The dominant sandstone escarpment of the Arnhem Land Plateau ranges in height from 30 metres to 300 metres, and includes the rock shelters and outliers of Ubirr and Nourlangie. In some areas the escarpment is eroding by up to one metre every thousand years.
It is believed that 140 million years ago much of Kakadu was under a shallow sea. The prominent escarpment wall formed sea cliffs and the Arnhem Land plateau formed a flat land above the sea. Today the escarpment, which rises to 330 metres above the plains, extends over 500 kilometres along the eastern boundary of the park. It varies from vertical cliffs in the Jim Jim Falls area to stepped cliffs and isolated outliers in the north.
The rock platforms of the plateau are dissected by a network of chasms and gorges. The top of the plateau is a harsh, dry place. Water drains away quickly. In most areas soil is scarce. The small patches of soil that are to be found consist mostly of coarse sand and leaf litter trapped in rock fissures or shallow depressions. Sparsely distributed pockets of woodland and open forest have developed on these coarse soils.
Along the escarpment, creeks have etched deep incisions to form gorges in which tall monsoon forests have developed. Water seeping from rock walls and the deep alluvial soils provide an important micro-environment for plants and animals. Many animals rely on these areas for refuge during the drier months. The dominant plant species is Allosyncarpia ternata, a large, spreading, shady tree that is found only in the Kakadu and Arnhem Land region.
Chestnut-quilled rock pigeons
Many animals living in the stone country of Australia's Top End are found nowhere else in the world. Among the endemic animals that can readily be seen at Ubirr and Nourlangie are the chestnut-quilled rock pigeon and the black wallaroo (the male black wallaroo is dark brown to black while the female is grey). Among the endemic animals less often seen are the white-throated grass wren, the rock ringtail possum, the giant cave gecko, Leichhardt's grasshopper, and the Oenpelli python, which at up to four metres long it is one of Australia's longest snakes.
Freshwater crocodiles live across the top end of Australia in the upper reaches of the larger rivers and creeks. They nest in the late dry season (September and October) along sandy banks, where they lay their eggs in carefully dug holes and then cover them. During the breeding season they are more aggressive-they have attacked visitors in plunge pools such as the one at Maguk. Please make sure you read the crocodile warning signs in plunge pool and gorge areas and read other information carefully.
Other, more widespread stone country species are the peregrine falcon, the ghost bat, the short-eared rock wallaby (often seen at Ubirr) and Merten's water monitor.
Plants growing in the stone country and on the outliers must survive extremely hot, waterless conditions for many months each year. Among the best examples of plants well adapted to these harsh conditions are the resurrection grasses, which dehydrate in the absence of moisture and spring back to life within 24 hours of rain.
Monsoon forests often develop in the cool, moist gorges that dissect the stone country. They are generally dominated by Allosyncarpia, a large, spreading, shady tree restricted to the Kakadu and Arnhem Land region.
More widespread and easily recognisable stone country and outlier plants are spinifex and the sandstone pandanus; both can be found at Nawurlandja.
- Allosyncarpia Allosyncarpia ternata An-binik
A large, hardy evergreen that is restricted to the stone country of Kakadu and Arnhem Land.
- Native ginger Curcuma australasica
An attractive leafy annual that grows from a tuber. Its hot pink flowers can be seen at Ubirr and Nourlangie in the wet season. It is also related to the turmeric plant, a native of Asia.
- Pityrodia jamesii
A shrub that grows on rocky areas in pockets of sandy soil. Pink-white flowers appear in September to December. The shrub's sticky, fragrant leaves are the main food of Leichhardt's grasshopper.
- Sandstone pandanus Pandanus basedowii An-more
Grows only in the sandstone areas of Kakadu and Arnhem Land.