More odds and ends

At the moment I'm preparing the grand annual update to The Compact Thescelosaurus. If you've been following along over the past couple of years, it doesn't take being Sherlock Holmes to guess what it will be. Actually, it doesn't even take being 1940s Radio Program Watson to guess, but do act surprised. In completely unrelated news, there's a taxon published this week I recommend you see: the little croc Thikarisuchus xenodentes. (What, you were expecting the theropod?) T. xenodentes, from the Cenomanian-age Blackleaf Formation of Montana, had a sharply triangular skull in top view and strongly differentiated teeth, among them the expected bitey teeth at the very front and long, low, narrow teeth at the back. This small croc may have had a taste for plants, or perhaps sliced up insects.

Something we love around here at Equatorial Minnesota is historical content about Minnesota geology. You may be familiar with the Minnesota Geological Survey's publication archive. It turns out that there are a bunch of MGS field notebooks scanned and available via the University of Minnesota libraries. You can find notebooks there from paleontologists such as Frederick Sardeson, Robert Sloan, and Clinton R. Stauffer, all of whom curiously have last names starting with "S". (Also interesting: Sardeson's field notebooks all have the left-hand pages filled out, but right-hand pages are much less frequently used. Was Sardeson a lefty?) There are also materials from the Department of Earth and Environmental Science.

Do you prefer to see your geology in the field? I came across a nice section of the St. Peter through Platteville under the I-94 bridge, on the east bank of the Mississippi. I'd been there years before but don't remember being so impressed with it. Maybe it wasn't as well-exposed then, or maybe I just wasn't experienced enough to appreciate it. The Platteville interval at the top was definitely exposed, but maybe this lower exposure is the result of more recent erosion or something. It definitely bears further photography and investigation, as it has a great view of the Glenwood.

The important part looks like this.

And here's what it looks like if I annotate all over it. I'm not entirely confident with the thickness of the Hidden Falls Member; for some reason the Mifflin is really grungy here. The Glenwood bits are open to interpretation (the Nokomis gets "Glenwood/St. Peter" because it's technically in the Glenwood but can't be distinguished from the St. Peter in well logs or gamma logs). The Tonti Member makes up most of the St. Peter Sandstone, if you're curious. There may be some Carimona at the very top. 

Oh yeah, also saw Bridal Veil Falls nearby, which has, y'know, seen better days, but is still running, after a fashion.

Or maybe you'd like some historical trivia? One of the things I wanted to find more about for the Mammoth Cave National Park paleo inventory was whatever became of the type specimen of Lithodrumus veryi, a Mississippian coral possibly collected from the park. It was described in 1904 by George Greene and the specimen has been lost to science since at least 1944. Greene's collection went first to the American Museum of Natural History and then to the National Museum of Natural History, but Lithodrumus veryi apparently went missing, as Easton (1944) couldn't find it at the AMNH. Just a few days ago I was looking up images of tabulate corals when I came upon a post at Louisville Fossils and Beyond that stated one cabinet had not been sold, and its contents were eventually going to the Indiana State Museum. Hope springs eternal! I've sent a message to the Indiana State Museum to see if perhaps L. veryi's type is there.

It's supposed to look like this (Greene 1904: Plate 49). Have you seen it, by any chance?

So that's what's going on around here. (Oh, that and some unusually intense bot "readership", or the whole of Hong Kong has suddenly discovered a passionate interest in the Ordovician fossils of Minnesota and Elliot Formation prosauropods. Maybe I'm cynical.) Tune in for the next post!

References

Allen, H. J., E. W. Wilberg, A. H. Turner, and D. J. Varricchio. 2025. A new, diminutive, heterodont neosuchian from the Vaughn Member of the Blackleaf Formation (Cenomanian), southwest Montana, and implications for the paleoecology of heterodont neosuchians. Journal of Vertebrate Paleontology e2542185. doi: https://doi.org/10.1080/02724634.2025.2542185

Easton, W. M. 1944. Revision of Campophyllum in North America. Journal of Paleontology 18(2): 119–132.

