Bearreraig Bay - The Storr, Skye - an excursion
|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.|
- 1 Excursion 18 Bearreraig Bay–The Storr (Figure 32)
- 2 (i) The coastal. section of Middle Jurassic sedimentary rocks at Bearreraig Bay
- 3 (ii) The Lower Tertiary amygdaloidal plateau lavas of The Storr (landslipped in part)
- 4 References
- 5 Appendix 1: Glossary of petrological names and terms
- 6 Appendix 2: Glossary of fossil names
- 7 Appendix 3: Glossary of place names and grid references
Excursion 18 Bearreraig Bay–The Storr (Figure 32)
Purpose: To examine Middle Jurassic sedimentary rocks and Lower Tertiary amygdaloidal plateau lavas.
Aspects covered: Middle Jurassic sedimentary rocks; plateau lavas (frequently amygdaloidal); landslipped material; moraines.
- Loch Leathan–Storr Lochs Dam–Bearreraig Bay– Storr Lochs Dam—(return to road)
- Old Man of Storr–The Storr—(return to road)
- 4 kilometres
- 4 kilometres (or 8km if Locality 8 is included)
- 4 hours
- 3 hours (or 6 hours if Locality 8 is included)
General comments: Coastal exposures are involved (Localities 1 and 2 of Part (i)) and, therefore, low tide conditions are necessary. The path through the forested ground at the beginning of Part (ii) is subject to closure during periods of bad (wet) weather as it can degrade to a quagmire (impassable due to the muddy conditions). The two parts of this excursion can be undertaken separately.
This excursion is split into two parts:
- The coastal. section of Middle Jurassic sedimentary rocks at Bearreraig Bay
- The Lower Tertiary amygdaloidal plateau lavas of The Storr (landslipped in part)
The Storr is a 719m summit 2km west of the Portree-Staffin (A855) road, 11km (7 miles) north of Portree and 18km (11 miles) south of Staffin. Ample parking for cars, minibuses and coaches is available in the lay-by on the west side of the road at the edge of the small forest, 200m north of Loch Leathan. Bearreraig Bay is 700m due east of the lay-by, at the base of the steep coastal cliffs. The local geology can be divided into two parts. East of the road Jurassic strata (2F), intruded by dolerite sills (9H), dip at a shallow angle to the west. West of the road these rocks are overlain by a spectacular sequence of Lower Tertiary plateau lavas (3D) forming the cliffs of The Storr. The lavas have been involved, in part, in massive, post-glacial landslips (11D) which have bodily transported material eastwards. The pinnacles of the Old Man of Storr and Needle Rock are landslipped outliers of the lava pile. Glacial moraines (11B) and peat (11H) mask parts of the underlying solid geology.
(i) The coastal. section of Middle Jurassic sedimentary rocks at Bearreraig Bay
From the lay-by, walk 500m south along the main (A855) road to the minor road north of Loch Leathan and thence east over the Storr Lochs Dam to the south side of the Bearreraig River. Follow the road east to the top of the water pipeline and descend the concrete stairway, adjacent to the pulley railway, into Bearreraig Bay.
Locality 1 [NG 5176 5276]
From the generator station, it may be noted that the cliffs around the bay are composed of the Middle Jurassic marine Bearreraig Sandstone Formation (2F). The following members are recognised within the locally-exposed sequence (thicknesses only approximate) (modified from Hudson and Morton 1969 and Morton 1965, 1976):
|Rigg Sandstone Member|
|Coarse gritty sandstone with comminuted shell debris||5|
|Alternating bands of dark grey, shaly sandstone and medium grey, sandy limestone in beds 0.6m to lm thick in the lower part and 0.3 to 0.6m thick in the upper part||65|
|Holm Sandstone Member|
|Soft, white sandstone with calcareous concretions (doggers), typically less than 1m in largest dimension, and lenticles of calcareous sandstone, becoming darker towards the base||35|
|Udairn Shale Member|
|Soft, dark grey, shaly sandstones with nodules of grey, sandy limestone. Beds of harder rock, 0.6 to 1m thick, occur in the upper part||70|
|Ollach Sandstone Member|
|Thinly-bedded, calcareous sandstones. Massive, soft, yellow-brown sandstone with concretions and lenticles of calcareous sandstone|
|Thin-bedded calcareous sandstones with shaly partings||15|
|Dun Caan Shale Member|
|Dark grey, sandy shales with fucoid marks and concretions of sandy limestone||15|
On the shore, 100m south of where the Bearreraig River flows into the Sound of Raasay, below the High-Water Line, the Dun Caan Shale Member crops out. These fine-grained sedimentary rocks contain carbonaceous ("fucoid") marks and large concretions of sandy limestone. Fossils present include: belemnites, and pleuromyid bivalves in life position. The Ollach Sandstone Member may be examined in the small cliff above the foreshore. Fallen blocks from higher stratigraphic units (many of which are fossiliferous) may be examined on the beach.
Locality 2 [NG 5178 5274]
150m south of where the Bearreraig River flows into the Sound of Raasay the gradational boundary between the Dun Caan Shale Member and the 011ach Sandstone Member can be examined in detail. In the lower part of the sandstone sequence four shale layers contain abundant ammonites (Graphoceras). Other fossils present in the Ollach Sandstone Member include: bivalves, gastropods, belemnites, crinoid ossicles, and fossil drift wood. The remainder of the sequence in the vertical cliffs shows prominent horizons of calcareous concretions, varying in size along strike, with typical sizes up to 50cm long and 20cm thick. Larger examples are present. Follow the Bearerraig River upstream, on the north bank, to the first bend, 30m west of the generating station.
