Present to 66 million years ago
On Illinois’ western and southern borders are Paleogene formations of sand and clay with a fossil fauna of Turritella gastropods, bryozoans, clams, oysters, other bivalves and gastropods, crabs, mackerel shark, and other fish. However, for most of the Cenozoic Illinois and Wisconsin were exposed and subject to erosion.
Pleistocene - 0.012 to 2.58 million years ago
During the last 2 million years (associated with rapid changes in cooler and warmer climates), large continental glaciers advanced and retreated over this area four times shaping the landscape into what we see today.
In places, these deposits contain the bones of Ice-Age mammals. During the cooler periods, when the glaciers advanced, the area was covered by tundra and home to mammoths, muskoxen, and snowshoe hares. As the glaciers retreated, the tundra gave way to spruce and hemlock forests with mastodons, stag mooses, giant short-faced bears, ground sloths, and giant beavers. Farther south in Illinois fossils of jaguars, peccaries, and armadillos are found.
Holocene - Present to 12,000 years ago
As the glaciers retreated, different plant zones followed the retreating ice. Pollen grains recovered from lake sediment cores document these changes in the plant communities. About 15,000 years ago, there were forests dominated by spruce trees, followed by a mixed conifer deciduous forest dominated by ash and spruce spreading across the region. Later, a rapid decline in spruce and ash pollen and an increase in elm and oak pollen occurred. Nine thousand years ago, the environment became drier and elm trees declined while grasses and other prairie species and oak trees dominated the communities. At 5,000 years ago, oak savannas began to dominate the landscape.