Interior Plains

right|thumb|300px|The Interior Plains are highlighted in red.

The Interior Plains is a vast international region that spreads across the Laurentian craton of central North America. It extends along the east flank of the Rocky Mountains from the Gulf Coast region to the Arctic Beaufort Sea. In Canada, it encompasses the Canadian Prairies separating the Canadian Rockies from the Canadian Shield, as well as the Boreal Plains and Taiga Plains east of the Mackenzie and Richardson Mountains. In the United States, it includes the Great Plains of the West/Midwest and the tallgrass prairie region to the south of the Great Lakes extending east to the Appalachian Plateau region.

Geologic history

A series of tectonic plate collisions in the crust that formed the center of the North American continent laid the groundwork for the modern-day interior plains. Mountain building and erosion around the plains as well as flooding from inland seas provided sediments that make up the rock strata of the interior plains.

Proterozoic Eon (2500 to 539 million years ago)

Between 2.0 and 1.8 billion years ago the Hearne-Rae, Superior, and Wyoming cratons were sutured together to form the North American/Laurentia craton in an event called the Trans-Hudson Orogeny (THO). The tectonic activity that followed along the edges of the four main cratons sparked mountain building in those regions. The formation of the Himalayas is following the same process after the Indian Plate began colliding with the Eurasian Plate about 10 millions years ago.

The interior of Laurentia remained relatively flat and became a basin for eroded sediment from mountains at the beginning of the current time period, the Phanerozoic Eon. The only remaining outcrops from this orogeny in the interior plains are in the Black Hills of South Dakota. The sediments that formed the Black Hills were granite and different types of igneous rocks, which make up the basement of bedrock in central North America. However, much of the Black Hills sediment has been metamorphosed and deformed, so it is uncertain what the conditions were like at the time of their formation. This coincided with the formation of Pangea around 300 MYA, when the Appalachians were at their peak height. The central plains of Laurentia were subjected to deposition of eroded sediment from these mountains. The oldest sediments from this period are felsic igneous rocks and granite that have since been metamorphosed, while the younger sediments are made up of sandstone, shale, limestone, and coal. Sediments deposited in the interior plains from this era are currently buried deep beneath the surface where they are difficult to study.

Mesozoic Era (252 to 66 million years ago)

Around 220 MYA, the supercontinent Pangea broke apart, and the North American continent began to move west and isolate itself. For much of this period the interior plains were covered by inland seas. During the Jurassic period, the Sundance Sea formed along the western coast of the North American continent and extended from northern Canada to the interior plains, covering parts of Wyoming, Montana, North Dakota, and South Dakota. Coquina and sandstone layers from marine deposition were deposited on top of rock layers from the Paleozoic Era. During the Cretaceous period, another inland sea called the Western Interior Seaway was formed. This body of water extended from present-day Alaska to the Gulf of Mexico and covered almost all the interior plains west of the current boundary of the Mississippi River. Limestone-shale couplets, as well as carbonate layers, are commonly found in sedimentary deposits from this inland sea. Towards the end of this period, the inland seas began to drain due to uplift from the formation of the Rocky Mountains.

Glacial history

2.6 million years ago at the start of the Pleistocene Epoch, the Laurentide Ice Sheet began to spread southwards to cover North America down to the northern Great Plains on the western side of the Interior Plains and down into most of Minnesota and Wisconsin. The Laurentide Ice Sheet had a large influence on the morphology of the Interior Plains during the end of the Pleistocene. During the retreat, the Laurentide scoured numerous pockets of sediment. Upon the plate's melting, those pockets were filled, resulting in the kettle lakes. The Great Lakes and both Great Slave Lake and Great Bear Lake of Canada were formed by the Laurentide. During the retreat, the Laurentide gouged and filled the glacial paleolake McConnell in northern Canada. As the region uplifted and rebounded isostatically from the mass of the ice sheet, paleolake McConnell was split into Great Slave Lake and Great Bear Lake. Great Slave Lake's basin formed under the 4-kilometer thick Keewatin Dome that today is the deepest lake in North America. A vast amount of smaller lakes were formed as well and serve an integral part of ethos in the surrounding regions. For example, Minnesota is often referred to as “the Land of 10,000 Lakes” due to the number and widespread recreational use of the state's lakes.

Much of the loess distributed within the Interior Plains has its origin in glaciers. In glaciated conditions, sand and silt-laden meltwater originating from alpine glaciers in the Rocky Mountains generated alluvial deposits at their base. This alluvium was then distributed throughout the Interior Plains by strong winds. Due to climate change, the average temperature of the Interior Plains is increasing and the region is becoming more arid. Because of the increase in rainstorm intensity, rain-driven erosion will grow as a factor of soil erosion in the Interior Plains.

Fluvial processes

Civil engineering projects have altered the fluvial geomorphology of the Interior Plains. Normal sediment transport by river and channel systems is interrupted by river-blocking structures such as dams and flow regulators. Before 1900, the estimated annual sediment transport by the Mississippi River to the Gulf of Mexico was 400 million tons. However, in the early 20th century, engineering projects including dams were created on the Missouri River, meander cutoffs, river training, bank revetments, and soil erosion control have reduced the annual transport rate to between 100 and 150 million tons of sediment per year. The artificial structures trap suspended sediment from traveling as it would in an un-engineered river.

Due to a warm climate and evapotranspiration rates surpassing precipitation rates, The Nebraska Sand Dunes are an example of the sand and loess during the epoch. These dunes were formed during the Pleistocene by Northwesterly winds depositing alluvial silt and sand. That loess is so prevalent in the Interior Plains is evidence of significant aeolian erosion, as deposits are generally accumulations of wind-blown dust.

thumb|right|Loess Hills in western Iowa along I-80.

Following World War I, wheat farming in the fertile loess soil of the Interior Plains swelled. The expansion of farmland eliminated many prairies containing soil-stabilizing grasses. While droughts in the region were common,

Current land use

Grassland and shrubland make up the largest portion of the Interior Plains within the United States, at 44.4 percent. Land used for cattle-grazing is included under this classification, which sustains nearly 50 percent of all United States beef cattle. By far, wheat comprises the largest portion of the agricultural yield in region; combined, wheat exports from the Interior Plains make up more than half of the world's exports. Other significant crops produced in the region include barley, corn, cotton, sorghum, soybeans, and canola, which is particularly important to Canadian exports.

Interior Plains in Canada

The Interior Plains of Canada are one of seven physiographic areas included in the highest level classification in that country. That country calls this primary classification level "region." For some of the seven regions, a subregion schema is provided. For other physiographic regions (such as the Interior Plains and Appalachian Uplands) subregions are not developed, but the tertiary level (called "division" in Canada) is used in the mapping data.

The following list is of the 14 physiographic divisions in the Interior Plains of Canada. Further information can be found at https://atlas.gc.ca/phys/en/index.html

Alberta Plain
Alberta Plateau
Anderson Plain
Colville Hills
Cypress Hills
Fort Nelson Lowland
Great Bear Plain
Great Slave Plain
Horton Plain
Manitoba Plain
Peace River Lowland
Peel Plain
Peel Plateau
Saskatchewan Plain

Interior Plains in the United States

The following is a breakdown of the secondary (provinces), and tertiary (sections) physiographic areas of the Interior Plains portion in the United States:

Central Lowland

Dissected Till Plains
Eastern Lake
Osage Plains
Till Plains
Western Lake
Wisconsin Driftless

Great Plains

Black Hills
Central Texas
Colorado Piedmont
Edwards Plateau
High Plains
Missouri Plateau, Glaciated
Missouri Plateau, Unglaciated
Pecos Valley
Plains Border
Raton