The following scheme shows: -
THE SUCCESSION OF THE SEDIMENTARY ROCKS IN CAITHNESS AND SUTHERLAND
Recent The post-glacial peat of Caithness moors; buried forests at Keiss foreshore.
Pleistocene - Glacial calcareous drift in north-east of the county (from Moray Firth region): glacial sandy drift of west and south (from Sutherland).
Newer Fossils consist of marine fauna (wanting in Caithness).
Many of the shells found in these strata arc of extinct species (wanting in Caithness).
(Wanting in Britain)
6 Lower and Oligocene
Represented in Isle of Mull leaf bed.
The shells are almost wholly of extinct species.
Sands and clay (wanting in Caithness).
8 Middle Occurs in south of England.
The London clay.
Upper Chalk and Ware
Surrey white chalk with flints.
Lower Clay and Sands
Weald clay in south of England.
12 Upper Freshwater clay beds between Kintradwell and the Ord; beds above the Brora coal.
13 Middle Carbonaceous shales at Brora.
14 Lower Shales and calcareous sandstones at Strathsteven, Brora.
15 Lias Shales and sandstones with plant remains at Dunrobin: sandstones and limestones in Golspie burn.
16 Upper (Wanting in Britain)
17 Middle (Wanting in Britain)
18 Lower Red sandstone of Lancashire and Cheshire
Old Red Sandstone
Magnesian limestone in Yorkshire and Cumberland
20 Upper English coal strata.
21 Lower Limestones of Lanark and Fife, with shales and ironstone.
22 Upper Yellow sandstone of Dunnet - a continuation of the Orkney beds.
23 Middle Caithness flagstones at Castk-town, etc. (with fossil fish and plants)
Braemore shales and flagstones with plant remains and conglomerates of Morven.
Silurian and Ordovician
Upper The metamorphic rocks of the central highlands and uplands of Caithness. 26
Lower The schists of the Caithness uplands underneath peaty moors.
Upper Quartzites in Caithness uplands as at Scaraben: also in west of Sutherland.
Lower Slates and shales (lower fossils) in Wales.
Extend from Cape Wrath to Lochinver, granitic in appearance, but folded and contorted.
The geology of Caithness is rendered somewhat simple by the fact that the surface of the county is made up mainly of two rock formations. The upland parts consist in a large measure of stratified schists, common all over the Scottish Highlands, and Cambrian quartzites. The lowland country, on the other hand, is composed mainly of Old Red Sandstone.
Overlying the metamorphic schists is the sandy glacial drift, covered with peat and dreary moorlands of heather, sedges, and coarse grasses.
Overlying these primeval strata are a series of conglomerates, flagstones, and sand tones in masses which may be said to form the Old Red Sandstone. The same formations occur in Orkney, east of Sutherland, and Ross. The fossil remains, chiefly freshwater fishes, show that the Old Red was formed by deposits in a freshwater lake. In the west of the county geologists can point to the shore lines of this ancient inland sea. Sir Archibald Geikie has estimated that the total thickness of the Old Red Sandstone deposits in Caithness and Orkney exceeds 18,000 feet. The oldest beds are the conglomerates formed by the detrition of the Silurian schists on which the lake rested. These conglomerates occur at Berriedale and form some of the higher mountains in the south-east of the county. Morven, Maiden Pap, and Smean are mainly conglomerates, while Scaraben, Salvoich, and Scalabsdale are mainly quartzite. The upper beds of the Old Red Sandstones occur in the extreme north of Caithness, extending into Orkney. The middle Old Red is exemplified by the flagstones of Thurso and the red marls and sandstones of John o' Groats district.
In the course of time the older deposits of the Red Sandstones were tilted and convoluted by pressure into mounds and hillocks, which in turn were worn down by the denuding forces of rain, frost, and wind. To these later periods of the Old Red Sandstone also belongs a sedimentary formation near Dunnet Head, where the traces of animal life are chiefly fossil remains of freshwater fishes.
The soil derived from wearing down the flagstones and sandstones became the subsoil of lowland Caithness. Above it lies the boulder clay of the Glacial Period. The detrition of the flagstones produced a soil containing traces of fertilising ingredients not found in the more ancient gneisses and schists of the interior. The boulder clay of the north-east, formed from ice drifts off the Moray Firth basin, possesses considerable elements of lime, shells, and other organic remains contributing to the fertility of the soil, with the result that north-eastern Caithness became a fruitful land in striking contrast with the barren uplands.
The ice movements during the Glacial Period brought masses of gravel and fragments of rock from the interior into the western and south-western regions of the county, now forming the ridges and eminences of upland Caithness. These boulders and the gravel in which they lie are obviously derived from rocks different from those on which they rest. The drifting masses carried by the ice-flow eroded hollows out of the soil. These hollows in many cases became the beds of lakes or formed marshes, which in time became peat mosses.
In the vast period that has elapsed since the flagstones of Caithness were laid down the earth's crust has been subject to upheaval and subsidence, greatly altering the surface of the country. The raised beaches so familiar in the east of Sutherland scarcely occur in Caithness. The remains of trees and plants in the peat mosses furnish ample evidence that Caithness was at one time covered with forests of pine, birch, alder, and willow, and that the climate, as well as the elevation of the surface, has varied greatly during the prolonged periods of geological time.