Geology & Fossils
Sea World, Midwest
Beneath fertile soils, blankets of sedimentary rocks from the Pennsylvanian Period, which ended about 280 million years ago, cover most of central and southern Illinois. Marine invertebrates are especially common in some Mississippian formations in the southern Shawnee Hills and northern Ozarks.
Fossils of Paleozoic marine invertebrates are found in the northern lowlands as well. These fossils indicate that Illinois lie under shallow seas during much of the Paleozoic Era, as did neighboring states.
Mazon Creek
Illinois’ most famous fossil site is Mazon Creek. Scientists think the area was a delta where one or more large, sluggish rivers entered a shallow subtropical sea. The sea in turn covered most of what is now Illinois during the Pennsylvanian Period, about 300 million years ago.
Scientists have found and described more than 350 types of plants and 320 animals from fossils from Mazon Creek, including both land-dwelling and marine animals. The most famous is a bizarre creature known as the Tully Monster, which was adopted as Illinois’ state fossil.
Rapid burial in the delta’s soft sediments created some remarkable fossils. Some are soft-bodied animals that are rarely found as fossils. Color patterns can even be seen on the skin of some of these animals! Traces of gills, internal organs, and hatchlings with yolk sacs can be seen on fossil fishes. Tentacles can be seen on fossils of animals similar to jellyfishes.
About 140 insect fossils have been found and named here. Forest dwellers included more than 60 species of millipedes, centipedes, scorpions, and spiders, and spider allies. Nine known small amphibians preyed upon them. More than 34 species of freshwater fishes, shrimps, and horseshoe crabs inhabited ponds and streams in forests that grew on shore. In the near-shore waters of the delta front were fishes and a variety of invertebrates, including mollusks, shrimps, and worms.
Fossils in Our Lives
Plant fossils found in Pennsylvanian rocks remind us that Illinois wasn’t always under water during the Paleozoic. One of the world’s great coal deposits is found in southeastern Illinois, in the Illinois Basin.
About 300 million years ago, during the Pennsylvanian Period, the equator ran through Illinois. Like today’s Amazon or Congo, the Illinois Basin was cloaked in lush tropical forests and coastal swamps. Where rivers emptied into the sea, they deposited sediments in vast deltas. Deposited along with the sediments were dead plants and animals the rivers also carried.
Forests grew on these deltas and other coastal lowlands. Slight movements of the continent allowed shallow seas to invade, burying forests, which became coal.
About 80 distinct coal layers are recognized, each representing a former forest that flourished on ancient deltas. The coal layers vary in thickness from a few inches to 15 feet.
Today, Illinois is a leader in the production of bituminous coal. Illinois communities named for coal include Carbon Cliff, Carbondale, Glen Carbon, Coal City, and Coal Valley.
Illinois’ Very Own Ice Age?
Perhaps as few as three glaciers reached Illinois during the Ice Age. About 500,000 years ago, the third glacier extended to a latitude of 37.5 degrees North in Illinois, setting a record. No Ice Age glacier had reached so far south. Named the Illinoian Glaciation, it covered 90% of the state.
But the Ice Age wasn’t always cold. Between glaciations, the climate warmed and the glaciers retreated. About 300,000 years ago, the climate became so warm that temperatures did not fall below freezing even during the coldest winter months. During these year-round, frost-free centuries, a distinctive soil formed on the glacial till. Scientists first recognized this soil in Sangamon County, in central Illinois, near the capital city of Springfield. They named this warm period between glaciations the Sangamonian Interglacial.
About 75,000 years ago, temperatures once again dropped. Snow began accumulating in Canada. Over a period of centuries, the snow piled into ice and began spreading southward once again. The Sangamonian Interglacial period had ended.
But the fabulous soils that made Illinois great remained. Enriched by prairie grasses, loess developed into the fertile black loam soil that blankets the broad central section of the state. Its rich soils made Illinois one of the nation’s most productive agricultural states. An abundance of minerals made it a leader in manufacturing. Extensive waterways enabled Illinoisans to easily export agricultural and manufactured goods, allowing Chicago to grow into one of America’s great cities.
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