Moine geology of West Glenelg and Loch Hourn, Loch Hourn region - an excursion

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From: Strachan, Rob, Friend, Clark, Alsop, Ian, Miller, Suzanne (Editors). A Geological excursion guide to the Moine geology of the Northern Highlands of Scotland.: Edinburgh Geological Society, Glasgow Geological Society in association with NMS Enterprises, 2010.

By John Ramsay

Fig. 6.1 General geological map of the Glenelg and Loch Hourn region.
Fig. 6.4 Map of the Moine and Lewisian rocks in the Arnisdale-Corran region.
Fig. 6.5 Detailed map of the coastal exposures on Rudha Camas na Caillin (Locality 6.6B).

Excursion 6 West Glenelg and Loch Hourn is composed of the following articles:[edit]

Excursion 6 West Glenelg and Loch Hourn, Loch Hourn region[edit]

Locality 6.4 Road section west of Arnisdale [NG 8320 1118][edit]

Road section west of Arnisdale (Fig. 6.1). Relationships between D2 and D3 structures within Moine psammites.

From Upper Sandaig drive along the road in the direction of Arnisdale. The road climbs to a hilltop [NG 7802 1302] where it is worth stopping to view to the SW of the Tertiary Volcanic islands of Rhum and Eigg. Continue downhill, passing into a forestry plantation (cattle grid) and through roadside outcrops situated in the complex core zone of the Loch Hourn-Loch Duich D3 antiform. On emerging from this forest (cattle grid), the road passes close to the shore line passing the cottage at Rarsaidh. After about 0.5km, park by roadside exposures of gently inclined Moine psammites which are Locality 6.4 [NG 8320 1118] (Fig. 6.1). Proceed by foot along the road towards Arnisdale.

The Moine psammites progressively change dip from SE to west as the trough line of a D3 syncline is crossed. Excellent roadside exposures on the north of the road show sections of typical Moine psammites with thin bands of pelitic material with well developed D3 crenulation cleavage. As one proceeds along the road to the SSE, the bedding surfaces of the Moines are exposed and one can see the development of two distinct crossing lineations: a D2 quartz feldspar rodding is cut and refolded by strongly developed D3 crenulation folds. The dip of the bedding gradually increases as one passes through the hinge of the D3 synformal fold (the Arnisdale Synform, (Fig. 6.4)). The dip gradually increases to vertical: the D3 linear features remain constantly oriented, with sub-horizontal axes, whereas the D2 lineations become progressively steeper, passing through the vertical to plunge towards the south and SE at the top of the hill [NG 8370 1054]. The angular relations between the constant D3 direction and the variable D2 lineation seen here indicates that the D3 fold geometry cannot be that of a simple flexural slip fold. In fact, this geometry is directly comparable with the small scale lineation refoldings seen at Loch Monar (Excursion 8), only here the deformed lineations are on a regional scale. The geometrical distribution of the D2 lineations is best explained by the flow kinematics described at Loch Monar, with the principal fold-forming flow (‘a’-direction) calculated from the intersection of the mean D2 great circle and the axial planes of the D3 folds (Ramsay, 1960). The significance of this locality in understanding the kinematics of superimposed folding is of fundamental importance here and elsewhere in the Scottish Highlands (Ramsay, 1960).

Return to the vehicle, and drive along this same road descending into the village of Arnisdale. Continue through the village and where the coast road bends abruptly to the left [NG 8468 1006] find a park place by a gate. Walk 400 m south along the beach to the rocks at Crudh ‘Ard [NG 8464 0963].

Locality 6.5 Crudh ‘Ard [NG 8464 0963][edit]

Crudh ‘Ard (Fig. 6.4). Relationships between D2 and D3 structures within Moine psammites.

This locality is situated close to the hinge of the D3 Ben Sgriol Synform. On the east side of the outcrop, Moine psammites dip steeply to the east on the overturned limb of the synform and these psammites (ii) become folded in the hinge (M-shaped D3 folds) of the main fold which is occupied by semi-pelitic rocks (Fig. 6.4). This is an excellent outcrop to study the deformation of the D2 linear structures around the hinges of small scale D3 folds. The D2 quartz-feldspar rods undulate over the D3 hinges. In the more pelitic parts the D3 folds show excellent coarse crenulation structures. Before leaving this locality it is good idea to view the hillside on the south face of Ben Sgriol above Arnisdale village. The Ben Sgriol Synform is extremely well displayed in this face. To the left (on Creag an Fhithich), ribs of Moine psammite dip eastwards (to the right) at ~20°. The actual curve of the fold hinge is seen just to the right of a prominent stream (Allt an Fhuarain) and further to the right the psammites turn through the vertical into an overturned position.

Return to the vehicle and drive along the road to a large parking place just before the village of Corran.

