Quartz sandstone, Cambrian Salterella Grit

From road-cuts near Skiag Bridge, Loch Assynt

Outcrop

Quartz sandstone; Cambrian Salterella Grit, Loch Assynt
This is a view of the uppermost part of the Salterella Grit in a road-cut exposure. You can see horizontal bedding marked by colour (and probably also by grain size) differences, and vertical grooves, which are weathered-out parts of vertical burrows like those in the Pipe Rock. The darker rock just visible at the top of the picture is the base of the Durness Limestone. The notebook is 20 cm long.

Hand specimen

Quartz sandstone; Cambrian Salterella Grit, Loch Assynt
In a fresh hand specimen the Salterella Grit is a fairly coarse, even-grained grey quartz sandstone. It has a slightly browner and rougher weathered surface than the basal quartzite, mainly because the material cementing the quartz grains is different, and is not as hard as the silica cement in the quartzite. Pits and hollows in the surface suggest that there is carbonate in the cement.

Thin section

Quartz sandstone; Cambrian Salterella Grit, Loch Assynt
The Salterella Grit is a quartz sandstone that is cemented in part by carbonate material. In this view, the outlines of fairly well rounded quartz sand grains are clearly defined. The carbonate crystals in the cement appear to stand out in relief, and help to outline the sand grains.

Plane polarized light, field of view 6 mm across.

Quartz sandstone; Cambrian Salterella Grit, Loch Assynt
Between crossed polars the quartz grains appear white through shades of grey to black. Between them, the small, pearly-speckled grains are carbonate (dolomite).

Crossed polars, field of view 6 mm across.

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