Mima Mounds
Encyclopedia
Mima mounds is a term used for 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.
s—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. The theory's author, George Cox, a retired zoologist at San Diego State University, says that a gopher family often can move up to 5 tons of earth a year. That's one-twentieth of the soil in an average Mima mound. Cox also asserts, most North American mounds are in gopher territory, and many gophers actually live in mounds. However, the only pocket gophers ever spotted on Mima Prairie built their burrows between the mounds, not in them . It is unclear whether gophers created the mounds or migrated to them because the mounds provided handy home sites.
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.
s 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.
data and published by researchers at Washington Department of Natural Resources
showed that the retreat of the ice-age glaciers could account for the mima mounds' formation. A feature of this theory is that meltwater from the glaciers carried gravel to sun cups, where it was deposited and formed mounds after glaciers had fully retreated.
The mima mounds located in the California Floristic Province (west of the Sierra Nevada and Cascade
ranges from Southern Oregon to Baja California. Coincidentally, mima mounds are located in Washington and Oregon State, west of the Cascade Range, and likewise throughout California west of the Sierra Nevada Range.
Silts are also capable of this type of 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, the composition allows water to swell and shrink in a similar fashion to clay, but on a smaller scale.
in Thurston County, Washington, a designated National Natural Landmark
.
, 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.
Idaho
Idaho is a state in the Rocky Mountain area of the United States. The state's largest city and capital is Boise. Residents are called "Idahoans". Idaho was admitted to the Union on July 3, 1890, as the 43rd state....
, Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
, and Washington, that are composed of loose, unstratified, often gravelly sediment that is an overthickened A Horizon
Soil horizon
A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time...
. 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
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
, hardpan
Hardpan
In soil science, agriculture and gardening, hardpan or ouklip is a general term for a dense layer of soil, usually found below the uppermost topsoil layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water...
, claypan
Claypan
In geology, a claypan is a dense, compact, slowly permeable layer in the subsoil having a much higher clay content than the overlying material, from which it is separated by a sharply defined boundary. Claypans are usually hard when dry, and plastic and sticky when wet. They limit or slow the...
, 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
Mississippi River
The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...
as illustrated by Cox. and Washburn First, they are found in a strip consisting of northwest Baja California
Baja California
Baja California officially Estado Libre y Soberano de Baja California is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is both the northernmost and westernmost state of Mexico. Before becoming a state in 1953, the area was known as the North...
, western and northcentral California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, and southcentral Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
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
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
, and central Colorado
Colorado
Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...
, and central Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
where they are typically called "prairie mounds". Third, they are found in an area containing parts of East Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
, western Louisiana
Louisiana
Louisiana is a state located in the southern region of the United States of America. Its capital is Baton Rouge and largest city is New Orleans. Louisiana is the only state in the U.S. with political subdivisions termed parishes, which are local governments equivalent to counties...
, southeast Oklahoma
Oklahoma
Oklahoma is a state located in the South Central region of the United States of America. With an estimated 3,751,351 residents as of the 2010 census and a land area of 68,667 square miles , Oklahoma is the 28th most populous and 20th-largest state...
, and southern Missouri
Missouri
Missouri is a US state located in the Midwestern United States, bordered by Iowa, Illinois, Kentucky, Tennessee, Arkansas, Oklahoma, Kansas and Nebraska. With a 2010 population of 5,988,927, Missouri is the 18th most populous state in the nation and the fifth most populous in the Midwest. It...
where they are commonly known as either "pimple mounds" or "prairie mounds". Finally, isolated patches of Mima mounds also are found in Iowa
Iowa
Iowa is a state located in the Midwestern United States, an area often referred to as the "American Heartland". It derives its name from the Ioway people, one of the many American Indian tribes that occupied the state at the time of European exploration. Iowa was a part of the French colony of New...
, eastern North Dakota
North Dakota
North Dakota is a state located in the Midwestern region of the United States of America, along the Canadian border. The state is bordered by Canada to the north, Minnesota to the east, South Dakota to the south and Montana to the west. North Dakota is the 19th-largest state by area in the U.S....
, and northwest Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...
.
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
Soil horizon
A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time...
lying either on a more or less flat or slightly, but noticeably depressed, clayey B horizon
Soil horizon
A soil horizon is a specific layer in the land area that is parallel to the soil surface and possesses physical characteristics which differ from the layers above and beneath. Horizon formation is a function of a range of geological, chemical, and biological processes and occurs over long time...
. 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
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
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
Pliocene
The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...
