Along the western margin of the county from Reay on the north coast to the Scaraben Hills there is a narrow belt of country which is occupied by metamorphic rocks of the types found in the east of Sutherland. They consist chiefly of granulitic quartzose schists and felspathic gneisses, permeated in places by strings and veins of pegmatite. On the Scaraben Hills there is a prominent development of quartz-schists the age of which is still uncertain. These rocks are traversed by a mass of granite sometimes foliated, trending north and south, which is traceable from Reay southwards by Aultnabreac station to Kinbrace and Strath Helmsdale in Sutherland. Excellent sections of this rock, showing segregation veins, are exposed in the railway cuttings between Aultnabreac and Forsinard. A rock of special interest described by Professor Judd occurs on Achvarasdale Moor, near Loch Scye, and hence named Scyelite. It forms a small isolated boss, its relations to the surrounding rocks not being apparent. Under the microscope, the rock consists of biotite, hornblende, serpentinous pseudo-morphs after olivine and possibly after enstatite and magnetite, and may be described as a mica-hornblende-picrite. The remainder of the county is occupied by strata of Old Red Sandstone age, the greater portion being grouped with the Middle or Orcadian division of that system, and a small area on the promontory of Dunnet Head being provisionally placed in the upper division.
By means of the fossil fishes, Dr Traquair has arranged the Caithness flagstone series in three groups, the Achanarras beds at the base, the Thurso flagstones in the middle, and the John o' Groats beds at the top. In the extreme south of the county certain minor subdivisions appear which probably underlie the lowest fossiliferous beds containing the Achanarras fauna. These comprise (1) the coarse basement conglomerate, (2) dull chocolate-red sandstones, shales and clays around Braemore in the Berriedale Water, (3) the brecciated conglomerate largely composed of granite detritus seen at Badbea, (4) red sandstones, shales and conglomeratic bands found in the Berriedale Water and further northwards in the direction of Strathmore. Morven, the highest hill in Caithness, is formed of gently inclined sandstones and conglomerates resting on an eroded platform of quartz-schists and quartz-mica-granulites. The flagstones yielding the fishes of the lowest division of the Orcadian series appear on Achanarras Hill about three miles south of Halkirk. The members of the overlying Thurso group have a wide distribution as they extend along the shore on either side of Thurso and spread across the county by Castletown and Halkirk to Sinclairs Bay and Wick. They are thrown into folds which are traversed by faults some of which run in a north and south direction.
They consist of dark grey and cream-coloured flagstones, sometimes thick-bedded with grey and blue shales and thin limestones and occasional intercalations of sandstone. In the north-west of the county the members of the Thurso group appear to overlap the Achanarras beds and to rest directly on the platform of crystalline schists. In the extreme north-east there is a passage upwards into the John o' Groats group with its characteristic fishes, the strata consisting of sandstones, flagstones with thin impure limestones. The rocks of Dunnet Head, which are provisionally classed with the upper Old Red Sandstone, are composed of red and yellow sandstones, marls and mudstones. Hitherto no fossils have been obtained from these beds save some obscure plant-like markings, but they are evidently a continuation southwards of the sandstones of Hoy, which there rest unconformably on the flagstone series of Orkney. This patch of Upper Old Red strata is faulted against the Caithness flagstones to the south.
For many years the flagstones have been extensively quarried for pavement purposes, as for instance near Thurso, at Castletown and Achanarras. Two instances of volcanic necks occur in Caithness, one piercing the red sandstones at the Ness of Duncansbay and the other the sandstones of Dunnet Head north of Brough. They point to volcanic activity subsequent to the deposition of the John o' Groats beds and of the Dunnet sandstones. The materials filling these vents consist of agglomerate charged with blocks of diabase, sandstone, flagstone and limestone.
An interesting feature connected with the geology of Caithness is the deposit of shelly boulder clay which is distributed over the low ground, being deepest in the valleys and in the cliffs surrounding the bays on the east coast. Apart from the shell fragments, many of which are striated, the deposit contains blocks foreign to the county, as for instance chalk and chalk-flints, fragments of Jurassic rocks with fossils and pieces of jet. The transport of local boulders shows that the ice must have moved from the south-east towards the north-west, which coincides with the direction indicated by the striae. The Jurassic blocks may have been derived from the strip of rocks of that age on the east coast of Sutherland. The shell fragments, many of which are striated, include arctic, boreal and southern forms, only a small number being characteristic of the littoral zone.