Loch Coruisk - Druim Hain, 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 25 Excursion 11 Loch Coruisk–Druim Hain, location map

Excursion 11 Loch Coruisk–Druim Hain (Figure 25)[edit]

Purpose: To examine layered basic and ultrabasic rocks of the Cuillin Complex.

Aspects covered: banding, brecciation and layering within the Layered Peridotite Series; layering and blocks within the Outer Layered Eucrite Series, the Inner Layered Allivalite Series and the Inner Layered Gabbro Series; the Druim nan Ramh Eucrite; a fault zone marginal to the Inner Layered Gabbro Series.

Route: Loch nan Leachd–Allt a' Chaoich–Allt Beag (the Nameless Burn)–Meall na Cuilce–Loch Coruisk–Allt a' Choire Riabhaich cairns north of Sgurr Hain–Druim Hain.

Distance: 12 kilometres (excluding access into and out of the area, see below).

Time: 12 hours (see General comments, below).

General comments: Loch nan Leachd is at the head of Loch Scavaig (Figure 25). During the summer months it may be reached by a tourist boat service from Elgol (on the west side of the Strathaird Peninsula). No timetable is available. To ensure the return journey by boat it may be necessary to negotiate times with the boat operator. Alternative access may be gained by walking from Kirkibost on the Strathaird Peninsula [NG 5450 1720] into Camasunary Bay [NG 5130 1884], thence across the Abhainn Camas Fhionnairigh and following the indistinct path around the southern side of Sgurr na Stri to Loch nan Leachd (10km, 4 hours). It is important to note that to walk in from Kirkibost, around the Sgurr na Stri Peninsula, to Loch nan Leachd, and out from Druim Hain (the last locality of the excursion) via Loch na Creitheach to Kirkibost will take approximately 8 hours. Furthermore, on the inward journey, the so-called Bad (or Ladies') Step on the west side of the Sgurr na Stri Peninsula will have to be negotiated with care, but without difficulty. Also, during periods of heavy rain, the Allt a' Chaoich may be impossible to cross (see Locality 1, below). It is advisable, therefore, to allow 12 hours for the completion of this excursion and only to undertake it in fine weather. A third procedure is to take the boat into Loch nan Leachd and to walk out from Druim Hain via Loch na Creitheach to Kirkibost. The three alternatives which have been suggested are illustrated in (Figure 25).

Locality 1 [NG 4902 1928][edit]

On the west side of Loch nan Leachd there is a small (56m) hill. From here the general disposition of the various rocks of the southern part of the Cuillin Complex may be noted (4A). To the west, on the other side of Loch na Cuilce, the main ridge of the southern part of the Cuillin Hills rises dramatically from sea level to over 900m O.D. The southern part of the ridge is composed of the Gars-bheinn Gabbro (4B), together with large sheets of basalt and dolerite, variously interpreted as: (1) large blocks of lava which foundered into the Cuillin magma chamber (Harker 1904); and, (2) intrusions (Bailey 1952; Hutchison 1966a). Recent work suggests that (2) is more likely (3D, 9E). North of the main ridge the deeply-eroded An Garbh-choire marks the outcrop of the Layered Peridotite Series (4D). These rocks weather to a distinct brownish-orange and are readily identified from a distance on a clear day. Below these rocks (to the NE), in the Meall na Cuilce area, the lower units of the Outer Layered Eucrite Series (4F) crop out. The basin of Loch Coruisk has been scoured out of the Zone II members of this series. To the NE of the loch the ridge of Druim nan Ramh marks the line of the Druim nan Ramh Eucrite (4H). The Zone I rocks of the Outer Layered Eucrite Series (4F) crop out locally and exhibit good examples of crystal lamination and slump structures. Xenoliths of basalt, dolerite, allivalite and peridotite are present throughout the series. Also common are large plagioclaseclinopyroxene segregation pods and veins, with crystals in excess of 4cm long. Presumably, these have developed in response to a more hydrous environment within the magma chamber. From here, cross the Coruisk River at the stepping stones and walk around the head of Loch na Cuilce, noting the similar igneous features on that side of the river, and particularly good layering on the SE side of Meall na Cuilce. Continue past the Coruisk Memorial Hut and the mouth of the Allt a' Chaoich (also referred to as the 'Mad' Burn and best crossed near the High-Water Line). From here, proceed SW to the northern side of the Allt Beag (sometimes referred to as the 'Nameless' Burn).

Locality 2 [NG 4824 1954][edit]

