Gilgai
Encyclopedia
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 well
s and pump
s 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 kangaroo
s.
Gilgais remain of great ecological significance as a source of water for animal and plant life.
Australian English
Australian English is the name given to the group of dialects spoken in Australia that form a major variety of the English language....
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
Acacia cambagei
Acacia cambagei, commonly known as Gidgee , Stinking wattle or Stinking gidgee is an endemic tree of Australia. It is found primarily in semi-arid and arid Queensland but extends into the Northern Territory, South Australia and north-western New South Wales. It can reach up to 12 meters in height...
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
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
and the southern United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. 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
Indigenous Australians
Indigenous Australians are the original inhabitants of the Australian continent and nearby islands. The Aboriginal Indigenous Australians migrated from the Indian continent around 75,000 to 100,000 years ago....
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 well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...
s and pump
Pump
A pump is a device used to move fluids, such as liquids, gases or slurries.A pump displaces a volume by physical or mechanical action. Pumps fall into three major groups: direct lift, displacement, and gravity pumps...
s 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
Feral
A feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...
pigs and kangaroo
Kangaroo
A kangaroo is a marsupial from the family Macropodidae . In common use the term is used to describe the largest species from this family, especially those of the genus Macropus, Red Kangaroo, Antilopine Kangaroo, Eastern Grey Kangaroo and Western Grey Kangaroo. Kangaroos are endemic to the country...
s.
Gilgais remain of great ecological significance as a source of water for animal and plant life.