Prairie

Natural history of the prairie

The post-settlement prairie

Resources

A prairie is a natural vegetation type in which perennial herbaceous plants predominate, particularly species of grasses. The word “prairie” comes from the French pre´rie (later, prairie), meaning meadow. The term was first applied to the swath of mid-continental North American grassland in the 1600s by French Jesuit missionaries and explorers, because the landscape resembled, on a much vaster scale, the familiar agricultural meadows of western Europe. Thus, geography and nomenclature came together to distinguish the North American prairie from similar grasslands elsewhere in the world: the steppes of central Asia, the pampas of South America, and the veldt of southern Africa.

Until the settlement era, the central prairie of North America stretched from southern Alberta, Saskatchewan, and Manitoba south to mid-Texas, and from the foothills of the Rocky Mountains eastward into Indiana. It covered about 1.4 million sq mi (3.6 million sq km). Outlying patches occurred in Ohio, Michigan, Kentucky, and southwestern Ontario. A similar vegetation type went under the names of “plains” or “downs” in the northeastern United States.

The general trend toward increasing rainfall and increasingly rich soil from west to east in mid-continental North America gave rise to a descriptive classification of the prairie. Its western edge, on the high plains, became known as shortgrass prairie, because shorter grasses grew on its generally poorer and drier soils. A transitional zone running north to south along the ninety-eighth meridian, through Alberta, Saskatchewan, the Dakotas, Nebraska, Kansas, and Oklahoma, became known as mixed-grass prairie. The richest, eastern sector, which bulged eastward from the ninety-eighth meridian through Illinois and into northwestern Indiana, became known as the tallgrass or “true” prairie. This scheme gradually evolved into the one used by modern biologists to classify prairies, which takes into account soil, bedrock, and vegetation types and has many divisions. The tallgrass prairie is the major subject of this article.

A native prairie is sprinkled with brilliantly colored flowers of broadleafed (or dicotyledonous) plants that often exceed the height of the grasses. Some prairie grasses attain a height of 6.6 ft (2 m), and sometimes more, if soil and moisture conditions are favorable. Early settlers’ descriptions of grasses taller than a person on horseback were probably exaggerated and reflected a tradition of romanticizing the landscape. Intermixed with the predominant grasses are broad-leaved plants called forbs, which lend color and diversity to the vegetation. Besides the grasses (family Poaceae), such as little and big bluestem and Indian grass, common prairie plants are species of legumes (Leguminosae), or flowering peas and clovers, and composites (Asteraceae), such as sunflowers, goldenrods, black-eyed susan, asters, and coneflowers.

Natural history of the prairie

Most of the prairie has developed since the most recent Ice Age, as determined from the dating of fossilized pollen grains to about 8,300 years ago. The retreating glaciers left a central strip of flat or slightly depressed topography overlying clay soil, or in the western states, rocky dolomite shelves. Climate, weather, soil, and topography then created the initial conditions for the prairie to develop. The central prairie is subject to the stresses of extreme changes in temperature over the course of a year, drought, occasional accumulation of standing water just below the ground surface, and drying westerly winds from the Rocky Mountains. That situation favored the growth of plants with hardy root systems and underground growing points, but whose aerial (or aboveground) parts could die back each year. Perennial grasses and low, hardy shrubs could survive in such a climate; unprotected trees could not. It is thought that the post-Ice Age climate set the stage for the development of the prairie, with soil types and frequent fires then favoring the growth of grasses and forbs.

Fire does not start a prairie, but it is a crucial factor in maintaining it. The pre-settlement fires were landscape-wide and moved rapidly, driven by westerly winds that traveled unimpeded across the plains. The aerial parts of prairie plants burn, but the roots, which in perennial grasses form a deep, thick tangle underground, do not. The fast-moving fires also consume litter, the dried stalks and plant remains that had died in previous seasons and fallen to the ground. Removal of litter gave the next season’s growth greater access to air and sunlight. The burns also killed shrubs and trees, which might otherwise have invaded the prairie and displaced its species. Some prairie fires were started by lightning; others were set by Native Americans, who saw the advantage to their horses and to the bison herds they hunted of having fresh vegetation to eat.

Bison, the primary grazers on the prairie, contributed to upkeep of the ecosystem by consuming young shoots of trees and shrubs along with their main food of grasses and forbs. Although they were massive animals, their wide-ranging habit ensured they would not

remain in one spot to churn up and destroy the roots of prairie grasses, as fenced-in cattle would later do.

