The oldest rock formations in the park are a mixture of sedimentary rock, laid down in a large geological depression called the Pine Creek Geosyncline, and igneous or volcanic rocks. They date from about 2500 million years ago, about half the age of the earth. The layered sequence of sedimentary and volcanic rock was changed under conditions of extreme heat and pressure into schist, gneiss, quartzite and marble. This was part of a major mountain building event, the Top End Orogeny, about 1800 million years ago. These ancient rocks contain the uranium-bearing bed referred to as the Cahill Formation, source of the Ranger, Koongarra and Jabiluka deposits.
The mountains formed during the Top End Orogeny would have once been several thousand metres high but were eroded over 100 million years to form a wide plain. The ancient rocks can be seen near the base of the escarpment. In the dry, monsoonal climate of that time, the rocks and soil were deeply leached as rainwater percolated down and evaporated from the ground surface, leaving behind iron and other minerals dissolved in the water to form a hard brown crust, or laterite. These deeply weathered rocks underlie the lowlands of Kakadu and are now covered by geologically recent deposits of sand and gravel. They can be seen in road cuttings along the Kakadu Highway, between Cooinda and the Bowali Visitor Centre. They look like blocks of dark brown laterite, usually with worm-like tubes on the surface. The colour of the laterite varies from brown on the surface to orange-brown and mottled orange-white lower down. The changing colours reflect the greater degree of leaching, or loss of iron and other minerals, during weathering.
About 1650 million years ago a huge river system, perhaps associated with a climatic change, spread over the plain, eroding much of the weak, crumbly rock on the surface. Life had not yet evolved on earth, so there was no vegetation to hold the soil and rocks together, which may be why the sediments deposited during this time were in very thick layers. Quartz sandstone up to 1000 metres thick was deposited across the entire region.
Ripple marks in the rock show that these sediments came from the north-west and tell scientists about the environment when the sediments were deposited. Flash floods caused beds of sand up to 10 metres thick and several kilometres across to be dumped very suddenly; in the long, quiet periods between these floods thinly bedded sections of the sandstone were deposited.
This sandstone is called the Kombolgie Formation; it forms the escarpment and outlier country and can be easily seen at Nourlangie. Look for polished pebbles set in the smaller sandstone matrix and imagine the force of the river that carried them there.
The next 1500 million years were geologically stable and the sandstone layers consolidated. During this time the first land plants appeared.
About 140 million years ago, during the Mesozoic era, a shallow sea spread across most of Kakadu from the north. The escarpment and outliers were sea cliffs at the edge of a vast, shallow sea. About 100 million years ago the seas receded.
The rate of retreat of the escarpment at its weakest points has been estimated at about one metre every 1000 years. At places such as Jim Jim Falls the face of the sandstone is strongly armoured by a layer of iron and silica. As water seeps through the rock the water 'takes' these minerals with it and then deposits them on the surface of the rock, forming a tougher rock face. Where this occurs the rate of retreat of the escarpment is much slower.
The Kombolgie Formation is remarkable in that it has remained relatively stable over such an extraordinary length of time. There has been no significant folding of the sediments since formation and exposed faces can be viewed in almost the same condition as they were when deposited. Countless gorges in the plateau have formed along joints and cracks and have eroded into a criss-cross pattern of deep cracks that look like knife cuts from the air.
In an area to the west of West Alligator Head it is possible to see beach ridges dating from about 8000 years ago, when sea levels were about 10 metres higher than they are today. Also during this period mud deposits containing many marine animals accumulated on the estuarine plains. As the sea retreated, rivers cut down into the mud deposits and in some places fossils of mud lobsters, snails, bivalve shellfish and wood fragments were preserved in the mud.