iron nodule p.19
Plate.2.3 – (left to right) Crinoid stem, Gastropod, Belemnite fossil p.19
Plate.2.4 – Cut iron nodule around a central particle p.19
Plate.3
Plate.3.1 – Pecten fossils within the Boulder Rock formation p.22
Plate.3.2 – Large carbonate concretion within karst weathered Boulder
Rock formation p.22
Plate.4 – Hand sample of the Iarann formation – note belemnite casts within
the lithology p.25
Plate.5
Plate.5.1 – Karst weathering exploiting both bedding and cleavage within the
Churchton formation p.29
Plate.5.2 – Outcrop showing clear bedding and cleavage planes within the
Churchton formation p.29
Plate.6 – Sample of the Snow Granite formation, note the white phenocrysts
of plagioclase p.31
Plate.7 – 2 Sills intersect a dyke in the valley of the Allt Fearns p.34
Plate.8

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Plate.8.1 – The Beinn Na’ Leac fault, note the ridge of scree p.37
Plate.8.2 – The Eyre fault, here the north end of the fault meets the sea p.37
Plate.8.3 – A large fissure parallel to the Beinn Na’ Leac fault p.37
Plate.8.4 – Internal image of the fissure in Plate.8.3, note the conchoidal
fracture still visible on either face p.37
3.3 List of models
Model.1 – Stand Systems of each Jurassic formation displayed as a function of
accommodation space and relative time p.12
Model.2
Model.2.1.1/2 – Models for the occurance of iron nodule horizons within the
FIM formation p.20
Model.2.2.1/2 - Models for the occurance of iron nodule horizons within the
FIM formation p.20
4.0 METHODOLOGY
Mapping tools included GPS (accurate to 100cm), rulers and tapes measures (accurate to 0.05cm),
and hand lenses of x10 and x40 magnification. Plane measurements were taken in the format
Strike/Dip Dip direction to 1 degree accuracy. Formations are named by their closest iconic
geographic feature or notable lithological attribute. Global factors considered included a -3 degree
mitigation to account for magnetic declination on Raasay.

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Mapping was undertaken over the course of 6 weeks interpreting lithologies and mapping contacts,
structures and exposures onto 1:10000 scale topographic basemaps. Location names are given in
italics with co-ordinates given to 5 figures in brackets.
5.0 INTRODUCTION
The Isle of Raasay is located east of The Isle of Skye forming part of the Inner Hebrides of western
Scotland (see location in Appendix.6). The island spans 62km2 in total over a distance of 21km
from its northern to its southern tip. The highest peak on the island, Dùn Caan (GR:NG:57909-
39487), is 0.444km high – other hills in the area range between 0.2 and 0.3km.
Of the 164 people living on Raasay, as of 2011 census data, (Scotlandscensus.gov.uk., 2017), most
reside in the islands biggest village Inverarish (GR:NG:55600-35900) on the southwest coast.
Notable locations here include Raasay House, The Ferry Terminal, Post Office and The
Oystercatcher House B&B.
The mapping area chosen was initially everything south-east of (GR:NG:54640-38000) - during
mapping this boundary was altered to include an area north of Oskaig (GR:NG:54640-38190) and
exclude the northeast of Beinn Na’ Leac (GR:NG:58529-36207) due to dangerous terrain there.
Geological exposure is clearest around the cliffs of the western coast and along the north-south
trending faults that scatter the southern area. Terrain is moderate consisting mainly of moorland,
however peat bogs, cliffs and fissures make the landscape increasingly treacherous particularly
toward the summit of Beinn Na’ Leac in the east. Large regions of nonexposure occur across the
east coast where forest and overgrowth conceals the majority of the basement outcrop. Weather
varies throughout the year but consisted of cold rain, with some clear sunny days, for the duration
of this study.

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6.0 STRATIGRAPHY
The Raasay Sequence comprising of 9 sedimentary deposits is broken into 3 periods of Tonian,
Permo-Triassic and Jurassic sedimentation. Tonian sandstones of the Eyre formation are bounded
by a paraconformity to the overlying Permo-Triassic conglomerates of the Gleann formation with
a time gap of ~500 Ma. A second paraconformity occurs between the Gleann formations Permo-
Triassic sediments and the Jurassic sequence of marine calc-arenites, shales and mudstones of
which the remaining 7 lithologies are composed of – this time gap is shorter, ~50 Ma. The Snow
Granite formation, which intruded after the lithification of the sedimentary sequence, has an
unconformable contact to all of the sedimentary lithologies exposed on Raasay.
Contact exposures on Raasay are sparse; the nature of these contacts could not be seen in direct
exposure so are concluded in discussion from paleomagnetic and fossil evidence – A Stratigraphic
summary is provided in Appendix.7.
7.0 SEDIMENTARY LITHOLOGIES
7.1 Introduction
Southern Raasay was observed to comprise of 9 sedimentary deposits which are summarised below
from oldest to youngest. Formations are named by those given in this study with their official
names (as discussed in the wider literature) provided in brackets. They consist of deep to shallow
marine, foreshore, deltaic and alluvial deposits:
- The Eyre formation (Sithean Glac An Ime member, Applecross formation) is a 130m thick deposit
of a Lithic Subarkose of Tonian, Neo-proterozoic age. This was deposited as a distal alluvial fan.

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- The Gleann formation (Stornoway formation) is a 50m thick deposit of a Subarkosic
Conglomerate of Permo-Triassic age. The formation was deposited as a series of coarse alluvial
fan bodies sharing a similar source to the Eyre formation.
- The Rail formation (Ardnish formation) is a 39m thick deposit of alternating Fossiliferous
Biomicrite and Mudstone of Sinemurian, Lower Jurassic age. It was deposited in a shallow marine
environment.
- The Fossiliferous Iron Mudstone formation (Pabba/Pabay Shale formation) is a 300-370m thick
Ferrous Shale of Late Sinemurian, Lower Jurassic age. This was deposited in a deep, near anoxic
marine environment.
- The Boulder Rock formation (Scalpa Sandstone formation) is a 120m thick Fossiliferous Calc-
arenite of Pliensbachian, Lower Jurassic age. This formation was deposited in a prograding
proximal or distal deltaic environment (dependant on further thin-section analysis) displaying
successive coarsening-up sequences.
- The Leabaidh formation (Portree Shale formation) is a 10m thick deposit of Fossiliferous Shale
of Toarcian, Lower Jurassic age. This formation was deposited in a deep marine environment.
- The Iarann formation (Raasay Ironstone formation) is a 5m thick deposit of Ferric Oomicrite of
Toarcian, Lower Jurassic age. This formation was deposited in a deep marine setting with ooids
washed in by storm events.
- The Bhonaid formation (Beinn Na’ Leac member, Bearreraig Sandstone Series) is a 10m thick
deposit of Calc-arenite of Aalenian, Middle Jurassic age. This formation was deposited in the
foreshore environment.

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- The Churchton formation (Druim An Fhuarain Sandstone member, Bearreraig Sandstone Series)
is a 235m thick deposit of Quartz Arenite of Aalenian, Middle Jurassic age. This formation was
deposited in a shallow marine environment as tidal sand waves.
A model of the relative order and evolution of system tracts for the 7 Jurassic formations is
summarised in Model.1. (Abbrevations: Lowstand Systems Tract (LST) – Transgressive Systems
Tract (TST) – Maximum Flooding Surface (MFS) – Highstand Systems Tract (HST) and Falling
Stage Systems Tract (FSST)).
7.2 Eyre Formation
Location: The Eyre formation, named for the proximity of its outcrop to Eyre Point
(GR:NG:57533-34173), is the oldest lithology exposed in the mapping area. It is most visible west
of Eyre, forming the cliffs along Raasay’s southern coast. The Eyre formation extends northeast
also, but is covered by a dense forest limiting observation.
Description: The Eyre formation is composed of a well cemented medium-grain (0.5-1mm⌀)
matrix containing sparse 5-30mm clasts. These sub-rounded clasts form ≤5% of the rock volume
and are randomly orientated within the matrix. From hand specimen the matrix is seen to compose
of: 51% Alkali Feldspar, 44% Quartz, 3% Biotite, and 2% Muscovite. The Eyre formations
lithology was therefore concluded to be a Lithic Subarkose.

