Australia: The Land Where Time Began

A biography of the Australian continent 

Uluru (Ayers Rock) see Flared Slopes see Mt August, the actual biggest single rock in the world

Uluru

About 440 km km southwest of Alice Springs.

The traditional land of the Anangu Aboriginal people encompasses two of Australia's culturally significant landmarks: Ayers Rock/Uluru and The  Olgas (Kata Tjuta) to the west of Uluru. To the east is Mt Connor, a mesa (a flat-topped hill).

Uluru is an inselberg composed of a coarse sandstone, arkose, that was considered to be the largest single rock in the world, though the actual largest single rock is Mt Augustus in Western Australia. It has remained largely unchanged for the past 40 million years, just slightly smaller and more rounded than it would have been originally. It is 8.8 km in circumference and rises 348 m from the flat surrounding desert. One of the most popular attractions about Uluru is the way it changes colour throughout the day and with distance and weather conditions.

Inselbergs such as Uluru, bornhardts of granite terrains, are similar to structures present in limestone areas, as well as areas of conglomerate, such as Kata Tjuta. They are also represented in the Meteora Complex of central Greece, and in sandstone in West Africa. Evidence suggests that in all these cases they have resulted from fracture-controlled subsurface weathering, the regolith that formed adjacent to joints being subsequently eroded preferentially, followed by pronounced scarp-foot weathering that produced flared slopes, and in limestone, caves and horizontal slots causing slope steepening and an abrupt piedmont angle to be formed. In most parts of the vast plains of the arid areas of Australia it is possible to see one or other of the various uplands on the horizon.

The process that led to the formation of Uluru began about 600 Ma (White 1994) or 550 Ma in the Cambrian, (Twidale & Campbell, 2005) when material eroded from the Petermann Ranges was deposited in the shallow inland sea, later to be buried and compressed to form the the coarse sandstone, arkose. According to Twidale & Campbell (2005), the material was deposited in the piedmont of the Musgrave Ranges.  About 100 million years later, about 439-400 Ma in the Devonian, a large area of the sandstone was folded, tilting it to an angle of 85o. The rock is coarse grained and has occasional beds of conglomerate, in some cases containing pebbles up to 1 inch in diameter. The arkose is greenish grey, but a thin patina of feldspar fragments and clay give it its distinctive red colour, deriving from small amounts of iron oxide in the form of haematite and goethite (White, 1994; Twidale & Campbell, 2005).

Because of its singular nature, being composed of hard sandstone with few cracks or faults, erosion has had little chance to grind it down as usually happens with uplifted rocks. As the surrounding softer material has been eroded away the visible part of the rock has grown, because the surrounding surface wears away faster than the rock. The present rock is being eroded by sloughing off layers of approximately equal thickness from all parts of the rock, by a process called spalling, or sheet erosion, or exfoliation, that was believed to result from the diurnal temperature variations that can be quite large in the desert environment, maintaining the shape while slowly reducing the size (White, 1994). It has been suggested that the mechanism of sheet erosion may not be the heating and cooling of the rock as has been accepted for some time, suggesting instead strong horizontal compressive crustal stresses may be responsible for sheet fractures, based on evidence from field work quoted by Twidale & Campbell (2005). see Physical Weathering (Twidale & Campbell, 2005).

The rock of Uluru is exposed as bare rock over nearly the entire structure. The domed crest is bevelled and gently sloping, the steep bounding slopes meeting the surrounding plains at a steep angle. On the ribbed crest there are ridges that reach up to 4 m in height, and between some of the ridges the linear depressions coincide with bedding planes, though in other cases the depressions develop along strata, bedding planes reinforced by siliceous infilling being associated with the adjacent ridges. In many of the depressions there are rock basins (gnammas), most being shallow pans with flat floors that can be 40 cm deep, with some hemispherical basins being up to 2 m deep. There are annular rings around the margins of some, and in some cases the floor is covered with sand (Twidale & Campbell, 2005).

On the northern and southern flanks the steep sides are ribbed, and the ends of the dipping strata are exposed. In the rare times of heavy rain the runoff can flow down some clefts, leading to the formation of potholes by scouring by these turbulent streams that have been described as spectacular when they are flowing. A fretting appearance on the upper parts of the eastern slopes has been produced by preferential weathering of bedding planes, that is well exposed in the formation that has been called 'the Brain'. Scarp-foot weathering has produced cliff-foot caves and flared slopes at the base of slopes, especially on the southern wall. The same process has produced gaping mouth caves on the southern face of the Rock (Twidale & Campbell, 2005).

