Figure 17. Boomer sub-bottom profile showing hyperbolic reflections within Quaternary glacial drift (Qd) in the northernmost St. Clair River area that are interpreted to be cobbles, boulders, and coarse-sediment lenses. The undifferentiated Quaternary unit (Qu) pinches out, exposing Qd at the riverbed. Vertical scale is depth in meters (zero depth is 175.9 m IGLD 1985 based on water level, at the time of data acquisition, at the NOAA gage at the mouth of the Black River), using a sound velocity of 1,450 m/s in water and 1,650 m/s in sediment. (See table 1 for explanation of stratigraphic units and unconformities; see fig. 10 for profile location.) Click on figure for larger image.