Picts are assumed to have been the descendants of the Caledonii and other Iron Age tribes that were mentioned by Roman historians or on the world map of Ptolemy. Pictland, also called Pictavia by some sources, achieved a large degree of political unity in the late 7th and early 8th centuries through the expanding kingdom of Fortriu, the Iron Age Verturiones. By the year 900, the resulting Pictish over-kingdom had merged with the Gaelic kingdom of Dál Riata to form the Kingdom of Alba (Scotland); and by the 13th century Alba had expanded to include the Brittonic kingdom of Strathclyde, Northumbrian Lothian, as well as Galloway and the Western Isles.