_Guide to the_
GEOLOGIC MAP OF ILLINOIS
_Illinois State Geological Survey
Educational Series 7_
STATE of ILLINOIS
Otto Kerner, Governor
[Illustration: SEAL OF THE STATE OF ILLINOIS]
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
[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
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.
[Illustration: Figure 1—A mantle of glacial drift covers the bedrock
in much of Illinois.]
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.