Greene, G. K. 1904. Contribution to Indiana palæontology 1(17): 168–175.

How to complicate the Glenwood

Recently I've been doing some work with geological stratotypes in National Park Service areas. The naming of geological formations is not unlike naming a fossil species of organism (or living species for that matter), except the type locality is also the type "specimen", and you don't dig up the type locality and put it in a museum (although you could certainly take a core). Anyway, there are a few stratotypes located within Mississippi National River and Recreation Area. I've alluded a couple of times to the type locality of the St. Peter Sandstone being the bluff under Fort Snelling. This locality is no longer accessible due to protective measures, but the formation *is* exposed in the immediate vicinity. The type locality of the Hidden Falls Member of the Platteville is in Hidden Falls Park (Sloan 1956). Sloan didn't state specifically where, but I have a pretty good idea. Finally, there are two units named from the section exposed at Lock and Dam No. 1.

Beautiful exposure, even if you can't climb around on it.

The Twin Cities Basin is just a small part of the area where the St. Peter–Glenwood–Platteville–Decorah sequence is exposed, and across this area, the rocks differ and the names differ. Reconciling nomenclature across states is part of what Mossler (2008) was about. An example of outstate attention on the Twin Cities Basin is Templeton and Willman (1963), from the Illinois Geological Survey. This publication brings the Illinois nomenclature into the Twin Cities, via a section at Lock and Dam No. 1 (p. 226–227). If you're used to the local names, the Lock and Dam section requires a lot of translation. For example, the Platteville is considered a group, divided into three formations and nine members. The Templeton and Willman nomenclature has not taken off in Minnesota, which has preferred a simpler system, and I can't say I disagree. It seems like overkill to pack that many formal divisions into about 9 vertical meters (30 ft), many of which are not visually distinguishable at a distance of a few meters. Their divisions of the transition from the St. Peter to the Platteville are another matter.

Beginning with our old friend the Pecatonica and going down, Templeton and Willman (1963) divided the Lock and Dam rocks into the Chana Member and Hennepin Member of the Pecatonica Formation (Platteville Group), the Harmony Hill Member and Nokomis Member of the Glenwood Formation, and the St. Peter Sandstone. Two of these members were named from this section: the Hennepin Member and Nokomis Member. In turn:

My initial thoughts. Arrows point to the approximate center of the named unit, so I don't have to commit to contacts like the coward and cad I am. The scale boxes are *very* rough; I was able to put a scale bar on the outer wall, but this is some distance back, so prefer a larger number and don't take it too seriously.

...Or is this what Templeton and Willman intended? Might be easier if I could get closer.

The Chana Member, 28 cm thick (11 in), is basically what we recognize as the Pecatonica;

The Hennepin Member is 69 cm (27 in) thick, divided into three parts: 46 cm (18 in) of clay-rich greenish-gray limestone, and green shale, over 15 cm (6 in) of clay-rich greenish-gray slightly sandy limestone, over 8 cm (3 in) of brown dolomitic sandstone (Templeton and Willman 1963). This unit, or at least the upper part of it, can be easily distinguished visually beneath the blocky overlying rocks (pers. obs.; see also the photos at the end of this post—areas that look white are weathered). The other two parts I'm not as sure about. The Hennepin Member should be thicker than the Chana and the underlying Harmony Hill put together, but from my photos and observations, I see four distinct units over whitish sandstone. Three of them seem fairly similar in thickness: the classic Minnesota Pecatonica, a light-colored recessive unit, and a greenish-brown recessive unit. Below them is a thinner, even more recessive yellow-brown unit. My first thought was that the greenish recessive unit was the Harmony Hill Member, but if I put it in the Hennepin Member and assume that the 15 cm and 8 cm beds of Templeton and Willman (1963) are both included in it, we get something much closer to the overall relative thicknesses T&W found for the Hennepin versus the Chana. Plus, the underlying thin yellow-brown interval looks to be closer to their description of the Harmony Hill Member. The internal proportions are still out of whack, though (three roughly equal units in photos versus 28 cm over 46 cm over 15+8 cm in the publication). (In case you were wondering, there is a grand total of zero [0] photos of outcrops in T&W '63, which is regrettable. A single decent photo with a couple of arrows would have helped immensely.) At any rate, the Hennepin Member is more shaley than the description in T&W '63 implies, or, rather, is less carbonate-rich (Mossler 2008). The Minnesota Geological Survey regards it as the upper part of the Glenwood Formation, rather than the basal part of the Platteville (Mossler 2008);

