The archaean gneiss, extending in great hummocky stretches through Durness, Eddrachillis, and Assynt southwards into Coigach is the oldest rock formation in the British Isles and one of the oldest in the world. On this ancient foundation the stratified rocks have been super-imposed. The older groups - Torridon, Cambrian, Silurian and Old Red Sandstone - are disclosed in masses which appear in succession eastwards to the Pentland Firth. Through these formations intrusive masses of igneous rock have penetrated in several places. In the region of Ben Loyal and Ben Stomino this intrusive rock is syenite, resembling granite but containing traces of the dark mineral named hornblende. A mass of grey granite extends between Loch Migdale and Loch Laggan, near Bonar bridge where it forms Migdale Rock and Bell Bheallaich. Eastward from Loth and extending into Caithness, as far as Berriedale Water, lies another mass of granite which culminates in Ben Bheallaich near Helmsdale (1940 feet). The most extensive outcrop of granite in the county (also extending into Caithness) reaches from Kinbrace northwards to Sandside Bay and along the valley of the Halladale to Portskerra. The western archaean rock, which has been penetrated by numberless dykes of granite, syenite, and other igneous rocks, rises to a height of nearly 1500 feet in Glasbheinn and Ceannabeinne in Durness, and Ben Stroma in Eddrachillis.

Resting on the gneiss are great masses of Torridon sandstone, which rise into huge mountains of reddish brown conglomerate in nearly horizontal beds of great thickness. Quinag, Canisp, Suilbheinn in Assynt and Fasbheinn in Durness are mainly composed of this ancient rock, while near Cape Wrath sea cliffs of the Torridon rise from the water's edge to a height of nearly 1000 feet. Handa and Rhu Stoer are also Torridonian. Above the Torridon sandstone lie Cambrian formations of white quartzite, limestone, and thin shales, in which occur the fossil named, "Olenellus" the discovery of which enabled scientific men to fix the age of the Cambrian. These rocks form a hand stretching from the west side of Loch Eriboll to Loch More and thence by the Upper Oykell to Ullapool in Ross-shire. To this series belong the limestones of Durness and Assynt, which lie above the "Olenellus" beds and piped quartzites. The largest outcrop of limestone is at Stronechrubie in Assynt, and considerable masses occur at Durness and Eriboll. Owing to great earth disturbance in the Cambrian period quartzite beds have been tilted over the more recent pipe-rock formation, on Cranstackie, Foinnebheinn, Arcuil, Beinn Uidhe, and Ben More.

Silurian metamorphic schists cover the whole of central Sutherland eastwards from the "Great Thrust" to Strath Halladale and Strath Ullie. Along this "Thrust" rocks have been heaved up, overlapped and crushed together by the action of colossal natural force. Its line can be followed from Whitenhead southward along the east side of Loch Eriboll till it crosses Loch More and Loch Gorm and, after following the east side of Loch Ailsh, sweeps westward to the Cromalt Hills. The Silurian country is dreary and bare, consisting largely of boggy heather-clad moorland with occasional elevated masses like Ben Hope, Ben Hee, Ben Clibreck, and Beinn Armuinn. On the top of Beinn Armuinn traces occur of the Old Red Sandstone, which was formed above the schists and stretches along the eastern side of the county from Meikle Ferry to Helmsdale - tentacles as it were of the main Old Red formation in Caithness. Similar tentacles of a smaller scale stretch along the north coast to Cnocfreicadain at Tongue.

In a narrow strip between the Old Red Sandstone and the Moray Firth, extending from Golspie to the Ord, are outcrops of a succession of shales, sandstones, coal and limestone belonging to the secondary or newer rocks. At Dunrobin these sandstones and limestones contain a good deal of plant remains, probably carried down by some ancient river to the sea. At various points from Dunrobin to Brora beds of plant refuse have formed into coal, of which the chief bed lies between the upper and lower Oolite at Brora.

Except the boulder clay these beds of Lias and Oolite (Jurassic) are the most recent rocks to be found in the north of Scotland. Resting upon them and stretching over the whole country lie masses of gravel and sand, marking the former presence of ice. Smooth eroded outcrops of gneiss disclose glacial action not merely in the valleys but on the tops of the high mountains.

The metamorphic Silurian schists contain in irregular quantities gold, silver, iron, and other minerals, which occur in various localities. The gold-diggings in the Suisgill and Kildonan burns in 1869 were carried on in the gravel and debris from these schists.