Climate, bison, and fire, maintained the dynamic boundary between prairie and forest. The prairie was not devoid of trees, however. Cottonwoods, green ash, and box elder grew as a riparian community along riverbanks, and long fingers of forest extended into the prairie, often bounded on their western edges by a watercourse that served as a natural firebreak. During periods without fire, plum trees and crabapple could take hold at the edges of the prairie. Copses of trees and patches of flowers interrupted the “seas of grass” and gave an overall more mosaic appearance to the prairie.

The post-settlement prairie

For a millennia, the North American prairie (bordered on the north, east, and south by forest) existed as a complex ecosystem that supported rich life, including aboriginal human cultures. Within the span of a human lifetime, however, it was almost entirely eradicated by conversion into agricultural land-use.

The early settlers, reliant on forests for building materials, firewood, fencing, and hand-crafted implements, initially distrusted a land on which few or no trees grew. That changed with the discovery that the tallgrass prairie could be converted into some of the richest cropland on the continent. Vast acreages went under the plow; other areas were overgrazed by domestic livestock. The assault on the central prairie began in earnest in the 1820s and was sped up by the opening of the Erie Canal, in 1825. The development of steamship routes on the Great Lakes and the westward expansion of the railroad system, in the 1850s, also facilitated large, westward population movements. By the beginning of the Civil War, most of the tallgrass prairie had been put to the plow. The widespread availability of barbed-wire fencing by 1890 released ranchers and farmers from their greatest dependency on wood and marked the final domestication of the prairie.

In the pre-settlement period, almost 60% of Illinois, then nicknamed the Prairie State, was covered by tallgrass prairie. In the post-settlement era, however, only about 0.01% of the original prairie was left. Prairie originally covered 85% of Iowa; in the post-settlement period 0.02% remained. The western states, with an overall drier climate and soils less suitable for agriculture, fared somewhat better, but no state retained more than a small fraction of its original prairie.

KEY TERMS

Forb— A perennial, herbaceous, broad-leafed (or dicotyledonous) plant.

Grass— Any member of the family Poaceae, characterized by long narrow leaves with parallel venation and reduced flowers; usually found on seasonally dry, flat lands. The cereal grains are grasses (barley, corn, oats, rice, rye, wheat).

Island habitat— A small area of ecosystem, surrounded by a different kind of ecosystem. The species in the island habitat cannot live in or penetrate the surrounding environment.

Relic prairie— A remnant of prairie, usually small, that has never been plowed or overgrazed; virgin prairie.

Most prairie today represents “island” habitat, existing in isolated patches rather than as a continuous extent of natural vegetation. Island communities are more vulnerable to natural and human-caused disturbances, and experience a higher rate of species disappearance than non-island ecosystems. Typical islands of native prairie, called relics, include small cemeteries that coincidentally preserved the prairie; small preserves in arboreta and demonstration projects; and areas such as railroad embankments in cities where development was restricted or the land was considered unsuitable for building on. About 30% of the remaining prairie in Illinois exists in tiny islands of less than one acre.

The loss of the prairie was part of a broader economic movement that involved both industrialization and the development of commercial agriculture. The economic development of the former prairie states resulted in the almost total eradication of a large unit of natural vegetation. Efforts are under way to restore large tracts of reconstructed prairie that might support relatively small numbers of breeding bison. Seeding with native plants and the use of controlled burns are crucial parts of the management system being used to achieve this ecological restoration. However, the formerly extensive tallgrass prairie will never be totally recovered, because its essential land-base is needed to provide food and livelihoods for an ever-increasing population of humans.

Resources

BOOKS

Encyclopedia of the Great Plains, edited by David J. Wishart. Lincoln, NE: University of Nebraska Press, 2004.

Moul, Francis. The National Grasslands: A Guide to America’s Undiscovered Treasures. Lincoln, NE: Bison Books, 2006.

Savage, Candace. Prairie: A Natural History. Vancouver, B.C.: Greystone Press, 2004.

Stuckey, Ronald L. “Origin and Development of the Concept of the Prairie Peninsula.” In The Prairie Peninsula—In the “Shadow” of Transeau: Proceedings of the Sixth North American Prairie Conference Columbus, OH: Ohio State University, 1981.

Whitney, Gordon G. From Coastal Wilderness to Fruited Plain: A History of Environmental Change in Temperate North America 1500 to the Present. Cambridge, U.K.: Cambridge University Press, 1994.

