Fish Creek Canyon

You never know where interesting geology will turn up. I've been poking around the Twin Cities for years and I'm still coming across places I'd never dreamed were there. Case in point: today's pictorial topic, Fish Creek Canyon. I know the name sounds like a place you might find in Montana or Idaho, where the water is cold, the fish are biting, and the bears are waiting for you to wander away from your cooler, but this particular Fish Creek Canyon is just about where St. Paul, Maplewood, and Woodbury meet. It's not a huge canyon, and I don't imagine the fish are very large these days, but it's the kind of little hidden backyard gem that makes exploring worthwhile. I visited it with a friend back in late November, not long before our on-off winter hit the "on" switch for the first time.

Down in the valley on a great fall day, with Fish Creek in view.

Fish Creek is spread across two city jurisdictions. Maplewood has Fish Creek Natural Area, which is mostly the heights above 494 on the south and east and Highway 61 on the west. If you scout around, you'll find that this is one of a handful of small bluff-top parks between Battle Creek Regional Park and 494 overlooking 61. Adjacent on the north is the actual canyon of Fish Creek, which is within St. Paul and owned by Ramsey County Parks but apparently not organized at the moment, per se. The area is also largely within Mississippi National River and Recreation Area (so I'm also *officially* curious). The bluff-top lands are well worth walking around in their own right. There are no official access points to the canyon from the bluff-top, but it is possible to reach the canyon via social trails, leading to a steep descent into the narrow valley.

The view from the bluff, with the November afternoon sun on a clear day.

The canyon itself is not vast or deep. The water power that carved the original ravine has long since dwindled to a hoppable creek. And yet, what the creek has cut for itself is such a perfect little feature, complete with a miniature waterfall.

You're walking along, and then the creek disappears.

A miniature waterfall.

There ought to be gnomes.

It widens a bit going down (note that this feature was put in by people, presumably for water management, although it's not bad for aesthetics, either).

The rock that has been cut through here is the St. Peter Sandstone in its case-hardened form, producing the steep-sided, narrow slot. If you've driven Highway 61 near here, you may have noticed how the St. Peter sinks out of view for most of the stretch between Battle Creek and Camels Hump in Cottage Grove. It's still there, it just only shows its face sporadically, and this is the most picturesque place to find it. Interestingly, the location of Frederick Sardeson's "Highwood" collecting locality for St. Peter Sandstone fossils was supposedly a little north of here, about a mile and a half south of Battle Creek Park on 61. This works out to about the area where the ravines now occupied by Highwood Avenue and Springside Drive empty out, so our old friend was successfully trying his luck out here back in the 1890s.

Sometimes the St. Peter feels like crumbling, and sometimes it feels like holding a wall.

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.

Finding waterfalls

A couple of months back, a friend pointed out a news items covering a slot canyon in Crosby Farm Park, so we decided to see if we could find it. We were successful in locating what's also called Homer's Odyssey (must be a pun based on the proximity to Homer Street), and it turned out to be one of the most unusual natural features of the Twin Cities. It's certainly worth a visit the next time you're in the park, although I can imagine it could be difficult to access during a year when it rains more than once every three months or so.

Approaching the entrance.

Looking back through the slot.

Inside the feature, which is kind of like a tall roughly cylindrical void.

Looking up at the top.

The canyon is almost like someone took a knife and cut straight down. Granted, it's not as if fresh St. Peter Sandstone shows much in the way of resistance to erosion, but I do wonder if there was a pre-existing weakness that focused this erosion so narrowly. Maybe there were voids in the sandstone or something similar?

Active waterfalls, waterfalls reduced to a trickle, and former waterfalls are scattered all over the Twin Cities metro. Some former falls, like the one in Cottage Grove Ravine Park or the abandoned falls northeast of Minnehaha Falls, have been high and dry for a long time due to the loss of their water source in the mists of time, but many have lost their flow due to drainage control measures in the past 150 years or so. Some could be reanimated with different priorities, while others have been buried or destroyed.

