Australia: The Land Where Time Began
Between about 1,800 and 1,400 Ma the sediments that later formed the rocks were deposited in the sea. Later folding and earth movement resulted in mostly sandstones and conglomerates about 500 m thick. This formation has 2 main sets of joints at right angles to each other. The Katherine River is one of the rivers with its head waters on the Arnhem Land Plateau. Its course follows a zigzag path along these intersecting joints. About 25 Ma it began cutting the Katherine Gorge through the ancient rocks of the plateau, primarily sandstones and conglomerate. Many other side fissures have been formed by weathering and erosion along the line of other joints, and there are a number of faults along which the rocks on either side have moved in relation to each other. There are 2 main sets of joints that run at right angles to each other. The river flows along one set of joints and then the other, then back to the original in the form of the zigzag pattern. Many side fissures have been formed by weathering and erosion, and there are a number of faults where large blocks on either side of a joint have moved.
About 15 km long, the gorge is like a very large corridor of stone, with 60-90 m high walls that are mostly sheer, though with many caves and overhangs in them. The Katherine River follows a zigzag course in which there are 13 straight sections that join at sharp, right-angled turns. For nearly the full length of the gorge the water surface is enclosed directly by the 2 walls, the result being that can viewed only from a boat or from the cliff tops.
There are 2 distinct seasons in the gorge, the Dry season in winter and the summer Wet season. When the monsoon arrives in the Wet season the river becomes a raging torrent that occasionally uproots trees and carries huge amounts of debris, the water level rising by up to about 18 m and what were small rapids in the Dry season become waves that are 3 m high. During the Wet most of the surrounding are is flooded and the gorge is completely inaccessible.
Following the end of the monsoon rain the river settles down to its placid state in the Dry. In this season it is in the form of a series of placid reaches that are up to 30 m deep reflecting the vivid orange walls. Small foaming rapids are formed by rock bars between some of the reaches, and the water surface in other parts of the river becomes slightly choppy because of the wind, though it is mostly tranquil in the Dry season.
Jedda Rock is a smooth, vertical 64 m high cliff face and Smitt's Rock is a prominent outcrop between the 4th and 5th sections of the gorge.
Though vegetation in the gorge is relatively sparse, there are cool alcoves and other places where they can gain a foothold. Lush, tropical rainforest species, including tree ferns, orchids and lilies, figs, Ficus spp. and Pandanus, as well as a fan palm that has been described as exquisite, Livistonia inernis, that is topped by delicate, weeping fronds on a slender trunk up to about 10 m high. Many trees, such as red-barked bloodwood, Eucalyptus dichromophloia, and paperback, Melaleuca spp., are present on the sandstone plateau above the gorge as well as on the banks of the river at both ends of the gorge . There is also an ancient cycad, Cycas calcicola, endemic to this region, that has foliage that is smoky grey.
There are many birds living in the gorge, cormorants, darters, rainbow birds and little fairy martins, Petrochelidon ariel, that build bottle-shaped nests that are high on the cliff walls. There are not many waterfowl in the area as the water is too deep to form the swampy habitats required by these birds.
There are 35 species of fish living in the waters of the Katherine River in the gorge, among which are the giant perch or barramundi, Lates calcarifer, and there are also freshwater or Johnston River crocodile, Crocodylus johnstoni, and tortoises. On land there are kangaroos, wallabies, bats, emus, Dromaius novaehollandiae, Gould's sand goanna, Varanus gouldii, and thousands of butterflies that congregate in Butterfly Gorge, a subsidiary crevice leading from the second section of the main gorge.
The Aboriginal people of the area are the Djauan, who included the gorge in their home country for about 18,000 years. At the present the descendants of this tribe live mostly in towns or on cattle stations (ranches). The paintings made by their ancestors can still be seen on the walls of the gorge. The rock art of the Djauan Aboriginal People differs from that of other Australian tribes in that it is not always in shelters or caves, often being on open walls where it is protected from the sun, wind and rain. Most of the art is of ancestral Dreamtime beings and local animals.
Katherine Gorge Flood geomorphology2
Katherine Gorge is a narrow, deep system of canyons cut into resistant sandstone. The region is subject to a tropical monsoonal climate with rare flood events that reach large magnitudes, that flush sediments of sand sized particles, accumulations locally being preserved as slackwater deposits at the mouths of tributaries. Hydrologic analysis of palaeofloods by the use of step-backwater flow modelling allows the making of quantitative estimates of flows that are geomorphollogicaly significant. According to the authors2 for an annual exceedence probability of 10-2, the gorge may produce discharges of 6,000 m3/s, with mean velocities of 7.5m/s and flow depth of 15-45m, and stream powers as high as 1 x 104 m2 and bed shear stresses of 1.5 x 103 N/m2. The transport of boulders with an intermediated diameter of 3 m or more that occurs at riffles at large scales requires such flows. Pools reach depths of up to 45 m at maximum stage, preferentially at vertical intersections in the rock, that are believed to develop by intense hydraulic action. The development of pools and riffles thus reflects the boundary of channels at times of peak flow. There are also other indicators of especially intense flow that include potholes, flutes, abraded facets on the surfaces of rock bars, and on upland bedrock surfaces, the development of scabland.
Sources & Further reading
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|