Sioux Quartzite-Morton Gneiss-Blue Mound State Park

Sioux Quartzite-Morton Gneiss-Blue Mound State Park

One of the most curious Rocks in Blue Mound State Park. It is aprox. 1.6 billion years old. It has recently re-emerged. With the breaking of the WPA containment dam/swimming pond at the Blue Mounds this boulder has been completely unseen for over seventy five years. It can be found aprox. 100 yards west of the breached dam. During the summer it is hard to see because of the volunteer trees that have grown up since the draining of the pond.
P.S. More about the 4″/6″ diameter circles later! bk


Today I’ve consulted with my friend the state Paleontologist for Illinois. I asked for a simple explanation of these “rock moons” occasionally found Sioux Quartzite (Morton Gneiss). Alas, the answer was not simple, redox reaction, oxidation and braided rivers had something to do with it and all of this was little understood by a non professional like myself. However a few things of notation: Most of these moons are 1 to 2 inches in diameter.
The boulder that is depicted has moons that are 4 to 6 inches in diameter, quite stunning and never before seen at the Blue Mounds (to my knowledge). Because of its proximity to the WPA dam it makes me believe that it was in situ and removed during construction. It almost has the appearance and surface of a glacial erratic. It is large enough that a small bulldozer would have quite an effort to move it out of the “soon to be flooded” area. I suspect it was left and covered with the rising water, not to be seen for at least a generation or two. The area that was once a man made pond is now covered in thick vegetation. It gives a much different perspective of what this area might have looked like to the indigenous peoples. Seek out this boulder late summer or fall and enjoy another miracle of nature….at Blue Mound State Park. billkeitel

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