 Scandinavian-held territories included the Northern Isles of Orkney and Shetland, the Hebrides, the islands of the Firth of Clyde and associated mainland territories including Caithness and Sutherland.The historical record from Scottish sources is weak, with the Irish annals and the later Norse sagas, of which the "Orkneyinga Saga" is the principal source of information, sometimes contradictory although modern archaeology is beginning to provide a broader picture of life during this period.There are various competing theories that have addressed the early colonisation process, although it is clear that the Northern Isles were the first to be conquered by Vikings and the last to be relinquished by the Norwegian crown.