Region 1: The Central Lowland

The Central Lowland is a flat-lying region located between the Appalachian Mountains to the east and the Great Plains to the west (Figure 4.5). It extends from the Canadian Shield in the north to the Atlantic Coastal Plain in the south and is part of the North American craton (the older, stable part of the continent).

Figure 4.5: Physiographic divisions of the Central Lowland.

The Central Lowland is composed of flat-lying Precambrian metamorphic and igneous rocks overlain by Paleozoic and Mesozoic sedimentary rocks. The Mesozoic sediments found in the region were eroded from the Rocky, Ozark, and Ouachita mountains, then carried to and deposited in the Western Interior Seaway that covered the area. Glacial erosion and deposition during the last ice age modified and smoothed much of the region’s surface, leaving behind thick layers of Cenozoic sediment and drift. Today, rivers running through this region, including the Missouri and Red rivers, have contributed significantly to erosion.

The Central Lowland has a generally smooth and flat topography, generated by glacial scouring during the ice age, as well as by the presence of enormous glacial lakes and erosion from catastrophic outbursts of meltwater. During the Quaternary, a 152-meter-thick (500-foot-thick) ice sheet flattened the landscape, leaving behind the gently rolling hills and shallow lakes that spread throughout the Drift Prairie today (Figure 4.6). The ice sheet also left behind layers of till, clay, gravel, and wind-blown silt (loess), which contributed to the area’s rich agricultural soils. The Red River Valley, directly to the east in North Dakota, is a flat lowland area that marks the former floor of glacial Lake Agassiz, which was once the largest freshwater lake in North America (Figure 4.7). About 160 kilometers (100 miles) west of the Red River Valley, the Missouri Escarpment—a ridge extending southeast from Canada to south-central South Dakota—separates the Drift Prairie from the Great Plains. The escarpment was formed when catastrophic floods at the end of the ice age carved a huge canyon, channeling future ice movements and flattening the surrounding area. The Missouri Escarpment is the remnant of this canyon’s west and southwest wall.

Figure 4.6: The Drift Prairie near Bottineau, North Dakota. The rolling landscape of the Turtle Mountains is visible in the far distance.

Figure 4.7: The extent of glacial Lake Agassiz during the Pleistocene. North Dakota’s Red River Valley follows the bed of this ancient lake.

Several landforms throughout North Dakota’s Central Lowland were formed as ice-thrust features—the weight and pressure of the advancing glacier displaced large bedrock slabs, shoving and thrusting large masses of rock and sediment, and depositing them a short distance from their original position. Geologists speculate that ice-thrust features occur so prominently in central and eastern North Dakota because the region’s water drains northward. Ice sheets advancing from the north prevented groundwater drainage, leading to increased pressure within the ground. Eventually, the weight of the glacier caused pieces of terrain to pop out of the ground, thrusting them a short distance forward and relieving the built-up pressure (Figure 4.8). Ice-thrust features are often accompanied by topographic depressions (usually lakes) located to the north, from which the material was displaced. Examples include Steele Lake near Anamoose, Medicine Lake and its adjacent Grasshopper Hills, and even Devils Lake, the largest natural lake in North Dakota. The Turtle Mountains of northernmost North Dakota, a plateau lying 600 meters (2000 feet) above sea level and 90 - 120 meters (300 - 400 feet) above the surrounding land, are another example of ice-thrust terrain that was later smoothed and rounded by further glacial erosion.

Figure 4.8: Ice-thrust features are created when pressure from the weight of an advancing glacier is released by thrusting a piece of terrain forward.

The Dissected Till Plains, extending from southeast South Dakota through Nebraska, are an area of rolling hills ripe with fertile soil. The area was initially scoured and flattened during the pre-Illinoian glacial stage; during the Wisconsinian, great quantities of loess accumulated there. Glacial runoff later led to erosion that sculpted the area into valleys and hills. Today, the Missouri River cuts across the plains at the border between Nebraska and South Dakota and runs south along Nebraska’s eastern border, forming wide floodplains that support a complex environment of sandbars and wetlands (Figure 4.9).

Figure 4.9: Sandbars in the Missouri River, viewed from Mulberry Bend Scenic Outlook near Dixon, Nebraska.