Marjorie Pannell

Prairie

A prairie is a natural vegetation type in which perennial herbaceous plants predominate, particularly species of grasses . The word "prairie" comes from the French prérie (later, prairie), meaning meadow. The term was first applied to the swath of mid-continental North American grassland in the 1600s by French Jesuit missionaries and explorers, because the landscape resembled, on a much vaster scale, the familiar agricultural meadows of western Europe . Thus, geography and nomenclature came together to distinguish the North American prairie from similar grasslands elsewhere in the world: the steppes of central Asia , the pampas of South America , and the veldt of southern Africa .

Until the settlement era, the central prairie of North America stretched from southern Alberta, Saskatchewan, and Manitoba south to mid-Texas, and from the foothills of the Rocky Mountains eastward into Indiana. It covered about 1.4 million sq mi (3.6 million sq km). Outlying patches occurred in Ohio, Michigan, Kentucky, and southwestern Ontario. A similar vegetation type went under the names of "plains" or "downs" in the northeastern United States.

The general trend toward increasing rainfall and increasingly rich soil from west to east in mid-continental North America gave rise to a descriptive classification of the prairie. Its western edge, on the high plains, became known as shortgrass prairie, because shorter grasses grew on its generally poorer and drier soils. A transitional zone running north to south along the ninety-eighth meridian, through Alberta, Saskatchewan, the Dakotas, Nebraska, Kansas, and Oklahoma, became known as mixed-grass prairie. The richest, eastern sector, which bulged eastward from the ninety-eighth meridian through Illinois and into northwestern Indiana, became known as the tallgrass or "true" prairie. This scheme gradually evolved into the one used by modern biologists to classify prairies, which takes into account soil, bedrock , and vegetation types and has many divisions. The tallgrass prairie is the major subject of this article.

A native prairie is sprinkled with brilliantly colored flowers of broadleafed (or dicotyledonous) plants that often exceed the height of the grasses. Some prairie grasses attain a height of 6.6 ft (2 m), and sometimes more, if soil and moisture conditions are favorable. Early settlers' descriptions of grasses taller than a person on horseback were probably exaggerated and reflected a tradition of romanticizing the landscape. Intermixed with the predominant grasses are broad-leaved plants called forbs, which lend color and diversity to the vegetation. Besides the grasses (family Poaceae), such as little and big bluestem and Indian grass, common prairie plants are species of legumes (Leguminosae), or flowering peas and clovers, and composites (Asteraceae), such as sunflowers, goldenrods, black-eyed susan, asters, and coneflowers.

Natural history of the prairie

Most of the prairie has developed since the most recent Ice Age, as determined from the dating of fossilized pollen grains to about 8,300 years ago. The retreating glaciers left a central strip of flat or slightly depressed topography overlying clay soil, or in the western states, rocky dolomite shelves. Climate, weather , soil, and topography then created the initial conditions for the prairie to develop. The central prairie is subject to the stresses of extreme changes in temperature over the course of a year, drought , occasional accumulation of standing water just below the ground surface, and drying westerly winds from the Rocky Mountains. That situation favored the growth of plants with hardy root systems and underground growing points, but whose aerial (or aboveground) parts could die back each year. Perennial grasses and low, hardy shrubs could survive in such a climate; unprotected trees could not. It is thought that the post-Ice Age climate set the stage for the development of the prairie, with soil types and frequent fires then favoring the growth of grasses and forbs.

Fire does not start a prairie, but it is a crucial factor in maintaining it. The pre-settlement fires were landscape-wide and moved rapidly, driven by westerly winds that traveled unimpeded across the plains. The aerial parts of prairie plants burn, but the roots, which in perennial grasses form a deep, thick tangle underground, do not. The fast-moving fires also consume litter, the dried stalks and plant remains that had died in previous seasons and fallen to the ground. Removal of litter gave the next season's growth greater access to air and sunlight. The burns also killed shrubs and trees, which might otherwise have invaded the prairie and displaced its species. Some prairie fires were started by lightning ; others were set by Native Americans, who saw the advantage to their horses and to the bison herds they hunted of having fresh vegetation to eat.

Bison, the primary grazers on the prairie, contributed to upkeep of the ecosystem by consuming young shoots of trees and shrubs along with their main food of grasses and forbs. Although they were massive animals, their wide-ranging habit ensured they would not remain in one spot to churn up and destroy the roots of prairie grasses, as fenced-in cattle would later do.

