Illinois is divided into 14 natural divisions. These natural divisions are described by the differences in topography, glacial history, bedrock, soils, and the distribution of native plants and animals found within. From the unglaciated Wisconsin Driftless Region in northwestern Illinois to the vast fertile soils of the Grand Prairie to the often flooded Coastal Plain of southern Illinois, Illinois is home to an incredible variety of habitats and species.
The Driftless Division (drift is glacially deposited debris) is characterized by rugged terrain. This area of Illinois lacks the granite boulders present in much of the Midwest and has deep ravines and valleys. From this evidence we know it escaped the glaciers of the Pleistocene and provided a haven for certain plants and animals to survive the glacial periods.
The area has the state’s highest point—Charles Mound. The soils are composed of wind-blown loess, disintegrated rock, and flood deposits. At one time most of the landscape was hardwood forest. Although the glaciers missed this area, debris from their melt waters blocked the southeast outlet of the Apple River, causing it to cut a new channel. As the river cut through the masses of limestone, dolomite, and shale to form its new channel, it also formed a rugged and picturesque canyon.
This region of rolling, glaciated topography is drained by the Rock River. The soils of this division are thin and are either loess (wind-blown sediment) or glacial till. Two sections make up the division. The Freeport Section is underlain with dolomite and limestone. Outcrops and “dells” occur along streams. The Oregon Section (south central Ogle County) is underlain with sandstone that has formed bluffs, ridges, and ravines.
Prairie once occupied the larger expanses of upland while forests were equally abundant along watercourses. White pine, Canada yew, and yellow birch—northern forest relicts—can still be found in this division. Prairie knobs (islands of prairie that were either too hilly or too troublesome to farm) support downy yellow painted cup and profusions of pale purple coneflower. Castle Rock State Park and Nachusa Grasslands are representative sites of this division.
Only 10,000 years ago, this division was covered by glaciers. Glacial landforms such as kames (conical mounds of glacial debris), moraines (long ridges of glacial debris), and eskers (a ridge of sand and gravel from an ancient embedded glacial stream) are common. The old bottom of Lake Chicago (the ancestor of Lake Michigan) is now occupied by the city of Chicago. Sand dunes of varying sizes occur along Lake Michigan. The soils are derived from lakebed sediments, peat, beach deposits, and glacial drift, and range in texture from sand and gravel to silty clay loams. In addition to a variety of prairie and forest communities, this division also has fens (wet prairies with an alkaline water source associated with calcareous springs and seeps), marshes (common because of the poorly drained soils), sedge meadows, and bogs. The only true bogs in the state and all of the state’s glacial lakes are found here, as is a natural beach-and-dunes association. The area is divided into four subsections—the Morainal Section, Lake Michigan Dunes, Chicago Lake Plain, and Winnebago Drift. To experience this division, visit Illinois Beach or Moraine Hills State Parks or Volo Bog State Natural Area.
This area, the largest natural division of the state, is a vast plain formerly occupied by tall-grass prairie. The grassland landscape was so unusual that early travelers had to turn to the sea for analogies, evoking “a sea of grass” or “a vast ocean of meadow-land.” In time this landscape came to be known as “prairie.” The fertile soils are young and high in organic content. They were developed from deposited loess, lakebed sediments, and glacial drift. Natural drainage was poor resulting in many marshes and prairie potholes. The prairies were a veritable wildflower garden containing several hundred species of grasses and forbs.
While much of the prairie has been lost, there are remnants and restorations including Goose Lake Prairie State Park, Midewin National Tallgrass Prairie, Allerton Park, and Kennekuk Cove County Park. Spitler Woods State Natural Area shows a prairie grove habitat and is one of the largest stands of old growth woods in Central Illinois
Consists of level to rolling uplands interspersed with deeply cut rivers and ravines with well-developed floodplains. It is a land of deep, forested ravines with intervening flat prairie openings. The area was covered by Illinoian age glaciers. Bedrock outcroppings are common in some locations.
The Galesburg Section is north of the Illinois River valley; both the Spoon and La Moine rivers drain this area. The amount of prairie here once almost equaled the amount of forest. One interesting habitat type is the dry-mesic barren, also known as an oak opening. In the spring at Argyle Hollow Barrens Nature Preserve look for the very unusual bi-colored bird’s foot violets growing among the mounds of lichens and mosses.
The Carlinville Section is southeast of the Illinois River valley. Macoupin Creek and the Illinois River are the major streams that drain this section. The original vegetation of this section was forest, with only 12% of the area in prairie. Very little prairie or forest remains today in this section.