Locality 6.6 Corran to Rudha Camas na Caillin [NG 85 08][edit]

Corran to Rudha Camas na Caillin (Fig. 6.4). D1 interfolding of Lewisian gneisses and Moine psammites.

Cross the bridge over the River Arnisdale and proceed southward along the foreshore of a raised beach for about 300m to the first prominent coastal outcrops at Locality 6.6A [NG 8500 0886]. These outcrops show the contact relations between strongly banded Lewisian gneisses forming the western part of the anticlinal strip of Western facies Lewisian (Fig. 6.4). The Moine rocks are a mixture of psammitic and muscovite-biotite pelitic bands with bedding planes parallel to the Lewisian banding. A strong D2 lineation is developed plunging down the dip of the layering to the ESE. In the Lewisian rocks, the distinctions between the original deformed gneissic bands and fresh augen and veins of new D2 quartz-feldspar are clear. Continue southwards. If the tide is low it is possible to skirt the outcrops along the shore; if not, it is best to follow a small footpath in the woods above to coastal exposures into a sandy bay. There are very clean outcrops of Moine psammites with strongly developed D1 isoclines plunging towards the south and SSE, with D2 migmatite veining. Leave the footpath and aim for the low rocky headland of Rudha Camas na Caillin which is Locality 6.6B [NG 8500 0800] (Fig. 6.4), (Fig. 6.5). The western Lewisian D1 anticlinal core, which has been narrowing to the south on account of its southern plunge, splits into three narrow southward plunging isoclinal fold cores at this locality and perfect exposures of all the Moine-Lewisian contact relations and three sets of superposed folds are to be seen (Fig. 6.5). The lithological differences between Lewisian and Moine occur at knife sharp contacts, yet there are no anomalous high strain zones at these contacts. Clear D1 southward-opening synformal folds are seen in the Moine psammites, and D1 southward closing antiformal folds in the Lewisian and the Moine-Lewisian relationships appear to be those characteristic of sets of isoclinal D1 folds. All these D1 folds have been overprinted by Z-shaped D2 folds which plunge to the ESE accompanied by a strong rodding, especially well developed in the hinge zones of the D2 folds. The refolding of small D1 isoclines in the cores of the D2 folds is exceptionally clear. Locally within the banded Lewisian gneisses, small, angular, almost kink-like D3 folds deform the D2 linear fabric. These are not present in the Moine psammites presumably because the massive recrystallised nature of the psammites proved too competent for such folds to form. These outcrops are exceptionally clear in the view that they present of the differences between Moine and Lewisian and the successively overprinted folds of the three deformation events: they should on no account be missed. All the geological relationships seen in this region and shown in (Fig. 6.4) and (Fig. 6.5) confirm Clough’s interpretation that the primary relationship between Lewisian and Moine is that of early isoclinal folding.

If the visitor has time it would be instructive to spend a further half hour proceeding eastwards along the Loch Hourn coast where there are many examples of SSE-plunging isoclinal D1 folds (this represents the synclinal zone between the two western facies anticlines (Fig. 6.4) superposed by D2 folds and intense SE-plunging linear fabrics associated with the development of migmatite veins).

Return to Corran where a welcome cup of tea can be obtained in ‘Sheena’s Tea Hut’. Return along the road through the village of Arnisdale and just after entering a forestry plantation west of Rarsaidh one crosses a cattle grid. Park in a large site on the left of the road at [NG 8094 1193].

Locality 6.7 Loch Hourn [NG 8193 1181][edit]

Loch Hourn (Fig. 6.1). Western facies Lewisian gneisses; Strathconon Fault Zone.The outcrops to be visited are coastal exposures which lie directly below the park site. One can descend directly through the forest, but the recent state of the newly-cut trees with a jungle of small branches knit together with thorny brambles is not encouraging. The best solution is to re-cross the cattle grid and descend to the foreshore at some convenient place at Leac Glas, then walk towards the west along the shore. At Locality 6.7 ((Fig. 6.1), [NG 8193 1181]) banded coarse-grained Lewisian hornblende-gneisses of western facies with lenticular masses of ultrabasic rock are cut by discordant amphibolite sheets which are clearly intrusive basic dykes. These dykes completely lack any gneissic material, but are cut by en-echelon quartz veins.

If time allows, it is instructive to proceed westward from these outcrops to visit the zone of faulting associated with the main Strathconon fault. Lewisian gneisses and Moine psammites are traversed by NE-SW-trending zones of shattered rock with subsidiary faults, and both become intensely crushed towards the main fault plane seen at Sgeir a’Chuirn-uisge [NG 8014 1181]. Some mono-clinal and conjugate folds, probably related to the fault movements, are found with their kinked sectors fractured and brecciated. Occasional dykes of brick-red-coloured late Caledonian lamprophyre show partial brecciation.

References[edit]

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