On the western, northern and eastern sides of Uluru the bedrock is at shallow depths, outcropping at the surface in places. The steep walls of bedrock adjoin rock platforms in places, and in the case of Little Uluru, there is a small bedrock dome with blocks and boulders resting on top of it. Between Uluru and Kata Tjuta the maximum recorded depth of the bedrock, of Cambrian age, is 108 m, reaching a minimum depth of 4 m below the surface 4 km northwest of the Rock. On the southern side there is about 20 m of sand over the bedrock close to the Rock, reaching to about 70 m past the point where there is a fault that is roughly parallel to the southern flank. An earlier hill-plain junction is indicated by prominent slope breaks 35-60 m above the present level of the surrounding plain. In the formation known as the Kangaroo Tail and scarp-foot caves there are sheet structures and fractures. Piles of large angular blocks have slipped to the base after having broken from some of the sheet structures (Twidale & Campbell, 2005). 

Mechanism of Formation

After a long period of compression and compaction, the sediments were folded during the Alice Springs Orogeny. The history of the rocks comprising Uluru resulted in a very hard, extremely resistant rock. During this orogeny the sedimentary layers were tilted to 85o. The rock type making up Uluru is now called the Uluru Arkose, since it is been found that it had a different history from the rocks of The Olgas. The rocks forming the Kata Tjuta Olgas are still called the Mt Currie Conglomerate. The new information that caused the name change is that it is now known that the sediments that formed Uluru were carried by palaeochannels from the south. Those of the Mt Currie Conglomerates were from the west and south-west. 

Both these inselbergs have a covering that has been hardened by weathering, deriving its rich colour from iron concentrated in the outer layer. Inside caves of Uluru that penetrate below the weathered layer the rock is grey-white to pinkish. The textured appearance of this fresh rock results from the coarse-grain made up of various sizes of grain that are poorly sorted. The textured appearance is added to by the presence of fine striations resulting from cross-bedding of the sediments.

It is not completely understood how these inselbergs have survived so much better than the same type of rocks that surround them, that have been eroded down much further, now being below the surface around the inselberg. It is thought that the reason could have been that the surviving above-surface rocks were from elevated ridges of the original rock, possibly forming palaeodivides, therefore being subjected to less chemical weathering by the groundwater when the rock is below the surface.

Uluru is composed of the same rock as the bedrock that surrounds it, but the rest of the bedrock is at a shallow depth beneath the present surface of the plains, or at its highest, outcrops at the surface, but never as far above the land surface as the Rock. It has been suggested that north-south compression was superimposed on the structural grain of the rock, that was northwest-southeast, after the period of mountain building that occurred in the Devonian. As a result of the compression, some rock compartments were exposed, which made them less vulnerable to attack of moisture than the surrounding, adjacent rock compartments that were weaker. According to this suggestion, the result was formation of Uluru, as well as the alignment of Uluru, Kata Tjuta and Mt Connor (Twidale & Campbell, 2005).

According to this suggested mechanism, Uluru would have been less susceptible to weathering once it was high enough above the surrounding plains to shed water more rapidly. The surrounding areas were weathered, becoming more susceptible to erosion, and as the size and height of Uluru gradually increased, the amount of runoff would also have increased, leading to increased infiltration around its base, which would increase the rate of subsurface weathering, particularly around the base. Twidale & Campbell suggest this is a case of positive feedback, reinforcement effect, according to which topographic features tend to be enhanced automatically once they are formed.

A deep palaeochannel has been discovered between The Olgas and Uluru. It has an in-filling of about 100 m, the sediments dating to the Late Cretaceous to Eocene. This implies that up to 75 million years ago, the rock massifs that today's Inselbergs are a part of, created elevated landscape features. It is thought these uplands were in the form, in the case of Uluru, of a soil-covered dome. In the case of The Olgas, it was probably a complex of low rock domes (White, 1994).

According to Twidale & Campbell, this palaeochannel was carved out of Cambrian strata. Fossiliferous lignite from the Maastrichtian - latest Cretaceous, about 70 Ma, has been found in the deepest parts of the valley. They suggest that the flattish bevelled surface of Uluru is part of this Maastrichtian surface, based on lateral projection of the Cretaceous-Cambrian unconformity. The weathering of the surrounding rocks was caused by wash from the hill. It was at that time, when the level of the plain was 35-60 m above the present level, that the formation of the gaping-mouth caves began. The caves were exposed following the removal of the weathered material by erosion. The flared slopes and cliff-foot caves were formed by a subsequent period of subsurface weathering to about 5 m deep. The bedrock level was lowered to its present level by another period of erosion, after which it was covered by deposits of alluvia and wind-blown detritus (Twidale & Campbell, 2005).

