The Geology of South Raasay Dissertation

Jonathan Edwards The Geology of South Raasay
i
The Geology of South Raasay
Jonathan Edwards
Department of Earth Sciences, Durham University
2016
This Dissertation is submitted in Partial Fulfilment of the
Requirements for the Degree ‘F600, BSc Geology’

ii
Abstract
A 14km2 area of South Raasay was mapped over an eight-week period in the summer of
2015. Nine sedimentary and three igneous formations were recognised and mapped at
1:10,000 scale. The sedimentary basement is composed of Torridonian aged, braided river
sandstones and conglomerates that were deposited in an alluvial fan system. The alluvial
fan is interpreted to have prograded during the deposition of these rocks. The Torridonian
formations are unconformably overlain by Jurassic age, shallow to deep marine
mudstones and sandstones. Two marine transgressions and regressions are proposed to
have taken place, which correlate with other regional base level changes. Cyclicity was
identified within the Suisnish Mine Formation and it is hypothesised to have been
influenced by orbital mechanisms. Significant hematite alteration of the Beinn na’ Leac
cement was also observed, which is inferred to be a result of hydrothermal fluids exolved
from the adjacent granophyre. The sedimentary succession has been intruded by an
expansive granophyre sill, a micro-gabbroic sill, along with abundant basaltic NE-SW
trending dykes. These igneous rocks are suggested to be part of the North Atlantic
Igneous Province due to their similarity to other Inner Hebridean igneous bodies. During
the Tertiary, regional extension associated with the opening of the North Atlantic resulted
in the development of a series of prominent extensional dip-slip, and conservative strike
slip faults. Many of these faults show strain partitioning relationships such as the Beinn
na’ Leac Fault. Recent, quaternary glaciation is identified by the presence of erratics and
glacial till. Raasay shows little economic promise due to low economic mineral
abundance and high export costs.

iii
Table of Contents
Abstract ii
Table of Contents ii
Figures iii
Acknowledgements iii
Chapter 1: Introduction 1
Chapter 2: Stratigraphy 2
2.1 Sequence Stratigraphy of Jurassic Sediments 2
Chapter 3: Sedimentology 4
3.1 Eyre Point Formation 5
3.2 Boulder Valley Formation 6
3.3 Suisnish Mine Formation 9
3.4 Suisnish Beach Formation 12
3.5 Na Fèarns Formation 15
3.6 Ribbon Formation 16
3.7 River Footpath Formation 18
3.8 Borodale Forest Formation 19
3.9 Beinn na’ Leac Formation 20
Chapter 4: Igneous Geology 24
4.1 Càrn nan Eun Formation 24
4.2 Osgaig Point Formation 26
4.3 Raasay Dyke Complex 11
Chapter 5: Structure 30
5.1 Introduction 30
5.2 Brittle Deformation 30
5.2.1 Beinn na’ Leac Fault 30
5.2.2 Osgaig Fault and Suisnish Fault 32
5.2.3 Eyre Fault 32
5.2.4 Other Faults 33
5.3 Dyke Emplacement 33
Chapter 6: Geological History and Economic Potential 34
6.1 Geological History 34
6.2 Economic Potential 35
Chapter 7: Conclusions 37
References 39
Appendix 1: Arran Fieldwork 40
Appendix 2: Raasay Clean Copy Map 46

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List of Figures
Figure 1: Geometry of Raasay Island. 1
Figure 2: Sequence Stratigraphy Interpretation of Jurassic Raasay Sediments 2
Figure 3: Geological Column of the Raasay Sedimentary Succession 4
Figure 4: Sketch log of Eyre Point Formation 5
Figure 5: This section under cross-polarised light showing the Eyre Point
Formation Mineralogy 5
Figure 6: Sketch log of the Boulder Valley Formation 7
Figure 7: Image showing the Boulder Valley Formation at outcrop scale 7
Figure 8: Rose diagram showing clast orientations. Note the E-W modal
abundance along with the spread of data 7
Figure 9: Boulder Valley Formation Depositional Environment Block Diagram.
Raasay succession indicated by red pole 8
Figure 10: Image showing the paraconformity between the Eyre Point
Formation (below) and the Boulder Valley Formation (above). 8
Figure 11: Sketch log of Suisnish Mine Formation 9
Figure 12: Stereonet showing bivalve pole orientations. Note the high density
of points close to the great circle. 10
Figure 13: Image Showing Calcified Bivalves within the Wackestone Units 11
Figure 14: Field Sketch of Suisnish Mine Formation 11
Figure 15: REDFIT spectral analysis curve of the Suisnish Mine Formation with
a 95% chi2 significance line plotted in green. The two statistically
significant peaks are highlighted in blue 12
Figure 16: Wavelet spectral analysis plot for the Suisnish Mine Formation with
a cone of influence plotted 12
Figure 17: Sketch log of Suisnish Beach Formation 14
Figure 18: Sketch log of the Raasay Ribbon Group, showing the three
constituent formations. 18
Figure 19: View of the hillside at locality 156 where gradient changes have
allowed the reconstruction of concealed stratigraphic contacts. 18
Figure 20: Thin section of the Beinn na’ Leac Formation showing hematite
mineralisation (accented by red shading). 22
Figure 21: Stereonet showing joint plane as poles of the Càrn nan Eun Formation 26
Figure 22: Image showing contact between the Suisnish Beach Formation and
Càrn nan Eun Granite. Note the feeder dykes present beneath the sill 26
Figure 23: Rose diagram showing joint plane strikes of the Càrn nan Eun
Formation 26
Figure 24: Thin section under cross polarised light of the Osgaig Point Formation 28
Figure 25: Field sketch of Columnular Jointing in the Osgaig Point Formation 28
Figure 26: Image Showing Concentric Pillow Lava within the Osgaig Point
Formation. 28
Figure 27: Field sketch of the Raasay Dyke and Sill Complex 29

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Grid References and Names
Four to six figure grid references are provided in brackets after locations using the notation
(GR: 070 295). The Ordinance Survey code for Raasay (NG) has been omitted from these
grid references, as it remains the same for all locations. All bearing measurements were made
with a magnetic declination of -3°. Location and formation names are italicised (e.g. Beinn
na’ Leac Formation).
Acknowledgements
The author would like to thank Professor Mark Allen for his guidance and support during this
study. Gratitude is also expressed towards the Department of Earth Sciences at the University
of Durham, for providing financial support in the undertaking of this research and supplying
necessary health and safety training. The hospitality of Raasay House is also acknowledged,
without which this project may have not been possible.
Figure 28: Schematic map showing the major fault names and their
displacements in South 31
Figure 29: Schematic map showing the structure present at Beinn na’ Leac.
Strain partitioning between dip-slip and strike-slip displacement is
evident. Note the distribution and consistent strikes of the fissures. 32
Figure 30: Block diagram showing relative motions of the Càrn nan Eun,
Osgaig Point, and Inverarish Forest Fault Blocks. 33
List of Plates
Plate 1: Suisnish Beach Formation
1) Field sketch of the Suisnish Beach Formation’s concretions
2) Image showing concretions in the Suisnish Beach Formation
3) Image showing external cast of ammonite in the Suisnish Beach
Formation 15
Plate 2: Beinn na’ Leac Formation
1) Image of iron mineralisation, accented in red, showing its
curvilinear nature along a slump structure
2) Image of hand specimen showing dendritic iron mineralisation
3) Image of bivalve external cast. Note its size and ribbed morphology 23

