Main image on top: Coast at Old Wick

Much of Caithness is underlain by Middle Devonian flagstones (map) – thinly-bedded siltstones and sandstones which cleave to give sheets of rock which have used extensively in the past as paving, tiles and field boundaries. These sedimentary rocks were laid down around 370 million years ago in the huge lake. The sedimentation was rhythmic – driven by similar Milankovich astronomical cycles which determined the tempo of the Ice Age. The cycles are repeated again and again in layers of rock with a cumulative thickness of some 4 km thick. the sequence is:

  1. laminite – dark grey organic silt, often with fish remains
  2. inter-layered dark shale, siltstone and limestone, with polygonal cracks
  3. coarse siltstones with ripple bedding
  4. shale, siltstone and fine sandstone, with ripple marks and desiccation cracks.

The sequences represent the response of the lake to changes in rainfall and evaporation by progressive shallowing of the anoxic floor of the lake, with first wave action and disturbance and finally exposure and drying out.

At Dunnet Head the succeeding group of Upper Old Sandstone are coarser and tougher rocks, although sandstone beds with calcareous cement weather to distinctive pitted surfaces. Similar rocks dominate Hoy.

Beneath these rocks lie the basal conglomerates and breccias of the Lower Devonian. The Sarclet Conglomerate is a particularly distinctive and attractive stone, with pebbles of granite, schist, quartzite and basalt set with in a red sandstone matrix (Johnstone and Mykura, 1989). These sediments rest on and are largely derived from the Moine metamorphic rocks of east Sutherland and the Caledonian granite intrusions, of which the Helmsdale Granite is the best exposed.

Ancient mud cracks in flagstones near Thurso

Credit: British Geological Survey

Sarclet Conglomerate
Traditional Caithness flagstone fencing

© Copyright David Bremner / Geograph

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