Creag Strollamus, 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 22 Excursion 8 Creag Strollamus, location map

Excursion 8 Creag Strollamus (Figure 22)[edit]

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

Aspects covered: the Broadford Gabbro; Torridonian sedimentary rocks; hydrothermally-altered Lower Tertiary plateau lavas; the Outer Granite of the Eastern Red Hills Centre; interbasaltic conglomerates; Jurassic sedimentary rocks.

Route: Allt Fearna–Creag Strollamus–Loch Cuil na Creig–Allt Apoldoire–Allt Strollamus–(Allt Fearna).

Distance: 8 kilometres.

Time: 8 hours.

General comments: A relatively short excursion on low ground which examines a wide variety of rock-types. The starting and finishing points of this excursion are 1.5km (1 mile) apart. For coach parties a turning space and pick-up point is available on the south side of the road at the disused quarries [NG 6030 2664] 1.5km (1 mile) further west along the main (A850) road from the starting point.

Creag Strollamus is a 265m hill close to the Broadford-Portree (A850) road, 4km (2.5 miles) NW of Broadford. Parking for several cars, minibuses and coaches is available in the large lay-by on the old bridge over the Allt Fearna, south of the main road.

Locality 1 [NG 6150 2610][edit]

From the old bridge proceed due west towards the crags at 100m O.D. on the east side of Creag Strollamus. 30m beyond the corner post of the fence, and up to the base of the crags, the Broadford Gabbro crops out. A particularly good exposure is located 200m west of the corner post, in the vicinity of a small burn. Here, a N-S elongate mass, 25m x 15m, forms a 2m-high knoll. The gabbro is medium- to coarse-grained and does not show any evidence of mineral layering. The dominant minerals are clinopyroxene (often altered to green aggregates of amphibole + chlorite + epidote) and plagioclase, in an ophitic textural arrangement. Olivine is not present. In places, the Broadford Gabbro contains anastomosing veins, 1–3mm wide, containing chlorite, epidote and calcite. Further details of this intrusion are presented in Section (D) of Chapter 7.

Locality 2 [NG 6128 2600][edit]

This 10m-high crag is composed of highly-altered Torridonian sandstones, siltstones and grits (2B), which commonly have a bleached appearance. Internal structures are not obvious and detailed examination is required in order to deduce a general dip of the strata of 40–50° to the east. The exposed faces are frequently parallel to bedding. These strata are, in places, strongly jointed and cut by basic (basalt and dolerite) and acid (felsite) dykes. From the northern end of this exposure proceed uphill in a NW direction. Keep to the heather-covered slopes and away from the steep and tree-covered crags on the north side of Creag Strollamus which run parallel to the BroadfordPortree road. Continue past a prominent V-shaped gully to a point due north of the summit of Creag Strollamus and on the north of a 20m-wide, prominent gully trending 140°. As an aid to finding this locality, it should be noted that on the shoreline, in line with the gully, is a large, wedge-shaped spit of sand.

Locality 3 [NG 6070 2656][edit]

On the north side of this gully highly-altered Lower Tertiary plateau lavas (3D) crop out, some of which are porphyritic, with phenocrysts of plagioclase (1–3mm). These lavas are intensely fractured and veined with secondary epidote and chlorite. Amygdales of dark green chlorite (1–2mm) tend to stand proud on weathered surfaces. Structural data, indicating the orientation of these lavas, is difficult to determine. Proceed south, across the gully, towards the summit of Cieag Strollamus.

Locality 4 [NG 6070 2654][edit]

On the other side of the gully the plateau lavas are cut by a 100m-wide dyke-like extension of the Broadford Gabbro (7D). The margins of this intrusion are typically doleritic, although, in places, are basaltic. This dyke can be traced to the NW, but does not extend to the coast. Walk across the dyke, towards the summit of Creag Strollamus, onto a distinctly porphyritic facies of the Outer Granite of the Eastern Red Hills Centre (7F, part (iii)). This rock contains phenocrysts of white alkali feldspar (2–3mm) set in a fine-grained felsitic groundmass of quartz and alkali feldspar. Locally, this intrusion is intensely sheared. The outcrop of the granite can be traced south to the summit of Creag Strollamus.

