Geography of Illinois - Geology

Geology

Illinois is mainly 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 therefore mainly Quaternary, with rocks buried deeply. The wealth of the State mainly arises from this abundance of soil.

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 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.