1
Chapter 1: Introduction
The Isle of Raasay is a small landmass, located in the Inner Scottish Hebrides. Situated ~3km
off the East coast of the Isle of Skye, the island is 21km long and occupies a total area of
~63km2 (figure 1). It has a total population of ~160 people with the majority of the
populous concentrated in the village in Inverarish (GR: 5554, 3571), which is
positioned on the SW coast of the island. Raasay’s terrain is rugged, but
never mountainous, with large hills such as Càrn nan Eun (GR: 5581,
3753) and Beinn na’ Leac (GR: 5923, 3671) providing good rocky
exposure. The low-lying Borodale Wood (GR: 5569, 3660), to the
South, offers less exposure due to the vegetation cover, and private
land ownership in Inverarish makes exploring this region difficult. The
South of the Island is characterised by several NW/SE trending normal
faults, which have caused deep gashes in the landscape. These faults
have been exploited by glacial activity and therefore the topographic
features are often infilled with marshy, quaternary material and can be difficult to traverse.
Due to the island’s location on the west coast of Scotland, precipitation and strong winds
occur regularly make fieldwork challenging. A southerly region of the island covering an area
of ~14km2 was mapped at 1:10,000 scale over an 8-week period, from the 13th of August, to
the 7th of October, in the year 2015.
This report aims to present a comprehensive overview of the geology of Raasay, using
observations made during the surveying of the area. These observations will then be linked to
the primary scientific literature in order to gain a deeper understanding of how the island’s
geology relates to its regional context.
Figure 1: Geometry
of Raasay Island
South Raasay
(Mapping Area)
North
Raasay
N
1km

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Chapter 2: Stratigraphy
2.0 Introduction
The stratigraphy of Raasay consists of Torridonian fluvial sandstones and conglomerates,
capped by a paraconformity, with Jurassic marine sandstones and mudstones resting on top
and a roof of conformable granophyre sill. The main boundary of note is the paraconformity,
which represents and estimated time gap of ~700Ma.
2.1 Sequence Stratigraphy of the Jurassic Sediments
Four sequence boundaries have been interpreted for Raasay’s Jurassic sedimentary succession
(figure 2). These are defined by abrupt changes in the sedimentary facies, usually associated
with base level transgression and a depositional hiatus. The basal boundary of the first
sequence is the paraconformity, which separates the Torridonian basement from the Jurassic
sediments. This sequence is dominated by shelf slope facies that fines upwards over the
course of a Highstand Systems Tract (HST), bound by a maximum flooding surface at its top.
This HST encompasses the Suisnish Mine Formation. The next sequence represents a marine
regression from shelf slope to shallow marine facies in the form of a Regressive Systems
Tract (RST). This includes the Suisnish Beach, Na Fèarns, and Ribbon Formations. Following
on from this, there is a short-lived Transgressive Systems Tract (TST), returning the
depositional environment to a shelf slope facies. This allowed for deposition of the River
Figure 2: Sequence Stratigraphy Interpretation of Jurassic Raasay Sediments

3
Footpath and Borodale Forest Formations. Finally, the uppermost sequence represents a
Lowstand Systems Tract (LST) which resulted in the deposition of the Beinn na’ Leac
Formation (figure 2). These sequence stratigraphic interpretations correlate with Morton
(1989), who also identified four systems tracts over this timeframe; A, B, C, and D (second
column, figure 3).

4
Chapter 3: Sedimentology
3.0 Introduction
Raasay presents a diverse sedimentological record, which indicates several changes in the
geological history of the island. Both carbonate and siliciclastic sediments are represented
across nine formations ranging from fluvial conglomerates, to laminar marine mudstones. The
oldest sequences are the Torridonian age, ‘Eyre Point’ and ‘Boulder Valley’ formations.
These successions were deposited under unidirectional, fluvial
conditions of moderate to high energy, in braided river and
alluvial fan environments respectively. They therefore show
features such as channelized sediments and imbrication. A
significant paraconfomity segregates these sediments from
much younger, Jurassic age sediments on top. These Jurassic
sediments were deposited under much lower energy conditions
in a shallow to deeper marine setting. They therefore
occasionally contain abundant marine fauna including
ammonoids and bivalves. The oldest Jurassic formation is the
‘Suisnish Mine’ formation, of Sinemurian age, which is
deposited on top of the dominant paraconfomity. This is
followed by the slightly younger ‘Suisnish Beach’ formation,
which is of particular interest as it shows a cyclic fluctuation
between a shallow marine, wackestone producing depositional
environment, with a deeper marine, laminar mudstone producing depositional environment.
‘Na Fèarns’ formation, ‘Ribbon formation’, ‘Borodale formation’, and finally ‘Beinn na’
Leac’ formation at the top of the sedimentary succession of Aalenian to Bajocian age. Over
the course of the deposition of the observed Jurassic sediments, there are two main cycles of
base level transgression and regression, which accounts for the variation in grain size of the
Figure 3: Geological
Column of the Raasay
Sedimentary Succession

5
sediments, and changes in the apparent biofacies. Recent Quaternary deposits are glacial in
nature consisting of erratics and till material that have infilled areas of topographically low
relief.
3.1 Eyre Point Formation
3.1.1 Spatial Distribution and Outcrop Style
The ‘Eyre Point Formation’ outcrops along the southernmost coast of Raasay from Rubha na
Cloiche (GR: 5629 3372) to the road at North Fèarns (GR: 5891 3547) occupying and
approximate area of 0.7km2. The formation tends to form small
ledges (2m-6m high) along the coast due to the steep gradients with
particularly good outcrops along the EW road that runs along the
south coast. Exposure is limited at South Fèarns due to a steep
NE/SW trending slope, covered by a dense deciduous forest. The
Eyre Point Formation was observed at 9 different localities across
the mapped area.
3.1.2 Lithology and Structural Observations
The Eyre Point Formation (figure 4) consists of a clast supported,
medium-sand grade arkose with sub-rounded clasts. The majority of
the formation is well sorted, however there are some anomalous
pebble-grade clasts present that are much larger than the
surrounding matrix. There are also uncommon, poorly sorted,
lenticular units present that are ~4cm thick and up to ~20cm
wide. The mineralogy of the formation is dominated by
quartz (51%) and feldspar (orthoclase (38%) and microcline
(5%)), which is accompanied by muscovite (2%) and biotite
Figure 4: Sketch log
of Eyre Point
Formation
Figure 5: This section under
cross-polarised light showing the
Eyre Point Formation Mineralogy

6
(4%) along with opaque minerals such as hematite (<1%) and ilmenite (<1%) (figure 5).
Cross bedding is observed at many horizons in the formation and shows an overall eastward
palaeocurrent. The high proportion of orthoclase gives the lithology and overall pink colour.
The Eyre Point Formation is very competent and well joined (~17cm spacing) with some
calcified veins.
3.1.3 Regional Context and Interpretations
The Eyre Point Formation shows classic sedimentary structures associated with a braided
river system on an alluvial fan such as cross bedding and channelized sediments. The well
sorted nature would indicate that these sediments were deposited in a fairly distal part of the
alluvial fan, however, the prominent feldspar component to the clasts show that deposition
occurred close to the sediment source. Williams (2001) suggests that these sediments were
derived from the footwall of the extensional Minch fault scarp. This fault also would provide
a gradient change to allow the deposition of the alluvial sediments. Selley (1965) classified
the Torridonian rocks of Raasay into three main facies; red facies, grey facies, and basal
Facies. The observations presented above as well as the stratigraphic location of the Eyre
Point Formation suggests that it belongs to the ‘red facies’, which is also interpreted to be a
braided river depositional environment (Selley, 1965).
3.2 Boulder Valley Formation
The ‘Boulder Valley Formation’ is exposed along the southern flanks of Suisnish Hill to the
North of the road, from Braemore to South Fèarns. It occupies an area of approximately
0.8km2 over two blocks that are offset dextrally by the Eyre Fault. The terrain is steep with
outcrops exposed in cliff sections ranging from 1m to 11m high. Much like the Eyre
Formation, exposure is limited on the East coast of the island, due to a steep incline covered