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Structure: Structures in the Eyre formation (Visible in
Figure.1) included planar laminations, cross bedding,
coarsening up and down sequences, and hummocks –
post-deposition formations also include low amplitude
flame structures and erosional channel horizon. Bed
thickness was on average 56cm ranging from 21-96cm.
The formation is estimated to be greater than 130m
thick.
Interpretation: The 50% angular feldspar grain matrix
shows the relative immaturity of the bedload deposited,
and indicates the erosional source is close. The
coarsening up/down sequences, presence of rounded clasts and erosional channels through the
upper beds shows this is a fluvial alluvial fan with a prograding trend. The presence of hummock
structures (Figure.1) suggests these sediments were deposited above the storm wave base in a
marine environment – however commonly related structures in this setting, such as tidal ripples or
cross-bedding, were not observed and this observation alone does not provide enough evidence for
this environment. This formation is interpreted to be a near-proximal, prograding alluvial fan
deposit.
Discussion: The Eyre formation is discussed as the Sithean Glac An Ime member in the literature
and is part of the larger Applecross formation. The Applecross formation as a whole is recognised
as red arkoses and conglomerates, alike the overlying Stornoway formation. The Raasay deposits
are agreed to be arkoses with dominant quartz, alkali feldspar and plagioclase (Stewart and
Donnellan, 1992) with lithics of quartzite, jasper, chert and feldspar porphyry (Williams, 1966).

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The Sithean Glac An Ime member itself is not well studied however the Applecross formation as
a whole has been Rb/Sr analysed and dated to 761 ±17 Ma placing this formation around the
Tonian, Neoproterozoic (Moorbath, 1969).
7.3 Gleann Formation
Location: The Gleann formation is named after the Scottish Gaelic for ‘valley’, in reference to the
valley north of Eyre point where it is exposed most prominently, often by strike-slip faulting at
(GR:NG:57200-34100). The Gleann is exposed in the valley and hills west of Eyre point, though
is both eroded at the coast and obscured by forest to the west of the central Eyre Fault.
Description: The Gleann formation consists of large, polymict,
point-contacted clasts (50% rock volume) with a well
cemented coarse-grained (0.5-2mm⌀) matrix (50% rock
volume) between. The clasts are subrounded-subangular and
unsorted with long axis measurements of ~10-150mm. 4 clast
lithologies were determined: a medium-grain, brick-red Lithic
Subarkose, a rounded Quartzite, a micaceous bedded Arkose
and a matte-grey Carbonate. The matrix is composed of 80%
Quartz, 10% Alkali Feldspar, 7% Muds, and 3% Muscovite.
The Gleann formation was determined to be a Subarkosic
Conglomerate.
Structure: The structure of the Gleann formation (shown in
Figure.2) consists of both clastic layers (96-181cm thick) and
non-clastic (3-12cm thick) breaks of the subarkosic matrix composition. The clastic layers show

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some imbrication, though in no dominant direction – there is no grading of clasts, large 15cm
boulders are deposited mid-layer parallel to 2cm clasts. The Gleann formation is ~50m thick.
Interpretation: By the massive bedding, and the large unsorted, angular clast sizes in this formation
the Gleann formation is interpreted to be a series of gravity flows which are triggered by the
unstable overloading of an alluvial fan in a LST. The deposition of the coarse grained beds of
subarkosic arenites between these flows, lacking any of the large clasts is an expression of the finer
distal background sedimentation which these gravity flows run down onto leaving undulating
erosional contacts on their base.
Discussion: The Gleann formation is discussed as the Stornoway formation in the literature. There
are no fossils present in the large clastic sequences. (Storetvedt and Steel, 1977) summarise the
Stornoway formation as successive coarse conglomeratic alluvial fan bodies which that grew out
onto local paleoslopes. The Stornoway formations age has been disputed – Torridonian, Devonian
and Permo-Triassic ages have been proposed however paleomagnetic evidence suggests a Permo-
Triassic age (Storetvedt and Steel, 1977). The formation thickness varies geographically from 4km
to 50m (as observed in this study) though is generally found to be 1.2km thick (Trewin, 2002).
7.4 Rail Formation
Location: The Rail formation is only exposed in two small regions across the south of the island –
at the Mine Workings (GR:NG:55500-34200) through which the mine rail was cut, and in the gorge
of Allt Fearns (Running from GR:NG:58532-36635 to GR:NG:58960-35466) that divides the
regions of North Fearns and South Fearns. The Rail formation is otherwise largely covered by
granites, trees and peat bogs.

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Description: The Rail formation consists of alternating deposits of carbonate and mudstone. The
carbonate layers are karst weathered along a regular cleavage plane (116/89E) (Plate.1.1) toward
the base of the sequence and are fossiliferous. Carbonate layers predominantly contain several
‘slug-like’ gryphaea (5-10% rock volume) (see Plate.1.3) as well as less common bivalve and
belemnite fossils (1% rock volume). The mudstone layers contain these gryphaea to a lesser extent
(2-3% rock volume) and are mainly restricted to the upper few centimetres of the mudstone bed in
each instance. All gryphaea are orientated in a life mode assemblage (see Plate.1.2). By a Dunham
classification the carbonate component in this formation was considered in-field to be a
Wackestone, however due to the low fossil percent content (<10%) through most of the Rail

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formation it may be better named through the Folk classification scheme as a Fossiliferous
Biomicrite.
Structure: In the lower log sequence, carbonate beds range from 6-21cm thick (average 10.3cm).
Mudstone beds range between 2-34cm (average 11.2cm). Bedding is planar in these lower
sequences but can be heavily bioturbated by narrow <0.5mm⌀ and convoluted in localised sections
(see Plate.1.3). Higher in the log bedding becomes distinctly thicker (45-77cm) and becomes less
convoluted as bivalve fossils become less numerous. The Rail formation is ~40m thick – a log of
this formation was taken and drawn in field and is attached as Appendices 1, 2, 3 and 4.
Interpretation: The carbonate lithology and presence of marine gryphaea fossils in a lifemode
assemblage evidences a shallow marine depositional environment within the high productivity
photic zone. The bedding and relationship between the carbonate and muddier layers is unclear –
the carbonates likely formed in-situ as background sedimentation with the finer muds coming down
from the continent into the basin during periods of heavy weathering. The regular alternation of
these beds may allude to orbital forcing altering the climate – wetter periods would increase the
bedload and carry the muds further out into the basin to cover the carbonates – these can accumulate
thickly during these periods as the muds reduce the light levels in the water column and inhibit the
growth of forams. The introduction of these nutrients later favours burrowing organisms which
would form the narrow vertical bioturbation seen across the beds. Another mechanism for the
introduction of this material is through turbidites – however the gryphaea are too large to be
deposited amongst the muddier material – the rapid overpressure of this theory would however
explain the convoluted bedding as fluid escape features. The Rail formation is interpreted to
represent a HST as carbonate sedimentation caught up with sea level.

