NITMILUK NATIONAL PARK #4

NITMILUK NATIONAL PARK #4

Another view when the Katherine Gorge becomes narrow with its high and steep walls cutting in the sandstone plateau. There are several right angle bends where the river has followed lines of weaker rock.

“Nearly two billion years ago the present-day area of Nitmiluk was covered by shallow seas and braided stream depositing masses of sediment on underlying base rock. Subsequently the sediment was compressed to form sandstone and conglomerate layers up to 2000 metres thick in a formation called the Kombolgie Subgroup; these layers can still be seen in the gorge walls today. After the sandstone and conglomerate had formed and while it was still deep underground, tectonic movement in the base rock caused cracks and faults in the sandtone. Many faults and fractures were at right angles to each other; some were filled with dolerite from hot volcanic magma - these dolerite dikes were up to ten metres wide and several kilometres long running through the sandstone.

Eventually the mass of sandstone and conglomerate was exposed and subjected to normal weathering. The hard sandstone resisted stream erosion but dolerite dikes and fractured rock in the fault zones were more easily eroded; dolerite is particularly prone to chemical weathering in the tropics. The early Katherine River therefore followed a zig zag course along the lines of faults and dikes. Eventually the river eroded the deep gorges we see today along that erratic course. Long and narrow secondary gorges formed running away from the river where dolerite weathered away leaving fertile, dark soil in the shelter of high, steep walls retaining remnant forest habitats.”

Contrasting with the gorges, the top of the sandstone plateau remains an especially harsh environment of sandstone broken by deep chasms and gullies. During the Wet Season, water remains trapped for months in warm pools and swamps; in the Dry Season the area is extremely arid with surface temperatures up to 60 degrees C.
(https://www.nitmiluktours.com.au/the-jawoyn/)