Gilgai

This article is about the landform. For the town, see Gilgai, New South Wales.

Gilgai is an Australian English word referring to a small, ephemeral lake formed from a depression in the soil surface. Gilgais are commonly a few metres across and less than 30 cm deep, however in some instances they may be several metres deep and up to 100 metres across. Gilgais are common and widespread in Australia but also occur in isolated locations worldwide. Gilgais are also commonly called Melon holes in Northern Australia. In the United States these landforms were called hogwallows in the 19th and early 20th centuries.

There are various theories of the origin of gilgai. A popular theory is as follows: Gilgais form on certain clay soils due to the swelling of the clay when wet and subsequent shrinkage upon drying. This action causes the soil to crack when dry and loose soil material then fills these cracks. When the soil swells upon subsequent re-wetting the soil pressure cannot be dispersed into the now-full cracks and the soil is forced sideways causing a mound to form between cracks and a depression to form at the location of the crack. The process is then further exaggerated by the depressions holding water and thus becoming wetter and swelling more than the mounds, causing even greater shrinkage and cracking. In addition, the cracks channel water deeply into the soil causing even greater swelling and subsequent cracking of the depression areas. Each cycle of swelling, shrinkage and cracking becomes more exaggerated and the landscape eventually becomes covered by a repeated pattern of mounds and depressions. The depressions are gilgais which become filled with water during the wet seasons.

Australia has an abundance of cracking clay soils and a large areas dominated by very pronounced wet and dry seasons providing ideal circumstances for the formation of gilgais. Brigalow and gidgee soils are particularly suited to gilgai formation. Small areas outside Australia also have the necessary soil types and rainfall variability for gilgai formation, including central Russia and the southern United States. Gilgais are structurally similar to the soil polygons of frigid regions, however soil polygons are formed by repeated rounds of soil shrinkage and swelling due to the freezing of soil water rather than the shrinkage and swelling caused by dehydration and rehydration that leads to gilgai formation.

Gilgais were an important source of water for Indigenous Australians and enabled people to seasonally forage over areas that lacked permanent water. Gilgais were important for similar reasons for early Australian pastoralists, allowing stock to seasonally graze areas that lacked permanent water. The introduction of water wells and pumps has reduced the value of gilgais to humans as a source of water. Gilgais are now generally considered a nuisance by farmers. The movement of soil associated with gilgai formation damages infrastructure including building foundations, roads and railway lines and the undulations produced interfere with crop harvesting. The presence of seasonal water in grazing land makes it more difficult to control stock and provides a water supply for vermin such as feral pigs and kangaroos.

Gilgais remain of great ecological significance as a source of water for animal and plant life.

See also

• Vertisol

References

• Alekseeva, T.V. & Alekseev, A.O. 1997 "Clay mineralogy and organization of finely dispersed material of gilgai soils (Stavropol Krai)" Eurasian soil science 30:8 867-876
• McManus, K. 1999 "Mound Theory, Gilgai and PSD Analysis" Proceeding, 8th Annual Australia and New Zealand Conference on Geomechanics, Hobart
• Wilson, J.W. 1964 "Vegetation patterns in an unusual gilgai soil in New South Wales," The Journal of Ecology, 52:2 379-389

Further reading

• Anthony J. Parsons, A. D. Abrahams, ed (2009). Geomorphology of Desert Environments. Springer. pp. 109–112. ISBN 978-1402057182.