Gleann Torra-mhichaig - Coire nam Bruadaran - Belig, 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 26 Excursion 12 Gleann Torra-mhichaig–Coire nam Bruadaran–Belig, location map

Excursion 12 Gleann Torra-mhichaig–Coire nam Bruadaran–Belig (Figure 26)[edit]

Purpose: To examine granites and pyroclastic rocks of the eastern part of the Western Red Hills Centre.

Aspects covered: the Beinn Dearg Mhor Granite; the Loch Ainort Granite; ferrodiorite of the Marscoite Suite; a xenolith of pyroxene granulite within the ferrodiorite; the Marsco Granite; the Meall Dearg Granite of the Srath na Creitheach Centre; the Outer Layered Eucrite Series; intrusive agglomerates; the Glas Bheinn Mhor Granite of the Eastern Red Hills Centre.

Route: Gleann Torra-mhichaig–Allt Coire nam Bruadaran–Eas a' Bhradain–Coire nam Bruadaran–Am Fraoch-choire–Druim Eadar Da Choire–Coire na Seilg–Belig–Coire Choinnich–Loch Ainort.

Distance: 10 kilometres.

Time: 8 hours.

General comments: In good weather conditions the colour contrast between many of the rock-types to be examined on this excursion enables them to be distinguished readily from some distance away.

The southern end of Gleann Torra-mhichaig is on the BroadfordPortree (A850) road, 16km (10 miles) from Broadford and 24km (15 miles) from Portree. On the east side of the road, below Beinn Dearg Mhor, is a large parking place overlooking Loch Ainort. The road immediately to the south is through a prominent cutting within the Beinn Dearg Mhor Granite.

Locality 1 [NG 5304 2790][edit]

Here, within the cutting 100m south of the parking area, the Beinn Dearg Mhor Granite (6F) is cut by basic dykes of the Beinn Dearg Type (9B, 9C). Obvious features of this granite include the general rusty appearance and mottled red coloration, both caused by the breakdown of ferromagnesian minerals. When fresh, this rock is greenish-blue, with glassy phenocrysts of anorthoclase in a glomeroporphyritic arrangement. Clinopyroxene and hornblende are the dominant ferromagnesian minerals. The groundmass consists of a granophyric intergrowth of quartz and alkali feldspar. The Beinn Dearg Type dykes are up to 3m wide and commonly show brecciated margins. They are generally feldsparphyric dolerites with occasional amygdales of zeolite. Return to the parking place and proceed south towards Broadford for 2km (1.2 miles) to the head of Loch Ainort. Ample parking for cars and minibuses or 2 coaches is available on the east side of the road 100m north of the bridge over the Allt Coire nam Bruadaran, below Eas a' Bhradain. Proceed 150m upstream from the waterfall (the waterfall is itself 150m upstream from the bridge over the river).

Locality 2 [NG 5318 2642][edit]

The eastern portion of the Western Red Hills Centre (6A) is dominated by granites, members of the mixed-magma Marscoite Suite (6H), and agglomerates (6B). Many of the rock-types can be distinguished from this locality. At the head of Coire nam Bruadaran the NW face of Marsco is composed of: the Southern Porphyritic Granite (6G) on the lower slopes (from Coire nam Bruadaran NW over Mam a' Phobuill); the Marsco Granite (6J) in Coire nan Laogh; and, the Glamaig Granite (6D) on the upper ridge of Marsco. Both the Southern Porphyritic Granite and the Marsco Granite can be traced east over Druim Eadar Da Choire. The summit of Belig is composed of highly-altered, basic lavas of the Lower Tertiary plateau sequence (3D), flanked to the west by the Outer Layered Eucrite Series (4F) of the Cuillin Complex on Garbhbheinn, and to the north (on the northern slopes of Belig and in the upper part of Coire Choinnich) by agglomerates. The Scar on the northern face of Belig consists of a large block of eucrite within the agglomerates. On the N-S -trending ridge of Belig, between the lavas and the agglomerates, the Marsco Granite crops out. To the east, the Glas Bheinn Mhor Granite (7E) of the Eastern Red Hills Centre crops out on the red hill of that name. In the immediate area, as well as on the NE-SW -trending ridge of Leathad Chrithinn, NW of Loch Ainort, the Loch Ainort Granite (6F) of the Western Red Hills Centre crops out. This granite, when fresh, is greenish-blue and contains glassy phenocrysts of anorthoclase in a glomeroporphyritic arrangement. In thin-section, the groundmass of this rock consists of quartz and alkali feldspar in a granophyric intergrowth, together with pyroxene, amphibole and Fe-Ti oxides. Proceed SW along the west side of the Allt Coire nam Bruadaran to its confluence with the Allt Mam a' Phobuill. Continue up the Allt Mam a' Phobuill for approximately 400m to the confluence of a southern tributary within a prominent area of boulder-strewn ground.

