Missouri ranks 21st in size among the 50 states. Its area of 180,533 sq km (69,704 sq mi) includes 2,119 sq km (818 sq mi) of inland water. The state’s distances at their maximum are 587 km (365 mi) from east to west and 513 km (319 mi) from north to south. The mean elevation is about 240 m (about 800 ft). Within Missouri are found three of the major physiographic provinces of the United States: the Central Lowland, the Ozark Upland, or Ozark Plateau, and the Gulf Coastal Plain. Each of these physiographic regions and its subdivisions, or sections, has a distinctive combination of topography, soils, and natural vegetation.
A large part of the Central Lowland in Missouri constitutes a section called the Northern Plains, or the Dissected Till Plains. This section occupies almost all of the state north of the Missouri River. The Northern Plains occupy not only northern Missouri but also adjacent portions of Kansas, Nebraska, and Iowa.
The name Dissected Till Plains suggests the origin of this area’s topography. Till plains are gentle plains composed of rock and soil particles and fragments left behind by retreating glaciers. In Missouri, continental glaciers once reached approximately as far south as the Missouri River, which marks the southern edge of the Northern Plains. After they retreated, the glaciers left behind the gentle surface of a till plain.
The glaciation of northern Missouri occurred relatively early in the sequence of ice-sheet advances and retreats in North America. Therefore there has been time since glaciation for stream erosion to roughen the original gentle surface of the till plain, and the plain has been dissected, or cut up, by the action of rivers deepening and widening their valleys.
A succession of river valleys bordered by belts of hilly country characterizes the landscape of the Northern Plains. Between these dissected areas lie gently rolling or almost flat areas, which are the remnants of the original till plain. The most level land in the Northern Plains lies in a narrow belt just west of the Mississippi River, where dissection has scarcely begun. Geographers sometimes treat this narrow band of the Central Lowland in eastern Missouri as a separate section, which they call simply the Till Plains. The Till Plains extend eastward into Illinois, where they cover almost the entire state. In Missouri glacial till is usually from 15 to 30 m (50 to 100 ft) thick.
The vegetation prior to European settlement in the Northern Plains consisted of both forest and prairie. The flat floodplains of the rivers and the adjacent belts of hills were the most wooded sections, with oak especially prominent. Areas at some distance from the major streams tended to be covered with prairie grasses interspersed with patches of woodland.
The Osage Plains form another section of Missouri’s Central Lowland. They are often called the Western Plains. This section lies south of the Northern Plains and west of the Ozark Upland. The Osage Plains extend into Kansas, Oklahoma, and Texas.
These plains in southwestern Missouri lay south of the limit of glaciation. Their surface, having received no glacial deposits, reflects the results of erosion of the underlying bedrock and is generally smoother than that of the Northern Plains. Occasional lines of low hills have been formed where a relatively hard layer of rock has resisted erosion and stands out above the rest of the terrain. However, the relief is not impressive in this section of Missouri, nor are the wide shallow valleys cut by the streams. "Missouri" © Emmanuel BUCHOT, Encarta, Wikipedia
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