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Discussion: The Rail formation is discussed as the Ardnish formation in the literature. The Ardnish
contains Gryphaea arcuate which have been used to date the formation to either the Semicostatum
Zone or Bucklandi Zone of the Sinemurian, Lower Jurassic (Morton, 2004). The formation is
between 36m (Hesselbo and Jenkyns, 1998) and 42m thick (Morton and Hudson, 1995) (Morton,
2004) – concordant with the observations of this study. The Rail formation also encompasses the
underlying Breakish formation, described by (Morton, 1999) which was not recognised in-field.
Outcrops of the Breakish are mainly concealed by the forests covering Raasay’s eastern coast.
7.5 Fossiliferous Iron Mudstone Formation
Location: The Fossiliferous Iron Mudstone (FIM) formation, is continuously exposed along the
coast south from Inverarish. The FIM is dominantly concealed by moorland, peat and granites
inland. Some localities such as exposure in the gorge west of Beinn Na’ Leac display excellent
section through the stratigraphic column.
Description: The FIM formation is recognised by its finely laminated, thickly bedded and well-
cemented mud sediments separated by horizons of iron nodules and iron-rich beds. The mud
sediments consist of very fine-grained (<0.01mm⌀) micaceous clay minerals, as a result the exact
mineral composition was not approximated from hand specimen.
There is a high fossil content (5% rock volume) in this formation - predominantly bivalves,
ammonoids (visible in Plate.2.1), belemnites, gastropod with less common broken gryphaea and
crinoid fossils (Plate.2.3). Elliptical iron nodules, which deform the surrounding laminations in the
FIM (see Plate.2.2), account for only (<1% rock volume) and occur in laterally extensive beds. By
the mud and fossil content the FIM formation is classified as a Shale, considering the notable iron
nodules it was specified to be a Ferric Shale to distinguish from the Leabaidh formation.

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Structure: Bedding was transgressively logged perpendicular to bedding through the largest nodule
in each deposition break horizon identified; the thickness of shale deposition was recorded between
each nodule. Shale beds varied between 1-400cm in thickness - often bedded in 20-40cm layers
lower in the sequence becoming thicker 3m-4m deposits higher in the log. This log has been
represented as a bar chart in Appendix 5 where the potential influence of orbital forcing on nodule
formation (In the case of Model.2.1) was scrutinised – the expression of orbital forcing within the
sequence does not seem to reliably occur, but could appear should more of the formations exposure
be logged and analysed in this nature. Laminations within the shale were continuously 0.5-1mm
thick throughout these beds. Iron nodules ranged from 2-9cm thick (Plate.2.4), on average 3.5cm

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with laminations deforming under and over the iron nodules. The FIM formation ranges between
300-370m thick. The eastern exposures of FIM readily break into tabular flakes through the valley
of the Allt Fearns - conversely around the west coast of Raasay the FIM was notably more
cemented and cleaves on sub-vertical planes of 180/76W.
Interpretation: The fine clay sediment of the FIM formation evidences a transgression to a deep
marine setting. The presence of ammonoids and belemnites and absence of the shallower gryphaea
also supports the interpretation of a deeper marine environment while broken, incomplete fossil
fragments of shallower organisms may come from similar environment to the Rail formation. This
would mean a long-lasting period of high sea levels following the deposition of the Rail formation
to deposit such a thick formation of shale. The formation of the nodules is debated – these could
accumulate syn-depositionally at the surface as they are seen in laterally continuous horizons (Seen
in Model.2.1), or from iron-rich fluid flow along the beds as the muds became compressed and
dewatered (Seen in Model.2.2). In either scenario, laminations are seen to deform above and below
the nodules and therefore these nodules must predate the formations final impermeable
compaction. The fissile nature of the FIM formation seen locally through the Allt Fearns is thought

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to be produced by the brittle deformation zone of the Beinn Na’ Leac faults cataclaysis and the
following erosion by the Allt Fearns.
Discussion: The FIM formation is discussed as the Pabba Shale or Pabay Shale formation in the
literature. (Hesselbo and Coe, 2000) interpret the deposition of this formation as a result of the
regional deepening that occurred during the Semicostatum Zone of the early Sinemurian. The
presence of Liasidium Variabile in this formation dates the Pabay Shales to the Oxynotum Zone of
the Late Sinemurian, Lower Jurassic (Brittain et al., 2010). Deposition of the Pabay Shales was
therefore between 196.5 and 189.6 Ma (Hesselbo and Jenkyns, 1998).
7.6 Boulder Rock Formation
Location: The Boulder Rock formation, named for its boulder-like concretions, is most visible in
its large 40m cliff sections across the coast east of Beinn Na’ Leac and more low-profile exposures
south of the Ferry Terminal (GR:NG:54518-36320). Inland, exposure is relatively poor and
contacts are largely inferred from topography.
Description: The Boulder Rock lithology is complicated consisting of a well-cemented fine-grained
(<0.5mm⌀) matrix. The Boulder Rock formation is predominantly quartz-based, but contains thin
(0.05mm) flaser bedding of dark, convoluted layers thought to be fine coals or muds. The
mineralogical composition was observed in hand specimen to be 90% Quartz, 5% Coals/Muds, 3%
Cement and 2% Muscovite. Lower in the formation dense layers of calcified pectens and oysters
(Plate.3.1)can be found, with less common bivalves and corals also present. Also notable are
rounded ‘boulder clasts’ (0.5m-4m wide) of a resistant, coarse-grained (1-2mm⌀), grey, >95%
Quartz lithology that occur on the west coast. On the east coast, carbonate concretions (~10-
100cm⌀) (Plate.3.2) form within a more calcareous, honeycomb-weathered variety of the Boulder

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Rock lithology. Considering the variation within the Boulder Rock, this formation can be broadly
classified as a Sublitharenite grading to a more calcareous composition lower in the sequence.
Structure: A log was not taken of this formation, though a continuous cliff exposure east of Beinn
Na’ Leac of >40m could continuously be logged with rope and climbing equipment. Bedding
within the formation is difficult to distinguish due to the algae coatings on coastal exposures but
appears to be on the scale of 0.2-1m. Sedimentary structures such as flaser and convolute bedding
are present in the upper sequence within beds. The Boulder formation is estimated to be 80m thick.
Interpretation: With the lithological variation over numerous outcrops in this formation an
environmental interpretation is difficult. The mature quartz sediment and presence of marine fossils
(oysters and pectens, bivalves or corals) common to most localities evidences a sustained shallow
marine environment over the course of this layers deposition. Coal or mud beds within the sequence
suggest an near or distal coastal deltaic environment respectively. The ‘boulder clasts’ share a
similar lithology and structure to the surrounding boulder rock so these are likely a result of

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diagenesis and nodulation rather than the initially interpretation of clasts – calcite fossils could
dissolve and reprecipitate to form nodules post-deposition. The shallower depositional
environment shows a large change in sea level between the deposition of the FIM and this boulder
rock. This formation is interpreted as having been deposited in a LST. Clearer examples of the
Boulder Rock deposits are needed to observe the structure and test the proposed theory of the
formations coastal deltaic depositional environment.
Discussion: The Boulder Rock formation is referred to as the Scalpa Sandstone formation in the
literature. (Hudson, 1983) recognises the massive sandstone beds to have a general coarsening
upward sequence and mentions the nodular cementation of the sandstones. (Hudson, 1983)
expresses that no detailed sedimentological work had been published however the formation has
been measured to be 123m thick on Raasay (Howarth, 1956). These sediments were deposited
between the Ibex Zone of the Lower Lias (Pliensbachian, Lower Jurassic) (Howarth, 1956) and
the Tenuicostatum zone of the Upper Lias (Toarcian, Lower Jurassic) (Morton, 2002).
7.7 Leabaidh Formation
Location: The Leabaidh (Scottish-Gaelic for ‘bed’) formation overlies the Boulder rock formation
and was only observed in a small area near the centre of the mapping area. Poor regional exposure
allows only limited observation – the clearest exposure is excavated at the Rassay No.1 Mine
(GR:NG:56842-36427) where the overlying Iarann formation was strip mined away a century ago
to aid the war effort. This formation is the oldest of three lithologies referred to as the ‘Band
Group’, these three lithologies are thin (~10m) and map as a ‘band’ across central Raasay.
Description: The Leabaidh formation is of a similar lithology to the FIM formation. The Leabaidh
consists of finely laminated micaceous clay sediments (>0.05mm) and contains numerous

