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To find out what was happening
in Illinois during specific time periods of Earth history, scroll
down the time period of choice. A nice, printable geologic time
table from the Illinois State Geologic Survey can be be viewed.
This site gives more detail about the specific time period divisions
and general character of the sediments deposited at that time.
PRECAMBRIAN:
The Precambrian time period is divided into the Phanerozoic and
the Proterozoic Eras. During the Precambrian not a lot happened
in Illinois. Cascade style volcanics
and granite intrusions occurred 1.5 billion years ago. Later, at
1.15 billion year ago the rifting (separating) of continental plates
created a weak zone along which the Mississippi River later formed.
This is the zone of the New
Madrid Earthquake. top
CAMBRIAN:
Illinois was emergent for most of the Cambrian period. Toward the
end of the Cambrian the sea came in and deposited the sands and
muds that created the dolomites, sandstones and shales now found
in Illinois. top
ORDOVICIAN:
Illinois was covered by shallow seas during the Ordovician. Marine
limestones and dolomites were deposited. In the late Ordovician
sands were deposited. The St. Peter Sandstone, which formed from
these sands, creates the backbone for Starved Rock, Buffalo Rock,
and Mattheissen
State Parks. top
SILURIAN:
Once again shallow seas covered Illinois. The seas at this time
were teeming with life. Near Chicago was an extensive reef,
with corals, hard-shelled organisms (like snails and clams), trilobites,
crinoids, and more. These reef limestones are now quarried for construction
materials. top
DEVONIAN:
Deep stagnant basins occurred in Illinois during the Devonian. The
sediments deposited in these basins would later form thick black
shale deposits, which are also found in Indiana and Kentucky. Important
changes in life occured during the Devonian,
such as the development of fish and amphibians. top
MISSISSIPPIAN:
Shallow seas once again invaded Illinois. Limestone were deposited
which form the caves
and karst topography of Western Illinois. The
lead and zinc deposits of Northwestern Illinois, Iowa, and Wisconsin
were formed at this time. top
PENNSYLVANIAN:
Coastal swamps
dominated the Illinois landscape. These swamps later formed the
vast coal
deposits that underlie 2.3 of the state. Land plants were abundant
strange swamp creatures
exsisted, including the state fossil, the Tulley
Monster.top
CRETACEOUS:
The Age of Dinosaurs! But not in Illinois. Illinois was emergent
during this time, except for some near shore clays and sands that
were deposited. However, three exciting things were happening in
Illinois. A meteorite
hit outside of Chicago, the beautiful state mineral,
fluorite, was formed, and strange igneous rocks, called cryptovolcanic
structures, were intruded below the surface.top
TERTIARY/QUATERNARY:
During the Tertiary marine sands and clays were deposited in Southern
Illinois. These absorbent clays, called Fuller's Earth, are used
as kitty litter! The Ice
Age occurred during the latest part of the Quaternary, the Pleistocene
epoch. Illinois was covered by four different continental glaciers,
filling in the river valleys and carving the Great Lakes. When Illinois
was not covered by ice, huge animals
roamed the area. top
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