Grey dolomitic limestone, Cambrian Durness Limestone

From road-cuts near Skiag Bridge, Loch Assynt

Outcrop

Grey dolomitic limestone; Cambrian Durness Limestone, Loch Assynt
This exposure lies just above the road, and shows the typical appearance of a weathered outcrop of the grey Durness Limestone. It is a grey dolomite, fine-grained and bedded in units 10 to 20 cm thick. Lichens do not grow on the rock surface, and the lime-rich soil discourages the growth of heather. Instead, areas underlain by the limestone are grassy, with a variety of lime-loving wild flowers. Rainwater slowly dissolves the surface of the outcrop, eating into the bedding planes to make the layering stand out more strongly, and etching out small vertical fractures into deeper grooves.

Hand specimen

Grey dolomitic limestone; Cambrian Durness Limestone, Loch Assynt
The cut surface of the limestone is dull grey, revealing few features. No bedding is visible at this scale, but tiny pale veinlets trace wavy paths across the surface in at least two directions. Fine limestones commonly fracture, and solutions passing through the cracks fill them up with more carbonate, precipitated from solution.

Thin section

Grey dolomitic limestone; Cambrian Durness Limestone, Loch Assynt
The Durness Limestone is mostly formed from carbonate mud, and is made of fine-grained interlocking dolomite crystals. Bedding is not usually visible. However, there are irregular seams of dark material traversing the rock, like that shown in this view. These probably form when the rock is put under pressure and compacted: the carbonate, being slightly soluble, gets preferentially dissolved along certain layers, leaving the dark non-carbonate material behind in a concentrated mass.

Plane polarized light, field of view 2.5 mm across.

  Grey dolomitic limestone; Cambrian Durness Limestone, Loch Assynt
This view shows one of the pale veinlets in detail. Compaction of the rock compresses it in some directions but opens up tiny fractures in others. The fractures fill with more carbonate crystals, but these new ones are free of the tiny grains of opaque matter that give the rock its grey colour. In thin section the vein material is clear, and the host rock is clouded with opaque dust.

Plane polarized light, field of view 1 mm across.

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