Moine geology of Loch a’ Bhraoin, Braemore and Loch Broom. Option B. Corrieshalloch Gorge and Loch Broom - an excursion

From Earthwise
Jump to navigation Jump to search
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 Simon Kelley

Fig. 9.1 Geology of the Moine rocks between the Fannich mountains and Ullapool, showing the localities to be visited.
Fig. 9.3 Sketch map for Locality 9.9.

Excursion 9 Loch a’ Bhraoin, Braemore and Loch Broom is composed of the following articles:[edit]


Option B. Corrieshalloch Gorge and Loch Broom[edit]

From the bridge [NH 1585 7500], return to the road and drive back along the A832 to the Braemore junction (Fig. 9.1). The first locality of this option lies about 1km along the A835 towards Ullapool. Parking is available for many cars and coaches in the Corrieshalloch Gorge car park on the north side of the road.

Locality 9.7 The Corrieshalloch Gorge [NH 204 782][edit]

The Corrieshalloch Gorge (Fig. 9.1). Initial deformation associated with the Moine Thrust in psammites and pelites.

Exposures in the immediate vicinity of the car park are not very informative. Walk westwards downhill along the A835 to a large roadside cutting at [NH 2007 7849]. Care should be taken here as traffic can be fast-moving along this stretch of road. The exposures here are interbedded psammites and pelites of the Inverbroom Psammite (Fig. 9.1). The psammites acted as rheologically competent layers and suffered relatively little deformation during the thrusting. Note that the quartz veins and pegmatites cross the psammites at high angles. Pelite rock types on the other hand acted as less competent layers and there is widespread evidence for simple shear across these layers, giving shear strain values as high as eight. Quartz veins and pegmatites are highly deformed within the pelites, indicating shear towards the WNW. A particularly important aspect of the metamorphic fabric within the pelites is the development of shear bands, creating lenses of undeformed, pre-mylonitic fabric between 0.25mm and 10cm long, and indicating overthrusting towards the WNW.

No stop is complete without seeing the Falls of Measach, formed by runoff water during the last glacial retreat. The view from the suspension bridge is truly spectacular, especially after heavy rain.

Return to the parking area and follow the road towards Ullapool. Approximately 6km further on, turn left along the single track road signposted for Letters, Ardindrean and Rhiroy. (This road is unsuitable for coaches; the turning place at Rhiroy is only large enough for cars or minibuses). Turning right at the first crossroads, follow the road past the chapel and along the shore of Loch Broom. The road along the southern shore of Loch Broom originally reached only as far as the first houses and was extended to its present length in the mid 1930s. Prior to this time the only way in and out was by boat. The population which was at its peak around 1846 lived by crafting and fishing, but the number of people living in this community has now dwindled to less than 35% of its earlier peak, excepting weekend visitors and geologists.

Locality 9.8 Loch Broom [NH 171 858][edit]

Loch Broom (Fig. 9.1). Increasing deformation associated with the Moine Thrust Zone mylonites.

As the level of the road rises above the shore of Loch Broom, about 1km past the chapel, massively bedded Inverbroom Psammites and interbedded pelites are exposed just above and occasionally at the roadside. Early pegmatites within the psammitic rock types lie at low angles to the foliation and exhibit pinch and swell structures (incipient boudinage). The fabric in the pelitic rocks is dominated by shear bands, and the grain sizes of the pelites have been reduced from their peak metamorphic sizes (250-500 µm) to typical mylonitic sizes (10-50 µm), apart from resistant muscovites that are rotated toward parallelism causing a strong planar fabric.

The crags of Cnoc an Droighinn [NH 149 888] represent the end of an ENE-trending ridge reaching to the Moine Thrust in the west (Fig. 9.1) and continuing across Loch Broom to the east. The structure of the ridge is an anticline in the Moine rocks, caused by a ramp structure in the underlying thrust zone. The ramp structure is exposed at Dundonnell and as such can only be recommended as a separate excursion. Coarse-grained psammites and pebble bands in the Inverbroom Psammite can be traced from Cnoc an Droighinn into the Moine mylonites at [NH 118 874], demonstrating that the mylonite foliation is not parallel to the earlier bedding/foliation planes. About 0.5km towards Rhiroy, the wood between the road and the shore gives way to open pasture. Parking is available in two long passing places less than 100m further along the road.

Locality 9.9 Loch Broom shoreline [NH 156 892 to NH 149 911][edit]

Loch Broom shoreline (Fig. 9.3). Traverse across the Moine Thrust.

The traverse is best attempted at low tide, though this is not essential (consult the Ullapool Tourist Office for times of the tides).