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belemnite (1x6cm-1.5x9cm) and ammonoid cast fossils (4-7cm⌀) exposed across bedding surfaces.
The formation is fissile, and the lack of exposure is attributed to streams which readily incise into
its lithology. From the limited observations above, the Leabaidh lithology is classified as a Shale.
Structure: Little structure was observed in the Leabaidh formation – only fine (1mm) planar
bedding was recognised. The Leabaidh is thought to be ~10m thick.
Interpretation: The deposition of this layer is thought to represent another rise in sea level to form
a deeper marine setting. Bivalves and ammonoids likely fell out from the water column on death
to rest in these beds - they would otherwise be allochthonous considering the particle size of the
formation. The fine sediment of this formation suggests a TST as the sea level deepened and
sedimentation rate was relatively slow. These presence of clay particles and high bio-productivity
suggests a near deltaic fan to provide the material and nutrients for this formation.
Discussion: The Leabaidh formation is discussed as the Portree Shale formation in the literature.
There are few detailed observations of the formation as it is poorly exposed in Southern Raasay.
The Portree shale formation is a fossiliferous shale, containing the ammonoid Dactylioceras which
were observed to be clearly exposed at the Raasay No.1 Mine – from these the formation has been
dated to the Toarcian, Lower Jurassic (Hudson 1983). The formation is confirmed to be 10m thick.
7.8 Iarann Formation
Location: The Iarann formation (Scottish-Gaelic for ‘iron’) overlies the Leabaidh formation and is
solely exposed in small man-made outcrops excavated by mining or footpath erosion. Observation
comes from only a handful of small ~2m thick localities so little is known of the depositional
structures. The Iarann formation is the central member of the Band Group.

25
Description: The Iarann formation (Hand
sample visible in Plate.4) is composed of
>1mm⌀ ooids supported in a clay sediment
matrix. The ooids are most commonly solid,
however some consist of a thin 0.01mm shell
surrounding a fine, orange-stained powder
within. The matrix is dark and fine grained but
a mineralogy cannot be determined from hand sample. As well as containing ooids the matrix
preserves the 3-4cm x 0.7cm⌀ casts of belemnites which have since weathered out. Throughout
this formation are thick ~1cm veins of a dense dark iron mineralisation. The lithology is identified
as a Ferric Oomicrite.
Structure: Bedding is observed to be quite thin (~1-2cm) and planar with no obvious fluvial or
deformation structures in the sediment. The thick iron veins cut through the oomicrite in planar and
sub-vertical orientations – likely following weaknesses in the cleavage and bedding of the
oomicrite. The Iarann formation is observed to be ~5m thick.
Interpretation: By the fine, planar material of the matrix the Iarann formation was likely deposited
under calm conditions below the wave base in a shallow marine environment. Ooids, which formed
under agitated conditions closer to the foreshore, are allochthonous. Ooids are thought to be washed
out from the foreshore during storm events. Belemnites which inhabited the water column may
also be transported from elsewhere; whether these are genetically related to the belemnites in the
Leabaidh formation could not be determined in field.
These clay sediments are thought to come from a distal deltaic source which slowly accumulated
over time, similar to the genesis of the Leabaidh formation. Iron mineralisation is thought to be a

26
post-depositional feature – iron rich fluids flowed through fractures in the lithology developing
veining. The fluids may exploit point-contacted ooids in the formation partially replacing them,
but preserving the calcite structure of those deeper in the otherwise impermeable matrix. The
formation was likely deposited in a HST as accommodation space decreased within the basin and
sea level became relatively shallow.
Discussion: The Iarann formation is discussed as the Raasay Ironstone formation in the literature.
(Lee, 1920) describes the Raasay Ironstone formation as oolitic, grading up from shaly matrix at
the base to an echinodermic limestone matrix cemented with granular calcite. Ammonites, from
within the mine, evidence a series of zones – specifically Hildoceras bifrons dates the formation to
the Subcarinatum zone of the Toarcian, Lower Jurassic (Lee, 1920). Regarding the iron
mineralisation, analysis by (Lee, 1920) determined the green mineralisation of chamosite to
discolour the Ironstone – (Kearsley, 1989) also observes the development of siderite in an
‘eggshell’ array around the ooid followed by further internal crystallisation. The source of the iron
required for this mineralisation is unclear, however (Hallam, 1966) discusses a few potential
mechanisms noting proximal rivers may have provided a concentrated iron source. Also mentioned
by (Hallam, 1966) is the work of (Carroll, 1958) who proposes iron may be able to coat clay
minerals before deposition and subsequent reaction with kaolinite, as present in the ooids, could
lower the redox potential of the iron to a more soluble form allowing localised mineralisation –
issues with this, highlighted by (Hallam, 1966), is the magnitude of mineralised chamosite
observed may be greater than possible the effect of this process given the small percentage of
kaolinite present in the mud-dominated formation.

27
7.9 Bhonaid Formation
Location: The Bhonaid (Scottish-Gaelic for ‘cap’) formation forms the uppermost member of the
Band Group and is the most prominent of the three observed. It occurs as thick (5-10m) ridges
which can easily be seen from the topography. This erosional behaviour often alludes to the trend
of the Band Group through the non-exposure of the moorland through which it occurs.
Description: The Bhonaid is composed dominantly of moderately-cemented, angular, fine-grained
(>0.1mm⌀) quartz with occasional ~1cm thick interludes of a slightly muddier matrix. Short
(0.2mm) flaser beds of a darker composition, alike those observed in the Boulder Rock formation,
are common throughout the sequence. The cement is calcareous as evidenced by the honeycomb
weathering of the bedding. Rare (2mm⌀) iron-rich hollows can be found within fresh surfaces. This
lithology, although similar to the Boulder Rock formation, lacks any fossils or concretions and in-
field is distinguished by this difference. The Bhonaid formation is identified as a Calc-arenite.
Structure: Structurally the bedding is of evenly thick (5-10cm) beds deposited in planar layers with
occasional low-angle crossbedding. No jointing within the formation was observed though the
Band Group as a whole is observed to be regularly faulted. The offset of the prominent ridges of
Bhonaid formation often indicates the location and motion of these faults. The Bhonaid formation
is observed to be 10m thick.
Interpretation: From the calcareous cementation, the Bhonaid formation is interpreted as a being
deposited in a shallow marine environment. The source of fine quartz sediments could be from the
near shore. An overall increase in grain size through the formations of the Band Group - from the
Leabaidh shales to the Iarann ferric oomicrite to this calc-arenite - suggests the evolution from a