Forests grew on deep regoliths throughout the Eocene. At this time Uluru and The Olgas were minor landscape features, but they would have affected the topography. Beginning in the Oligocene, this regolith was stripped away, and Uluru and Kata Tjuta (The Olgas) became more prominent as the level of the surrounding country was lowered. The forests were gradually replaced by arid plains, the inselbergs standing out more as time passed.

The Uluru Arkose and the Mt Currie Conglomerate of the area were both being eroded by deep sub-surface weathering - chemical erosion by humic acid in the soil. The resistant rock of the drainage divides were undergoing surface weathering, rounding of edges and etching of the ridges. The caves that are now 60 m and 30 m above the surrounding surface on the sides that rock were begun when these parts of the rock were still below the surface of the surrounding land. They are examples of scarp-foot weathering connected with the height of the surrounding slope at that time. At the present foot of the inselbergs, 'wave caves' result by the lowering of the surrounding plains by 3-5 m in recent times, most likely during the very arid Pleistocene Ice Age.

Twidale & Campbell disagree with others who believe that Uluru had about the same appearance 60 Ma and earlier as it does at the present. They suggest that it was a soil-covered low hill. According to their suggestion the development of its present dramatic appearance resulted from the stripping of the regolith, exposing the upper surface that was ribbed and dimpled, and the development of the steep bounding slopes occurred during the last 70-60 million years.

The stages of formation are: (White, 1994)

  • Early Mesozoic- the Uluru Arkose formed a drainage divide - a low, soil-covered rise.
  • Late Mesozoic-Early Tertiary - the terrain on both sides of the divide is lowered by erosion, exposing the rock at the top. The debris from  the erosion forms deposits in the valley between Uluru and Kata Tjuta. The sediments from both inselbergs combine in the lake that forms in the valley. Erosion starts to undercut the iron-hardened capping of the exposed rock as the gradient steepens, and chemical weathering in the run-off zone around the base of the rocks weakens the underlying rock.
  • Block collapse steepens the sides of the inselbergs as chemical weathering at and below the soil surface undercuts the exposed rock.
  • Late Tertiary - the rock takes its shape and the valley floor sediment covers the lake and as a result of the drying climate a sandplain forms
  • At present the undercutting of Uluru by sub-surface weathering continues, fallen blocks litter the deflation-produced platform from the Pleistocene Ice Age.

Fauna of Uluru

The top of Uluru seems to be as inhospitable a place as could be imagined. Bare rock blasted day after day by the hot desert sun, yet there are several creatures that live there. One is a shield shrimp (Triops australiensis). The eggs of this shrimp lie dormant for years until the rare occasions when sufficient rain falls to form pools in the surface gullies. The first shrimps to hatch are all females, which then produce more eggs without having mated, and succeeding generations of female shrimps continue to lay eggs that always hatch only females. By the time the pools start to dry up males start to hatch. The males mature rapidly and fertilise the eggs of any females producing eggs at that time. When the water in these pools evaporates completely the fertilised eggs dry out and remain dormant until the next sufficiently heavy rain. Under the same circumstances the unfertilised eggs die. 

Aboriginal Significance of Uluru

The people of the Aboriginal tribes living in the area have dreamtime stories about every nook and cranny of the rock. One story is about the Windulka (mulga-seed men), who came from the Petermann Ranges. By coincidence, the story told by geologists is that it was sediment from the erosion of the then much higher Petermann Mountains that were consolidated to form the rock of the future Uluru hundreds of millions of years ago. Unfortunately this coincidence no longer exists, as it is now known that the sediments that formed Uluru actually came from the south. 

There is an excellent cultural centre located within the park. There are also a number of walks that cater for different fitness levels. There is a park entry fee. Uluru accommodation can also be found at the local visitor centre.

Sources & Further reading

  1. Mary E White, After the Greening, The Browning of Australia, Kangaroo Press, 1994
  2. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd

Links

  1. Geology
  2. Uluru/Ayers Rock
  3. Ayers Rock (Uluru)
Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 16/11/2012

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading