Schwegman's Natural Divisions of Illinois

​About 85 percent of what is now Illinois was covered by glaciers at least once during the Pleistocene Epoch (1.6 million to 10,000 years ago) of the Cenozoic Era. The glacial periods affecting Illinois are known as the pre-Illinoian, Illinoian and Wisconsinian. Only the extreme northwestern and extreme southern parts of the state along with Calhoun County and parts of Pike, Jersey, Monroe and Randolph counties were not glaciated. No one is sure what caused this ice age. It could have been due to a cyclic pattern of factors relating to the earth's orbit and tilt on its axis; shifts in the Gulf Stream in the Atlantic Ocean; reversals in the earth's magnetic field; volcanic activity; galactic dust clouds; or other reasons. The evidence does show that the glaciation occurred as the result of abrupt climatic changes, not gradual ones. Ice sheets began to grow from regions near the North Pole at this time when the summers were about 7 to 13 degrees Fahrenheit cooler than those of today, and the winter snows did not completely melt.

Low maximum temperatures, not low minimum temperatures were necessary for the glaciers to develop. Because the time of cooler conditions lasted tens of thousands of years, thick masses of snow and ice accumulated to form glaciers. As the glacial ice thickened with more snow, its great weight caused it to flow outward at the edges, often for hundreds of miles. As the ice sheets expanded, the areas in which snow accumulated also increased in size. Glaciers were able to continue to grow until the climate warmed enough so that the rate of melting was greater than the rate of expansion.

Pleistocene glaciers and the waters melting from them changed the landscapes that they covered. Some sections of the glaciers in northern Illinois were about 2,000 feet thick, while other areas of the state were covered by ice masses about 700 feet thick, still as tall as a 60-story building. The glaciers moved the land they overrode, leveling and filling many valleys. Moving ice carried colossal amounts of rock and earth, for much of what the glaciers wore off the ground was kneaded into the moving ice and carried along, often for hundreds of miles.

The continual floods released by melting ice carved new waterways, deepened old ones and partly refilled both with sediments as great quantities of rock and earth were carried beyond the glacier fronts. According to some estimates, the amount of water drawn from the sea and changed into ice during a glaciation was enough to lower the sea from 300 to 400 feet below its level today. Consequently, the melting of a continental ice sheet provided a tremendous volume of water that eroded and transported sediments. In most of Illinois, glacial and meltwater deposits buried the old hill-and-valley terrain and created the flatter land forms which would become the prairies. Glaciers left a mantle of soil and buried deposits of gravel, sand and clay over about 90 percent of the state.

Pre-Illinoian (1.6 million to 300,000 years ago) glaciers invaded Illinois from the west and east. There may have been several glaciers advancing into Illinois during this period, but not much evidence of them remains because it was so long ago and wind, water and other glaciers have mostly destroyed it.

The Illinoian stage glaciation was extensive in Illinois. At this time glaciers extended to the most southern point that they have ever reached in the northern hemisphere. That place was in Illinois, near Carbondale. About 85 percent of what is now Illinois was covered by this ice sheet.

The Wisconsinian glaciation started about 15,000 years ago and covered much of the northern and east-central parts of our state. The Illinois area of this glaciation would generally become the Grand Prairie natural division. The moraines and Lake Michigan in northeastern Illinois are all remnants of this glacial period. About 12,000 years ago the climate warmed, and the glaciers began to melt and retreat, forming large lakes. As the melting continued, the lake waters eventually eroded their banks and created enormous floods. The flood known as the Kankakee Torrent was mainly responsible for the deposition of sand along the Illinois River, where sand prairies developed.

Wisconsin Driftless Division (no sections in this division)

Rock River Hill Country Division
Freeport Section - The Freeport Section includes most of the Rock River Hill Country Division. It is characterized by rolling hills and the presence of dolomite and limestone bedrock. Limestone caves are present.

Oregon Section - The Oregon Section has sandstone bedrock and distinctive plants, including ground pine, rusty woodsia and oak fern. These plants are more commonly found in areas farther north in the United States.

Northeastern Morainal Division
Morainal Section - This section contains the moraines and related geologic features resulting from late advances in the Wisconsinian glaciation period. Most of Illinois' glacial lakes and peatlands are found here.

Lake Michigan Dunes Section - The Lake Michigan Dunes Section is distinctive for its unique plants that grow on the dunes and beaches. Plant succession from shifting sand to stabilized sand results in a variety of species. Beach grass, trailing juniper and bearberry are three examples.

Chicago Lake Plain Section - This flat, poorly-drained area is composed of the lake bed sediments of glacial Lake Chicago. Long ridges of shore-deposited sands are conspicuous features. A few natural lakes exist near Calumet City. The original vegetation of this section was prairie and marsh with scrub-oak forests on sandy ridges.

Winnebago Drift Section - The Winnebago Drift Section includes gravel hill prairies that once extended along the east bluffs of the Rock River valley into Wisconsin. The section has fairly good drainage. Wet prairies and marshes exist in the sand area along Coon Creek. The prairies contain many plant species which are more common in prairies further west of Illinois.

Grand Prairie Division 
Grand Prairie Section - This section includes the part of Illinois that was affected by the late stages of the Wisconsinian glaciation, that is outside the Northeastern Morainal Division and that does not include outwash and sand areas. The Shelbyville and Bloomington moraines form the boundaries of this section. Black-soil prairie, marshes and prairie potholes are common in this poorly-drained area. The Kankakee mallow is found in this section, growing only on an island in the Kankakee River.

