Decorah, Iowa

After leaving Pikes Peak State Park, we headed northwest toward the Minnesota border, passing through Decorah, Iowa along the way. When I was an undergraduate student I spent a lot of time collecting in the Decorah Shale, which outcrops around Northfield, Minnesota as well as in its type area around the town of Decorah. As we drove through town we saw the exposure shown above and had to stop for a few minutes!

The Decorah is the shale that make up the lower half of the section in the picture. The upper limestone is the Dunleith Formation. Both units are Ordovician, and both are fossiliferous. One of the more common fossils in the Decorah is the “gumdrop bryozoan” Prasopora, colonies of which form hemispherical domes on top of bits of debris. We don’t have many of these in the VMNH collection, so we collected several for the museum and for PHCC’s teaching collection:

As it turns out, the entire Decorah Formation is present in this outcrop, as the car was sitting on the top surface of the underlying Platteville Formation. The bottom 2-3 meters of the Decorah was covered under debris and not visible. This was unfortunate, because the bottom of the Decorah contains the Deicke and Millbrig K-bentonite beds, the remnants of ashfalls from huge volcanic eruptions that occurred in Virginia during the Ordovician. The same K-bentonites are also present in the Eggleston Formation (for example, near Hungry Mother State Park) meaning that part of the Decorah in Iowa and Minnesota exactly correlates with part of the Eggleston in Virginia.

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