The Harmony Hill Member is 23 cm (9 in) of yellow-green shale (Templeton and Willman 1963);

The Nokomis Member is also tricky. Templeton and Willman (1963) described it as 330 cm (110 in) thick, divided into 10 to 18 cm (4 to 7 in) of silty white sandstone, over 58 cm (23 in) of very silty, greenish-buff to red-brown, thin-bedded, partly ferruginous sandstone, over 224 cm (78 in) of very silty white to yellow-buff sandstone. Mossler (2008) noted that an interval of silty sand is commonly found in Minnesota between standard Glenwood and standard St. Peter, and this interval has frequently been included in the Glenwood. (The Glenwood is a bigger deal outside of Minnesota.) However, this interval is difficult to distinguish from standard St. Peter in well logs and natural gamma logs, so for practical purposes Mossler (2008) recommended including it in the St. Peter. It is also difficult to identify a difference just by looking at the outcrop from the vantage point of the landing. The best I can do is identify a couple of slope breaks that may indicate changes in mineralogy which are not otherwise apparent from color or bedding at that distance. A contact in the vicinity of the lower slope break would give a thickness in the vicinity of the quoted figure.

The breaks are easier to see in oblique view than straight on. Note also how plants like to colonize the Glenwood–upper Nokomis interval.

References

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

Sloan, R. E. 1956. Hidden Falls Member of Platteville Formation, Minnesota. Bulletin of the American Association of Petroleum Geologists 40(12):2955–2956.

Templeton, J. S., and H. B. Willman. 1963. Champlainian Series (Middle Ordovician) in Illinois. Illinois Geological Survey, Champaign, Illinois. Bulletin 89. [large file]

Conodonts

A common yet easily overlooked type of fossil is the conodont (or "conodont element", should we choose to be picky and/or technical). They are easy to overlook because they are sub-millimeter-scale fossils, which makes them just barely visible to the naked eye if you look really hard. Most of the time, though, all you can do is to take bulk samples of rock and run them acids to liberate these tiny fossils; conodont fossils are made of calcium phosphate, so they will not be dissolved by some acids that do dissolve calcium carbonate, i.e. limestone. Having dissolved your limestone, you can then inspect the remainder under a microscope. Ideally, you will have some little pointy toothy bits, which are the conodont elements.

The Glenwood Formation

Beneath the Twin Cities, sandwiched snugly between the crumbling off-white edifice of the St. Peter Sandstone and the proudly jutting gray to tan Platteville Formation, is a gray to brown layer about 3 to 5 feet thick (or about a meter to a meter and a half for our metric friends), composed primarily of shale. This is the Glenwood Formation, sometimes known as the Glenwood Shale (which is a bit of misnomer because of the sand content). Like a fawn or baby bird, it hides from the eye, although we know it must be there, because heavens knows where else we would get adult deer and birds, and the Platteville has yet to collapse onto the St. Peter. The only reliable way to find it is to look for places where both the Platteville and St. Peter are exposed, and then check the area between them. Even then, you might not be able to see it well; for example, at Minnehaha Falls, the Glenwood is so deeply recessed you can't see much at all. The most obvious exposures are at Lock & Dam 1 and Dayton's Bluff.

The Amazing Four-Sided Herringboned Ice Cream Cone (with creamy polyp center?)

