Glen Brittle and the Western Cuillin Hills, Skye - an excursion

From Earthwise
Jump to navigation Jump to search
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 30 Excursion 16 Glen Brittle and the Western Cuillin Hills, location map
Figure 13 Geological sketch-map showing the position of intrusive complexes and dyke swarms within the British Tertiary Volcanic Province
Figure 4 Geological sketch-map of the Cuillin Complex
Figure 12 Geological sketch-map of the Sea of the Hebrides (modified from Binns et al. 1974)

Excursion 16 Glen Brittle and the Western Cuillin Hills (Figure 30)[edit]

Purpose: To examine the outer units of the Cuillin Complex.

Aspects covered: interbasaltic conglomerates; hydrothermally-altered plateau lavas; the Ring Eucrite; rocks of the Border Group; the Outer Layered Allivalite Series; intrusive tholeiite sheets; cone-sheets; non-xenolithic peridotite dykes; a volcaniclastic pipe; the Layered Peridotite Series; xenolithic peridotite dykes.

Route: Glenbrittle House–Eas Mor–Loch an Fhir-bhallaich–Allt Coire Lagan–East Buttress of Sron na Ciche–Sgurr Sgumain–Loch Coir' a' Ghrunnda–Coir' a' Ghrunnda–An Sguman–Loch Brittle (Glenbrittle House).

Distance: 16 kilometres.

Time: 10–11 hours.

General comments: An arduous excursion on high ground, best undertaken in good weather. Excellent views from the ridge area.

Glen Brittle lies on the west side of the main Cuillin ridge at the head of Loch Brittle. Follow the Broadford–Portree (A850) road to Sligachan (26km (16 miles) from Broadford and 14km (9 miles) from Portree). Take the Dunvegan (A863) road along Glen Drynoch to the Carbost (B8009) road (a distance of 8km (5 miles)). From here, follow the Carbost road, past the head of Loch Harport, as far as Merkadale (2.5km (1.5 miles)) and thence take the minor road signposting Glen Brittle. Descend into Glen Brittle and proceed as far as the bridge over the Allt Coire na Banachdich (a further distance of 11km (7 miles)). Parking for at least 10 cars, 4 minibuses or two coaches is available on both sides of the road, 100m north of the bridge over the river. Coaches may turn in the area south of the bridge.

Locality 1 [NG 4116 2156][edit]

From the path on the north side of the Allt Coire na Banachdich (starting at the sheep/cattle pens) the general geology of the Western Cuillin Hills area may be noted. To the west, plateau lavas (3D) crop out on Creag na Laire, Beinn Staic and Beinn a' Bhraghad. To the east, the ground as far as the lower to middle parts of Coire Lagan, Coire na Banachdich and Coire a' Ghreadaidh is composed of the Ring Eucrite (4B) intruded by numerous tholeiite sheets (9E). The Ring Eucrite gives way, abruptly, in the upper crags, to the Border Group suite of rocks (4C) which are marginal to the Outer Layered Series of the Cuillin Complex (4A). The Border Group can be traced from Sron na Ciche, across Coire Lagan and around the west side of the Sgurr Dearg ridge into Coire na Banachdich, and thence NE between Sgurr nan Gobhar and Sgurr na Banachdich. The main ridge of the Cuillin Hills, from Sgurr Sgumain in the south to Sgurr a' Ghreadaidh in the north, is composed of rocks of the Outer Layered Series cut by numerous tholeiite sheets (9E). Dykes of the regional swarm (9B) and members of the Cuillin group of cone-sheets (9D) cut all of the units mentioned.

Locality 2 [NG 4134 2162][edit]

200m NE of the sheep/cattle pens, Lower Tertiary interbasaltic fluviatile conglomerates (3B) crop out in the upper part of the exposures near the road. These conglomerates are intercalated with thin horizons of sandstone and siltstone (best seen in the small stream) and are flat-lying. Clast-types include: porphyritic granophyre, amygdaloidal basalt, and Jurassic sandstone. It is suggested by Meighan et al. (1981) that the granophyre clasts were derived from the slightly older Rhum volcanic centre, which can be seen to the south of the present locality (Figure 13). Return to the path and proceed SE to the Allt Coire na Banachdich.