28
TST (Leabaidh) to a HST (Iarann) developing into a FSST (Bhonaid). The absence of any marine
fossils observed in this formation implies this environment was not inhabited during its deposition.
Discussion: The Bhonaid formation is discussed as the Beinn Na’ Leac member in the literature. It
is described as calcareous sandstone which forms the second lowest member of the Bearreraig
Sandstone Series. (Morton, 1965) observed this member to contain calcareous concretions and
lenticels of another calcareous sandstone – these features would argue for a high-energy foreshore
depositional environment. This conclusion would support the earlier interpretation of a shallowing
sequence however more detail would be needed to confirm the theory of a deltaic source and
deposition mechanism for these sediments – it is more likely this is worked material deposited at
the foreshore as the sustained high energy needed to produce calcareous nodules would likely
inhibit the growth of deltaic fan structures. The lack of fossils in this carbonate formation also
supports the shoreface environment where this material could be broken down beyond recognition
in hand specimen. (Morton, 1965) recognises the exposures of the Beinn Na’ Leac member around
Beinn Na’ Leac of the Bearreraig Sandstone Series in general to have been deposited across the
Discites subzone of the Sowerbyi zone of the Lower Bajocian, Middle Jurassic and the Opalinum
zone of the Aalenian, Middle Jurassic – field relationships determine the latter Aalenian age for
this occurrence as the base of the overlying Churchton formation is of similar Aalenian age. Of
note, between the Beinn Na’ Leac member and the Iarann formation lies the Caan Shale member
of the Bearreraig Sandstone Series – this member is was not in the mapping area and outcrops
further to the north near to Dun Caan.
7.10 Churchton Formation
Location: The Churchton formation, named after Churchton Bay (GR:NG:54800-36300) where it
was first observed, is the thickest sedimentary lithology on Raasay and marks the youngest

29
sedimentary formation observed in the region. It is widely exposed across the top of the mapping
area in the steep hills and valleys.
Description: The Churchton formation is composed of moderately-cemented, medium to fine-
grained (0.2-1mm⌀) quartz grains. The cementation between grains is a carbonate, noted from the
golf-ball karst weathering of outcrops. Large ‘mouth-shaped’ weathering features (visible in
Plate.5.1), up to 1.5m wide, develop along bedding and cleavage planes. The mineral composition,
as observed from hand specimen, is dominated by >95% Quartz with the rest composing of calcite
cement. Over the vast exposure of this formation only a single bivalve fossil was located at
(GR:NG: 56170-37200). Given the mineralogy this lithology is classified as a Quartz Arenite.
Structure: The Churchton formation exhibits herringbone cross-stratification and cross-bedding,
though it is difficult to recognise given the homogenous nature of the formation and the low-angle
cleavage planes within the beds (exposures like those visible in Plate.5.2 demonstrate plane
surfaces within the Churchton formation). The formation is strongly faulted by both dextral strike-
slip and normal faults which offset the clear boundary between the Churchton formation and the

30
intruded Snow Granite formation. Calcite veins develop sub-vertically along joints parallel to these
strike-slip motions in a north-south orientation. The Churchton formation is estimated to be in
excess of 250m thick.
Interpretation: The Churchton formation is mature, well-sorted, quartz-rich sediment which
evidences a long history of erosional reworking before final deposition – the high energy
requirements for this to occur implies a shoreface depositional environment given the calcareous
cement. Cross-bedding and herringbone cross-stratification further reinforce this interpretation
evidencing a bi-directional flow. This conclusion continues the interpretation from the Band Group
group of a shallowing sea with evidence of a LST. To determine the overall thickness of the
Churchton formation the upper contact needs to be observed.
Discussion: The Churchton formation is discussed as the Druim An Fhuarain Sandstone member
in the literature. The Druim An Fhuarain is also a member of the Bearreraig Sandstone Series,
studied in detail by (Morton, 1965) who recognises the similar whitish, cross-bedded quartzite beds
identified in field. Interbedded sandy, bioclastic limestone (not observed in-field) was also noted
by (Morton, 1965) within the formation. The sandstones are fossiliferous, though only the lower
part of the formation exposed near Faoilean, Isle of Skye (GR:NG:56763-20186) evidences the
crinoids, pelecypods and bryozoans observed by (Morton, 1965). From comparison to the Mull
faunal sequence the brachiopods can date the base of this formation to the Scissum Zone of the
Aalenian, Middle Jurassic (Morton, 1965). Perhaps there is an issue of preservation of any fossil
casts due to the karst weathering of exposures. The formation is logged to be 235m thick, from the
cross-section estimate of 250m the formations upper contact is not much further north of the area
mapped. (Morton and Hudson, 1995) concluded the formation to be deposited as linguoidal tidal
sand waves concordant with observations and interpretations given here.

31
8.0 IGNEOUS FORMATIONS
8.1 Introduction
3 Igneous formations were mapped on Raasay. The largest of these, the Snow Granite formation is
a large lateral plutonic intrusion that is well exposed across southern Raasay and contacts all
sedimentary formations. The second largest, the Battery formation, is a large basaltic sill which
intrudes into the youngest sedimentary deposit, the Churchton formation – as the Snow Granite
and Battery formations showed no contact in-field the relative age relationship between these could
not be determined in-field. Finally the Dyke formation comprises of an extensive complex of dykes
which are spread evenly across Raasay and are observed to intrude through the second oldest
sedimentary deposit, the Boulder Rock formation.
8.2 Snow Granite Formation
Location: The Snow Granite formation named for its white speckled appearance when wet is the
most exposed lithology across southern Raasay. The Snow Granite formation is dominant to the
north of the mapping area and occupies a large portion of the central western region and forms the
bedrock beneath Inverarish.
Description: The Snow Granite formation is characterised
by its coarse-grained, porphyritic texture of white
phenocrysts of plagioclase (2-6mm⌀) against a quartz-
rich, greyer matrix (<2.5mm⌀) (Hand sample visible in
Plate.6). Its mineralogical composition is 55% Quartz,
30% Orthoclase, 10% Plagioclase, 5% Biotite. Given the

32
high silica content of this formation this has been classified as a Granite.
Structure: Jointing within the Snow Granite is of variable intensity ranging from regular 1-5cm
sub-vertical sets to wider 1-2m sub-vertical joint spacing with additional low angle joints with 0.5m
spacing.
Interpretation: The Snow Granite is interpreted as the plutonic intrusion of a large lateral body of
granitic magma that, from its coarse-grained porphyritic texture, has experienced two stages of
cooling. The sub-vertical jointing within this formation developed as a result of contractional
cooling and further in response to extensive tectonic deformation. The intrusion is relatively aged
to be younger than the lithification of the Raasay Sedimentary sequence and older than the strike-
slip deformation of the region.
Discussion: The Snow Granite formation is discussed as the Raasay Granite in the literature. The
lithology is described broadly in the work of (Davidson, 1935) as acidic acknowledging the
variation in composition from marginal felsitic or spherulitic lithologies with a granophyre core. A
porphyritic texture was noted within all these lithologies, these phenocrysts are agreed to have
developed after the initial intrusion. The intrusion structure is regarded as a sill (Davidson, 1935)
however due to the discordance of this intrusion from the bedding of the Raasay sedimentary
sequence this is disputed here to be a lateral plutonic intrusion.
8.3 Battery Formation
Location: The Battery formation is named after Battery Wood (GR:NG:54300-36500) where this
lithology was first observed. The formation outcrops across the north-western coastal region of the
mapping area spanning from Oskaig (GR:NG:54762-38165) to the Ferry Terminal.