Climate, bison, and fire, maintained the dynamic boundary between prairie and forest. The prairie was not devoid of trees, however. Cottonwoods, green ash, and box elder grew as a riparian community along riverbanks, and long fingers of forest extended into the prairie, often bounded on their western edges by a watercourse that served as a natural firebreak. During periods without fire, plum trees and crabapple could take hold at the edges of the prairie. Copses of trees and patches of flowers interrupted the "seas of grass" and gave an overall more mosaic appearance to the prairie.

The post-settlement prairie

For a millennia, the North American prairie (bordered on the north, east, and south by forest) existed as a complex ecosystem that supported rich life, including aboriginal human cultures. Within the span of a human lifetime, however, it was almost entirely eradicated by conversion into agricultural land-use.

The early settlers, reliant on forests for building materials, firewood, fencing, and hand-crafted implements, initially distrusted a land on which few or no trees grew. That changed with the discovery that the tallgrass prairie could be converted into some of the richest cropland on the continent . Vast acreages went under the plow; other areas were overgrazed by domestic livestock . The assault on the central prairie began in earnest in the 1820s and was sped up by the opening of the Erie Canal, in 1825. The development of steamship routes on the Great Lakes and the westward expansion of the railroad system, in the 1850s, also facilitated large, westward population movements. By the beginning of the Civil War, most of the tallgrass prairie had been put to the plow. The widespread availability of barbed-wire fencing by 1890 released ranchers and farmers from their greatest dependency on wood and marked the final domestication of the prairie.

In the pre-settlement period, almost 60% of Illinois, then nicknamed the Prairie State, was covered by tallgrass prairie. In the post-settlement era, however, only about 0.01% of the original prairie was left. Prairie originally covered 85% of Iowa; in the post-settlement period 0.02% remained. The western states, with an overall drier climate and soils less suitable for agriculture, fared somewhat better, but no state retained more than a small fraction of its original prairie.

Most prairie today represents "island" habitat , existing in isolated patches rather than as a continuous extent of natural vegetation. Island communities are more vulnerable to natural and human-caused disturbances, and experience a higher rate of species disappearance than non-island ecosystems. Typical islands of native prairie, called relics, include small cemeteries that coincidentally preserved the prairie; small preserves in arboreta and demonstration projects; and areas such as railroad embankments in cities where development was restricted or the land was considered unsuitable for building on. About 30% of the remaining prairie in Illinois exists in tiny islands of less than one acre.

The loss of the prairie was part of a broader economic movement that involved both industrialization and the development of commercial agriculture. The economic development of the former prairie states resulted in the almost total eradication of a large unit of natural vegetation. Efforts are under way to restore large tracts of reconstructed prairie that might support relatively small numbers of breeding bison. Seeding with native plants and the use of controlled burns are crucial parts of the management system being used to achieve this ecological restoration. However, the formerly extensive tallgrass prairie will never be totally recovered, because its essential land-base is needed to provide food and livelihoods for an ever-increasing population of humans.

Resources

books

Coupland, Robert T., ed. Natural Grasslands: Introduction andWestern Hemisphere. Ecosystems of the World 8A. New York: Elsevier, 1992.

Madson, John. Tall Grass Prairie. Helena, MT: Falcon Press, 1993.

Smith, Daryl D. "Tallgrass Prairie Settlement: Prelude to the Demise of the Tallgrass Ecosystem." In Recapturing a Vanishing Heritage. Proceedings of the Twelfth North American Prairie Conference, edited by Daryl D. Smith and Carol A. Jacobs. Cedar Falls: University of Northern Iowa, 1992.

Stuckey, Ronald L. "Origin and Development of the Concept of the Prairie Peninsula." In The Prairie Peninsula—In the
"Shadow" of Transeau. Proceedings of the Sixth North American Prairie Conference Columbus: Ohio State University, 1981.

Whitney, Gordon G. From Coastal Wilderness to FruitedPlain: A History of Environmental Change in Temperate North America 1500 to the Present. Cambridge, England: Cambridge University Press, 1994.

Marjorie Pannell

KEY TERMS

Forb

—A perennial, herbaceous, broad-leafed (or dicotyledonous) plant.

Grass

—Any member of the family Poaceae, characterized by long narrow leaves with parallel venation and reduced flowers; usually found on seasonally dry, flat lands. The cereal grains are grasses (barley, corn, oats, rice, rye, wheat).

Island habitat

—A small area of ecosystem, surrounded by a different kind of ecosystem. The species in the island habitat cannot live in or penetrate the surrounding environment.