Locality 3 [NG 4140 2144][edit]

Hydrothermally-altered basic lavas of the Lower Tertiary plateau sequence (3D) are exposed in the stream bed and on the banks of the Allt Coire na Banachdich. On the south bank the lavas contain thin infills of conglomerate. Follow the north bank of the river, upstream, over lavas seen cropping out in the gorge, to the waterfall of Eas Mor. From here, looking into the gorge, note altered Ring Eucrite (4B) forming prominent onion-skin weathering surfaces. These rocks are cut by numerous dolerite and basalt dykes (9B). As there are few easy routes into and out of this gorge, these relationships should be best appreciated from this vantage point. Similar unweathered, coarse-to fine-grained Ring Eucrite, also intruded by dykes, also crops out around the waterfall. Cross the river above the waterfall and follow the track, uphill, over further poorly-exposed Ring Eucrite, past the NE side of Loch an Fhir-bhallaich. En route, note olivine-rich minor intrusions, which weather to a prominent brownish-orange, on the south side of Sgurr nan Gobhar and SW side of Sgurr Dearg. From the SE end of the loch, the near-vertical buttress wall on the north side of Sron na Ciche can be seen. Half way along this buttress, and running upwards to the east, is a prominent boulder-strewn gully. Mid-way up this gully, on the Cioch Buttress face, there is a distinct change in rock-type, with the lighter Ring Eucrite (4B) giving way (to the east) to the darker rocks of the Border Group (4C). This contact is the subject of Locality 4, below. In order to get to this locality, follow the southern branch of the path across the Allt Coire Lagan to the base of the north face of Sron na Ciche. From here, proceed up the gully to the contact.

Locality 4 [NG 4440 2040][edit]

The sharp contact between the Ring Eucrite (light grey) and the Border Group (brown) dips at a steep angle to the east. At this locality, many of the features which typify the Border Group, such as a marginal xenolithic facies and a zone of 'wispy banding' (4C), appear to be missing. Within a few tens of centimetres of the contact, the younger Border Group rocks are fine-grained, passing inwards, over a distance of approximately 20m, to the east, into a less olivine-rich allivalite (the White Allivalite (4C)). To the north, this contact can be seen on the West Buttress of Sgurr Sgumain, dipping at a much shallower angle than at the present locality. Proceed to the top of the gully, noting en route several basalt and dolerite dykes (9B), some up to 3m wide, cutting the White Allivalite. Also present are cone-sheets (9D), up to 1m thick, dipping at a shallow angle to the east. Dispersed throughout the White Allivalite are examples of net-veined segregations, and xenoliths of various block-types, up to several metres across.

Locality 5 [NG 4480 2048][edit]

20m before the top of the gully there is an intrusive tholeiite sheet (9E) within layered allivalites. This sheet is identified readily by its fine-grained, dull grey coloration, and is cut by numerous veins of calcite giving the rock a sheared appearance. Continue to the top of the gully, where rocks of the Outer Layered Allivalite Series (4E) crop out. Proceed NNW along the path towards the top of Sgurr Sgumain. Within the first 100m, note a distinct brownish-orange, tortoise-shell -weathering peridotite dyke (9I), similar to those already seen from a distance on the south side of Sgurr nan Gobhar and the SW side of Sgurr Dearg. Above this sheet, towards Sgurr Sgumain, is a highly fractured intrusive tholeiite sheet which is a member of the so-called Main Ridge Complex (9E) (Figure 4). These rocks vary greatly in form, from welded intrusive breccias to xenolithic intrusive units. This particular sheet is an amygdaloidal intrusive breccia. Further details of this group of rocks are presented in Section (9E) of Chapter 9. Proceed to the summit area of Sgurr Sgumain and look NNE. The exposures on the west side of Sgurr Mhic Choinnich illustrate the large number of cone-sheets (9D) which cut the various members of the Cuillin Complex. The cone-sheets are readily discerned as relatively flat-lying, ribbon-like features on the steep back wall of Coire Lagan, above the spectacular stone chutes (12E). Return down the path to the top of the gully. The NW side of Coir' a' Ghrunnda, between the loch and the top of the gully, is composed of rocks of Unit 1 of the Outer Layered Allivalite Series (4E). Layering is not particularly well-developed, although examples of slump structures are found in the vicinity of the top of the gully. 100m SSE of the top of the gully is a volcaniclastic breccia pipe (4N), cut by basalt and dolerite dykes. The pipe is elongate, with a long axis of 40m trending NW-SE and a short axis of 25m trending NE-SW. The angular to sub-rounded, matrix-supported blocks within this pipe are typically 5–10cm across, although examples up to 2m across can be found. They consist of: basalt, dolerite, gabbro, eucrite, allivalite, and peridotite; all set in a tuffaceous matrix. Descend to Loch Coir' a' Ghrunnda and thence to the middle of the high ground which forms the prominent rock barrier on the SW side of the loch.

