By the History Center of Lake Forest-Lake Bluff
As the Earth’s axis and orbit path have changed over millennia, massive ice sheets have grown in size, melted, and changed positions around the globe. Lake Forest and Lake Bluff have been covered in ice as recently as 12,000 years ago.
Global shifts in temperature have followed a predictable pattern over the past 700,000 years, occurring in 100,000-year cycles of glaciation and melt. At the peak of the last Ice Age, our region’s average temperature was about 6.2 degrees lower than today.
Woolly mammoths roamed Illinois until about 13,000 years ago. Poster by Oscar Rabe Hanson, artist for the North Shore Line.
As the Laurentide ice sheet, which was the last glacier to cover our area, melted and refroze repeatedly over its 25,000-year life, it shifted the size of the lake and created new shorelines, which remain as high ground roads around the area. Some of those old shorelines that are used as roads today include Green Bay Road, Milwaukee Avenue and parts of the interstate system.
While the lake fluctuated in size, Glacial Lake Chicago (about 60 feet higher than present-day) was the largest the lake became. The current Lake Michigan level came about 2,500 years before today, fed by the melting of the ice sheet as it sat over Canada. The smallest lake actually created dry ground that made it possible to walk across the lakebed from approximately the modern locations of St. Joseph’s in Michigan to Glencoe in Illinois. That access brought wildlife into the region again, and along their paths, the Paleo Indians followed, hunting and settling the region as the land returned to more temperate environments.
Mastodons (Mammut americanum) and woolly mammoth (Mammuthus primigenius) were unrelated large elephant-like creatures that roamed Illinois until about 13,000 years ago. The wooly mammoth, more equipped to handle the cold, seems to have navigated the northernmost territories just at the southern edge of the retreating Laurentide ice sheet, while the more temperately inclined mastodons are believed to have lived well south of the land where Lake Forest and Lake Bluff are now.
While mastodons grazed in the flat plains of southern Illinois, where archeologists have uncovered numerous fossils, the mammoths were inclined to dine on the shrubs and trees that thrived in the northern region of the state. While their fossils are rarer, evidence of the animals have been found in the Volo Bog and in Kenosha. Although several post-glacier species are extinct, including both the mammoths and mastodons, many of the species that lived side by side with them 10,000 years ago are still with us, including white-tailed deer, bald eagles, and bobcats.
From the last Ice Age period—the Wisconsin Glaciation Episode—only two significant ice sheets remain: the Antarctic Ice Sheet and the Greenland Ice Sheet, both of which are now melting, for the first time in 700,000 years.
To find out more about this and other topics related to our local natural history, check on the History Center’s new exhibit, Four Degrees of Difference: From Glaciers to Global Warming, which opened on Jan. 25. Visit www.lflbhistory.org to learn more.