Beinn an Dubhaich, Skye - an excursion

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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.
Figure 18 Excursion 4 Beinn an Dubhaich, location map

Excursion 4 Beinn an Dubhaich (Figure 18)[edit]

Purpose: To examine the Beinn an Dubhaich portion of the Outer Granite of the Eastern Red Hills Centre and the surrounding country-rocks.

Aspects covered: the Outer Granite (Beinn an Dubhaich portion) of the Eastern Red Hills Centre; a "25-Foot" raised beach; Cambro-Ordovician carbonates; deformed dykes of the Lower Tertiary regional swarm; Lower Jurassic sandstones, limestones and shales; a gabbro dyke; Triassic conglomerates; Cambrian quartzites; Torridonian sandstones; contact metamorphism and skarn development within the Cambro-Ordovician carbonates.

Route: Camas Malag–Allt nan Leac–Allt Poll a' Bhainne–Glen Boreraig–Allt nan Leac Beinn an Dubhaich–Kilbride–(Camas Malag). Distance: 9 kilometres.

Time: 8 hours.

General comments: Coastal exposures are involved and, therefore, low tides are important (but not critical).

Proceed to Kilbride on the Broadford-Elgol (A881) road, 8km (5 miles) SW of Broadford. 1.5km (1 mile) west of Loch Cill Chriosd the road branches. Follow the less-prominent left-hand (south) fork over the cattle grid, along the gated-road, to Camas Malag. NW of the road is a large marble quarry, the working face of which exposes deformed basic intrusions (9P). The marble is used either as aggregate or as a source of agricultural lime. Ample parking and turning space for cars, minibuses and coaches is available on the grass-covered "25-Foot" raised beach SW of the road.

Locality 1 [NG 5826 1928][edit]

From this locality several features of the general geology of the surrounding area may be noted. Due west, on the Strathaird Peninsula, Lower Tertiary plateau lavas (3D) overlie Middle and Upper Jurassic strata (2F), all of which dip at a shallow angle to the west. Further west, the irregular ridge of Blaven marks the eastern edge of the Cuillin Complex (4A). Basic dykes of the Lower Tertiary regional swarm (9B) cut the layered rocks which crop out on Blaven and are frequently inweathered or outweathered. 4km to the north of the present locality the Inner Granite (7G) of the Eastern Red Hills Centre (7A) forms the prominent summits of Beinn na Caillich, Beinn Dearg Mhor and Beinn Dearg Bheag. Directly east, the southern (Beinn an Dubhaich) portion of the Outer Granite (7F) of the Eastern Red Hills Centre intrudes the core of a broad anticlinal structure within Cambro-Ordovician carbonates (2D). Dykes of the Lower Tertiary regional swarm (9B) are common throughout the surrounding area and are particularly well-exposed along the coast. However, they are rare within the relatively late-stage granite which crops out on Beinn an Dubhaich. At Camas Malag a "25-Foot" raised beach (11C) lies immediately to the NE of the present-day beach. The cobbles and sand of this raised beach can be examined in the banks of the small stream directly to the SW of the small bridge at the western end of the bay. Cobbles of the following rock-types are identified readily: granite, basalt, dolerite, Lewisian Gneiss, Moine Schist, Torridonian sandstone, Cambro-Ordovician carbonate, and Jurassic sandstone, limestone and shale. The wave-cut platform at the NW end of the Camas Malag is composed of the Beinn an Dubhaich portion of the Outer Granite (7F). This is a slightly porphyritic leucogranite, containing phenocrysts of alkali feldspar and quartz. The main mafic minerals are hornblende and biotite. Rare, fine-grained, mafic inclusions, typically rounded and up to 5cm across, are dispersed throughout the volume of this intrusion. Proceed to the headland on the south side of the Camas Malag.

