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Title: Guide to the Geologic Map of Illinois - Educational Series 7
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Language: English
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                             _Guide to the_
                        GEOLOGIC MAP OF ILLINOIS

                   _Illinois State Geological Survey
                         Educational Series 7_

                           STATE of ILLINOIS
                         Otto Kerner, Governor


                   William Sylvester White, Director


                          John C. Frye, Chief
                            URBANA, ILLINOIS

                          42517-20M 2 (78783)

                             _Guide to the_
                        GEOLOGIC MAP OF ILLINOIS

                            GLACIAL GEOLOGY

    [Illustration: Decorative capital]

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

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.

    [Illustration: Figure 1—A mantle of glacial drift covers the bedrock
    in much of Illinois.]


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

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

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

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.

    [Illustration: 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

 CENOZOIC “Recent Life”
 Age of Mammals
       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
       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
 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
    Age of Invertebrates
    Silurian            Principally dolomite and limestone
    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
                        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.

                           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.

    [Illustration: Figure 3—When rocks are down-warped and the surface
    is leveled by erosion, the youngest rocks are preserved in the
    center and the older rocks are exposed at the margins.]

Because the entire region, including the basin, has been eroded by rain,
ice, wind, and many rivers and streams, the youngest Paleozoic rocks
(top-most layers) are preserved only in the middle of the basin, as
shown in figures 3, 6, and 7. This is why the map shows the coal-bearing
rocks of Pennsylvanian age in the center of the state and the older
rocks cropping out in successive bands around the margins of the basin
in southern, western, and northern Illinois.

Distribution of bedrock in northern Illinois is influenced by a broad
upfold or arch (see figures 4, 6, and 7). The map (plate 1) shows the
older rocks (Cambrian and Ordovician) exposed at the center of the arch
and surrounded by younger Silurian and Devonian rocks. Another elongated
upfold (anticlinal belt) extends from the vicinity of Dixon
southeastward into Indiana. The crest of the upfold is indicated on the
map by patches of Silurian and Devonian rocks in Champaign and Douglas

    [Illustration: Figure 4—When rocks are upfolded and the surface is
    eroded, the older rocks are exposed at the crest of the dome or

In extreme southern Illinois and in north-central Illinois, the rock
layers are broken by great faults (see figures 5 and 6) which displace
the layers of rocks by as little as a few inches to as much as 3,000

The structural map of Illinois (figure 6) shows the position of the
major geologic structures. The big basin is indicated by shading, with
the darkest pattern showing where the basin is deepest. The same strata
that lie at sea level at the basin’s outer edge are downwarped to 6,000
feet below sea level in its deepest part. Axes of some small anticlines
and locations of major fault zones also are shown.

    [Illustration: Figure 5—When rocks are faulted by earth stresses,
    the layers of rock are displaced or offset.]

    [Illustration: Figure 6—Major structural features of Illinois—the
    Illinois Basin, anticlines, and faults. The increasing depth of the
    basin is shown by progressively darker patterns.]

    Position of major anticline
    Major faults

    [Illustration: Figure 7—The cross section from north to south
    through Illinois shows the strata down-warped into the basin.]

                           HISTORICAL GEOLOGY

Reading the history of the earth’s crust is like reading a mystery
story. The geologist must examine, or read, each rock layer, from the
youngest at the top to the oldest at the bottom, searching for clues to
its origin, age, and development from which he can reconstruct the past.

One of the clues to the past is the fossil remains of plants and animals
found in the rocks. For instance, if a certain coral is known to have
lived only during one span of time, all rocks containing that coral
fossil must have been formed within that span.

Records of wells (well logs) and rock cores or samples collected when
test holes are drilled into the bedrock also reveal much about the
sequence of rock strata beneath the surface. The kinds of rocks
encountered tell, in addition, something about ancient geography, for
some were formed on land and others were deposited in long-departed

Such knowledge is extremely important in finding and developing mineral
resources such as coal and oil.