Locality 3 [NG 5200 2552][edit]

100m NW of this confluence the main river forms a small gorge with 20m of continuous exposure of a marginal, chill facies of the Southern Porphyritic Granite (6G) in the stream bed. This rock consists of obvious phenocrysts of alkali feldspar (3–5mm) in a blue-green, felsitic groundmass. A few metres upstream from three rowan (mountain ash) trees (on the south bank) normal facies of the Southern Porphyritic Granite may be examined in the stream bed. Details of the mineralogy of this granite are presented in Section (6G) of Chapter 6. From here, proceed SSE to the top (south) of the waterfall of the Allt Coire nam Bruadaran.

Locality 4 [NG 5206 2480][edit]

Here, the light-coloured Southern Porphyritic Granite crops out. The phenocrysts which typify this granite are of quartz and alkali feldspar, set in a granophyric intergrowth of the same two minerals. Continue south from the waterfall for 150m to where the main river is cut by a NW-SE -trending tract of bright green grass which marks the ring-dyke outcrop pattern of the Marscoite Suite of rocks (6H). This approximately 80m-wide ring-dyke separates the Southern Porphyritic Granite to the north from the Marsco Granite to the south and consists of the mixed-magma rock-type marscoite, the Southern Porphyritic Felsite, and a basic rock, ferrodiorite. It is the ferrodiorite, rich in calcium and phosphorus, which gives rise to the bright green grass. In the west bank of the river the ferrodiorite is identified by its distinctive onion-skin weathering associated with brown soils, although more fresh material crops out in the east bank. Enclosed within the weathered ferrodiorite is a large (at least 1m x 0.5m) xenolith of Lewisian Gneiss (2A). This rock is a two-pyroxene basic granulite with a distinctive, coarse-grained foliation. In part, the xenolith has been recrystallised to pyroxene-amphibole hornfels by Lower Tertiary thermal metamorphic events (Thompson 1981). Continue south along the Allt Coire nam Bruadaran, over Marsco Granite (6J), to the head of the .corrie, where there is the remains of an old deer fence.

Locality 5 [NG 5190 2430][edit]

The non-porphyritic Marsco Granite is a distinctive blue rock when fresh and weathers to a pale brown. Further details on this granite are presented in Section (6J) of Chapter 6. From this locality, close to the fence, there is an excellent view of the main Cuillin ridge to the SW, with Druim nam Ramh below, and the red hill Meall Dearg in the foreground. The dome-shaped summit of Ruadh Stac lies directly to the south, with the irregular ridge of Blaven, forming the eastern margin of the Cuillin Complex, to the SSE. To the SE the lower granitic slopes of Garbh-bheinn are capped by rocks of the Outer Layered Eucrite Series (4F). The conical hill to the north of Garbh-bheinn is composed of Marsco Granite, as is the south face of Marsco to the west. Looking NE, down Coire nam Bruadaran, the flat-capped hill of Dun Caan on Raasay (an outlier of the north Skye lava field) is seen in the distance. The red hills on the north side of the corrie are composed of the Beinn Dearg Mhor Granite. Walk east, around the southern side of the 489m conical hill composed of Marsco Granite, towards a saddle on the lower part of the N-S trending ridge of Garbh-bheinn at the head of the Allt nam Fraoch-choire. En route note, to the SE, the clear boundary relationship of the Meall Dearg Granite of the Srath na Creitheach Centre (5B) with the overlying, but older, Outer Layered Eucrite Series which forms the main Garbh-bheinn ridge.