Descend through the field to the shoreline where the Inverbroom Psammite is exposed 9A (Fig. 9.3). Massive psammite banding seen in the Inverbroom Psammite further from the thrust is broken up into irregular lenses of psammite separated by small shear zones or shear bands. The lenses vary in size, reaching up to 3m in length. Lenses of remaining psammite are increasingly broken into smaller lenses or flattened into the new foliation as the process of shear band formation increases in intensity towards the thrust. The early foliation in all pelitic horizons is broken up by shear bands, all indicating thrusting towards the WNW (Kelley & Powell, 1985).

A few metres further along the shore, 9B, minor folds ranging from close to isoclinal in style deform the foliation. The folds exhibit a wide range of styles and axial trends, though their axial planes coincide to within 10°. The minor folds in these exposures are most probably of one generation apart from late kink zones. Curved hinge lines in some exposures indicate that the folds, which formed during the shearing event causing the mylonitization, nucleated perpendicular to the movement direction and were rotated towards it during progressive deformation (Kelley & Powell, 1985). This pattern of minor shear-related folds is characteristic of shear zones (Alsop & Holdsworth, 2004a and references therein).

The rocks develop a stronger planar foliation, 9C, with thinly interbanded psammitic and pelitic mylonites resembling slates in some exposures. However, the increase in the intensity of planar structures is disrupted by shears (see 9A) and minor folds (see 9B) (Fig. 9.3).

The already complicated pattern of the psammitic mylonites is further disrupted by kink zones that are related to brittle movement of the thrust, 9D. These exposures are less than 50m above the thrust plane, and the mylonites become extensively brecciated as the thrust is approached with thin zones of cataclasite occurring parallel to the earlier foliation.

The thrust plane is exposed as a sloping exposure, 9E, of Torridonian sandstone forming the footwall, as the Moine mylonites that formed the hanging-wall have been eroded. The sandstones do not have any apparent internal structures, but are heavily brecciated and cut extensively by quartz veins. A few metres further on, the sandstones exhibit sedimentary layering and cross-bedding that are undeformed. The Torridonian sandstones are not thick in this thrust sheet (the Kinlochewe Thrust sheet) as shown by the outcrop of coarse-grained Lewisian amphibolitic gneisses on the shoreline at Blarnalearoch [NH 148 912] (Fig. 9.3).

Return to the parking place via the shoreline or follow the stream at the Lewisian exposure to the track and return along the road.

The two stone forts marked as brochs on the 1:25,000 map, Dun Lagaidh [NH 142 913] and Dun an Ruigh Ruadh [NH 149 901], are two of the rare stone forts on the west coast of Ross-shire. Not much remains of their walls, but Dun an Ruigh Ruadh is easily accessible above the road at Rhiroy. They were excavated by Mackie (1975) who found evidence of occupation at Dun Lagaidh as early as the 7th century BC; thick deposits suggest a lengthy period of use. A second phase of occupation gave rise to a circular dry stone walled defensive structure, though it was not a broch. Mackie considers that the structure was an Iron Age galleried dun, which would have been built earlier than the classic brochs of Glenelg and the Western Isles. Later occupiers in the 12th and 13th centuries repaired the structures using mortar. For those continuing towards Ullapool, one further locality situated just above the Moine Thrust is worthy of a visit.

Locality 9.10 The Moine mylonites [NH 149 922][edit]

The Moine mylonites (Fig. 9.1). Roadside exposures of the Moine mylonites on the A835.Park in the large lay-by on the south side of the road at [NH 151 920]. The geology in the road cutting immediately opposite is now rather obscured by wire mesh, so walk carefully along the north side of the road for about 200m to the NW. A large road cutting exposes flaggy Moine psammitic and semi-pelitic mylonites that are only a few metres above the Moine Thrust (although this is not exposed on the road section). Quartz and pegmatite veins occur nearly parallel to the intense foliation and both carry the mylonitic stretching lineation (plunging towards 110°). In contrast to the previous locality, at least two sets of folds are present here. A series of smallscale intrafolial isoclines were probably formed during early stages of mylonitization. A more prominent set of mesoscopic folds have S-geometry and are open in style; axes plunge to the SE, down the dip of the fold axial planes. These folds deform the mylonitic foliation and carry a tight axial-planar crenulation fabric in pelitic bands. The two sets of folds are analogous to the ‘F2’ and ‘F3’ folds described within the Morar Group in Excursions 10 and 13, and are thought to have formed during a single phase of progressive ductile deformation associated with westerly-directed overthrusting of the Moine rocks onto the Caledonian foreland.

References[edit]

At all times follow: The Scottish Access Codeand Code of conduct for geological field work