The land area of the county (to high-water mark and exclusive of inland water) is 438,833 acres. Inland lochs are numerous, especially in the parishes of Latheron, Halkirk, and Reay, and over an area of 7139 acres. The foreshores extend to 2717 acres, while tidal waters cover 131 acres. Thus the gross area of the county extends to 448,822 acres or 701 square miles.

Caithness forms an isosceles triangle, with its apex at Drumhollistan, the extreme westerly point of the county, just over 30 miles distant from Duncansby and the Ord. The base of the triangle runs for 40 miles in a north-easterly direction along the Moray Firth, from the Ord to Duncansby, with an outward bulge from Sarclet Head to Noss Head, a gentle inland sweep at Dunbcath and a more pronounced indentation at Sinclair Bay. The north side of the county runs westerly from Duncansby to Drumhollistan, with a northward bulge containing bold and rocky promontories reaching out to Dunnet Head. There is one considerable indentation forming Thurso Bay, with smaller bays at Crosskirk and Sandside. The coastline from the Ord round to Drumhollistan, following indentations, extends to about 90 miles. The inland border of the county touches Sutherland along its whole extent and runs in a southerly direction from Drumhollistan to the Ord, with a westward sweep at Cnoc Gaineimh (1391 feet) about 20 miles south of Drumhollistan, where the water of Berriedale and the Thurso river take their rise.

In Pont's map of Caithness (about 1608), the Halladale is given as the north-western boundary between Sutherland and Caithness, but the watershed has been recognised as the boundary since the erection of the shires. In 1892 Strathhalladale, which, though in Sutherland, formed part of the parish of Reay, was detached from Reay and added to the parish of Farr. A detached part of the parish of Thurso (near Dorrery) was added to Halkirk.

There is a striking contrast between the flat, fertile, and well-cultivated country in the north-eastern angle of Caithness and. the inland expanse of barren, waterlogged moor, gradually rising from the northern flats until it attains its greatest elevation in the cone of Morven.

The contour of Caithness is dependent upon its geological structure. Where the archaean metamorphic rocks prevail the country consists of barren heatherclad mountains interspersed with many peat mosses, lochs, and bogs, and the scenery closely resembles that of the northern Highlands. The lowland country, on the other hand, in aspect, as in geological structure, resembles the "Laigh" of Moray, the rich haughs of Easter Ross, and the fertile Orkneys. Like the Orkneys, too, Caithness is nearly devoid of trees. In the summer and autumn the rich cornfields, with the pasture and meadow lands, on which splendid herds of cattle comfortably graze, redeem the flatness of the scene. In former times before the days of advanced agriculture, visitors to Caithness were repelled by the unrelieved bareness of the scenery and described the country as "awesome and forbidding".

If Caithness suffers from lack of trees there is compensation in its coast scenery. The deep geos, the protecting stacks and headlands resound with the crash and roar of huge Atlantic breakers. The elevation of the rocks and promontories adds to the effect. Nowhere in Britain is there finer rock scenery than all the coasts of Caithness.

The Loch of Watten (Norse "Water"), 5 miles long by 1 1/2 miles broad, is the largest lake in the county, and next to it in size are Lochs Calder, Shurrery, and More. There are no fewer than twenty-four lochs in the parish of Halkirk, and a large number also in Latheron and Reay. St John's Loch in Dunnet, like Loch IMa Nathair in Strathnaver, enjoyed in olden times a reputation for its healing virtues. Invalids resorted to it in large numbers, especially at Midsummer (St John's Day), plunging into its waters and going through certain ceremonies which were expected to result in an effctive cure.