VEGETATIONAL CHANGES IN WESTERN ILLINOIS DURING THE ONSET OF LATE WISCONSINAN

Peats and silts near Biggsville, western Illinois, have yielded the most detailed paleoecological record available in the midcontinent for the time when Late-Wisconsinan glaciers began to advance. These deposits were analyzed for pollen, vascular-plant macro- fossils, bryophytes, insects, and stratigraphy, and date from 28 000 to 22 000 BP. Upland vegetation was a Picea-Pinus banksiana forest from 27 900 to 26 600 BP. From 26 600 to 22 700 BP, Picea and Larix laricina were the dominant trees, and Pinus had all but disappeared. This change probably reflects a climatic cooling when glaciers began to advance into the Midwest. A decrease in Picea pollen percentages and total pollen accumulation rate, an increase in Cyperaceae and other non-arboreal pollen, and the ap- pearance of Selaginella selaginoides after 22 700 BP suggest a further cooling, and indicate an open environment as Late-Wisconsinan glaciers advanced into Illinois. Locally, a small pond, which accumulated organic silts and supported a rich aquatic flora, was present from 27 900 to 27 000 BP. Peatlands surrounding the pond were dom- inated by rich fen bryophytes and a diverse wetland herbaceous flora. A Picea mariana muskeg replaced the pond and fen between 27 000 and 23 500 BP, the peat became in- creasingly woody, and the wetland became dominated by P. mariana, Larix laricina, and Betula glandulosa. Peat deposition ceased by 23 500 BP when loess accumulation began; small ponds again were present in the lowland, but they supported only a sparse aquatic flora. By 21 400 BP, the basal loess and paleosol had capped the peats, and subsequently rapid loess deposition buried all previous deposits.