Gilgai and hogwallows are the same thing. They are naturally occurring landforms that are called microrelief. 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 some special mechanism or process that starts and maintains these landforms. At stake is an unknown 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 have a flat or tableland area between the microknolls without clearly defined microlows. These landforms can have 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.
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.