Australia: The Land Where Time Began

A biography of the Australian continent 

Gilgai & mukkara

An Australian Aboriginal word that has been accepted in the scientific literature for the puffs and depressions associated with cracking and swelling clays. Other terms for this phenomenon are crab hole, Bay of Biscay soil, melon hole, corduroy soil, etc.

These are found in a number of different soils that are mostly clays that swell and crack, also texture-contrast soils that have a thick subsoil clay horizons. They are most common where clays are 5 m or more deep. The can be on plains with a gentle slope that have a wet-dry regime, alternatively, subject to more intense drying that occurs more frequently. They take several forms, closed depressions, or sometimes a sinkhole inside mounds; open-ended troughs between linear banks on clay plains. After rain they contain water, but uniformity of conditions for cropping is lacking. In places such as the Brigalow country of southern Queensland that have been cleared for cultivation they make the conditions very difficult for grain cropping.

The most widely accepted, though not unanimous, mechanism for the formation is that when the clay dries and cracks, the surface material accumulating in the cracks partially blocks them. When they are subsequently wetted the swelling caused increased pressure as a result of the blockage, displacing the subsoil. The depressions deepen further as mounds develop.

Another suggestion is that the cracks cause differential wetting, and roots making paths for the penetration of water, that amplifies the effects of partial blocking. The optimum gilgai formation is determined by the exchangeable sodium percentage (ESP) of the clay.

A fuller explanation has been written by Twidale & Campbell, Source 3

Gilgai are small mounds and depressions that form in patterns from random to ordered. They are found on clay soils with a high expansion coefficient with seasonal water content changes, overlying thick clay horizons in the subsoils, on plains that are level or gently sloping. Generally they are found only in soils that are 60 cm deep.

A number of Gilgai types have been recognised, a very common type of which is small mounds and subcircular depressions, of varying size and spacing, displaying an irregular pattern. There is a height difference between the high and low points that are adjacent of about 30 cm, the inter-mound distance varying from 3-10 m. The development of a linear or wavy pattern of gilgai is a variation from the basic form, elongated features extending along the contour. On the deep clay soils on the Brigalow land of Queensland there are large gilgai that reach up to 2 m high and are up to 100 m apart.

The development of these micro-relief  forms involves a number of mechanisms, individually and in combination. An important factor is the swelling and shrinking of clay subsoils, gilgai being associated with clay subsoils that are expansive and deep, and occur in areas where the soils are subjected to cycles of thorough wetting and drying. According to the authors, the processes involved in gilgai development are not completely understood. One hypothesis has been more generally supported, it suggests that when the clay is dry major cracks develop as a result of contraction that are perpendicular to the surface. The volume available for expansion the next time the soil is wet is reduced by the entry of soil from the surface and near-surface by falling in or being washed into the deeper cracks. In the region between the cracks heaving relieves the pressures that result. The debris that falls into the cracks when the clay is dry is restricted to the finer material and the clay swells when it is wet,  thrusting up heterogeneous debris. Coarser material becomes concentrated as a surface layer by this mechanism. This is one reason for the stone mantle that is characteristic of many arid regions. During the subsequent drying of the soils the mounded area between the cracks tends to be maintained. As the process is repeated the subsoil is progressively displaced, a mound developing between the cracks and adjacent to the cracks, a depression. This hypothesis is supported by observations. Puffs and depressions are revealed in exposed sections.

In the cracks there is a wedge-shaped fill of loamy A-horizon material. Lag concentrations of martials, in the form of carbonate nodules and gravel, that are otherwise confined to the subsoil between and beneath mounds, are found in the mounds of some gilgai. The deep subsoil clay and the clay exposed on the mounds are similar to each other. On black earth soils, tongues of calcareous clay, that is of a light colour, rise to the surface beneath mounds.

Gilgai are known to develop very rapidly under some conditions, hummocks and hollows reappearing in about 2 years after they have been smoothed by grading or cultivation, and there are instances in which fence posts have been pushed out of the ground in as little as 3 weeks after being placed in the ground. The tendency to gilgai formation is eventually eliminated by continuous cultivation. For engineering projects such as road building, laying of pipelines and the construction of buildings, gilgai are a serious problem. The walls of houses in parts of Adelaide have developed serious cracks that are caused Bay of Biscay soils. The frames of windows and doors in wooden houses are pushed out of alignment.


Sources & Further reading

  1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
  2. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
  3. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd.
Last updated  02/04/2020



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