Stoer Bay: Sediments of the Stoer Group (Torridonian)

Red siltstones and sandstones on the shore of the Bay of Stoer. In the distance, a little offshore, the rock shaped like a shark's fin is part of the outcrop of the volcanic mudflow of Stac Fada (see below).

On the coast at Stoer bay, a succession of red rocks, dominated by sandstones and siltstones, preserves a record of the Precambrian environment in their textures and structures. They suggest a semi-arid climate where migrating river systems brought in coarse and fine sediment, where lakes formed and dried out, the sediment surface sometimes covered with blooms of single-celled life, at other times shrinking and cracking in the sun.

These sandstone layers increase in bed thickness and also in grain size upwards. The base of each bed commonly cuts down slightly into the bed below, forming a shallow channel. These upper layers are river deposits - with time, deeper channels and more vigorous streams brought in larger amounts of coarser sand. The bedding surfaces at the bottom of the photo have ripple-marks on them, like lake-shore sand-flats.

View from above of the ripple-marked surfaces on top of beds of fine sandstone. Measuring the alignment of these ripples tells us about ancient current directions.

This pattern of polygonal cracks was formed in a muddy siltstone, probably a lake deposit, as it dried out. The cracks became filled with light-coloured sand from the next thin layer of sediment, and now they stand out in bold contrast.

This set of fragmented thin layers in purple sandstone has the pitted weathering usually developed by carbonate-rich material. Laminated structures like this are believed to form when microbes (commonly called blue-green algae, but more strictly cyanobacteria) formed a thin mat that bound the sediment surface together. Cyanobacteria were one of the characteristic simple life forms of the Precambrian era.

Rock types at Stoer Bay

D.J. Waters, Department of Earth Sciences, June 2008