The strata

Stratum

In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

 of the bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 geology of Illinois are dominated by Pennsylvanian

Pennsylvanian

The Pennsylvanian is, in the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly . As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain...

 age rocks which occur within the Illinois

Illinois

Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

 Basin.

Illinois Basin

The Illinois Basin

Illinois Basin

The Illinois Basin is a Paleozoic depositional and structural basin in the United States, centered in and underlying most of the state of Illinois, and extending into southwestern Indiana and western Kentucky...

 is a northwest-southeast asymmetrical structural basin that is filled with more than 4000 meters of Paleozoic

Paleozoic

The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...

 sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s. The basin covers most of Illinois, and extends into western Indiana and western Ketucky. The basin is bounded to the north by the Mississippi River

Mississippi River

The Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...

 and the Kankakee Arch, to the east by the Cincinnati Arch

Cincinnati Arch

The Cincinnati Arch is a broad structural uplift between the Illinois Basin to the west, the Michigan Basin to the northwest and the Appalachian Basin and Black Warrior Basin to the east and southeast. It existed as a positive topographic area during Late Ordovician through the Devonian Period...

, and to the south by the Ozark uplift

Tectonic uplift

Tectonic uplift is a geological process most often caused by plate tectonics which increases elevation. The opposite of uplift is subsidence, which results in a decrease in elevation. Uplift may be orogenic or isostatic.-Orogenic uplift:...

 and Pascola Arch.

Rocks

The oldest rocks exposed in Illinois, are Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

, and occur along the Rock River in Ogle County. The youngest are Tertiary (Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

) and occur in southwestern Illinois. Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period sediments onlap the Paleozic units in extreme southern Illinois.

A bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

 geologic map

Geologic map

A geologic map or geological map is a special-purpose map made to show geological features. Rock units or geologic strata are shown by color or symbols to indicate where they are exposed at the surface...

 was published by Kolata et al., 2005 and is available at the Illinois State Geological Survey.