I had some errands to do in St. Paul on Saturday and ended up showing my mother around some of the localities I'd been to last year for a project. There was the overlook north of the Ford Bridge, across from the Temple of Aaron, where I'd seen a shell bed in the Magnolia Member of the Platteville, but the site turned out to be a lot more interesting with a year of experience under my belt. This time, we could follow the shell bed around the outcrop, and I could pick out the overlying Carimona Member of the Decorah Shale.
|The shell bed, highlighted by a helpful graffiti artist.|
There's a bed near the base of the Carimona called the Deicke K-Bentonite. It's a couple of inches thick and is recessed compared to the sandwiching limestone. The bentonite used to be volcanic ash, and substantially thicker before compression set in. It records one of the biggest known volcanic eruptions in Earth's history, a blowout that took place off of what is now the east coast of the United States approximately 454.6 million years ago. Back then, volcanic island arcs and little chunks of continental crust were lining up for the chance to pile into the east coast, and much of what's east of the Appalachians is underlain by these stray blocks. If you're near the transition from the well-exposed Platteville Formation to the recessive Decorah Shale and you know what to look for, you can put your hand on the brief moment in time when the ash started falling and didn't stop until long after the seafloor ecosystem was buried. It's not just here, either: the Deicke shows up all over the eastern US and similar bentonite beds are found in the Ordovician rocks of Scandinavia. As you might guess, an ecosystem based around small immobile filter-feeders does not take well to burial in ash; a local extinction event has been recognized since at least the 1920s (Sardeson 1926).
We continued north, stopping at Shadow Falls and going along the goat paths that follow the bluffs. Spring is in swing here, at least for the birds. A bald eagle flew by perhaps 20 to 30 feet away, chased by a crow. Warblers flitted through the branches, blue jays voiced their annoyance, nuthatches skittered around tree trunks, and woodpeckers drummed above us. Shadow Falls is one of the more popular geological sites (and sights) in the Twin Cities. It's not a bad place to get acquainted with the Platteville and Decorah, if you watch your step. The broad platform formed by the Platteville, so beloved by University of St. Thomas students, also shows the Carimona. Actually, if you felt like it, you could more or less follow the goat paths from the first overlook to Shadow Falls and be walking at about the level of the Carimona the whole way (you'd have to cheat at the Grotto, which leads into St. Thomas's south campus, but you get the idea).
I had one more site in mind: the Winchell Trail south of the Short Line Bridge. The Winchell Trail is an old Native American trail that runs below the bluffs on the Minneapolis side of the river. The name refers to Newton Horace Winchell, who today is best known for his work with the first version of the Minnesota Geological Survey, but counted archaeology among his other interests. South of the Short Line Bridge the trail level ends up close to the base of the Mifflin Member of the Platteville, so the overlying upper Platteville is happily eroding all over the place. It doesn't take a lot of looking to find large blocks of Platteville where one surface is "paved" with molds and casts of brachiopods, or where a band of "eyelashes" can be seen (these being shells in cross-section). These are just more expressions of the kind of shell beds we had seen at the first overlook in the upper Platteville. I have heard that there was once a quarry for Platteville stone down here, which would explain some of the geography. The Platteville was once quarried all over the Twin Cities for building stone, back in the 19th century, and if you go through the 19th and early 20th century documents digitized by the Minnesota Geological Survey you will find frequent references to the former quarries.
|I know I've used it before, but it's a nice block and site-appropriate.|
Sardeson, F. W. 1926. Pioneer re-population of devastated sea bottoms. Pan-American Geologist 46:273–288.