Mima mounds
[2] Theories for the origin of Mima mounds include burrowing by pocket gophers; accumulation of wind-blown (aeolian) sediments around vegetation to form coppice dunes or nebkhas; seismic ground shaking by major earthquakes, though none have been observed to form Mima mounds; and shrinking and swelling of clays in hog-wallow or gilgai landforms.
This type of landscape typically has a shallow basement layer such as bedrock, hardpan, claypan, or densely bedded gravel.
[6][7][8][9] In the northwestern United States, Mima mounds also occur within landscapes where a permanent water table impedes drainage, creating waterlogged soil conditions for prolonged periods.
[7] Mima mounds are found in northwest Baja California and adjacent San Diego County, where again they are an integral part of vernal pools' landscape.
[10][11][12][13] Excavations made into the Washington mounds show that underneath a blanket of prairie grass lies a mixture of loose sand, fine gravel, and decayed plants.
Vernal pools form where an impermeable or very slowly permeable layer underlies small and shallow depressions and creates a perched water table.
The impermeable or very slowly permeable layer typically consists of either soil horizons such as duripans or claypans or bedrock in the form of volcanic mud or lava flows.
This impermeable layer locally impedes drainage and creates perched water levels and causes the formation of vernal pools within the intermound depressions that are associated with Mima Mounds.
Vernal pools are typically small, shallow, and complex ephemeral wetlands that only have internal drainage because they are hydrologically isolated from perennial inflow by a ring of Mima Mounds.
For example, in the Modoc Plateau region of California, numerous vernal pools are found on the surface of volcanic mudflows and basalt lava flows where Mima Mounds are completely absent.
[19] One theory on the origin of Mima mounds is that they were created by small burrowing rodents such as pocket gophers (Thomomys talpoides) of the endemic North American family Geomyidae.
Nevertheless, the fact that the surface area of a typical Mima mound is similar to the size of an individual gopher's home range is consistent with the theory they were constructed by the rodents.
[3] Another major theory concerning the origin of pimple and prairie mounds argues that they are either coppice dunes or nebkhas formed by the accumulation of wind-blown sediments around clumps of vegetation.
[23] In addition, the ages demonstrated that the mound grew gradually over time through the accumulation of material, which supports the gopher hypothesis.
In 2017, Corina Tarnita and several of her colleagues published a paper in Nature which explained these and other related self-organised vegetation patterns by means of a general theory which integrates scale-dependent feedbacks and the activities of subterranean ecosystem engineers such as termites, ants, and rodents.
Mima mounds are also present at the Scatter Creek Unit and Rocky Prairie, located in southern Thurston County, Washington.
