 Tym-Poronaiskaya Valley separates the two ranges.Susuanaisky and Tonino-Anivsky ranges traverse the island in the south, while the swampy Northern-Sakhalin plain occupies most of its north.Crystalline rocks crop out at several capes; Cretaceous limestones, containing an abundant and specific fauna of gigantic ammonites, occur at Dui on the west coast; and Tertiary conglomerates, sandstones, marls, and clays, folded by subsequent upheavals, are found in many parts of the island.