                            ECONOMIC GEOLOGY

Minerals produced commercially in Illinois include crude oil, coal,
limestone, dolomite, clay, sand, gravel, fluorspar, tripoli, ganister,
novaculite gravel, silica sand, and the metals zinc and lead. The
distribution of mineral industries, shown in the several maps of figures
8 through 12, is, of course, related to the distribution of the rocks
(plate 1). For example, the coal mines are scattered along the margin of
the area of Pennsylvanian rocks where the coals are at relatively
shallow depths.

The mineral fuels, coal and petroleum, are the leading mineral products
of Illinois, making up about 70 percent of the annual value of all
minerals produced in the state. For many years coal was at the top of
the list, but petroleum now holds first place.


In recent years production of petroleum has averaged about 78 million
barrels each year, making Illinois the eighth largest petroleum
producing state. The major oil area is the deep part of the Illinois
Basin in the southcentral and southeastern parts of the state, but
significant discoveries have been made recently in central and western
Illinois. There are about 490 oil fields in Illinois, ranging from a few
to several thousand acres in size.

Oil has been discovered in rocks of the Pennsylvanian, Mississippian,
Devonian, Silurian, and Ordovician Systems, but the Mississippian are
the most productive and account for about 76 percent of our total oil

    [Illustration: Figure 8—Oil pools in Illinois as of January 1, 1961.
    The pools are concentrated mainly in the Illinois Basin and along
    the LaSalle Anticlinal Belt.]

    [Illustration: Figure 9—Active coal mines are scattered around the
    margin of the basin where the coals are at relatively shallow



Illinois ranks fourth in the nation among states that produce bituminous
coal, the type of coal most widely mined in the United States. Coal is
used by many manufacturing industries, by public utilities companies
that burn coal to generate electricity, and by individuals for heating

All Illinois coal deposits are found in the Pennsylvanian System. Of the
many coals, only a few are thick enough to be mined profitably. The coal
is of bituminous rank, but it varies in quality. Illinois coal reserves
in strata more than 28 inches thick are estimated at 137 billion tons
and are larger than those of any other state.

Around the edge of the coal basin where the coal lies at shallow depths,
it is commonly mined in open pits (strip mines). However, in the past,
most coal has been mined underground, and more than half of Illinois
coal is still produced from such mines.

                         Limestone and Dolomite

There are two types of carbonate rocks in Illinois, limestone and
dolomite. In appearance they are almost indistinguishable, although they
are different chemically. Limestone is composed mainly of calcium
carbonate (the mineral calcite), whereas dolomite is calcium magnesium
carbonate (the mineral dolomite).

Limestone deposits are found at many places in the western,
southwestern, and southern marginal portions of the state. They are
largely of Mississippian and Ordovician age. Lesser deposits of
Pennsylvanian limestone are found in the central portion of the state.
In the northern fifth of the state, dolomites of Silurian and Ordovician
age are exposed at many places.

Limestone and dolomite are quarried for a variety of uses, including
aggregate for concrete, agricultural limestone, railroad ballast, chips
for bituminous roads, and for use in metallurgical and chemical
processes. They also are quarried for building stone at some places, and
marble is produced in southern Illinois.

    [Illustration: Figure 10—Limestone and dolomite are quarried at many
    places, but fluorspar, zinc, and lead are produced in restricted
    mineralized areas.]

    Quarry, Limestone or Dolomite
    Cement or Lime
    Zinc and Lead

Limestone and shale or clay are burned together in kilns to produce
portland cement. Portland cement is manufactured from a Pennsylvanian
limestone in LaSalle County and from an Ordovician limestone in Lee

Limestone or dolomite also is burned in kilns to make lime.
Mississippian limestone is used in Adams County and Silurian dolomite is
used in Cook County for lime making.

                         Clay and Clay Products

Illinois commercial clays are found in glacial, Cretaceous, Tertiary,
and Pennsylvanian deposits. Pennsylvanian shales also are sources of
clays. Glacial clays, which occur as loess (wind-blown materials),
alluvial (water-deposited) sediments, and glacial till, are widespread
in the state. Cretaceous and Tertiary clays, at the extreme southern tip
of the state, are valuable for the manufacture of heat-resistant bricks
and coatings and for floor-sweeping compounds.

