Strollamus to Torrin, 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 23 Excursion 9 Strollamus to Torrin, location map

Excursion 9 Strollamus to Torrin (Figure 23)[edit]

Purpose: A general excursion to examine sedimentary, metamorphic and igneous rocks.

Aspects covered: the Outer Granite of the Eastern Red Hills Centre; hydrothermally-altered Lower Tertiary plateau lavas; Middle and Upper Jurassic sedimentary rocks; Lewisian Gneiss; interbasaltic conglomerates; agglomerates; rhyolite lavas; an ignimbrite; the Inner Granite of the Eastern Red Hills Centre; metamorphosed and non-metamorphosed Lower Jurassic sedimentary rocks; Cambro-Ordovician dolostones.

Route: Strollamus–Allt Strollamus–Allt na Teangaidh–Creagan Dubh–Srath Beag–Allt an t-Sratha Bhig–Allt Slapin–Torrin.

Distance: 8 kilometres.

Time: 6–7 hours.

General comments: A non-circular excursion most suitable for coach parties. Parties should be dropped off at the disused quarries on the Broadford-Portree (A850) road 5km (3 miles) NW of Broadford. The pickup point is at the marble quarry on the east side of Torrin (on the Broadford-Elgol (A881) road, 9km (5.5 miles) SW of Broadford). Turning space for coaches is available at both places.

Locality 1 [NG 6030 2664][edit]

From the drop-off point at the road entrance proceed 75m NE along the metalled track (which runs parallel to the main road) to a small disused quarry SE of the track. This quarry is within the Outer Granite of the Eastern Red Hills Centre (7F). The granite has a mottled appearance on weathered surfaces. When fresh it is light grey with prominent off-white phenocrysts of feldspar (up to 5mm) and smaller (1–2mm) crystals of quartz. Clots of hornblende and biotite can also be identified. The second quarry, located at the turn-off, is within Lower Tertiary hydrothermally-altered plateau lavas (3D). This rock is bluish-green, devoid of obvious structures such as columnar jointing, but contains breccias, amygdales and veins of secondary minerals such as carbonates, epidote and chlorite. There is no obvious evidence for defining individual flows. Proceed SW along the metalled track to a point 25m beyond the Allt Apoldoire, opposite a small pedestrian gate. Here, and for 25m to the SW, on the south side of the track, are exposures of the Upper Jurassic Staffin Shale Formation (2F). These strata are cut by a 2m-wide, altered, basic dyke. The Jurassic strata are highly fractured and rust-red weathered. When fresh they are bluish grey and relatively hard due to thermal metamorphic effects by the nearby granites. Bedding strikes NW-SE and dips at approximately 20° to the NE. Continue west along the track to the Allt Strollamus. In so doing, note thermally-metamorphosed sandstones, shales and limestones of the Middle Jurassic Great Estuarine Group (2F) on the south side of the track. Within the Allt Eoghainn an apophysis of the Outer Granite (7F) crops out.

Locality 2 [NG 5936 2650][edit]

A few metres south of the old bridge, sandstones, calcareous sandstones and limestones, from near the top of the Middle Jurassic Bearreraig Sandstone Formation (2F), have been thermally-metamorphosed by the nearby granites to quartzites and calcsilicate -bearing marbles (7I). The path to Torrin begins 20m west of the old bridge, but as the best exposures are within the stream bed, proceed south along the west bank of the river, up the succession. The initial and dominant lithology of the Bearreraig Sandstone Formation is a white sandstone which contains large, rounded masses of buff-coloured material, possibly representing calcareous concretions. Within these strata small acid intrusions are found. For example, 150m south of the old bridge, at a bend in the stream, a granite sheet forms a prominent exposure in the stream bed and in the west bank. Further south, in the vicinity of the first small east-flowing confluence of the Allt Strollamus, a small mass of brecciated granite is exposed. In the section south of this confluence there is a 35m-wide exposure of porphyritic felsite. Continue upstream to an obvious water main marker on the east bank of the Allt Strollamus.

Locality 3 [NG 5910 2590][edit]

The boundary between the Bearreraig Sandstone Formation and the overlying Great Estuarine Group is located here, 175m north of the confluence of the Allt Strollamus and the Allt na Teangaidh. To the south, exposures of the Great Estuarine Group consist of indurated black shales interbedded with thin, blue limestones, which occur close to the base of the group and dip to the SE at 40–50°. Both rock-types contain the bivalve Neomiodon. The stream runs parallel to the strike of the bedding at this point. Typically, the shales are more resistant to weathering and stand proud, whilst the limestones are generally inweathered. A 1m-thick, rusty-orange -weathering, porphyritic felsite sill is intruded into these strata 30m north of the confluence. Proceed up the east side of the Allt na Teangaidh, over exposures of steeply-dipping Great Estuarine Group strata, easily observed for example at a small concrete weir and further south. These rocks are intruded by dykes of the Lower Tertiary regional swarm (9B). Continue upstream to obvious exposures at the bottom of a prominent, cascading waterfall.

