_Educational Series 5_
_Guide to_
ROCKS AND MINERALS
OF ILLINOIS
_Illinois State Geological Survey_
[Illustration: uncaptioned]
STATE of ILLINOIS
William G. Stratton, Governor
DEPARTMENT of
REGISTRATION and EDUCATION
Vera M. Binks, Director
First printing 1959
Second printing 1960
ILLINOIS STATE GEOLOGICAL SURVEY
John C. Frye, Chief
URBANA, ILLINOIS
Printed by Authority of the State of Illinois
_Guide to_ ROCKS AND MINERALS OF ILLINOIS
Illinois has so long been known as the Prairie State that at first
glance it seems a most unlikely place in which to collect rock and
mineral specimens.
But Illinois has a surprising wealth of rock and mineral resources, not
only to be collected as interesting specimens but to be put to practical
and profitable use.
The rich prairies that gave the state its nickname are themselves
derived from ancient rocks, worn and changed by millions of years of
action by weather, water, wind, plants, and animals. Unmeasured depths
of rock underlie the prairies, hills, and valleys, and in some parts of
the state are exposed in outcrops, canyons, and river valleys. Boulders
and gravel brought in by the glaciers thousands of years ago are strewn
over many parts of the state.
These resources are of great value. Besides the rich agriculture based
on the rock-derived soil, much of our industry, manufacturing, and
transportation is dependent on rock and mineral materials. Every county
in Illinois possesses some rocks and minerals that either are being used
or have potential future value.
The Illinois State Geological Survey several years ago began to prepare
sets of typical rocks and minerals of Illinois for use by the schools
and other educational groups in Illinois. This booklet is designed to
furnish a brief geological background and explanation of these common
Illinois rocks and minerals. It also should be useful to the student or
amateur interested in making his own collection.
Even though Illinois has no mountain ranges or deep canyons, the geology
of the state has many complexities. In fact, the very flatness of our
topography is a complicating factor because in order to study the
geology at many places in the state it is necessary to use information
from mines and descriptions (logs) and samples (cores) of the rock
penetrated during drilling of deep wells. There are also geophysical
methods of learning something about the rocks beneath the surface.
[Illustration: Fig. 1.—Geologic map of Illinois.]
CAMBRIAN
ORDOVICIAN
ORDOVICIAN-SILURIAN
ORDOVICIAN-SILURIAN UNDER DRIFT
DEVONIAN-MISSISSIPPIAN
PENNSYLVANIAN UNDER DRIFT
CRETACEOUS-TERTIARY
KEY
Wisconsin glacial drift
Illinoian glacial drift
Pennsylvanian boundary
The complexity of Illinois geology is not produced by the upturning and
sharp folding of rock layers such as can be seen in the Rocky Mountains,
but rather by the changes in composition, thickness, and character of
the rock layers that are only gently warped or relatively flat. At
several places in the state, especially in the southern part, faults, or
breaks, in the rock layers do occur, but over much of our area this is
not common.
The presence of usable minerals at considerable depth is known at many
places; coal is mined from depths greater than 800 feet, and oil is
produced from saturated rock layers, called pay zones, several thousand
feet below the surface. Lead and zinc ores, fluorspar, silica sand,
limestone, sand, gravel, clay, and shale are all produced at shallower
depths. However, the student can see only those rocks and minerals that
are to be found at or near the surface. For that reason the following
paragraphs describing their geologic occurrence deal only with surface
geology.
The youngest of the major geologic divisions of our rocks is called the
Pleistocene, which is the scientific name for the “Ice Age” deposits.
During this relatively recent period of geologic time, which began about
a million years ago, glaciers flowed southward from Canada and spread a
layer of “glacial drift” over all of the state except the northwest
corner, the southwest edge of the state along the Mississippi River, and
extreme southern Illinois (fig. 1).
Most of the glacial deposits that we see were formed by the last two of
the four major periods of glacial advance, the Illinoian and the
Wisconsin. The Illinoian was the most extensive, reaching as far south
as Carbondale and Harrisburg. The Wisconsin, so called because its
deposits are so widely spread in that state, reached only to Mattoon and
Peoria.
The glacial drift is the youngest and uppermost of the divisions of the
rock column (fig. 2). Within the drift can be found the widest diversity
of rock and mineral types—quartzite, schist, and other metamorphic
rocks; granite, gabbro, and other igneous rocks; and of course the
sedimentary rocks, limestone, dolomite, sandstone, shale, and even
pieces of coal, which occur in bedded layers of the older rocks in
Illinois.
Sand and gravel were carried and deposited by flowing streams before,
during, and after glaciation, but the major deposits were made while the
glaciers were melting. They contain a wide variety of rock and mineral
types.
[Illustration: Figure 2—Diagram of layers of rocks in Illinois.]
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
As shown by the diagrammatic rock column (fig. 2), rocks placed in the
divisions called early Cenozoic and Mesozoic are next in age to the
Pleistocene. The map (fig. 1) shows that the Cenozoic and Mesozoic rocks
occur only in the extreme southern tip of Illinois because only that
part of Illinois was covered by a northward extension of the forerunner
of the Gulf of Mexico in which the deposits of sand, gravel, and clay
were laid down.
The next older division of Illinois rocks is called Pennsylvanian—or
“Coal Measures”—because during the last century they were first
extensively described by geologists working in Pennsylvania.
The Pennsylvanian is one of our most important groups of rock strata
because it contains all of our minable coal beds, as well as important
deposits of limestone, shale, clay, sandstone, and some oil and gas. The
Pennsylvanian rocks are very widespread in Illinois, occurring under the
glacial drift from depths of a few feet to several hundred feet
throughout about two-thirds of the glaciated area (fig. 1).
Next below the Pennsylvanian are the Mississippian rocks. We in Illinois
are particularly interested in this division of rocks because they take
their name from the excellent exposures along the Mississippi River
valley in western Illinois, southeastern Iowa, and eastern Missouri.
They are composed of extensive beds of limestone and cherty limestone,
sandstone, and shale.
Mississippian rocks are of great economic importance in the structural
area known as the Illinois Basin, where they are the most important oil
producing rocks. They also contain our fluorspar deposits and along the
valley bluffs are an excellent source of limestone for quarrying.
Rocks older than the Mississippian—except for small areas along the
Mississippi and Illinois River valleys—are found at the surface only in
the northern quarter of the state and locally in Hardin County near the
southern tip of the state. They are nonetheless economically important
because from these older rocks are produced lead and zinc, some oil and
gas, silica sand, limestone, dolomite, and shale.
On the generalized rock column (fig. 2) these older rocks are grouped
into two units. The uppermost contains the Devonian and Silurian and the
lower contains the Ordovician and Cambrian. In general they include
dolomite, limestone, and shale, with sandstone at several places,
especially in the lower unit.