Pennsylvanian clays and shales are the most important source of clay for
manufacturing such products as pottery, stoneware, drain tile, sewer
pipe, flue tile, building tile, brick, and special heat-resistant

                            Sand and Gravel

Deposits of sand and gravel, found in many parts of Illinois, provide
large quantities of material for concrete, railroad ballast, road
gravel, building sand, molding sand, and other uses. Glacial deposits
are the principal sources of sand and gravel, but in extreme southern
Illinois where there is no glacial drift the main source of supply is
Tertiary gravel and Devonian chert gravel.

    [Illustration: Figure 11—The clay industry is scattered widely in
    Illinois. Many manufacturing plants are located at the clay pits.]

    Pit or Plant

Almost every county has some sand and gravel, but the major areas of
production are northeastern Illinois and some of the major rivers.
Considerable tonnages of sand and gravel are dredged from bars in the
channels of the Mississippi, Ohio, and Wabash Rivers.

                              Silica Sand

Silica sand is used in making glass, for sand blasting, as grinding
sand, as molding sand for casting steel, and in the fracture treatment
of oil wells to increase oil production. Some of the sand is finely
pulverized into a powder that is used as a filler, a fine abrasive, as a
ceramic material, and for many other purposes.

This special sand, composed of grains of the mineral quartz (silica),
comes from a sandstone of Ordovician age that is mined in LaSalle and
Ogle Counties.

                          Tripoli and Ganister

Tripoli, or “amorphous” silica, is mined from Devonian rocks in
Alexander County. The silica is finely ground for use as a polishing
agent, as a filler, in buffing compounds, and for other purposes.

Ganister, a granular material with a high silica content, also is mined
in extreme southern Illinois. It is used in making products that can
withstand high temperatures.


For many years Illinois has led the nation in the production of
fluorspar, accounting annually for about half the total United States
output. The industry is centered in Hardin and Pope Counties where the
deposits occur in veins and beds in rocks of the Mississippian System.
Reserves of fluorspar are adequate to supply the industry for many

    [Illustration: Figure 12—Sand and gravel is produced at many places
    in the state. Silica sand is produced only from a sandstone in
    northern Illinois. Tripoli is found only in southern Illinois.]

    Sand and Gravel
    Silica Sand
    Molding Sand

Fluorspar is used in the manufacture of hydrofluoric acid, as a flux in
the manufacture of steel, in the refining of aluminum, for the
manufacture of fluorine compounds used in a variety of products, in
ceramic glazes, and for other special purposes.

                             Lead and Zinc

Lead and zinc are the only metallic minerals mined in Illinois. Deposits
are found in Jo Daviess County in northwestern Illinois and in the
fluorspar area of Hardin and Pope Counties in extreme southern Illinois.
In northwestern Illinois the ore occurs in rocks of Ordovician age, but
in southeastern Illinois it is associated with fluorspar in rocks of
Mississippian age.

Between 1820 and 1865, the mining area of which northwestern Illinois is
a part was the nation’s principal producer of lead ore. In recent years
production of zinc has increased and that of lead has decreased.

                             Water Supplies

Water, too, is a mineral resource, and our most necessary one. The
source of nearly all water is rain and snow, which collect in lakes and
streams or seep into the ground to form our two types of water
supplies—surface water and ground water.

Ground water in Illinois collects underground in deposits of sand and
gravel or in the porous sandstone or limestone layers of bedrock. These
rock materials are called “aquifers” because of their ability to hold
water and to allow it to flow into wells that are drilled into them.

The aquifers are not equally distributed throughout Illinois, so
ground-water supplies are abundant in some parts of the state but are
scarce in others. Some Illinois cities depend wholly or partly on
surface supplies from lakes and rivers.

The sand and gravel aquifers yield more ground water than any others in
Illinois. The major sand and gravel aquifers were deposited by streams
and lie along river valleys. Some of these river valleys are no longer
visible to us because they were buried by glacial drift. They are
recognized, however, from well records or drilling records.

The northern third of Illinois has the most abundant supplies of ground
water. Bedrock formations of Devonian through Cambrian age are favorable
aquifers and supply drinkable (potable) water at depths of from 200 to
2,000 feet. These formations, however, lie much deeper in the Illinois
Basin, where the water they furnish is too salty for most uses.