7
by deciduous forest that runs from Eyre to South Fèarns. The Formation was observed at 11
localities across the mapped area.
The Boulder Valley Formation (figures 6 & 7) is composed of poorly sorted polymict
paraconglomerates (~70% matrix) with a channelized base
and four clast compositions: arenite (39%), arkose (45%), a
schistose metamorphic lithology (15%), and a white
metamorphic lithology (1%) (See figure %). Clasts are well
rounded and very poorly sorted, ranging in size from 2mm
to 86mm. The matrix is sand grade and is similar in
composition to the arkosic Eyre Point Formation. Some
lenticular shaped units are present which are composed
exclusively of matrix material. These range in thickness
from ~1m thick (e.g. locality 63) to ~15cm thick (e.g.
locality 125) and tend to show parallel laminations at their base. The clasts in the
conglomerate show an overall E-W alignment, however there is notable spread in the data
(figure 8). The formation is absent of any fossil.
Figure 6: Sketch log of the
Boulder Valley Formation
Figure 8: Rose diagramshowing
clast orientations. Note the E-W
modal abundance along with the
spread of data
Figure 7: Image showing the Boulder
Valley Formation at outcrop scale

8
3.2.3 Interpretations and Regional Context
The Boulder Valley Formation was likely deposited under high energy, fluvial conditions
within river channels. This provides an explanation for the poor sorting of the conglomerates,
rounding and apparent E-W alignment of the clasts, and the presence of lenticular shaped
units. The high degree of variation of clast
alignments (figure 8) is likely due to
deposition within a braided system on top of
an alluvial fan (figure 9), where flow
direction is observed to vary as a fan matures.
The coarse nature of the clasts indicates that
deposition occurred proximal to the source
of the sediment of the alluvial fan, a great
distance from the fan margin. This also explains
the high orthoclase component within the
matrix. The parent rock for the metamorphic
and arenitic clasts is not recognised across the
mapped area, however it is likely that some of
the metamorphic material was derived from the
Lewisian Complex that outcrops to the North of
the Screapadal Fault and underlies the Eyre
Point Formation (e.g. Storetvedt and Steel,
1977). The Boulder Valley Formation is formally known as the Stornoway Formation and
there is significant debate in the literature (Morrison, 1887; Woodward, 1914; and Storetvedt,
1977) concerning the formation’s age due to a lack of fossil material and the paraconformity
present at its base. The main age proposals are Torridonian, Devonian and Permo-Triassic.
Figure 9: Boulder Valley Formation
Depositional Environment Block Diagram.
Raasay succession indicated by red pole
Figure 10: Image showing the
paraconformity between the Eyre Point
Formation (below) and the Boulder Valley
Formation (above)

9
Observations from this study suggests that the Torridonian age interpretation is most likely
due to several reasons:
- The mineralogy and interpreted depositional environment is similar to that of the Eyre
formation. This infers that both formations were derived from a similar sediment
source, and were deposited in a similar setting - such as a prograding alluvial fan.
- The Boulder Valley Formation is characteristically dissimilar to the Jurassic, marine
sedimentary rocks found in South Raasay. Storetvedt (1978) argues that this is
because the formation is an onshore representation of the Mesozoic marine succession,
however, the compositional difference makes this unlikely.
- An undulating paraconformity was observed between the Boulder Valley Formation
and the Suisnish Mine Formation, indicating that a significant time gap had occurred
before the Jurassic.
3.3 Suisnish Mine Formation
The Suisnish Mine Formation outcrops across a locally confined area (~0.3km2) on Suisnish
Hill close to a disused iron hopper that was used for the iron mine works (GR: 5555, 3432). It
is also exposed in South Fèarns (GR: 5815, 3498), however this area is
inaccessible due to a steep, densely forested slope. The best outcrops
occur as cliff sections ~8m high that run parallel with, and on the east
side of the disused railway (GR: 5566, 3464). The formation was
observed at five localities across the mapped area.
The Suisnish Mine Formation (figures 11 and 14) consists of an
interbedded succession of fine grained, dark grey, laminar siltstones
Figure 11: Sketch
log of Suisnish
Mine Formation

10
Figure 12: Stereonet showing
bivalve pole orientations.
Note the high density of points
close to the great circle
with poorly sorted, sparse biomicrites/wakestones. Bedding is planar with units varying in
thickness from ~10cm to ~140cm. The siltstones have a micaceous mineralogy (~10%) which
is very similar to the composition of the wackestone matrix. However, the wackestone units
sometimes show cross bedding and contain large (~4cm) calcified bivalves with prominent
concentric growth lines (figure 13). Ammonoids are also present, although rare. The bivalves
all have similar orientations (figure 12) and are found in the greatest abundance at the base of
the wackestone units (figure 13). Sub-vertical joints (112/78N) are present which cross cuts
the bedding planes however; jointing is more abundant in the more competent laminar
siltstones.
The Suisnish Mine Formation was likely deposited within a marine basin due to the presence
of ammonoids and bivalves. The low energy features such as parallel laminations indicate that
the height of the water column was substantial, though the
presence of bivalves infers that deposition still occurred
within the photic zone. The abundance muscovite and
siliciclastic matrix shows that these rocks were deposited
fairly close to a continental sediment input, such as on a
continental shelf. The Suisnish Mine Formation is known
formally in the literature as the Ardnish Formation and Lee
Figure 14: Field Sketch of
Suisnish Mine Formation
Figure 13: Image Showing Calcified
Bivalves within the Wackestone Units

11
(1920) compared the rocks observed in South Raasay with sections at Hallaig Shore and
classified these rocks as Sinemurian age (lower Jurassic) using biostratigraphy.
3.3.4 Cyclic Analysis
The Suisnish Mine Formation represents a rapid changing depositional setting from
wackestone producing, shallow marine environment, to laminar siltstone producing, deeper
marine environment. 10 cycles are observed in South Raasay are three possible interpretations
for these changes: fluctuating; (1) energy, (2) sediment input, and (3) oxygen levels, or
perhaps a combination of the three. Climatic and/or base level allocyclic controls are
potentially therefore a dominant force on the sedimentation.
(1) Fluctuations relating to the energy of the water column associated with base level changes
could lead to the deposition of this formation. During periods of acquiescence, the bivalve
community could flourish and be deposited with a mudstone matrix. During waters that are
more turbulent however, the organisms are less adapted, and so do not survive, while slightly
coarser grained, thicker siltstone units are deposited in their place.
(2) Variations in sediment influx is an alternative hypothesis. Due to the filter feeding nature
of bivalves, an increase in sediment flux and deposition would result in the suffocation of
these organisms and a deposition of thicker units, absent of bivalves. Contrastingly, when
sediment influx was low, bivalve communities could develop and form the bivalve dominant
units.
(3) Cyclicity of the oxygen state of the sediment associated with base level fluctuation could
also allow the deposition of this formation. Transgression would lead to anoxia of the
sediments and the deposition of bivalve free units, while regression would allow bivalve
respiration and the deposition of wackestone.

