Orkney rocks are mainly formed from mud and sand deposited in a freshwater lake during the Devonian period (400 million years ago). The sub tropical climate along with evaporation and rainfall led to the formation of layers of marginal sediments of sand, silt & mud and a finely laminated dark grey to black sandstone. Extensive and repeated growths of algae formed distinctive layered structures. (text copied from 'West Mainland information leaflet' (Orkney Tourist office).
The views from Black Craig clearly show the layered rock formations in the cliffs.
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