Claire-Bibby-Report

University of Aberdeen
GL4023 Geological Map Project 2015-2016.
Geological Interpretation of the Durness region,
including an investigation into the elongation of the
Lewisian Gneiss.
51228583: Claire Bibby
BSc Geology and Petroleum Geology

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University of Aberdeen
Geological Map Project 2015-2016.
Declaration of academic integrity
I declare that this piece of work is my own and does not contain any unacknowledged work from
other sources.
Signed:
Name (print): CLAIRE BIBBY Date: 29/01/16

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Contents
1.0 Introduction............................................................................................................................................ 5
2.0 Lithologies of the Area........................................................................................................................... 5
2.1 Upper Dolostone – Durine Formation...................................................................................... 5
2.2 Middle Dolostone – Sangomore Formation.............................................................................. 6
2.3 Lower Dolostone – Sailmhor Formation .................................................................................. 7
2.4 Pipe Rock – Pipe Rock Member................................................................................................ 8
2.5 Quartzite Breccia....................................................................................................................... 8
2.6 Quartzite – False-bedded Quartzite Member............................................................................ 9
2.7 Muscovite-Chlorite Mylonite – Oystershell Rock................................................................... 10
2.8 Gneiss – Orthogneiss .............................................................................................................. 10
3.0 Geological History of the Area ............................................................................................................ 11
3.1 Published Stratigraphy ............................................................................................................ 11
3.2 Observed Stratigraphy............................................................................................................. 12
3.3 Geological History .................................................................................................................. 14
3.4 Comparative Geological History............................................................................................. 16
4.0 Detailed analysis of the effects of Crustal Extension observed at Ceannebienne Bay......................... 16
4.1 Hypothesis............................................................................................................................... 16
4.2 Aims ........................................................................................................................................ 16
4.3 Method .................................................................................................................................... 16
4.4 Results..................................................................................................................................... 17
4.5 Discussion ............................................................................................................................... 18
4.6 Conclusion............................................................................................................................... 20
6.0 Bibliography......................................................................................................................................... 22
7.0 Appendix.............................................................................................................................................. 23
7.1 Appendix A : Boudin measurements used for Figure 15 and 16............................................. 23
7.2 Appendix B: Cross Sections of Leirinmore and Sango Bay. .................................................. 24
List of Figures
Figure 1a Geological map of the British Isle……………………………………………………...……5
Figure 1b Topographic map of the Durness region…………………………………………..…….....5
Figure 2a Digitalised Field Sketch of Upper Dolostone………………………………………….…..6
Figure 2b Photograph of Upper Dolostone………………………………………………………….....6
Figure 3a Digitalised Field Sketch of Middle Dolostone………………………………….……….....7
Figure 3b Photograph of Middle Dolostone…………………………………………………….……...7

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Figure 4a Digitalised Field Sketch of Lower Dolostone…………………………………….…….….7
Figure 4b Photograph of Lower Dolostone……………………………………………….…...……….7
Figure 5a Digitalised Field Sketch of Pipe Rock…………………………………….……..…...……..8
Figure 5b Photograph of Pipe Rock……………………………………………………….…………….8
Figure 6a Digitalised Field Sketch of Quartzite………………………………………..…….………..9
Figure 6b Photograph of Quartzite…………………………………………………………............….9
Figure 7 Annotated photograph of Herringbone Cross-stratification ……………....…………9
Figure 8 Digitalised field sketch of a mylonitic fabric………………………………………..……10
Figure 9a Digitalised Field Sketch of Gneiss……………………………………………...……..…..11
Figure 9b Photograph of Gneiss .….….….….…..…….….….….….….….….….....….............11
Figure 10 Published stratigraphic column…………………………………………………….……...11
Figure 11 Observed stratigraphic column………………………………………………………...…..12
Figure 12 Digitalised model of normal fault theory one…………………………………………….13
Figure 13 Digitalised sketch of Horst and Graben system………………………………………….15
Figure 14 Simplified model of Boudin unit……………………………………………….........….….17
Figure 15 Distribution of Boudins, pie chart……………………………………………...………….17
Figure 16 Elongational values, divided bar chart…………………………………………..……….18
Figure 17 Annotated photograph of symmetrical Boudin…………………………………….…….19
Figure 18 Classification of Boudins …………………………………………………….….…..19
Abstract
The research area being discussed is located in the North-west Highlands of Scotland, on the
Hebridian Terrain, the foreland of the Moine Thrust. The purpose of this paper is to establish a
geological history based on observed evidence, gathered over a thirty five day mapping
excursion, completed in the summer of 2015. This geological history will be compared to that
derived by others. The second aim is to analyse the elongation within the Lewisian Gneisses by a
sampling method used on Ceannabienne Beach, a hypothesis is set for this and can be accepted,
revealing that Pegmatite is the main lithology to be affected by Boudinage. An overall history
can be drawn, suggesting a period of metamorphism underlies a sedimentary sequence formed
within a transgressive environment.

