Gilgai

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Gilgai are small, ephemeral , damp areas of the ground, pools, water holes or lakes that have formed in tub-like depressions in the ground. They are mainly found in warm climates and are underlain by swellable clay soils . In a broader sense, all micro-relief forms are referred to as Gilgai, which have a morphology consisting of shallow troughs and adjacent flat elevations. Gilgai are therefore a collective name for surface shapes with regular structures that arise from seasonal changes in humidity .

etymology

The name Gilgai goes back to a word from the language of the Aborigines of Australia and means small water hole . In English they are u. a. Also known as melonholes , crabholes , hogwallows or puff and shelf formations (melon holes, crab holes, pork pillars, blow-out and shelf formations), in French as mottes , mottureaux or moutons (Charente) or as truchines or beurlins (Vendée).

description

A Gilgai landscape in Queensland

Gilgai Lakes are usually several meters in diameter and less than 30 centimeters deep, but in exceptional cases they can be up to 100 meters wide and reach a depth of several meters. The differences in relief ( vertical interval ) in Gilgai forms in Holland are only a few centimeters, but are generally 50, 80 and 100 centimeters and in Queensland they reach 180 centimeters. The related Mima Mounds in western Washington even show relief differences of 250 centimeters. Gilgai structures usually form on absolutely flat topography, but they can also arise on relatively low slopes.

In a broader sense, Gilgai are not only designed as negative relief forms (depressions), but also as positive relief forms (elevations). The basic individual forms vary in both cases from circular, through elliptical, elliptical-elongated to furrow-like or wall-like elongated. Gilgai usually appear in socialized groups, which can demonstrate a low to relatively high degree of order. The ordered structures can in turn be arranged according to one or more preferred directions. In particular, the shapes created when the slope is inclined are often parallel to the general incline or lie diagonally. Some of the ordered structures sometimes have quite complex geometries. The basic shapes are mostly hemispherical in elevation, with the sides falling flat. The upper part is flattened on steeply sloping sides. Completely analogous forms also exist in the negative relief.

Gilgai are mostly associated with soils that are capable of swelling, such as Vertisol . The required amounts of precipitation are between 150 and more than 1500 millimeters / year.

Ultimately responsible for their formation is argilliturbation , which unfolds its effectiveness through repeated cycles of swelling and subsequent shrinking of the clay-rich soil.

In dry periods, cracks form in the soil, which are then backfilled with loose soil material. When the soil is moistened again, the resulting overpressure cannot affect the filled cracks, but is forced to move sideways. As a result, slight bumps form between the cracks and pits just above the cracks. The process experiences positive feedback because the water in the depressions lasts longer. Due to the increased moisture, the depressions swell more than the elevations, which in turn leads to further contraction and cracking. Over time, the swelling / shrinking / cracking processes become more and more significant until finally the landscape is covered by a repeating pattern of bumps and depressions. The surface water then collects in the depressions during the rainy season.

Typology

In their early work in 1951, Hallsworth and Robertson were able to distinguish six basic structure types:

  • Normal Gilgai
  • Melon Hole Gilgai
  • Stone Gilgai
  • Network Gilgai
  • Linear or wavy Gilgai
  • Tank Gilgai

Normal Gilgai

This is the most common form of Gilgai. It is characterized by irregularly arranged elevations and shelf-like depressions. Their height differences are sometimes minimal and hardly noticeable, but at a wavelength of 15 meters they can reach up to 3 meters. If the elevations take on a circular shape, they are called puffy gilgai in English . The soils usually show a threefold division into A 1 , A 2 and B horizons, which is often lost in the center of the depressions. The A 1 horizon is a Solod (Solodischer Planosol or steppe bleaching earth), the depressions consist of humus gley and the underlying B horizon, which is criss-crossed by vertical cracks, is usually made of brown clays. In swelling areas (English puffs ), the sub-floor can penetrate directly to the surface.

Melon Hole Gilgai

Melonenloch Gilgai consist of large elevations (English mounds ), which are separated by rather complex, flat depressions (English shelves ). The sinks have one or two drainage holes and are typically 1 to 3 meters wide and 15 to 20 centimeters deep. The elevations are underlaid by brown clay, while the depressions are covered by a layer up to 40 centimeters thick made of dark brown to gray clay, which in turn sits on top of gray clay.

Stone Gilgai

Stone Gilgai, in turn, can be divided into three sub-types:

  • Circular stone gilgai.
  • Step stone gilgai.
  • Stone polygons.

Stone Gilgai are the most common Gilgai species in the deserts of Australia. They are very similar to the patterned ground found in high mountains and in polar landscapes . Their quite wide and flat elevations are covered with stone.