Locality 3 [NG 5164 5274]
Here, the Udairn Shale Member (17m above the base of the unit) may be examined. Siderite nodules are present throughout. Fossils are not common, but include: ammonites (Graphoceras), belemnites, and fragments of drift wood. Continue upstream through the Udairn Shale Member to the lower waterfall.
Locality 4 [NG 5154 5254]
At the base of the lower waterfall the Udairn Shale Member is exposed; passing upwards gradationally in the waterfall face into the Holm Sandstone Member. Two to three metres below the top of the Udairn Shale Member are ammonite-rich shale horizons. Behind and above the main waterfall is a large, weathered-out recess in the Holm Sandstone Member. The Holm Sandstone Member is light-coloured, with a limonitic cement, and contains doggers of calcareous sandstone and sandy limestone, together with carbonaceous fragments. This rock is best examined in the recess and is most easily approached from the NW bank. To gain the highest ground proceed along the recess behind the main waterfall and up the gully on the SE bank of the river.
Locality 5 [NG 5140 5242]
From the top of the main waterfall, up to the third waterfall, the lower part of the Rigg Sandstone Member crops out, consisting of alternating beds of shaly sandstone (75cm thick) and calcareous sandstone, with gradational boundaries. Continue to the top (lip) of the third waterfall, via the SE bank, and rejoin the river course. 5m back from the waterfall lip is the mould of an ammonite 30cm in diameter. Continue upstream to the Storr Lochs Dam over a 4m-thick dolerite sill which forms a small waterfall and which is partially obscured by trees and shrubs.
Locality 6 [NG 5124 5240]
The upper part of the Rigg Sandstone Member, consisting of calcareous grits, crops out at the base of the Storr Lochs Dam. They are thinly-bedded, coarse sandstones, frequently cross-bedded, and contain comminuted shell debris (of unrecognisable fossils).
Return across the top of the dam and along the minor road to the main (A855) road.
(ii) The Lower Tertiary amygdaloidal plateau lavas of The Storr (landslipped in part)
Proceed north to the lay-by and thence along the marked track (see General Comments, above), through the forest, to the base of the pinnacle of the Old Man of Storr (2km).
Locality 7 [NG 4996 5398]
Follow the path between the Old Man of Storr and Needle Rock, thence towards Coire Faoin, to the west side of the Old Man. Plateau lavas (3D) may be examined at the base of the Old Man of Storr and on the cliff face of The Storr. These lavas form part of the Beinn Edra Group, lowest in the plateau lava sequence, and are typically nonporphyritic basalts, but with a few porphyritic flows and a mugearite towards the top. One of the most striking features of these lavas is the presence of amygdales of zeolites. These include: analcime (trapezohedral crystals); chabazite (clusters of rhombohedra up to 5mm); thomsonite (white or bluish, compact, fibrous masses); stilbite (cream-coloured, radiating bundles); laumontite (long, delicate fibres); mesolite (white, hair-like fibres), plus other, less-common varieties. Other amygdale minerals are chlorite and opaline silica (Anderson and Dunham 1966). Although zeolites can be recovered from the lava outcrops, weathered-out samples of up to 3cm across, showing the crystal characteristics noted above, are best collected from the soil and rock screes in the vicinity of the path on the SW side of the Old Man of Storr. Also present in these screes are blocks of bright red laterite representing complete alteration of the lava to a soil, together with lateritised lava representing an intermediate stage of alteration (3C). The lavas exposed on the Old Man of Storr and Needle Rock, plus other major outcrops found away from the main cliff face of The Storr, are not in situ, having moved east as part of a landslipped mass (11D). The way-up of these landslipped blocks may be deduced from laterite profiles, with the most altered material occurring towards the original top of the flow, and grading downwards into unlateritised lava. A typical example may be examined at the recessed base of the Old Man of Storr, above the scree slopes. The sequence exposed on the cliff face of The Storr consists of at least 24 flows, although there may be a further 120m of lava concealed below the scree (11E) and landslipped material (11D) (Anderson and Dunham 1966). Red, lateritised tops to flows (3C) are common throughout the Storr lava sequence. Also to be noted in the cliff face are prominent vertical gullies, formed by the inweathering of dykes of the Lower Tertiary regional swarm (9B).
Follow the path from the Old Man of Storr. north between the Storr cliff face and Needle Rock. From the highest point of the path, south of where it crosses an old fence and trends NW around into the corrie NE of The Storr, it is possible to identify several of the landslipped blocks on the slopes east of the cliff face. For example, the pillars of the Old Man of Storr and Needle Rock, together with smaller blocks around the small lochan east of Needle Rock. The Storr landslip is considered to be of the mature type and, therefore, stable (11D). The dip of the flows exposed on the Old Man of Storr and the surrounding major landslipped blocks, to the east, suggests a simple toppling mechanism from the cliff face.
Looking west, individual lavas of the Storr cliff section are also readily distinguished, occurring as flat-lying units which are, on average, 10m thick. The steepness of the cliff face, however, makes detailed examination of these lavas very difficult. From here, either, return to the road, or, follow the track into the shallow corrie on the NE side of The Storr, and thence SW uphill to the summit. It is stressed that a good distance be kept from the vertical cliff face. The cliff edge is unstable and liable to collapse.
Locality 8 [NG 4954 5404]
The highest lava in the sequence crops out around the summit of The Storr and is a grey, platy mugearite (3D). From the summit of The Storr the spectacular landslips (11D) of this part of north Skye are readily discerned. Below, to the east, landslipped material can be traced as far as the road (in part, covered by forest). To the west of the summit, the moraine-covered (11B) dip slope of the plateau lava sequence (3D) can be traced to Loch Snizort Beag on the west side of the island.
Return down the corrie on the NE side of The Storr and thence by the path to below the cliff face, past Needle Rock, to the track through the forest, down to the road.
|At all times follow: The Scottish Access Codeand Code of conduct for geological field work|