On the north side of the Allt Beag, Zone I rocks of the Outer Layered Eucrite Series (4F) crop out. These rocks are plagioclase-olivine-clinopyroxene orthocumulates, without the development of good layering. Proceed approximately 500m up the north side of the Allt Beag to an elevation of 150m O.D., to where continuous exposures of the Layered Peridotite Series (4D) may be examined in the immediate vicinity of the burn. En route to this elevation note isolated outcrops of peridotite on the north side of the stream, together with xenoliths of peridotite (up to 1m across) within the Outer Layered Eucrite Series (4F). The peridotites are readily identified by their distinct brownish-orange weathering characteristics (due to the breakdown of olivine, releasing iron) and 'tortoise-shell' fracture pattern. The main outcrop of the Layered Peridotite Series lies to the south of the Allt Beag, which here flows along the southern side of the rock basin formed mainly by the upper tributaries of the Allt a' Chaoich. Proceed upstream on the south bank of the Allt Beag through peridotites of the so-called Zone I (4D). At the SE side of the rock-basin the Allt Beag turns sharply to the south. Follow the stream to where it dies out. The area to the south consists of continuous exposures of the Zone I peridotites and these form the south and SW side of the rock basin draining from the Coire a' Chruidh and Coire Beag. From these rock-faces proceed north to a knoll in the confluence area of the upper tributaries of the Allt a' Chaoich. This knoll consists of banded Zone II peridotites. To the north of the knoll are large erratics of brecciated Zone III peridotites, consisting of angular fragments of brownish-orange-weathering peridotite in a matrix of paler, feldspathic peridotite. The presence of feldspathic peridotite veins and associated brecciation of the host-rocks suggests that significant post-depositional movement of intercumulus liquid has taken place, resulting in the segregation of cumulus crystals and the liquid trapped in the interstices of the crystal pile. From here, proceed north and across the Allt a' Chaoich and then continue SE down the north side of that stream. In this area the rocks of the Outer Layered Eucrite Series (4F) contain a significant proportion of xenoliths of peridotite derived from Zones I, II and III of the Layered Peridotite Series, together with fragments of allivalite and eucrite. The block concentration decreases up the sequence, to the NE. In the vicinity of the blocks deflection and distortion of layering within the eucrites can be seen, suggesting that the blocks slumped into an unconsolidated crystal-liquid mush. Continue to the SE end of the small lochan at [NG 4806 1992].

Locality 3 [NG 4806 1992][edit]

SE of the lochan a large boulder sits on a peridotite inlier. 15m WNW of this boulder black, streaky, discontinuous layers of chrome spinel, up to 1cm thick, occur within the peridotite and dip at a steep angle to the west and NW. Proceed to the small gully SE of the lochan. The gully is, in part, controlled by an inweathered cone-sheet. Carefully descend down the NE side of the gully to the shore of Loch na Cuilce. Return to the Coruisk River and cross the stepping stones. The path leading from Loch Coruisk to the cairns north of Sgurr Hain (at [NG 5012 2138]) crosses Zones I and II of the Outer Layered Eucrite Series (4F). North of where the main path is joined by a less distinct path which runs along the north side of Loch Coruisk, the Druim nan Ramh Eucrite (4H) crops out.

Locality 4 [NG 4950 2064][edit]

South of Loch a' Choire Riabhaich, in the vicinity of the main path, the Druim nan Ramh Eucrite (4H) is exposed on strongly-jointed, glacial pavements and crags. It is a very distinctive, coarse-gained rock but does not show any of the layering or associated features found in the Outer Layered Eucrite Series. Locally, individual outweathered crystals of clinopyroxene achieve lengths of 10cm, or more. Olivine-rich facies can be recognised by their distinct brownish-orange weathering. Dykes and cone-sheets are particularly common within this intrusion. Further description and interpretations of the Druim nan Ramh Eucrite are presented in Section (4H) of Chapter 4. Due east of Loch a' Choire Raibhaich, the path climbs steadily up through the poorly-exposed Inner Layered Allivalite Series (4I). These pale-coloured, laminated rocks may be examined in sporadic exposures above (to the SE of) the path. At approximately 250m SW of the cairns (which are at [NG 5012 2138]), the path passes over a fault zone, cut by basalt and dolerite intrusions, which is between 50 and 100m wide.

Locality 5 [NG 5012 2116][edit]

The fault rocks (4K) are exposed on the higher ground east of the path and mark out a steep-sided, arcuate outcrop pattern. In good weather they may be distinguished from the coarser-grained allivalites by their lighter colour and more compact and fractured appearance. The fault rocks may be identified from the path, as they occur in crags just before (to the south of) a short but distinct zig-zag bend. They also form the low ground between the northern side of Sgurr Hain and the cairns at Druim Hain. The dominant rock-types are tuff and breccia, cut by dolerite and basalt intrusions, all of which have been subjected to severe cataclasis. Blocks of brownish-orange peridotite are present within the fault rocks. Proceed to the cairns.

Locality 6 [NG 5012 2138][edit]

In the vicinity of the cairns, and NW along the Druim Hain ridge, the Inner Layered Gabbro Series (4K) crops out. These rocks constitute the structurally-highest part of, and most evolved compositions preserved within, the Cuillin Complex (4A). Layering dips to the north, at between 40 and 50°, towards a focal point below Meall Dearg at the southern end of Glen Sligachan. These rocks offer some of the best examples of layering within the Cuillin Complex and a good example of such layering may be found directly west of the most southerly cairn. Xenoliths of highly-altered (hornfelsed) eucrite and gabbro occur throughout the series. Further details of the Inner Layered Gabbro Series are presented in Section (K) of Chapter 4.

Return to the cairns and, either, proceed down from the most southerly cairn along the path previously ascended to Loch nan Leachd and thence by boat to Elgol (by previous arrangement), or, descend due east to the northern end of Loch na Creitheach—a convenient route being along the southern side of the most southerly, unnamed tributary which flows east into the Abhainn Camas Fhionnairigh. Cross over to the north bank of this small stream at a point before it flows into the Abhainn Camas Fhionnairigh. The crossing of the Abhainn Camas Fhionnairigh can be difficult and should be done with care. Within the ground north of the loch are some particularly muddy oxbow lakes and these should be avoided. Follow the Sligachan-Kirkibost path south along the east side of the loch to a small stone wall NW of An t-Sron. From here, proceed through the small col between the summit of An t-Sron and the ridge of Blaven. The path is indistinct at this point, but from the col the more obvious path skirts the southern side of Blaven. Follow this path SE to where it crosses the Abhainn nan Leac and thence join the main track on the east side of Camasunary Bay. From here, continue east to Kirkibost (total distance from Druim Hain to Kirkibost is 8km, 2–3 hours).

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