12
When deposition rate for the formation is assumed broadly constant at outcrop scale, time can
be used as a proxy for height up the section. Therefore, the cyclic nature of the sediments can
be analysed using spectral frequency analysis. Redfit analysis has identified two dominant
frequencies that are statistically significant that may have resulted in the deposition of these
sediments (Figure 15). Wavelet analysis shows a similar bimodal distribution with a dominant
low frequency signal, along with a higher frequency component (figure 16). Wavelet analysis
also shows a slight temporal change in the higher frequency cycles, though this may just be an
artefact of the cone of influence or due to the small dataset size. Unless the bedding planes of
the formation are dated by absolute means, it is not possible to calculate a value for the
frequency of the two cycle cycles, though they can be correlated with other research that has
been done on the subject.
Nhnhn
3.4 Suisnish Beach Formation
The Suisnish Beach Formation occurs in two locations across the South of the island. A small
area (~0.6km2) outcrops along the South West coast from Suisnish to Suisnish Point (GR:
5518, 3532), and a much larger area (~1.2km2) is present to the West of the Beinn na’ Leac
fault, along the road (GR: 5813, 3627). Outcrops tend to be floor building, because of the
Left - Figure 15: Redfit spectral analysiscurve of the Suisnish Mine Formationwith a 95% chi2
significance line plotted in green. The two statistically significant peaks are highlighted in blue
Right - Figure 16: Wavelet spectral analysis plot for the Suisnish Mine Formation with a cone of
influence plotted

13
formation’s soft nature and its shallowly dipping bedding planes. With that said, the
formation has been deeply incised by the Allt Fearns River providing good vertical sections.
The Suisnish Beach Formation was observed at 22 localities across the mapped area.

14
The formation (figure 17) consists of a dark grey, finely laminated
siltstone containing red coloured nodular structures (plate 1.1 and 1.2)
and ammonoids (plate 1.3). The mineralogy of the rocks is difficult to
identify, though an abundance of muscovite mica (~10%) is recognised.
The siltstone is very soft and crumbles easily. Planar bedding varies in
thickness from a few millimetres to ~10cm thick, however most units
are at the lower end of this scale. Bedding planes dip consistently by
~20°, though the strike is ~30° different between outcrops to the NE
and outcrops to the SW of the island. The red coloured concretions
make up ~5% of the formation and are fine-grained. These features are
also much harder and denser than the surrounding siltstone and show
concentric layering. Multiple concretions often occur along the same
bedding plane and surrounding bedding planes appear to deflect around the structures. The
ammonoids observed in the formation are only ~5.2cm in diameter and show little
Figure 17: Sketch
log of Suisnish
Beach Formation

15
ornamentation except for some minor ribbing. In terms of structures, the formation contains
one consistent joint plane throughout (097/82S) which are spaced ~14cm apart. However, the
joints do not intersect more than a few tens of beds and are minor features.
The Suisnish Beach formation was likely deposited within a similar marine setting to the
Suisnish Mine formation; however, the abundant bivalves and wackestones are no longer
present. A hypothesis that explains this change is a marine transgression, which has resulted
in the depositional environment becoming too deep and stagnant for the bivalves to survive.
This would also explain the greater abundance of ammonoids as the biofacies has shifted from
an ‘inner shelf’ environment to a more ‘outer shelf’ environment. The Suisnish Beach
formation is known formally in the literature as the Pabay Shale Formation, which has been
Plate 1: Suisnish Beach
Formation
1) Field sketch of the Suisnish
Beach Formation’s concretions
2) Image showing concretions
in the Suisnish Beach
Formation
3) Image showing external cast
of ammonite in the Suisnish
Beach Formation
1 2
3

16
biostratigraphically dated using dinoflagellate cysts (Brittain et al., 2010) and ammonites
(Oates, 1978) as upper Sinemurian age. The transgression that is interpreted to have led to the
deposition of this formation is therefore likely a local expression of the regional deepening
that took place during the early Sinemurian (Hesselbo and Coe, n.d.). At this time, many
basement highs across the country were permanently submerged beneath the Jurassic sea such
as the Ordovician-age Durness Limestone (e.g. Farris et al., 1999). In terms of the
concretions, the features are likely syn-depositional, which explains the deflected bedding.
The red colour of the concretions is likely a result of iron mineralisation, which is also
reflected by density and hardness. Therefore, these features would most likely have formed
within the sulphate reduction zone.
3.5 Na Fèarns Formation
The Na Fèarns Formation is a spatially expansive formation that outcrops over an
approximate area of 1.4km2 across the South of Raasay. It is situated along the East coast of
the island at North Fèarns (GR: 5952, 3609), as well as further West, throughout the central
part of the island until Inverarish Hotel (GR: 5645, 3644). Prominent exposure is situated
along Inverarish Burn (GR: 5653, 3699) where the river has stripped away superficial
deposits, however steep valley sides makes this area inaccessible. Exposure is very poor on
the hill to the East of Inverarish Burn (GR: 5771, 3655) because of thick glacial deposits,
though its presence is inferred. Outcrops are in the form of 5m-8m cliff sections and the
formation was observed at 12 localities across the mapped area.
The Na Fèarns Formation is dominated by medium-sand grade, quartz arenite sandstones,
with some siltstone interbeds. The sandstones are thickly bedded (>1m), while the siltstone
units are much thinner (~30cm). The formation is generally well sorted and the clasts are sub-

17
rounded in shape, though there is an overall coarsening upwards trend throughout the units.
Concretions, similar to those observed in the Suisnish Beach Formation are present, though in
a much lower abundance and are dark grey in colour as opposed to rusty red. The mineralogy
of the Na Fearns Formation is mostly quartz (>95%), though the rock’s weathered surfaces
are darker brown in colour which is not what was observed from the other quartz arenite
formation on the island, the Beinn na’ Leac Formation. Hummocky cross stratification is
dominant in places such as at locality 145, where the sediments appear to be subdivided into
discreet ‘packets’. Calcified solitary corals are present within the sandstone units, which have
a diameter of ~0.5cm. These specimens show fibrous radial structures that could be
interpreted as septa. Regarding structure, few joints were present within the formation (~8m
spacing) as it is less competent than other sedimentary lithologies in the area. At locality 67
some slicken-lines were present in the Na Fearns Formation, and this is hypothesised to be
due to the nearby strike-slip fault (see Chapter 5).
The Na Fèarns Formation is hypothesised to have been deposited within a marine
environment; however, this environment is much shallower to the one that created the
underlying Suisnish Beach Formation. This is because the grain size of the rocks is coarser
than the Suisnish Beach siltstones and an overall coarsening upwards is recorded in the
succession. The presence of corals indicates that the formation was deposited proximal to a
reef system, and therefore within the photic zone and above the storm-wave base. This
provides an explanation for the hummocky cross-stratification that has developed due to
bilateral wave action. The Na Fèarns formation is known formally in the literature as the
Scalpay Sandstone Formation; however, relatively less research has been conducted on the
sequence. This is likely due to their inaccessible distribution on the island.
3.6 Ribbon Formation

18
The Ribbon Formation is the oldest member of the Raasay Ribbon Group (figure 18), which
is a thin group of three sedimentary formations that spans a few tens of metres. In map view,
the feature curves with the topography much like a piece of ribbon. Prominent faulting of this
feature has resulting the creation of five main blocks, which are
situated from Borodale Wood (GR: 5575, 3618) to the east coast of
the Island. The Raasay Ribbon Group is particularly visible in the
hillside at locality 156 (figure 19), where gradient changes show the
contact relationships. Outcrops of this Ribbon Formation are often
small ledges (~2m2) and are poorly preserved because of weathering.
The Ribbon Formation outcrops over an area of approximately
0.2km2 and was observed at just 3 localities across the mapped area.
The Ribbon Formation consists of well-sorted, matrix supported,
rusty red coloured oomicrite, which is interbedded with darker grey
mudstones. The ooids are pale cream in colour and both ovular and
concentric in nature. The ooids consist of ~30% of the mineralogy.
Muscovite clasts are also present (~25%), while the remaining ~45% is composed of a fine-
grained calcareous mud. Graded bedding is present in the oolite on centimetre scales. Bedding
is planar (dipping 171/16W) and units vary in thickness from ~1.5cm to ~10cm.
Figure 19: View of the hillside at locality 156 where gradient changes have allowed the
reconstruction of concealed stratigraphic contacts.
Figure 18: Sketch log
of the Raasay Ribbon
Group, showing the
three constituent
formations