Circular stone gilgai are characterized by a fine-grained inner depression, which is surrounded by a slightly raised stone ring. The roughly circular depressions have a diameter of about 3 meters, whereas the diameter of the stone ring reaches up to 8 meters. Circular stone gilgai are formed with only a minimal slope. The subsoil under the stone ring is rich in clay and has a silty crust in which pebbles are embedded. The soil in the area of ​​the depression is much more sandy in the upper 30 to 50 centimeters, but below this depth it resembles the soil under the stone circle with fairly frequent, coarse clasts. Circular stone gilgai form irregular patterns but occasionally develop networks.

Stepped stone gilgai require a gradient of 0.5 to 6 ° for their formation. They emerge from ordinary stone gilgai under the action of gravity, whereby the stone walls are elongated. The stone walls on the higher side are relatively steep and lie on fine-grained soil. There are often sinkholes at their base. The stone walls on the valley side are flattened and comparatively poor in stone cover, as some of the stones have slipped down the valley. They lie on sandy ground.

Stone polygons ( sorted stone polygons ) have a diameter of 40 to 80 centimeters. They are marked by pebbles made of Silcrete , which sit on a plaster of smaller fragments coated with desert varnish. In the interior of the depression there are no longer any Silcrete pebbles, but the fragment fraction. The recess consists of fragments of loose, sandy clay that sits on the clay pillars of the sub-floor, which are criss-crossed with shrinkage cracks. The thickness of the clay layer is usually greater in the depression than in the outer area of ​​the polygons.

Network Gilgai

The morphology of Netz-Gilgai, engl. lattice gilgai is complex. These include both interrupted elevations that run parallel to the collapse of the terrain, as well as more coherent forms that are arranged like a network in different directions. With a diameter of around 5 meters, its depressions are 20 centimeters lower than the shoulders of the trough. The ground, which is a little over 1 meter thick, consists of black clay, which is underlaid by yellow-brown clay. In the area of ​​the trough shoulders, very dark brown clay is pressed, sometimes carbonated tubers and softer soil secretions are also carried away here.

Linear or wavy Gilgai

Linear or undulating Gilgai are tied to gradients that can fall between 0.25 ° and 3 °. Their elevations and depressions are continuous and run at right angles to the contours of the terrain. With a diameter of 4 meters, they are only 5 to 10 centimeters deep. During the dry season they look a bit puffed up. They are also made of black clay, which is a bit thicker at 1.5 meters. Yellow-brown clay is also pressed on the edges.

Tank Gilgai

As the name suggests, Tank-Gilgai have a rectangular, tank-shaped arrangement. These very large Gilgai can have depressions 10 to 20 meters long and 15 to 20 meters wide. At the same time, they reach a depth of 60 to 150 centimeters. Their interior is underlaid with gray clay, which is covered by a thin dark brownish-gray layer of clay. There is brown clay under the shoulders.

Emergence

Knight (1980) differentiates between four development mechanisms for Gilgai:

  • Elevation between cracks.
  • Elevation of the ground over cracks.
  • Shrinkage over cracks.
  • Elevation of the ground due to the load.

The mechanism of soil elevation between cracks can in turn be divided into three sub-mechanisms, all of which start with the compression of the soil:

  • The soil compression leads to an extrusion of soil material by means of plastic flow, which in turn causes the Gilgai shoulders to lift.
  • The soil compression causes the soil material to break in blocks. Penetrating moisture and material slipping into the cracks that have formed build up pressure in the clays below the surface and ultimately lead to desk-like lifting out. This is probably the most common Gilgai explanation.
  • The soil compression causes oblique push-ups.

However, Knight (1980) criticizes all of these mechanisms because of their difficult mechanical feasibility.

There are two models for lifting the floor over cracks :

  • The ground uplift occurs through the cumulative accumulation of internal oblique thrusts.
  • The floor is raised by a vertical, desk-like approach. This model goes back to Howard (1939) and is used by Ollier (1966) to explain Stein-Gilgai.

The mechanism of shrinkage over cracks is hypothetical and comes from McGarity (1953). As a result, elevations should arise in the area between the cracks. The depressions come to lie outside the cracks and are a result of the soil sagging as a result of drying out.

