Knowledge Bank

Late Wisconsin tills in Midwestern U.S.A. contain varying abundances of igneous and metamorphic rocks derived from the Precambrian Shield of North America. The relative abundance of these clasts is determined by the distance from their source province. The Wabash and Powell-Union City Moraines of Ohio and Indiana indicate that the abundance of igneous and metamorphic clasts in the +5 mesh fractions of till decreases southward and with distance from their source. Samples from the Cuba Moraine of Ohio and from the Port Huron, Galt, Orangeville, Paris, and Dummer Moraines of Ontario, Canada show different rates of change of the abundance of igneous and metamorphic clasts compared to the Wabash and Powell-Union City Moraines. Complications due to additions of previously deposited tills, variable bedrock lithology and different rates of melting are major causes of this difference. Understanding of these factors combined with glacial mechanics may lead to an empirical model that relates the abundance of clasts to the distance of transport.