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.

The return of "Dinosaur skeletal anatomy"

As promised earlier, I've reworked the skeletal anatomy section of the old website and added it here. I have rewritten parts of it, particularly to improve the section covering vertebrae, and have substituted different skull figures. I also strongly encourage you, if you have not done so already, to marvel in some armored dinosaurs, the ankylosaurid Zuul crurivastator, and the unnamed Suncor nodosaurid. (Also, that's a heck of a plesiosaur that the Tyrrell has to go with the nodosaur!)

The return of "Anatomical terms of location"

Having done a relatively large number of dinosaur-related posts in the past few months, I've found myself running into anatomy and anatomical terms of location (dorsal, lateral, etc.). Given that not everyone knows all about the jargon, parenthetical glosses tend to slow things down, and I had perfectly serviceable glossaries for these subjects on the late Thescelosaurus!, I decided to revisit that information. I've started by putting the anatomical terms of location on their own separate page. Wikipedia has a useful summary as well, but if you aren't a wiki fan, like having the information on hand here, or just like diagrams featuring the excellent Wild Safari Sauropelta, this is for you. Eventually, I plan to put up skeletal anatomy as a page, and probably a geologic time scale as well (or at least a link to one).

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).

The Age of Homebodies

To expand on an offhand comment in the previous post, to give a nod to everyone staying at home tonight on New Year's Eve, and to set up a convenient directory of fossil groups:

The Upper Ordovician of the Twin Cities metro includes, in ascending order, the St. Peter Sandstone, the Glenwood Formation, the Platteville Formation, the Decorah Shale, and a tiny remnant of the Cummingsville Formation (and yes, they each deserve an explanatory post). There are a few other formations above the Cummingsville Formation in southeastern Minnesota, but that, as they say, is for another time. In terms of fossils, we're mostly looking at the Platteville and Decorah. Calling the St. Peter Sandstone "sparsely fossiliferous" is an understatement, the Cummingsville Formation of the metro is more or less limited to a few lucky blocks near where St. Paul, West St. Paul, Mendota Heights, and Lilydale meet, and although the Glenwood Formation has a decent microfossil assemblage (if you need a hand lens or microscope to see what it is, it's a microfossil), it's been stingy with most other fossils. What we do have from these three formations is not radically different from the Platteville and Decorah, at least not in terms of phyla and classes and so forth.

Mississippi National River and Recreation Area

One more bit of exposition...

The loving attention of generations of continental glaciers has covered most of the metro area with copious glacial drift. There are plenty of stories here for future posts, but for now I'll just make the observation that because of this, if you want to find bedrock outcrops in the Twin Cities, you need to find places where the glacial deposits have been cleared out. The best places for this are along the Mississippi River.

Many of you are familiar with Mississippi National River and Recreation Area, also known as MNRRA ("Minn-Ra") or MISS. MNRRA cuts a narrow strip from Riverside Park in Anoka County to Hastings, 72 miles (116 km) long. The National Park Service does not own most of the land within the boundaries; instead, MNRRA is a partnership park, in which the Park Service works in concert with a number of different landowners. State, county, city, business, educational, and any number of private entities are found within the river corridor. At this time, the acreage owned by the NPS includes some islands and the Coldwater Spring parcel between Minnehaha Park and Fort Snelling State Park. The river bluffs within MNRRA are the best places in the Twin Cities to observe the rocks and fossils of the metro. Of course, right now is not the best time to visit, but you can still stop by the Visitor Center in the lobby of the Science Museum of Minnesota. Many of the sites in the map (below) are of interest for geology and paleontology, and will be covered at some point. The site I have in mind for the next entry, though, is a bit outside...

Geology 101: Sedimentary Rocks

Before we get too far into things, we ought to establish some background. If words like sandstone and limestone aren't part of your daily vocabulary, it wouldn't be very fair of me to go on about them without letting you in on why they are important, would it?

First of all, rocks fall into three broad groups: igneous, metamorphic, and sedimentary. The first two are not of pressing concern at this time and so will only be mentioned briefly, although at some point we will return to them. Igneous rocks form by cooling from a molten state, i.e. magma or lava. They include rocks like granite, one of the patron rocks of countertops, and basalt, a common dark stone which is often what you get from flowing lava. Metamorphic rocks are produced by subjecting rocks to heat and/or pressure. The minerals of a rock are altered by these factors, and sometimes banded textures appear. Metamorphic rocks include marble, the other patron rock of countertops, and schist (foliated; sheet-like thin layers) and gneiss (banded), beloved for their pun value (many geologists are addicted to terrible puns). Of course, if you ramp up the forces of metamorphism too much, the rock melts, and you're back around to igneous processes.