Paleontology and geology
Continental glaciers covered nearly 90 percent of the state 130,000 years ago. When the massive ice sheets melted, ground up rock debris was left behind as a blanket of glacial “drift” more than 120 meters (360 feet) thick in places. Quaternary fossils are found in this “drift,” as well as in sinkholes, fissures, caves, and lake deposits that were not affected by glaciers. Glaciations were interrupted by warm intervals, or interglacials. Fossils from the colder times include spruce logs and woolly mammoth, while fossils of giant tortoises represent warmer conditions. Other important fossils include the mastodon, the giant beaver, the stag moose, and the Jefferson's ground sloth. Although not shown on this map, Quaternary glacial deposits occur on the bedrock throughout most of Illinois.