The Déroc waterfall is located almost 100 feet (30 m) above the glacial valley of the Gambaïse stream. This brook is a tributary of the Bès river that crosses the village of Marchastel. It adds up with the outlet of the Salhiens lake and together become a cascade. The waterfall is home to a real Aubrac treasure, a cave with a ceiling made up of huge basalt columns. This geological formation results from the solidification and thermal contraction of a basaltic lava flow.