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VOCABULARY: aulacogen, brachiopod, dynamic, emergent, orogeny, Pangaea, radiometric, stromatolites

HOW OLD IS THE EARTH? Radioisotope data from meteorites and moon rocks indicate ages of 4.5 to 4.7 billions years before the present, and scientists consider this time to represent the beginnings of our solar system. It is curious that the oldest rocks found on Earth have radiometric ages of only 3.8 billion years old.

These ancient rocks may record one of the first mountain building events of the young planet Earth. Earth has been a dynamic planet for a long, long time. Continents have come together and broken apart several times before Pangaea, the most recent time when all the continents were together as one. Top

RODINIA ROCKS! All continents were united in a supercontinent called Rodinia. 1.7 billion years ago and began to break apart 1.5 b.y. ago. This time of continental breaking and stretching was accompanied by volcanic activity. During this time Illinois was on the edge of the continent. Volcanoes may have dominated our landscape - violent, eruptive volcanoes, like Mt. St Helens, whose rocks are similar to those found very deeply buried beneath the Illinois corn fields. Top

The separated continents began to move together during what is called the Grenville Orogeny, 1.2 b.y. ago. As the continents pushed together, sediments and rocks got compressed and pushed upward. Lofty mountains probably marked the zone of joining, creating another supercontinent, Pannotia.

THERE'LL BE A WHOLE LOT OF SHAKIN' GOIN' ON! Years of erosion followed, and 1.15 b.y. ago the sutured supercontinent once again began to break apart. Cracks developed along what is now the Gulf of Mexico and up what is now the Mississippi Valley. Cracking continued along the Gulf but not along the Mississippi Valley; the continents separated, leaving behind a "failed arm" or aulacogen, which would be an area of weakness for the rest of geologic time - a zone along which one of the world's largest rivers flows; a zone we know as the New Madrid Earthquake System. Top

DISCOVERED - LIFE ON EARTH! During the ages of volcanism associated with plate motion, the earth's atmosphere slowly changed to one that could support primitive life forms. Among the oldest fossils are stromatolites, found in rocks 3 b.y. old. The fossil record in Illinois, however, did not begin until the Paleozoic Era, which began 542 m.y. ago. During the earliest Paleozoic, the Cambrian Period, the Midwest was emergent, so Cambrian rocks are scarce. It was not until the end of this time that the Cambrian sea slowly covered Illinois with a thin blanket of sand.

By the beginning of Ordovician time (490 m.y.) the sea had deepened in the Midwest, and Illinois was the site for the chemically deposited limestones and dolomites that are so abundant today.The Appalachian Mountains were on the rise during the middle Ordovician Period, as the continents began to come together to form Pangaea.Sandstone and shale began to wash into the shallow Illinois sea as rivers carved away at the emerging highlands to the east. The St. Peter Sandstone that resulted from this sand now forms the ridges near Starved Rock and Matthiessen State Parks and around Oregon, Il. Top

TROPICAL ILLINOIS! The next time period, the Silurian (455 m.y.), is well represented in Illinois. Silurian limestone is the dominant type of bedrock of northeastern Illinois. Life flourished in widespread warm seas. Illinois ws equatorial during the Silurian,and reef development, teeming with sponges, corals, and shelled organisms, was extensive, extending from what is now the Ozarks to New York. Along I-80 outside Chicago is a large quarry, the Thornton Quarry, which is in the Silurian reef limestones and dolomites.

By the late Paleozoic all the continents gradually united to form the last super continent, Pangaea. Because of the mountain-building activity during part of the Devonian Period (405 m.y. ago), Illinois was emergent, and the sedimentary record is absent. But by late Devonian, the seas had returned. A deep basin had developed in the Midwest, called the Illinois Basin, and black, organic-rich sediments were carried into the stagnant basin forming a thick layer of black shale found in Illinois, Indiana, and Kentucky.

During the Devonian the evolution of amphibians began. Until the appearance of these vertebrates, the Devonian landscape was dominated by insects, spiders, and primitive plants. The diversification of life on land was underway.Top

GET THE LEAD IN! Widespread seas returned to the Illinois area during the Mississippian Period (355 m.y. ago). The resulting limestones are thick and rich in fossils, such as crinoids (sea lilies) and shelled organisms. Trilobites, on the decline since the Devonian, would soon be extinct, as the more adaptive fishes replace them in the ecological niche.

Near the end of the Mississippian, the structural effects of the rising Appalachians were felt in Illinois in two ways. The lead and zinc mineralization around Galena occurred at this time, as hot brines moved upward through the rocks, carrying lead and zinc with them. Millions of years later, French explorers to northern Illinois discovered lead ore (galena) mines used by the Native Americans. These deposits proved to be very important during the westward expansion of a fledgling nation. Illinois' first industry developed from the mining and processing of the lead for ammunition. These limestones are the sites of many of Illinois' caves and karst topography.Top

COAL-CAN YOU DIG IT? Rivers flowing from the rising Appalachians carried sediment that was deposited throughout Illinois. Broad deltas spilled into the Illinois Basin, setting the coastal floodplain environment for a very important time in Illinois history: the Coal Age.

