The Ozark Plateau, or Ozark Upland, occupies most of Missouri south of the Missouri River. It extends into adjacent parts of Arkansas and Oklahoma. The region is also called the Ozark Mountains and sometimes the Ozark Hills or simply the Ozarks. In overall form the Ozark Upland is an uplifted dome elongated in a southwest-northeast direction. The highest part of the dome in Missouri extends from the southwestern corner of the state northeastward to the Saint Francois Mountains. Large areas along the crest of this dome are between about 375 and 500 m (about 1,200 and 1,600 ft) high. This is not high compared to the Appalachian or Rocky mountains, but it is markedly higher than the surrounding plains. Southeastward from the crest the upland descends fairly steeply, so that it is only about 120 m (about 400 ft) above sea level at its junction with the plains along the Mississippi River. On the northern side of the crest the descent is somewhat more gradual.
Topography in the Ozark Upland does not correspond in any simple way to elevation. As in the Northern Plains, the roughness of the local topography depends on the degree to which streams have cut valleys into the surface, which originally was quite smooth. The most dissected areas show picturesque tangles of deep stream valleys and intervening ridges with steep slopes. They are found near the Current and Black rivers in the southeast, near the White River in the extreme southwest, and near the Osage and Gasconade rivers in central Missouri. In the most deeply dissected areas, principally those on the southern slope of the dome, stream valleys have cut 120 to 210 m (400 to 700 ft) into the upland surface. The surface has been reduced to a series of narrow interstream ridges.
In contrast to the hilly sections, large areas near the top of the dome and some toward its lower edges are relatively undissected. They give the appearance of monotonous, rolling plains. There are deeply entrenched stream valleys in this area, but they are so widely spaced that the casual observer may well be unaware of them. A name often applied to most of this region is the Salem Upland. Part of it, in the southwest, is called the Springfield Plateau. The Springfield Plateau adjoins the Osage Plains. Its surface is almost as gentle as that of the plains, except for occasional river valleys that have made cuts of 60 to 90 m (200 to 300 ft) into the plain.
Most of the Ozark Upland is composed of sedimentary rocks, principally soluble limestones and dolomites, also known as carbonate rocks. Over many thousands of years surface and underground waters have burrowed the uplands into a labyrinth of thousands of caves, springs, and sinkholes. It is known as karst topography. The carbonate rocks cover a hidden core of older, harder igneous rocks. One section of the Ozark Upland, however, differs in character from all other sections. This is the Saint Francois Mountains, at the eastern end of the crest of the dome. Only in these mountains have the sedimentary rocks been sufficiently eroded away so that the underlying igneous rocks are exposed.
They form the rounded, knoblike peaks of an old mountain range. The peaks project, in isolation or in clusters, between 230 and 300 m (750 and 1,000 ft) above the surrounding sedimentary basins. One of these knobs, Taum Sauk Mountain, reaches 540 m (1,772 ft) above sea level and is the highest point in Missouri.
However, the Saint Francois Mountains area is not generally as rugged as some of the lower, stream-dissected areas.
Before the time of white settlers, forests covered most of the Ozark Upland. These forests consisted of many species of trees, most of which were deciduous hardwoods. Oaks were the most widespread. Mixed with the hardwoods were stands of softwoods, including cedar and pine. They were minor elements in the forest except in the southeast, where pines locally made up a large proportion of the timber. In most areas the forest was relatively open, with abundant grasses growing among the trees, and could be considered a woodland or savanna.
In the west the forest was thinner than in the east and was interspersed with large areas of prairie grasses. Almost all of this Ozark forest is gone, but large areas are covered with small second-growth timber and scrub. In national and state forests and other managed lands the forest has returned in dense stands. A portion of the broad Gulf Coastal Plain that extends across the South from Texas to Florida also extends northward into southeastern Missouri. This section is known as the Mississippi Alluvial Plain or simply as the Southeastern Lowland. It is also called the Bootheel because of its shape.
The Southeastern Lowland is the lowest, flattest, wettest, and most fertile part of Missouri. Its flat surface seems almost featureless to the observer, although isolated ridges stand from 3 to 60 m (10 to 200 ft) above the level plain. This lowland rises less than 120 m (400 ft) above sea level in the north and less than 90 m (300 ft) in the south. The lowest point in Missouri, at 70 m (230 ft), is in the Southeastern Lowland where the Saint Francis River exits the state. Before settlement much of the region was covered with standing water and a dense, swampy forest. It is now largely cleared and artificially drained. The Southeastern Lowland was the focus of some of the highest magnitude earthquakes in U.S. history.
In 1811 and 1812 several earthquakes of magnitudes above 8 on the Richter Scale shook the region around New Madrid, causing some lands to sink, others to rise, and affected the course of the Mississippi River. The threat of severe earthquakes continues in the region. "Missouri" © Emmanuel BUCHOT, Encarta, Wikipedia
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