Locality 2 [NG 5822 1886][edit]

This headland is partly composed of granite, giving way, to the south, after a prominent gully, to Cambro-Ordovician carbonates (Ben Suardal Dolostones). These carbonates have been recrystallised to calc-silicate -bearing marbles by the granite and have undergone substantial de-dolomitisation. Throughout these steeply-dipping, fine-bedded, alternating light- and dark-banded rocks are numerous chert nodules, which have reacted to varying extents with the carbonates, producing concentric bands of calc-silicate minerals, including: talc, tremolite, diopside, and forsterite (70. It has been suggested that these irregular-shaped, cherty nodules are organic in origin, consisting of structures such as worm casts (Peach et al. 1910). The Cambro-Ordovician carbonates are cut by deformed dykes of the Lower Tertiary regional swarm (9P). These often near-vertical minor intrusions are typically less than 1m wide and are predominantly basalts or dolerites. The most obvious feature shown by the dykes is the development of boudin structures, with the necks of the individual boudins exhibiting remarkable amounts of thinning. The dykes have well-developed chilled margins, even within the necks, suggesting that cooling was contemporaneous with deformation. Little, if any, penetrative tectonic fabric is present within these dykes, which is in contrast with the country-rocks, which contain cleavages which are deflected into the boudin necks. It is likely that these dykes were intruded whilst the district was being subjected to radial compressive forces, during the emplacement of the nearby Outer Granite. A good example of these relationships is shown by the third dyke which crops out on the headland south of the near-vertical granite-marble contact. 10m south of this dyke, good examples of large chert nodules within the marbles may be examined. Walk 500m south, around the headland over Cambro-Ordovician carbonates (marbles), to where large slabs of well-bedded Lower Jurassic limestones, sandstones and shales (2F) form a prominent feature above the High-Water Line. Traversing the marbles the following features should be noted:# The development of a typical limestone pavement containing clints and grykes

  1. The highly folded nature of the strata
  2. The presence of basic dykes of the Lower Tertiary regional swarm
  3. Approximately midway (250m) between Localities 2 and 3 the Triassic Camas Malag Formation crops out (2E). These thinly-bedded sandstones, siltstones, mudstones and conglomerates are nonfossiliferous and represent lacustrine and alluvial fan deposits. They dip at 25–35° to the west and are most easily examined on the near-vertical back wall of the beach.

Locality 3 [NG 5852 1832][edit]

The unconformity separating Cambro-Ordovician carbonates (2D) and the overlying Lower Jurassic strata of the Upper Broadford Beds (2F) occurs where the clint and gryke pavement of the former gives way to the predominantly heather-covered ground of the latter. Directly north of the mouth of the Allt nan Leac is a 60m-wide inlier of Cambro-Ordovician carbonates, exposed above the High-Water Line. Unconformably above the rocks of the inlier is the following Lower Jurassic sedimentary sequence (modified from Hallam 1959):


(TOP)
4. Sandstones and sandy shales; Arnioceras (aff. semicostatum) and ?Euasteroceras spp. in a sandstone bed 10m from the top 100
3. Thin, wavy and laminar, sandy shales and sandstones; Arnioceras (aff. semicostatum) common; Gryphaea (arcuata), Oxytoma (inequivalve) 5
2. Thin, laminated limestones and subsidiary shales with derived Gryphaea at the base 3
1. Thin limestones and shales (shell beds); Gryphaea and Oxytoma common. Fragments of Cambro-Ordovician carbonate and chert at the base of the sequence 1.5
(BASE)

Unit 1 crops out at the mouth of the Allt nan Leac.

Units 2 and 3 crop out directly south of that river.

Unit 4 crops out south of a 3m-wide, multiple, basalt dyke which also cuts Unit 3 on the foreshore. The rocks of Unit 4 are exposed on a 25m-high cliff face that can be traced south as far as the Allt Poll a' Bhainne.