33
Description: The Battery formation is a mafic fine grained igneous (<1mm⌀) lithology. It is mainly
composed of a black, blocky, pyroxene or amphibole crystals (<1mm⌀) (indistinguishable in hand
specimen) with vitreous olivine (0.5mm⌀ phenocrysts). These are supported by a fine-grained
matrix of white plagioclase, biotite and other darker phases. It is difficult to observe in hand
specimen the percent composition but a mineralogical estimate constitutes of: 60%
Pyroxene/Amphibole, 25% Olivine, 10% Plagioclase, 5% Biotite. This formation has been
classified as a Basalt.
Structure: The Battery formation also displays cooling
texture and structures – Columnar jointing (>6m*1m⌀)
(Figure.3) was identified along the coast near Oskaig
as well as concentric pillows of basalt near the Ferry
Terminal. The latter has a finer grained structure with
no phenocrysts at its core as seen in the general
lithology. The formation also appears to have thick
bedding, inferred from the regular dip and strike of the exposed surface around the coast – the
overall thickness of the formation is at least 110m.
Interpretation: From the pillows and relatively small crystals the Battery formation it is determined
to be an extrusive as these features are typically associated with lava quenching. The layering and
columnar jointing result from thick periodic extrusions onto a terrestrial surface – the columnar
jointing could develop in very thick flows that cooled slowly due to their size. Phenocrysts of
olivine evidence a brief stage of cooling before extrusion and crystallisation of the finer matrix.
Discussion: The Battery formation is discussed as the Little Minch Sill-complex in the literature.
The lithology, although difficult to resolve in field, was seen to consist dominantly of pyroxenes

34
or amphiboles with weathered olivine phenocrysts – observations made by (Davidson, 1935)
determine the complex to be formed of Picrite Basalts (These are typically pyroxene rich with
magnesium-rich olivines (50-65%)) and Crinanites (25-50% olivine). The feeding branches of the
Little Minch sill complex are estimated to have a thickness of ~250m (Gibb and Gibson, 1989)
with the exposure at Raasay no thinner than 90m (Davidson, 1935) and, as observed in field, at
least 110m thick. Observations and interpretations of pillow basalts in this study are likely the
effect of onion-skin weathering on the basalts creating concentric pillow-like features. The bedding
of the sill are the result of subsequent intrusions across a plane of weakness – columnar jointing is
observed to develop across these internal injection contacts, therefore the intrusion of the Little
Minch Sill complex has been interpreted to have formed as rapid successive pulses of picritic and
crinanitic magmas (Gibb and Gibson, 1989).
8.4 Dyke Formation
Introduction: The Dyke formation covers a broad area of
the mapped region and is observed to cut all formations bar
the Churchton, Snow Granite and Battery Formations.
Dense occurrences of these dykes can be clearly observed
south-west of Beinn Na’ Leac in the FIM formation. It is
possible the density of these dykes is expected to continue
throughout most of southern Raasay unexposed beneath the
moorland.
Description: The dykes of Raasay are typically black, fine-
grained intrusions occasionally displaying a vesicular core
in larger dykes which formed as gas exsolved from the magma. The mineralogy is determined to

35
be mafic from the black colour common to all of these intrusions. The minerals present have glassy
or dull lustres which are thought to be olivine and pyroxene respectively however thin section
analysis is required to confirm these interpretations as grains are only <0.5mm⌀ with a finer matrix.
The lithology is considered to be a basalt.
Structure: Dykes range from 6-400cm in width. Dykes were observed to trend N-Sand NWW-SEE
orientation however dykes and sills of this composition were observed to open subvertically and
subhorizontally in all orientations (Example shown in Plate.7).
Interpretation: The mafic composition of the Dyke formation is markedly different to the silicic
Snow Granite formation so likely comes from a different melt source.
Discussion: The Dyke formation is discussed as the North Britain Paleogene Dyke Suite in the
literature which developed as part of the North Atlantic Igneous Province. The Raasay Sub-swarm
is a part of the regional Skye Dyke Swarm which trends NW-SE across Britain (Trewin, 2002) –
this is reflected in the measurements made in this study.
9.0 STRUCTURE
9.1 Introduction
The brittle deformation of Rassay is apparent in the large dextral strike-slip faults which cut across
from the northeast to the southwest. Other large motions include the detachment of Beinn Na’ Leac
by low-angle normal and dextral strike-slip faulting. Other structures analysed include the jointing
within the Snow Granite formation, orientation of the Dyke formations, as well as the overall
regional tilt of sedimentary deposits and the Battery formation sill intrusion. An isometric view of
Raasay’s structure, showing the larger regional movement, is visualised in Appendix.9.

36
9.2 Brittle Deformation
5 Major strike-slip faults occur on Raasay, the largest of these is named as the Eyre Fault stretching
NE-SW from Oskaig to Eyre Point (GR:NG:54660-38415 to GR:NG:57475-34195). 1 Major low-
angle normal fault occurs around Beinn Na’ Leac (GR:NG:59000-37270 to GR:NG:58360-36200)
and joins into a Major strike-slip fault (GN:NG:58360-36200 to 59000-35540) - interpreted to have
detached the hill from the bedrock. Other faults recognised include 4 Minor mesoscale dextral
strike-slip faults, 1 sinistral Riedel Shear, 4 mesoscale normal faults, 2 fissures, 4 faults of
unidentified nature.
A value for transform faults is calculated to compare the partitioning of deformation across Raasay
from: Fault Magnitude = Fault movement/Fault length. This can only serve as an estimation of the
Fault Magnitude (Me) as both the fault movement and fault length are estimated from field

37
interpretations and are bounded by the mapping area. The use of this however is to provide a
‘ballpark’ proxy value for the average stress field across Raasay’s dextral strike-slip deformation.

38
9.2.1 Eyre Fault
The Eyre Fault (Redrawn here in Figure.4) stretches a total distance of 5km with an offset of 250m
(Magnitude estimate (Me) = <0.05). The line of this feature is recognised by the offset of
lithological contacts, easily traced through the valleys and cliffs that have formed as a result of its
movement. The Eyre Fault cuts through all lithologies present on Raasay giving it a relative date
of younger than the Battery formation. The fault is most apparent north of Oskaig (Shown in
Plate.8.2), where a cliff of Battery formation to the left is opposed by one of Snow Granite.
9.2.2 Beinn Na’ Leac Fault
The Beinn Na’ Leac Fault (Visible in Plate.8.1), once restored, has a displacement offset of 800m
along its fault surface – although no fault length is observed in this motion due to erosion, the
significantly larger fault displacement than the surrounding transform faults implies the magnitude
of this fault is estimated to be far greater evidencing stronger extensional stress or a longer active
history. The current angle of the normal fault plane is 13 degrees - as such a large movement is
unlikely to develop along this angle it is suggested that this fault was established before the regional
tilting of Raasay by ~20 degrees. An extensional fault of this scale is more feasible on the combined
restored angle of 33 degrees – conchoidal fracture on the surface planes (Shown in Plate.8.4) within
fissures associated with this fault indicate this movement happened relatively recently to still
preserve these features.
9.2.3 Other Faults
Notable faults include a secondary major fault that stretches from (GR:NG:56240-37160 to
GR:NG:55470-38900) and further north. This fault runs parallel to the Eyre Fault and is interpreted
to act as a strain partitioning fault. The fault is >1.9km in length and has an offset of 315m giving

39
a magnitude estimate of (Me = <0.16) – if this fault has developed under the same stress as the
Eyre fault it should have the same magnitude. From this assumption this interpretation can be tested
as the fault should have a length of 6.3km.
Two parallel dextral strike-slip faults run north-south through the centre of the mapping area again
responding to the same stress field. The leftmost has an offset of 440m along a length of >2.1km,
the rightmost has an offset of 450m along a length of >1.5km. These give magnitude estimates of
(Me= <0.21) and (Me = <0.30) respectively. Between these two features a Riedel Shear Fault has
been inferred (from stratum contours and topographic stream channel formations) to have
developed at 45 degrees to the strike-slip motion – this provides an explanation for the sudden
offset in the contact location and is plausible given the stresses that develop between the two north-
south parallel faults which bound it.
Complimentary to the deformation history of the Beinn Na’ Leac fault are two fissures (The larger
of the two is shown in Plate.8.3) that open through the detached hill in a northeast-southwest
orientation perpendicular to the main normal fault. These are 8m and 1m wide each and, in the
larger of the two, radial conchoidal fractures (Plate.8.4) were observed on opposing fault surfaces
evidencing the lack of motion across this plane and it’s relatively recent opening. From topography
many more of these fissures could be inferred parallel to these over Beinn Na’ Leac’s northwest
slope however most have been filled in by debris and muds after opening. These fissures are
thought to grow down to the base of the detached upper block and formed along pre-existing
weaknesses caused by the northwest-southeast extensional stress field.