To experience this area, visit Siloam Springs and Arglye Lake State Parks.
Dry sand prairie and scrub oak forest, dominated by black and blackjack oak, are the natural vegetation. The division is divided into two parts—Illinois River and Mississippi River sections. The meltwaters of the Wisconsin glacial episode greatly affected the formation of both the Havana sand deposit (Illinois River) and the Hanover-Oquawka sand deposit (Mississippi River). The Havana sand was formed as meltwater cascaded down the prehistoric course of the Illinois River. The Hanover-Oquawka deposit occupies an expanse between the bluffs on the east and the Mississippi River on the west. In both of these areas, tremendous floods carried a huge volume of sand and gravel downstream at breakneck speed, depositing material when the water slowed. Look for prickly-pear cactus, three-awn grass, little bluestem, and Indian grass in Sand Ridge State Forest or Thomson-Fulton Sand Prairie Nature Preserve.
The rivers and floodplains of the Mississippi River from Wisconsin to the Missouri River in Calhoun County. It also stretches along the Illinois River and its major tributaries south of LaSalle. This division is divided into two sections—Illinois River and Mississippi River. Broad floodplains and gravel terraces formed by glacial floodwaters characterize the terrain. Both big rivers have oxbow lakes, but backwater lakes are found only in the Illinois River and its major tributaries south of LaSalle. The Sportsman’s and General Guide described this area in 1877: “The most noted sporting grounds in Central Illinois, if not in the whole State, lie upon the Illinois River . . . The game here is of great variety and abundance . . .” Much of this land was originally forested and forests still occur along the broad floodplains. Prairies, marshes, and moist savannas also occurred. One unique habitat in this division was originally classified as a springfed bog or a “hanging fen.” This rare natural community is characterized by sloping peat at the edge of a moraine.
The narrow band of river bluffs and rugged terrain along the Mississippi River floodplain from Rock Island County to St. Clair County and the lower Illinois River valleys make up this division. Bedrock cliffs and outcrops of limestone and sandstone are common along the river bluffs. Hill prairies occur atop south and west-facing bluffs, while oak-hickory forests predominate in the ravines and on cooler, north- and east-facing slopes. Deep deposits of loess (windblown silt) form high bluffs. Two peaks in Pere Marquette State Park are examples of loess deposits—McAdams Peak and Lovers Leap. The division is divided into two sections— Glaciated and Driftless. The topography of the Glaciated Section is a result of the Pleistocene glaciers—the Illinoian and pre-Illinoian stages. Unusual habitats found here include limestone glades, loess hill prairies, and glacial-drift hill prairies. The topography of the Driftless Section is rougher because the area escaped the glaciers. Limestone and sandstone outcrops frequently occur. Unusual habitats found here include limestone glades and loess hill prairies. Pere Marquette State Park is found in this section.
This division includes the Mississippi River and its floodplain from Alton to the Thebes Gorge. The Mississippi River is muddy here due to the silt load brought in by the Missouri River. The soils have developed from this alluvium and are either finely textured and well-drained with areas of sand, or clayey with poor drainage. The division is divided into two sections—the Northern, which originally contained prairies, marshes, and forest, and the Southern, which was densely forested. The Northern Section contains the American Bottoms, a montage of swales, backwater lakes, ridges, and river terraces. When Charles Dickens visited the area in 1842 all he could hear was the loud chirping of frogs and all he could see on the “unwholesome, steaming earth” was mud, mire, brake (overgrown marshland) and brush. Horseshoe Lake, which is a U-shaped ox-bow, is now a state park. The Southern Section’s forests have not only pin, overcup, and cherrybark oaks, but also species associated with bottomland swamps. Look for pumpkin ash, swamp cottonwood, and bald cypress in La Rue Swamp and listen for the calls of green treefrogs.
This division encompasses the area south of the Shelbyville Moraine, the Sangamon River, and Macoupin Creek. The Illinoian glacial episode reached its southernmost limit just beyond this division. The bedrock consists of sandstone, limestone, coal, and shale. A layer of thin soil with poor internal drainage, usually loess or till, covers the bedrock. Many of the soils have a high clay content leading to “claypan” subsoil. From north to south the glacial till becomes thinner.