At the headland 300m south of the Allt nan Leac the Unit 4 sandstones and sandy shales are cut by a 10m-wide olivine gabbro dyke. Inland, this steep-sided intrusion widens and forms a prominent ridge running ESE into Glen Boreraig. Continue south along the beach from the dyke to a point 100m south of the Allt Poll a' Bhainne (200m south of the Allt a' Ghairuillt) to where the overlying (Lower) Pabba Beds (2F) he conformably upon the Broadford Beds. The Pabba Beds are composed predominantly of well-bedded shales which contain calcareous concretions ("doggers"). These concretions are typically lens-shaped, up to 50cm thick, and several tens of centimetres long. The Pabba Beds can be traced south to Rubha Suisnish and thence east towards Boreraig. Return north along the beach for 200m to the mouth of the Allt a' Ghairuillt and thence upstream to the main track [NG 5902 1768]. From the small bridge over the Allt a' Ghairuillt proceed ENE along the break in slope which marks the northern side of Glen Boreraig and continue until the southern side of the valley of the Allt nan Leac is reached (a distance of approximately 1km).

Locality 4 [NG 6000 1816][edit]

Within the upper reaches of the Allt nan Leac Triassic strata (2E) crop out on the southern side of the valley. These continental sedimentary rocks were deposited in alluvial fans and form a number of prominent exposures, a good example being in the vicinity of a small group of rowan (mountain ash) trees associated with solution hollows in the underlying Cambro-Ordovician carbonates (2D). In detail, these rocks are conglomerates containing matrix-supported clasts derived, predominantly, from the underlying Cambro-Ordovician carbonates. Continue east along the south side of the valley for 300m to a prominent rock-face which is composed of Lower Cambrian quartzite (2D) (the Kilchrist-Suisnish footpath runs across the base of this face). The quartzites are relatively pure, not obviously bedded, and are highly fractured. 300m further east (up the valley), marked by a minor break in slope, the quartzites give way to prominent red sandstones and grits of the Applecross Group of the Torridonian sedimentary sequence (2B) which dip to the SW. In places, these rocks contain thin veins of quartz. The Cambrian quartzites unconformably overlie the Torridonian strata and both have been thrust over the underlying Cambro-Ordovician carbonates as part of the Kishorn Thrust Sheet (10B). The line of the thrust plane is marked by the conspicuous depression which forms the E-W -trending upper reaches of the Allt nan Leac. From here, proceed NW towards a point between the two obvious summits of Beinn an Dubhaich. Note the distinct grey colour of the topographically lower Ben Suardal Dolostones, in contrast to the white colour of the upper, although stratigraphically lower, Strath Suardal Dolostones (2D). This relationship is due to the form of the anticlinal structure which trends approximately E-W in the vicinity of Beinn an Dubhaich (2D). The Strath Suardal Dolostones are in contact with the granite of Beinn an Dubhaich (7F), which forms the southern part of the Outer Granite of the Eastern Red Hills Centre (7A). The boundary is generally steeply-inclined and is particularly clear due to the obvious differences between the two rock-types involved. Pass between the two summits of Beinn an Dubhaich, on the east side of Loch nan Learg, thence north down the grassy slope on the north side of Beinn an Dubhaich, through a small coppice of silver birch, and across the flat ground in a line towards Kilbride House. Rejoin the granite-carbonate contact at the foot of the slope and follow it NW towards Kilbride.

Locality 5 [NG 5902 1982][edit]

25m south of the fence bounding the cottage at [NG 5904 1994] is a small (8m x 3m), magnetite-dominated skarn (7I) at the boundary between the granite and the Cambro-Ordovician carbonates (Ben Suardal Dolostones). The skarn has the form of an elongate pod, parallel to the contact. The skarn material has weathered to a distinctly rusty-red colour due to the oxidation of the magnetite. Other minerals include: diopside, cuspidine, clinohumite, and monticellite. This skarn belongs to the Group (II) described in more detail in Section (7I) of Chapter 7.

Follow the fence WSW and rejoin the Kilbride–Camas Malag road and thence proceed SW to Camas Malag.

References[edit]

Appendix 1: Glossary of petrological names and terms[edit]

Appendix 2: Glossary of fossil names[edit]

Appendix 3: Glossary of place names and grid references[edit]

At all times follow: The Scottish Access Codeand Code of Conduct for Fieldwork .