40
9.2.4 Fault Magnitudes
Considering the above estimations of fault magnitude across Raasay of <0.05, <0.16, <0.21 and
<0.30 it is evident these are inaccurate given the constraints and error margins of mapping. Should
these faults have all formed under homogenous stress conditions the magnitude would be expected
to be similar. Assuming the lowest magnitude of 0.05 to be closest to the regional figure, the total
length of the faults within this mapping area can be estimated to extend far further beyond the
mapped area and beyond land exposures.
9.3 Jointing Within The Snow Granite Formation
Stereonet analysis of jointing in the Snow Granite formation
(Visualized in Figure.5) shows a broad NW-SEplane of extension
created from the opening of two sets of joints – these sets open
roughly on the planes of 174/70E and 105/70N. Most of these
fractures open at sub-vertical angles and maintain this trend across
the various granite provinces and exposures exposed on Raasay.
The jointing across these two axes opened on the cooling and
contraction of the granite after the initial intrusion to the surrounding extensive field. The date of
the intrusion is thought to post-date the regional tilting, as these joints retain their assumed original
vertical orientation.
9.4 Dyke Formation Intrusion
The Dyke formation was mapped and measured across Raasay at each occurrence to attempt to
recognise regional strain patterns from dominant extension directions. Of 40 Dyke planes observed,
measurements showed several varying strike and dip planes. Although (Figure.6) shows extension

41
by dyking broadly occurred in all directions two dominant planes
are interpreted from (Figure.6) in N-S and NWW-SEE
orientations.
From the above observations it is interpreted that the intrusion of
these dykes occurred under similar regional stresses to the Snow
Granite formation which exhibited similar opening plane trends.
9.5 Regional Tilt
Due to the many fault blocks that have activated on Raasay,
assuming the overall regional tilt is unreliable from a handful of
measurements – as a mitigator for this error several dip/strike
measurements were averaged to give a broad figure for each
formations dip/strike. Analysis of the dip/strike measurements
made on Raasay, shown in (Figure.7), show sediments all average
out to a concordant slope from which Raasay was concluded to
have an overall regional tilt of 20 degrees northwest. This tilt is thought to have developed with
the continued subsidence of the sedimentary basin after the deposition and lithification of Raasay’s
observed sedimentary sequence.
9.6 Interpretations and Discussion
The sedimentation on Raasay is interpreted to be controlled by a large extensional basin to the
northwest. The continued development of this basin post-lithification of the Raasay sediments
created large detachment faults such as the Beinn Na’ Leac fault to accommodate associated
stresses. Once a regional tilt of 20 degrees had established the Snow Granite formation intruded

42
and cooled producing-sub vertical jointing on contraction. The Battery formation and dykes
subsequently intruded through planes of weakness in the bedrock. The Eyre fault and other strike-
slip faults grew and accommodated the extension of this basin. The area was then subject to glacial
erosion forming the current topography. (Graham and Ryan, 2000) summarises some knowledge
of extension in the Inner Hebrides Basin to have developed during the Triassic and Jurassic. The
basin subsided on NNE-SSW oriented half graben structures delimited by the Minch fault. The
local Little Minch Basin in which the Raasay sediments also developed tectonically, however some
debate as to whether sediments were accommodated by faulting or thermal subsidence has been
disputed with seismic and gravitational data. A graphic diagram of this chronology is available in
Appendix.8.
10.0 ECONOMIC POTENTIAL
10.1 Introduction
The economic potential of Raasay’s geological resources is broadly devalued by the absence of
regular transport to and from the island. Resource extraction now and in the past has always
required the additional expense of ferrying or shipping material to the mainland. Material would
be difficult to remove from inland quarries as the existing road network comprises of small, single
track roads. Establishing new roads across the moorland that could accommodate large vehicles
would be expensive.
10.2 Water
Numerous freshwater lochs such as Loch a’ Mhuilinn (GR:NG:55268-36753) and other lochs
around Càrn nan Eun (GR:NG:55745-37525) are present on Raasay which once filtered can act as
a sustainable fresh-water drinking supply.

43
10.3 Peat
Peat bogs form in the upland valleys of central-southern Raasay, these can be used as fuel by the
immediate population but serve little use to the wider market as they are no longer a demandable
energy resource. The wet climate would also hinder drying this peat with traditional methods.
10.4 Energy
Raasay’s topography and steep rivers could be harnessed as a sustainable energy source. Although
the island is connected to the grid the power consumption could be lowered by installing wind
turbines across the western hills. The energy of the Inverarish Burn (which flows from
GR:NG:57478-38743 to GR:NG:55364-35653) passing through Inverarish could also be
harnessed with a small hydro power dam system as the impermeable granite bedrock is ideal for
retaining a lake.
10.5 Churchton Formation
As a Quartz Arenite this formation has the potential to be mined for use in glassmaking due to its
high quartz content (>95%) and easily removed calcite cement. The formation is widely exposed
and abundant across the hillsides of central-southern Raasay. Debris outcrops (Seen forming a
ridge in Plate.8.1) are abundant across the west face of Beinn Na’ Leac which are already of a
manageable scale (1-4m) to transport with heavy machinery, access to this area is difficult however
with no established roads currently present.
10.6 Iarann Formation
The ferrous iron content of the Iarann formation was successfully mined from mid-1916 to supply
iron during WW1 (Draper and Draper, 1990). Should the old infrastructure be rebuilt and replaced
material could be mined again, though the economic feasibility of this given the competitive

44
international market would make extraction relatively expensive. The old mine infrastructure is
present at Raasay No.1 Mine (GR:NG:56682-36719) and Raasay No.2 Mine (GR:NG:55820-3629)
– the latter was relatively unproductive in its active history though given the ease of transport from
these locations the site is worth re-evaluation to determine if it is workable with modern mining
techniques.
10.7 Rail Formation
As a carbonate rock the potential use of this lithology once crushed as a cement component is
possible. The proximity of the exposed outcrops (GR:NG:55569-34396) to the Old Ferry Terminal
(GR:NG:55478-34168), less than 250m, makes this source readily transportable as well. However
the interbedded muds may dilute this lithology too much to practically process and use on an
industrial scale when carbonates can be extracted from purer sources en-masse elsewhere in the
UK.
10.8 Snow Granite Formation
The Snow Granite formation is viable for use as road aggregate, paving and countertop slabbing
due to its high erosive resistance and appearance. Outcrops are well exposed and the formation is
abundant in uninhabited moorland – thick, easily-mined outcrops are present at (GR:NG:57941-
35860) close to the existing road network. Scree slopes of material are also present here, which
could be processed with a small rock crusher to provide an aggregate component for small road
repairs on the island.

45
10.9 Battery Formation
Basalt is also viable to use as road aggregate and gravel as it is highly resistant to erosion and
impermeable. Outcrops are well exposed across the west coast and could be removed and shipped
cheaply for export or processed on Raasay for local purposes.
11.0 GEOLOGICAL HISTORY
The Geological history of Raasay is summarised below:
Precambrian (Tonian, Neoprotezoic) Deposition of the Eyre formation age as a distal
alluvial fan.
- Paraconformity – ~500 Ma -
Permo-Triassic Deposition of the Gleann formation as series of coarse alluvial fan bodies.
- Paraconformity – ~50 Ma -
Lower Jurassic (Sinemurian) Deposition of the Rail formation in a shallow marine
environment.
(Late Sinemurian) Deposition of the FIM formation in a deep near-anoxic
environment.
(Pleinsbachian) Deposition of the Boulder Rock formation in a proximal
pro-deltaic fan.
(Toarcian) Deposition of the Leabaidh formation in a deep marine
environment.
(Toarcian) Deposition of the Iarann formation in a deep marine setting.