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1.0 Introduction
Durness is located between two sea lochs, Loch Eriboll to the East and the Kyle of Durness to
the West, in the North West Highlands of Scotland (see Figure 1a and 1b). The Moine Thrust
lies to the South East of the research area, meaning Durness lies within the Hebridean Terrane.
This is one of the terranes formed as part of the Caledonian Orogenic Belt. This terrain is
bounded to the South with the Northern Highland Terrane, by the Moin Thrust. (Park, Stewart
and Wright. 2002)
Figure 1a (left) and 1b (right): a) Geological map of the British Isles (British
Geological Survey) b) Topographic map of the Durness region, black box shows
mapped area (addapted from Digimap)
The research area covers ~15km2
between the town of Durness and South to Loch Duail and
Loch Uamh Dhadhaidh. The lithologies making up this geological region include the Gneiss,
belonging to the Lewissian Complex, ageing back to the Archean period. These Gneisses underly
a Cambrian Sandstone Formation and a sequence of Ordivician Limestones, these groups are
then divided further, into formations.
This report aims to unfold the geological history of the area, both startigraphically and
stucturally. Also contained within this paper there is a focus on the crustal extention that has
occurred within this Gneiss, allowing Boudinage to occur.
2.0 Lithologies of the Area
Within the research area a total of 8 rock types were observed. Within certain lithologies there
were intrusions observed but not mapped. These will be included in the descriptions of the host
lithology.
2.1 Upper Dolostone – Durine Formation
The unit of Upper Dolostone was observed in the most North Westerly headland of Sango Bay
and continued along the shoreline, to the end of the study area. On obtaining a clean surface an
overall grey colour was noted, this was noted to be upper fine grained. This grey colour was
tainted with a very light pink colour. This was presumed to be Orthoclase, though no further tests
were undertaken to confirm this. Acid was applied in several places and results varied, with

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small amounts of fizzing observed at sea level while there was a more aggressive reaction further
inshore.
There was a very obvious presence of Chert within this Dolostone. These upper fine to lower
medium grained white “patches” were seen across the unit area, particularly across the headland
at the north of Sango Bay. These inclusions contained a sparkly mineral, presumed to be
Muscovite Mica and were able to be scratched, ruling out the possibility of it being Quartz.
Many fractures were observed running in random directions, this suggests a series of fracturing
events. These fractures were seen with a red/orange infill due to the presence of Iron oxide. Due
to the Iron being contained to only the fractures, this suggests a fluid flow after the fracturing has
occurred.
Figure 2a (left) and 2b (right): a) Digitalised field sketch and b) Photograph showing Chert inclusions
within the Upper Dolostone. Heavily fractured, fractures containing Iron precipitate.
2.2 Middle Dolostone – Sangomore Formation
Middle Dolostone is the formation most observed within the Durness Group, in the studied area.
Based on stratigraphy this lithology sits conformably between the Upper Dolostone and Lower
Dolostone. In observed surface geology it sits as the stratigraphy says in that it directly overlies
the Lower Dolostone, bound by a unit of Chert. There are two observed units of the Middle
Dolostone spread across the headlands at Leirinbeg, separated by the inlet leading to Smoo Cave.
It is believed these two units were at one stage connected but have been separated by a normal
fault. This has been observed in two prominent features within the area; an offset between the
North Western and South Eastern headlands and Breccia observed in two separate locations (NC
4202 6746 and NC 41896704).
Middle Dolostone appears to have a greater lime content than Lower Dolostone but less than
Upper Dolostone. This was derived through the application of acid onto clean surfaces. Like the
overlying unit there are red infilled fracture and veins, suggesting the continuation of the iron
precipitate.

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Infilled fractures (~1mm – 2mm) were also observed, in cross-sectional view. The material
filling the fractures appeared similar to the Chert that forms a boundary between this unit and the
Lower Dolostone.
Figure 3a (left) and 3b (right): a) Digitalised field sketch and b) Photograph: Middle Dolostone surface
outcrop showing weathered patches and Chert filled fractures.
2.3 Lower Dolostone – Sailmhor Formation
Like the Middle Dolostone, above, there are also two units of the Lower Dolostone across the
headland. This unit of Dolostone is distinguishable from the two formations above through the
mottled appearance observed on clean surfaces. This pattern consists of dark grey patches within
a lighter grey matrix. The dark grey patches are found to be lower medium grained while the
light areas are finer grained, both appear well sorted.
Running through both dark and light patches, are very thin (approx. 0.1mm) veins of what on
first observation were noted to be quartz. On further analysis of field notes and of the
surrounding areas it would make considerably more geological sense to be Chert, due to its
presence above the unit, as explained in 3.2 Upper Dolostone. This mineral also appears in clasts
(approx. 0.5mm), through the unit.
As well as the Chert clasts, there were patches of iron of a similar size, shown in a
reddish/orange colour, though these were less common. This unit was heavily fractured and due
to the angle of bedding on the water line this was easily measurable.
The Lower Dolostone conformably underlies the Middle Dolostone and conformably overlies the
pink tinged Pipe Rock.
Figure 4a (left) and 4b (right): a) Digitalised field sketch and b) Photograph Mottled pattern within the
Lower Dolostone. Very thin veins of Chert running through patches.

