Kakadu National Park - Landforms

Landforms

There are six main landforms in Kakadu National Park: the Arnhem Land plateau and escarpment complex, known as the stone country; the outliers; the lowlands; the southern hills and basins; the floodplains; and the tidal flats. Each landform has its own range of habitats. Kakadu’s varied landscapes and the habitats they contain are features that contributed to its listing as a World Heritage Area.

Most of Kakadu was under a shallow sea approximately 140 million years ago, with the escarpment wall formed from sea cliffs and Arnhem Land from a flat plateau above the sea. Currently the escarpment rises 330 metres above the plateau and extends approximately 500 kilometres along the East edge of the park and on into Arnhem Land. The escarpment varies from near vertical cliffs in the Jim Jim Falls area to isolated outliers and stepped cliffs in the North.

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 in most areas. Sparse pockets of open forest and woodlands have developed in these areas. However 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.

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.

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.

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.

Kakadu’s coast and the creeks and river systems under tidal influence (extending about 100 kilometres inland) make up this landform. The shape of the estuaries and tidal flats varies considerably from the dry season to the wet season. During the dry season tidal action deposits silt along the river beds and banks. During the wet season the river beds are eroded by the floodwaters and large quantities of fresh and saline water flow out across the tidal flats, where silt is deposited. Large silt loads are also carried out to sea, some of the silt being deposited as a nutrient rich layer on the sea floor, contributing to the muddy waters that characterise Kakadu’s coastline.

The estuaries and tidal flats are home to an array of plants and animals adapted to living in the oxygen-deficient saline mud. The dominant habitats are mangrove swamps and samphire flats. Where freshwater springs occur along the coasts and river banks, isolated pockets of coastal monsoon rainforests form.