Pollen analysis of Wisconsinan sediments from eleven localities in northern and central Illinois, combined with the results of older studies, allows a first general survey of the vegetational changes in Illinois during the last glaciation.
In the late Altonian (after 40,000 B.P.), pine was already the most prevalent tree type in northern Illinois. Probably because of the influence of the last Altonian ice advance to northern Illinois, pine migrated to the south and reached south-central Illinois, which was at that time a region of prairie, with oak and hickory trees in favorable sites. Likewise in the late Altonian, spruce appeared in northern Illinois. Spruce also expanded its area to the south during the Wisconsinan, reaching south-central Illinois only after 21,000 B.P., in the early Woodfordian. Deciduous trees (predominantly oak) were present in south-central Illinois throughout the Wisconsinan. Their prevalence decreased to the north.
The vegetation during the different subdivisions of the last glacial period in Illinois was approximately as follows:
Late Altonian: Pine/spruce forest with some deciduous trees in northern and central Illinois; prairie and oak/hickory stands in south-central Illinois; immigration of pine.
Farmdalian: Pine/spruce forest in central Illinois; deciduous trees and pine in south-central Illinois, with areas of open vegetation, perhaps similar to the present-day transition of prairie to forest in the northern Great Plains.
Woodfordian: Northern and central Illinois ice covered; in south central Illinois, spruce and oak as dominant tree types, but also pine and grassland.
During the Woodfordian, pine and spruce disappeared again from south-central Illinois, and oak/hickory forest and prairie again prevailed. The ice-free areas of northern Illinois become populated temporarily with spruce, but later there is proof of deciduous forest in this region.
Pollen investigations in south-central Illinois have shown convincingly that deciduous trees could survive relatively close (less than 60 km) to the ice margin. Therefore the frequently presented view that arctic climatic conditions prevailed in North America during the last glaciation far south of the ice margin can be refuted for the Illinois area, confirming the opinion of other authors resulting from investigations of fossil mollusks and frost-soil features.
The small number of localities investigated still permits no complete reconstruction of the vegetation zones and their possible movements in Illinois. During the Altonian and Farmdalian in Illinois, a vegetational zonation probably existed similar to that of today in North America. As the ice pushed southward as far as 39° 20′ N. lat in the early Woodfordian, this zonation was apparently broken up under the influence of a relatively moderate climate. In any case, the Vandalia area, which was only about 60 km south of the ice, was at that time neither in a tundra zone nor in a zone of boreal coniferous forest.