Interior Plains

Between 2.0 and 1.8 billion years ago the Hearne-Rae, Superior, and Wyoming cratons were sutured together to form the North American craton, Laurentia, in an event called the Trans-Hudson Orogeny (THO).[2] This event was like the Indian plate colliding with the Eurasian plate, which formed the Himalayas. After initial collisions during the THO, tectonic activity at the edges of the four main cratons sparked mountain building. 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.[3] 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.[2]

This period has a large importance in Earth’s history as it saw the Cambrian explosion and Permian extinction. When global sea level rose and continents became partially submerged, the oceans had an explosion of complex life, which was the first time an event like this occurred on Earth. However, the center of Laurentia remained above sea level and as the continent moved east towards other supercontinents like Gondwana, the Appalachian Mountains began to form around 400 MYA.[4] 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.[5] 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.[6]

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.[7] 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.[8] 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.[9] Towards the end of this period, the inland seas began to drain due to uplift from the formation of the Rocky Mountains.[7]

The Laramide Orogeny event was when the western Cordillera was formed due to the flat-slab subduction of the Farallon Plate under the North American Plate. This created the frontal range of the Rockies from Montana through New Mexico. The outcrops seen at the surface of the Rockies are made up of sandstone, granite, and limestone; as well as metamorphic rocks uplifted from the Proterozoic Period. The interior plains have remained relatively flat during this period and recent sedimentation is from erosion of the newly formed Rocky Mountains as well as continued erosion from Appalachia. In general, Rocky Mountain sediment is deposited on the plains west of the Mississippi River, and Appalachian sediment is deposited to the east of the Mississippi River.[10]

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.[19] 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 100-150 million tons of sediment per year. The artificial structures trap suspended sediment from traveling as it would in an un-engineered river.[17]

While average annual temperatures vary significantly between the northern and southern portions of the Interior Plains, the climate is characterized by susceptibility to droughts due to generally low annual precipitation.[20]

Due to a warm climate and evapotranspiration rates surpassing precipitation rates,[20] the southern Interior Plains are highly susceptible to droughts and soil erosion. A significant feature of aeolian erosion in the Interior Plains is the ubiquitous loess deposits. The deposits were placed by winds during the Pleistocene epoch.[21] The Nebraska Sand Dunes are an example of the sand and loess during the epoch.[22] 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.[23]

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.[24] While droughts in the region were common,[20] during the following drought, aeolian soil erosion was exacerbated by the reduced soil-holding prairie grasses. Dust storms eroded hundreds of millions of tons of topsoil, causing dust storms for months in the historical region known as the Dust Bowl. On May 12, 1934 alone, an estimated 200 million tons of wind-eroded topsoil were transported to the Atlantic Ocean.[24]

In response to the rapid aeolian erosion, soil preservation methods were implemented. In the years following the Dust Bowl, 18,500 miles (29,800 km) of shelterbelt were planted by the Works Progress Administration to reduce wind intensity.[25]