Camasunary Bay, Skye - an excursion
|From: Bell, B.R. and Harris, J.W. An excursion guide to the geology of the Isle of Skye : Geological Society of Glasgow, 1986. © 1986 B.R. Bell & J.W. Harris. All rights reserved.|
- 1 Excursion 10 Camasunary Bay (Figure 24)
- 2 References
- 3 Appendix 1: Glossary of petrological names and terms
- 4 Appendix 2: Glossary of fossil names
- 5 Appendix 3: Glossary of place names and grid references
Excursion 10 Camasunary Bay (Figure 24)
Purpose: To examine rocks associated with the SE margin of the Cuillin Complex.
Aspects covered: Jurassic sedimentary rocks; Lower Tertiary lavas and dykes; Torridonian sedimentary rocks (some of which have been subjected to high-temperature mobilisation (rheomorphosed)); a "25-Foot" raised beach; Jurassic marbles; the margin of the Cuillin Complex; cone-sheets; the Outer Layered Eucrite Series; the Coire Uaigneich Granite.
Route: Kirkibost–Am Mam–Abhainn nan Leac–Camasunary Cottage–Sgurr na Stri–Abhainn Camas Fhionnairigh–(Kirkibost). Distance: 12 kilometres.
Time: 8 hours.
General comments: An excursion which examines layered rocks of the Cuillin Complex whilst staying on relatively low ground. The Abhainn Camas Fhionnairigh has to be forded in order to examine Localities 7 and 8. Therefore, during periods of heavy rainfall, the use of wellington boots is advisable.
Proceed to Kirkibost on the Broadford-Elgol (A881) road, 19km (12 miles) SW of Broadford. 500m SW of Kirkibost the Camasunary footpath joins the main road. Space for 10 cars, 4 minibuses or one coach is available on the west side of the road. Coaches may turn either at the Drinan turn-off, 1.6km (1mile) to the south, or, further south, in Elgol, 100m beyond the Post Office, at a former petrol station.
Locality 1 [NG 5450 1720]
Looking NE from the beginning of the footpath, note the trap topography of An Carnach, with Lower Tertiary plateau lavas (3D) overlying the lower cultivated ground composed of Upper Jurassic shales, sandstones and limestones (2F), all of which dip at a shallow angle to the west. To the NNW, individual lava flows (at least 20) may be identified readily on Slat Bheinn. In the immediate area, Jurassic strata belonging to the Great Estuarine Group are found, although poorly exposed. Proceed 1.5km (1 mile) along the footpath over Jurassic strata (Great Estuarine Group and Staffin Bay Formation) to where a gate crosses the path.
Locality 2 [NG 5326 1776]
Exposed on the path, 30m west of the gate, are basic lavas which occur at the base of the Lower Tertiary plateau sequence of Strathaird (3D). These lavas are distinctly green and contain visible secondary chlorite, epidote, albite, calcite and quartz. Secondary zeolites are rare. Fresh olivine is uncommon and is typically replaced by aggregates of chlorite, serpentine, talc, magnetite and carbonate. Primary features of the lavas, such as flow structures, have been destroyed by the pervasive hydrothermal alteration which occurred in Lower Tertiary times (12F). From here, continue NW along the footpath, upwards through the lava sequence, to Am Mam.
