The lithostratigraphy, radiocarbon ages, and pollen and plant macrofossil assemblages in a low terrace on the Iowa River beyond the Wisconsinan glacial boundary reveal regional alluvial relationships and indicate that valleys were important corridors during vegetational change in the Midwest. Basal pebbly sands probably represent a valley train from a central Iowa glacial advance ∼25,000 to ∼21,000 yr B.P. Overlying organic silts indicate low energy conditions from ∼21,000 to ∼15,000 yr B.P. Pebbly sands of an upper valley train were deposited from ∼14,000 yr B.P. until 10,000 yr B.P., about 1,500 yr after the Des Moines Lobe had retreated out of the Iowa River Basin.
Pollen and plant macrofossils from the top of the upper pebbly sands indicate that the area was covered by mixed conifer-hardwood forest taxa whose pollen percentages do not match those predicted by models of vegetational change in eastern North America. Boreal elements with general distribution over 500 km to the north at 10,000 yr B.P. persisted in peatland habitats. The valley environment contained a variety of edaphic and microctimatic conditions that supported diverse plant communities. During glacial-interglacial transitions, medium to large valleys served as migration corridors for plant taxa and harbored riparian communities without modern analogs.