Australia: The Land Where Time Began

A biography of the Australian continent 


Flared Slopes see Wave Rock

According to the authors, 'A consideration of the origin of flared slopes  provides a good example of the way in which to approach geomorphological problems, and also illustrates how an analysis of the evolution of a relatively minor form throws light on the genesis of larger features, and indeed can be related to general geomorphological theory.'

In Australia, the Eyre Peninsula and the southwest of Western Australia are places where there are many examples of flared slopes. In both these parts of the country they are on granite bedrock, but in other parts of the continent they occur on bedrocks of other types, such as on Arkose all along the base of the southern side of Uluru (Ayre's Rock) in the arid part of the Northern Territory, reaching heights of 4 m. Arkose is a sandstone with feldspar fragments. In the Kata Tjuta (The Olgas) complex, they are found as isolated features on the western side of the conglomerates forming Kata Tjuta. In the southern Flinders Ranges there are small flared sidewalls on low sandstone residuals. In the Gawler Ranges, a silicic volcanic rock massif, to the north of Eyre Peninsula, 3 m high flares are present on the southern margin. They tend to be poorly formed and isolated, with the exception of an area to the south of Nonning Homestead.

At a site in the United States, City of Rocks, New Mexico, where there is also an area of silicic volcanics, of rhyolite tuff, and on residual dolomite blocks and ridges in Spain, near Cuenca, in the Ciudad Encantada (Enchanted City), there are well developed side walls. A wide range of rock types, as well as those mentioned previously, are reported to have developed flared slopes of varying scales, amphibolite, basalt, limestone and diorite. The common factor to all instances of flared slopes, whatever the rock type, is a massive structure with few open fractures.

The climates flared slopes are found in ranges from cold regions, or uplands that are seasonally cold, as in the City of Rocks, Idaho, and in California, the Sierra Nevada. In southeastern Australia, the Kosciuszko and Mt Buffalo regions; In central Spain, the Sierra Guadarrama, to the warm humid regions such as are found in northern Australia; in southern Brazil, the Rio de Janeiro area, and West Malaysia. They have also been found in southern France, Zimbabwe and in South Africa, the Cape Province. On Kangaroo Island, Remarkable Rocks, and western Eyre Peninsula they are present in coastal environments, as well as in the Sahara Desert, among other places. Flared slopes have been found in a wide variety of lithological, climatic and topographic localities. They are especially well represented in southern and southwestern Australia on granite, where they are more common and better developed than elsewhere.

There are a number of features that characterise most of the flared slopes of southern Australia. They are most often present around the base of hills, but also on higher slopes, and in the uplands, in clefts and valleys. They are also found on boulders, and on points and spurs they display a higher degree of overhang. Wave Rock is an exception to this. In some instances 2 or 3 concavities link to comprise the overall flare. In stepped inselbergs, some higher inselbergs, the flares can be divided into several distinct zones. The flared zones are not always horizontal, whether at high levels or at hill bases. Most are parallel with the slope-soil/regolith or rock platform junction, being inclined and irregular. The southern sides of hills flares are most common location of flared sidewalls on Eyre Peninsula. According to the authors, the most important feature, incipient flared slopes shaped in bedrock, have been observed in excavations, beneath the natural land surface, covered by weathered bedrock in situ.

According to the authors, a number of suggestions have been made in popular magazines as to the mechanism of formation of flared slopes such as Wave Rock, such as marine wave action, wind erosion or glacial erosion. The authors suggest these proposed mechanisms can be rejected, based on the known evidence. The basal undercutting could be explained by wave action and other marine agencies, but if this were the case marine sediments and fossils should be present at the base of flared slopes. No such marine sediment or fossils have been found. At least some of the flared slopes are relatively young, though their ages have not been directly determined, and there have been no marine incursions high enough to reach sites where the formations have been found in places such as the Sierra Nevada on the Iberian Peninsula, in Zimbabwe or central Australia in the timeframe necessary to account for the younger of the flared slope formations. The other problem with the wave action mechanism is that it could not account for the subsurface forms. There are relic sand ridges on the Eyre Peninsula of desert conditions prevailing in the Late Pleistocene. Based on various lines of evidence it has been concluded that the desert dunes advanced from the northwest, clearly indicating that the prevailing winds at the time were from the north and northwest. If the formation of the flared slopes of the region were the result of sand blasting they should be on the north and west sides of the hills, but they are on the southern side.

Runoff from higher ground in desert areas tends to be concentrated around the base of hills, allowing the plants to grow better around the bases of the residual, as around Uluru. This vegetarian would reduce the effect of aeolian erosion, protection the base from sandblasting. The subsurface developments cannot be explained by aeolian erosion either. Formation of flared slopes by glacial action can also be ruled out, because the area has not been affected by glaciation during he last 250 million years.

