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.
[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
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.
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.