Assurne

Aise, Aisurne, Arcise, Arsune, Assume, Ausurne, da Fume, de Furne, du Furne, Sourne, Surpe

A salt-water strait that marked the boundary between the kingdoms of Surluse and Logres; or separating Arthur’s Britain from Galehaut’s land of Sorelois in the Prose Lancelot. It could only be crossed by two bridges: the North Wales Bridge (North Wales Causeway) and the Irish Bridge, both of which were long, dangerous, and partially submerged. In the midst of the waterway was the Lost Island, where Galehaut had a residence - the l'Isle Perdue castle.

According to the Livre d’Artus, the Assurne separated Britain from Gorre (Gore), which has similarities with Sorelois. R. S. Loomis thought that it was the river Severn.