The Allt Daraich section, 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 28a Excursion 14 The Allt Daraich section, map
Figure 28b Excursion 14 The Allt Daraich section—generalized vertical section

Excursion 14 The Allt Daraich section (Figure 28a), (Figure 28b)[edit]

Purpose: To examine the relationships between hybrid rocks of the northern Marscoite Suite.

Aspects covered: hydrothermally-altered and brecciated plateau lavas; dioritic glamaigite; glamaigite; marscoite; Glamaig Granite.

Route: Sligachan–Allt Daraich–(Sligachan).

Distance: 5 kilometres.

Time: 4 hours.

General comments: An excellent stream-section with well-exposed contacts between hybrid rocks and granite.

Sligachan lies at the head of Glen Sligachan on the Broadford-Portree (A850) road. It is 26km (16 miles) from Broadford and 14km (9 miles) from Portree. Ample parking for cars, minibuses and coaches is available in the area, for example, on the south side of the road 100m east of the new bridge over the River Sligachan. Proceed south along the Loch Coruisk path, which starts between the two bridges on the old road, on the east side of the River Sligachan. Details of the view looking south into Glen Sligachan are provided in Locality 1 of Excursion 15. Follow the path for 250m, to where it diverges from the course of the Allt Daraich. Continue along the rough path on the SW side of the Allt Daraich. In passing, note the Lower Tertiary plateau lavas (3D) which crop out within the stream bed, cut by a NW-SE -trending dyke of the regional swarm (9B), giving the stream its distinctly straight form. These lavas are, in places, intensely altered and contain numerous generations of cross-cutting, anastomosing veins of secondary minerals such as calcite and epidote (12F). At the waterfalls, continue upstream between the old deer fence and the Allt Daraich for a further 1km, past a small confluence on the south side of the stream. At [NG 5002 2966], a 3m-high knoll of plateau lava adjacent to a small stone wall upon a point-bar is reached. On the NE bank of the stream, immediately opposite, are the ruins of an old stone bothie. The Allt Daraich section runs from here, SE, for approximately 500m.

Locality 1 [NG 5002 2966][edit]

The section to be examined is shown in (Figure 28b). The rock-types involved within this steep-sided ring-dyke (6H) are:

  1. marscoite—a pale grey rock with obvious (up to 1cm) xenocrysts of plagioclase (andesine) with distinctly rounded, corroded margins
  2. glamaigite—a distinctive, obviously inhomogeneous rock consisting of dark, round inclusions set in a paler 'matrix'. Xenocrysts of plagioclase (andesine) occur within both components
  3. dioritic glamaigite—a more homogeneous form of glamaigite, which has the appearance of a 'mafic' granite

The order of intrusion is: (1), then (2), then (3). Further details of these rock-types are presented in Section (H) of Chapter 6.

Country-rocks to the ring-dyke are:

(1) hydrothermally-altered and brecciated plateau lavas (3D)—pale grey, typically non-porphyritic, and veined with calcite, epidote and chlorite; (2) The Glamaig Granite (6D)—a medium-grained, hornblende- and biotite-bearing granite containing characteristic, small (1–2cm), rounded to sub-angular, mafic inclusions.

Proceed upstream from the knoll. The first 85m consists of hydrothermally-altered and veined plateau lavas (3D), (12F). Close to the margin of the ring-dyke, the lavas are cut by a 3m-wide apophysis of dioritic glamaigite ((a) on (Figure 28b). There is then a break in the exposure for approximately 15m before relatively continuous exposures of dioritic glamaigite (b) occur. Continue upstream through a further length of no exposure then more dioritic glamaigite (c) to the outcrop of (heterogeneous) glamaigite (d). 20m beyond is a small mass of marscoite (e). Proceed upstream for approximately 100m to the next exposures. Here, glamaigite is found, interspersed with a narrow mass of marscoite which crops out in the SW bank of the stream (f). Within the stream bed, below where the old deer fence crosses the stream, extremely heterogeneous glamaigite is found, containing veins of more leucocratic material (g). Continue upstream, into a narrow gorge, to a well-exposed, sharp contact between glamaigite and marscoite (h). The glamaigite intrusion cuts the marscoite intrusion. Proceed further upstream to (i), where marscoite is chilled against Glamaig Granite (again well-exposed in the stream bed).

Return to Sligachan.

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