The Strathaird Peninsula, 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 17 Excursion 3 The Strathaird Peninsula, location map

Excursion 3 The Strathaird Peninsula (Figure 17)[edit]

Purpose: To examine Middle to Upper Jurassic sedimentary rocks and their relationship to Lower Tertiary dykes and plateau lavas.

Aspects covered: Middle and Upper Jurassic sedimentary rocks; Lower Tertiary basic and xenolithic ultrabasic dykes; Lower Tertiary plateau lavas.

Route: Elgol–Port na Cullaidh–Cam Mor–Cladach a' Ghlinne–Glen Scaladal–Ben Cleat–Cnoc Breac–(Elgol).

Distance: 11 kilometres.

Time: 10 hours.

General comments: Coastal exposures are involved and, therefore, low tide conditions are necessary (see Locality 6, below).

Elgol is located on the west side of the Strathaird Peninsula, 24km (15 miles) south of Broadford on the A881 road. The road down to the coast from the village is very steep (in places, 1 in 4). Ample parking for cars and minibuses is available at the scenic viewpoint, not at the Port na Cullaidh road end, where vehicles may only turn. Coaches should proceed only as far as a turning area 100m beyond the Post Office, at a former petrol station.

Locality 1 [NG 5160 1360][edit]

The coastal section to be examined in this excursion runs from Elgol in the south to Glen Scaladal half way up the west coast of the peninsula. General features of the geology of the surrounding area may be noted from the beach at Port na Cullaidh. The Strathaird Peninsula is built predominantly of Middle to Upper Jurassic sedimentary rocks (2F) overlain by the Lower Tertiary plateau lava sequence (3D), all of which dip to the west at a shallow angle. The lavas form a resistant cap on top of the sedimentary units, for example, on Ben Cleat and Ben Meabost. Dykes of the Lower Tertiary regional swarm (9B) cut all of the rock-types exposed on the peninsula. To the north, the main Cuillin ridge runs from Gars-bheinn in the south to Sgurr nan Gillean in the north and marks out the western portion of the Cuillin Complex (4A). Remnants of the eastern sector of the complex are preserved on Blaven, north of Camasunary Bay, which is situated at the head of Loch Scavaig on the west side of the peninsula. Beyond are the granites of the Srath na Creitheach (5A) and Western Red Hills (6A) Centres, with the peaks of Ruadh Stac and Marsco being particularly obvious. Due west of the peninsula is the flat-lying island of Soay, which is composed of Torridonian strata (2B). Also visible from the west coast of the Strathaird Peninsula is the Lower Tertiary igneous complex on the island of Rhum, some 20km south of Skye (Figure 12) and (Figure 13). Most of the sedimentary rocks to be examined during this excursion have been thermally altered, to some extent, by the Lower Tertiary igneous activity (3D), (12F). Fossils are often recrystallised to coarse-grained calcite or have been dissolved. Also, shales are typically less fissile and sandstones are much harder. Arcuate warps within the strata around the intrusive centres cause their positions, relative to a sea level datum, to vary quite markedly. Several horizons are therefore encountered more than once along this coastal section (Figure 17). The Middle to Upper Jurassic sequence, as exposed on Strathaird, consists of (modified from Hudson and Morton 1969, Sykes 1975 and Harris and Hudson 1980):

STAFFIN SHALE FORMATION

(Middle Callovian–Kimmeridgian) Camasunary Siltstone Member (7.5m+) Camasunary Sandstone Member (91.4m) Scaladal Sandstone Member (32.6m) Tobar Ceann Siltstone Member (19m)

STAFFIN BAY FORMATION

(Upper Bathonian–Lower Callovian) Cam Mor Sandstone Member (9m)

GREAT ESTUARINE GROUP (Bathonian)

Skudiburgh Formation (18m) Kilmaluag Formation (23m) Duntulm Formation (8m) Valtos Sandstone Formation (24m)

Lealt Shale Formation (46m)

Lonfearn Member

Kildonnan Member

Elgol Sandstone Formation (15m) Cullaidh Shale Formation (6m)

BEARRERAIG SANDSTONE FORMATION (Bajocian)

Garantiana Shale Member (12m)

Druim an Fhuarain Sandstone Member (470m)

For the majority of this excursion the strata are examined going up the succession.

From the end of the road at Port na Cullaidh walk south, keeping above the beach, past the jetty over a thin but extensive dolerite sill until the first coastal exposures of sandstone are reached. Access to the beach is most easily gained by descending the cliffs at the back of the beach.