Locality 6 [NG 5260 2420][edit]

In the vicinity of the saddle four rock-types may be examined. The junction between the Meall Dearg Granite (5C) and the Outer Layered Eucrite Series (4F) shows severe alteration of the latter against the former, indicating the granite to be of a younger age. Both these units are cut by the still-younger Marsco Granite (6J). An elongate (WNW-ESE) mass of agglomerate crops out approximately 100m below (WSW of) the saddle and can be traced up to the saddle. The small dark exposures of these rocks stand proud of the surrounding Marsco Granite and consist of unsorted, sub-angular fragments of fine-grained, basic and acid rock-types in a pale green matrix. In proceeding up to the saddle, note a block of orange-weathering peridotite within the Outer Layered Eucrite Series 20m up on the west side of the Garbh-bheinn ridge, close to the old deer fence. Looking east from the top of the saddle, the N-S -trending ridge of Belig forms a distinctive topographic feature. The most southerly part, including the summit, is composed of hydrothermallyaltered plateau lavas (3D). Approximately two-thirds of the distance along the ridge, north from the summit, the lavas give way to pale-weathered Marsco Granite. This in turn gives way, at the northern end of the ridge, to agglomerates containing a large slab of eucrite. Beyond Belig, to the NE, is the "whale-backed" ridge of Glas Bheinn Mhor, composed of the granite of that name (the oldest of the major acid intrusions of the Eastern Red Hills Centre (7E)). From the saddle proceed into the upper part of Coire na Seilg, where highly-altered members of the Outer Layered Eucrite Series (4F) of the Cuillin Complex (4A) crop out.

Locality 7 [NG 5308 2414][edit]

Immediately to the north of the prominent waterfall in the Allt Coire na Seilg the eucrites may be examined. They are coarse-grained, occasionally layered, basic rocks which are cut by several generations of anastomosing veins of epidote and/or chlorite. In places the rock is stained white due to feldspar alteration. Follow the base of the eucrite crags on the east side of Coire na Seilg into the headwaters of the Eas a' Chait, staying close to the line of an old deer fence. NW of the summit of Belig the eucrite gives way, abruptly, to the plateau lavas. Upon reaching the western slopes of Belig, climb gradually upwards to the NE, towards the crags at the northern end of the ridge. The route taken may cross over the eastern end of a generally poorly-exposed mass of polygenetic agglomerates, consisting of sub-angular to rounded blocks of obvious quartz porphyry, granite and eucrite, typically less than 5cm across, set in a matrix of comminuted particles of the blocks (6B). Clearly exposed on the west side of Belig is the boundary between the hydrothermally-altered plateau lavas and the Marsco Granite. This boundary dips at a steep angle to the south. The lavas have been crushed, with chlorite and epidote as common secondary minerals within numerous anastomosing veins. Continue north along the western slope of Belig to the boundary between the Marsco Granite and grey agglomerates.

Locality 8 [NG 5420 2480][edit]

These matrix-supported agglomerates consist of sub-angular to rounded blocks up to 1m across, although typically most of the fragments are less than 20cm across. Block-sizes vary markedly over short distances. The dominant block-types are: granite, gabbro, eucrite, basalt, dolerite, and sedimentary material. The matrix is pale green and is composed of fragments, up to 0.5mm, of the block-types. Further descriptions of the agglomerates are presented in Section (B) of Chapter 6. From this locality look west to the eastern side of Druim Eadar Da Choire where a prominent, grass-filled gully marks the line of the ring-dyke of the Marscoite Suite of rocks from Coire nam Bruadaran. The brown soils represent ferrodiorite degradation. At the foot of the gully the ring-dyke trends NE across the lower (northern) part of Coire na Seilg.

Descend into the Coire Choinnich and thence to the main road.

Road-side exposures of the Glas Bheinn Mhor Granite (7E) of the Eastern Red Hills Centre may be examined along the SE side of Loch Ainort. This intrusion is the oldest of the large acid plutons of the centre and contains phenocrysts of sodic plagioclase (1–5mm). The dominant dark minerals are biotite and a Ca-rich amphibole. Also present are small druses containing calcite, fluorite, zeolite and an Fe-rich epidote. Small clots of mafic minerals are dispersed throughout the volume of this intrusion. Further details of this granite are presented in Section (7E) of Chapter 7.

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 .