Information provided by David Pittman
INTRODUCTION
The dynamic geologic changes in the Midwestern region is an amazing story difficult to comprehend. Migrating continents traveled to the equator and back, mysterious upheavals totally eliminated large portions of geologic history, and ice sheets hundreds of feet tall swept over and over across most of the state. Who could make this stuff up? Yet there is scientific documentation to support this fantastic story. Like most of Northern and Central Illinois, the geology of Root Cemetery is primarily defined by the giant oceans that once covered the state and by the more recent ice ages that churned the landscape like bulldozers. A portion of the large Illinois River valley, with its slow-moving backwater lakes and miles-wide gravel bed, once hosted the mighty Mississippi. Welcome to the modern geology of Chillicothe, Illinois.
BIG PICTURE EARTH
Once upon a time, about 15 billion years ago, the vast area of space we call our Milky Way Galaxy began to concentrate its random gases. The forces of gravitational attraction and centrifugal force created the beginnings of our solar system, with our Sun at the center of a series of rings of material that eventually would concentrate themselves into planets.
Around 4.6 billion years ago, Earth formed into layers 3,800 miles thick. The inner core was solid iron and nickel, surrounded by an outer core of molten iron and nickel. The upper area in Illinois is called the continental crust and is between and 16 and 56 miles thick. Most of the state is underlaid by a basement rock called granite, which is one to one and a half billion years old. Above this are different rock layers whose varied depth and combination tell the story of migrating continents, ancient oceans, long vanished rivers, and the ice age.
About 570 million years ago, the basement granite foundation of Illinois began to sink from the Earth’s surface and became covered by oceans. Its deposits of soft sand, mud, and lime became sedimentary rocks such as shale, sandstone, limestone, coal, and dolomite. Coral reefs formed in part because the entire continent was situated near the very warm equator. Some of the best fossil records of early plants and animals formed in this time can be found in Illinois. Peoria county has multiple seams of coal with fossils of giant ferns. Coal was an important part of the local mining economy until the late 20th century. Coal Hollow Park, just north of Chillicothe, offers a glimpse into the coal mining history of our area.
MISSING 150 MILLION YEARS OF ROCK
Illinois geologic history has a mysterious vacancy in its geologic record from about 280 to 34 million years ago. For reasons not yet clear, Illinois contains no rocks or fossils from the Permian, Triassic, or Jurassic periods, and a portion of the Cretaceous period. No fossils are found for the times of reptiles when dinosaurs lived and when huge marine reptiles swam in the seas. Most of these giants died off about 65 million years ago, in a sudden extinction often called the K-Pg impact event. There have been at least four other mass extinctions on our earth, but this was the most recent.
ICE AGE
The story of the Central Illinois rocks resumes about 34 million years ago. A series of ice ages recorded across North America began around 1.8 million years ago, moving down from the Arctic and changing the landscape. The last two glacial periods are called the Illinoian (300,000 – 125,000 years ago) and the Wisconsin (75,000 – 10,000 years ago). This last glacial period shaped most of our area. Chillicothe was buried under ice hundreds of feet in height, near the southern edge of the Wisconsin glacier.
As the Wisconsin glacial period ended, the glaciers receded slowly north, scraping the land, sometimes depositing large amounts of debris. Long low hills called terminal moraines were formed this way and are now, because of their elevation, often a favorite site for windmill farms. Their most common elements are sand and gravel, but there is a wide diversity of rocks and minerals including metamorphic rock (quartzite, schist, granite, gabbro) and sedimentary rock (limestone, dolomite, sandstone, shale, coal beds). These are all part of the glacial drift, and almost all of them can be found in Central Illinois.
Today most of the state has a top layer of finely ground soil at least 100 feet thick. This soil consists of wind-blown light particles and glacial drift, a mixture of ground-up rock debris, much of it from Canada. It is among the best agricultural soil in the world. There are occasional large boulders called erratics found in the area that were carried from the far north by the glaciers and then left as the ice melted away.
OLD MISSISSIPPI RIVER BED
About 30,000 years ago, the wide Illinois River Valley between Hennepin and Spring Bay was actually the home of the mighty Mississippi River. Fed by icesheet melt and water from the glacial lakes to the north, the Mississippi flowed from Rock Island to Hennepin and then south almost to the Chillicothe area.
This wide valley carried much of the water from southern Wisconsin and the Great Lakes. At Spring Bay, the ancient Mississippi then flowed southeast into eastern and southern Tazewell County, where it joined a now vanished river called the Mahomet River, near the present town of Delavan. The Mahomet River carried water from as far away as West Virginia. Today the Mahomet Aquifer is a deep underground water source located hundreds of feet below the surface in the old riverbed.
KANKAKEE TORRENT
About 24,000 years ago, the Illinois River valley was largely filled up with buried glacial debris from the Wisconsin Glacier. The Mississippi River had been diverted 60 miles west to its current channel by a layer of ice known as the Lake Michigan lobe, which forced the river to flow 60 miles west to its current course.
A series of sudden, catastrophic floods known as the Kankakee Torrent occurred when a giant lake south of Lake Michigan burst its natural dam, causing other lakes with natural dams of moraines to break, releasing a huge volume of water.
The floods removed the old glacial debris from the Illinois River Valley but initially were stopped by larger and thicker terminal moraine deposits near Spring Bay. The flood water created a large temporary lake in eastern and southern Tazewell County.
The large gravel bed that surrounds Chillicothe was created at this time as a part of the gravel beds from Hennepin to south of Peoria. Eventually, new pulses of torrent water breached the older Spring Bay moraine dam. Water flowed southeast of Chillicothe to a few miles south of modern Bartonville, where it joined the existing Kickapoo Creek channel. From this point, the modern Illinois River Valley continues south to join the Mississippi River near St Louis.
The area around Peoria with its wide lakes and sloping hills was formed by the Wisconsin glacier that temporarily dammed the flood waters, rather than by erosion of the Mississippi River valley.
The Illinois River is now a very shallow, slow-moving body of water unable to wash out the sediment that is deposited by its tributary streams, a primary reason for its minimal depth. Smaller watersheds, especially the Kankakee, Des Plaines, and Fox Rivers provide the majority of the water flow of the modern Illinois River.
LOCALLY AT ROOT CEMETERY
Directly west of Root Cemetery, hills rise 100 feet to meet the uphill prairie plains, on the western edge of Blue Ridge Road and the small unincorporated town of Northampton. The hills are the Buda moraine and Providence moraine, formed at the end of the Wisconsin Glacial Episode. The historic Peoria to Galena Road is here, first surveyed in 1850s alongside the Peoria Coach Road first surveyed in 1835. Like all moraines, these were temporary stationary ice fronts on the margin of the glacier, a combination of rocks and minerals deposited by diminishing Wisconsin glacier.
Root Cemetery is on the western edge of a large gravel quarry with a 25-foot exposure of creek bed on its eastern property boundary. The soil is light brown, essentially free of rock debris and indicative of loess, a windblown material excellent for farming.
REFERENCES
A View of the Past: An Introduction to Illinois Geology, Christopher J Schuberth, Illinois State Museum, 1986
Correspondence with Professor Ed Stermer, ICC Science, Technology, Engineering and Mathematics.
Geology of the Mackinaw River Watershed, Mclean, Woodford and Tazewell counties, Illinois, C. Pius Weibel and Robert S Nelson, IDNR, 2007
Guide to the Geology of Kankakee River State Park Area, Kankakee County, Illinois, Wayne T. Frankie, Illinois State Geological Survey, 1997
Illinois Ice Age Legacy, Myrna M Killey. State of Illiinois, IDNR, 2007