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2.4 Pipe Rock – Pipe Rock Member
Pipe rock is found within the Eriboll Sandstone Formation, from the Cambrian era. It sits
conformably above the Quartzite and below the mottled Lower Dolostone. A unit of
approximately 0.47km2
was observed over the Leirinmore and Sangobeg Sands area. This
lithology was found to be upper medium grained to lower coarse grained and had an overall deep
red/purple colour. This colouration was found to be iron oxide staining and varied in
concentration throughout the area. The colourless grains were unable to be scratched, confirming
the body of the unit was quartz based.
Small patches of a black, flakey and glassy mineral were found within small outcrops on the
Sangobeg Sangs, this is presumed to be due to the normal fault between this unit and the Gneiss,
to the South.
The defining feature to differentiate the Pipe Rock and the Quartzite is the bumpy pattern
observed on the top and bottom surfaces of each bed. These bumps appear to have a finer grain
size than the main body of the unit. Bumps on the top and bottom were not seen to be connected
but it can only be assumed there is a relationship between the two.
Large outcrops of brecciated Pipe Rock were observed to the North of the Pipe Rock unit, at the
contact to the Lower Dolostone. This Breccia appeared almost like a clast supported
conglomerate, the clasts being predominantly quartz. On this outcrop the bumps were
considerably less obvious, in both size and quantity. This leads to the belief that these bumps are
able to be weathered meaning they are made of a softer material than the unit body.
Figure 5a (left) and 5b (right): a) Digitalised field sketch and b) Photograph Algae covered “bumps” on
the top surface of a Pipe Rock outcrop. Moderately fractured.
2.5 Quartzite Breccia
The Quartzite Breccia was only observed as one 7m long outcrop running NE-SW, to the North-
West of Sango Bay. The main crystalline body of the outcrop appears the same as the Quartzite
seen towards Loch Uamh Dhadhaidh, to the South East of the overall research area. The pink
Quartzite appears heavily and randomly fractured, no overall trend was observed. Within this
relatively small outcrop there are several large patches, of angular clasts, covering the majority

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of the surface area of the outcrop. This suggests a Breccia, known to be formed by a brittle fault,
aiding the structural story of the area.
2.6 Quartzite – False-bedded Quartzite Member
Quartzite is the purest form of sandstone and can be observed in various locations across the
studied area. Quartz grains are easily identifiable as they are clear, rounded and glassy, causing it
to reflect light. There is an overall pink tinge on the outcrops, most likely due to iron oxide.
These Quartz grains appear to be cemented together by more Silica. Outcrops of this lithology
were unable to be scratched, another indicator of Quartzite.
Quartzite units were identified by a depositional sedimentary structure cross-stratification. This
feature forms mostly in the small scale (NC 4347 6398) suggesting cross-lamination, but there
was evidence of larger bedded features (NC 4532 6400). As well as cross-stratification, Flaser
bedding is observed in two separate locations, the unit in which it occurs appears to vary in
thickness.
As well as cross-bedding and cross-lamination, herringbone cross-stratification was observed
around the Bàgh uamh Dhadhaidh Quartzite headland.
Figure 7: Annotated photograph of Herringbone cross-
stratification. More than one re-activation surface visible.
Figure 6a (left) and 6b (right): a) Digitalised field sketch and b) Photograph of large scale Cross-
stratification with a close view of a layer of flaser bedding.

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2.7 Muscovite-Chlorite Mylonite – Oystershell Rock
On Sango Bay a fine grained dark grey lithology is observed, noticeably different to
neighbouring rock types due to the obvious fabric. The majority of the outcrops were made up of
a dark platy mineral that was easily broken, presumed to be Biotite. Within this Biotite, many
small and very sparkly minerals were observed, these showed the characteristics of Pink leading
to the inclusion within the field name. Through the dark grey minerals, there is an overall green
colouration. This is likely to be Chlorite minerals, which are often found in metamorphic rocks.
There are numerous layers of white and pink transparent rock, similar to the Pegmatite seen
within the Gneiss (see 3.8 Gneiss – Orthogneiss), of varying thickness (up to ~4 cm) across all
outcrops. These layers include large (~0.5cm – 2cm) crystals, predominantly quartz based. As
well as these light layers, there appeared to be thin veins of an orange mineral, potentially iron
oxide, folding through clean surfaces. These veins were observed in two contrasting views
meaning enhanced knowledge of the feature’s 3D shapes.
Figure 8: Digitalised field sketch of a mylonitic fabric (S-C).
As previously mentioned, within this Mylonite there is a strong S and C fabric. This aids in the
construction of the geological history of the research area.
2.8 Gneiss – Orthogneiss
Gneiss makes up a large proportion of the study area and through the symbolic banding, is easily
identifiable. The majority of the dark bands are made up of Felsic minerals, especially Biotite.
The observed Biotite units tend to be thinner than others, at approximately 1cm, though in
certain locations it appeared that several thin bands had formed in close proximity to one another
and looked to be one unit. The lighter bands were coarse, pink and crystalline, identified as
Pegmatite and could be up to approximately 6m in width. There were also thinner light bands of
Quartz, these tended to be what was folded in planar view.
Banding was best seen in vertical cross-sectional view, especially as the foliation across the unit
is greater than 40° and often above 80°. Crustal extension is also visible in this angle. Extension
is calculated through the presence of Boudinage, especially in the Ceannebienne Bay area (see
6.0).

