The area around Isleornsay, 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.

Excursion 1 The area around Isleornsay (Figure 15)[edit]

Purpose: To examine rocks within and immediately below the Moine Thrust Sheet, and the Moine Thrust Plane. (Figure 15)

Aspects covered: Lewisian Gneiss ('normal' gneiss, amphibolite, serpentinite, 'phyllonitic' gneiss from close to the Moine Thrust Plane, mylonites); the Moine Thrust Plane; Torridonian sedimentary rocks (Beinn na Seamraig Group).

Route: (Broadford–Isleornsay)

  1. Ord turn-off–Loch Baravaig
  2. Ord turn-off–Loch nan Dubhrachan
  3. Allt Duisdale
  4. Ard Ghunel

Distance:

  1. 2 kilometres
  2. 200 metres
  3. 2 kilometres
  4. 2 kilometres

Time:

  1. 2 hours
  2. 30 minutes
  3. 2 hours
  4. 2 hours

General comments: Part (iv) involves coastal exposures and, therefore, low tide conditions are necessary. The excursion should be organised such that Part (iv) is undertaken when the tide is most favourable.

This excursion is split into four parts:

  1. The serpentinite at Loch Baravaig
  2. Foliated gneisses at Loch nan Dubhrachan
  3. The Moine Thrust Plane in the Allt Duisdale
  4. Amphibolites on the Ard Ghunel

On the Sleat Peninsula three thrust sheets of Pre-Cambrian rock-types occur with a faulted Cambro-Ordovician foreland sedimentary sequence exposed beneath the lowest thrust plane (10B). The main movements of the thrust planes were to the west, during Lower Palaeozoic times. The tectonic units, in descending order, are:

MOINE THRUST SHEET:

Moine Schists (2C)

Lewisian Gneiss (2A)

Moine Thrust Plane

TARSKAVAIG THRUST SHEET:

Moine Schists (2C)

Lewisian Gneiss (2A)

Tarskavaig Thrust Plane

KISHORN THRUST SHEET:

Torridonian sedimentary rocks (2B)

Kishorn Thrust Plane

FORELAND SEQUENCE:

Cambro-Ordovician sedimentary rocks (2D)

Torridonian sedimentary rocks (2B)

(i) The serpentinite at Loch Baravaig[edit]

From Broadford proceed south along the Broadford-Armadale (A851) road for 16km (10 miles) to the Ord turn-off. Parking for cars, minibuses or a coach is available on the west side of the road at the turnoff. Continue on foot south along the main road for 200m to a gate on the east side of the road. From the gate, the serpentinite mass which is to be examined in this part of the excursion lies 1km to the ESE. Go through the gate and cross a stream over steeply-dipping, foliated (E-W trending), acid gneiss (2A), keeping to the north side of the gated fence. Where the fence ends, continue ESE over the top of the 107m hill. This summit is composed of thinly foliated, acid gneiss containing large quartz segregations. In detail, this gneiss consists of 2–3mm-thick, dark bands rich in hornblende and biotite, together with leucocratic bands (up to 1 cm thick) rich in feldspar and quartz. Continue to the prominent ridge 200m ESE of this summit.

Locality 1 [NG 6810 1000][edit]

The serpentinite (2A) forms a distinct NE-SW trending ridge with an obvious trough on its NW side and Loch Baravaig to the SE. The ridge and trough feature can be traced to the NE for at least 1km. The serpentinite is blue-grey, with a light brown, weathered surface, and lacks an obvious foliation. Under the hand-lens the rock is medium-grained with brown specks of material which represent altered crystals of Fe-rich carbonate. Magnetite segregations, in the form of elongate (2–3cm) crystals, associated with pink-weathered dolomite and magnesite, occur as veins up to 15cm thick. These segregations are readily examined in exposures above the scree slopes on the NW side of the mass.

Return to the road and thence to the Ord turn-off.

(ii) Foliated gneisses at Loch nan Dubhrachan[edit]

(ii) Proceed north from the turn-off, on foot, for 90m along the main (A851) road to the first road-side exposures (a 2m-high rock-face).

Locality 2 [NG 6742 1040][edit]

The rocks here are Lewisian acid gneisses and Lewisian amphibolites (2A). The lower 1m of the exposure is a dark green, medium-grained amphibolite with a distinct foliation (strikes 060°, dips 15° SE). Although predominantly amphibolite, this rock contains patches of quartz plus feldspar. The upper 1m of the exposure is a coarsely foliated, acid gneiss containing prominent augen structures. The augen are up to 20cm long and consist of glassy grey quartz and dull white feldspar. Separating the dominant augen are thin, irregular bands rich in hornblende and biotite. These two minerals also occur as single crystals dispersed throughout this acid gneiss.

