Stratigraphic studies of the glacial geology of the northwestern margin of the Allegheny Plateau of northeastern Ohio and northwestern Pennsylvania include evidence for multiple pre-Wisconsinan glaciations. Near the former eastern margin of the Scioto lobe at Mount Gilead, Ohio, Millbrook, Gahanna, and Chesterville tills have an inferred Illinoian age based on thermoluminescence ages of 146 000 ± 25 000 and 124 000 ± 16 000 BP from an overlying silt interpreted as loess. Northampton Till overlies Millbrook Till of the Killbuck lobe and its correlative Mogadore Till of the Cuyahoga lobe, but does not overlie the correlative Titusville Till of the Grand River lobe. There are signficant differences in carbonate contents among these tills. Millbrook Till and its equivalents contain more kaolinite than other tills, and the Northampton Till contains more clay. Millbrook Till and its correlatives represent the most extensive Illinoian glaciation of the plateau, whereas the other Illinoian tills were deposited by ice that flowed a shorter distance onto the Allegheny Plateau. Butler, Mapledale, and Slippery Rock tills, formerly assigned to an Illinoian glaciation, are pre-Illinoian in age. Scarcity of materials for the dating and the lack of recognizable paleosols separating the tills from Wisconsinan deposits inhibits the reliability of the chronological classification of these tills.