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Figure 9a (left) and 9b (right): a) Digitalised field sketch and b)
Photograph of banding within deformed Gneiss. Bands of dark
felsic minerals(Biotite) and light mafic minerals (Pegmatite).
3.0 Geological History of the Area
3.1 Published Stratigraphy
The stratigraphic column of the Hebridean Terrane is thought of as being continuous across the
foreland of the Moine thrust. This stratigraphy is split into five groups seen below:
Figure 10: Published stratigraphy
including the Torridonian Sandstone
and An t-Sron Group (Cawood, Merle,
Strachan and Tanner, 2012)

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Cawood, Merle, Strachan and Tanner (2012) determine the age range of this succession, dividing
it into a lower section, named the Ardvreck Group and an upper, Carbonate sequence. These
Ordovician Carbonates contain the Durness Group and are approximately 750m thick. The
Ardvreck Group can be sub-divided into the Eriboll and An t-Sron Formations, both of Cambrian
age. The Eriboll Formation contains the Basal Quartzite (75-125m) and the burrow containing,
Pipe Rock (75-100m). The An t-Sron Formation is also made up of two separate members. The
lower member is the Fucoid Beds consisting of Dolomitic Siltstone with a thickness of around
12-27m and the upper unit is a Quartz Arenite named the Salterella Grit (<20m). The oldest units
are the Torridon Group and the Lewisian Complex. The Torridon Group is made up of fluviatile,
cross-bedded Sandstones, deposited by a braided river (up to 7km) (Krabbendam, Mendum and
Goodenough, 2008). The Lewisian Gniess forms the basement rock for the North-west of
Scotland. Due to this the thickness is unknown.
3.2 Observed Stratigraphy
In the study area, an area that should theoretically have the same stratigraphy, stratigraphic units
are missing. The Ardvreck Group contains only the Eriboll Group, meaning the relatively thin
An t-Sron Formation has not been observed.
Figure 11: Observed Startigraphy
for the Durness region.
There are possible reasons for the absence of the An t-Sron formation, within the Durness region.
These theories remain purely speculative as there is no field evidence of the formation. The first
theory details a large scale normal fault running across the Hebridean Terrain. A sketch diagram
overleaf shows the lithologies in stratigraphic order being displaced:

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Figure 12: Digital representation of theory one. Showing a
large normal fault running across the North-west of Scotland,
explaining the absence of the An t-Sron Group. Normal fault
downthrown direction is shown by a red arrow.
The blue rectangle in the sketch represents the location of the Durness study area, in the overall
fault system. Due to the offset of the normal fault and the location of Durness, the An t-Sron
Formation does not appear in the stratigraphic column.
A second theory to explain the absence of the beds could be that the group has been eroded
away, leaving behind an unconformity. Though there is no large time gap, the units are expected
to be very thin and therefore would not take as long as others, for example the Torridonian
Sandstones, to be deposited and eroded away. An unconformable contact was not observed
between the over and underlying units but the contacts were in fact faulted, due to extension,
making an unconformity less clear even if it were to be present.
The final theory does not follow the path of the units being missing but that they were simply not
observed. Fucoid Beds are Dolomitic Siltstones meaning they have a very low permeability. This
means water is unable to flow through the units, therefore will congregate on the unit surface. If
there were to be a Fucoid surface outcrop, water would provide the ideal environment for a bog
to form, allowing wetland organisms to grow. With this type of coverage viewing an outcrop
would be nigh impossible. A similar situation could arise with a Salterella Grit surface outcrop,
though in this instance the outcrop is likely to have a high permeability. As the water flows over
and into the bed, nutrients will be left behind providing a home for the growth of organisms.
Lichen, for example, has the characteristics to grow on harsh surfaces, including rocks. Over
time plant civilisation could build and again the outcrops will be completely encompassed.
Within the study area there was no observed evidence suggesting there was a large scale fault
between the Durness Group and the Pipe Rock. Due to the size of the area this is likely to be a
substantial feature and would be obvious within the known history of the area (see Chapter 4.).
For this reason theory one is implausible.
Due to the presence of these units on the East side of Loch Eriboll, the unconformity would have
to be very short and the erosion would have occurred in a very localised area. With no evidence
of an erosional event in the geological history of the field area, this concept is highly unlikely to
have taken place.
Within the study area there were relatively large areas of no-exposure, these were filled in with
inferred boundaries of known lithologies. Theory three suggests that An t-Sron members were

