Assynt shore

The road section between Inchnadamph and Lochinver, in the vicinity of Skiag Bridge, is regularly visited by student parties. The reason is that the section provides a nearly complete transect through the classic stratigraphy of NW Scotland. All the main foreland units are represented, from Lewisian, Torridonian and then the full Cambrian succession into the Durness limestone. For the most part there are no thrusts so a simple application of trigonometry allows students to calculate the thickness of the Cambrian units, a necessary preparatory step to understanding the structural complexities elsewhere. The best option is to work gradually from older to younger rocks, which means starting in the west.

There are nice outcrops of Lewisian gneisses to be found on the shore of Loch Assynt a couple of hundred metres off the A837 [NC 213251]. There are good examples of gneissic band that can be measured in 3D together with a strong mineral lineation. There is also a basic dyke that can be traced out along the shore. However, back at the road there are dramatic outcrops in the road cut. Here Torridonian sediments rest directly upon Lewisian gneisses. Take care approaching the rocks - the traffic can be very fast-moving and unexpected. But it's worth it. The contact between the two rock types is an unconformity. This represents an ancient land surface, over 800 million years old. It represents a gap in the geological record - the time between the last metamorphism and deformation within the Lewisian and the deposition of Torridonian sediments. Amazingly this is a period of one and a half billion years.

The unconformity can be traced for several hundred metres east along the road section. It undulates, cutting up and down across the section. These undulations represent small hills that were buried by the accumulation of Torridonian sediments. The sediments are nearly horizontal, banking in the ancient topography. Looking up you can see a further 600m of Torridonian heading up towards the top of Spidean Coinich (764 m), the southern summit of Quinag.

The top of Spidean Coinich is made of quartzites. These rocks form a prominent inclined white band that dips eastwards, towards the head of Loch Assynt. So we would predict that the quartzites should cross the road. The expectation is realised at a small bay [NC 228247] nearly 2 km from the base of the Torridonian. The boundary between quartzites and Torridonian is not exposed here - you'd have to walk up the slopes of Quinag for that. But we can get it close enough. This is another unconformity. The Torridonian strata are truncated against the quartzites. But unlike the base ofsf the Torridonian, the sub-Cambrian unconformity is planar. The time gap represented by this unconformity is some 250 million years.

The quartzites crop out on the roadside in a series of small outcrops and a low cliff. These are great places to check them out. While the Torridonian is red-purple coloured, the quartzites are bright white. This reflects their far greater quartz content, nearly 99% in most samples. In weathered outcrops there are yellow smudges that represent chemically altered feldspar (difficult to detect in fresher samples - it's slightly milkier than quartz). The quartzites contain dramatic cross-bedding, inclined depositional surfaces contained between the main bedding planes. These represent small sand-waves, heaped up by vigorous tidal currents on a shallow-marine shelf some 550 million years ago.

The road climbs out of the bay and passes through a cutting [NC232246]. In this section the nature of the quartzite changes. The cross-bedding effectively disappears and the bedding becomes a little thinner than before. You need to look really closely to see the critical feature here - the quartzite is jam-packed with fossil worm burrows. They are like very long pencils, perpendicular to bedding and give this quartzite its special name - the Pipe Rock. The pipes can be picked out on bedding planes as small studs, the size of pencil ends. In good lighting conditions and with patience you can back-track through the road cutting to find where the pipes first appear. This is the oldest direct evidence of complex multi-cellular life in NW Scotland.

The most obvious worm burrows are found in road cuttings at Skiag Bridge [NC235244]. Here the Pipe Rock is tinged red - a consequence of iron staining (the rock is still nearly 100% quartz). The iron staining enhances the burrows - although it does not pick them out perfectly. Perhaps you can see why this aspect of the quartzite is informally called the "bird-shit Pipe Rock".

The geology at Skiag Bridge.

Continuing along the loch-side road things begin to change abruptly. The top of the crag at the road junction marks a change in the landscape. More readily eroded rocks lie on top - the Fucoid Beds. These are brown-weathering silts and sands which have a small carbonate content (as a cement). There is a wealth of fine detail in these rocks that include a range of fossil burrow types. From a distance it appears that the transition up from the Pipe Rock is abrupt but it actually occurs transitionally over the space of less than a metre.

The Fucoid beds are overlain by another band of quartzites called the Salterella Grit (old maps and books call this the Serpulite Grit). It has a carbonate cement which means that it has subtly different lichen cover to the Pipe Rock. The transition up from the Fucoid Beds can be picked out in the roadside and in a low cliff on the shore [NC 237240]. The Salterella is only a few metres (and a few beds) thick. It passes up into a gritty dark grey carbonate, the basal part (the Ghrudaidh Formation) of the Durness units. The road section here contains clear worm burrows that penetrate into the Salterella Grit.

The Cambrian units, up from the Torridonian, as seen on this transect are all dipping at about 12 degrees towards ESE. They form a so-called conformable succession. There is no evidence for prolonged periods of erosion. So the succession tells a story of progressive environmental changes (although all the ancient environments represent shallow marine settings) as the sediments were laid down horizontally, bed by bed. The whole pile was then tilted to the ESE. But our story does not end here. The road cuttings in the Ghrudaidh limestones contain a bedding-parallel band of dark igneous rock. This was intruded as magma into the limestones, forming a horizontal sheet - called a sill. These are amongst the most northerly, outlying representatives of the foreland igneous rocks of Assynt.

Immediately above the base of the limestones comes the start of the Moine Thrust Belt. The road section continuing towards Inchnadamph runs within Durness carbonates (with occasional sills) but these strata are not gently dipping. Rather they are thrown into folds and repeated by thrusts. As the terrain is made up of carbonates it is difficult for the untrained eye to pick out these structures. The Sole Thrust (the regionally lowest thrust structure) can be found in the low cliffs on the headland of Ardvreck Castle. To gain an appreciation of the complexity of thrusts hereabouts there are two options. First you could track carefully up one of the small stream sections that drain the hillside overlooking the road to Kylesku (A837; e.g. GRs NC 235250, NC 234256). These sections contain numerous alternations of Fucoid Beds and Salterella, repeated on thrusts. In these sections the thrusts lie at the upper boundary of each Salterella Grit unit, before the Fucoids come in again.

An alternative strategy is to examine the cliffs from the A837 that lie south of Inchnadamph. This section, known as Stronchrubie, gives a natural, albeit rather oblique, section through the lower part of the imbricates. Here Durness carbonates are stacked up on faults and the bedding is made to dip more steeply to the east that it does on the foreland. These sections have been gazed upon by over a century's geologists.

To reprise the geology of this area it is worth gazing across Loch Assynt onto the northern escarpment of Beinn Garbh. The low slopes are made up of Lewisian gneiss, the intermediate by Torridonian and on top we'd expect the Cambrian quartzites. So it's a great place to understand the two unconformities.

But in practice things are more complicated. The Cambrian and Torridonian strata are intruded by a series of generally gently-dipping sheets of igneous rock - felsite. Although of a different composition, these are part of the same suite of igneous rock as the sills in the Durness carbonate seen on the road side.

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