Figure 18. Boomer sub-bottom profile showing the seismic stratigraphy where Lake Huron meets the head of the St. Clair River. Vertical scale is depth in meters (zero depth is 176.0 m IGLD 1985 based on water level, at the time of data acquisition, at the NOAA gage at Fort Gratiot), 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.