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present, but evidence of this had been well concealed. Based on the three arguments posed, this
deems the most logical.
The An t-Sron Group is not the only Group to be missing from the research stratigraphic log.
Torridonian Sandstones were also not observed. The reason for this is believed to be easier to
derive than the other absent beds. The unit below the Torridonian Sands is the Lewisian Gneiss
and this is aged back to the Archean time period. The formation situated above the sand in the
published stratigraphy is the observed cross-bedded Quartzite. This is known to have been
deposited during the Cambrian period. This leaves a time gap of approximately two thousand
million years where there appears no new rock activity. This gap is likely to represent an
unconformable surface. It can be presumed that the Torridonian Sandstone was in fact deposited
onto the study area but before the Quartzite was deposited the Torridonian Group was eroded
away. Though this unconformity was not observed, a boundary between these two units
(Quartzite and Gneiss) was observed. It can now be concluded that this was in fact the
unconformity representing the absent Sandstone.
3.3 Geological History
Using the stratigraphic column it is known that Gneiss forms the basement to the area. It is
believed, due to minerals present, that this is a Granitic protolith that has undergone
metamorphism at great depths, creating this metamorphic unit. Throughout the body of the
lithology there is a strong near vertical foliation. This is the first suggestion of ductile
deformation. This process of deformation is reinforced by the presence of Boudins (see Chapter
5) created through Boudinage. For these to have been observed crustal elongation requires to
have occurred. The dark felsic bands are another key to the understanding of the ductile
deformation. These bands contain Biotite minerals that have been elongated to a large extent.
This deformation occurred at a great depth where temperatures were high. It was then brought to
the surface to allow deposition to younger lithologies.
After the deformation of the Gneiss, as discussed above (see Chapter 4.2), the Torridonian sands
were deposited. Due to the absence of this layer the depositional environment of this lithology is
unknown, though there is the possibility that it would be the same as that of the Quartzite sitting
above.
The Quartzite is observed to have both large and small scale cross-stratification, aiding the
identification of the beach environment in which they would have been deposited. Cross-
stratification occurs in a tidal environment with a bidirectional fluid flow, in this case sea water.
The Pipe Rock overlying the Quartzite is made of the same overall body but in this layer no
cross-stratification is observed, this leads to the conclusion that the depositional environment has
changed. The bumps observed are presumed to be burrows. A suggestion for these vertical
burrows could be that if the environment has gone through a period of transgression and is now
in a high turbidity, shallow marine environment, the organism could be burrowing for shelter. As
the full geometry of these burrows is unknown this theory is purely speculatory.
Above the Sandstone units lies the Dolostone within the Durness Group. These Dolostones are
likely to have originally formed as Limestones in a warm, shallow marine environment with no

15
input of clastic materials and where carbonate muds accumulate. This suggests the continuation
of the foreseen transgressive sequence. At a certain depth it can be presumed that the water
became magnesium-rich allowing the calcite minerals to be replaced by Dolomite. The filtering
through of the magnesium-rich waters is likely to have been what caused the mottled patches
observed within the Middle Dolostone unit.
While the units within this group are being deposited pressure is building up and causing an
overpressure on the Sandstone beds. This overpressure is applying heat to the units and is
beginning the process of metamorphism. This is the change in the Eriboll Sandstone Formation
from Sand to Quartzite.
After the stratigraphic units have been deposited and metamorphosed to their current state there
was a period of tectonic uplift, lifting the newly formed stratigraphy above sea level.
Within Sango Bay there are two units that appear somewhat out of place, the unit of Gneiss and
the Muscovite-Chlorite Mylonite. These units appear between the three units of Dolostone. The
Mylonite is believed to be a shear zone that has moved with the Gneiss. This is likely to be an
isolated appearance of the Moine Thrust. This would require the thrust to have continued over
from Eriboll on the East of the sea loch, and protruded into the conformable sequence observed
in Durness.
All stratigraphic units from the research column are now in place. The lithology has now gone
through deposition, some have been metamorphosed in the process and all have been uplifted.
The next event is likely to explain the faulting system across the area. A number of normal faults
are observed running in a general North-east South-west direction, this suggest the same event
has caused these faults, this event is likely to be a period of crustal extension. From cross-
sections created for this area (see Appendix B) a series of domino faults can be observed: these
are likely to have started at the most South-easterly fault dipping to the North-west which in this
case is between Gneiss and Quartzite. These faults then continue to the North-west. Domino
faults cause no internal deformation which is why the conformable sediments can be seen tilting
at a constant angle.
It is believed that there may be two stages of extension; the first running South-east to North-
west and the second, smaller event, in reverse. This explains the few faults dipping to the South-
east. The point between the two stages of faulting is believed to have occurred in Sango Bay.
The two thrust beds appear topographically lower than the limestone units to either side, and are
bounded by normal faults, this fosters the idea of a Horst and Graben system, seen below (Figure
13), with the thrust beds being the Graben.
Figure 13: Basic sketch of a Horst and Graben
feature, similar to that seen across the Sango Bay

