Gilgai and hogwallows are the same thing. They are naturally occurring landforms called microrelief that only occur in expansive clays. The higher areas have been called microknolls, mounds, and microridges and the lower areas are called depressions, microlows or microdepressions. According to Gustavson (1975) the maximum relief produced by these landforms is about 18 inches.The three basic forms are mounds and depressions, polygons (with microridges) and linear that are elongated in the direction of slope. The expression "hogwallows" appeared first (in print) in the United States but the Australian expression "gilgai" has become the more commonly used word. These landforms are caused by montmorillonitic expansive soils. The study of these landforms is compelling because the resulting micro-topography is anomalous with regard to surface moisture and the expected shrink-swell behavior. Low spots collect water, this should make these areas heave and disappear. So we would expect expansive soils to dampen out low areas, but the exact opposite occurs with these landforms. This means that there must be a special mechanism or process that starts and maintains these landforms. At stake is some process that produces extraordinary differential movements in expansive clays.
There is a similar class of microrelief that are sometimes confused with gilgai that are not related with expansive soils. These landforms are different because they sometimes have a flat or tableland area between the microknolls without clearly defined microlows. In other cases there are apparent sinks or microlows separated by flat or tableland without any defined microhigh areas. These similar landforms can also have a much higher relief than gilgai and are more appropriately called pimple mounds, mima mounds, hillocks, prairie mounds, nebkhas, and hummocks. In Texas there are pimple mounds in the same general area as gilgai. But studies of these similar types do not reveal a connection with expansive clays.
To find these landforms you usually have to look at historical images taken between December and March because during this period gilgai may be holding water and this makes them more visible in aerial photography. This is fairly typical of aerial photos in the southeastern United States. Higher resolution images are also a requirement. There is also a chance the ground surface will be dry even it the images are made during the optimum period. Presently the extreme rural areas of Mississippi and Texas do not have adequate photographic coverage to map and too fully define the distribution. This is unfortunate because these landforms could help map unknown areas where damaging expansive clays exist.
2. DIFFERENTIAL REBOUND IN A PREBUCKLING MODE: In part II of this blog, I will contrast the difference between a prebuckling internal stress and a buckling internal stress and show how a prebuckling stress creates differential rebound and the initial mounds and depressions.
The Origin of Hogwallows and Gilgai - PART II The conclusion of this blog.