It is believed that during unstable decades from 711 to 756 in al-Andalus, power struggles between Arab and Berber factions, together with newly converted Visigoths (muwalladi), led to a division of territory: richer agricultural lands in the Guadalquivir basin fell under Arab control, and mountainous areas such as the Serra de São Mamede were generally held by Berber clansmen. Arab sources refer to the area north of the Guadiana as Bi:lad al-Barbar or Lands of the Berbers.[23] Following the chaotic decades of invasion, by the later 8th century CE Marvão would have formed part of the Western thughūr ('march' i.e. buffer area or boundary zone) ruled by a marcher lord, or ka'id from the old Lusitanian capital of Mérida.[24] One of three marches, this was known as the Lower March (al-Tagr al-Adna) or Distant March (al-Tagr al-Aqsa).