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3.4 Comparative Geological History
The Gneisses encompassing a large portion of the field area is reported to be derived from a
Granite-gabbro unit that also encompasses schist that is identified as deformed sedimentary rock
(Phemister, 1960. Cited in: Swett, 1969). Metamorphism is thought to have been metamorphosed
and deformed in two separate events; the Inverian Event (2.4 – 2.2Ma) and the Laxfordian Event
(1.7Ma). (Weaver, B.L. and Tarney, J. 1981). This contradicts the single stage of elongational
deformation thought to have occurred, stated in the geological history derived from field
evidence. During the two published events the granulites were retrogressed to amphibolite-facies
(Weaver, B.L. and Tarney, J. 1981).
Through field research it was derived that the Dolostones formed in a warm, shallow marine
environment. This can be confirmed by the work of Harland and Gayer, Sett and Smit and
McKerrow et al (1972, 1972, 1991, 1992. Cited in: Park, R.G. et al. 1997). It is stated that the
separation of Laurentia, Baltica and Gondwana is where the Iapetus Ocean emerged, South of
the equator. On the South-western coast of Laurentia there was no clastic input and the
Carbonates, which now make up the Durness Group, began to form. The marine transgression
derived from the depositional environmental change is also backed by Park (1997) where it is
confirmed that the transgression continued throughout the Cambro-Ordovician.
The mottling of the Dolostones has been debated since the early twentieth century, when Peach
et al (1907. Cited in: Swett, K. 1969) named it the “Leopard Stone”. This pattern could represent
two phases of dolomitization occurring at different times or different rates. The first phase may
be due to a local "unmixing" of metastable sedimentary mixtures (Sujkowski, 1958. Cited in
Swett. 1969), though the migration would be limited by a low diffusion porosity. The later phase
is likely to be either considerably faster or slower permeation by the magnesium-rich fluids
(Swett, 1969). This theory generally backs that stated in Chapter 4.3 Geological History.
4.0 Detailed analysis of the effects of Crustal Extension observed at Ceannebienne Bay
4.1 Hypothesis
The process of Boudinage is more likely to occur in one lithology than any others within the
Gneiss at Ceannebienne Bay (can appear in literature as Tràigh Allt Chàilgeag).
4.2 Aims
The aim of this focused report is to collect sufficient data to calculate an elongation value for
each unit, to determine a trend within lithologies.
4.3 Method
Around the Ceannebienne Bay area thirty units containing Boudins were randomly selected,
whereupon each individual Boudin was measured and added together. The overall length of the

17
unit was then also measured. The lithology and location of the unit were noted, along with a
photograph.
With these measurements the elongational value was calculated:
𝐸𝑙𝑜𝑛𝑔𝑎𝑡𝑖𝑜𝑛 =
𝑙 𝑜 − 𝑙1
𝑙 𝑜
In this equation lo is the original length of the unit before any amount of deformation (see Figure
14 below) and l1 is the accumulative lengths of the Boudins.
Figure 14: Simplified model of the Boudins. l1 represents Boudin features while lo is
the overall length of the unit. Thic could be used to understand the elongation
equation.
4.4 Results
A table can be seen in Appendix A detailing all collected data and the elongation values. From
simply calculating the most common lithology within the data it can be seen that Pegmatite was
the lithology that contains the most Boudins. It can be seen on the pie chart below (figure 15)
that Pegmatite units made up over three quarters of this study, seconded by Quartz units, sitting
considerably lower at a tenth percentile. Elongated Biotite appears in three of the five rock types.
These were separated out of clarity, however if they were to be accumulated this new category
would overtake Quartz and be the second most deformed lithology. This would not alter the
overall conclusion from this study.
Figure 15: Pie chart representing the distribution of units containing Boudins
between lithologies.
76.67%
10%
6.67%
3.33%
3.33% Pegmatite
Quartz
Elongated Biotite
Elongated biotite with
pegmatite
Elongated biotite with
quartz and pegmatite
intrusions

18
Having drawn the conclusion that Pegmatite yields the highest number of Boudins within the
sample, the elongation values were evaluated to establish a trend. The data was split into bins of
0.099 and a frequency table was created. A divided bar graph (figure 16) is used to display this
data.
Figure 16: Divided bar chart split into bins of 0.099. This shows the lack of
overall trend in the elongational values deduced from the Pegmatite units.
Pegmatite is the most common lithology to have experienced Boudinage.
It can be seen that there is not one dominant bin. The majority of elongation values fell between
0.100 and 0.399 and also between 0.600 and 0.699.
4.5 Discussion
Boudins are known to be the broken up pieces that once formed a continuous layer, or in this
case band, within the Gneiss, formed through the process of Boudinage. Boudinage involves the
stretching of a layer until it breaks into fragments, often thought of as the opposite of folding
(Fossen, 2010). Boudinage occurs when there is a high competence (viscosity) contrast between
the surrounding units and the layer being stretched. With no viscosity contrast a layer will simply
elongate without breaking apart.
Luepold (2014) suggests that Boudins are related to structures that are a result of layer parallel
orientation extension, of a more competent layer lying between less competent layers or amongst
a less competent matrix. This therefore suggests that in this area of study Pegmatite has a higher
viscosity than that of the surrounding Gneiss.
Boudinage can occur under thee kinetic classes; no-slip Boudinage, Antithetic Slip Boudinage
and Synthetic Slip Boudinage (Goscombe, Passchier, Hand, 2004). The Boudins observed at
Ceannabienne are believed to be a result of no-slip Boudinage, due to their symmetry. Again,
this category can be divided up based on appearance. Drawn Boudins is the deduced overall
name for the study samples.
Drawn Boudins are known to be symmetrical with no obvious block rotation (Figure 17). These
Boudin units vary in the degree of separation between each Boudin block and the degree of
thinning of the interconnecting neck zone. In the Ceannabienne area Boudins are seen in both
necked and tapered form. Necked Boudins involve the blocks being connected by a neck, while
tapered Boudins appear completely isolated, or can be delicately connected by a very thin neck.