, and older coastal plain sediments. Rarely, pimple mounds that occur on these hillslopes are elongated in an upslope-downslope direction.
Structure
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. Bernard Hallet, a geologist at the University of Washington explains that this is unusual, as most land surfaces have a top layer of organic material, dead plants and animals, no more than half a meter (1.6 ft) thick, while Mima mounds are made of organic-rich soil more than 2 meters thick.Theories
Pocket Gophers
One major theory concerning the origin of Mima mounds argues that they were created by pocket gopherPocket gopher
The pocket gophers are burrowing rodents of the family Geomyidae. These are the "true" gophers, though several ground squirrels of the family Sciuridae are often called gophers as well...
s—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. The theory's author, George Cox, a retired zoologist at San Diego State University, says that a gopher family often can move up to 5 tons of earth a year. That's one-twentieth of the soil in an average Mima mound. Cox also asserts, most North American mounds are in gopher territory, and many gophers actually live in mounds. However, the only pocket gophers ever spotted on Mima Prairie built their burrows between the mounds, not in them . It is unclear whether gophers created the mounds or migrated to them because the mounds provided handy home sites.
Aeolian Origin
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 HoloceneHolocene
The Holocene is a geological epoch which began at the end of the Pleistocene and continues to the present. The Holocene is part of the Quaternary period. Its name comes from the Greek words and , meaning "entirely recent"...
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.
Seismic Activity
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 waveSeismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...
s 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.
Glaciation
A 2009 study using lidarLIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
data and published by researchers at Washington Department of Natural Resources
Washington Department of Natural Resources
The Washington Department of Natural Resources manage over of forest, range, agricultural, and commercial lands for the people of Washington State. DNR also manages of aquatic areas which include shorelines, tidelands, lands under Puget Sound and the coast, and navigable lakes and rivers...
showed that the retreat of the ice-age glaciers could account for the mima mounds' formation. A feature of this theory is that meltwater from the glaciers carried gravel to sun cups, where it was deposited and formed mounds after glaciers had fully retreated.
Vernal Pools
Vernal Pools are shallow depressions that fill with water during winter rains. Water accumulates below the soil surface and above a layer of hardpan or impermeable substance, creating a seasonally perched water table. Water is prevented from percolating underground, leaving evaporation as the only means of escape.The mima mounds located in the California Floristic Province (west of the Sierra Nevada and Cascade
Cascade Range
The Cascade Range is a major mountain range of western North America, extending from southern British Columbia through Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as the North Cascades, and the notable volcanoes known as the High Cascades...
ranges from Southern Oregon to Baja California. Coincidentally, mima mounds are located in Washington and Oregon State, west of the Cascade Range, and likewise throughout California west of the Sierra Nevada Range.
Shrinking and Swelling of Silts
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 capable of this type of 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, the composition allows water to swell and shrink in a similar fashion to clay, but on a smaller scale.
Washington
The phenomena can be seen at the Mima Mounds Natural Area Preserve, Capitol State ForestCapitol State Forest
The Capitol State Forest is a state forest in Thurston and Grays Harbor counties of the U.S. state of Washington. It includes part of the unusual Mima Mounds geologic feature.The Capitol State Forest is managed by Washington Department of Natural Resources...
in Thurston County, Washington, a designated National Natural Landmark
National Natural Landmark
The National Natural Landmark program recognizes and encourages the conservation of outstanding examples of the natural history of the United States. It is the only natural areas program of national scope that identifies and recognizes the best examples of biological and geological features in...
.
Literature
Mima mounds have been featured in the writings of Laird BarronLaird Barron
Laird Samuel Barron is an award winning author and poet, much of whose work falls within the horror, noir, and dark fantasy genres. He has also been the Managing Editor of the online literary magazine Melic Review. He lives in Olympia, Washington.-Biography:Mr...
, 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.
External links
- Mima Mounds and similar landforms Perigeezero
- Do Soils Need Our Protection? by Dr. Ron Amundson
- Mima Mounds preserve is fragile, mysterious, oregonlive.com
- Photos taken by The Seattle Times
- Mima Mounds of Thurston County: A Study of Evapotranspiration,Geologic History & Myths from The Evergreen State CollegeThe Evergreen State CollegeThe Evergreen State College is an accredited public liberal arts college and a member of the Council of Public Liberal Arts Colleges. It is located in Olympia, Washington, USA. Founded in 1967, Evergreen was formed to be an experimental and non-traditional college...
environmental analysis program