As exposures of two older tills beneath the Illinoian are rare, the recent finding of such an exposure near Winchester, in western Illinois, is of special interest (see figure 1).
West of the Illinoian drift border, in eastern Iowa and Missouri, there are two drifts older than the Illinoian, each deposited by an ice-sheet coming from the Keewatin field. One of these drift sheets, and perhaps both, extends eastward across the Mississippi River underneath Illinoian drift for a distance which has heretofore not been regarded as very great. A driftless area in Calhoun and southern Pike counties marks the eastern border of the Kansan drift from the Keewatin field and the western border of the Illinoian drift from the Labrador field. North of this driftless area the Kansan drift border runs underneath the Illinoian, and its position from this area northward to the driftless area in Jo Daviess County . . .