A gilgai is a small,

ephemeral Ephemerality (from the Greek word , meaning 'lasting only one day') is the concept of things being transitory, existing only briefly. Academically, the term ephemeral constitutionally describes a diverse assortment of things and experiences, fr ...

lake formed from a surface depression in expanding clay soils. Gilgai is also used to refer to the overall micro-relief in such areas, consisting of mounds and depressions. The name comes from an

Australian Aboriginal Aboriginal Australians are the various indigenous peoples of the Mainland Australia, Australian mainland and many of its islands, excluding the ethnically distinct people of the Torres Strait Islands. Humans first migrated to Australia (co ...

word meaning small water hole. The pools are commonly a few metres across and less than deep but, in some instances, they may be several metres deep and up to across. Gilgais are found worldwide wherever there are cracking clay soils and marked wet and dry seasons. Gilgais are also called melonholes, crabholes, hogwallows, or puff and shelf formations.

Formation

Gilgais are thought to form in

vertisol A vertisol is a Soil Order in the USDA soil taxonomy and a Reference Soil Group in the World Reference Base for Soil Resources (WRB). It is also defined in many other soil classification systems. In the Australian Soil Classification it is c ...

s through repeated cycles of swelling when wet and subsequent shrinkage upon drying. That action, known as argillipedoturbation, causes the soil to crack when dry, allowing loose soil material to fill the 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. That process is then exaggerated as the depressions hold water and thus become wetter and swell 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 hold surface water during wet seasons.

Distribution

Australia has an abundance of cracking clay soils and large areas dominated by pronounced wet and dry seasons, providing ideal circumstances for gilgais to form. Central

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

and several parts of the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

, including

South Dakota South Dakota (; Sioux language, Sioux: , ) is a U.S. state, state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota people, Dakota Sioux ...

Mississippi Mississippi ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South regions of the United States. It borders Tennessee to the north, Alabama to the east, the Gulf of Mexico to the south, Louisiana to the s ...

, and

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

also host the formations. Gilgais are structurally similar to the

patterned ground Patterned ground is the distinct and often symmetrical natural pattern of geometric shapes formed by the deformation of ground material in periglacial regions. It is typically found in remote regions of the Arctic, Antarctica, and the Outback ...

of frigid regions, but

periglacial Periglaciation (adjective: "periglacial", referring to places at the edges of glacial areas) describes geomorphic processes that result from seasonal thawing and freezing, very often in areas of permafrost. The meltwater may refreeze in ice wedg ...

soil polygons are formed instead, by repeated freeze-thaw cycles.

Relation to humans

Gilgais were an important water source for

Indigenous Australians Indigenous Australians are people with familial heritage from, or recognised membership of, the various ethnic groups living within the territory of contemporary Australia prior to History of Australia (1788–1850), British colonisation. The ...

, enabling them to forage over areas that lacked permanent water. Similarly, they allowed the stock of early Australian pastoralists to graze those areas. The introduction of wells and

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

s reduced the value of gilgais to humans as a source of water. Many farmers now generally regard them as a nuisance. The movement of soil associated with gilgai formation damages infrastructure, including building foundations, roads and railway lines, and the undulations 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

feral A feral (; ) animal or plant is one that lives in the wild but is descended from domesticated individuals. As with an introduced species, the introduction of feral animals or plants to non-native regions may disrupt ecosystems and has, in som ...

animals and

kangaroo Kangaroos are marsupials from the family Macropodidae (macropods, meaning "large foot"). In common use, the term is used to describe the largest species from this family, the red kangaroo, as well as the antilopine kangaroo, eastern gre ...

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

Crayfish Crayfish are freshwater crustaceans belonging to the infraorder Astacidea, which also contains lobsters. Taxonomically, they are members of the superfamilies Astacoidea and Parastacoidea. They breathe through feather-like gills. Some spe ...

burrow in the wet basins and ants build up the mounds, magnifying the formations through

bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a ...

.Stone, E. L. (1993). "Soil burrowing and mixing by a crayfish". ''Soil Science Society of America Journal'' 57: 1096–1099.

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 * *{{cite book, author= Andrew S. Goudie, title=Arid and Semi-Arid Geomorphology, year=2013, publisher=Cambridge University Press, pages=105, isbn=978-1-107-00554-9 *Beckmann, G. G., Thompson, C. H., and B. R. Richards. 1984. Relationships of soil layers below gilgai in black earths. In J. W. McGarity, E. H. Hoult and H. B. So (eds.) ''The Properties and Utilization of Cracking Clay Soils''. Reviews in Rural Science no. 5. Armidale, NSW, University of New England, pp. 64–72. *Costin, A. B. 1955a. A note on gilgaies and frost soils. Journal of Soil Science 6: 32–34. *Hallsworth, E. G. and G. G. Beckmann. 1969. Gilgai in the Quaternary. ''Soil Science'' 107: 409–420. *Hallsworth, E. G., Robertson, G. K., and F. R. Gibbons. 1955. Studies in pedogenesis in New South Wales. VII. The ‘‘gilgai” soils. ''Journal of Soil Science'' 6: 1–31. *Jensen, H. I. 1911. The nature and origin of gilgai country. ''Proceedings of the Royal Society''. NSW 45: 337–358 *Knight, M. J. 1980. Structural analysis and mechanical origins of gilgai at Boorook, Victoria, Australia. ''Geoderma'' 23: 245–283 . *McManus, K. 1999 "Mound Theory, Gilgai and PSD Analysis" ''Proceeding, 8th Annual Australia and New Zealand Conference on Geomechanics, Hobart'' *Ollier, C. D. 1966. Desert gilgai. ''Nature'' 212: 581–583. *Paton, T. R. 1974. Origin and terminology for gilgai in Australia. ''Geoderma'' 11: 221–242. *Stephen, I., Bellis, E., and A. Muir. 1956. Gilgai phenomena in tropical black clays of Kenya. ''Journal of Soil Science'' 1–9. *White, E. M. and R. G. Bonestell. 1960. Some gilgaied soils in South Dakota. ''Soil Science Society of America Proceedings'' 24:305–309. *Wilson, J.W. 1964 "Vegetation patterns in an unusual gilgai soil in New South Wales," ''The Journal of Ecology'', 52:2 379-389 Australian English Lakes * Australian Aboriginal words and phrases

Formation

Distribution

Relation to humans

See also

References

Further reading