This division is divided into two sections. The Effingham Plain Section which is a relatively flat plain drained by the Kaskaskia River, contained mostly prairie, while the Mt. Vernon Hill Country Section has rolling, hilly topography and was mostly forested. Found in both sections are post oak flatwoods which occur on the hardpan, clay soil. Trees do not live excessively long in a flatwood forest. They undergo a regular cycle of 150 to 200 years, and when they reach a certain size, the trees are inevitably blown down because of their shallow root penetration in the clay soil. Post Oak Rest Area along I-57 is a good example of this community type.
The eastern border of Illinois contained the great trees that made up the last stronghold of the eastern deciduous forest. Traces of this magnificent forest still remain in the landscape surrounding theWabash and Vermilion rivers. This division is divided into three sections: Bottomlands, Southern Uplands, and Vermilion River. The Wisconsin glacial-episode impacted the Vermilion River Section, while all three sections were influenced by the earlier Illinoian glacial episode. This division is a transition zone between forest and prairie, but lowland and upland forests dominate the landscape, containing a great diversity of trees: beech, tulip poplar, cottonwood and several species of oak—pin, overcup, cherrybark, bur, Shumard, and swamp white. The understory is a garden of spring ephemerals with a nearly continuous cover of forest vegetation. In addition to a large number of tree species, the state’s only National Wild and Scenic River is found here with several species of fish found nowhere else in the state, including the bluebreast and harlequin darters. To experience this division visit the Russell Duffen Nature Preserve in Vermillion County and Beall Woods State Park in Wabash County.
A narrow band from northern Monroe County to Alexander County makes up the Ozark Uplift, a domelike geologic structure of exposed, ancient bedrock centered in the Ozark Mountains of Missouri. Great limestone bluffs mark the edge of the Mississippi Valley. Sandstone ravines in Randolph County and the sinkhole region of Monroe County, with its caves and springs, make this landscape unique in Illinois. This division is divided into three sections. The Northern Section was glaciated during the Illinoian glacial episode and is underlain with relatively pure limestone. The Central Section, which was also glaciated during the same period, is underlain with sandstone. The Southern Section is unglaciated and underlain with cherty limestone. Prior to settlement most of this division was forested and a rich assemblage of tree species can still be found here. This is the only division where shortleaf pine occurs naturally. Hill prairies occurred on the river bluffs of the Northern Section. Unique organisms include the eastern coachwhip snake and the plains scorpion. Visit Pine Hills in Union County and Fults Hill Prairie Nature Preserve in Monroe County to experience this division.
Just south of where the Illinoian glacier stopped lie massive escarpments—the backbone of southern Illinois. The landscape is characterized by high, east-west sandstone cliffs that form the Greater Shawnee Hills. Lower hills underlain by limestone and sandstone are known as the Lesser Shawnee Hills. The topography is very rugged, with many bluffs and ravines. Clear, rocky streams widened and deepened the ravines forming canyons, shelves, steps, and shelter bluffs. Where the slopes are steep, bare rock is exposed. Most of this division was forested, yet openings occurred—barrens and glades. Barrens are grassy openings found on rocky, south- facing slopes that have only a thin layer of soil. Vegetation includes small, gnarled, and twisted blackjack and post oaks. Prairie grasses and the occasional blazing star grow here. Glades are open expanses of bedrock on bluff tops, dominated by red cedar. Although prairie grasses such as little bluestem occur, the ground is likely to be covered with moss and lichens. This division is divided into two sections, the Greater Shawnee Hills, of which Ferne Clyffe State Park is a good example, and the Lesser Shawnee Hills. Cave in Rock State Park is found in this section.
South of the Shawnee Hills, the land flattens, the drainage is poor, and frequent flooding occurs. Only knolls and ridges of the Cretaceous Hills break the broad plain of alluvium from the Cache, Ohio, and Mississippi River bottoms that make up this division. The coastal plain of Illinois resembles lands that surround the present-day Gulf of Mexico. This division is divided into the Cretaceous Hills and the Bottomlands sections. The Cretaceous Hills extend in a narrow band from the Mississippi to the Ohio rivers. They are low hills made of gravel, sand, and clay and are remnants of the more broadly spread Cretaceous deposits in Kentucky and Tennessee. The hills are low and contain many seep springs. Plants associated with northern bogs can be found here, including sphagnum moss and a profusion of ferns. The Bottomlands Section contains southern swamps of bald cypress and water tupelo at their northern most limits. The trees may be surrounded by a thick green blanket of duckweed, its surface broken only by the ribbon of a swimming cottonmouth. Heron Pond State Natural Area, with its long boardwalk, is an excellent place to discover this division.