The sheet of drift in northeastern Iowa, which, in the recent literature relating to Pleistocene geology, is known as the Iowan, has certain individual characteristics that differentiate it sharply from the other drift-sheets of the Mississippi valley. Before McGee’s classic work on the region under consideration, no one had suspected that more than one sheet of till was represented in the Pleistocene deposits of Iowa; but the studies of McGee in Iowa and of Chamberlin in Wisconsin led to the very general recognition of the multiple character of our drift deposits. Other investigators, using the methods of observation employed by the pioneer workers named, and guided by the criteria for discriminating different drift-sheets which they pointed out, have been able to map, with a fair degree of accuracy, the limits of the successive ice invasions.
Previous Work on the Iowan
The earlier work on the Pleistocene of northeastern Iowa . . .