Locality 6 [NG 4506 2008][edit]

Looking north towards the back wall of the corrie several features may be noted. First, the obvious contact between the grey tholeiite sheet capping Sgurr Sgumain and the underlying brown rocks of the Outer Layered Allivalite Series. The summit of Sgurr Alasdair is composed of the same sheet. Second, to the SE of the loch, below Caisteal a' Garbh-choire, the near-vertical contact between the Outer Layered Allivalite Series and the distinctly brownish-orange Layered Peridotite Series (4D) may be noted. The rocks of the Layered Peridotite Series contain abundant xenoliths of feldspathic peridotite and allivalite. They are also intensely veined with feldspathic peridotite. Numerous blocks of this tortoise-shell -weathering material litter the glaciated corrie floor. To the SW, the Isle of Rhum dominates the skyline (Figure 12) and (Figure 13). On the east side of this island are the summits of Askival and Hallival, which are composed of layered allivalites (type-locality) and peridotites. Further to the west are lavas overlying granites. To the east of Rhum is Eigg, whilst further south is Muck. To the west is Canna. All three of these flat-lying islands are dominated by plateau lavas of Lower Tertiary age. In the immediate vicinity, the rock barrier is composed of rocks of the Outer Layered Allivalite Series. From here, the descent to the base of the glacially-scoured (11B) Coir' a' Ghrunnda involves a traverse over three distinct topographic features (steps) referred to as "boiler plates". Below the rock barrier, in the NW wall of the corrie, on the lip of the uppermost boiler plate, the steeply-inclined (dipping to the NE) boundary between the Outer Layered Allivalite Series and the White Allivalite of the Border Group is again seen. This boundary can be traced across the corrie into the NW face of Sgurr nan Eag. At the lip of the middle boiler plate, in the NW and SE walls of the corrie, the almost-vertical gully marks the boundary between the White Allivalite and the Ring Eucrite. Along the SE side of the corrie, at the level of the upper and middle boiler plates, another prominent gully, trending NE-SW, marks the line of a 6m-wide, xenolithic, ultrabasic dyke of the Ben Cleat type (9I). This dyke can be traced for at least 500m. The xenoliths are of coarse-grained, ultrabasic material and the dyke shows only slightly finer-grained margins. The dyke has weathered to a pale, rusty-yellow and contains numerous, closely-spaced, longitudinal joints which can be traced individually along strike for several metres. These joints typically do not cut any of the xenoliths. The olivine content across this dyke, unlike many of the Ben Cleat type, shows very little variation. Further details of the petrology and genesis of these dykes are presented in Section (9I) of Chapter 9. Basic dykes of the regional swarm (9B) cut this dyke. A dyke of similar composition occurs in the NW side of the corrie, again within a small, linear gully. This dyke can be seen at the top of the middle boiler plate, and its course is followed by a small river along the NW margin of the lowest plate for at least 150m.

The path out of the corrie follows a line NW of the Allt Coir' a' Ghrunnda and skirts around the base of Sron na Ciche. Once out of the corrie note to the south an arcuate, brownish-orange intrusion of peridotite cropping out on An Sguman. Descend the hillside WNW, via the main path, to Glen Brittle, and thence north to Glenbrittle House.

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 .