Illinois ranks 25th in size among the states of the Union, with an area of 149,997 sq km (57,914 sq mi). That includes 1,958 sq km (756 sq mi) of inland water and 4,079 sq km (1,575 sq mi) of Lake Michigan over which the state has jurisdiction. The greatest north-to-south dimension of the state is 610 km (379 mi), and the greatest east-to-west distance is 343 km (213 mi). The mean elevation is about 180 m (600 ft). Illinois includes parts of four major natural regions, or physiographic provinces, of the United States: the Central Lowland, the Interior Low Plateaus, the Ozark Plateaus, and the Gulf Coastal Plain. The extensive flatlands of the Central Lowland occupy nearly all of Illinois, whereas the other three regions cover only small areas of the state. The Interior Low Plateaus and the Ozark Plateaus form a strip of hilly land across the southern part of the state. The flat alluvial lands of the Gulf Coastal Plain cover a small section of extreme southern Illinois.
The Central Lowland and the Interior Low Plateaus are subdivisions of a broader region known as the Interior Plains. The Ozark Plateaus form a section of the larger Interior Highlands region, and the Gulf Coastal Plain is part of the Coastal Plain.
The Central Lowland covers all but a small area of Illinois. Most of the Central Lowland is a level or slightly undulating plain, crossed here and there by broad, low ridges. The flatness of the land is a result of glacial action that occurred during the last Ice Age, which ended about 10,000 years ago, when great ice sheets advanced and retreated across the region. After the ice eventually retreated northeastward, the old preglacial landscape was left buried under a thick cover of glacial deposits, called drift, or glacial drift. Much of the drift is made up of clay and boulders, which together are called till, and form moraines.
The so-called Till Plains form the typical flat farmlands of Illinois. The fertile Till Plains area of east central Illinois is known as the Grand Prairie. The low ridges found in some areas are terminal moraines that were piled up during the periods of glaciation by stagnant ice sheets.
In some areas, especially in the Grand Prairie and in south central Illinois, the land is so flat that the taller buildings of cities on the plain can be seen for great distances across country. Partly because of the flatness, rainwater does not readily drain away, and before ditches and drains were dug, much of the land was swampy for at least part of the year. There are still large tracts of such wetland along the Wabash, Kaskaskia, and Big Muddy rivers today. Farther west, between the Illinois and Mississippi rivers, lie the Quincy hills, which are broken by deeply cut stream valleys. The extreme northwest is also an area of hills. Together with neighboring sections of Wisconsin and Iowa it forms the Driftless section, or Wisconsin Driftless section. Only the earliest of the ice sheets covered this section, and nearly all of the drift deposited has long since been eroded.
Charles Mound, which is 376 m (1,235 ft) above sea level and the highest point in Illinois, lies on a long hilly ridge in the Driftless section. However, this is only 180 to 210 m (600 to 700 ft) above the general level of the Grand Prairie and much of the rest of the state.
The Interior Low Plateaus portion of southern Illinois was not covered by the ice sheets, and its high ridges and bluffs afford magnificent panoramic views across the lowlands. Much of the region is now part of Shawnee National Forest. The Ozark Plateaus cover a small section of southwestern Illinois. Much of the land is forested and is too rugged and rocky for farming. Limestone, which is soluble in water, underlies much of the region, and small depressions, known as sinkholes, are found where the limestone has been dissolved. The Gulf Coastal Plain, at the southern tip of Illinois, is an extremely flat area where the land has been built of alluvial deposits from the Ohio and Mississippi rivers. Much of the region is cultivated and is very productive. Because of the fertile soils and the great river there, early settlers in this section of Illinois gave the region the nickname Little Egypt. "Illinois" © Emmanuel BUCHOT, Encarta, Wikipedia
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