Like the Michigan and Permian Basins, the Illinois Basin is a structural bowl of Precambrian basement rock within which younger layers of sedimentary rock have accumulated. This bowl, which covers the southern 75% of Illinois, the southwestern 40% of Indiana, western Kentucky and a small portion of northwest Tennessee, is surrounded by structural "arches," uplifts of the deep, ancient Precambrian rock; these include the Kankakee Arch to the northeast, the Cincinnati Arch to the southeast, the Wisconsin Arch to the north, the Mississippi River Arch to the northwest, the Ozark Uplift to the west and the Pascola Arch to the southwest.
Three miles deep at its center, this broad basin of Precambrian rock, 1.3 billion years old, accumulated layers of sediment throughout much of the Paleozoic Era (from 600 to 270 million years ago); the great majority of these deposits occured within shallow seas, which invaded and retreated from the basin at least 50 times during that period, while others were carried in by streams or deposited within vast wetlands. As the basin filled in from the Cambrian to the Pennsylvanian Periods, these layers of sediment dipped from the surrounding arches toward the center of the structural depression; at the surface, older sediments are thus found at the periphery of the basin while the youngest (Pennsylvanian) cover its center. Following this prolonged period of deposition, which was intermittently disrupted by uplift and erosion, the Illinois Basin has undergone surface molding by the Pleistocene Glaciers and numerous streams; the glaciers flattened northern portions of the basin and coated them with a thick layer of glacial till while streams have carved southern portions into a maze of hills and valleys.
Travelling across the Illinois Basin today, one sees no evidence of the Precambrian bowl that underlies the region; indeed, its edge only outcrops in limited areas of southern Wisconsin and southeastern Missouri. Glacial erosion and till have produced the flat, productive Corn Belt across much of Illinois and western Indiana while Carboniferous sediments of the basin have been mined for their coal and drilled for their oil. As in most regions of our globe, the surface topography of the Illinois Basin only hints at the miles of sediment and complex geologic formations that lie below.