Relic prairie

—A remnant of prairie, usually small, that has never been plowed or overgrazed; virgin prairie.

Prairie

An extensive temperate grassland with flat or rolling terrain and moderate to low precipitation. The term is most often applied to North American grasslands , which once extended from Alberta to Texas and from Illinois to Colorado, but similar grasslands exist around the world. Perennial bunchgrasses and forbes dominate prairie flora . In dry climate prairies, a lack of precipitation prohibits tree growth except in isolated patches, such as along stream banks. In wetter prairies, periodic fires are essential in preventing the incursion of trees and preserving open grasslands. Historically, huge herds of large mammals and scores of bird, rodent, and insect species composed native prairie fauna . During the nineteenth and twentieth centuries most of North America's native prairie habitat and its occupants disappeared in the face of agricultural settlement, cattle grazing, and fire suppression. Today the geographic extent of native prairie plants and animals is severely restricted, but pockets of remaining prairie are sometimes preserved for their ecological interest and genetic diversity.

While grasses make up the dominant vegetation type in all prairies, grass sizes and species vary from shortgrass prairies, usually less than 20 inches (50 cm) high, to tallgrass prairies, whose grasses can exceed 7 ft (2 m) in height. Following the gradual east-west decrease in precipitation rates on the Great Plains, eastern tallgrass prairies gradually gave way to midgrass and then shortgrass prairies in western states. On their margins, prairie grasslands extended into a mixed "parkland," or "savanna," with bushes and scattered trees such as oak and juniper. Characteristic prairie grasses are perennial bunchgrasses, which have deep, extensive root systems and up to a hundred shoots from a single plant. Bunchgrasses typically grow, sprout, and flower early in the summer when rainfall is plentiful in the prairies, and their deep root systems make the long-lived plants resistant to drought , hail, grazing, and fire. In addition to the dominant grasses, vast numbers of flowering annual and perennial forbes—herbaceous flowering plants that are not grasses—add color to prairie landscapes.

Prairie plant species are well adapted to live with fire. Under natural conditions fires sweep across prairies every few years. Prairie fires burn intensely and quickly, fed by dry grass and prairie winds, but they burn mainly the dry stalks of previous years' growth. Below the fire's heat, the roots of prairie plants remain unharmed, and new stalks grow quickly, fertilized by ashes and unencumbered by accumulations of old litter. Before European settlement, many prairie fires were ignited by lightening, but some were started by Indians, who had an interest in maintaining good grazing land for bison , antelope, and other game animals.

Well into the nineteenth century, prairie habitats supported a tremendous variety of animal species. Vast herds of bison, pronghorn antelope, deer, and elk ranged the grasslands; smaller mammals from mice and prairie dogs to badgers and fox lived in or below the prairie grasses. Carnivores including grey wolves , coyotes, cougar, grizzlies, hawks, and owls fed on large and small herbivores. Shorebirds and migratory waterfowl thrived on millions of prairie pothole ponds and marshes. Most of these species saw their habitats severely diminished with the expansion of agricultural settlement and cattle grazing range. Prairie soils, mainly mollisols, are rich and black with a thick, fertile organic layer composed of fine roots, microorganisms , and soil . Where water was available, this soil proved ideal for growing corn, wheat, soybeans, and other annual crops. Where water was scarce, nutritious bunchgrasses provided excellent grazing for cattle, which quickly replaced buffalo, elk, and antelope on the prairies in the middle of the nineteenth century.

Prairie habitat and animal species have also lost ground through fire suppression and wetland drainage . Farmers and ranchers have actively suppressed fires, allowing trees, exotic annuals, and other non-prairie species to become established. Widespread wetlands drainage has seriously diminished bird populations because of habitat loss. Especially in wetter northern and eastern regions, canals were cut into prairie wetlands, draining them to make arable fields. This practice continues today, even though it destroys scarce breeding grounds for many birds and essential migratory stopovers for others.

In recent decades increased attention has been given to preserving the few remaining patches of native prairie in North America. Public and private organizations now study and protect prairie grasslands and wetlands, but the long-term value of these efforts may be questionable because most remnant patches are small and widely separated. Whether genetic diversity can be maintained, and whether habitat for large or ranging animals can be reestablished under these conditions, is yet to be seen.

[Mary Ann Cunningham Ph.D. ]

RESOURCES

BOOKS

Cushman, R. C., and S. R. Jones The Shortgrass Prairie. Boulder, CO: Pruett Publishing Co., 1988.

