Coarse sandstone, Stoer Group (Torridonian)

These sandstones lie just above the Lewisian Gneiss basement at Clachtoll, NW of Lochinver.

Outcrop

Interbedded coarse and medium-grained sandstones, Stoer Group (Torridonian), Clachtoll
Brown sandstones were laid down by rivers flowing in the valleys cut in the Lewisian Gneiss surface. Some are softer and darker-coloured, some are paler and better cemented. They lie on top of, and also grade sideways into, the comglomerates that lie on the gneiss surface itself. The gneiss, with its thin covering of conglomerate, is just out of the picture to the right. The white patches on the outcrop are lichens. The compass-clinometer is about 10 cm long.

Hand specimen

 Coarse-grained sandstone, Stoer Group (Torridonian), Clachtoll
Individual sand grains are up to 4 mm across. The bedding is shown by changes in grain size from layer to layer. Glassy white grains are quartz; pinkish-orange grains are feldspar. Strictly, this is an arkose, a feldspar-rich sandstone. The matrix has the purplish-brown colour characteristic of the Torridonian rocks.

Thin section

 Coarse-grained sandstone, Stoer Group (Torridonian), Clachtoll
Poorly sorted angular grains are cemented by a dark orange-brown matrix. Clear grains are quartz; cloudy grains are feldspar. Note the great range in particle sizes, the angularity, and the large amount of feldspar. These all imply that the material has not spent a great deal of time being subjected to weathering and transport processes at the Earth's surface.

Plane polarized light, field of view 7 mm across.

 Coarse-grained sandstone, Stoer Group (Torridonian), Clachtoll
This view under crossed polars makes it clear that most of the larger clasts are actually fragments of Lewisian Gneiss rather than individual grains of quartz or feldspar. Each clast contains a number of crystals of feldspar and quartz, and they closely resemble the textures of the underlying gneisses. The dusty, bright-coloured material in the feldspar grains is very fine mica crystals, the beginnings of chemical breakdown of the feldspar while it was still part of the gneiss.

Crossed polars, field of view 7 mm across.

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