The Platteville Formation revisited

For the person interested in fossils in the Twin Cities, the Decorah Shale is money in the bank. It's a cooking pot that never empties, a gas tank that's always full. If you have a patch of it, you cannot lose. The Platteville Formation is more like a lottery ticket. If you pick up a random piece, the chances are good it will have nothing, or maybe an imperfect brachiopod mold or two, or some "eyelashes" from shells in cross-section, or half of a burrow. Even when you do find a chunk that's loaded with fossils, usually it's 95% brachiopods and 5% snails, with a couple of crinoid columnals, bivalves, or bryozoans for variety. Every so often, though, you will come up with something unusual. It's true that the Decorah also rewards in-depth exploration, but the "floor" of discovery is so much higher in the Decorah that the feeling when you do find something out of the ordinary in the Platteville is much different. It's more of an accomplishment. The universe has rewarded your perseverance, has conspired with taphonomy, lithification, and erosion to put someone with the proper skills and inclination (i.e. you) in this place at this time to observe and appreciate this fossil. (I will refrain from pulling out the conulariid again.)

I've seen a few of these big snails; not sure about the genus yet.

In terms of preservation, the Decorah Shale is a strictly representational artist dedicated to faithful reproduction of the fossils, thanks to relatively mild conditions for fossilization and diagenesis (the stuff that happens during and after the formation of sedimentary rocks, like replacement of calcite with dolomite). Thanks to dolomitization, the Platteville Formation of the Twin Cities Basin is a sort of minimalist impressionist, retaining only some essential essence of a given fossil while losing most of the fine details. (It also has a thing for sparkles, what with all of the fine dolomite crystals.)

And then we've got blocks like these, doing a decent job of imitating the Decorah. I think I can place the source to a specific bed in the upper Mifflin, but it may be very localized.

On Saturday, I was the guest paleontologist for the Second Saturday fossil event at Coldwater Spring. (I'm the tall one with the facial hair.) After having spent a lot of time along the gorge, I think it is fair to say that Coldwater Spring is one of the best places in the Twin Cities to be in close contact with the Platteville Formation, if not the best. It is certainly the best place to take people of all ages and experience levels to see Platteville rocks and fossils. In most locations on the gorge, the Platteville is a brooding presence capping whitish bluffs of St. Peter Sandstone, inaccessible to all but the most reckless. At some places where a ravine joins the gorge, such as Shadow Falls, Minnehaha Falls, and a few locations on the Minneapolis side of the river, you can walk around parts of the Platteville, but you also are stuck on narrow paths where you've got the Platteville on at least one side, sometimes two (the other side is the one above your head), and the steep slope of the bluffs on the other. This can be chancy when you're on your own and is not feasible for groups of non-professionals, and even when you do go, you usually only get to see the lower part of the formation. By virtue of erosion and some human modification, Coldwater Spring allows you to appreciate the Platteville at close range on level ground. The gentle slope of the bike path trail means it's a short walk from the lower Platteville exposed at the south end of the park to the upper Platteville at the north end. This chance alignment also means you can get right next to the contact with the Glenwood at more or less level ground as well. October is also one of the best times to visit: the vegetation is dying back so you can see the rocks, the temperature and humidity are comfortable, mosquitoes and ticks are in retreat, and the ground is is not saturated with spring snowmelt.

The park is also great for these fossil walks because of the fossiliferous building stone and the presence of several areas with lots of small eroded blocks of the Platteville. I can bring families to the building stone used in the parking area and near the Spring House to give them an idea of what the fossils look like, and then the kids can rummage around in the loose stone. It's a great time: if your family is here, you're probably already the kind of kid who likes to rummage around in rocks; the Platteville is a reliable producer of shelly fossils, so everybody should get to see something; there's that paleontologist guy who can tell you what you've found, and if it's really interesting he'll call everybody over to see what it is; and there are also lots of interesting bugs and spiders and so forth if you're striking out on the fossil front, without anything too dangerous (one of the perks of exploring in Minnesota, although we do scare the bejeezus out of many innocent pillbugs). I get lots of questions, and nobody seems to mind that it's "catch and release" here (NPS property). I've been around the rock pile a few times, so I'm jaded. It takes more than a couple of brachiopod molds to get my interest. But if this is your first time to visit fossils in the field, to turn over a rock and maybe be the first person to ever see the brachs on that particular slab, you can't do much better.

First shells!