Guide to the Geologic Map of Illinois

by Illinois State Geological Survey

BEDROCK GEOLOGY

Beneath the glacial drift of Illinois many layers of rocks overlie a base of ancient crystalline rocks that in Illinois occur at depths of 2,000 to as much as 15,000 feet. The geologic map ([plate 1], in pocket) is drawn as if the mantle of glacial drift had been removed to expose the layers of bedrock, which are largely limestone, shale, and sandstone.

The key on the map shows the age sequence of the rocks, arranged with the youngest at the top, and gives the names that geologists have assigned to the various systems of rocks. Each system consists of rocks that were deposited during a long period of time. The complete sequence of rocks might be likened to a book of earth history, and each system likened to a chapter. Systems are divided into formations, which might be regarded as pages in the book.

As shown on the generalized rock column in [figure 2], the rocks next older than the glacial drift (Pleistocene) are the Tertiary and Cretaceous sands, gravels, and clays, mostly unconsolidated. They occur only at the extreme southern tip of Illinois and were deposited when that area was covered by a northward extension of the Gulf of Mexico. The Tertiary rocks are shown on the map in grayed pink (T) and the Cretaceous in red-violet (K).

Next older than the Cretaceous are the Pennsylvanian rocks, named for the state of Pennsylvania where they are well exposed and were first studied.

The Pennsylvanian System is divided into two areas on the map (P¹ and P²). The rocks shown in lightest gray (P²) lie above the No. 6 Coal in the sequence and those in medium gray (P¹) lie below it. The No. 6 Coal is one of the thickest, most valuable coals in the state. Pennsylvanian rocks occur under the glacial drift, and their thickness ranges from a few feet to as much as 3,000 feet.

Figure 2—Diagram of layers of rocks in Illinois. The oldest rocks are at the bottom, the youngest at the top. Names are the standard ones applied by geologists to the subdivisions of the geologic eras.

Era			General Types of Rocks
	Period or System and Thickness
		Epoch
CENOZOIC “Recent Life”
Age of Mammals
	Quaternary 0-500′
		Pleistocene or Glacial Age
			Recent—alluvium in river valleys
			Glacial till, glacial outwash, gravel, sand, silt lake deposits of clay and silt, loess and sand dunes; covers nearly all of state except northwest corner and southern tip
	Tertiary 0-500′
		Pliocene	Chert gravel; present in northern, southern, and western Illinois
		Eocene	Mostly micaceous sand with some silt and clay; present only in southern Illinois
		Paleocene	Mostly clay, little sand; present only in southern Illinois
MESOZOIC “Middle Life”
Age of Reptiles
	Cretaceous 0-300’		Mostly sand, some thin beds of clay and, locally, gravel; present only in southern Illinois
PALEOZOIC “Ancient Life”
	Age of Amphibians and Early Plants
	Pennsylvanian 0-3,000′ (“Coal Measures”)		Largely shale and sandstone with beds of coal, limestone, and clay
	Mississippian 0-3,500’		Black and gray shale at base; middle zone of thick limestone that grades to siltstone, chert, and shale; upper zone of interbedded sandstone, shale and limestone
	Age of Fishes
	Devonian 0-1,500’		Thick limestone, minor sandstones and shales; largely chert and cherty limestone in southern Illinois
	Age of Invertebrates
	Silurian 0-1,000’		Principally dolomite and limestone
	Ordovician 500-2,000’		Largely dolomite and limestone but contains sandstone, shale, and siltstone formations
	Cambrian 1,500-3,000’		Chiefly sandstones with some dolomite and shale; exposed only in small areas in north-central Illinois
ARCHEOZOIC and PROTEROZOIC
			Igneous and metamorphic rocks; known in Illinois only from deep wells

The Pennsylvanian System contains many different kinds of rocks, including all of our minable coals. It also contains important deposits of limestone, shale, and clay, and at places oil and gas.

Next below the Pennsylvanian are the rocks of the Mississippian System, shown in blue on the map (M¹ and M²). The lower and middle Mississippian rocks (M¹) are largely limestone in and near the areas mapped, but in the central and eastern part of the state where they are buried under Pennsylvanian rocks they contain much siltstone and cherty limestone. The upper Mississippian rocks (M²) consist of a succession of sandstone, shale, and limestone formations.

This system of rocks takes its name from the Mississippi River because there are excellent exposures of these strata along the Mississippi Valley in western Illinois, southeastern Iowa, and eastern Missouri.

The Mississippian rocks are a source of limestone, fluorspar, zinc, and ganister, and are of greatest economic significance in southeastern Illinois where they are the most important of our oil-producing rocks.

The Devonian (D, dark gray on the map), Silurian, (S, violet), Ordovician (O, light pink), and Cambrian (C, deep pink) rocks, in the order named, are older than the Mississippian strata. In general, they include dolomite, limestone, shale, and sandstone. Except for small areas along the Mississippi and Illinois River Valleys, these older rocks are found at the surface only in the northern quarter of the state and locally in Alexander, Hardin, Jackson, Monroe, Pike, and Union Counties. They are nevertheless economically important because they yield limestone, dolomite, silica sand, oil, zinc and lead, tripoli, novaculite, and novaculite gravel.

The rocks of the Cambrian through Pennsylvanian Systems belong to the Paleozoic Era. The Paleozoic rocks overlie crystalline rocks, such as granites, that extend to unknown depths in the crust of the earth. The crystalline rocks are not exposed in Illinois but are encountered in the drilling of some deep wells and may be seen in the nearby Missouri Ozarks and in central Wisconsin.