The mechanism of elevations due to the load can again be divided into two parts. The first explanation is based on density inversions within the soil layers. The resulting compensatory movements lead to the floor lifting in places. The second approach is based on liquefied soil, which works its way up along the cracks through the solid soil layer.

meaning

For the Aborigines, Gilgais were a very important water resource, which made it possible for them to stay even in areas without a permanent water supply. Later the first settlers benefited from the Gilgais as water for their herds of cattle. However, the introduction of wells and pumping systems has meanwhile reduced the value of the Gilgais. In fact, some farmers now often regard Gilgais as a nuisance, as the earth moving associated with them affects physical infrastructure such as foundations, roads and railways. The undulating topography is also detrimental to harvesting machines. The seasonal pools make it difficult to control the herds, and they also attract unwanted visitors such as feral pigs and kangaroos .

Nevertheless, Gilgai still have great ecological importance, as they provide vital water supplies for animal and plant communities. The pools are home to crabs and the surrounding elevations ants , which use bioturbation to raise the existing hills even further.

Occurrence

Gilgai are widespread worldwide, but they are bound to swellable clay soil underlays and clearly pronounced, seasonal dry times, which cause the clay crust formed during the wet season to break open. They often form on soils that have a strongly contrasting soil texture. They are undoubtedly most common in Australia, but they can also be found in Western Europe , southern Russia , the Middle East , India , Africa, and the United States .

Occurrence in detail:

Web links

Commons : Gilgai  - collection of images, videos and audio files

Individual evidence

  1. ↑ Randall J. Schaetzl, Sharon Anderson: Soils: genesis and geomorphology . Cambridge University Press, Cambridge UK 2007, ISBN 0-521-81201-1 , pp. 283 .
  2. ↑ F. Verger: Mottureaux et gilgais . In: Annales de Géographie . tape 73 , no. 398 , 1964, pp. 413-430 .
  3. ↑ EG Hallsworth, GK Robertson: The nature of gilgai and melonhole soils . In: Australian Journal of Science . tape 13 , 1951, pp. 181 .
  4. ↑ a b C. D. Ollier: Desert Gilgai . In: Nature . tape 212 , 1966, pp. 581-583 .
  5. ↑ JA Mabbutt: Desert landforms . MIT Press, Cambridge MA 1977.
  6. ↑ a b J. A. Mabbutt: Pavements and patterned ground in the Australian stony deserts . In: Stuttgart Geographical Studies . tape 93 , 1979, pp. 107-123 .
  7. ^ RU Cooke, A. Warren: Geomorphology in deserts . University of California Press, Berkeley 1973.
  8. ↑ JC Dixon: Aridic soils, patterned ground and desert pavements . In: AD Abrahams, AJ Parsons (Eds.): Geomorphology of Desert Environments . Chapman & Hall, London 1994, ISBN 0-412-44480-1 .
  9. ↑ a b M. J. Knight: Structural analysis and mechanical origins of gilgai at Boorook, Victoria, Australia . In: Geoderma . tape 23 , 1980, pp. 245-283 .
  10. ↑ EG Hallsworth et al. a .: Studies in pedogenesis in New South Wales. VII. The gilgai soils . In: Journal of Soil Science . tape 6 , 1955, pp. 1-31 .
  11. ^ A b E. M. White, RG Bonestall: Some gilgaied soils of South Dakota . In: Soil Science Society of America Journal . tape 24 , 1960, pp. 305-309 .
  12. ^ A. Howard: Crab-hole gilgai and self-mulching soils of the Murrumbidgee Irrigation Area . In: Pedology . tape 8 , 1939, pp. 14-18 .
  13. ^ JW McGarity: Melon hole formation in the Richmond River District of New South Wales . In: Proceedings of the Australian Conference on Soil Science . tape 2 , 1953, p. 1-7 .
  14. ↑ TA Paton: Origin and terminology for Gilgai in Australia . In: Geoderma . tape 11 , 1974, p. 221-242 .
  15. ↑ EG Hallsworth, GG Beckman: Gilgai in the Quaternary . In: Soil Science . tape 10 , 1969, p. 409-420 .
  16. ^ SA Harris: The classification of gilgaied soils: some evidence of northern Iraq . In: Journal of Soil Science . tape 10 , 1959, pp. 27-33 .
  17. ^ MF van Oosten: Soils and Gilgai microrelief in a Central African river plain in the light of the Quaternary climatic changes . In: Boor en Spade . tape XI , 1961, p. 126-148 .
  18. ^ SF Kuipers: Bodemkunde (Zesde Druk), NV Ed .: Mij. Tjeenk Willink. Zwolle 1958.
  19. ^ LP White, R. Law: Channeling of alluvial depression soils in Iraq and Sudan . In: Journal of Soil Science . tape 20 , 1969, p. 84-90 .
  20. ↑ CS Denny: Fans and pediments . In: American Journal of Science . tape 265 , 1967, pp. 81-105 .