According to Dr Munro the shell and rubbish heaps excavated by Laing in 1866 belong to the period of transition from the palaeolithic to the neolithic age. If this is correct they are among the oldest traces in Scotland of human civilization. Among the other monuments of greatest antiquity in Caithness are the long, horned sepulchral cairns belonging to the neolithic period, usually containing chambers of one or more compartments, in which are found human remains, flint implements, rude ornaments, and pottery.
The circular cairns throughout the county are of later date, but they also yield relics of the neolithic period. The pottery is more elaborate in design while the implements also indicate a more advanced people. Articles of bronze are now to be fonnd as well as flints. In these cairns the burial cists are smaller than those of the long cairns and have no access from without.
The stone circles found all over England and Scotland belong to the Bronze Age. Burial cists frequently occur within these circles, and some authorities believe that they were erected for religious purposes at a time when forms of ancestor worship prevailed. Stone circles, which are numerous in Sutherland, were at one time more common in Caithness, but they have nearly all been swept away. In a number of cases only a single boulder or stone now remains where at one time there was probably a stone circle. More frequently in Caithness rows of stories or boulders are to be found which are also referable to the Bronze Age and associated with burial cairns or cists. Some of the standing stones in the county are of great size, such as "Stone Lud" at Bower.
These huge stones sometimes occur in pairs, as at Yarrows and Achvarasdal. Stones of later date mark the burial place of some noted Celtic maormer or Norse warrior.
Of the dwellings of the earlier neolithic race no traces remain. The dwellings of the Bronze Period were circular, and "hut circles", though not so numerous in Caithness as in Sutherland, occur at Greenland in Dunnet, at Warehouse, and at various places in Latheron, such as Forse, Langwell and Berriedale.
After the Bronze Period came what is called the Iron Age, associated with the advent of the Celtic people who over-ran Britain some five centuries B.C. The Celts of the Iron Age had simpler burial customs than the neolithic races. They placed their burial cists in the earth without any cairn or other permanent mark. These cists, usually small, contain along with human remains specimens of the Celtic art of the period. The chief monuments to this people are their fortified dwellings or brochs. There are remains of evidences of 145 brochs in Caithness, most of them now little more than green mounds. The late Sir F.T. Barry of Keiss excavated twenty-four brochs in different parts of the county and was thus enabled to furnish detailed plans and measurements of these structures. The brochs served both as dwellings and as places of strength. Roman relics, chiefly glass and pottery, have been found in some of the broch; a fact which would indicate that they were in use in the earliest centuries of our era. Near some of the brochs are hill forts or fortified enclosures, such as occur at Holborn Head, and at Loch Watenan, near Wick. Castle Linglas, on the shores of Sinclair Bay, at one time supposed to be an old Norse stronghold, is really a broch. The excavations of some of the brochs yielded querns, weaving combs and spindles, as well as articles of personal adornment. These structures are referable to a period which must have extended over many centuries. The latest of them might have been in use when the Gaelic missionaries reached Caithness. The builders of these brochs were people of relatively advanced culture, possessing a good practical knowledge of farming and high skill in masonry. They knew how to choose the best cultivable land in the county for their settlement, while the skilful design of their brochs has been much admired.
The earth houses, of which examples occur at Langwell in Latheron and Ham in Dunnet, belong to the Iron Age. Probably owing to the obdurate quality of the subsoil these structures are not so numerous in Caithness as in Sutherland, but several examples survive of galleried dwellings.
The early Christian monuments consist chiefly of sculptured stones, sometimes rudely inscribed with ogham script and having interlaced Celtic ornamentation. The stones with carved symbols are probably of pre-Christian date. The Skinner Stone, whose fragments arc preserved in Thurso Museum, and the Ulbster Stone, now in the grounds of Thurso Castle (brought thither from St Martin's Church at Clyth), have beautifully incised Celtic crosses on both sides. Sculptured stones with various symbols and ogham characters were found at Keiss some years ago, and are now in the National Museum at Edinburgh. An incised stone with a runic inscription, also in Thurso Museum, is to he assigned to the Norse Christian period, which dates from the death of Sigurd the Stout in 1014.
Of the Norse Pagan Period prior to that date few relics survive. Two brooches, a bracelet, and a pin, all of the Pagan Period, were found in a burial cist at Castletown in 1786. Brooches and bracelets were also found near Wick about 1837, and at other places in the county at subsequent dates. Two interesting sculptured stones with incised symbols are preserved at Sandside House.
The hill forts on Ben Freacadain and Cnoc an Ratha in Reay, and at Garrywhin in the parish of Wick are probably to be referred to the Iron Age.