Calcareous siltstone, Cambrian Fucoid Beds

Road-cut near Skiag Bridge, Loch Assynt

Outcrop

Calcareous siltstone, Cambrian Fucoid beds, Loch Assynt
The Fucoid beds are quite variable in appearance in composition. Some are rich in carbonate, almost like limestones, others are dark and flaky, more like shale. This outcrop is quite rich in carbonate, but shows narrow dark layers marking where the bedding planes are. The orange-brown colour, fine grain size, and relative softness distinguish this rock unit from the sandstones below and above. The upper part of the outcrop shows the typical pits and cavities caused by acid groundwater dissolving part of the carbonate in the rock. The hammer shaft is 30 cm long.

Hand specimen

  Calcareous siltstone, Cambrian Fucoid beds, Loch Assynt
This view is of a bedding plane. It shows a dark-stained uneven surface with ridges and furrows. The rock unit got its name from the early belief that these markings were the compressed remains of seaweeds (fucoids). Now it is thought that they are the trails left by feeding creatures on the sea floor. The rest of the specimen shows the orange-brown colour characteristic of the rock unit.

Thin section

  Calcareous siltstone, Cambrian Fucoid beds, Loch Assynt
This thin section view shows the silt-sized grains of sediment, much of which is dolomite (calcium magnesium carbonate), and also branching irregular bands of dark material.

Plane polarized light, field of view 3 mm across.

Calcareous siltstone, Cambrian Fucoid beds, Loch Assynt
This is an even more magnified view of the carbonate-bearing siltstone. Grey and yellow grains are quartz, dolomite grains are bright-speckled, and there are one or two elongate flakes of mica.

Crossed polars, field of view 1 mm across.

D.J. Waters, Department of Earth Sciences, May 2003