West of the Illinois River and at the southern tip of the state, the
Mississippian, Devonian, and Silurian limestones are the chief aquifers.
Their yield, however, is small.

In the Illinois Basin all these formations are overlain by Pennsylvanian
rocks which offer only small, scattered supplies of ground water.


A city or farmer needs a new water supply. Where can suitable
water-bearing formations be found? A new limestone quarry is considered.
Where are the most suitable limestone deposits? A coal mine is being
planned. How deep must a shaft be sunk to reach the nearest minable coal
and what will the mining conditions be? A land owner has a clay deposit
on his land. What uses can be made of it? A road is to be constructed.
Are sand and gravel or limestone deposits available nearby for
construction materials? A farmer wants to know whether there might be
oil on his property. Are oil-bearing structures present and, if so, at
what depth?

These are typical questions brought to the Illinois State Geological
Survey by the industries and people of Illinois. From its accumulated
store of information and experience, the Survey provides answers. This
storehouse of knowledge is the result of the Survey’s continuous program
of geologic exploration, mapping, research, and organization and
interpretation of data.

The Survey explores and maps the geology and mineral resources of the
state, does research to find new and improved uses for our mineral
resources, and organizes and interprets basic geologic information
gathered from well logs, rock samples, and field studies, putting them
all into usable form.

Such knowledge is shared with the public through publications, maps,
lectures, and personal consultations with industries, drillers,
engineers, land owners, teachers, and others in need of geologic

The discovery of facts about the rich mineral resources of Illinois has,
in effect, only begun, and it is the task of the Geological Survey to
continue to collect and apply new information so that our resources can
be used to the best advantage.


The Educational Extension Section of the Geological Survey conducts six
field trips each year, in various parts of the state, for teachers,
students, and laymen. It also assembles and distributes rock and mineral
collections for Illinois educational groups, gives lectures, prepares
exhibits, and identifies rocks and minerals for the public.

Educational Extension publications, such as this book, are simplified
discussions of geological subjects. Available for 25 cents each, these

    Educational Series 4: Guide for Beginning Fossil Hunters, by Charles
          W. Collinson.
    Educational Series 5: Guide to Rocks and Minerals of Illinois.
    Educational Series 6: Field Book of Pennsylvanian Plant Fossils of
          Illinois, by Charles Collinson and Romayne Skartvedt.

Many technical discussions of the various phases of Illinois geology
also are published by the Geological Survey. Regional reports on the
geology and mineral resources of some areas are available in many school
and public libraries or they may be purchased from the Survey. Some of
the regional reports include:

    Beardstown, Glasford, Havana, and Vermont Quadrangles, Bulletin 82,
    Buda Quadrangle, Circular 275, no charge.
    Carlinville Quadrangle, Bulletin 77, $1.00
    Chicago Region, Bulletin 65, Part 1, 50 cents.
    Marseilles, Ottawa, and Streator Quadrangles, Bulletin 66, $1.00.

“Mineral Production in Illinois in 1959,” Circular 300 (no charge), is
one of a series of annual economic summaries. “Caves of Illinois,”
Report of Investigations 215, price 50 cents, will be available by
September 1961.

         Illinois State Geological Survey Educational Series 7
                  24 pages, 1 plate, 12 figures, 1961

    [Illustration: Illinois State Geological Survey, Urbana Educational
    Series 7—Plate 1—North]

    [Illustration: Illinois State Geological Survey, Urbana Educational
    Series 7—Plate 1—Central]

    [Illustration: Illinois State Geological Survey, Urbana Educational
    Series 7—Plate 1—South]

    T  Tertiary (Pliocene omitted)
    K  Cretaceous
    P²  Pennsylvanian (Above No. 6 Coal)
    P¹  Pennsylvanian (Below No. 6 Coal)
    P³  Pennsylvanian (Above No. 6 Coal)
    M²  Mississippian (Upper)
    M¹  Mississippian (Middle and Lower)
    D   Devonian
    SD   Silurian and Devonian
    S   Silurian
    O   Ordovician
    C   Cambrian
    F  Fault
    OSDM   Complex faulted area
  Physiographic base map by James A. Bier
  Price, 25 cents


                          Transcriber’s Notes

—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook
  is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by

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