Travelling Australia
Katherine Gorge
Katherine Gorge
Katherine Gorge is a narrow gorge with high and steep walls twisting and turning through the sandstone plateau. The Katherine River river runs through the gorge in a series of long, deep sections separated by rapids in the Dry Season; these rapids prevent tour boats or canoes travelling unhindered up and down the river. Sections between rapids are known as gorges numbered from First Gorge where the river leaves the gorge at the edge of the sandstone plateau. The Second Gorge has steeper, higher walls than the First Gorge and goes through several right angle bends where the river has followed lines of weaker rock. A frequently used picture of Katherine Gorge is along a straight, narrow and very steep-sided part of Second Gorge known as Katherine Canyon (picture above).
Katherine Gorge tour boat
Tour boat grounded on sand at the upstream end of First Gorge. Most passengers have disembarked and walked to a similar boat waiting for them at the end of the Second Gorge.
Boat tours leave from the boat wharf outside the gorge and near the Nitmiluk (Katherine) National Park visitor centre. Tours go to two, three, or more gorges depending on tourist preferences and the time they wish to spend sightseeing. After leaving the wharf, boats move upstream into the First Gorge and up that gorge to the rapids at the upstream end where the boats run up onto a sandbank and passengers walk around the rapid to board another boat waiting in the Second Gorge. The return trip is in the reverse direction along the same gorges.

The Visitor Centre has an excellent description of the geology of Katherine Gorge as well as informative displays of the aboriginal explanation of the gorge.
Katherine Gorge - page 2
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.
Katherine Gorge
Beyond the kayak is a fault in the sandstone previously filled by dolerite which has weathered away leaving a gap filled with protected fertile soil supporting remnant forest.
  Nitmiluk Visitor Centre.
  Australian Geographic map "Nitmiluk National Park".