Terrain change west of Reay, moving from the smooth slopes developed on the Devonian flagstones to the craggy, ice-moulded surfaces of the conglomerates and granites

Cut flagstones ready for use

Ancient mud cracks in flagstones near Thurso

 Sarclet Conglomerate

Geology and scenery

The influence of rock type and structure is probably nowhere more apparent than in Caithness. The county exhibits two basic forms of terrain:

  • the plain of Caithness, with its gentle slopes and low hills and ridges streamlined by glacial erosion
  • the hilly relief to the south and west, including isolated hills and ridges plus extensive peat-covered plateau, and generally showing few signs of glacial modification.

The plain is developed across Devonian flagstones and sandstones. The dips of the Old Red Sandstone are generally low and control the detail of the many gentle slopes. The rock is easily quarried by glacier ice or by waves but its matrix of silt and sand is compact and strongly cemented, making for a durable rock. Coastal cliffs and platforms show strongly geometric outlines, with cliff faces often following the rock strike and geos exploiting major joints, or faults and igneous dykes.

The inselbergs of southeast Caithness are developed in resistant conglomerates and quartzites. Parts of this terrain are very old, exhumed from beneath the Old Red sandstone cover rocks, so that on parts of Scaraben the 380 million year old scree still clings to ancient hill slopes. The Langwell Forest covers a sequence of ridges and valleys developed over the last 60 million years. The ridges follow resistant bands of quartzite and the valleys have been opened out in more easily weathered igneous and metamorphic rocks. The hill country of the north coast has knuckles of conglomerate and granite rising out of the peat-covered low ground formed in diorite and sandstone. It has been heavily moulded by the ice sheets moving northwards into the Moray Firth. The remote plateaux at the heads of the Dunbeath and Glutt waters are peat-covered remnants of erosion surfaces created by prolonged weathering and erosion before the Ice Age.

Geology map  Memoir Part 1  Memoir Part 2