Locality 5 [NG 6070 2606][edit]

From the summit of Creag Strollamus the following geological panorama can be observed:# South, the Inner Granite (7G) of the Eastern Red Hills Centre (7A) forms the summits of Beinn na Caillich and Beinn Dearg Mhor

  1. SW and west, the scree-covered, rounded granite mass of Beinn na Cro forms the western portion of the Outer Granite (7F), part (i)) of the Eastern Red Hills Centre (7A). The prominent, heather-covered ridge of Glas Bheinn Bheag is composed of Jurassic sedimentary rocks (2F). Beyond is the scree-covered, elongate ridge of Glas Bheinn Mhor, forming the granite of that name (7E). In the distance are the summits of Marsco, Beinn Dearg Mhor and Glamaig, which are composed of various granites of the Western Red Hills Centre (6A)
  2. North, the island of Scalpay is dominated by Torridonian sedimentary rocks (2B) dipping to the east, overlain by Jurassic (2F) and Cretaceous (2G) sedimentary rocks which crop out in the grass-covered valley of the Allt Stapaig, above which (to the east) are Lower Tertiary plateau lavas (3D)
  3. East, the low-lying ground of Strath Suardal, south of Broadford, is composed of Lower Jurassic sedimentary rocks (2F). Beyond Broadford Bay are the summits of Sgurr na Coinnich and Ben Aslak, which are composed of Torridonian sedimentary rocks (2B).

From the summit of Creag Strollamus proceed SW, downhill, towards Loch Cuil na Creig.

Locality 6 [NG 6060 2600][edit]

100m SW of the summit, the crags are composed of highly-altered Torridonian strata (2B). Nevertheless, these rocks show clear evidence of bedding and cross-bedding. Slump structures and micro-faults are also evident. The dominant rock-types are sandstones, grits and siltstones, with obvious magnetite-rich layers up to 5mm thick. Further to the SW, the Torridonian strata give way to highly-altered plateau lavas (3D), the junction being marked by a distinct break in slope. These lavas form the low-lying ground around Loch Cuil na Creig, extending SW to the Allt na Teangaidh. They are dark green and are cross-cut by several generations of anastomosing veins of epidote and chlorite (1–3mm wide). Horizons rich in amygdales of chlorite and calcite are common throughout these lavas and frequently reach thicknesses in excess of 50cm. Proceed to the NW end of Loch Cuil na Creig to where the Allt Apoldoire flows north out of the loch. Continue north along the Allt Apoldoire for 100m and thence 20m to the east.

Locality 7 [NG 6026 2606][edit]

Here, Lower Tertiary interbasaltic conglomerates (3B) crop out. They are crudely bedded and dip to the east at 15–20°. These rocks are composed of sub-angular to rounded cobbles of dominant red Torridonian sandstone and grit, together with lesser amounts of Jurassic sandstone, limestone and shale, and Lower Tertiary basalt and dolerite. The cobbles are typically less than 10cm across, although blocks as much as 50cm across are present. The matrix is a dark, grey-green mud and the cobbles are typically matrix-supported. Proceed WSW over poorly exposed plateau lavas for 1km to the confluence of the Allt na Teangaidh and the Allt Strollamus.

Locality 8 [NG 5904 2570][edit]

The first exposures, 20m downstream from the confluence, are of indurated, black shales interbedded with thin, blue limestones, close to the base of the Great Estuarine Group, dipping to the SE at 40–50°. Both rock-types contain the bivalve Neomiodon. 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. 175m north from the confluence the boundary between the Great Estuarine Group and the underlying Bearreraig Sandstone Formation crops out (2F). Continue downstream and down section through a 35m-wide porphyritic felsite intrusion, with phenocrysts of quartz (2–3mm) and alkali feldspar (2–4mm), into the underlying Bearreraig Sandstone Formation, which is dominated by white, thermally-metamorphosed sandstones, with rare grit and pebble horizons. In addition, large, rounded masses of buff-coloured material are present, possibly representing calcareous concretions within the sandstones. At the sharp bend of the Allt Strollamus, 150m south of the old bridge, a small granite sheet (7F, part (iii)) intrudes the sedimentary rocks. A few metres south of the old bridge, sandstones, calcareous sandstones and limestones, from near the top of the Bearreraig Sandstone Formation, have been thermally-metamorphosed by the nearby granites to quartzites and calc-silicate bearing marbles (7I).

Return ENE along the old track to where it joins the Broadford-Portree (A850) road at the disused quarries (pick-up point for coach parties) and thence east to the lay-by at the Allt Fearna (1.5km (1 mile)) (for car and minibus parties).

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 .