Region 2: The Great Plains

The Great Plains, a broad plateau that is home to intermediate and short grasslands, stretches for 3200 kilometers (2000 miles) from the Canadian interior south to the Mexican border. Its 800-kilometer (500-mile) wide expanse is sandwiched between the Central Lowland and the Canadian Shield to the east and the Rocky Mountains to the west.

Conditions in the Great Plains become increasingly drier as one travels from east to west. Highly fertile Mollisols with a thick, black top horizon are found in the region’s eastern extent. These soils allow for the greatest productivity and are often associated with intensive agricultural operations. As one moves westward, decreasing moisture and vegetation impacts soil development, making soils thinner and less productive, which naturally produces shorter grasses (Figure 8.15). The central Great Plains are dominated by dry Mollisols belonging to the suborder Ustolls, which form in semi-arid conditions. These soils can become even more dusty and dry during drought conditions (Figure 8.16), limiting crop yields and leading to damaging dust storms such as those that occurred during the Dust Bowl of the 1930s. In the western Dakotas, western Nebraska, eastern Montana, and eastern Wyoming, the decreased precipitation and lower soil fertility provides for a localized agricultural economy based heavily in rangeland livestock—these states are leaders in the production of beef cattle and sheep. Crops here often require irrigation from local aquifers or various surface water impoundments.

Figure 8.15: Seemingly endless stretches of rolling short and intermediate grasses dominate the drier soils of the Great Plains.

Figure 8.15: Seemingly endless stretches of rolling short and intermediate grasses dominate the drier soils of the Great Plains.

Figure 8.16: Dust rises from dry Ultisols in the Great Plains’ Prairie Pothole Region.

Figure 8.16: Dust rises from dry Ultisols in the Great Plains’ Prairie Pothole Region.

In many western areas of the Great Plains, the soils are heavily influenced by existing sedimentary rock material lain down during the uplift of the Rockies and the deposition of Mesozoic sediments. The erosion of exposed Cretaceous marine shales produces Vertisols, soils that experience drastic fluctuations in volume when exposed to water (Figure 8.17). Locals refer to such soil as “gumbo” and consider it to be unworkable and impassable when wet. It is also a major engineering concern for structures involving roads and buildings due to its predisposition to shifting and developing creep or slow mass movement of earth. This same shifting ability discourages the formation of any distinct horizons. Clayey Vertisols are highly alkaline and water restrictive, inhibiting crop yields and forcing most agricultural usage into rangeland grazing.

Figure 8.17: Cracked Vertisols in central Montana.

Figure 8.17: Cracked Vertisols in central Montana.

Alfisols are scattered throughout Montana as well as concentrated in the Black Hills of South Dakota. These soils generally form in forested areas as a result of weathering processes that leach minerals from the surface layer into the subsoil, where nutrients are retained. The Black Hills’ unique geology makes them a forested oasis amidst a sea of grassland—and perfectly suited to the development of Alfisols.

See Chapter 2: Rocks to learn more about Black Hills geology.

Aridisols are present throughout Wyoming on the western edge of the Great Plains, approaching the Rocky Mountains. These soils, which have no viable agricultural use, occur where the ground remains dry throughout most of the year due to limited precipitation. Consequently, Aridisols show very little evidence of leaching, and they contain abundant accumulations of clay. The Powder River Basin in eastern Wyoming, which receives 2351 centi-meters (920 inches) of precipitation each year, is one example of an arid expanse dominated by Aridisols.

Entisols, young and unstable soils lacking in horizons, are found where erosion and deposition occur faster than the rate of soil formation. Both the soils that overlay loess structures in eastern Nebraska and the rapid erosional surfaces of the Badlands of South Dakota, North Dakota, Montana, and Wyoming exhibit similar Entisol characteristics (Figure 8.18). In the Sandhills of north-central Nebraska and south-central South Dakota, the underlying Ogallala Formation’s sandy conglomerate has contributed to the sediment load needed to form the aeolian or windblown formations found in the area. Fully 52,000 square kilometers (20,000 square miles) of land is covered in sand dunes and sand sheets, which were largely created from windblown material eroded by glaciers during the late Pleistocene. Recent surveys of the dunes, which are currently covered with a thin veneer of grassland vegetation, have suggested that they were active within the last several thousand years, and may become active again in the event of a severe drought.

See Chapter 4: Topography for more information about the Sandhills.

Inceptisols can be found scattered throughout grasslands and lightly forested areas in Montana, South Dakota, and Nebraska.

Figure 8.18: The Badlands of South Dakota form new soil quickly due to the phenomenal average erosion rate of approximately 2.5 centimeters (1 inch) per year.

Figure 8.18: The Badlands of South Dakota form new soil quickly due to the phenomenal average erosion rate of approximately 2.5 centimeters (1 inch) per year.