<P> Chasms and gorges form a network that dissects the rocky platforms on the plateau . The plateau top is a harsh, dry environment where water drains away quickly and top soil is scarce . Sparse pockets of open forest and woodlands have developed in these areas . Creeks have carved deep gorges in the escarptment in which tall monsoon forests grow . These areas form microclimates for plants and animals and often serve as a refuge during the dry season . Allosyncarpia ternata, a large shady tree found only in the Kakadu and Arnhem Land, is the dominant plant species . </P> <P> The outliers are essentially pieces of the Arnhem Land plateau that have become separated from the plateau complex by erosion . They were islands in the ancient seas that once covered much of Kakadu . The gently undulating lowland plains stretch over much of the Top End . Travelling anywhere in Kakadu, you cannot help noticing the lowlands--they make up nearly 70% of the park . The soils are shallow and often overlie extensive sheets of laterite (ironstone) and a thick profile of strongly leached rocks . </P> <P> During the wet season water carried down from the Arnhem Land plateau often overflows from creeks and rivers onto nearby floodplains . Alluvial soils carried in the floodwaters add nutrients to the floodplains . Nutrient - rich soils along with an abundance of water and sunlight make the floodplains an area of prolific plant and animal life . During the dry season the water recedes into rivers, creeks, and isolated waterholes or billabongs . 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 . </P> <P> 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 retreating Arnhem escarpment; they are of volcanic origin and are extremely old (2500 million years). This landform is characterised by rugged strike ridges separated by alluvial flats . </P>

Australia has two principle river systems. what are their names