Springfield Section - The Springfield Section is part of the area covered by the Illinoian glaciation. Prairies grew on this land in presettlement times. It has better drainage than the younger Grand Prairie Section. Deep loess (a wind-blown silt) deposits support dry hill prairies along the lower Sangamon River. Large areas of floodplain forest grow in the valley of the lower Sangamon River and its tributaries.

Western Section - The Western Section was covered by the Illinoian glaciation. This well-drained land was predominantly prairie in presettlement times.

Green River Lowland Section - The valley of the Green River and the lower Rock River was formed by glacial meltwaters. Much glacial outwash was deposited, and sand flats and dunes developed. The section originally had many marshes and wet prairies. Scrub-oak forests grew on the sandy ridges, and floodplain forests were present along the rivers. Sand prairies were found on the sand flats and dunes. Most of this section has been disturbed by grazing, cultivation and drainage.

Kankakee Sand Area Section - The sand of the Kankakee Sand Area Section was deposited by the Kankakee Flood during the later stages of the Wisconsinian glaciation. Sand prairie and marsh were the predominant vegetation of this section before the land was drained for cultivation. Scrub-oak forests exist on drier sites. The primrose violet is restricted to this section in Illinois. The clear, well-vegetated, sand-bottomed streams contain fishes like the weed shiner, ironcolor shiner and least darter.

Upper Mississippi River and Illinois River Bottomlands Division
Illinois River Section - The Illinois Section of this division is characterized by its backwater lakes and forest vegetation. Spring bogs exist along the river bluffs.

Mississippi River Section - The Mississippi River Section is made of several, separated bottomlands along the Mississippi River, from the Wisconsin border to Calhoun County. Most of the prairies of this section have been drained for agriculture. Forests are found along the river inside levees and on river islands.

Illinois River and Mississippi River Sand Areas Division
Illinois River Section - This section differs from the Mississippi River Section by the absence of several plant and animal species.

Mississippi River Section - This section has several plant and animal species which are absent from the Illinois River Section including false heather and rock spikemoss. Both of these plants form large mats that stabilize dune blowouts.

Western Forest--Prairie Division
Galesburg Section - The Galesburg Section is the area of the Western Forest-Prairie Division that lies north of the Illinois River valley. At the time of settlement, there were about equal amounts of forest and prairie in this section, with forests mainly along the tributaries to the Illinois River.

Carlinville Section - The Carlinville Section of this division is the land southeast of the Illinois River valley. Originally it was covered mostly by forest, with prairie accounting for about 12 percent of the area.

Middle Mississippi Border Division
Glaciated Section - The topography of this area was modified by the pre-Illinoian and Illinoian glaciation stages. Limestone underlies most of this section and may often be seen in cliffs along the river bluffs.

Driftless Section - This area of the state is apparently unglaciated. It has many sinkholes and sinkhole ponds.

Southern Till Plain Division
Effingham Plain Section - The Effingham Plain Section is a relatively flat plain drained by the Kaskaskia River. It originally was mostly prairie. Post oak flatwoods are characteristic of the uplands. Sanctuaries for the greater prairie chicken exist in this section.

Wabash Border Division
Bottomlands Section - The Bottomlands Section of this division encompasses the bottomland forests, sloughs, marshes and oxbow lakes in the floodplains of the Wabash River, Ohio River and their major tributaries. Bottomland forests are the main vegetation type with wet prairie and marshes associated with the sloughs.

Southern Uplands Section - The Southern Uplands Section contains the dry and mesic upland forests on the bluffs along the Wabash River. Some sandstone ravines support an unusual combination of plant species.

Vermilion River Section - The Vermilion River Section is characterized by rugged topography and the beech-maple forests in the ravines along the Vermilion River and its tributaries. The beech-maple forest represents a climax, deciduous forest type of the northeastern United States, which is found in Illinois only in the extreme eastern and southern portions.

Ozark Division
Northern Section - The Northern Section of the Ozark Division has limestone bedrock, caves, sinkholes and a unique combination of plants and animals. Plants found only in this section include stiff bedstraw and slender heliotrope. They grow on hill prairies or exposed limestone ledges.

Central Section - The Central Section of this division has sandstone bedrock. Its forest and other floral components are distinctive. Bradley's spleenwort and Harvey's buttercup are species only in this section of this division.

Southern Section - The Southern Section has significantly different bedrock, topography, glacial history, forest composition and animals from the other two sections in the division. Black spleenwort and shortleaf pine are two of the unique plants found here.

Lower Mississippi River Bottomlands Division
Northern Section - The Northern Section is distinguished by its forest composition, presence of wet prairies and marshes and absence of coastal plain trees. The bottomlands of this section near East St. Louis are called the "American Bottoms."

Southern Section - The bottomland forests of the Southern Section contain a greater number of tree species than those found in the Northern Section and also include some swamp species typical of the coastal plain.

Shawnee Hills Division
Greater Shawnee Hills Section - The Greater Shawnee Hills Section has sandstone bedrock and distinctive plants. Filmy fern and French's shooting star are unique plants in this section. French's shooting star may have grown in this area since before glacial times. Ravines and ledges along streams support plant species, like club mosses and sphagnum, which are normally found in more northern areas of the Midwest.

Lesser Shawnee Hills Section - The Lesser Shawnee Hills Section has limestone bedrock and sinkholes. The fluorspar deposits in Hardin County are world famous. Caves are common in the limestone bluffs. Wild mock orange is a distinctive plant.

Coastal Plain Division
Cretaceous Hills Section - This section is named for the fossil beds from the Cretaceous period that are found in the rolling hills here. Also present in the hills are sands, gravels and clays.

Bottomlands Section - The Bottomlands Section encompasses bottomland forests, oxbow lakes, sloughs and rivers. This area includes the remnants of the once vast bald cypress and tupelo gum swamps along the rivers.

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