The farther back you go, the less familiar the lifeforms. Obvious, no? And yet the pattern is not a simple one to one relationship. For example, the Cambrian was a relatively brief time in which the invertebrates collectively decided that "anything goes" and did their darnedest to fulfill that maxim. Many groups didn't hack it and disappeared before the Ordovician. From the Ordovician to the Permian, the shallow seas were filled with bryozoans, brachiopods, and crinoids, with growing numbers of rugose and tabulate corals, mollusks, and fish. The dominant groups of the Paleozoic were greatly reduced or wiped out altogether at the end of the Permian. Almost all of the enigmatic or otherwise difficult-to-classify groups kicked the bucket by the Permian–Triassic extinction, with a few exceptions such as the bellerophont snails (or monoplacophorans), the conodonts, and today's entry, which all persisted into the Triassic for reasons known only to them. The post-Cambrian bryo-brach-crinoid seafloor communities were replaced by Mesozoic communities dominated by mollusks, stony corals, echinoids (sea urchins), and cartilaginous and bony fish. With the end-Cretaceous subtraction of the ammonites, belemnites, certain groups of bivalves, and marine reptiles, and the addition of marine mammals, this becomes the typical modern marine assemblage. Today, many of the extinct Paleozoic groups appear strange, which is a bit unfair because they were just being the best filter feeders/detritivores/algal symbionts they could, and because there are plenty of unusual things alive in the ocean this very instant. Some of them, however, seemingly went out of their way to stand out. One example follows below the fold.

Where to see metro geology, part 1: Lock and Dam No. 1

As the snow and ice begin to melt away and the temperature rises above freezing, it once more becomes possible to see the geology of the metro without outfitting a polar expedition. While there's something to be said for iced-over waterfalls and the peace and stillness of a fresh winterscape, it's darned hard to take field notes with choppers on or fogged-up glasses, and frankly if you're doing it to impress people, it won't work. Spring is not a bad time to do some exploring; you don't have to contend with the bugs, the generous Minnesota humidity isn't in full swing yet, and the ground vegetation hasn't obscured many areas (and given ticks places to loiter). The most obvious drawback to springtime geology is the saturation of the ground, particularly in places where the Decorah Shale is at or near the surface. The Decorah likes to form a thick gray-green mud that both cakes footwear and makes the ground admirably slippery, particularly on slopes where you'd just as soon prefer that it wasn't slippery. Throw in the leaf litter from the previous fall, and your April hike can get a lot more interesting. A less-obvious hazard is the insidious action of freeze and thaw cycles. In short, rocks that weren't fractured and liable to fall or slide in the autumn can get that way by the spring.

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).

Scolecodonts and other signs of worms

Clinton R. Stauffer had one of the most convenient field areas it is possible for a geologist to have. He worked for the University of Minnesota from 1914 to 1944, producing a number of papers on paleontology and stratigraphy; they include a study of the Paleozoic of Minnesota (Stauffer and Thiel 1941), a list of Pleistocene mammal finds from the state (Stauffer 1945; an update would be greatly appreciated!), and several descriptions of microfossils (Stauffer 1930, 1933, 1935a, 1935b). He did a fair amount of collecting for the university, and a fair amount of collecting within the university. "Aha," you may say if you are familiar with the geography; "the campus is split by the river, with prominent and accessible bluffs." This is quite correct, but he did not limit his on-campus collecting to the bluffs. For example, during the construction of Northrop Auditorium in 1927, he obtained rocks from the excavation for the heating shaft (Stauffer 1930), and described a number of conodonts and other fossils from this material (Stauffer 1930, 1933, 1935a). If he wanted comparable material from other locations, it was only a matter of miles to southwestern St. Paul/southeastern Minneapolis, where he worked extensively in the Ford Plant/Fort Bridge/Minnehaha Creek/Lock & Dam 1 area (this was convenient both in terms of location and time; the dam, auto plant, and bridge were all completed during this time frame, so there was a lot of disturbed ground and excavated rock to pick through). It wasn't all roses and brachiopods, though; those bluffs on campus are not a place for anyone who have a healthy respect for gravity and large heavy rocks. The combination of a narrow footpath and overhanging rock makes the area about the most hazardous I have seen for Twin Cities geology.

The bluffs south of the Washington Avenue Bridge. Not recommended.