46
(Aalenian) Deposition of the Bhonaid formation in a foreshore environment.
(Aalenian) Deposition of the Churchton formation in the shallow marine
environment.
Paleogene Development of the Beinn Na’ Leac fault
Regional tilt of ~20 degrees to the north-west associated with the continued
opening of the Little Minch basin
Intrusion of the Dyke formation followed by the intrusion of the Snow
Granite formation and intrusion of the Battery formation sill.
Development of the dextral strike-slip faulting across Raasay such as the
Eyre fault and other extensive faults to accommodate further development
of the Little Minch Basin
Quaternary Period of glacial erosion to modern topography
Development of fissures along the Beinn Na’ Leac fault
12.0 CONCLUSIONS
The main conclusions of this study are summarised here below:
- Two significant paraconformities are identified between the Gleann and Eyre formations
(~500 Ma) as well as the Gleann and Rail formation (~50 Ma).
- Observations of sedimentary structures in the Eyre formation associated with a
marine/lacustrine environment indicate this alluvial fan prograded into water.
- Gravity flows within the Gleann formation are triggered by overloading at the proximal
fan, no evidence of climactic or tectonic forcing was interpreted from the field.

47
- Orbital forcing may influence the deposition of iron nodules within the F.I.M. formation,
cycles can be interpreted as eccentricity and precession but more extensive logging and
analysis is required to verify this.
- Fault cataclaysis of the immediate F.I.M. formation from the movement of the Beinn Na’
Leac fault dictated the path of the Allt Fearns.
- Large calcareous and later diagenic nodules are recognised to have grown within the
formation, these indicate the a long period of groundwater flow to accumulate.
- The Band Group, consisting of the Leabaidh, Iarann and Bhonaid formations, is interpreted
to represent a general TST-HST-FSST sequence following the regional deepening
evidenced in the F.I.M. formation.
- Regional NW tilting, associated with the further subsidence of the Little Minch Basin is
thought to predate the intrusion of Raasay’s igneous formations based upon the sub-vertical
orientation of cooling joints measured in the tilted sediments.
- Conchoidal fracture within the Beinn Na’ Leac fissures dates this extension to the
Quaternary period and may be associated with glacial activity during this time.
- The Churchton formation is identified as Quartz Arenite, an abundant silica lithology of
Raasay. The formation could be extracted for glass – however the percentage of carbonate
cement may be too great. Further thin section analysis is required to conclude its value.
- The Snow Granite and Battery formations are concluded to act as good aggregate materials
and may be processed and incorporated for local use on the islands infrastructure.
- Wind and/or Hydro power turbines could be established on Raasay to provide a sustainable
energy source.

48
13.0 REFERENCES
13.1 Dissertation References
Brittain, J., Higgs, K. and Riding, J. (2010). The palynology of the Pabay Shale Formation (Lower
Jurassic) of SW Raasay, northern Scotland. Scottish Journal of Geology, 46(1), pp.67-75.
Carroll, D. (1958). Role of clay minerals in the transportation of iron. Geochimica et
Cosmochimica Acta, 14(1-2), pp.1-28.
Davidson, C. (1935). XVI.—The Tertiary Geology of Raasay, Inner Hebrides. Transactions of the
Royal Society of Edinburgh, 58(02), pp.375-407.
Draper, L. and Draper, P. (1990). The Raasay Iron Mine. 1st ed. Dingwall: L. & P. Draper.
Gibb, F. and Gibson, S. (1989). The Little Minch Sill Complex. Scottish Journal of Geology, 25(3),
pp.367-370.
Graham, J. and Ryan, A. (2000). IAS Dublin September 2000. 1st ed. [Dublin, Ireland]: Dept. of
Geology, Trinity College Dublin, pp.41-58.
Hallam, A. (1966). Depositional Environment of British Liassic Ironstones Considered in the
Context of their Facies Relationships. Nature, 209(5030), pp.1306-1309.
Hesselbo, S., Oates, M. and Jenkyns, H. (1998). The lower Lias Group of the Hebrides Basin.
Scottish Journal of Geology, 34(1), pp.23-60.
Howarth, M. (1956). The Scalpa Sandstone of the Isle of Raasay, Inner Hebrides. Proceedings of
the Yorkshire Geological Society, 30(4), pp.353-370.

49
Hudson, J. (1983). Mesozoic sedimentation and sedimentary rocks in the Inner Hebrides.
Proceedings of the Royal Society of Edinburgh. Section B. Biology, 83, pp.47-63.
Kearsley, A. (1989). Iron-rich ooids, their mineralogy and microfabric: clues to their origin and
evolution. Geological Society, London, Special Publications, 46(1), pp.141-164.
Lee, G. (1920). The Mesozoic rocks of Applecross, Raasay and North-East Skye. 1st ed.
Edinburgh: HMSO.
Moorbath, S. (1969). Evidence for the age of deposition of the Torridonian sediments of north-
west Scotland. Scottish Journal of Geology, 5(2), pp.154-170.
Morton, N. (1965). The Bearreraig Sandstone Series (Middle Jurassic) of Skye and Raasay.
Scottish Journal of Geology, 1(2), pp.189-216.
Morton, N. (1999). Middle Hettangian (Lower Jurassic) ammonites from Isle of Raasay, Inner
Hebrides, and correlation of the Hettangian-lowermost Sinemurian Breakish Formation in the Skye
area, NW Scotland. Scottish Journal of Geology, 35(2), pp.119-130.
Scotlandscensus.gov.uk. (2017). Area Profiles | Census Data Explorer | Scotland's Census. [online]
Available at: http://www.scotlandscensus.gov.uk/ods-web/area.html [Accessed 18 Jan. 2017].
Stewart, A. and Donnellan, N. (1992). Geochemistry and provenance of red sandstones in the
Upper Proterozoic Torridon Group in Scotland. Scottish Journal of Geology, 28(2), pp.143-153.
Storetvedt, K. and Steel, R. (1977). Palaeomagnetic evidence for the age of the Stornoway
Formation. Scottish Journal of Geology, 13(3), pp.263-268.
Trewin, N. (2002). Geology of Scotland. 1st ed. London: The Geological Society, p.319, 397-400.

50
Williams, G. (1966). Palaeogeography of the Torridonian Applecross Group. Nature, 209(5030),
pp.1303-1306.
13.2 Additional Map & Software References
Allmendinger, R., Cardozo, N. and Fisher, D. (2012). Structural geology algorithms. 1st ed.
Cambridge: Cambridge University Press.
Cardozo, N. and Allmendinger, R. (2013). Spherical projections with OSXStereonet. Computers
& Geosciences, 51, pp.193-205.
Google.co.uk. (2005). Cite a Website - Cite This For Me. [online] Available at:
https://www.google.co.uk/maps [Accessed 19 Jan. 2017].
Inkscape. (2003). Sodipodi.
Microsoft Office Powerpoint 2013. (2013). Microsoft.
Microsoft Office Word 2013. (2013). Microsoft.
SedLog. (2009). Dimitrios Zervas, Royal Holloway University of London.
Stereonet 9. (2006). Richard W. Allmendinger.
14.0 APPENDICES
Appendices, as well as digital copies of this document, the figures, plates, models, dissertation
progress diary and clean copy map are contained within the attached CD at the back of this
document.
Printed copies of Appendix.1 to Appendix.9 follow overleaf:

54
Appendix.6 – Location Map of the Southern Raasay field area.

55
Appendix.7–StratigraphicColumnoftheRaasaySedimentarySequence.

56
Appendix.8–ModelfortheformationofRaasay.

57
Appendix.9 – Regional and fault movement.

58
Appendix.10–CrosssectionsofRaasay(seeAppendix.11forprofiles).

59
Appendix.11 – Clean copy map of Raasay.

The Geology of Southern Raasay

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