Mima mounds // are low, flattened, circular to oval, domelike, natural mounds found in the northwestern United States, Idaho, Oregon, and Washington, 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. Within the northwestern United States, they are typically 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.
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.
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. Not all mima mound features have the same structure though. One mound in Washington had a very complex soil profile: A horizon is a black sandy loam (due to charcoal content from aboriginal burning on the prairies), B horizon is a gravelly sandy loam, C horizon is an extremely gravelly sand.
Vernal pools are shallow surficial depressions that seasonally fill with water during winter and spring rains and dry up during dry summer months. They get their name from the recognition of the seasonality of the habitat and the springtime flora associated with them. 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.
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. However, within California, 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.
One major theory concerning the origin of Mima mounds argues that they were created by pocket gophers—small, burrowing rodents with fur-lined "pockets," or pouches, in their cheeks. The theory is that gophers tunneling into loose soil run into a gravel layer below. Unable to burrow any farther, the gophers start building upward and outward. It is unclear whether gophers created the mounds or migrated to them because the mounds provided handy home sites.
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. For example, based on grain-size data of and optically stimulated luminescence ages obtained from pimple mounds in the south-central United States, Seifert and others concluded that these mounds consisted of wind blown sediments that accumulated during prolonged late Holocene droughts. They suggest that although they superficially resemble the mima mounds of the northwestern United States, the pimple mounds of south-central United States have a greatly different origin from them.
Andrew Berg, a geologist with the U.S. Bureau of Mines in Spokane, proposed that Mima and pimple mounds were the result of very intense ground shaking resulting from major earthquakes. He formulated this hypothesis while building a dog house. As he hammered together sheets of plywood coated with volcanic ash, he noticed that the hammering vibrations caused the ash to heap into small mounds that looked a lot like miniature Mima mounds. From that observation, Berg hypothesized that vibrations from violent earthquakes could have formed the Mima mounds. According to Berg, the soil on the Mima Prairie is like volcanic ash, and the layer of rock below that is like a plank of wood. When seismic waves move through the hard ground and bump into faults, or large fractures in the ground, the waves bounce backward. Those ricocheted waves collide with other seismic waves from the quake, and between the collision points, the soil rises and forms mounds. Berg claims that Mima mounds occur only in seismically active areas—areas where the ground is unstable and many earthquakes occur. The area where the Washington Mima mounds are found experienced a major earthquake about 1,000 years ago.
Shrinking and swelling of clays
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When clay is exposed to large amounts of water, the water collects between the clay minerals (which are flat planes). Due to the shape of the minerals, the water travels in between the compacted layer, thus "swelling" the clay-bed into mound-like features. Silts are also related with this geomorphologic feature; however, silt is coarser-grained sediment so the minerals do not "hold" water in the same way. Silt is more penetrable than clay is, but since the area around the base of the mima mounds are a loamy soil. Shrink/swell soils are most often related to landforms called "hog wallows" or "gilgai" that can look similar to mima mounds.
Mima mounds have been featured in the writings of Laird Barron, in which they have been alluded to as the source or symptom of various supernatural or unexplained events. The climax of his story "Proboscis" takes place at mima mounds located in Washington. An extensive reference list pertaining to the geology of silt mounds can be found here and geoarchaeology of silt mounds here.
- 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.
- 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.
- United States Dept. of Agriculture, 1990. Soil Survey of Thurston County, Washington. 283 p.
- Bauder, E.T., A.J. Bohonak, B. Hecht, M.A. Simovich, D. Shaw, D.G. Jenkins, and M. Rains. (2009) A Draft Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of Vernal Pool Depressional Wetlands in Southern California. San Diego State University, San Diego, California
- Riefner, R.E. and D.R. Pryor. (1996) New locations and interpretations of vernal pools in Southern California. Phytologia. 80(4):296-327.
- Smith, D.W., and W.L. Verrill (1998) Vernal Pool-Soil-Landform Relationships in the Central Valley, California. in: C.W. Witham, E.T. Bauder, D. Belk, W.R. Ferren Jr., and R. Ornduff, eds., pp. 15-23, Ecology, Conservation, and Management of Vernal Pool Ecosystems – Proceedings from a 1996 Conference. California Native Plant Society, Sacramento, California.
- Riefner, R.E., S. Boyd, and R.J. Shlemon (2007) Notes on native vascular plants from mima mound-vernal pool terrain and the importance of preserving coastal terraces in Orange County, California. Aliso. 24:19-28.
- Keeler-Wolf, T., D.R. Elam, K. Lewis, S.A. Flint (1998) California Vernal Pool Assessment Preliminary Report. Department of Fish and Game, State of California, Sacramento, California.
- Seifert, C.L., R.T. Cox, S.L. Forman, T.L. Foti, T.A. Wasklewicz, and A.T. McColgan (2009) Relict nebkhas (pimple mounds) record prolonged late Holocene drought in the forested region of south-central United States. Quaternary Research. 71(3):29-339.
- Berg, A.W. (March 1990). Formation of Mima Mounds, A seismic hypothesis 18 (3). Geology. pp. 281–285. doi:10.1130/0091-7613(1990)018
- Linda Ashton (June 24, 1990). A Theory on Those Mysterious Mounds / Geology: Odd features on American landscape have long been a puzzle. A researcher says they are related to earthquakes. Los Angeles Times. Retrieved 2010-05-10
- [www.washington.edu/qrc/mimahelentour], Quaternary Research Center, Mt. St. Helens and Mima Mounds Tour, accessed 1 Dec. 2009.
- Heinrich, P.V., 2008, General Pimple Mound References (Version 5.0)
- Heinrich, P.V., 2008, Geoarchaeology of Pimple Mound References (Version 4.0)
- Burnham, L.H., and Johnson, Donald L., ed. (2012). Mima Mounds: The Case for Polygenesis and Bioturbation. The Geological Society of America.
- Walter, D., and A. Bryant (2001) Mima Mounds of Thurston County: A Study of Evapotranspiration,Geologic History & Myths, Environmental Analysis Program, The Evergreen State College, Olympia, Washington.
- Anonymous (2008) Mima Mounds: Mystery hides in vast prairie Photos taken by The Seattle Times.