In prehistoric times the Western Isles and the and the west coast of Scotland north of the Firth of Clyde seem to have formed a geographical and cultural unit quite distinct from the central and southern mainland but closely connected with the far north, with the counties of Sutherland and Caithness and the Orkney and Shetland Isles. Together all these territories form a highland and island zone which has recently been designated as one of the four main natural regions of Iron Age cultures in Scotland and called the Atlantic Province [I]. The distinctiveness of the cultures which occupied this maritime province not only in the Iron Age but at all periods emphasizes the effectiveness of the mountain barriers which hinder contact with the south and central mainland and also, by contrast, the relative ease with which it was colonized by sea. The spread of Neolithic chambered tombs in the 3rd millennium B.C. and the Norse settlements of the 9th century A.D. onwards show how seaborne migrants from distant regions could quickly occupy the province, and a similar series of immigrations seems to have taken place in the Iron Age.