Smith, R. L. Ecology and Field Biology. 3rd ed. New York: Harper and Row, 1980.

Whittaker, R. H. Communities and Ecosystems. 2d ed. New York: Macmillan, 1975.

PRAIRIE

The prairie consists of the flat or moderately hilly lands of the nation's middle section, also called the Great Plains. Oklahoma, Kansas, Nebraska, South Dakota, and North Dakota are mostly prairie. Midwestern states of Illinois and Iowa also contain some prairie lands. Primarily covered with tall grasses (which the pioneers described as a sea of grass), the prairie receives low to moderate rainfall each year. Summers in prairie regions are generally very hot and winters harshly cold. These climatic conditions combined to delay settlement of the region. Though ranchers found grasses suitable for grazing livestock, inadequate rainfall did and does make farming difficult. Further, because the region lacked trees, building on the prairie was limited.

Passage of the Homestead Act in 1862 granted settlers up to 160 acres (64 hectares) of frontier land in exchange for building on or cultivating prairie land. Thus many farmers moved their families onto the Great Plains. In the 1870s Russian Mennonites, who introduced a variety of winter wheat, settled Kansas.

A common crop used by early settlers was turkey red wheat, which could be planted in the fall and harvested in the early summer. The crop, which can withstand cold temperatures, received the benefit of the moisture caused by spring snowmelt and was harvested before the scorching summer. Cultivation of the grain spread, and in 1894 wheat became Kansas' principal crop, earning the state the nickname "Bread-basket of America."

In other parts of the Great Plains farmers adopted dry farming techniques. (A technique by which some fields are allowed to lie fallow every other season so that soil can store up enough moisture and nutrients to support the next year's crops.) Wheat was found to be well suited to dry farming, but it was not used exclusively. Some farmers also grew corn. The end of the nineteenth century saw the invention of the steel plow and the improvement of the McCormick reaper; subsequently, these machines were a great boon to farm production.

The introduction of winter wheat, the development of dry farming methods, and innovations in agricultural machinery combined to make possible the settlement of the American prairie. As a result, between 1863 and 1900 about half a million families became homesteaders in the West. Most of them settled on the Plains, which became one of the world's leading wheat-producing regions by the early 1900s.

See also: Dry Farming, Homestead Act, Homesteaders, Westward Expansion

Prairie

The term prairie is an ecological term used to describe a geologic plain covered by mostly grass. Prairies have been subdivided into smaller, more specific categories by the type of vegetation they support. Short grass and long grass prairies historically covered most of the central portion of the United States. However, the grasses have been replaced by urbanization and agriculture, but the plain still exists.

The Great Plains of the United States support one of the most famous prairies in the world. As with all prairies, the area is supported underneath by a firm bedrock . In this case, the bedrock is composed of limestone deposited by a relatively continuous series of ancient seas that advanced and retreated across America and Canada for millions of years. Dolomite , containing high levels of magnesium, is the primary building block of the bedforms .

Overlying the bedrock are massive and extensive fossil coral reefs . These ancient reefs are quite impressive. They began to form in the warm shallow seas after the Silurian about 400 million years ago. Growth was intermittent as the seas transgressed (grew) and regressed (receded) in a cyclic pattern.

During the last 1.9 million years, the entire upper North American continent was covered with ice . The ice sheets grew and shrank according to global climate fluctuations. The grinding of the massive ice sheets produced a fine sediment called glacial till . The meltwaters of the glaciers moved the till away from the sheets and out onto the dolomitic plain in a process known as glacial drift. The drift formed many distinctive structures including eskers, moraines , and kettles. The sequences of advancing ice are recorded in the layers of the sediments. What was once considered a distinct pattern of a few ice advances is now understood to be a complicated chronology of at least 29 different episodes. The last identifiable age of ice deposition is called the Wisconsin glaciation . This event stripped much of the prairie of high physiographic features while depositing the characteristic soils of the current prairie.

The soils that are high in carbonates are not very rich for growing vegetation. Trees are exposed to extremes of temperatures and varying precipitation . They do not fare as well as the hardier grasses. Consequently, the evolution of grasses has been intimately tied with the development of the prairie. The prairies have been threatened by the introduction of foreign grass types. Some areas of the prairie have been set aside as national grasslands and non-native grasses are sought out and removed. The majesty of the prairie still exists in these historic places.

See also Glacial landforms; Ice ages; Marine transgression and marine regression; Soil and soil horizons