Locality 3 [NG 5250 1800]
At Am Mam, 15m south of the path, a large doleritic dyke (at least 10m wide) forms a prominent feature, trending 150° (9J). This intrusion consists of euhedral calcic plagioclase megacrysts (typically up to 10mm) and olivine phenocrysts (1–2mm), set in a fine-grained, basic groundmass. A vertical contact to this dyke is exposed on its west side, together with a 30cm-thick skin of contact-metamorphosed plateau lava. This dyke has tholeiitic affinities and may have had a liquid composition similar to that advocated for the primary magma of the Cuillin Complex (12C). Proceed along the path into the east side of Camasunary Bay. At [NG 5220 1870] the path crosses a small burn at a narrow wooden bridge. Look NNE up the valley of the Abhainn nan Leac, with Blaven on the west and Slat Bheinn on the east. The lower, heather-covered slopes of Blaven are composed predominantly of Lower Jurassic strata (2F), whilst the upper, bare rock-faces are eucrites of the Outer Layered Series of the Cuillin Complex (4F), cut by shallow-dipping, concordant cone-sheets (9D) and numerous large, vertical dykes of the Lower Tertiary regional swarm (9B). The crags below the summit of Slat Bheinn are composed of plateau lavas (3D), underlain by Upper Jurassic strata. Separating the Lower and Upper Jurassic rocks, and marked by a spring-line parallel and to the east of the Abhainn nan Leac, is the Camasunary Fault (10D). At the head of the valley, marked by a broad, steep band of scree, is the Coire Uaigneich Granite (4L), forming a near-vertical intrusion marginal to the Cuillin Complex. Proceed along the main path, into the bay, to where another bridge crosses the Abhainn nan Leac.
Locality 4 [NG 5182 1864]
In the vicinity of the bridge, relatively undeformed Torridonian siltstones, sandstones and grits (2B) are well-exposed. These sedimentary rocks show occasional cross-bedding and grading features. Basaltic and doleritic dykes of the Lower Tertiary regional swarm (9B) cut these rocks and are seen in the stream bed south of the bridge. Follow the path to the far (west) side of Camasunary Cottage.
Locality 5 [NG 5156 1884]
Camasunary Cottage sits upon a "25-Foot" raised beach (11C). The hummocky ground 500m to the NW constitutes the irregular boundary between the Cuillin Complex and Torridonian country-rocks (2B). Also present (at [NG 5118 1922]) is a small outcrop of Lower Jurassic marble, encircled and partially covered by a 15m x 10m area of bright green grass. This rock-mass has been variously interpreted as: (1) in situ material above Torridonian strata (Harker 1904; Carr 1952); and, (2) a fault-block (Almond 1960). Traversing NW from the cottage in a straight line towards the marble outcrop, cross the raised beach and reach the Torridonian strata. Veering slightly to the north, proceed along the NE side of the valley running up to the marble. Along this traverse the following features may be noted, becoming more obvious towards the margin of the Cuillin Complex:
- Bedding and cross-bedding within the Torridonian strata become distorted (rheomorphosed)
- Dykes depart from normal linear trends
- Dyke margins become more irregular
- Veins of felsitic and granitic material cut the dykes and Torridonian strata
- The presence of "breccias", within the Torridonian country-rocks, consisting of gabbro, dolerite and basalt fragments in a granitic host
The source of the felsitic and granitic material noted in (d) and (e), above, is believed to be from the melting of Torridonian sedimentary rocks at depth (4M). Proceed 150m NW from the marble outcrop onto flat-lying exposures of eucrite.
Locality 6 [NG 5110 1934]
The margin of the Cuillin Complex here consists of a coarse-grained, unlayered eucrite (4B), which invades partially-melted Torridonian sedimentary rocks (2B) in the form of large, irregular lobes. Chilled marginal facies of the eucrite have not been identified. The eucrite is itself cut by veins of granite and felsite which were generated by the melting of Torridonian strata. This feature is referred to as back-veining. The area immediately to the NW of this locality consists entirely of unlayered eucrite, cut by numerous dykes of the regional swarm (9B). The eucrite is composed predominantly of clinopyroxene (augite) and plagioclase (bytownite to labradorite), together with lesser amounts of olivine, orthopyroxene (often pseudomorphed by bastite) and Fe-Ti oxides. Large oval or circular segregation pods and veins are common, containing large (up to 20mm) crystals of augite and plagioclase. These crystals have developed, presumably, in response to a more hydrous environment within the magma chamber. Also present within the eucrite are large, basic xenoliths (eucrite, dolerite and basalt), up to several metres across. The margin of the complex may be traced SE, across the Abhainn Camas Fhionnairigh, north of the small group of sand spits, onto the Sgurr na Stri Peninsula. Here, the eucrite is in contact with plateau lavas (3D) which have been contact-metamorphosed up to pyroxene hornfels facies (within 300m of the contact). The contact is inclined to the north at approximately 40° and is marked by a distinct break in slope at around 150m O.D. Within the lavas, primary (igneous) clinopyroxene has been replaced by granules of secondary (metamorphic) orthopyroxene and clinopyroxene. In the most severe case of this contact metamorphism, which occurs on the Sgurr na Stri Peninsula, plagioclase is also closely involved in this granulitic textural arrangement of secondary minerals. Xenoliths of metamorphosed basalt are a common feature within the eucrite close to this part of the contact.
