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THE PENNSYLVANIAN GEOLOGICAL PERIOD
IN ILLINOIS  325 to 285 Million Years Ago

During the Pennsylvanian Geological period, from 325 to 285 million years ago, Illinois was a vastly different place that what we see today. The Great Lakes, the gently rolling prairies, and the Mississippi, Missouri and Ohio Rivers that we are so familiar with, did not exist until the end Ice Age, less than 20,000 years ago.

During the Ice Ages the Illinois Glaciation from 500,000 years ago to 125,000 years ago, covered over 9 million square miles, from New York City westward to St. Louis and Seattle, as well as nearly all of Canada. Glaciers spread southward into the Midwest from two centers of ice accumulation in western and eastern Canada. At it's maximum it reached the northern slopes of the Shawnee Hills deep in southern Illinois. This huge glacier sloped upward towards the north, and 75 miles from it's leading edge the glacier was over a mile thick. Farther up the slope the glacier was as much as 2 miles thick. As the glaciers advance they grind and pulverized the earth below them, and when they retreated the uncovered surface was blanketed by as much as 300 feet of wind blown soil that was left behind.

The great rivers, the Missouri, Mississippi, Ohio and Illinois did not reach their current course until relatively recently. ( See Glacial Diversion of the Mississippi River. )

To put this into a historical perspective, the great lakes, which hold one-fifth of the fresh water on the earth's surface, did not reach their current shape and size until 9,000 years ago, about the time the ancient city of Jericho was founded.  Also, the water level of the lakes did not stabilize at the current level until approximately 2,500 years ago, about the time that Pericles began construction of the Parthenon on the Acropolis in Athens Greece.

THE PENNSYLVANIAN LANDSCAPE

 

The land forms that we are so familiar with are a modern replacement for a terrain that, during the Pennsylvanian period, was a cross between the bayous of southern Louisiana and the rain forests of Central and South America, similar to the drawing to the above. At one time or another during the Pennsylvanian Period almost all of Illinois was underwater. The area where the coal was formed was a vast swamp along the edge of a huge inland sea called the Chicago Sea, that stretched from Ohio to the west coast of North America.

The land was more heavily forested than in the drawing above, and were probably quite similar to some of the swamps in the Cache River Natural Area, in Johnson and Polaski Counties in extreme Southern Illinois.

During this time Illinois was located approximately 20 degrees south of the equator, with the year around tropical environment that accompanied the location. There was no seasonal  variation so trees and vegetation grew continuously pilling up tremendous amounts of leaves and plant material that turned into peat and then eventually into the coal we mine.  The types of plants that existed at the time were very different from that we have today, but two insects that we would recognize have been found as fossils. The earliest know cockroaches come from this time period, and giant dragonflies with 30" wingspans.

Below is a picture of a Pennsylvanian Period, coal forming swamp reconstructed as a living Biome. This period is famous for its vast coal swamps, such as the one depicted here. Such swamps produced the coal from which the term "Carboniferous", or "carbon bearing" comes from.


Image from the Field Museum of Natural Nistory. Text from A View of the Past, An Introduction to Illinois Geology, Christopher J. Schuberth, Illinois State Museum.

The numbers on the illustration above show the following Plants and animals:
1.  Lepidodendron tree 
2.  Sigillaria tree
3.  Neuropteris fern

4.  Sphenophyllum plant
5.  Calamites Tree
6.  Cordaites tre
7.  Annularia leaves

In addition, this diorama also shows 2 insects. Just above the number "2" on the left tree you will find a   giant cockroach, and if you look very carefully between the rightmost tree marked "2" and to the right tree (# 5) you will see a giant dragonfly.

PAGE LAST UPDATED: MARCH 4, 2002