The term clue or cluse comes from the Latin clausus, meaning closed. It designates a narrow passage dug by a torrent perpendicular to an impassable mountain range. The cluse of Sisteron is lodged between the Tithonic limestone strata of the Rocher de La Baume on the right and the softer marls of the citadel on the left. It was dug by the torrential waters of the Durance whose action the harder walls of the Rocher de La Baume resisted better, hence their relatively closed passage as a cluse.