The islands include four types of topographical formation: volcanic, high limestone, low platform, and coral atoll. The Palau barrier reef encircles the Palau group, except Angaur Island and the Kayangel atoll. The reef encloses a lagoon (1,267 sq km/489 sq mi) on the western side, containing a large number of small elevated limestone islets known as the Rock Islands. Babelthuap and Koror, with peak elevations of 217 m (713 ft) and 628 m (2,061 ft), respectively, contain elevated limestone and volcanic formations. Arakabesan, Malakal, and several small northern islands are volcanic formations. Peleliu and Angaur are low-platform reef islands.