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 Patterns in nature are visible regularities of form found in the nature, natural world. These patterns recur in different contexts and can sometimes be modelled mathematically. Natural patterns include symmetries, trees, spirals, meanders, wav ...

. Mima mounds can be seen at the Mima Mounds Natural Area Preserve in

Washington state Washington, officially the State of Washington, is a state in the Pacific Northwest region of the United States. It is often referred to as Washington State to distinguish it from the national capital, both named after George Washington ...

. "Mima" is a name derived from a

Chinook Jargon Chinook Jargon (' or ', also known simply as ''Chinook'' or ''Jargon'') is a language originating as a pidgin language, pidgin trade language in the Pacific Northwest. It spread during the 19th century from the lower Columbia River, first to othe ...

term meaning "a little further along" or "downstream". Theories for the origin of Mima mounds include burrowing by

pocket gopher Pocket gophers, commonly referred to simply as gophers, are burrowing rodents of the family Geomyidae. The roughly 41 speciesSearch results for "Geomyidae" on thASM Mammal Diversity Database are all endemic to North and Central America. They ar ...

s; accumulation of wind-blown ( aeolian) sediments around vegetation to form coppice dunes or nebkhas;

seismic Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...

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 heuweltjies of the South Western Cape region of South Africa are of an origin far different from either.

Distribution

Pacific Northwest and southern Cascadia

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 In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of bed ...

hardpan In soil science, agriculture and gardening, hardpan or soil pan is 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 ...

, 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. They also are found in south central

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

, and in western and north central

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

, where they are typically known as "hogwallow mounds". Within this strip they are often a part of the landscape local to vernal pools.

Southwestern U.S. and northwestern Mexico

Mima mounds are found in northwest

Baja California Baja California, officially the Free and Sovereign State of Baja California, is a state in Mexico. It is the northwesternmost of the 32 federal entities of Mexico. Before becoming a state in 1952, the area was known as the North Territory of B ...

and adjacent

San Diego County San Diego County (), officially the County of San Diego, is a county in the southwest corner of the U.S. state of California, north to its border with Mexico. As of the 2020 census, the population was 3,298,634; it is the second-most populous ...

, where again they are an integral part of vernal pools' landscape. Mima mounds occur outside the western coastal North America in three major regions between the

Cascade Range The Cascade Range or Cascades is a major mountain range of western North America, extending from southern British Columbia through Washington (state), Washington and Oregon to Northern California. It includes both non-volcanic mountains, such as m ...

, the

Sierra Nevada The Sierra Nevada ( ) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primari ...

, and the

Sierra de Juárez The Sierra de Juárez, also known as the Sierra Juarez, is a mountain range located in Tecate Municipality and northern Ensenada Municipality, within the northern Baja California state of northwestern Mexico. It is a major mountain range in ...

in the west, and the

Mississippi River The Mississippi River is the main stem, primary river of the largest drainage basin in the United States. It is the second-longest river in the United States, behind only the Missouri River, Missouri. From its traditional source of Lake Ita ...

in the east. As described by Cox and by Washburn, these are: *Mima mounds are found in a roughly north-south strip of the Great Plains containing parts of north central

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

, and central

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

, and central

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

, where they are typically called "prairie mounds". *Mima mounds are found in an area containing parts of east

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

, western

Louisiana Louisiana ( ; ; ) is a state in the Deep South and South Central regions of the United States. It borders Texas to the west, Arkansas to the north, and Mississippi to the east. Of the 50 U.S. states, it ranks 31st in area and 25 ...

, southeast

Oklahoma Oklahoma ( ; Choctaw language, Choctaw: , ) is a landlocked U.S. state, state in the South Central United States, South Central region of the United States. It borders Texas to the south and west, Kansas to the north, Missouri to the northea ...

, southern

Missouri Missouri (''see #Etymology and pronunciation, pronunciation'') is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it border ...

, and most of

Arkansas Arkansas ( ) is a landlocked state in the West South Central region of the Southern United States. It borders Missouri to the north, Tennessee and Mississippi to the east, Louisiana to the south, Texas to the southwest, and Oklahoma ...

where they are commonly known as either "pimple mounds" or "prairie mounds". *Isolated patches of Mima mounds also are found in

Iowa Iowa ( ) is a U.S. state, state in the upper Midwestern United States, Midwestern region of the United States. It borders the Mississippi River to the east and the Missouri River and Big Sioux River to the west; Wisconsin to the northeast, Ill ...

