Going up (and down!) in section, southeastern Minnesota

I was recently on a trip to southeastern Minnesota for several days. While there, inevitably I ended up with some geological photos. I must confess that I've never spent any time down here outside of passing through, and this was actually the first occasion I'd spent any time above the Cummingsville, so it was nice to see the overlying units I'd only read about before. If you'd like to know what we're up against, I recommend the Minnesota Geological Survey's geologic atlas of Fillmore County (the trip wasn't entirely in Fillmore County, but you'll get the idea).

Arcola Bluffs Day Use Area

It's been a good while since our last St. Croix post, hasn't it? I wanted to let you in on a fun place I recently visited for the first time; Arcola Bluffs Day Use Area. This site is not widely known; there's this article (with much more artistic photos than my own), an NPS cultural landscape assessment from 2018 that weighs in at 204 mb (NPS 2018; absolutely worth it if you want a thorough understanding of the site, and also includes a section on Fairy Falls), and then some short pieces here and there, and that's about it. Visiting it, though, you'll discover great geology, views of the river and the historic Soo Line High Bridge, forest and prairie settings, and some evocative ruins.

Barn Bluff

One of the outstanding geological sites in southeastern Minnesota is Barn Bluff (He Mni Can to the Dakota, La Grange to the early French explorers*) in Red Wing. The bluff is a bedrock island oriented roughly east–west, adjacent to the Mississippi River just above the Lake Pepin section. If you were to have gone back to the early postglacial period, it would have been a literal island thanks to meltwater filling the Mississippi River valley.

*All of the names are kind of prosaic, actually. "He Mni Can" is "hill, water, wood", "La Grange" or "Lagrange" is "the barn" for its general shape, and the English name is just a translation of the French.

The view of the south side of the west end. Observant eyes may notice that the color of the lower outcrops change near the left side of the picture, from orangish on the left to banded green on the right. Read on for why!

Paleozoic Taxa of the St. Croix Valley

Back when I was working on the Saint Croix National Scenic Riverway project, I'd compiled a spreadsheet of all of the fossil genera and species that had been reported from the rocks exposed along the valley. There had been some talk of spinning it off as a separate thing, but that never happened, so I played with the idea of posting it here. I then forgot about it until recently looking through the backlog of half-formed ideas. The spreadsheet itself was all ready to go, so I figured "why not?"

It's pretty simple; a column of numbers so it can be sorted back to the original organization when I'm working, a column for the genus/species, a column for the broad classification, twelve columns for the formations (go here for a refresher; the names are abbreviated for space, but each one has a note providing the full name), a column for references (defined on the "References" tab), and a column for additional notes. If a given species is present in a particular formation, the corresponding cell is marked "Y" and filled blue; if there's some question, the cell is marked "?" and filled yellow. If it's not present, there's a dash and no color fill. To check it out, you can enter here. I would not be surprised if there has been some oversplitting, if for no other reason than the challenges of preservation (we're dealing with a lot of natural molds and casts of partial trilobites in sandstone; fragility and preservation fidelity leave something to be desired).

It's worth mentioning that there is a document that covers some of the same ground, Raasch (1950). In one sense it's more narrowly focused, on Cambrian trilobite biostratigraphy, but in another it's more diffuse, with a larger area of interest.

References

Raasch, G. O. 1950. Zonal range of Croixan trilobite genera in the upper Mississippi Valley. Cambrian Subcommittee Memorandum No. V. Illinois State Geological Survey, Urbana, Illinois.

Practical guide to St. Croix Valley sedimentary formations

Now that I've seen a fair amount of the Cambrian rocks of St. Croix National Scenic Riverway and the St. Croix Valley, it seems like a good time to set them out as was done for the MNRRA formations. This time around, we'll go to the base of the Cambrian sequence in Minnesota/Wisconsin and work our way up to where the sequence overlaps with the MNRRA rocks. One day I'll have to get into southeastern Minnesota and complete the Paleozoic sequence with the rest of the Ordovician and the Devonian.

As with the MNRRA formations, we're covering a fairly narrow span of time. The Cambrian formations were all deposited between about 500 to 491 million years ago based on biostratigraphic correlations. This includes some unconformities. One other note: I'm working from the Minnesota side of the St. Croix River, and I'm most familiar with the Minnesota names. Mossler (2008) harmonized the stratigraphic nomenclature of Minnesota's Paleozoic rocks with the schemes used in neighboring states, but there is still one difference: the Minnesota Geological Survey uses lithological terms in formation names, while the Wisconsin Geological & Natural History Survey doesn't. The upshot is slightly different names. For example, the units called the Jordan Sandstone and Oneota Dolomite on the Minnesota side of the river are called the Jordan Formation and Oneota Formation on the Wisconsin side. There isn't really a practical difference; the names just look different. In ascending order, the rock units we're most concerned with are the Mount Simon Sandstone, Eau Claire Formation, Wonewoc Sandstone, Tunnel City Group, St. Lawrence Formation, and Jordan Sandstone.

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).