The Find

A weekend or so back I traveled down to Whitewater State Park to participate in their Fossil Field Trip event. As part of this, visitors go to a site off of the park where they can hunt for fossils (participants can keep two). It was very well presented and attended! The site is in the Cummingsville Formation, which is the limestone that follows the Decorah Shale. (Actually, it's a bit more complicated than that: much of the Cummingsville Formation in Olmsted County is roughly the offshore equivalent of the more nearshore Decorah of the Twin Cities, so where we were is probably equivalent to a horizon in the upper half of the Decorah quarry walls at the Brickyard.) Unlike the distinctive green-gray Decorah, the Cummingsville presents as a tan unit. It's a shaly limestone at the site, so it doesn't quite have the strength of a pure limestone, but it does better than the Decorah.

Weathering Cummingsville at the site.

The Cummingsville is not as fossiliferous as the Decorah, even accounting for my different search image, and the site is a known fossil site, so the going was slow at the beginning. Then I found this:

Well, I did do a little prep work and applied water before taking this picture, and the rock is about palm-sized all told, but you get the idea.

I'd been hoping to find a receptaculitid, because those are common in the Cummingsville and I haven't found one, but this was something better. Even in an unprepared state it was obviously an echinoderm based on the plates, but what kind? A crinoid calyx? Something else?

If you know what the arrows are pointing to...

The prep work exposed the little vent-like features, meaning it was some kind of rhombiferan cystoid. The cleaning also made it more obvious where the plates met, which can be kind of confusing when there are all these ridges to mislead the eye.

A handy diagram! The dotted lines are where the plate edges become obscure or are lost.

If you don't know what it is, check this out:

Figure 1 from Parsley (1970), with red lines and dots added by me. Original image CC BY-NC-SA 3.0.

Yup, this is a Pleurocystites, or at least a good-sized chunk of one. Either P. squamosus (Parsley 1970) or P. strimplei (Brower 1999) would be appropriate for the Cummingsville; there's not really enough there to tell between them, but I'm not complaining! It's fun to be in the right place at the right time with the right knowledge and experience.

References

Brower, J. C. 1999. A new pleurocystitid rhombiferan echinoderm from the Middle Ordovician Galena Group of northern Iowa and southern Minnesota. Journal of Paleontology 73(1): 129–153. 

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.

Parsley, R. L. 1970. Revision of the North American Pleurocystitidae (Rhombifera-Cystoidea). Bulletins of American Paleontology 58(260): 135–213. 

Practical guide to MNRRA/metro-area bedrock geology

I can't believe I didn't put up a post like this earlier. Here is a thumbnail guide to the various bedrock formations exposed within MNRRA and, by extension, most of the Twin Cities metro. Further information can be found in Ojakangas and Matsch (1982) or Ojakangas (2009), if you'd prefer a nontechnical level of discourse, or Mossler (2008) if you want a technical overview. The maps published by Mossler and Tipping (2000) and Mossler (2013) (see previous post) are also useful.

The formations of interest are the Jordan Sandstone, Prairie du Chien Group, St. Peter Sandstone, Glenwood Formation, Platteville Formation, Decorah Shale, and Cummingsville Formation. We'll take it from the top, or rather, the bottom, going in ascending order from the oldest rocks that are exposed (the Jordan Sandstone; there are older rocks below it, but they aren't exposed within MNRRA or in the central Metro. You can find most of them along the St. Croix, though).