And even the best of them sometimes get caught short in droughts, as Minnehaha Falls shows in early October 2022.

Even beyond the named falls, there are many small-scale unnamed or obscure features. If you're looking to find some for yourself, you can often guess general areas to look from topo maps: look for valleys crossing steep contours. That's a telltale sign that a creek once crossed a bluff, or something similar. Or, if you'd rather try immediate adventure, get on one of the gorge trails (e.g., the Winchell Trail in Minneapolis or the unnamed goat path equivalents on the St. Paul side) and just keep your eyes open; they're there.

The falls in the Grotto, exiting the University of St. Thomas.

This is on the Minneapolis side, just across from E 36th Street. I've never heard that this one had a name, so it may well not have been doing more than a trickle when Minneapolis was developed, but it would have been substantial at one time.

Another area is south of Minnehaha Park, in the Coldwater Spring/dog park area. There is a secluded active waterfall southeast of Coldwater that has cut a modest slot in the St. Peter; again I'm not sure if it has a widely circulating name. Novelty Falls on the bluffs north of the dog park is very difficult to notice when the falls aren't falling; observation of the bluffs suggests there are several other places that may get mini-falls if there is a lot of rain or meltwater. Marks of water erosion at the top of the bluffs and water-worn hollows at the base will show you where flow has occurred.

A close view, low and up into the chute of the Coldwater waterfall. It's rather more impressive in person, as it tends to photograph poorly due to the confines of the site and the limited foot space. (Note that this area below the bluff has a tendency to become a respectable swamp following anything stronger than a light rain.)

Novelty Falls itself wasn't running in November, but these bluffs are very interesting!

They are also present on the south side of the Mississippi; for example, the Brickyard trail in Lilydale Regional Park passes right by one:

Here pictured during one of the few days this "winter" when there was snow on the ground.

(And yes, I am well aware that this blog has never covered the Brickyard...)

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]

Revisiting Coldwater Spring and Fort Snelling State Park

The same day I got out to Shadow Falls Park, I also stopped at Coldwater Spring and Fort Snelling State Park. These locations are geologically restricted to the Platteville down to the St. Peter, and the exposures are much more in the bluff mode of expression.

Lo, the type section of the St. Peter Sandstone, mostly behind a wall (probably for the best, given the way people use any handy outcrop of the St. Peter to practice their rock-carving skills)

What impressed me at these sites were the numerous rockfalls. It seemed like a lot, but I don't have the numbers to judge. It would actually be a simple project: somebody could periodically go along the paved path and document new falls by photos and GPS. The rub is that it's also a long-term project; you wouldn't get useful information from a couple of years.

At this fall, out of the Mifflin Member of the Platteville, it's easy to see the lighter color where the rocks used to be.

The outcrops of the Hidden Falls and Magnolia members just north of Coldwater have been taking a beating lately, although I'll grant that it's not as impressive when they only fall a few feet. There's both day-to-day attrition and larger collapses.

If you should happen to go below the paved path, down toward the river (say if you're going into the dog park), you can also see evidence of older disruption, where large blocks of Platteville have been displaced.

If the beds are noticeably tilted, that's a good indication they aren't where they started. (And also the whole "lying in jumbled heaps" thing, that's a good clue.)

A small, gentle waterfall southeast of Coldwater, unnamed as far as I know, running through a cleft in the St. Peter.

Given the geology, fossils weren't terribly diverse or well-preserved. There were the usual brachiopod beds exposed on some of the fallen blocks:

There were also these things. 99.999% says they're a couple of weathered burrows, although the coincidental weathering on the longer of the two made me think of segmented stems when I first saw them, which would be rather odd for the Platteville. Further study of the photo showed that the "segmented" feature was longer than I thought, and that the apparent segmentation was limited to the two lines.

Same photo, light red lines added to trace the features.

