The shore platform and the backing cliffs of glacial till
The wave-smoothed rock ramp at the rear of the platform
An erratic boulder on the beach. This quartzite-rich ORS conglomerate may come from the slopes of Scaraben
Significance: an extensive intertidal shore platform backed by cliffs developed in shelly till
Gills Bay is one of the longest stretches of low-lying rock coast on the northern shores of Caithness. Like other rocky shores in Caithness and Orkney where the land drops gently to the sea, Gills Bay displays broad shore platforms in the inter-tidal zone, up to 200 m wide. The seaward parts of the platform are covered with seaweed and appear to be areas of low activity. The rear of the platform carries boulders of a wonderful variety of rock types derived from erosion of the shelly till. These boulders form a storm beach on parts of the back of the platform and extend on to a structurally-controlled rock ramp which locally forms the rear of the platform. Continuing erosion of the ramp has in places created a low rock cliff and recent block removal is evident from sockets in its face that can often be matched to blocks on the rear of the platform. The rock surfaces exposed at the rear of the platform are often smoothed, a result of mobilisation of the boulder cover and abrasion during storms.
Whilst there is no doubt that development of the shore platform continues, the overall impression is that the platform has been largely exhumed from beneath a cover of shelly till. Support for this view is provided by the presence of striations on the backing ramp. The striations demonstrate that the wave-smoothed ramp, and by implication its fronting platform, existed before the last ice sheet crossed Caithness. Similar inherited shoreline features occur widely in Caithness and Orkney.
The crossing striae at Gills Bay probably relate to the passage of the last ice sheet across the north coast and into the Pentland Firth. The overlying shelly till is dark grey in colour, with many shells in its unweathered lower layers. Prominent also are large clasts of Cretaceous chalk derived from the outer Moray Firth - the puzzle of how chalk has been transported westwards from its subcrop remains unsolved. The boulder clay also contains many smaller, round stones of igneous and metamorphic rocks. The boulder cover on the platform provides many opportunities to spot distinctive and often attractive rocks derived from all over the Northern Highlands and carried by ice to the edge of the Pentland Firth.