Sunday, July 27, 2014

Where to see metro geology, part 5: Shadow Falls Park

Having worked the dinosaurs out of my system for a couple of weeks at least, it is time to return to the Twin Cities. Today's location is one of my favorite places. I first visited it when I was an undergrad at the University of St. Thomas around the turn of the century. This may surprise some of you (well, if this is the first post you've read), but the non-scholastic aspects of college life were utterly lost on me. I did some writing and reading, learned how to kind of flail at a guitar in a rhythmic fashion and sing at the same time, and started the world's drabbest-looking website on dinosaurs. It's amazing how these things will occupy you. In my hectic schedule I also found time to spend poking around the network of goat trails along the Mississippi gorge, which is where the subject of this post comes in.

Shadow Falls Park is a small St. Paul city park at the western terminus of Summit Avenue. It has been a city park since 1902, although at the time I stumbled upon it, its identity was not loudly identified to visitors. Today if you stop by, though, there is a panel of information detailing its history and geology. The park gets its name from the titular falls, a small cataract a short distance back from the river. The stream feeding it is a well-behaved trickler, and makes a suitable ice display in the winter. As you might have guessed from previous discussions on local falls, the thing holding it up is the Platteville Formation.

And that's what it looks like, up close. It's more impressive with some distance.

Shadow Falls has a long and distinguished history in the annals of Twin Cities geology, for two reasons: the rocks have excellent exposures, and this is one of the few places in the Cities where you can actually see the Decorah Shale. As Ojakangas (2009) put it, Shadow Falls has the “best exposed and most easily accessible exposure of the Paleozoic rock formations in the Twin Cities area”. The first reference is probably in Newton Horace Winchell's 1877 description of the geology of Hennepin County (also of interest as a historical document today). In this publication, he wrote about a fossiliferous green shale we can now recognize as the Decorah Shale as being present at “Finn’s Glen”. Finn's Glen is a slippery place, and I suspect that the real Finn's Glen may actually be the Grotto that leads into the south campus of the University of St. Thomas (or it might just be completely covered over). Fortunately, Winchell also provided a map, and it is clear that his "Finn's Glen" is our Shadow Falls. A few years later (1886), Edward Oscar Ulrich (better known for his work elsewhere, but brought to Minnesota by Winchell with several other members of the Cincinnati gang to describe fossils) named the crinoid Cremacrinus punctatus for a specimen (U.S. National Museum 89879) from "Finn's Glen", which again appears to be our Shadow Falls (Sloan et al. 1987). To check out this specimen, go to the USNM online database, then the tab "Search by Field", and type in 89879 for "Catalog Number - USNM". Click on the little box with the plus sign left of the first result, and you'll get a pop-up box with some photos (not recommended if you get jealous of other peoples' crinoids).

I do not know when Shadow Falls acquired its present name, but it was at least by the 1890s. There is an offhand reference in an education journal from 1899 that shows teachers knew it by that name and were taking students there on field trips. (See, kids? This is what happens when you skip the socializing in college and instead explore goat trails; you end up digging up references to geological sites in 19th century education journals.) In the 20th century, it became a particular favorite of University of Minnesota geologists, who considered it their “standard experimental Platteville section” for when they wanted to test out ideas (Sloan 2005). It has been featured in numerous publications (not counting field trip guidebooks and such), including Mazzullo and Ehrlich (1980) on the St. Peter Sandstone, Dokken (1987) on Platteville Formation trace fossils, Rice (1987) on brachiopods, Johnson (1988) on Glenwood Formation fossils, and Kolata et al. (1996) on the bentonite beds. Stratigraphic information can be found in Mossler (1985) and several of Sloan's publications, the most recent in Sloan (2005).

No doubt you are now rarin' to get out and see this site. A brief word beforehand, though: Shadow Falls is not as forgiving as the previously featured sites, which had many points of interest on level ground. Shadow Falls hugs the gorge, and to see the geology, you've got to get out on the goat paths. Most of them are innocuous, but the fact remains you are in a close association with steep drops. Respect the path and your own abilities. In terms of geohazards, there are many places where you can get right underneath Platteville Formation blocks that are merrily eroding away. One day, they will fall off, and they won't be too particular about what is underneath (or standing on top). Above the Platteville, the eroded Decorah Shale mantling the upper slopes and goat paths does its charming "turn into sludge when in contact with water" trick whenever it rains or the snow thaws. Your shoes and probably pant legs will be a mess if you ignore this basic fact, and when the ground is slippery you're always at a greater risk of falling. To sum up, don't try to do too much, minimize time spent under the outcrops, and let the ground dry out before hiking through.