Locality 2 [NG 5132 1318][edit]

600m south of Port na Cullaidh the Druim an Fhuarain Sandstone Member of the Bearreraig Sandstone Formation crops out on a flat-lying platform below the cliffs. In the vicinity of two small promontories, these marine sandstones exhibit dramatic trough cross-bedding, with amplitudes of up to 0.5m, which indicate a current flow direction to the NW. In returning north along the coast, the trough cross-bedded sandstones pass upwards into non-cross-bedded units containing ferruginous concretions. The numerous basic dykes (mostly basalts and dolerites) of the Lower Tertiary regional swarm which cut these sandstones are typically 1–2m wide. 150m south of Port na Cullaidh, at the base of the dolerite sill which forms a promontory, the upper part of the Bearrearaig Sandstone Formation is represented by the Garantiana Shale Member. Here, it is flat-lying, thinly-bedded and pyritiferous, the latter due to the thermal effects of the overlying sill. The sill is a non-porphyritic dolerite and interfingers the shale, having caused much contortion of the bedding. Veins of calcite are common within the shale. The dip slope of the sill may be traced inland south of the Allt Port na Cullaidh. Return north to the road end and proceed to the boulder-covered beach in front (west) of the school at Port na Cullaidh.

Locality 3 [NG 5162 1366][edit]

On this beach discontinuous exposures of the Cullaidh Shale Formation of the Great Estuarine Group may be examined. It is a hard, black shale, containing fish scales and gastropods and is rich in carbonaceous material. These strata are cut by outweathering dykes and sills. In the cliff face directly behind (east of) the school, the Elgol Sandstone Formation crops out. At the base of the cliff a gradation from the Cullaidh Shale Formation through a thin-bedded, bioturbated, dark sandstone into the Elgol Sandstone Formation is particularly clear. 120m north of the school the beds within the Elgol Sandstone Formation show peculiar careous (honeycomb) weathering characteristics. The gradation from shale into sandstone may be interpreted as a change in depositional environment from deep water to shallow water, with the sandstone having been deposited in a deltaic environment. Proceed 400m north into the next small bay. The best access to this bay is via a footpath above the boulder-covered beach.

Locality 4 [NG 5166 1410][edit]

On entering the south side of the Bay, the Elgol Sandstone Formation gives way to the overlying Lealt Shale Formation. At the base of the cliff in the middle of the bay a small outcrop of vent breccia, containing fragments of Jurassic sandstone, shale and limestone, together with dolerite and porphyritic basalt, may be examined. A few metres south of this outcrop, the following local sequence of bivalve-bearing limestones and shales within the Lealt Shale Formation is found (modified from Hudson and Morton 1969):


(TOP, north of the vent breccia, close to a thin sill) m
7. Limestone with septarian concretions; Isognomon 0.23
6. Shale with concretions and thin limestone beds; Isognomon at top 0.60
5. Limestone, laminated; top surface weathers to show elliptical domes—algal stromatolites. Lower part shows penecontemporaneous breccias 0.20
4. Limestone, fine-grained, dark; Placunopsis (socialis) and Cuspidaria (ibbetsoni) 0.15
3. Limestone, Placunopsis; fibrous calcite at base 0.05
2. Shale, septarian concretions; small Praemytilus at top 1.00
1. Limestone, Shelly 0.15
(BASE, south, close to an outweathered dyke crossing the beach)

Continue along the beach and out of this small bay (a distance of approximately 200m). The Lealt Shale Formation grades upwards into the Valtos Sandstone Formation.

Locality 5 [NG 5162 1444][edit]

The Valtos Sandstone Formation on Strathaird is dominated by sandy limestones. Mud cracks are commonly preserved on upper surfaces of individual beds, whilst load casts may be identified on basal surfaces. Beds show bioturbation, poorly-developed cross-bedding, and careous weathering surfaces. The small sea stack at this locality is composed of a thick, sandy limestone, close to the top of the formation. 100m beyond (north of) the stack, a NW-SE -trending composite dyke crops out on the beach. The marginal non-porphyritic basalt units are approximately 1m wide and flank a central portion of basalt containing megacrysts (up to 3cm) of plagioclase (9J). Sulphides are disseminated throughout the central portion of the dyke. A few metres north of this dyke are excellent exposures of the Duntulm Formation.

Locality 6 [NG 5166 1458][edit]

These consist of an alternating sequence of shales and limestones containing the oyster Praeexogyra (hebridica). Within this sequence there is a thin algal limestone horizon. Above are the basal members of the Kilmaluag Formation. The westward protrusion of the coastline here, consisting of numerous multiple dykes immediately south of the waterfall [NG 5164 1468], forms a barrier at high tide. If it is impassable, return to Elgol and follow the path on the west side of Bidein an Fhithich (1km return to Elgol, 2km walk north along path). Regain the beach at [NG 5180 1500].

Locality 7 [NG 5180 1500][edit]

The Skudiburgh Formation consists of mottled, silty mudstones which have been thermally-metamorphosed. They have a purplish-red or green coloration, are non-fossiliferous, and constitute the highest formation of the Great Estuarine Group. They crop out extensively north of the waterfall. Above, and further to the north, is a coarse marine sandstone (the Cam Mor Sandstone of the Staffin Bay Formation). Fallen blocks of this sandstone may be examined on the beach and contain brachiopods of the rhynchonella type. Gain the cliff-top path by scrambling up the grassy slope behind the beach, keeping away from rock faces. Proceed along the path to the southern end of the Carn Mor Rockfall (11E). Here, the sequence, exposed from the beach below to the crags above, consists of: Kilmaluag Formation (at sea-level); the Skudiburgh Formation; the Cam Mor Sandstone Member of the Staffin Bay Formation; and, the Tobar Ceann Siltstone Member of the Staffin Shale Formation. The path is at the level of the Skudiburgh Formation–Cam Mor Sandstone Member boundary. The boundary between the Cam Mor Sandstone Member and the Tobar Ceann Siltstone Member can be examined at the level of an upper path 50m further up the slope. The sandstones, siltstones and shales of the Tobar Ceann Siltstone Member contain abundant belemnites at this locality and these can be obtained from blocks within the screes adjacent to the path. Follow the upper path north to Glen Scaladal.

