The pictures posted here contain a lot of information. For those of you who are interested in lighthouses on the Great Lakes, this site, www.terrypepper.com looks well researched and extensive.
Here is a map of the ring of gypsum in Michigan from the Michgan Department of Natural Resources. (Their website contains a treasure trove of maps of all kinds.) Lower Michigan is a series of concentric rings formed when it was a shallow sea, the ring of coral reef limestones lies outside the ring shown here and mining it was a huge industry in Michigan.