The medieval earldoms of Orkney and Caithness were positioned between two worlds, the Norwegian and the Scottish. They were a maritime lordship divided, or united, by the turbulent waters of the Pentland Firth. This unlikely combination of island and mainland territory survived as a single lordship for 600 years, against the odds. Growing out of the Viking maelstrom of the early Middle Ages, it became an established and wealthy principality which...