History of Normandy – Wikipedia

Historians have few sources of information for this period of Norman history: Dudo of Saint-Quentin, William of Jumièges, Orderic Vitalis, Flodoard of Reims, Richerus and Wace.Diplomatic messages are the primary source of information for the succession of dukes.. Rollo of Normandy was the chief – the “jarl” – of the Viking population. After 911, he was the count of Rouen.

: https://en.wikipedia.org/wiki/History_of_Normandy