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).

I saw both show caves, Mystery Cave and Niagara Cave, and am not going to play favorites (although if you have time and opportunity for the 2-hour geology tour at Mystery Cave, and you don't mind carrying a battery-powered lantern as opposed to having lighting set up in the cave for you, that's quite an experience). (Oh, and with all the recent rain, Niagara Falls in Niagara Cave was running quite vigorously!) Both caves formed in the Galena Group, the mostly carbonate, sometimes shaly unit above the Decorah. In ascending order the Galena includes the Cummingsville, Prosser, Stewartville, and Dubuque Formations. The caves get as low as the Prosser in Niagara Cave but are mostly Stewartville into Dubuque. The Stewartville likes to make narrow passages and often has burrows, which are expressed as small pits or bumps (depending on sedimentology), whereas the Dubuque, with thicker shaly intervals, makes wider passages with more breakdowns (and the shale makes the cave floor a bit more fun, depending on how good your footwear's tread is). You can see fossils in both caves, in the Stewartville in particular (even if you don't count the burrows). Niagara Cave has an abundance of Fisherites, mostly in cross-section, in a narrow interval.

This is the Stewartville being particularly narrow-passaged about things in Niagara Cave. To be fair the cave developed along joints, and that does set a blueprint. (And my interior cave photos look, well, like they were taken inside a cave, so I won't torture you with too many.)

Up in the Dubuque in Mystery Cave; notice the change in bedding to almost cinder-block-like intervals separated by a mortar of shale. (Yes, the angles are a bit disorienting, but if you look at the people you'll see the tilt is in the walls, not the photographer.)

No scale, unfortunately (think instant waffles from the freezer, roughly), but this is a Fisherites in cross-section, looking a bit like railroad tracks. A simple pleasure, perhaps, but I'd never seen them in the wild.

Another stop was at Masonic Park north of Spring Valley, where the Prosser Formation is on excellent view. It has thin beds grouped in larger packages, and is quite pale. The Stewartville is reported to be present here as well; having only otherwise seen it in caves, I'm hesitant to say just where it is, but there is a definite change in the upper quarter or fifth to somewhat thicker beds that are a bit more yellowish-tan and slightly more resistant (the profile seems to flare out a bit going up). There were similar steep river bluffs visible on the Palisade Trail at Forestville State Park, perhaps a bit lower in section but not as visible due to plants. The well-known Rifle Hill Quarry is just outside of the park, but I did not make a stop (maybe another time).

The change is subtle and inconveniently near where the bluff runs out, so there are lots of plants hanging down over it. What is not obvious from the photo is that there is a creek at the base of the bluff; this is not a quarry wall!

The thin beds are in thicker bundles, something like how on a record there are tracks separated by bands, each track with many grooves. The geologic record!

Meanwhile, at Forestville... I'd have a hard time saying what this is with my couple of hours of experience, but it's Galena something, and the plant-rich interval halfway up puts me in mind of a contact. Cummingsville–Prosser?

Interestingly, the stratigraphy varies quite a bit in Fillmore County. The southwest part of the county has Devonian bedrock, but in the northeast in Rushford it's down as low as the Tunnel City Group (the old Franconia). If you were looking for one county in the state to see the entire Paleozoic column, you could do a lot worse than Fillmore! I was surprised to realize that Lanesboro was down in the Prairie du Chien. There are outcrops adjacent to the dam (which, despite appearances, is not a natural waterfall).

But it's worth a photo!

I went in geologically blind to Beaver Creek Valley State Park, in neighboring Houston County, and found the outcrops looked strangely familiar. They were in fact the Oneota (lower Prairie du Chien). Better yet, the first clue I had was a location where there was sandstone under a carbonate. It was the Jordan–Oneota contact, which I'd never seen before (there's supposed to be an exposure in the north part of Stillwater, but I get nervous outcrop-hunting in residential areas).

Behold, the Cambrian passing into the Ordovician. This contact's not too hard to spot. Springs and seeps here like to form near it.