19
3.6.3 Interpretation and Regional Context
The Ribbon Formation’s depositional environment is not easily interpretable. The presence of
öoids would indicate that the formation was deposited within a low latitude shallow marine
environment, where the water would be sufficiently agitated and warm enough to allow the
precipitation of calcite layers about a nucleus. However, this energy level is not reflected in
the muddy matrix which was likely deposited under much lower energy conditions. As a
result, it is hypothesised that the öoids are allochthonous, and were transported from a shallow
marine environment into deeper conditions. The mechanism for this transportation is likely
weather anomalies, such as storm events, which makes cyclic analysis redundant. This would
explain why some units are absent of öoids and the observed centimetre scale graded bedding.
Kearsley (1989) classified the öoids as secondary B4 subclass, because they contain fragile
kaolinite structures that would not withstand the abrasion involved with agitated formation.
As a result, weathering or diagenetic alteration of the original öoids has taken place to
produce these structures. It is probable that the Ribbon Formation marks the start of the
Toarcian sedimentary succession on Raasay as the organic rich and fine grained nature of the
formation characterises the early Toarcian anoxic oceanic event. The Ribbon Formation is
referred to formally as the ‘Raasay Ironstone Formation’, and it is well known for its high
iron content and extraction for ore leading up to and during the First World War (e.g. Draper
and Draper, 1990).
3.7 River Footpath Formation
The River Footpath formation (figure 18) is the intermediate member of the Raasay Ribbon
group. It therefore bears a similar spatial distribution to the Ribbon formation. It occurs
throughout the centre of the mapped area from Borodale Wood (GR: 5619, 3658) to the hill
West of Beinn na’ Leac (GR: 5749, 3690), and on the East coast of the island. The outcrop

20
style is of laterally continuous wall sections that tend to form the Eastern limit of Borodale
Forest due the topographic change associated with abrupt lithological change. The formation
is not very thick and therefore occupies a surface area of just 0.2km2. As a result, this
lithology was only observed at 5 localities across the South of the island.
The River Footpath formation presents a lithology consisting of very fine-grained, laminated
micaceous mudstones. Bedding is often only a few millimetres thick, and the well-developed
laminated texture lends the rock a slight cleavage. The lithology is also very soft, preserving
fingerprints when handled in hand specimen. Muscovite clasts are present in the lithology
(~5%) surrounded by a mud matrix (~95%), and the rock has an overall dark grey to black
appearance.
The very fine grained size of the River Footpath Formation would indicate that this formation
was deposited in very stagnant, low energy conditions. Although no marine fossils were
observed, it’s stratigraphic position would suggest that the palaeoenvironment was still a
marine basin.
3.8 Borodale Forest Formation
The Borodale Forest Formation (figure 18) is the youngest member of the Raasay Ribbon
Group and has a very similar distribution to the River Footpath and Ribbon Formations.
Outcrops tend to be in the form of laterally extensive cliffs between 3m and 6m high. These
cliffs can extend to up to 20m wide and mark the Eastern limit of the Borodale Forest. The
formation outcrops over a surface area of just 0.2km2 and was only observed at 5 localities
across the mapping area.

21
The lithology of the Borodale Forest Formation consists of quartz arenite, similar in
appearance and composition to the Na Fèarns Formation. Parallel bedding is well developed
at the base of the section, while cross bedding is present at the top of the formation. An
overall coarsening upwards trend is also observed from a medium-sand grade to coarse-sand
grade. The mineralogy is mostly quartz (>95%) with some biotite clasts present in low
abundance. The formation’s overall appearance is pale grey.
Due to the similarities with the Na Fèarns Formation, the interpretation for the Borodale
Forest Formation is very similar. A shallow marine environment is inferred to have produced
the texturally mature, sand-grade sediments. An overall increase in the energy environment is
observed which is illustrated by the shift from parallel bedding to cross bedding and
coarsening up section. The Borodale Forest Formation is known formally in the literature as
the Beinn na’ Leac Sandstone Member as part of the Bearreraig Formation.
3.9 Beinn na’ Leac Formation
The Beinn na’ Leac Formation is the most expansive sedimentary formation that occurs on
South Raasay, outcropping over an approximate area of 3.4km2. It forms the bedrock of the
prominent Beinn na’ Leac hill on the East coast of the island (GR: 5915, 3662) and fringes the
southern margins of the Càrn nan Eun granophyre (e.g. GR: 5593, 3706) and Osgaig Point
micro-gabbroic sill (e.g. GR: 5504, 3680). The formation is therefore offset by many of the
strike-slip faults that penetrate the Càrn nan Eun granophyre including the Eyre Fault. The
outcrop style of this formation is greatly dependent on the location. To the West of the island,
the outcrops are floor building making it difficult to identify temporal changes in the rock’s

22
Figure 20: Thin section of the
Beinn na’ Leac Formation
showing hematite mineralisation
(accented by red shading)
deposition. To the East on Beinn na’ Leac, on the other hand, very high cliffs up to 20m are
present along with treacherous fissures that make exploring this area quite hazardous.
Contacts between the Beinn na’ Leac Sandstone Formation and the Càrn nan Eun Granite are
very easy to identify, as the vegetation that grows on the sandstone is very grass dominated as
opposed to bracken dominated vegetation that grows on the granite. The Beinn na’ Leac
Formation was observed at 38 localities across the mapped area.
The Beinn na’ Leac formation consists of thick units of cross-bedded, medium grained,
arenitic sandstones, with a mature, well-sorted mineralogy dominated by quartz (>95%) and
some detrital orthoclase. Overall, the rock has a rusty red
appearance with a pale grey weathered surface. At
many localities, dark red veins are present (plate 2.2)
which are dendritic and curvilinear in nature. In cross
section, these veins show alteration of the calcite
cement to hematite (figure 20) and there is no
difference in grain size across the boundaries.
Furthermore, the formation shows features commonly
associated with dissolution on its weathered surfaces,
such as honeycomb weathering and channels. This is likely because of a calcite component to
the rock’s cement. Fossils are also present in low abundance such as at locality 14, where a
large (13.5cm length from umbo to anterior margin) bivalve external cast is present (plate
2.3). The specimen shows significant ribbing and has a jagged anterior margin. In terms of
structure, the formation is competent and contains many sub-vertical joints, which have
allowed large blocks to fall from outcrops under gravity. The formation dips ~20° degrees

23
NEE, though this varies slightly across the island as the formation has been offset and rotated
by a series of oblique slip faults (see Chapter 6).
The presence of marine fossils shows that the depositional environment is still marine;
however, the energy conditions are much more significant than previously observed in the
Raasay Ribbon Group. This indicates the reverting to a depositional environment similar to
what produced the Na Fèarns Formation. This interpretation can be presented due several
factors. These include the ornamentation and size of the fossils, medium-sand grade grain size
of the clasts, and presence cross-stratification. The dark red veins that occur throughout the
formation are probably post-depositional as there is no variation in grain size and the
compositional difference takes place in the cement rather than the matrix or clasts. Therefore,
Beinn na’ Leac Formation Plate
1) Image of iron mineralisation,
accented in red, showing its curvilinear
nature along a slump structure
2) Image of hand specimen showing
dendritic iron mineralisation
3) Image of bivalve external cast.Note
its size and ribbed morphology
3
21

24
it is hypothesised that the veins resulted from fluid migration through the rock that dissolved
the calcite cement and precipitate hematite in its place. Perhaps these fluids exolved from the
stratigraphically near granophyre sill (Chapter 5.1) and thus occurred at a similar time. If this
were the case, however, the mineralisation would be expected to be present other formations
on South Raasay. As a result, more observations need to be made in order to test this
hypothesis. The Beinn na’ Leac Formation is referred to formally in the literature as the
Druim an Fhurain Member (though this spelling is technically incorrect), which is the
youngest member in the Bearreraig Sandstone Formation.