, eastern

North Dakota North Dakota ( ) is a U.S. state in the Upper Midwest, named after the indigenous Dakota people, Dakota and Sioux peoples. It is bordered by the Canadian provinces of Saskatchewan and Manitoba to the north and by the U.S. states of Minneso ...

, and northwest

Minnesota Minnesota ( ) is a U.S. state, state in the Upper Midwestern region of the United States. It is bordered by the Canadian provinces of Manitoba and Ontario to the north and east and by the U.S. states of Wisconsin to the east, Iowa to the so ...

. 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 A and E horizons lying either on a more or less flat or slightly, but noticeably depressed, clay laden B 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 The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

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 The Early Pleistocene is an unofficial epoch (geology), sub-epoch in the international geologic timescale in chronostratigraphy, representing the earliest division of the Pleistocene Epoch within the ongoing Quaternary Period. It is currently esti ...

and

middle Pleistocene The Chibanian, more widely known as the Middle Pleistocene (its previous informal name), is an Age (geology), age in the international geologic timescale or a Stage (stratigraphy), stage in chronostratigraphy, being a division of the Pleistocen ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58

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.

Vernal pools

Vernal pool Vernal pools, also called vernal ponds or ephemeral pools, are seasonal pools of water that provide habitat for distinctive plants and animals. They are considered to be a distinctive type of wetland usually devoid of fish, and thus allow the saf ...

s 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 Silcrete is an indurated (resists crumbling or powdering) soil duricrust formed when surface soil, sand, and gravel are cemented by dissolved silica. The formation of silcrete is similar to that of calcrete, formed by calcium carbonate, and ferri ...

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.

Theories

Pocket gophers

One theory on the origin of Mima mounds is that they were created by small burrowing rodents such as

s ('' Thomomys talpoides'') of the endemic North American family '' Geomyidae''. Researchers in the 1940s found that Mima mounds tend to form in areas with poorly draining soils, so the "Fossorial Rodent Hypothesis" proposed that gophers build mounds as an evolutionary response to low water tables. It could be argued that gophers live in the mounds opportunistically but did not build them. Metal tracers implanted in a Mima mound field in San Diego demonstrated that gophers unexpectedly pushed soil upwards towards the center of the mounds instead of pushing the soil downward. 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 :(1) an initially flat surface can be specified, and :(2) time can be sped up. 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 loop Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop where the outcome of a process reinforces the inciting process to build momentum. As such, these forces can exacerbate the effects ...

s which amplify small features to create large scale patterns, a common facet of

self-organization Self-organization, also called spontaneous order in the social sciences, is a process where some form of overall order and disorder, order arises from local interactions between parts of an initially disordered system. The process can be spont ...

. 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. Radiocarbon ages down through a Mima Mound

Radiocarbon ages down through a Mima Mound

The publication of this modelling study received attention from the international press.

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 othersSeifert, 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. concluded that these mounds consisted of wind blown sediments that accumulated during prolonged late

Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

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, like vibrations that cause mounds on Chladni plates. 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 wave A seismic wave is a mechanical wave of acoustic energy that travels through the Earth or another planetary body. It can result from an earthquake (or generally, a quake), volcanic eruption, magma movement, a large landslide and a large ma ...

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. 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 The Carrizo Plain ( Obispeño: ''tšɨłkukunɨtš'', "Place of the rabbits") is a large enclosed grassland plain, approximately long and up to across, in southeastern San Luis Obispo County, California, United States, about northwest of Lo ...

(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.

Shrinking and swelling of clays

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. Shrink/swell soils are most often related to landforms called "hog wallows" or " gilgai" that can look similar to mima mounds.

Glaciers

Regina Johnson, with the Washington Department of Natural Resources (which oversees the Mima Mounds Natural Area Preserve), claimed that the mounds were formed by glacial processes. However, 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 began forming approximately 4600 years ago, long after the retreat of the glaciers. In addition, the ages demonstrated that the mound grew gradually over time through the accumulation of material, which supports the gopher hypothesis.

''Nature'' paper

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.

Washington

The phenomenon can be seen at the Mima Mounds Natural Area Preserve, a designated

National Natural Landmark The National Natural Landmarks (NNL) Program recognizes and encourages the conservation of outstanding examples of the natural history of the United States. It is the only national natural areas program that identifies and recognizes the best e ...

near Capitol State Forest in Thurston County, Washington. Mima mounds are also present at the Scatter Creek Unit and Rocky Prairie, located in southern Thurston County, Washington.

References

External links

{{Commons category
Mima Mounds: Mystery hides in vast prairie
Photos taken by The Seattle Times.
BBC article on the origins of mima mounds
Soil Patterns