Another suggestion is that the flared slopes could be formed by runoff from the bare hills, as the volume and velocity of the water increase downslope, leading to increased erosion, splash and scouring at the base of the hill creating concavities at the base. If this was the mechanism for the formation of flared slopes they would be expected to be best developed at the heads of valleys carved into the hills, as that is the situation where water volume and erosion would be expected to be at its greatest. There should also be no, or at least very little development of flares where the flow diverges, on spur points. These are just the situations of well developed flares, and they are at their least developed state at the heads of valleys. The subsurface formation of flared slopes doesn't fit with this proposed mechanism either.

The authors suggest that the runoff from the hills is involved in the formation of flared slopes, but not in the proposed mechanisms above, that all involve surface erosion. They suggest that as the water flows off the hills its spreads out around the base, infiltrating into the subsurface of the soil. The usual processes of weathering, solution, hydration, hydrolysis, especially of the mica and feldspar in the granite, forming a zone of regolith around the base of the hills.As water continues to infiltrate in the zone the regolith gradually becomes deeper, and as it does it comes in contact with the bedrock boundaries of the zone, including that of the hillside, spreading the formation of regolith into the rock of the hill, undermining and undercutting it. The surface soil dries in the dry season, that is summer in southern Australia, but there is enough subsurface water at depth to allow subsurface weathering to continue. There are no weaknesses in massive rocks for the weathering to work on to break up the rock, resulting in a smooth weathering front that forms concavities. Fractures are exposed to similar weathering allowing similar features to form. When the soil or regolith is removed by erosion the previously soil/regolith covered weathering front at the base of the hill is exposed as a flared slope, extending laterally into a depression that is exposed as a scarp-foot depression in places, described by the composite German word Bergfussniederungen. The scarp-foot weathering sometimes forms basal fretting, or develops into an alcove, tafone or cliff-foot cave.

The suggestion is that flared slopes form in 2 stages, subsurface weathering at either the base of the hill or along a fracture, and the removal by evacuation of the regolith that exposes the weathering front as an etch form (Twidale & Campbell, 2005). This proposed mechanism explains the incipient forms found at City of Rocks, New Mexico; at Veyrires, southern France; and the Calca Quarry, Chilpuddie Hill, on Eyre Peninsula, and especially at Yarwondutta Rock. It also explains why most of the high flared walls face south, the side away from the sun, where moisture tends to linger longer, though not precluding the less common slopes on the northern side of hills. Fractures favour subsurface weathering at Turtle Rock near Wudinna, where the flared wall is along a prominent fracture, and also at Wave Rock, where the structure is formed in an embayment that is fracture-defined.

The development of flares on boulders is also explained by subsurface weathering, as well as with flared zones displaying irregularities in the vertical position. The explanation suggests that multiple flares are a result of episodic minor lowering of the soil surface exposing concavities in the hill base. The tectonic, and hence topographic stability of the cratons, are believed to account for the exceptional development in southern Australia of flared slopes, especially on the cratons, where time has been allowed for intense scarp-foot weathering to develop. Subsurface weathering also accounts for blade-like structures, such as seen near Mt Manypeaks to the east of Albany, Western Australia, where the blade-like form developed from a thin slab that had separated from a larger block near Mt Manypeaks.

The authors suggest that in the subsurface erosion mechanism of flare slope formation, the level where the concave section of the hill formed by the subsurface erosion meets the convex part of the hill slope is the level of the original ground surface before the commencement of weathering. Stepped inselbergs where flared slopes occur at a number of levels on the residuals, suggest subsurface weathering occurred alternately with local base level lowering and regolith erosion. The authors suggest such forms are relevant to the wider argument of inselberg formation, suggesting the hills have increased in height relative to the surrounding plains over time as the plains have been lowered are a more rapid rate, persisting through several phases and cycles of the development of the landscape. This is relevant to general theories of the evolution of landscapes, as well as to the ideas about inselbergs.

A building block that can be used in the investigation of other forms is provided by the analysis of flared slopes and development of an apparently viable explanation for their formation. According to the authors, in their analysis, features that were characteristic of flared slopes were identified that required an explanation. The possible explanations that have been put forward were then examined and tested against the evidence uncovered in field work, the explanation that they regard as most likely to be correct emerged from this process, though they say it may change if further evidence indicates that the conclusion they arrived at is inadequate. 

Sources & Further reading

  1. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd
  2. Hellen Grasswill & Reg Morrison, Australia, a Timeless Grandeur, Lansdowne, 1981


Author: M. H. Monroe
Last updated 01/04/2011


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