In Provence, the term "clue" (a narrow gorge) was used to designate a narrow passage, dug by the torrents of a montain river, running perpendicular to a mountain range, apparently impassable. Another name often used was "cluse", this word comes from the Latin "clausus" which means "closed". The narrow gorge pass at Sisteron is located between the Baume rock and the Citadel rock. These geological formations were formed during the Secondary Era when large quantities of limestone sediment were deposited here at the bottom of the sea. The sedimentary layers were then folded twice: firstly, at the end of the Secondary Era during the creation of large east-west oriented folds, resulting in the moutnains of,Lure and Ventoux, and then secondly, during the Tertiary Era during the uprising of the Alps. The Durance river, with its torrential flow, completed the geological transformation by digging a trench, a gorge through softer black marl. The opposite side, made up of harder strata (tithonic limestone) has better resisted the action of the torrent, leaving only a narrow passage: the gorge. The Gorge of Sisteron is also a metaphorical "doorway". A climatic gateway: downstream, the area begins where olive tree were planted. A historical gateway: from the Dauphiné area, it allows you to pass into area of Provence. It is also an obligatory passage for transportation routes. It was in Sisteron that the first stone bridge over the Durance river was probably built in the 13th century. Since then, roads, canals and motorways have been forced to use this exceptional site.