Woodfordian till deposited by the juxtaposed Saginaw and Huron-Erie glacial lobes in southeastern Michigan appears similar in many respects but may be differentiated on the basis of certain mineralogical characteristics of clay-sized particles. This is apparent from X-ray diffractograms of basally oriented, magnesium-saturated, and glycerol-solvated specimens. Thirty-five till samples from the converging Kalamazoo (Saginaw lobe) and Mississinewa (Huron-Erie lobe) moraines and their interlobate tract have 7-Å/10-Å peak height ratios that differ significantly according to provenance. Every sample from locations within and proximal to the Kalamazoo moraine has a ratio of 0.91 or more. All but one of the samples from the Mississinewa moraine and areas to the east have ratios less than 0.91. Data from several other sources indicate that the same relationships also exist on a larger scale in southeastern Michigan. The findings are consistent with previous interpretations of the area's geology and may provide a reliable basis for the placement of the surficial boundary between drifts deposited in a complex interlobate area.