Return to the Ord turn-off.

(iii) The Moine Thrust Plane in the Allt Duisdale[edit]

Proceed 1.6km (1 mile) north along the Broadford-Armadale (A851) road to a gated track on the west side of the road. Careful parking for cars, minibuses or a coach is available near the track entrance. Follow the track through three gates, passing sheep pens and a small cottage. From the third gate proceed NNW over open ground to a stile on the south side of the Allt Duisdale.

Locality 3 [NG 6832 1274][edit]

Cross the stream and, 15m upstream, beneath the stream bank, phyllonitic Lewisian gneisses may be examined (2A). This rock is blue-green and thinly foliated. It contains secondary epidote and chlorite, together with sporadic porphyroblasts of hornblende, biotite and albite. Proceed upstream for 70m, passing more of these gneisses exposed in the south bank, and cross onto a small, grass-covered river terrace (on the south bank of the stream). Here, Lewisian gneisses crop out, whilst on the north bank are distinctly red Torridonian arkoses and sandstones of the Beinn na Seamraig Formation (2B). Staying on the south bank of the stream, a further 50m upstream the Moine Thrust Plane (10B), with Lewisian Gneiss overlying younger Torridonian strata, is clearly exposed in the stream bed for at least 20m along strike. The thrust plane dips to the south at a shallow angle. The following section may be examined:

(TOP)

Phyllonitic Lewisian Gneiss

Thrust (with fault gouge)

Mylonitised Lewisian Gneiss (1m)

outweathered quartz-feldspar mylonites

inweathered micaceous mylonites Thrust Torridonian strata

Beinn na Seamraig Formation

(BASE)

Within the mylonites the principal mineral lineation trends E-W, indicating the dominant transportation direction of the Moine Thrust Sheet. From evidence elsewhere it is concluded that all of the thrust sheets of the district were transported westwards (Peach et al. 1910). The 1m-thickness of mylonitised gneiss is highly contorted and constitutes a separate, small thrust sheet (see section above). The Torridonian strata consists of rust-red, well-bedded, micaceous arkoses and sandstones. A further 30m upstream, in an area of shallow waterfalls, are exposures of Torridonian strata on both banks, and more particularly high up in the south bank. These field relationships indicate the presence of a fault trending NW-SE, with a downthrow to the NE. The steep-sided gully further upstream is eroded along the line of this fault.

From here, gain the north bank of the stream and return to the road.

(iv) Amphibolites on the Ard Ghunel[edit]

Proceed north for 1.6km (1 mile) to the Isleornsay turn-off and continue for 1km (0.6 miles) along the minor (A852) road to the road end. Parking for cars, minibuses and coaches (with sufficient room to turn) is available beyond the hotel, at the pier. Walk back along the road to the junction signposting Camascross and gain the beach. Follow the indistinct path above the High-Water Line to the NE side of the Ard Ghunel. Care should be exercised when walking on any of the ground between the High- and Low-Water Lines as it is, in places, particularly muddy and soft underfoot.

Locality 4 [NG 7066 1214][edit]

The Lewisian Gneiss exposed on the small isthmus of land on the NE side of the Ard Ghunel is granulitic, consisting of alternating dark- and light-coloured bands (2A). The dark bands are rich in chlorite and actinolite, imparting the rock with a green colour. The actinolite needles frequently cross-cut the foliation of the rock and attain lengths in excess of 10mm. The light-coloured bands are dominated by feldspar and quartz. Also present are pink, granitic pods, often several centimetres across, typically aligned parallel to the foliation. Proceed SE along the coast for 100m to a large, dark green mass of Lewisian amphibolite which crops out on the beach. This rock is dominated by the amphibole actinolite and chlorite, together with minor sodic plagioclase. Within this amphibolite are large (at least 15cm x 45cm, and more typically 90cm x 150cm), often inweathered, zoned, ultrabasic pods. Matthews (1967) notes the following features:

  1. They are typically either spheroidal or lenticular, with their long axes parallel to the foliation of the surrounding rocks
  2. They consist of talc-dolomite cores (not present in at least half of the examples studied), surrounded by radiating needles of actinolite (coarsest nearest the core, finer-grained nearer the margins of the pod), all of which are encapsulated by a thin rim or zone which is rich in biotite

Matthews (1967) suggests that these zoned bodies have formed in response to metasomatic element transfer between ultrabasic masses and the more acid country-rock gneisses.

Return to the road.

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 geological field work