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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 3 to more than 50 m; in height 30 cm to greater than 2 m; 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. 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. 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. Similar or at least reminiscent phenomena occur on all continents and the proposed causes 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.〔Cramer MD, Barger NN (2013) Are Namibian “Fairy Circles” the Consequence of Self-Organizing Spatial Vegetation Patterning? PLoS ONE 8(8): e70876. doi: 10.1371/journal.pone.0070876〕 Furthermore, it has been argued that the possibly distinct heuweltjies of the South Western Cape region of South Africa are of an origin far different from either. ==Distribution== Within the northwestern United States, Mima mounds typically are part of what is commonly known as hog-wallow landscape. This type of landscape typically has a shallow basement layer such as bedrock, hardpan, claypan, or densely bedded gravel. 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.〔Cox, G.W., 1984a. ''Mounds of mystery.'' Natural History. v. 93, no. 6, pp. 36–45.〕〔Cox, G.W., 1984b, ''The Distribution and Origin of Mima Mound Grasslands in San Diego County, California.'' Ecology. v. 65, no. 5, pp. 1397-1405.〕〔Jackson, J.A., 1997, Glossary of Geology. American Geological Institute. Alexandria, Virginia.〕〔Washburn, A.L., 1988, ''Mima mounds: an evaluation of proposed origins with special reference to the Puget Lowland.'' Report of Investigations, vol. 29. Washington Division of Geology and Earth Resources, Olympia, Washington.〕 Mima mounds occur outside the northwestern United States in three major regions west of the Mississippi River as illustrated by Cox〔 and Washburn.〔 First, they are found in a strip consisting of northwest Baja California, western and northcentral California, and southcentral Oregon where they are typically known as "hogwallow mounds". Within this strip and the area of San Diego, they are often an integral part of the local vernal pool landscape.〔 Second, they are found in a roughly north-south strip of the Great Plains containing parts of northcentral New Mexico, and central Colorado, and central Wyoming where they are typically called "prairie mounds". Third, they are found in an area containing parts of East Texas, western Louisiana, southeast Oklahoma, and southern Missouri where they are commonly known as either "pimple mounds" or "prairie mounds". Finally, isolated patches of Mima mounds also are found in Iowa, eastern North Dakota, and northwest Minnesota.〔〔 Within 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 A and E horizons lying either on a more or less flat or slightly, but noticeably depressed, clayey B horizon. Pimple mounds range in diameter from 6 m to more than 45 m; in height 30 cm to greater than 1.2 m; 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. Rarely, pimple mounds that occur on these hillslopes are elongated in an upslope-downslope direction.〔Carty, D. J., 1980, ''Characteristics of pimple mounds associated with the Morey soil of southeast Texas.'' College Station, Unpublished M.S. thesis, Texas A&M University, College Station, Texas.〕〔Carty, D.J., J.B. Dixon, L.P. Wilding, and F.T. Turner, 1988, ''Characterization of a pimple mound-intermound soil complex in the Gulf Coast region of Texas.'' Soil Science Society of America Journal. v. 52, p. 1715-1721.〕〔Goodarzi, N.K., 1978, ''Geomorphological and soil analysis of soil mounds in Southwest Louisiana.'' Unpublished M.A. thesis, Department of Geography and Anthropology, Louisiana State University, Baton Rouge.〕〔Holland, W.C., L.W. Hough, and G.E. Murray, 1952, ''Geology of Beauregard and Allen Parishes.'' Geological Bulletin no. 27. Louisiana Geological Survey, Baton Rouge.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Mima mounds」の詳細全文を読む スポンサード リンク
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