Contrary to other rocks, limestones are karstic when water infiltrates, taking on carbon dioxide. It dissolves the rock and forms underground networks and caves, a phenomenon found in the Gorges du Tarn. Dolomitic rocks, loaded with magnesium carbonate, also resulted from the sea that spread out in the region during the Jurassic. More sensitive to erosion, they formed sculpted peaks known as lapiaz. The river carved the mountains for nearly 20 million years and left walls 1,312 to 1,968 ft (400 to 600 m) high.