Locality 7 [NG 5068 1884]
On the SE side of Sgurr na Stri, the eucrite along the margin of the Cuillin Complex (4A) is a coarse-grained, unlayered type (4B) which, in places, veins the country-rock lavas. Within a few metres of the contact crude layering is discernible, dipping at a shallow angle (15°) to the north. The dip of the layering gradually increases up the sequence. These layered rocks belong to Zone I of the Outer Layered Eucrite Series (4F). Layering is seen on a variety of scales (millimetres up to several tens of centimetres) and is defined by differing proportions and grain-size variations of pyroxene, plagioclase, olivine and Fe-Ti oxides. Features approximating to current flow structures, graded-bedding, flaser structures and wash-out channels may be identified within these rocks. Cutting the rocks of the Cuillin Complex are members of the Cuillin group of cone-sheets (9D), dipping at a shallow angle to the north, which are approximately concordant to the mineral layering. The cone-sheets are either dolerites or basalts, typically less than 1m thick, and dip towards a focal point below Meall Dearg, 4km to the north. Members of the Lower Tertiary regional dyke swarm (9B) are common on Sgurr na Stri. Proceed down the SE side of Sgurr na Stri to the west bank of the Abhainn Camas Fhionnairigh, opposite the present-day beach (11C) and approximately 100m downstream from the remains of the footbridge.
Locality 8 [NG 5094 1874]
The contact between the plateau lavas (3D) and the Coire Uaigneich Granite (4L) is exposed clearly on the west bank of the Abhainn Camas Fhionnairigh. These two contrasting rock-types (the dark green lavas and the pale grey granite) are readily distinguished from each other. Brecciation by the granite has led to the incorporation of blocks of the lava into the granite. It may be noted that the boundary between these two rock-types dips to the SE at an angle of approximately 60°. 10–20m south from this contact, where the granite is exposed on a wave-cut platform, several of its features may be observed. First, scattered xenoliths of Torridonian sedimentary rock, in various stages of digestion, may be identified. These xenoliths are typically 1–10cm across, although 'rafts' up to 5m long are found. Their margins are often irregular and diffuse. This foreign material is typically finer-grained and somewhat darker than the granite. Second, the granite is pale grey and contains needles of hypersthene, up to 1cm long (commonly altered to chlorite), together with less obvious phenocrysts of sodic plagioclase, tridymite (now inverted to quartz) and Fe-Ti oxide. The distribution of these minerals is irregular and all are set in a granophyric groundmass of alkali feldspar and quartz. Models for the genesis of the Coire Uaigneich Granite are presented in Section (4L) of Chapter 4. Cone-sheets cut the Coire Uaigneich Granite. The outcrop of the granite may be traced along the beach until it gives way, abruptly, to well-bedded Torridonian siltstones, sandstones and grits (2B), dipping at a shallow angle to the NW. The contact strikes 040° and dips at 35° to the NW.
Return to Kirkibost across Camasunary Bay, rejoining the main path east of the Abhainn nan Leac.
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