19
Figure 17: Annotated photography of a unit of
Pegmatite that has experienced Boudinage. The vast
majority of Boudins observed in the Gneiss at
Ceannabienne Bay were symmetrical, aiding in the
classification. A line of symmetry is shown in red.
Following through the classification scheme posed by Goscombe et al. the full geometry of the
Boudin train can be derived.
Figure 18: Goscombe et al (2004) poses a
classification scheme used to identify the geometry of
the observed Boudin.
Both Boudin Block Geometry and Boudin Train Obliquity are standard across all thirty study
samples, with Boudin Train Obliquity showing Foliation-parallel Boudin trains. Material
Layeredness however, varies slightly within the samples. Sample numbers four and eight are

20
seen to be Multi-Layer Boudinage as they are not simply made up of the one lithology. The other
samples are classed as Single-Layer Boudinage.
It is known that Pegmatite, the most dominant lithology in this study, is comprised of mainly
Quartz and Feldspar ribbons, with remnants of coarse-grained K feldspar phenocrysts (Gapais,
and Boundi, 2015). These units were observed containing large quantities of crystals of up to
2cm in diameter. This grain size changed across the units sampled, however remained
significantly larger than the grains within the Biotite or Quartz. The Biotite was very finely
elongated and easily broken apart, while the Quartz appeared coarse-grained in most parts.
This variation in the Pegmatite grain size is likely to be the cause of the wide variety in
elongation values, seen in Figure 16. If grain size was to remain consistent there would be a
predominant bin, as values would be clustered.
Due to the considerably higher grain size, Pegmatite is going to be the most resistant band within
the Gneiss. This means that no matter its surroundings a unit of Pegmatite is going to experience
Boudinage on extension. Quartz and Biotite are only going to form Boudins if their surrounds
are of a lesser competence, which will occur considerably less frequently than Pegmatite.
Otherwise the bands will simply become elongated in a linear manner.
4.6 Conclusion
At this stage the hypothesis of “The process of Boudinage is more likely to occur in one lithology
than any others within the Gneiss at Ceannebienne Bay” can be accepted. The Pegmatite is
considerably more suitable to undergo Boudinage than any of the other observed units, for the
reasons stated above. The shape of the observed Boudins also help piece together the strain
history of this area.
5.0 Summary
Durness is an area of complex metamorphic history. Through the course of this report it becomes
apparent that there is no definitive explanation for the current state of deformation, particularly
based on field evidence. Previous studies differentiate the Gneisses within the Lewisian
Complex, based on stages of deformation, though these stages and the boundaries between them
seem to vary between authors. This makes the story of the geological history uncertain. The
detailed analysis of the Boudins (see Chapter 5.0) within the Gneiss at Ceannabienne Bay gives
an insight into the ductile deformation that occurred at one stage, though it has been suggested
different stages of deformation could in fact produce the similar looking deformation features,
for example the Boudins studied.
With the Moine Thrust protruding over Sango Bay it could be expected that the Sedimentary
units would be exposed to high levels of distortion. On field observations and cross section it can
be seen that this is not the case. The Eriboll Sandstone Formation and the Durness Group can be
seen to lie conformably on top of one another.
This area provides both complex geology that has been hotly debated amongst academics and
uniform geology that can be understood from field observations. The field area contains

21
sufficient evidence for a comprehensive Sedimentary depositional sequence to be derived. It also
provides the ideal location to understand the concept of a foreland and its surroundings, with the
complex Moine system to the East and the sheared Mylonites to the North, towards Faraid Head.
The key factor in solidifying the lithological evidence was the ages of the rock units. These dates
placed the units within a known time scale from which the history would be developed. This
information was vital in explain the missing stratigraphy.
Substantially more time could be spent in this area, consolidating information and finding
physical evidence for theories developed in this paper.

