AN ABSTRACT OF THE THESIS OF Elizabeth C. Geiger, for the Master's degree in Geology, presented on August 28, 2008, at Southern Illinois University Carbondale. TITLE: PALEOECOLOGY OF PLEISTOCENE GASTROPODS IN GLACIAL LAKE DEPOSITS IN SOUTHERN ILLINOIS/MISSOURI MAJOR PROFESSOR: Dr. Scott Ishman During the Pleistocene Epoch in the central Midwest United States multiple glaciations and interglaciations occurred in response to the advance and retreat of glaciers. This study focused on the Illinoian and Wisconsinan glaciations, which are separated by the Yarmouthian and Sangamonian interglacials. The advance of glaciers during glacial stages caused major rivers and their tributaries to aggrade creating slackwater lakes in many tributary valleys to the Mississippi River. Gastropod assemblages, found in glacial lake deposits, were used to interpret paleoecology during major glacial advances. Gastropods were collected from four sites in southern Illinois and Missouri to compare the Illinoian and Wisconsinan glaciations to one another, as well as to compare similar aged glacial lake deposits. This study identified forty-eight gastropod species, which were analyzed using statistical procedures. Hierarchical cluster analyses resulted in the identification of gastropod assemblages that were interpreted using habitat classifications. The results indicate specific similarities and differences between the Illinoian and Wisconsinan glaciations regarding environmental and climatic conditions.