Guide to the Geologic Map of Illinois

Part 1

Guide to the GEOLOGIC MAP OF ILLINOIS

Illinois State Geological Survey Educational Series 7

STATE of ILLINOIS Otto Kerner, Governor

DEPARTMENT of REGISTRATION and EDUCATION William Sylvester White, Director

1961

ILLINOIS STATE GEOLOGICAL SURVEY John C. Frye, Chief URBANA, ILLINOIS

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS 42517-20M 2 (78783)

Guide to the GEOLOGIC MAP OF ILLINOIS

GLACIAL GEOLOGY

Although the age of the Earth is measured in billions of years, the face of Illinois is young—a mere 15,000 years old.

During the Ice Age, most of Illinois was repeatedly invaded by huge glaciers, sometimes towering a mile or more high, that carried embedded in them ground up rock materials they had gouged out of the bedrock to the north as they ponderously pushed south.

When the last of the glaciers melted from Illinois, about 15,000 years ago, the country that emerged looked far different from the preglacial land. Old hills and valleys had vanished, new ones had formed, and a mantle of unconsolidated rock material, the burden carried by the ice and dropped as the ice melted, lay over most of the region.

Most of this material, called glacial drift, was brought in by the ice during the last two of the four major periods of glaciation—the Illinoian period 100,000 to 150,000 years ago and the Wisconsinan 5,000 to 50,000 years ago. The older drift introduced during the Kansan and Nebraskan glacial periods is almost entirely buried beneath the later drifts.

The glaciers covered all of Illinois except the northwestern corner, the southwestern edge along the Mississippi River, and extreme southern Illinois, as shown in figure 1. In those areas the land is much as it was before the glaciers came. In the glaciated portion of the state, however, the bedrock generally is covered by the rock debris the ice carried from as far away as Canada. As the fringes of the ice melted, these loads of rock material were, in some places, dumped as ridges (moraines) which are the hills and mounds on the flat prairies of the present landscape. Such material also filled ancient river valleys, but new valleys were cut by torrents of water released by the melting ice.

WISCONSINAN GLACIAL DRIFT KANSAN GLACIAL DRIFT ILLINOIAN GLACIAL DRIFT

The glacial drift belongs to the youngest (topmost layer) of the major divisions of our rocks, which geologists have named the Pleistocene (scientific name for Ice Age deposits).

Most of the drift is an unsorted mixture of clay, pebbles, and boulders called “till,” but some glacial deposits consist of water-sorted sand and gravel carried and deposited by meltwater from the glaciers. Other materials were deposited by the wind—sand was piled into shifting dunes and fine silts were spread like a blanket over the land. This mantle of silt is called loess.

The glacial deposits contain a wide variety of rocks, some brought from regions to the north, others scoured from the layers of native rock in Illinois.

The limits of the Illinoian, the Wisconsinan, and the Kansan glaciations are shown in figure 1. Some of the more prominent moraines are sketched with dark gray lines on plate 1. Within this area, glacial drift covers the bedrock except along valleys where streams have cut through and removed it.

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.

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 Mostly sand, some thin beds of clay and, 0-300’ locally, gravel; present only in southern Illinois PALEOZOIC “Ancient Life” Age of Amphibians and Early Plants Pennsylvanian Largely shale and sandstone with beds of coal, 0-3,000′ limestone, and clay (“Coal Measures”) Mississippian Black and gray shale at base; middle zone of 0-3,500’ thick limestone that grades to siltstone, chert, and shale; upper zone of interbedded sandstone, shale and limestone Age of Fishes Devonian Thick limestone, minor sandstones and shales; 0-1,500’ largely chert and cherty limestone in southern Illinois Age of Invertebrates Silurian Principally dolomite and limestone 0-1,000’ Ordovician Largely dolomite and limestone but contains 500-2,000’ sandstone, shale, and siltstone formations Cambrian Chiefly sandstones with some dolomite and shale; 1,500-3,000’ 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.

Part 2

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.

STRUCTURAL GEOLOGY

The rock formations appear to lie flat in most of Illinois, but they are slightly inclined in most places. In some areas they are down-warped into basins and troughs (synclines), upfolded into domes and arches (anticlines), or broken by faults.

The largest structural feature in Illinois is a great spoon-shaped basin—the Illinois Basin—that extends southeastward into Indiana and Kentucky. The deepest part of the basin is in southeastern Illinois.

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