The Geologic History of Henry’s Farm

By Dr. Kris Huysken,  Department of Geosciences,  Indiana University Northwest.

The history of the land ultimately begins with geology, and the story of Henry’s Farm begins with the geologic history of central Illinois.  The landscape, soil, and water are all controlled by the geology of this region. 

Early Geologic History

Deep wells provide the water for all of the people and animals on Henry’s farm.  They also provide water for transplants, and during the rare times when irrigation is necessary to germinate seeds or keep plants going through droughts.  The rocks that serve as the local aquifer were formed early in Illinois’ history during a period of time known as the Paleozoic Era, which spanned from about 570 to 250 million years ago.  Tectonic forces had moved Illinois close to the equator and it was covered with shallow seas during much of this time.  Fossils of coral, crinoids, and many other marine creatures found in Illinois limestone attest to the variety of life teeming in the region during this time.   

Sediment deposited in these seas later turned into the sedimentary strata now lying below the surface.  Small fluctuations in sea level led to alternating deposition of limestone, shale, and sandstone.  The sandstones provide excellent aquifers in this region.  Small pores between grains of sand allow water to slowly move through the rock while filtering out impurities.  

Glacial History

The history of the land we see today begins with the last ice advance over eastern and central Illinois.  During the period of time known as the Pleistocene Epoch (approximately 1.6 million to 10,000 years ago) glaciers advanced over Illinois in response to climate cooling.  With them, the glaciers carried enormous loads of rock debris and finer sediment eroded from outcrops further north.  During warmer periods, known as interglacial periods, the melting glaciers deposited enormous loads of sediment.  The last ice advance that affected Illinois is the Wisconsin advance.  It began about 70,000 years ago and eventually extended into eastern and central Illinois when lobes of ice advanced south through the Lake Michigan lake basin.  The extent of the Wisconsin advance is marked by a blanket of glacial sediment called till, and is shown in green on the accompanying map of surficial deposits produced by the Illinois State Geological Survey.  By 10,000 years ago, the Wisconsin advance was completely over.

Features formed by glaciers give clues about their extent and behavior.  End moraines are ridges made from till that form when the glacier’s margin stays put for awhile.  Debris-rich glacial ice continuously deposits sediment at the glaciers margins.  When the glacier is stationary debris piles up forming a ridge that outlines the margins.  End moraines are important to geologists because they allow us to understand the location and shape of the glacier at different times throughout history.  End moraines outlining positions of glacial ice during the Wisconsin ice retreat are shown on the accompanying map in dark green.  Between end moraines, the sediment from the melting glaciers covers the area much like a blanket.  This type of moraine is known as ground moraine and is characterized by the gently rolling hills seen throughout much of Illinois.

Post Glacial History

When the glaciers receded, rivers and streams began to erode the moraine, initially carving small stream channels and later carving broad flood plains as rivers further eroded the glacial deposits.  Henry’s Farm lies in the Mackinaw River flood plain where periodic flooding deposits nutrient-rich silt creating rich soil.  On this satellite photograph, the meandering Mackinaw River and tributaries can be seen.  The tributaries are accentuated by thin forested areas.  The Mackinaw River is part of the Mississippi River drainage basin – the largest drainage basin in North America.