Prehistory to 1672
- Geologic History
- Early Peoples and Indian Civilizations
Geologic History
Prehistory means the time before man recorded events by writing. To find out about this
time we rely on information provided to us by scientists called geologists, who
study the earth, or archaeologists,
who study the artifacts ancient peoples left behind. Geologists use a geologic
timetable to describe the formation of the land and the development of
different life forms. To simplify their studies, geologists have divided the study of time
into 4 units: eons, eras, periods and epochs. They have determined the earth to be at
least 4 � billion years old! A geologic map
shows the layers of land which have developed in Illinois throughout its history.
Different rocks and minerals appeared as time passed because of glaciers,
erosion, volcanoes, earthquakes and other natural events. It is believed life began on earth
at least 3.8 billion years ago. For millions of years, a vast inland sea
covered Illinois. About 300 million years ago, the landmasses on the
Earth were joined together, and Illinois was located close to the equator.
| Fossils of
animals and plants that lived in Illinois during this time can be found in rock formations
in many parts of the state. One of the most significant discoveries of these fossils in
Illinois is the Mazon
Creek site. As time passed, the landmasses separated,
and the North American continent drifted to the north away from the equator to its present
location. From 18,000 B.C. to 16,000 B.C., an Ice Age enveloped
Illinois and glaciers
covered much of the state. As the Ice Age faded, the glaciers
retreated
and the climate became more suitable for other forms of
life. |
 The State
Fossil is known as the
Tully Monster. The first fossil of this animal was discovered by
Francis Tully in 1958. The Tully Monster was a soft-bodied carnivorous
marine animal that lived in Illinois 300 million years ago.
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Additional Resources
Study Guide Questions
- How old is the earth according to geologists? When
do they believe the first life on earth appeared?
- What was the environment in Illinois like 300 million years ago?
What type of animals and plants lived in Illinois at that time?
- At what site in Illinois have ancient fossils been
found?
- What has happened to the earth's landmasses since
300 million years ago?
- What animal life existed in Illinois after the last Ice Age around
10,000 B.C.? Why are many of these animals no longer present in Illinois?
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