25
Chapter 4: Igneous Geology
The Island of Raasay is host to three main Igneous Formations which outcrop across ~50% of
the mapped area. The most expansive of these is the Càrn nan Eun felsic granophyre which
forms the bedrock of Suisnish Hill and Càrn nan Eun. This granophyre is ‘sheet-like’ in shape
and unconformably rests on top of the sedimentary succession discussed in chapter 4. The
feature was likely responsible for many of the iron enrichment observed across the island due
to exolved hydrothermal fluids. The Osgaig Point micro-gabbroic sill is another significant
igneous formation that is exposed on the West coast of the island. It’s prominent columnar
jointing often resembles that of the famous Giant’s Causeway in Northern Ireland. The
feature was mistakenly identified as a lava plateau in early studies (e.g. Bradshaw and Fenton,
1982), however is more likely to be an intrusive sill due to it’s grain size. Finally, many
basaltic dykes have intruded into the sedimentary rocks of South Raasay, on the South East
coast, and along Allt Fèarns. Much of this igneous activity is interpreted to be a result of
development of the North Atlantic Igneous Province (NAIP) which took place in the mid-
Palaeocene to early-Eocene (Jolley and Bell, 2002). This could potentially explain the
compositional and observational similarities between the Giant’s Causeway and the Osgaig
Point Formation.
4.1 Càrn nan Eun Formation
The ‘Càrn nan Eun Formation’ outcrops across two regions within the mapped area; Càrn
nan Eun to the North West, East of Osgaig (GR: 5591, 3782) and Suisnish Hill to the South,
adjacent to Eyre Point (GR: 5615, 3523). The two regions occupy ~2.8km2 and ~4.1km2
respectively. Both Càrn nan Eun and Suisnish hill are of high relief and show abundant
bedrock exposure. The two areas have also been faulted which has been exploited by erosion,
leaving gullies (~70m deep), many of which have now been drowned resulting in 9 lakes such

26
as Loch Storab. The vegetation cover on these rocks is dominated by thorned plants and long
grasses making the formation’s presence distinctive, even in areas of low exposure. The Càrn
nan Eun formation was observed at 71 localities across the mapped area.
The lithology of these rocks is composed of a coarse-grained granite with a crystalline,
porphyritic texture. The coarse groundmass (2mm –
3mm) makes up ~60% of the rock, while the phenocrysts
(4mm-8mm) make up ~40% of the rock. The lithology
contains four dominant minerals giving it a felsic
composition, and overall pale cream appearance; quartz
(~35%), plagioclase (~26%), orthoclase (~25%), and
biotite (~14%). Quartz and orthoclase tend to form the
phenocrysts whilst plagioclase and biotite make up the
groundmass. Under the microscope, it is observed that intergrowth has occurred between
plagioclase and quartz in this lithology (e.g. Woodward, 1914). Grain size is not consistent
across the intrusion. Where the granite comes into contact with other formations, the
groundmass is significantly finer and darker grey in colour. The phenocrysts are also reduced
in size to ~0.9mm in diameter. This is characteristic of a chilled margin which was observed
in detail at locality 17 (p16). Xenoliths are also present within the granite which range in size
Figure 23: Rose diagramshowing
joint plane strikes of the Càrn nan
Eun FormationFigure 22: Image showing contact between the Suisnish
Beach Formation and Càrn nan Eun Granite. Note the
feeder dykes present beneath the sill
Figure 21: Stereonet showing
joint plane as poles of the Càrn
nan Eun Formation

27
from 1.1m to 2.4m. These xenoliths show planar bedding (~3mm thick) but have been baked
making lithological characteristics difficult to observe. Directly beneath the intrusion, feeder
dykes were observed which have a similar porphyritic composition to the overlying granite
(figure 22). Continued exposure to atmospheric conditions has resulted in the partial
decomposition of ferromagnesian minerals in the granophyre to be replaced with rusty
alteration products. In terms of structure, the Càrn nan Eun Formation is well-jointed due to
its competent nature (figures 21 and 23). Many of these joints are orientated to form vertical
columns (figure 22) and the substantial jointing has led to large blocks of the lithology to have
been dislodged and fall from outcrops.
The Càrn nan Eun granophyre was likely intruded at plutonic depths due to it’s coarse grain-
size. The porphyritic texture shows a two stage crystallisation process, whereby the
phenocrysts had a long time to crystallise before the conditions of the magma chamber
changed and the groundmass cooled at a slightly faster rate. The granophyre sill has been
interpreted to be ‘sheet-like’ in shape (e.g. Woodward, 1914) and it is discordant with the
underlying sedimentary units. This infers that a large time gap has occurred between the
deposition of the Jurassic sediments and the emplacement of the intrusion. The joint planes
are orientated in three main sets. These have likely formed as a result of a negative dilation of
the magma during cooling.
5.2 Osgaig Point Formation
The ‘Osgaig Point Formation’ is situated along the western coast of South Raasay from
Osgaig Point (GR: 5449, 3819) to the ferry terminal (GR: 5447, 3634), covering most of
Osgaig. The lithology occupies ~1.1km2. The area is low lying with poor exposure inland due

28
to drift cover. The best exposure is present along the coastal cliffs with Oskaig Point being
exemplary (locality 22, p20). A significant change in topography trending NNW-SSE marks
the eastern extent of the formation. This topographic anomaly is a result of a significant
strike-slip fault (see Chapter 6). The Osgaig Point Formation was obsered at 16 different
localities across the mapped area.
The Osgaig Point Formation consists of a crystalline, medium to coarse-grained lithology
with a porphyritic texture. The medium-grained groundmass
(~1mm) comprises ~80% of the rock and is composed of
pyroxene (55%) and plagioclase (25%) (figure 24).
The groundmass’ overall colour is a dark grey, which
is indicative of a mafic igneous composition. The
remaining 20% of coarse phenocrysts are all of an
olivine composition ranging in size from 3mm to 5mm.
Weathering of these phenocrysts has let to many being
removed from the groundmass and deposited in the
sands of Oskaig beach. The Osgaig Point Formation shows major columnar jointing (see
Figure 25: Field sketch of
Columnular Jointing in the
Osgaig Point Formation
Figure 26: Image Showing Concentric
Pillow Lava within the Osgaig Point
Formation.
Figure 24: Thin section under
cross polarised light of the Osgaig
Point Formation

29
figure 25, locality 22, p23). Much like with the Càrn nan Eun Formation, xenoliths are
present in the Osgaig Point Formation such as at locality 26 (p22). These show planar
bedding and are up to 10m by 6m in size. Alteration from the surrounding igneous material is
once again significant but a sedimentary protolith can be inferred. Finally, pillow lava
The Osgaig Point Formation’s overall grain size indicates that it was a feature that was
intruded at hypabyssal depths. This must mean that the rock formed intrusively as opposed to
extrusively. The columnular jointing illustrates that the magma experienced a negative
dilation during cooling. The jointing also identifies that the formation is probably a sill, as
these cracks usually form perpendicular to the intrusion’s wall or base. Olivine abundance and
groundmass grain size varies along the coastline, with greater abundances (up to 30%)
towards the south. It is hypothesized that this is because the intrusion has developed a slight
cumulate texture near its base due to the gravity settling of the olivine phenocrysts. Much like
with the granophyre, this intrusion was likely emplaced as a result of the North Atlantic
Igneous Province during the Tertiary.
4.3 Raasay Dyke Complex
The ‘Raasay Dyke and Sill Complex’ occurs across the whole mapping area. They feature in
high abundance on the north side of the road between Na Fèarns and Glen Lodge, along the
southern coast at Suisnish Point, as well as beside the river, west of Beinn na’ Leac. The
formation tends to stand several metres proud of surrounding lithologies, which is inferred to
be due to a greater resistance to weathering. There is very little correlation between the strike
trends of this formation.

