Midwestern United States - Physical Geography

Physical Geography

The vast central area of the U.S., into Canada, is a landscape of low, flat to rolling terrain in the Interior Plains. Most of its eastern two-thirds form the Interior Lowlands. The Lowlands gradually rise westward, from a line passing through eastern Kansas, up to 5,000+ feet in the unit known as the Great Plains). Much of the Plains are now converted land use-wise to farming.

While these states are for the most part relatively flat, consisting either of plains or of rolling and small hills, there is a measure of geographical variation. In particular, the eastern Midwest near the foothills of the Appalachian Mountains; the Great Lakes Basin; the Ozark Mountains of southern Missouri; the rugged topography of Southern Indiana and far Southern Illinois; and the Driftless Area of northwest Illinois, southwest Wisconsin, southeast Minnesota, and northeast Iowa exhibit a high degree of topographical variety.

Proceeding westward, the Appalachian Plateau topography gradually gives way to gently rolling hills and then (in central Ohio) to flat lands converted principally to farms and urban areas. This is the beginning of the vast Interior Plains of North America. As a result, prairies cover most of the Great Plains states. Iowa and much of Illinois lie within an area called the "prairie peninsula", an eastward extension of prairies that borders conifer and mixed forests to the north, and hardwood deciduous forests to the east and south.

Geographers subdivide the Interior Plains into the Interior Lowlands and the Great Plains on the basis of elevation. The Lowlands are mostly below 1,500 feet above sea level whereas the Great Plains to the west are higher, rising in Colorado to around 5,000 feet.

The Lowlands, then, are confined to parts of Minnesota, Iowa, Michigan, Ohio, Indiana, Tennessee, and Kentucky. Missouri and Arkansas have regions of Lowlands elevations but in the Ozarks (within the Interior Highlands) are higher. Those familiar with the topography of eastern Ohio may be confused by this; that region is hilly but its rocks are horizontal and are an extension of the Appalachian Plateau.

The Interior Plains are largely coincident with the vast Mississippi River Drainage System (other major components are the Missouri and Ohio Rivers). These rivers have for tens of millions of years been eroding downward into the mostly horizontal sedimentary rocks of Paleozoic, Mesozoic, and Cenozoic ages. The modern Mississippi River system has developed during the Pleistocen Epoch of the Cenozoic Eraut its rocks are horizontal and are an extension of the Appalachian Plateau.

Rainfall decreases from east to west, resulting in different types of prairies, with the tallgrass prairie in the wetter eastern region, mixed-grass prairie in the central Great Plains, and shortgrass prairie towards the rain shadow of the Rockies. Today, these three prairie types largely correspond to the corn/soybean area, the wheat belt, and the western rangelands, respectively.

Although hardwood forests in the northern Midwest were clear-cut in the late 19th century, they were replaced by new growth. Ohio and Michigan's forests are still growing. The majority of the Midwest can now be categorized as urbanized areas or pastoral agricultural areas.