Submerged Wisconsin Sand Ridges
Researchers from the Large Lakes Observatory used the multibeam sonar mounted on the University of Minnesota's research vessel, the Blue Heron, to investigate submerged sand ridges off the Wisconsin shore of Lake Superior. The surveyed area, depicted at right, is a 3.8 by 3.8 km area (2 by 2 nautical miles) with water depths between 13 and 32 meters.
Using the multibeam data, sediment samples and seismic reflection data the researchers identified varying lake bottom terranes. As seen on the images below, the near shore lake floor is composed of glacial till with very little modern sediment deposition, while off shore the lake floor consists of lacustrine (lake) clays. In between the off shore clays and the near shore tills are two different types of sand ridges. The older ridges are several kilometers long, about half a kilometer wide and rise approximately 3 to 6 meters above the lake floor. These ridges are relict drowned shoreface-attached sand ridges that formed when lake level was significantly lower than its current level, approximately 6,000 years ago. The younger ridges are oriented obliquely to the older ridges, are, at most 1.5 kilometers long, about 200 to 300 meters wide and rise approximately 1 to 3 meters above the lake floor. The younger ridges are believed to be sub-aqueous sand dunes that formed in deeper waters.
Below is a chart of western Lake Superior showing the location of the submerged sand ridges off of the Wisconsin shoreline. This is a section of NOAA chart 14966. You can look at more NOAA charts and the User's agreement online.