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Mima mounds are low, flattened, circular to oval, domelike, natural mounds that are composed of loose, unstratified, often gravelly sediment that is an overthickened A horizon. These mounds range in diameter from to more than ; in height to greater than ; and in density from several to greater than 50 mounds per hectare, at times forming conspicuous natural patterns. Mima mounds can be seen at the Mima Mounds Natural Area Preserve in Washington state.
"Mima" is a name derived from a Chinook Jargon term meaning "a little further along" or "downstream".
Theories for the origin of Mima mounds include burrowing by ; accumulation of wind-blown (aeolian) sediments around vegetation to form coppice dunes or ; 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.
Though the definitive Mima mounds are common in North America, it has not been shown that all North American mounds result from the same causes. Superficially similar phenomena occur on all continents, and the proposed causal factors do not occur in all regions that have been studied. Nor is it clear that all such mounds really are the same, either physically or functionally; for example, the so-called fairy circles of Southern Africa tend to be less mound-like and occur in different climatic and ecological conditions from Mima mounds. Furthermore, it has been argued that the possibly distinct of the South Western Cape region of South Africa are of an origin far different from either.
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. Mima mounds are named after the Mima Prairie in Thurston County, Washington.
They also are found in south central Oregon, and in western and north central California, where they are typically known as "hogwallow mounds". Within this strip they are often a part of the landscape local to vernal pools.
Mima mounds occur outside the western coastal North America in three major regions between the Cascade Range, the Sierra Nevada, and the Sierra de Juárez in the west, and the Mississippi River in the east. As described by Cox and by Washburn, these are:
Within Arkansas, Louisiana, Missouri, Oklahoma, and Texas, pimple mounds, also called locally "prairie mounds" and "natural mounds", consist of low, flattened, circular to oval, domelike, mounds composed of loose, sandy loam or loamy sand. Typically, these mounds consist entirely of a thickened loamy and sandy Soil horizon lying either on a more or less flat or slightly, but noticeably depressed, clay laden Soil horizon. Pimple mounds range in diameter from to more than ; in height to greater than ; and in density from several to greater than 425 mounds per hectare.
Unlike the Mima mounds of Oregon and Washington, "pimple mounds" are not limited to the relatively flat and poorly drained surfaces, i.e. late Pleistocene coastal and fluvial terraces: They also occur in abundance of the slopes, summits and crests of hills created by the deep erosion and dissection of unconsolidated and unlithified early Pleistocene and middle Pleistocene, Pliocene, and older coastal plain sediments. In rare cases, pimple mounds that occur on these hillslopes are elongated in the upslope-downslope direction.
Within California, vernal pools are quite commonly associated with Mima Mounds. These Mima Mounds are typically located on stable landforms that are greater than 100,000 years old. These landforms are characterized by strongly developed soils that usually have a relatively impermeable layer (claypan or silcrete duripan) in the subsoil. 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. Although the ponded water that fills vernal pools comes and goes throughout the year, it is present at least for a short time in most years.
Within California, however, the mound-depression microrelief associated with Mima Mounds is just one of a variety of geographic settings within which vernal pools occur. 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.
This uphill soil transport contrasts with the typical gopher behavior of pushing soil downhill, but can be overridden when soil is saturated. Consequently, gophers in mima mound fields seem to be aware of randomly distributed topographic highs and orient their burrowing accordingly in early mound creation stages. However, the mounds were already fully formed and the gophers may have just been maintaining them. 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.
Results from the tracer study were incorporated into a numerical model that simulated the burrowing behavior of gophers. The advantage of modelling in this case is that
In the computer simulations, mounds naturally emerged from randomly distributed topographic highs, and reached topographic steady state after several centuries of gopher activity, which could explain why nobody has ever witnessed the growth of one. Once the mound field reaches topographic maturity, the mounds feature more uniform spacing and hexagonal tessellation. Results indicated that formation of these mound fields are largely contributed by positive feedback loops which amplify small features to create fractal, a common facet of self-organization. The slow modeled mound growth rates and their spatial distribution agreed with field observations.
Although occupation of mounds by gophers does not by itself prove that gophers built the mounds since they could be living there opportunistically, to date, this is the strongest evidence for the origin of these enigmatic features. Moreover, the results from the computer model are supported by radiocarbon ages of organic material taken at regular intervals down through the middle of a typical mound in the Mima Mounds Natural Area Preserve (Washington state). These ages confirm that the mounds grow slowly through the accumulation of material, at the rate of about 2.6 cm/yr. publication of this modelling study received attention from the international press.
However, since this hypothesis has been proposed, there have been many large earthquakes throughout the world and none have been reported to have formed Mima mounds. In addition, Mima mounds have been gradually growing on the Carrizo Plain (California) since the 1980s when plowing of the fields was halted. These mounds have been forming in the absence of any large earthquakes. Furthermore, Berg's experiments were done using dry cohesionless sediment; when these experiments are attempted with moist sediment (ie, similar to real soils), mounds do not form. Finally, radiocarbon ages of organic material taken at regular intervals down through the middle of a typical mound in the Mima Mounds Natural Area Preserve (Washington) showed that the mound formed gradually over time, at the rate of about 2.6 cm/yr and, thus, could not have formed during a single event, like an earthquake. Therefore, there is no realistic experimental evidence or geological evidence supporting the 'earthquake' hypothesis.
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