Most of Caithness is underlain by flatstones of the Middle Old Red Sandstone (ORS) that are mid Devonian in age, and were deposited as muds and sands in a large lake. the famous fossils fish beds of Caithness and Orkney occur in these rocks. Later in the Devonian period the lake was filled with sediment and the area dried out. Locally there was uplift and erosion before the Upper Old Red Sandstone was deposited. Dunnet Head and most of the Island of Hoy consist of red and yellow sandstone of the Upper ORS. These sandstones have sedimentary structures that indicate an alternation of deposition by rivers (fluvial) and sand dunes (aeolian) in a semi-arid environment. A few fish scales have been found on Dunnet Head, but they are rate.
The eastern margin of Dunnet Head is controlled by the Brough Fault. This N-S fault separates the Upper ORS from Middle ORS flagstones to the east. The rocks are highly deformed along the line of the fault. Later, probably in Permian times, a small volcanic vent punched its way through the sandstones, and is now seen in the Burn of Sinigoe.
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