No abstract

Recent stratigraphic studies have characterized the sequence and nature of tills deposited in Iowa and Illinois by Quaternary Laurentide ice sheets. Basal tills within the sequence are each dominated by matrix-size materials and exhibit uniform matrix properties over hundreds or thousands of square kilometers. This uniformity suggests that matrix-size materials were effectively mixed for each basal till.

Organic material exposed within a small swale fill in Pit 6 of the Wedron Silica Sand Co. near Wedron in LaSalle County, Illinois, includes well-preserved pollen, plant macrofossils, and insect remains. This material occurs in slackwater sediment in the lower part of the Peddicord Formation, which was deposited as existing valleys were dammed by fluvial aggradation during the initial late Wisconsinan advance of Laurentide ice into the Wedron area. Wood from the organic horizon has a radiocarbon age of 21,460 ± 470 yr B.P. (ISGS-1486). The pollen spectrum is dominated byPicea, Pinus, and Cyperaceae. Plant macrofossils comprise a mix of boreal-forest taxa, includingPicea, Larix laricina, and the mossCampylium stellatum; subarctic species includingBetula glandulosa, Empetrum nigrum, andSelaginella selaginoides; along with the predominantly arcticVaccinium uliginosumvar.alpinum, Dryas integrifolia, andRhododendron lapponicum. The insect fauna contains the western montane ground beetleOpisthius richardsoni; several arctic-subarctic ground beetles includingDiacheila polita, Helophorus sibiricus, andPterostichus (Cryobius) caribou; and a diverse assemblage of insects that today inhabit the boreal forest. We interpret the biotic record to record a phase in the transition from closed boreal forest to open tundra as climatic conditions deteriorated in advance of continental glaciation.