What's a nice outcrop like you doing with a joint like this?

What can you expect to see?

When you arrive, there is a small parking lot, which is helpful because there aren't as many parking areas adjacent to the gorge on the St. Paul side as there are on the Minneapolis side. Whether or not there are spaces available depends on the event calendar and the time of day. Many weekends in the summer feature running events or clean-ups or what-have-you, and it is generally difficult to park during midday on weekends when there isn't an event and the weather is remotely tolerable, so mornings are best. There is a memorial obelisk near a fenced overlook, commanding a good view of the Mississippi; most river traffic is barges, but you might also see a crew team practicing or a fire boat exercise. The gorge is also crawling with large birds of prey, including bald eagles. Beneath the fenced platform is a slope of bleached yellowish clay leading to a few natural "steps" of limestone and dolomite, and then to a broad Platteville platform where there are usually a couple of college kids hanging out. (If something does happen to you, at least there's usually someone around.) Goat paths radiate from around the fenced area down to the platform and along the gorge of the Mississippi and the valley of the creek; if you can't tell which way to go, just wait from the fenced overlook until one of the locals heads down, and follow their lead. Once you hit the goat trails, it's just a matter of how far you feel like going. I'm going to start by going all the way down to the mouth of the creek, which under normal conditions is not too tricky. What you will see here is the lower part of the standard gorge/bluff outcrop, the St. Peter Sandstone, with its standard petroglyph-ery of recent vintage. Way up at the top, the outer edges of the Platteville platform are visible.

Either the perspective is really weird or the Platteville is practicing non-Euclidian geometry.

The broad part of the platform is mostly the Mifflin Member of the Platteville, which you can see by walking near the St. Peter/Glenwood/Platteville zone. The thin horizontal beds and jointing of the Mifflin are quite apparent. This is also the part where you want to avoid falling rocks, because you'll have the least warning and maneuvering space. Other exposures of the Platteville ring the valley between the Mississippi gorge and the falls, beneath the goat trail that winds around.

From the top down: Mifflin Member of the Platteville, big blocks of the Pecatonica Member, a largely concealed zone with the Glenwood Formation, and the St. Peter Sandstone.

The series of steps is primarily the Magnolia Member of the Platteville Formation and the overlying Carimona Member of the Decorah Shale. The Hidden Falls Member, because of its shaly content, erodes easily, which makes the overlying Magnolia less secure and causes it to be farther back from the edge. The nice thing here is that you can see the Carimona Member quite well; it is the highest step in the "staircase". In the photo below, it is the section of rock from about halfway up to about three-quarters up. You can see a couple of cut-ins, which represent more erodible beds of shale and bentonite. The lower cut-in in the photo below is the Deicke K-bentonite. If you are unfamiliar with the Deicke K, it happens to represent one of the biggest eruptions known. It can be traced over millions of square km (Bergström et al. 2004), and the eruption that produced it may have let loose over 400 times the material of Mount St. Helens (Ojakangas 2009), roughly 270 cubic miles of ash (about 1,120 cubic km), or a cube about 6.5 miles (10.5 km) on a side (Huff and Kolata 1990). Today, it's only about 3 in (7 cm) thick in the Twin Cities area, but before compression it may have been 11 in (27 cm) thick (Dokken 1987). As you might suspect, introducing this much ash was not a great thing for the immobile shallow-water suspension-feeders of the early Late Ordovician, and there appears to have been a local mass-extinction at this time (Sardeson 1926; Sloan et al. 2005).

The Carimona was formerly described as a member of the Platteville Formation in Minnesota, but it was reassigned to the Decorah Shale to make the terminology more congruent with neighboring states (Mossler 2008). Once you've seen it, it becomes much easier to pick out elsewhere. There is a similar exposure at an overlook near the Ford Bridge, but it is much harder to get to.

