Coarse feldspathic sandstone, Torridon Group (Torridonian)

From roadside outcrop on the north shore of Loch Assynt

Outcrop

Coarse feldspathic sandstone, Torridon Group (Torridonian)
This outcrop of coarse-grained purplish-red sandstone shows the structure known as cross-bedding. Prominent bedding planes (forming shadows on the rock face) are horizontal and spaced about 10 cm apart. Between these planes, and at an angle to them, you can see a fine layering, often marked by changes in grain size, dipping down to the right or to the left. Cross-bedding like this is common in the sediments laid down by broad, shallow rivers, where the water flows in many small channels separated by sand bars. Sand is deposited in the channels, and the streams migrate sideways, to and fro across the course of the river, leaving behind the layers of sand that formed on the sloping channel sides. Hammer shaft is 30cm long.

Hand specimen

Coarse feldspathic sandstone, Torridon Group (Torridonian)
This hand specimen shows characteristic textures of the coarse-grained Torridonian sandstones. The rock is made of alternating coarser and finer layers. In any one layer, however, there is a mixture of coarse and fine material - the rock is poorly sorted. The coarse grains are of many different colours, from white to dark red-brown, and are clearly composed of different materials. In fact, feldspar is as abundant as quartz, and the rock is, strictly speaking, an arkose or feldspathic sandstone.

Thin section

Coarse feldspathic sandstone, Torridon Group (Torridonian)
This view gives a good impression of the range of sizes of sand grains. Some are angular, some are more rounded. The degree of rounding should depend on how much time they have spent being transported in the river system. Clear grains are mostly quartz, cloudy grains are feldspar. Dark iron oxide material forms a thin coating on the grains and makes up part of the matrix or cement, giving the red-brown or purplish colour to the rock.

Plane polarized light, field of view 7 mm across.

Coarse feldspathic sandstone, Torridon Group (Torridonian)
Between crossed polars we see the great variety of different types of material making up the grains. There is quartz (e.g. large grain at left), potassium feldspar with "tartan" twinning (top right) and fragments of various rock types including quartzite, sheared quartzite, and Lewisian Gneiss. Some of these rocks occur nearby, such as the gneiss. Others, like the quartzites, do not, and must have been brought down the rivers from much further away.

Crossed polars, field of view 7 mm across.

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