Midwest Ordovician craters: Decorah and Rock Elm

Life in the Ordovician of the Upper Midwest wasn't all warm tropical seas and all the organic particles you could filter. We've already run into giant volcanic eruptions spreading ash far and wide. There was also a significant glaciation at the end of the Ordovician; the ice itself didn't get to tropical North America, but it did lead into major extinctions. We don't have to go to the poles or volcanoes off the coast of North America for dramatic geologic events, though: we only have to go as far as west-central Wisconsin and northeastern Iowa.

Where to see metro geology, part 6: odds and ends

And now, to complete, more or less, my other series:

I've already hit most of the best places to see Twin Cities bedrock geology up close, and there isn't really much need to flog this further. So, to wrap up things, I'm collecting a few stray thoughts on other areas. There is one other location that is supremely worthy of its own post, and if you are tuned in to the local fossil-hound scene you can guess exactly what it is, and also why there's not much of a point in discussing it now. Thus, the Brickyard of Lilydale will wait.

Previous entries:
Lock and Dam 1/Ford Bridge area
Minnehaha Regional Park
Coldwater Spring
Fort Snelling State Park
Shadow Falls Park

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.

Where to see metro geology, part 4: Fort Snelling State Park

It seems reasonably fitting for the Memorial Day weekend for the next stop to be at Fort Snelling State Park. Although the fort is certainly best known for its place in the history of Minnesota, it is also an early geological locality of note. Because of this, and because much of the original bedrock has been covered, this post will have a more historical bent.

Fossils in the St. Peter Sandstone 2: actual fossils

I'm not quite sure why, but "Fossils in the St. Peter Sandstone" is one of the more popular posts to date. Who am I to argue with the St. Peter Sandstone? Last fall I visited the paleontological collections of the University of Minnesota and took a number of photos. I've received permission to post examples (thank you, Dr. David Fox!), and among them are a few photos of actual St. Peter Sandstone fossils, as primarily collected by Sardeson.

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.

Fossils in the St. Peter Sandstone

The St. Peter Sandstone, a Middle–Late Ordovician unit that deserves the appellation "sandstone" like few other formations, has proven itself a very useful formation. Its extremely pure quartz sand is prized for various industrial applications, like glassmaking. It is readily excavated, so digging sewers and burying utilities is simplified. It has a tendency to form caves, which coupled with the ease of excavating makes it an ideal substrate for underground storage, cheese aging, mushroom growing, some types of brewing, and so on. More frivolously, it has provided a vast natural canvas for people who like to carve their names in things, and it has rewarded generations of graduate students looking for thesis and dissertation topics. At 100+ feet (30 m+) of uniform sand with few apparent bedding structures, and an off-white color that weathers to a kind of sickly gray, it is also an excellent natural soporific if you are not interested in any of these things. This most useful of formations is very much a bust paleontologically, which seems like some kind of a metaphor, but I'm not going to push things.

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).

Monoplacophorans

There are some things in life that just don't have a handy name. Take for example the subject of this entry, an obscure group of mollusks known from a few modern deep-water species and a more robust fossil record. Various members of this group have labored under the terms "Monoplacophora", "Tergomya", and "Tryblidiida", all of which could be mistaken for a disease, a planet in a science fiction series, or a piece of anatomy you'd rather not know about. (You can combine all three: "I got monoplacophora in my tryblidiida after I visited Tergomya. It's as bad as it sounds.")  At one point someone suggested that their vernacular name should be "gastroverm", which you'll probably agree was no help at all. The reason for this multitude of names has to do with the rules of classification; visit here or here if you want details (and don't say I didn't warn you), but the basic bit is that Tryblidiida includes all the modern forms, and is classified in Tergomya, which is in turn part of Monoplacophora, and "Monoplacophora" is something of a problem because its traditional definition includes some things that weren't all that closely related. However, everyone was kind of used to Monoplacophora, so they kept it around, except now it can have a precise definition, or be a "state of mind" including various odd extinct things from the Cambrian, which I shall omit. If you want to be picky, this is about tergomyans.