22
6.0 Bibliography
Cawood, P. A., Merle, R.E., Strachan, R.A. snd Tanner, P.W.G. 2012. Provenance of the
Highland Border Complex: constraints on Laurentian margin accretion in the Scottish
Caledonides. Journal of the Geological Society, London. 169 (1), 575-586.
Fossen, H. 2010. Chapter 14 - Boudinage. In: Structural Geology. New York: Cambridge
University Press, 271 – 283.
Gapais, D and Boundi, A.L.B. 2015. Pegmatite mylonites: origin and significance. In: Faulkner,
D.R., Mariani, E. and Mecklenburgh, J. Rock Deformation from Field Experiments and Theory.
A Volume in Honour of Ernie Rutter. London: The Geological Society of London, Special
Edition 409. 167-173.
Goscombea, B.D., Passchierb, C.W. and Hand, M. 2004. Boudinage classification: end-member
boudin types and modified boudin structures. Journal of Structural Geology. 26, 739–763.
Krabbendam, M., Mendum, J.R. and Goodenough, K.M. 2008. Lewisian, Torridonian and Moine
rocks of Scotland: an introduction - Tectonic Settingand Evolution of the Northern Highlands.
In: Mendum, J.R., Barber, A.J., Butler, R.W.H., Flinn, D., Goodenough, K.M., Krabbendam, M.,
Park, R.G. and Stewart, A.D. Lewisian, Torridonian and Moine Rocks of Scotland, Geological
Conservation Review. No. 34.. Peterborough: Joint Nature Conservation Committee. 9-20.
Leupold, M. 2014. Evolution of Pegmatite Boudins in Marble, Naxos (Greece). Field Course,
RWTH Aachen University.
Park, R.G., Stewart, A.D. and Wright, D.T. 2002. The Hebridean Terrane. In: Trewin, N.H. The
Geology of Scotland. 4th ed. The Geological Society, London, 45-80.
Swett, K. 1969. Interprtation of Depositional and Diagenetic History of Cambrian-Ordovician
Succession of Northwest Scotland. In: Kay, North Atlantic: Geology and Continental Drift, a
Symposium; Papers. California: American Association of Petroleum Geologists. 630-646.
Weaver, B.L. and Tarney, J.. (1981). Lewisian gneiss geochemistry and Archaean crustal
development models . In: Bickle, M.J., Brodholt, Buffett, B.A., Frank, M., Marty, B., Mather,
T.A., Shearer, P., Sotin, C., Stoll, H., Vance, D. and Yin, A. Earth and Planetary Science
Letters, 55. Amsterdam: Elsevier Scientific Publishing Company. 171-180.

23
7.0 Appendix
7.1 Appendix A : Boudin measurements used for Figure 15 and 16
No. LOCATION LITHOLOGY INDIVIDUAL
BOUDINAGE
LENGTH (cm)
UNIT
LENGTH
(cm)
ELONGATION
VALUE
1 4419 6551 Elongated biotite 66 176 0.625
2 4414 6561 Pegmatite 62, 48, 44, 18 292 0.414
3 4414 6561 Quartz 41, 15 86 0.349
4 4413 6561 Elongated biotite
with pegmatite
38, 122, 34, 30 293 0.235
5 4413 6566 Pegmatite 15, 13, 25, 13 175 0.623
6 4411 6566 Elongated biotite 750, 320 1350 0.207
7 4414 6562 Pegmatite 210, 60, 45 395 0.203
8 4416 6568 Elongated biotite
with quartz and
pegmatite
intrusions
275, 165, 367 1100 0.266
9 4418 6571 Pegmatite 35, 32, 41, 20 875 0.854
10 4418 6571 Pegmatite 55, 61 910 0.873
11 4418 6570 Pegmatite 38, 72, 45 227 0.317
12 4417 6570 Pegmatite 64, 45, 52, 40,
45, 50
367 0.193
13 4417 6576 Pegmatite 68, 62, 90, 96,
55
455 0.185
14 4416 6578 Pegmatite 71, 191, 105,
182, 251
861 0.071
15 4422 6589 Pegmatite 104 270 0.615
16 4422 6594 Pegmatite 34, 109, 31 247 0.296
17 4423 6591 Pegmatite 26, 21, 24, 39,
24, 46
276 0.348
18 4422 6589 Quartz 45, 21, 33 104 0.048
19 4417 6586 Pegmatite 55, 52, 28, 62, 7,
21
690 0.659
20 4407 6594 Quartz 24, 47, 34, 53 286 0.448
21 4417 6586 Pegmatite 69, 35 318 0.673
22 4410 6585 Pegmatite 78, 26, 43, 39,
19, 22, 61
605 0.524
23 4410 6585 Pegmatite 52, 17, 9, 18, 10,
17
190 0.353
24 4410 6585 Pegmatite 17, 17, 52, 9, 13,
4, 17
268 0.519
25 4442 6552 Pegmatite 67, 34, 42, 71 241 0.112
26 4441 6556 Pegmatite 28, 32, 34, 29 155 0.206
27 4442 6555 Pegmatite 55, 41, 26, 126 354 0.299
28 4442 6555 Pegmatite 87, 41, 73 302 0.334
29 4440 6571 Pegmatite 11, 15, 26, 58 125 0.120
30 4448 6557 Pegmatite 11, 14, 18 85 0.494

24
7.2 Appendix B: Cross Sections of Leirinmore and Sango Bay.
Leirinmore:
Sango Bay:

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