The Cambrian

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The Cambrian rocks of NW Scotland give rise to some particularly distinctive landscapes. The hard white quartzites dominate the craggy mountains of Foinaven, cap the Torridonian hills of Canisp and Quinag and even crop out on the summit of the highest peak, Ben More Assynt. In contrast limestones lie beneath the grassy slopes around Inchnadamph creating landscapes more like the Yorkshire Dales than the NW Highlands. Yet these distinctive rocks are not restricted just to NW Scotland. Very similar strata of the same age are found in Eastern Greenland and in the Appalachians.

Detail of the Eriboll Sandstone.The oldest Cambrian strata are the quartzites, which were deposited to a thickness of about 160m. The formal name of this unit is the Eriboll Sandstone Group, named after the type area of these quartzites at Loch Eriboll. Being quartzites these strata are almost entirely made up of grains of quartz - so clean in fact the units have been considered as suitable for the production of glass. Cross-bedded quartzites.The lower part of the quartzites preserve spectacular cross-bedding (which gives this part the name of "False-bedded Quartzite Formation, although most geologists call it informally the Basal Quartzite, a term introduced by Peach and Horne). These sedimentary structures tell a story of water currents that alternated in direction repeatedly up through the rock sequence. In the modern world this current behaviour is created by tides - so our best guess is that these quartzites were deposited on a shallow, tide-washed marine shelf.

A bedding plane in the Pipe Rock.After about 75m of cross-bedded quartzites the strata change abruptly - but you need to look carefully to spot the change. We're only half way through the whole quartzite sequence but at this point the cross-bedding is lost. It is replaced by vertical burrow structures - about the diameter of a pencil, penetrating down about a metre at a time perpendicular to the bedding. Skolithos in Pipe Rock.These are an example of a trace fossil (as distinct to a body fossil, the preserved form of a particular organism). These trace fossils are called Skolithos. In the recent geological record many of these types of structures are thought to have been worm burrows - an interpretation favoured for the ones in the quartzites. Regardless of their origin they give the distinctive strata a name - the Pipe Rock. In some beds the burrows have a different form, with funnel shaped profiles. This type of trace fossil is called Monocraterion and is generally interpreted as being an escape burrow, formed as the organism escaped from being buried by a sudden influx of sediment. worm burrows seen in profile in rock.The burrows in the Pipe Rock are generally interpreted as having formed in shallow-water marine conditions - similar to that in which the Basal Quartzite accumulated. So the difference in the palaeoenvironment is that during Pipe Rock deposition there were marine organisms that burrowed as part of their life style.

Fucoid beds above Loch Glencoul.The quartzites are overlain by a very distinctive but much thinner unit. It is brown-coloured and contains extensive burrowing. Some of these burrows are parallel to bedding planes and were originally misinterpreted as being the casts of seaweed (fucoides). These structures are now thought to be formed by animals but the name lives on for the rock unit is called the Fucoid Beds. These strata consist on thin bedded sandstones and siltstones cemented by carbonate. There is extensive evidence of sea-bottom grazing animals, including one of the chief predator groups of the Cambrian - Trilobites. Some of these fauna are important because not only do they place the strata as Early Cambrian age, they are also identical to trilobites found on the other side of the Atlantic in rocks of the same age. In contrast they are very different to the trilobites found in the same age strata in the British Isles south of Scotland. This led geologists to propose that originally northern Scotland was part of North America (an ancient continent called Laurentia) and that a now-gone ocean (called Iapetus) lay south of Scotland. The fossils tell of moving continents, part of the geological evidence for plate tectonics.

Parts of the Fucoid Beds contain desiccation cracks showing that they sporadically lay above sea level. So the best explanation is that they were deposited in a generally low-energy, coastal environment that supported abundant life. The best guess is that this was a large lagoon. The transition from Pipe Rock into the Fucoid Beds represents a relative fall in sea-level.

Only about 20m of Fucoid Beds were deposited at any given place, They pass up into a thin (<8m thick) quartzite unit called the Salterella Grit. This is a very clean sandstone indicative of a renewal of the relatively high energy environment of the older quartzites. The grit contains rare clusters of small (mm), conical shell casts (Salterella) and is cemented by carbonate. Collectively the Fucoid and Salterella are called the An t-Sron Group after their type locality on a promontory on the shores of Loch Eriboll.

Trace fossils in the Durness Limestone.Above the An t-Sron comes the Durness Group - an accumulation of hundreds of metres of carbonates (chiefly the calcium-magnesium carbonate called dolomite). The majority of these strata are chemical precipitates (probably enhanced by biological action) - indicating that the input of detritus into this part of the Cambrian sea had largely stopped. There are several different groups of strata within the Durness Group but in Assynt only the oldest two parts are found. The oldest is the dark grey Ghrudaidh Formation, a few tens of metres thick. These carbonates are overlain by the paler Eilean Dubh Formation. Both units contain rare body fossils but in general the alteration of these sediments into dolomite after deposition has destroyed most of the macro fossil content.

Durness Limestone at Inchnadamph.A key feature of the Cambrian strata of NW Scotland is its remarkable persistence, not only in the sequence of units described above but also in their thickness. It is a classic example of a "layer-cake" stratigraphy. It is tempting to explain these features as indicating that the palaeoenvironments change very abruptly at particularly times in the Cambrian and that, say, all the Fucoid Beds were deposited at the same time. However, modern geological thinking (actually the ideas are over a century old!) is that the quartzites and the Fucoids were deposited synchronously in different places and that the variation recorded geologically in a given place charts systematic changes in sea level. Imagine at a given instant quartzites are accumulating (and being burrowed) on an open shelf while in shallow water near shore Fucoid Beds are being deposited. As sea level falls the shelf becomes shallower so that Fucoid Beds are deposited on top of Pipe Rock. Then as sea level rises again quartzites (this time called the Salterella Grit) accumulate above the Fucoid Beds. In this way a rock sequence is built up but the transitions between different units are diachronous - they are not geological time markers. In many instances in the geological record this interpretation can be demonstrated by fossil content or other markers of geological time. Regrettably this isn't an option in the Cambrian strata of NW Scotland because only the palaeoenvironment represented by the Fucoid Beds preserved significant records of life. Nevertheless the strata are very distinctive and are ideal for recognising tectonic structures in the Moine Thrust Belt. But that's another story.

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