 During the dry season the water recedes into rivers, creeks, and isolated waterholes or [[billabong]]s. Kakadu's wetlands are listed under the Convention on Wetlands of International Importance (the Ramsar Convention) for their outstanding ecological, botanical, zoological, and hydrological features.The southern hills and basins cover a large area in the south of the park, including the headwaters of the South Alligator River.Rocks here have been exposed from beneath the [[Scarp retreat|retreating]] Arnhem escarpment; they are of volcanic origin and are extremely old (2500 million years).