Locality 8 [NG 5206 1618][edit]

The coastal cliffs south of Glen Scaladal are composed of rocks of the Valtos Sandstone Formation. At this locality they are dominated by alternating beds of shale and calcareous sandstone intruded by regularly-spaced dykes of the Lower Tertiary regional swarm (9B). Load-cast structures, mud crack infills and fibrous calcite lenses are common within the shales. South of the mouth of the Scaladal Burn a light grey, shelly limestone horizon (1m thick), containing the gastropod Viviparus (scoticus), forms a prominent feature running out to sea. Cross over the "25-Foot" raised beach (11C), east of the present-day beach, to the base of the coastal cliffs north of Glen Scaladal.

Locality 9 [NG 5194 1650][edit]

On the beach and in the cliff the Kilmaluag and Skudiburgh Formations are again exposed. The sequence is as follows (modified from Hudson and Morton 1969):


(TOP, north) m
SKUDIBURGH FORMATION
Mudstone, mottled grey and red -
Mudstone, red with calcareous nodules 0.6
Mudstone, grey with calcareous nodules 0.9
KILMALUAG FORMATION (appoximately 23m)
21. Marlstone with ostracods and pellets, rough weathering 0.8
20. Mainly marlstone, some beds rich in ostracods 1.8
19. Mudstone, grey with calcareous nodules 1.2
18. to 9. Breccia beds. Laminated marlstone, thin-bedded; some beds brecciated with the lamination in the clasts sub-horizontal, and surrounded by more brecciated material. The clasts are dolomite, the "matrix" calcite; both arrgillaceous. "Pipes" of more brecciated material penetrate less-brecciated beds to a depth of 0.3m. These beds weather brown and the clasts are
9. more resistant to weathering than the matrix 1.5
8. Mudstone, with 0.3m beds of nodular and ostracod-rich limestone. Viviparus near top. Fibrous calcite 5.3
7. Three thin, white-weathering marlstones and two grey mudstone beds, prominent near the base of the cliff 0.4
6. Shale or mudstone with nodular marlstones 1.5
5. Viviparus limestone 0.1
4. Shale, black; 0.2m. Praeexogyra (hebridica) -bearing marlstone near top 1.8
3. Poorly exposed marlstones with mudcracks. (Note within the cliff a dyke occupying a fault) 2.5
2. Shale with marlstones; mudcracks, mudflake and pellet conglomerates, some dolomitic, in marlstones 4.0
1. Shale, black, poorly-exposed (with sills) 2.4
(BASE, south)

From here, either, return along the cliff-top path to Elgol, or, proceed to Locality 10 by walking SE up the grass-covered southern side of Glen Scaladal to the saddle between Ben Cleat and Ben Meabost. From the saddle observe the plateau lava sequence (3D) on Beinn Leacach, on the north side of Glen Scaladal. At least 15 flows may be identified from the step-like (trap) topography which has developed. Typically, the hard centres of these lavas stand proud, whilst the softer bases and tops are inweathered. Follow the rough track on the SW side of the valley between Ben Cleat and Ben Meabost over poorly-exposed plateau lavas (3D) to a point a few metres NW of the highest point of the saddle.

Locality 10 [NG 5270 1566][edit]

Forming a distinctly rounded ("whale-back") ridge, approximately 15m long and 4m wide, a xenolithic ultrabasic dyke (9I) constitutes the only significant exposure in the low ground between Ben Cleat and Ben Meabost. It is a member of the Ben Cleat group of ultrabasic dykes and is characterised by numerous cognate xenoliths of feldspathic peridotite and allivalite (up to 30cm across). Details of this important group of minor intrusions are presented in Section (I) of Chapter 9. Walk 450m SSW over plateau lavas to the summit of Ben Cleat.

Locality 11 [NG 5238 1532][edit]

Close to the summit, other members of the Ben Cleat group of ultrabasic dykes crop out. The one 80m east of the summit trends N-S, is up to 5m wide, and is exposed for at least 200m along its length. The second dyke, 60m west of the summit, is also 5m wide, has a similar trend, and can be traced for over 300m south towards Cnoc Breac. This dyke is less-obviously xenolithic than the others. These dykes are readily distinguished from the more common basalt (s.l.) and dolerite dykes of the Lower Tertiary regional swarm (9B) by their rounded, bulbous forms and brownish-orange coloration.

Walk south, via Cnoc Breac, down the grassy slopes over Jurassic strata in the lower ground, to the road close to the corrugated building of the Free Presbyterian Church. From here, it is approximately 1.5km (1 mile) to the car-park at the Elgol Scenic Viewpoint.

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