Figure 16. Thickness of Quaternary undifferentiated glaciofluvial, glaciolacustrine, fluvial, and lacustrine deposits (Qu) in the northernmost St. Clair River area.  Qu is absent where Quaternary glacial drift (Qd) is exposed at the surface (purple area). The black line shows the location of the geologic section A-A’ (fig. 12). Grid-cell size is 20 m x 20 m. (2005 orthophoto base from Michigan State University, Remote Sensing & GIS Research and Outreach Services, USDA-FSA Aerial Photography Field Office.)