has not always dominated the Illinois landscape. Glacial ice, steppe
vegetation, and deciduous and coniferous forests have covered the state.
have advanced and retreated many times over the last 1.8 million years
(scientists refer to this time period as the Pleistocene).
During the last two glaciations, the Illinois and the Wisconsin, ice covered major portions of the state.
Wisconsin, the ice advanced into Illinois approximately 30,000 years ago.
It reached farthest south (at Shelbyville, Illinois) by 24,000 years ago.
It then advanced and retreated several times before finally disappearing
from the State by about 17,000 years ago.
During the Illinois, 186,000 to 128,000 years ago, ice covered most of Illinois.
what we know about the changing plant communities and past environments
in Illinois is based on fossil pollen. (use
back button to return here) Many plants are wind pollinated, and some of
this pollen falls or is washed into lakes and wetlands, where it is preserved
in the sediments.
have learned what the forests in different areas of Illinois looked like
by studying fossil pollen.
Pollen data from several sites in southern Illinois provide some information on plant communities present during the late Illinois and early Wisconsin Ice Ages, and the time between the two glaciers.
End of the Illinois glacier
and Central Illinois
These data show that when the Illinoisian ice retreated, a conifer
forest grew and covered southern Illinois. The dominant trees were spruce
and pine. Today, this type of forest exists along the Canadian border with
the United States and northward. So we know the climate must have been
Between glaciers (Interglacial)
between glacial periods first became warmer and wetter, and the conifer
forest was replaced (slowly and gradually) by a deciduous forest characteristic in more temperate climates. The dominant trees were oak, elm, hickory, and beech. As the climate continued to become warmer and drier, savanna-type vegetation developed. Grasses, ragweed, and oak became most common.
of the Wisconsin glacier
A deciduous forest developed at the end of the last interglacial as conditions became moister. As the Wisconsin ice advanced the climate became drier and cooler. Open spruce forests developed by the end of the Wisconsin glaciation. This type of forest prefers a cooler and drier climate than does a deciduous forest.
Pollen data from Biggsville, a site in northwestern Illinois, provide a record of plants growing during a period that occurred just before the Wisconsin ice advanced into Illinois - a time from approximately 36,000 to 30,000 years ago.
Wisconsin glacier arrives
beginning of this period, jack pine and spruce were common, but spruce
became more dominant as climate became cooler. Pine had disappeared by
the time the glaciers advanced into Illinois. The Wisconsin glacier overrode
a spruce forest (use back button to return here) as it advanced into
the glacier margin, tundra developed, and this persisted for some time after the glacier retreated. Sedges (grass-like plants) were common in the tundra vegetation.
spruce logs, Tonica Illinois