Locality 4 [NG 5912 2466][edit]

Here, Lewisian gneisses (2A) crop out. They consist of deformed, steeply-dipping, alternating light- and dark-coloured bands. Quartz, alkali feldspar and plagioclase are dominant in the leucocratic portions, whilst chlorite, epidote, biotite, pale green amphibole and pyroxene form the dark bands. Thin, anastomosing, secondary veins of epidote and chlorite are common throughout the outcrop. Thin-section studies of these gneisses indicate that they have undergone extreme hydrothermal alteration, which caused retrogression of the original, high-grade, metamorphic mineral assemblage to the secondary mineral assemblage now present. The presence of patches of quartz and alkali feldspar, in a granophyric intergrowth within the light-coloured bands, suggests that partial melting and subsequent crystallisation have taken place. It is likely that both the hydrothermal features and the evidence of partial melting can be attributed to higher-than-normal heat flow conditions during Lower Tertiary times (12F). Cross over to the west bank of the Allt na Teangaidh and proceed upstream for approximately 75m to where a 0.5m-thick dolerite dyke, trending WNW-ESE, cuts the gneiss at a NW-flowing tributary. In the vicinity of this confluence and the waterfalls, small (up to 5m across) outliers of hydrothermally-altered Lower Tertiary lava (3D) sit unconformably upon an irregular surface of gneiss. This relationship is seen more clearly at Locality 5, below. 20m further upstream (to the south) the gneisses are totally covered by the lavas. These flows are typically veined and contain amygdales (predominantly of carbonate, chlorite and epidote), together with patches of metamorphosed laterite. The mineralogies of the gneisses, lavas and patches of laterite (3C) are indicative of the greenschist metamorphic facies. From here, proceed WSW to the base of the crags of Creagan Dubh and walk around the foot of the crags, first towards the west and then SW. The crags consist of hydrothermallyaltered lavas inclined towards the north. Continue over the remains of a stone wall until an old wooden post fence is reached. The fence runs from the lochan in the valley floor up to the crags. 35m beyond the fence, to the south, is the first of two steep gullies within the crags; the second occurs a further 45m to the SW. The gullies have formed by the inweathering of Lower Tertiary dykes of the regional swarm (9B). 15m SW of the second gully, at the base of the crags, the unconformity between the gneisses and the lavas is exposed.

Locality 5 [NG 5862 2426][edit]

The unconformity, although locally variable, is flat-lying and can be traced for at least 20m along strike. The lowest 6–8m of exposure is of gneiss. The gneiss has an obvious foliation and is similar to the material described from Locality 4, above. Return 15m NE to the second gully, which allows a section through the overlying lava pile to be examined. 20m up this scree-filled gully (11E), on the NE side, interbasaltic conglomerates (3B) crop out. These conglomerates consist of large lenses, up to 5m thick, intercalated with the lava sequence. They can be traced along strike for up to 100m. The cobbles within these horizons are subangular to rounded, typically less than 10cm across, and are dominated by Jurassic sandstone, siltstone, shale and limestone (2F), together with lesser amounts of Torridonian sandstone and grit (2B). Return to the base of the crags and walk west down the grass-covered slope (consisting of poor exposures of the gneiss) to the path in the floor of the Srath Beag. Upon joining the path walk south over discontinuous exposures of the Outer Granite of the Eastern Red Hills Centre (7F) and continue to where the Allt an t-Sratha Bhig is joined by a major, west-flowing tributary off Beinn Dearg Mhor. From this confluence, proceed east up the nameless stream for approximately 300m to where the Outer Granite gives way to pyroclastic rocks.

Locality 6 [NG 5806 2328][edit]

Within the stream bed the following sequence is exposed:

TOP OF THE OBSERVED SEQUENCE—CUT BY THE INNER GRANITE (and topographically highest) m
Ignimbrite, much brecciated, with irregularities in fabric orientation 3.90
Ignimbrite, with well-developed fabric (cut by a 0.25m-thick, basic dyke) 7.30
(poorly exposed rhyolite) (8)
Rhyolite, with highly contorted flow-banding 0.65
Rhyolite, much brecciated with flow-banding 1.25
(poorly exposed rhyolite) (2)
Rhyolite, strongly flow-banded 1.10
Rhyolite, fine flow-banding 1.30
Rhyolite, no clear fabric orientation 1.60
Mixed, chaotic agglomerate, with many blocks of flow-banded tuff and Jurassic strata, all set in a tuffaceous matrix 1.90
BASE OF THE OBSERVED SEQUENCE—CUT BY THE OUTER GRANITE (and topographically lowest)

Return to the path and continue south to where the Allt an t-Sratha Bhig forms two prominent meanders. In this area the path crosses a small stream flowing off the west side of Beinn Dearg Mhor. This stream sinks above the path (to the east) into a deep swallow hole which can be seen immediately west of the path. Within the swallow hole there is a small cluster of trees. Care should be exercised in this area as the swallow hole is steep-sided, unstable and deep. The swallow hole occurs in Lower Jurassic limestones (2F) which crop out along the margin of the Outer Granite. These steeply-inclined strata can be observed in the tributary east of the path and control the course of that stream. Continue south along the path over Lower Jurassic limestones, sandstones and shales (also exposed in the stream bed of the Allt an t-Sratha Bhig). Due east of Cnoc Slapin, where the path crosses the Allt Slapin, a vertical, faulted contact between the Cambro-Ordovician Ben Suardal Dolostones (2D) and Lower Jurassic sandstones and shales (2F) is marked by a 2m-wide zone of highly brecciated material, predominantly shale, in the form of disoriented blocks.

Continue SSW over the dolostones to the Broadford-Elgol (A881) road and thence SE for 1km (0.6 miles) to the marble quarry on the east side of Torrin to the pick-up point.

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