The Moll Coast, 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 27 Excursion 13 The Moll Coast, location map

Excursion 13 The Moll Coast (Figure 27)[edit]

Purpose: To examine hybrid rocks and granites of the northern part of the Western Red Hills Centre.

Aspects covered: the Loch Ainort Granite; the Northern Porphyritic Felsite; hybrid (mixed-magma) rocks of the northern Marscoite Suite; the Maol na Gainmhich Granite; Torridonian sedimentary rocks. Route: Maol Ban–Luib na Moil–Maol na Gainmhich–Achadh Mor.

Distance: 3 kilometres.

Time: 2–3 hours.

General comments: An excursion involving road-side localities and therefore can be completed on foot (doubling the distance to 6km) or by a series of road-side stops by car, minibus or coach.

Maol Ban is on the coastal road which runs along the NW side of Loch Ainort between Sconser and Luib. Proceed along the Broadford-Portree (A850) road, 13km (8 miles) from Broadford or 27km (17 miles) from Portree, to the turn-off for the old coastal road at the southern end of Loch Ainort. From here, follow the old coastal road (no signposts) along the NW side of Loch Ainort for 5km (3 miles) to Maol Ban. Parking for 3–4 cars, 2–3 minibuses or one coach is available on the west side of the road at a small, disused, road-side quarry, directly inland from a sand-spit on the shore. In the immediate area, the Loch Ainort Granite (6F) crops out.

Locality 1 [NG 5648 2976][edit]

The Loch Ainort Granite (6F) is most easily examined in this road-side quarry. On weathered surfaces the granite is mottled dark green to rust red, but when fresh is greenish-blue. It is a porphyritic rock, with glomeroporphyritic crystals of anorthoclase, together with the dark minerals ferrohedenbergite and hornblende. The groundmass is dominated by quartz and alkali feldspar in a granophyric intergrowth. Veins of late-stage felsite occur in the back-wall of the quarry. Proceed NW along the road, over the Moll River, and thence over a second bridge. The traverse involved in Locality 2 commences 200m beyond the second bridge, at the beginning of a concrete barrier fence on the loch side of the road. The rocks are exposed in a road-cut on the landward (west) side of the road.

Locality 2 [NG 5616 3080][edit]

Here, the following continuous section (from south to north) is exposed (modified from Wager et al. 1965):

Northern Porphyritic Felsite (61)
Fractured, with phenocrysts of alkali feldspar (outweathered) and quartz
Typical unweathered rock is dark grey
(INNER, SOUTHERN MARGIN OF THE MOLL SHORE INTRUSION OF THE NORTHERN MARSCOITE SUITE (6H))
Marked by a distinct break in slope on the hillside west of the road, and located in the road-cut a few metres north of a culvert under the road. This contact is vertical
Marscoite, with chilled contact against the Northern Porphyritic Felsite. Pale grey, with obvious, large (1cm), white-weathering xenocrysts of plagioclase 20m
Glamaigite, with streaky appearance and veined with felsite. Gradational contact with marscoite on either side. An inhomogeneous rock of granitic appearance 30m
Marscoite, with chilled contact against the Maol na Gainmhich Granite (6E). Pale grey, with obvious, large (1cm), white-weathering xenocrysts of plagioclase 25m
(OUTER, NORTHERN MARGIN OF THE MOLL SHORE INTRUSION OF THE NORTHERN MARSCOITE SUITE (6H))
Not marked by a distinct break in slope. Located 10m south of a small culvert under the road, at the northern end of the continuous portion of the concrete barrier fence on the loch side of the road
Crushed Maol na Gainmhich Granite (6E)
Light-coloured, coarse-grained rock containing amphibole (arfvedsonite)

45m north of the marscoite–Maol na Gainmhich Granite contact, at a slight rise in the road, the granite gives way to an exposure of highly-altered, medium-grained, basic rock with obvious crystals of secondary biotite. This intrusion occurs within the Maol na Gainmhich Granite, although its relationship to the other rocks described above is uncertain. Proceed 500m north along the road, through Maol na Gainmhich Granite, to a small parking area on the landward (west) side of the road. Here, the granite gives way, abruptly, to a 70m-wide outcrop of Northern Porphyritic Felsite (6I). This outcrop is dominated by phenocrysts of alkali feldspar (up to 1cm) in a glomeroporphyritic arrangement, together with less-obvious phenocrysts of quartz. At the bend in the road, marked by a break in slope on the hillside to the west, is the near-vertical boundary between the felsite and country-rock Torridonian strata (2B). Further exposures of these non-fossiliferous, fine-bedded, highly-fractured, grey, green and pink grits, sandstones and siltstones may be examined along the coast, NE of the road.

Continue west along the road for a further 2km (1.2 miles) to the large road-side quarry at Achadh Mor. This quarry is within metamorphosed Torridonian grits, sandstones and siltstones (2B) which are worked as a source of aggregate. Rejoin the Broadford-Portree (A850) road 1km further west at Tormichaig.

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