Geography of Illinois

• Illinois has a maximum north–south distance of 390 miles (630 km) and 210 miles (340 km) east-west.
• Total area is 57,918 square miles (150,010 km²), ranked 25th in size of the 50 states.
• Water area is 2,325 square miles (6,020 km²); Lake Michigan accounts for most of this.
• Charles Mound in the northwest Driftless Area is the highest point in the state at 1,235 feet (376 m) above sea level.
• The Mississippi River at Cairo, Illinois is the lowest point, at 279 feet (85 m).
• Average elevation is 600 feet (180 m) above sea level.
• McLean County is the largest county in terms of land area (1,184 sq mi.), while Cook County is the largest county in terms of population (5,275,541 as of 2020), and the largest when land and water area are combined.[1]

Illinois has three major geographical divisions: Northern, Central, and Southern. Collectively, central and southern Illinois are often referred to within Illinois as "downstate Illinois" but with political developments since World War II "Downstate" now generally refers to all of Illinois outside of the Chicago metro area.

The city of Chicago and some of its older suburbs are heavily Democratic. While this tendency has historically been balanced by Republican voters in the suburbs, Democrats have significantly increased their suburban support in the past decade. Central Illinois's largely rural character helps to sustain a largely Republican voting pattern and widespread antipathy toward Chicago. The combination of poorer soil, coal mining and industrialization has caused the southern region to lean Democratic politically, especially in the urbanized region around Saint Louis, Missouri.

Rockford, Illinois is known as a center of liberal "gypsy moth" Republicans.

Illinois is predominantly covered with soil, hundreds of feet thick in many places. Much of this soil is called, appropriately, "Illite", and is the most common type of soil on the planet. The visible geology of the state is mainly Quaternary, with rocks buried deeply. The wealth of the state mainly arises from this abundance of soil and the favorable agrarian conditions it provides.

Illinois is a major coal-producing state. Much of the bedrock surface is of Pennsylvanian age, including "cyclothemes", regular sequences of limestone, shale and coal layers. Newer rocks may have once existed in the State, but these have been removed by repeated epochs of glaciation. Only tiny areas of newer bedrock exist. The chance of finding dinosaur bones in Illinois is almost nil:, the Pennsylvanian rocks were here before the dinosaurs.

Galena, a lead ore, has been mined in the northwest corner of the state. The ore was processed around Peru, Illinois, where river transport was available to bring the ore to a source of coal. About six tons of coal are needed for each ton of ore.

Fluorite, calcium fluoride, is the state mineral. It is mined in the southeastern portion of the state, around Metropolis. Fluorine gas is produced from fluorite. This is used in the production of uranium hexafluoride in Metropolis, Illinois for use at the gaseous diffusion plant in Paducah, Kentucky.

Geodes can be found in the older Mississippian outcroppings along the Mississippi River.

Igneous intrusions are found in the far southern areas of the State. Ages ago North America began to split roughly along the lines of the Ohio and Mississippi Rivers. Igneous rock flowed up into fissures, as a new ocean began to form. Had this continued, Illinois might have resembled the Arabian Peninsula, a broken terrain between two continents. This split, however, failed: the continent reclosed, leaving only a few igneous intrusions, and some oil fields, as evidence that it happened.

North of the Illinois River the bedrock surface is much older Ordovician sandstone. This sandstone yields ancient sand that is very useful in the production of glass, and in making molds for metal castings.

Large mammal species currently found in Illinois include whitetail deer and coyote, with the latter becoming urbanized in the Chicago area, as well as common in the rural areas. Furbearers commonly found include opossum, raccoon, mink, red and gray foxes, and muskrat. Commonly seen lagomorphs include the cottontail rabbit and commonly seen rodents include squirrels. Game birds found include Canada goose, mallard duck, ruffed grouse, wild turkey, and bobwhite quail. The northern cardinal is the official state bird. The state has approximately 300 species of birds, many of which migrate on the Mississippi Flyway. Extensive wetland restoration along the Mississippi River and Illinois River are attracting many species.

About two thirds of the state was originally prairie, with much of the area converted to rowcrop farming, while the southern third of the state remains heavily forested with oak, hickory, and maple. Also, several prairie plants.

Especially in the northern third of the state, intensive agriculture has made some species rare, while population numbers are increasing in the less-developed south. Large mammals generally no longer found in the wild in Illinois include American bison, gray wolf, American black bear, and elk.

Köppen climate types of Illinois, using 1991-2020 climate normals.

Illinois has a continental climate, with large temperature extremes not moderated by either mountains or oceans. Weather is influenced primarily by cold Canadian Arctic air in the winter, and warm, humid air from the Gulf of Mexico in the summer. High temperatures in the south of the state are about 10 to 12 °F. warmer than the north. Average annual temperature is 47 °F. in the north and 58 °F. in the south. Temperatures greater than 90 °F. occur about 45 days per year in the south, and 12 days per year in the north. Temperatures below zero °F. occur one day or less per year in the south, and 12 days per year in the north. Annual rainfall is 34 inches (860 mm) in the north and 46 inches (1,200 mm) in the south.

Prevailing winds are from the west. Average wind speeds "blowing off the prairie" are greater in the Chicago area. Statewide, the wind speeds average 8 to 12 miles per hour (13 to 19 km/h), and is greater in the winter than in summer.

Snowfall averages 30 inches (760 mm) in the north and 10 inches (250 mm) in the south, although the actual snowfalls vary greatly by year. In some years, southern Illinois receives very little snowfall, and may never get typical winter weather.

Most of the state has four distinct seasons. The southernmost part of the state is heavily forested and the trees are noted for having beautiful fall colors.

Tornadoes are a common hazard, primarily in the central part of the state, with peak months in June and April. Freezing rain is most common in central Illinois.

The Mississippi River is the nominal western boundary of the state, except for locations where the river has changed course. Kaskaskia, Illinois, the first state capital, is now west of the river. The Mississippi, due to a flood, destroyed the old town of Kaskaskia, and occupies what was the lower channel of the Kaskaskia River. A portion of Missouri is east of the river, near Grand Tower, Illinois. Similar "avulsions" are found along the Ohio River border with Kentucky and the Wabash River border with Indiana.

• List of ecoregions in Illinois

• Harris, Stanley E.; C. W. Horrell; Daniel Irwin (1977). Exploring the Land and Rocks of Southern Illinois: A Geological Guide. Carbondale: Southern Illinois University Press. ISBN 0809307405.
• Wiggers, Ray (1997). Geology Underfoot in Illinois. Missoula, Mont: Mountain Press Pub. Co. ISBN 087842346X.

• About Illinois Data and Statistics