Sandstone with small shelly fossils, Cambrian Salterella Grit

From road-cuts near Skiag Bridge, Loch Assynt

Outcrop

Quartz sandstone with small shelly fossils; Cambrian Salterella Grit, Loch Assynt
The Salterella shells are just large enough to make out in the outcrop, if the surface is clean and the light suitable. Here is a cluster of broken cross-sections of the tiny white cone-shaped shells in the grey-brown sandstone.

Hand specimen

Quartz sandstone with small shelly fossils; Cambrian Salterella Grit, Loch Assynt
The Salterella Grit is a light grey medium-grained sandstone when fresh, and carbonate-bearing parts of it weather to a brown colour. Rather irregular bedding lamination, marked by thin black layers, is visible on the cut surface. Cross sections of Salterella are also just visible on the right-hand half of the specimen.

Thin section

Quartz sandstone with small shelly fossils; Cambrian Salterella Grit, Loch Assynt
Salterella is a small shelly fossil found only in Lower Cambrian rocks in NW Scotland and North America. The shells are shaped like tiny ice-cream cones, and when sectioned appear as circular or pointed elongate shapes. In this rock the original shells are replaced by fine-grained carbonate and siliceous material. This view shows transverse and lengthways cross sections next to each other.

Plane polarized light, field of view 7 mm across.

Quartz sandstone with small shelly fossils; Cambrian Salterella Grit, Loch Assynt
Salterella is a small shelly fossil found only in Lower Cambrian rocks in NW Scotland and North America. The shells are shaped like tiny ice-cream cones, and when sectioned appear as circular or pointed elongate shapes. In this rock the original shells are replaced by fine-grained carbonate and siliceous material. This view shows transverse and lengthways cross sections next to each other.

Crossed polars, field of view 7 mm across.

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