The Carimona Member of the Decorah Shale includes the column starting about halfway up. The black object on top is the snout of a guitar case, to give you an idea of scale. Far up at the top there are people overlooking from the overlook.

The Decorah Shale's shaly member crops out a little bit along the slope up to the fenced overlook, particularly just below. This unit has never been very fond of outcrops, though, so most of what you see is this gray-green soil, which lets you know the Decorah is right there next to you even if you can't see it. The best place to observe this property is the quasi-bare patches on the north-facing slope of the valley between the valley's mouth and Shadow Falls.

I get a lot of mileage out of this picture for some reason.

You will also notice fossils here, most the typical Decorah BBC. Various publications have put this site 'round as a place to collect them, but in the aftermath of the 2013 accident at Lilydale, I made some inquiries with the St. Paul parks people and found from several different representatives that they do not permit fossil hunting here. I don't know if it had always been thus and nobody bothered to check, or if this was a recent change, but there you go. This is something that should be clarified on their official documents for the public.

References cited:

Bergström, S. M., W. D. Huff, M. R. Saltzman, D. R. Kolata, and S. A. Leslie. 2004. The greatest volcanic ash falls in the Phanerozoic: trans-Atlantic relations of the Ordovician Millbrig and Kinnekulle K-bentonites. The Sedimentary Record 2(4):4–8.

Dokken, K. 1987. Trace fossils from Middle Ordovician Platteville Formation. Pages 191–196 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Huff, W. D., and D. R. Kolata. 1990. Correlation of the Ordovician Deicke and Millbrig K-bentonites between the Mississippi Valley and the southern Appalachians. AAPG Bulletin 74(11):1736–1747.

Johnson, J. D. 1988. A new fossil fauna from the Glenwood Shale in southeastern Minnesota. Journal of the Minnesota Academy of Science 53(3):unpaginated.

Kolata, D. R., W. D. Huff, and S. M. Bergstrom. 1996. Ordovician K-bentonites of eastern North America. Geological Society of America, Boulder, Colorado. Special Paper 313.

Mossler, J. H. 1985. Sedimentology of the Middle Ordovician Platteville Formation, southeastern Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 33.

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, MN. Report of Investigations 65.

Ojakangas, R. W. 2009. Roadside geology of Minnesota. Mountain Press Publishing Company, Missoula, Montana.

Rice, W. F. 1987. The systematics and biostratigraphy of the Brachiopoda of the Decorah Shale at St. Paul, Minnesota. Pages 136–166 in R. E. Sloan, editor. Middle and Late Ordovician lithostratigraphy and biostratigraphy of the Upper Mississippi Valley. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 35.

Sardeson, F. W. 1926. Pioneer re-population of devastated sea bottoms. Pan-American Geologist 46:273–288.

Scott, C. B. 1899. Teachers’ field lesson—Fort Snelling. School Education 18(3):22.

Sloan, R. E. 2005. Minnesota fossils and fossiliferous rocks. Privately published, Winona, Minnesota. Available from the Minnesota Geological Survey.

Sloan, R. E., W. F. Rice, E. Hedblom, and J. M. Mazzullo. 1987. The Middle Ordovician fossils of the Twin Cities, Minnesota. Pages 53–69 in N. H. Balaban, editor. Field trip guidebook for the upper Mississippi Valley, Minnesota, Iowa, and Wisconsin. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook 15.

Sloan, R. E., M. D. Middleton, and G. F. Webers. 2005. Late Ordovician stratigraphy and paleontology of the Twin Cities Basin. Late Ordovician lithostratigraphy and biostratigraphy of the southern margin of the Twin Cities Basin. Pages 105–143 in L. Robinson, editor. Field trip guidebook for selected geology in Minnesota and Wisconsin. Minnesota Geological Survey, St. Paul, Minnesota. Guidebook Series 21.

Ulrich, E. O. 1886. Remarks on the names Cheirocrinus and Calceocrinus, with descriptions of three new generic terms and one new species. Minnesota Geological and Natural History Survey Annual Report 14:104–113.

Winchell, N. H. 1877. The geology of Hennepin County. Minnesota Geological Survey, St. Paul, Minnesota. Annual Report 5:131–201.

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