By Pennsylvanian time (300 m.y. ago), the final stages of the Pangaea plate collision resulted in a gradual uplift of the continent and withdrawal of the Mississippian seas. The Illinois region was left a coastal swamp, similar to the Everglades, the Dismal Swamp, or the Louisiana Bayous. Luxuriant forests covered the flat Illinois landscape. The area was along a coast with a large river meandering back and forth, forming sandy deltas which frequently change abruptly from one place to another. This river emptied into the Illinois Basin, forming a vast and wandering delta/swamp environment. The resulting accumulating organic deposits formed vast coal deposits, of great economic importance, which underlie about 2/3 of the state. Top

CREATURE FROM THE BLACK LAGOON! Much has been learned about the life forms of the Pennsylvanian Period in Illinois from the fossils from Mazon Creek. Special conditions of rapid burial and preservation have resulted in fossilization of both soft-bodied and hard-shell sea creatures and many terrestrial plants and animals. The area of Mazon Creek was along a coast, with both swampy, forested areas and shallow coastal marine zones. Organisms found in Mazon Creek concretions include jellyfish, hatching fish with egg sacs, clams, insects, amphibians, shrimp, crabs, and, of course Illinois own curious Tulley Monster. The Tulley Monster is a shrimp-like animal found nowhere else but the Mazon Creek area of Illinois. It is the state fossil.

Pennsylvanian sandstones and Mississippian limestones have been carved into the interesting formations seen at Garden of the Gods in Shawnee State Forest, in the southern part of the state. Top

METEORITE HITS CHICAGO! Following the Pennsylvanian Period is a huge gap in the Illinois geologic record. Illinois may have been emergent during the Permian Period (280 m.y. ago) and throughout most of the Mesozoic Era. Rocks of the three periods of the Mesozoic (Triassic, Jurassic, Cretaceous - the age of dinosaurs) are absent in Illinois, except for a small deposit of Cretaceous sediment in the southern part of the state. Some very important events occurred during this time which affected Illinois.

Many meteorite impact craters formed during the Mesozoic Era. As a matter of fact, some scientists attribute the triggering of the splitting of Pangaea and the Cretaceous extinctions of dinosaurs to cataclysmic meteorite impacts. Two such impact structures exist in Illinois, buried under hundreds of feet of glacial debris. One, which may be of Ordovician age, is near Glasford, Il. The second underlies Chicago's neighbor, Des Plaines. It has been determined to be post-Permian, and may be part of the Mesozoic meteorite madness. Top

VOLCANO EXPLODES IN ILLINOIS! The Cretaceous was a time of volcanism as well as impact in Illinois. A line of five volcanic crater-like features, called cryptovolcanic structures, extends westward from Illinois into Missouri. All of these are roughly circular, are associated with faults and fractured rock, and are accompanied by unusual (in Illinois) igneous rocks and mineral deposits. Magma deep below the surface moved upward, releasing gas and hot liquids in violent explosions in some areas. One of these places is the Hicks Dome cryptovolcanic structure, in the southern part of the state. During its formation it produced mineral-rich fluids that moved upward through rocks in southern Illinois near Rosiclare, leaving behind the beautiful and valuable deposits of fluorite, the state mineral.

The Cenozoic Period began 63 m.y. ago. Pangaea continued to move apart, as it continues to do today. In the West the moving plates crumpled the edge of the continent causing folding and faulting of rock and sediment and the formation of the Rocky Mountains. The Early Cenozoic was not as exciting in Illinois, however, as the general period of non-deposition of the rock record continued. Illinois was emergent at this time, except for the southern part of the state, where 400 feet of sand and clay were deposited by rivers as they emptied into the the shallow sea. These clays, called "Fuller's Earth." were found to have very absorbent properties, and, in 1947, a new Illinois industry was created around the mining and processing of this clay for catbox litter! Top

ILLINOIS PREPARES FOR BLIZZARDS! The Cenozoic Era was the beginning of modern life. Mammals occupied the air and water, but dominated the land; this was the Age of Mammals.Many types of mammals evolved, including human life forms, whose appearance would ultimately dominate and change our planet forever. The warm bloodedness of mammals allowed them to be more adaptive to changing environments and to be better able to survive harsh climates. During the Late Cenozoic, the Pleistocene, conditions changed and, in more northern latitudes the climate became colder as the Great Ice Age began.


The surface of Illinois was profoundly affected by the Ice Age, as massive glaciers moved across the state. The rolling plains comprising most of Illinois are made of hundreds of feet of glacial deposits. Ponds, bogs, and lakes dot the till which filled in valleys and covered most of the state with sands, gravels, and soil. Bluffs of wind blown glacial silt, called loess,developed along the Mississippi River Valley. The glaciers blocked rivers and caused them to change their direction of flow; they gouged out other river valleys, widening and deepening them. These would fill with water from the melting glaciers to form the Great Lakes.Not all of Illinois, however, was covered by glaciers. Near Galena, there is a driftless area around Apple River Canyon State Park. Top

GIANT ANIMALS RUN WILD! The thick, fertile glacial debris that covers the state makes excellent farmland. There is more than corn, however, that comes out of the glacial till. Mastodon and mammoth fossils have been discovered in Illinois. Fossil evidence indicates that stag moose and giant beaver, bison and tortoises roamed the grasslands and forests that stretched before the ice sheets.

The area around Chicago is not directly covered by glacial material. Chicago is flat; much flatter than our fertile rolling prairies. This is because Lake Michigan was once much larger than it is now, and the area occupied by Chicago was a mucky lake bottom. As the ice sheets advanced and retreated, the level of the lake lowered and rose. Islands, such as Blue Island, and spits appeared and disappeared with the rise and fall of the water level. The last and final retreat of the ice sheet left us with the magnificent lakeshore we have today. Top