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A Brief Upper Mississippi Valley Trip


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Last month I was able to make a brief trip to hunt the Ordovician rocks of the Upper Mississippi Valley. The stratigraphy up here was very confusing to me at first since a lot of the units are very similar looking. To that end, I am endeavoring to include more site pictures in my trip reports of this area, in the hope that it will assist others when collecting this area.

 

My first stop was in SE Wisconsin. Unfortunately the right of way was much narrower in person than on Google Earth so I did not feel comfortable collecting here. But it was a great site to observe the three lowest members of the Platteville Formation. The lower massive dolomite is the Pecatonica Member, the middle thinner bedded limestone layer is the Mifflin Member, and the slightly thicker bedded limestone above is the Grand Detour Member. These can be tough to differentiate on unweathered outcrops, but become quite distinctive after weathering.

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Next I checked out a couple of my favorite Maquoketa Formation stops in NE Iowa and had some luck, although I have nothing to show as everything is off being prepped. I also stopped at a very long roadcut exposing the top of the Decorah Formation and the entire Galena Formation. This picture shows the contact of the Ion Member of the Decorah and the overlying Dunleith Member of the Galena.

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The Ion is well known for having an abundance of the gumdrop bryozoan Prasopora. The one on the right below is itself encrusted with other bryozoans.

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This was the end of the first day. On the second day, I made my way up to SE Minnesota for the first time. The picture below shows the contact between the Prosser (Dunleith equivalent) and Stewartville (Wise Lake equivalent) Members of the Galena Formation.

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The Stewartville is well known for the lovely large gastropod molds that can be found, such as this very displayable Maclurites.

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I finished off the trip at two sites in the Maquoketa. Graptolites were probably the most commonly encountered fossil, although they weather quickly and you need to split rock to find nice fresh examples.

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And to finish off the report, here is the best of the trilobites I found. The cephalon has some damage, but it is otherwise a nice inflated specimen.

Anataphrus vigilans

Elgin Member, Maquoketa Formation

Fillmore Co., MN

Prepped by Malcolm T.

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Thanks for looking, hope you enjoyed!

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I did. 

Thank you for sharing.

It really is nice to see what the Formations and Members that some of my fossils come from actually look like. :)

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Fossildude19

Great report, Connor! 

Thanks for posting it. 

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Your specimens and your knowledge are both a real pleasure. I’m curious as to how many rocks you had to split to find that cool bunch of graptolites. 

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2 hours ago, tombk said:

Your specimens and your knowledge are both a real pleasure. I’m curious as to how many rocks you had to split to find that cool bunch of graptolites. 

They are locally very common. At one outcrop I found only a single graptolite. At another outcrop a mile over (both Elgin Mbr), most every rock was full of them.

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  • 2 weeks later...
On 8/25/2021 at 4:00 PM, connorp said:

They are locally very common. At one outcrop I found only a single graptolite. At another outcrop a mile over (both Elgin Mbr), most every rock was full of them.

True. Gumdrop bryozoan sure are common in upper part of Decorah in twin cities (similar layers). But its weird seeing Decorah getting capped by another layer of limestone! 

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