30
The Raasay Dyke and Sill Complex (figure 27) takes the
form of many discordant and concordant, fine grained
crystalline features, which span an order of magnitude in
thickness (3cm – 3.5m). All display an equigranular, fine-
grained texture and are of a dark grey colour. These
characteristics are diagnostic of and have led to the
interpretation that the lithology is a mafic basalt. Due to
the fine-grained nature of the lithology it is difficult to
quantify the mineral abundances, however the rock
contains both pyroxene and plagioclase.
The igneous dykes observed on Raasay were likely emplaced during the development of the
North Atlantic Igneous Province (see chapter 5)
Figure 27: Field sketch of the
Raasay Dyke and Sill Complex

31
Chapter 5: Structure
5.1 Introduction
Across South Raasay, several prominent faults have offset the geology causing a repeating of
the stratigraphy (figure 28). This is particularly noticeable in the Ribbon Group sediments
which occurs 5 times throughout the centre of the area. Faulting has also brought the Oskaig
Point Formation adjacent to the Càrn nan
Eun Formation, without which the
formation would not be observed in South
Raasay. Many of these faults have been
exploited by weathering and glacial
erosion resulting in deep scarps in the
topography. An overall tilting of the island
is also hypothesized due to a regular offset
of the bedding measurements in all formations by ~20° to the West. The emplacement of the
Raasay Dyke Complex appears to have been along similar azimuths. This would indicate that
their emplacement was to accommodate an E-W, regional extension event that may be
correlated with similar igneous formations across Northern Britain.
5.2 Brittle Deformation
Six prominent faults were observed on the South of Raasay. These consist of the Oskaig Fault
(from GR: 5442, 3633 to GR: 5475, 3830), Eyre Fault (from GR: 5513, 3735 to GR: 5755,
3409), Beinn na’ Leac Fault (from GR: 5905, 3555 to GR: 5887, 3707), Suisnish Fault (from
GR: 5685, 3499 to GR: 5581, 3396), and two faults which offset the Ribbon Group in the
centre of the mapped area.
5.2.1 Beinn na’ Leac Fault
Figure 28: Schematic map showing the major
fault names and their displacements in South
Raasay

32
The Beinn na Leac’ Fault (figure 29) is situated on the South East coast of the island. It trends
NNE-SSW to the west of Beinn na’ Leac (GR: 5857, 3644), before sharply curing to a NW-
SE direction, south of the hill and intersecting the coast. The fault is referred to in the
literature as the ‘Hallaig Fault’ (e.g. Smith et al., 2009), however over last two years it has
been called the Beinn na’ Leac Fault (Morton, 2014). This is a more appropriate name as the
fault defines the Western limit for the Beinn na’ Leac topographic high. Displacement along
the fault is normal in nature which has resulted in the down-throw of the Beinn na’ Leac
Formation adjacent to the older Suisnish Beach Formation. The mean dip of the Beinn na’
Leac fault block is 23°. This is slightly greater than the mean dip of formations on the other
side of the fault (19°). It is therefore apparent that a component of fault block rotation has
occurred which would provide a mechanism for the difference in dips across the fault. If this
is in fact the case, then the fault plane is likely listric at depth. Surrounding formation
thickness is not influenced by the fault, indicating that displacement occurred long after the
deposition of the sediments. Furthermore, the fault has offset many dykes that make up the
Raasay Dyke Complex. As discussed previously, these igneous features were likely emplaced
during the Palaeogene. Therefore, due to cross cutting relationships it can be inferred that the
Beinn na’ Leac Fault is a relatively recent feature that occurred within the last 66Ma. Smith et
Figure 29: Schematic
map showing the
structure present at Beinn
na’ Leac. Strain
partitioning between dip-
slip and strike-slip
displacement is evident.
Note the distribution and
consistent strikes of the
fissures

33
al. (2009) concluded that the displacement occurred at a similar age, and further interpreted
that the fault was caused by glacial loading of the fault block. It seems unlikely that glacial
loading is the only factor that initiated the faulting. Another factor, for example, could have
been a pre-existing plane of weakness within the Suisnish Beach Formation.
5.2.2 Osgaig Fault and Suisnish Fault
The Osgaig Fault is a curvilinear accessory fault
which branches off from the Eyre Fault at GR:
5514 3737 and curves to NEE-SWW before
intersecting the coastline (figure 30). It has a
normal slip sense, dipping towards the south,
though the displacement is hard to quantify due to
a lack of marker units. The fault’s footwall is
composed of the Osgaig Point Formation, while
the hanging wall is composed of the Beinn na’
Leac Formation.
The Suisnish Fault is similar in many respects to
the Osgaig Fault. It is an accessory fault to the Eyre Fault showing a normal slip sense and
dipping towards the south. It also has a similar trend and curvilinear form, branching off the
Eyre Fault at GR 1568 8349: and intersecting the coast at GR: 1558 8339. These two faults
are particularly interesting due to their relationship with the Eyre Fault (see 5.2.3).
5.2.3 Eyre Fault
The Eyre Fault is the most laterally extensive fault within the mapped area. It extends from
Oskaig Point to Eyre Point, trending NNW-SSE. Displacement along the fault is oblique,
strike-slip in nature. This is identified from striations observed at locality 144 (GR: 5592
3629). The magnitude of this displacement is estimated at ~640m. What is peculiar about the
Figure 30: Block diagram showing
relative motions of the Càrn nan Eun,
Osgaig Point, and Inverarish Forest
Fault Blocks.

34
Eyre Fault is that it broadly trends perpendicular to the other major faults in the region, and
has a different slip sense. These characteristics can be explained by a strain partitioning
hypothesis. This hypothesis is applicable as the nearby normal fault, the Osgaig Fault could
be applying stress to the surrounding rocks allowing the strain to partition from a normal slip
sense, to a strike-slip slip sense (figure 30).
5.2.3 Other Faults
Two other dominant faults are present in the mapped area. These are situated around
Inverarish Burn (GR: 5664, 3667 and GR: 5724, 3727) and are largely inferred due to a mask
of superficial deposits. Their inferred trend is broadly N-S, which is roughly in-between the
trend of the prominent dip-slip (e.g. Osgaig Fault) and strike-slip (Eyre Fault) faults in the
area. This likely means that these faults show both dip-slip and strike-slip components if they
formed during the same strain episode.
5.3 Dyke Emplacement
Fine grained, basaltic dykes have intruded into the sedimentary succession across the
mapping area (chapter 4.3). These dykes range in thickness from a few centimetres to several
metres. These features tend to be orientated NE-SW, and therefore likely accommodated
some of the extension that resulted in the development of the faults mentioned above. Indeed,
along an NE-SW transect of Suisnish Beach (GR: 5549, 3420), it was calculated that the crust
was extended by ~4%.

