Travelling Australia
Uluru and Kata Tjuta
Uluru and Kata Tjuta are made of different types of rock. Uluru is made of arkose, a course-grained sandstone rich in the mineral feldspar, formed from sandy sediment eroded from mountains composed mainly of granite. Kata Tjuta rock is a conglomerate containing pebbles and boulders of granite and basalt cemented by sand and mud. Both are sedimentary rocks formed by pressure of overlying material on deposits laid down after erosion.

Uluru and Kata Tjuta are near the southern margin of a geological structure called the Amadeus Basin which formed about 900 million years ago and received layer upon layer of sediment over several hundred million years. About 550 million years ago earth movement uplifted mountain ranges which immediately began to erode. Grasses and other plants had not then evolved so the bare terrain was exposed and huge amounts of sediment were washed away when it rained to form alluvial fans near the mountain ranges. As the mountain ranges were eroded, building of the alluvial fans slowed; by that stage the fans were at least 2.5 kilometres thick. The remains of these alluvial fans, one of arkose sand and the other of conglomerate, eventually became Uluru and Kata Tjuta.

Kata Tjuta
About 500 million years ago the region was again covered by a shallow sea; sand and mud were deposited on top of the alluvial fans which were buried by silt and other sediment. These overlying sediments compressed and cemented the arkose sand into arkose and the gravels of Kata Tjuta into conglomerate. Subsequently the sea receded from the Amadeus Basin and the rocks were folded, fractured and uplifted above sea level. By about 300 million years ago horizontal layers of arkose had been turned nearly ninety degrees to their present position, the Kata Tjuta conglomerates had been tilted fifteen to twenty degrees from the horizontal.

Uluru and Kata Tjuta - page 2
The ground surface then was much higher than the present tops of Uluru and Kata Tjuta but that surface eroded from 300 million to 65 million years ago when erosion slowed as the climate became drier; so dry that, by 30,000 years ago, sand dunes had begun to form in the area and remain there today. Despite this extensive erosion, Uluru and Kata Tjuta are the above-ground part of enormous slabs of rock extending far beneath the ground. The size of the slabs is not definitely known; they may extend as far as five or six kilometres underground.

The perimeter of the base of Uluru is 8.8 kilometres, the rock mass itself is 3.6 kilometres long and 2.4 kilometres wide, and stands 348.7 metres above the surrounding plain. There are no major joints or fractures visible in Uluru. Rain runoff from the rock surface has formed steep valleys with pot holes and plunge pools on the southern side. On the north-western side weathering has produced parallel raised ridges reflecting the sedimentary layers of Uluru. Differences in grain size or the strength of the cement caused the variation. The flaky surface comes from chemical decay of minerals in arkose. The red colour is caused by oxidation or rusting of the iron in the arkose; fresh arkose is grey in colour. As surface rocks are eroded, the release of pressure produces fissures parallel to the rock surface.

Kata Tjuta is a group of 28 rounded hills resembling domes, 27 kilometres west of Uluru. Mt. Olga itself rises 545.4 m above the surrounding plain, so it is actually higher (197.3 metres higher) than Uluru. The 28 main domes cover an area of 21.68 square kilometres. There are a number of smaller domes and ridges outside the main grouping, making about 61 domes in total. The main dome area is faulted by two sets of vertical joints which have acted as weakness zones for erosion, resulting in deep, narrow valleys, such as the Valley of the Winds, through the rock mass.

Uluru strata Uluru flaky surface
Kata Tjuta conglomerate Kata Tjuta outcrop
Top left: Nearly vertical ridging showing how much the rock has been tilted since these strata were laid down horizontally.
Top right: The flaky surface of Uluru is caused by chemical decay of minerals in arkose.
Bottom left: Kata Tjuta conglomerate containing rounded boulder cemented together by mud.
Bottom right: The strata in Kata Tjuta have been tilted only about fifteen degrees.
   "The Geology of Uluru National Park". Number 3 in the Uluru Park Note series, pub. Australian National Parks and Wildlife Service. July 1992.
  Australian Museum Online.