1. A residual portion of the roof of a subterranean stream which has not collapsed and is found bridging a valley. Normally a surface feature, but may be used to describe a similar occurrence in a cave system [20]. 2. A rock bridge spanning a ravine and not yet eroded away [10]. Synonyms: (French.) pont naturel; (German.) Naturbrucke, Felsbrucke; (Greek.) physiki gefyra; (Italian.) ponte naturale; (Russian.) estestvenny most; (Spanish.) puente natural; (Turkish.) dogal kopru; (Yugoslavian.) prirodni most, naravni most. See also natural arch.