35
Chapter 6: Geological History and Economic Potential
6.1 Geological History
The geological history of South Raasay is summarised in the table below.
AGE EVENT
PRECAMBRIAN
1) Torridonian: Deposition of Torridonian sandstones (Eyre Point
Formation) within a braided river system on the margin of an
alluvial fan.
2) Torridonian: Short time gap followed by the progradation of the
fan system resulting in the deposition of coarser fanglomerates
(Boulder Valley Formation) on top of a paraconformity.
PALAEOZOIC
3) Significant time gap, during which the upper surface of the
Boulder Valley Formation is weathered leaving an uneven surface.
JURASSIC
4) Sinemurian: Base level transgression allowing for the deposition
of marine shelf mudstones over a HST (Suisnish Mine Formation
and Suisnish Beach Formation) on top of a paraconformity.
5) Pleinsbachian: Base level regression (RST) resulting in a
coarsening of the sediments to medium-grade sandstones (Na
Fèarns, River Footpath, Borodale Forest Formations.)
6) Toarcian: Short-lived transgression (TST) allowing the
deposition of a thin band of soft mudstones (Ribbon Formation)
7) Bajorician: Regression resulting in the deposition of the Beinn
na’ Leac Formation over a Lowstand Systems Tract.
PALAEOGENE
8) Igneous activity associated with the development of the NAIP,
resulting in the emplacement of the Raasay Dyke Complex, Osgaig
Point Sill and Càrn nan Eun granophyre sill. Iron vein enrichment
in the Beinn na’ Leac formation also takes place.
9) East-West extension brought about by the opening of the North
Atlantic resulting in the development of prominent normal dip-slip
and strike-slip faults.
10) Regional tilting towards the West.
QUATERNARY
11) Glaciation exploits weaknesses in the bedrock, such as faults,
forming valleys and depositing glacial till and erratics.
12) Development of peat bogs.

36
6.2 Economic Potential
6.2.1 Iron
The island of Raasay has a history of exporting iron ore from mid-1916 to the end of the
Second World War (e.g. Draper and Draper, 1990). Signs indicating iron deposits were found
across the mapped area such as rust coloured rivers to iron rich concretions found in the
Suisnish Beach formation. Most of the iron was mined from the ironstone in the ‘river
footpath formation’, and transported to the south coast by a railway. Evidence for the
ironworks and railway are still prominent on the island such as a large hopper at Suisnish
Point (GR: 5549, 3420). The additional cost of transporting the ore off the island by ferry
likely made it uneconomical to mine after the war. In addition, its low iron content (Anderson
and Dunham, 1966) makes it unlikely to be economical in modern markets.
6.2.2 Peat
Raasay has an abundance of peat bogs across the south of the island. Most are fairly small,
spanning only a few tens of metres in diameter. Surrounding the lochs on Càrn nan Eun,
however, the peat deposits are much more expansive and difficult to traverse. Much like with
iron, the peat it is unlikely to be economical in UK markets due to the additional ferry
transportation costs and sporadic nature of the deposits. However, for island residents, the
peat could provide a useful fuel source to generate heat and electricity.
6.2.3 Water
The freshwater lochs of Càrn nan Eun are excellent resources of fresh water. Lochs such as
Loch Storab (GR: 5650, 3863) and Loch na Mna (GR: 5797, 3853) provide fresh water to the
residents of Inverarish.
6.2.4 Granite

37
The expansive granophyre intrusions are a suitable road aggregate resource due to its
resistance to erosion and impermeability. The use of these rocks may be more economical in
road construction on the island, as road stone from elsewhere would not have to be
transported onto the island. However, the demand for more roads of the island is currently
low, and lack of mining infrastructure would act against this resource being extracted.

38
Chapter 7: Conclusions
The most significant conclusions presented in this work are listed below:
1) Two paraconformities are present in the stratigraphic column, between the Eyre Point
Formation and Boulder Valley Formation (time gap of a few million years), and
between the Boulder Valley Formation and Suisnish Mine Formation (time gap of
approximately 700Ma).
2) The Eyre Point and Boulder Valley Formations are concluded to have been derived
from a similar sediment source and deposited under similar conditions to the
Torridonian Sandstones observed in Assynt.
3) The Boulder Valley Formation is interpreted to be of Torridonian age, rather than
Sinemurian age due to its resemblance to the Eyre Point Formation in terms of
composition and interpreted depositional environment.
4) The Beinn na’ Leac Formation has been enriched in hematite in the form of veins.
This hematite was likely sourced from hydrothermal fluids that exolved from the Càrn
nan Eun granophyre.
5) Marine transgression and regression occurred twice during the deposition of the
Jurassic age sediments, which is correlated with regional events such as the early
Sinemurian transgression that flooded the Durness basement high.
6) Two cyclic frequencies have been identified to be forcing the sedimentation on the
Suisnish Mine Formation. These are likely influenced by orbital mechanisms, though
the succession must be dated by absolute means to test this hypothesis.
7) Igneous activity is concluded to have taken place during the Tertiary as part of the
North Atlantic Igneous Province. This explains the similarities between the Càrn nan
Eun Granophyre and other felsic intrusions within the Inner Hebrides such as on
Arran, as well as similarities between the Osgaig Point Formation and Giant’s
Causeway in Northern Ireland.

39
8) The Osgaig and Suisnish Faults show normal dip-slip displacements because of strain
partitioning from the neighbouring strike-slip Eyre Fault. This mechanism is also
evident in the Beinn na’ Leac Fault.
9) The fissures present on Beinn na’ Leac are likely minor strike-slip faults that have
been exploited by weathering.
10) The region underwent NW-SE extension during the Tertiary that was accommodated
by the NE-SW trending basaltic dykes and normal faults, and NW-SE trending strike-
slip faults. This strain was likely a result of the opening of the North Atlantic.
11) The area has been subjected to glaciation in recent geological history resulting in the
deposition of erratics and glacial till in topographic lows.

40
References
Anderson, F. and Dunham, K. (1966). The Geology of Northern Skye: FW Anderson and KC
Dunham. HM Stationary Office.
Bradshaw, M. and Fenton, J. (1982). The Bajocian 'Cornbrash' of Raasay, Inner Hebrides:
palynology, facies analysis and a revised geological map. Scottish Journal of Geology, 18(2-
3), pp.131-145.
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.
Draper, L. and Draper, P. (1990). The Raasay Iron Mine: Where Enemies Became Friends.
Heritage Publications.
Farris, M., Oates, M. and Torrens, H. (1999). New evidence on the origin and Jurassic age of
palaeokarst and limestone breccias, Loch Slapin, Isle of Skye. Scottish Journal of Geology,
35(1), pp.25-29.
Jolley, D. and Bell, B. (2002). The evolution of the North Atlantic Igneous Province and the
opening of the NE Atlantic rift. Geological Society, London, Special Publications, 197(1),
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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. and Buckman, S. (1920). The mesozoic rocks of Applecross, Raasay, and northeast
Skye. Edinburgh: H. M. Stationery off. [printed by Morrisson and Gibb, limited, Tanfield].
Morrison, W. (1887). Precambrian conglomerate of Lewis. Transactions of the Edinburgh
Geological Society, 5(2), pp.235-242.
Morton, N. (1989). Jurassic sequence stratigraphy in the Hebrides Basin, NW Scotland.
Marine and Petroleum Geology, 6(3), pp.243-260.
Morton, N. (2014). Large-scale Quaternary movements of the Beinn na Leac Fault Block, SE
Raasay, Inner Hebrides. Scottish Journal of Geology, 50(1), pp.71-78.
Oates, M. (1978). A Revised Stratigraphy for the Western Scottish Lower Lias.Proceedings of
the Yorkshire Geological Society, 42(1), pp.143-156.
Smith, D., Stewart, I., Harrison, S. and Firth, C. (2009). Late Quaternary neotectonics and
mass movement in South East Raasay, Inner Hebrides, Scotland. Proceedings of the
Geologists' Association, 120(2-3), pp.145-154.

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Storetvedt, K. and Steel, R. (1977). Palaeomagnetic evidence for the age of the Stornoway
Formation. Scottish Journal of Geology, 13(3), pp.263-268.
Storetvedt, K. (1978). Structure of remanent magnetization in some skye lavas, NW
Scotland. Physics of the Earth and Planetary Interiors, 16(1), pp.45-58.

42
Appendix I: Arran Fieldwork
Dyke Extension Exercise

43
Dyke Extension Exercise

44
Logging Exercise

45
Logging Exercise

46
Logging Exercise

47
Logging Exercise

48
Appendix II: Raasay Clean Copy Map

The Geology of South Raasay Dissertation

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