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...)

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.

Some thoughts on St. Anthony Falls

If you're familiar with the history of Minneapolis, you probably know that beneath what we now call St. Anthony Falls are the remains of the natural St. Anthony Falls. The real falls went kaput quite a while ago; October 5, 1869, to be precise. The creation of the Hennepin Island tunnel is usually named as the culprit, but after seeing this photo I am beginning to suspect suicide. For various reasons, the presence of a waterfall or a reasonable facsimile thereof was required, so with the aid of the Army Corps of Engineers, the body was embalmed in wood and cement, and propped up in its accustomed place.

Granted, it's a pretty darn good job of embalming.

The Platteville Formation revisited

For the person interested in fossils in the Twin Cities, the Decorah Shale is money in the bank. It's a cooking pot that never empties, a gas tank that's always full. If you have a patch of it, you cannot lose. The Platteville Formation is more like a lottery ticket. If you pick up a random piece, the chances are good it will have nothing, or maybe an imperfect brachiopod mold or two, or some "eyelashes" from shells in cross-section, or half of a burrow. Even when you do find a chunk that's loaded with fossils, usually it's 95% brachiopods and 5% snails, with a couple of crinoid columnals, bivalves, or bryozoans for variety. Every so often, though, you will come up with something unusual. It's true that the Decorah also rewards in-depth exploration, but the "floor" of discovery is so much higher in the Decorah that the feeling when you do find something out of the ordinary in the Platteville is much different. It's more of an accomplishment. The universe has rewarded your perseverance, has conspired with taphonomy, lithification, and erosion to put someone with the proper skills and inclination (i.e. you) in this place at this time to observe and appreciate this fossil. (I will refrain from pulling out the conulariid again.)

I've seen a few of these big snails; not sure about the genus yet.

In terms of preservation, the Decorah Shale is a strictly representational artist dedicated to faithful reproduction of the fossils, thanks to relatively mild conditions for fossilization and diagenesis (the stuff that happens during and after the formation of sedimentary rocks, like replacement of calcite with dolomite). Thanks to dolomitization, the Platteville Formation of the Twin Cities Basin is a sort of minimalist impressionist, retaining only some essential essence of a given fossil while losing most of the fine details. (It also has a thing for sparkles, what with all of the fine dolomite crystals.)

And then we've got blocks like these, doing a decent job of imitating the Decorah. I think I can place the source to a specific bed in the upper Mifflin, but it may be very localized.

On Saturday, I was the guest paleontologist for the Second Saturday fossil event at Coldwater Spring. (I'm the tall one with the facial hair.) After having spent a lot of time along the gorge, I think it is fair to say that Coldwater Spring is one of the best places in the Twin Cities to be in close contact with the Platteville Formation, if not the best. It is certainly the best place to take people of all ages and experience levels to see Platteville rocks and fossils. In most locations on the gorge, the Platteville is a brooding presence capping whitish bluffs of St. Peter Sandstone, inaccessible to all but the most reckless. At some places where a ravine joins the gorge, such as Shadow Falls, Minnehaha Falls, and a few locations on the Minneapolis side of the river, you can walk around parts of the Platteville, but you also are stuck on narrow paths where you've got the Platteville on at least one side, sometimes two (the other side is the one above your head), and the steep slope of the bluffs on the other. This can be chancy when you're on your own and is not feasible for groups of non-professionals, and even when you do go, you usually only get to see the lower part of the formation. By virtue of erosion and some human modification, Coldwater Spring allows you to appreciate the Platteville at close range on level ground. The gentle slope of the bike path trail means it's a short walk from the lower Platteville exposed at the south end of the park to the upper Platteville at the north end. This chance alignment also means you can get right next to the contact with the Glenwood at more or less level ground as well. October is also one of the best times to visit: the vegetation is dying back so you can see the rocks, the temperature and humidity are comfortable, mosquitoes and ticks are in retreat, and the ground is is not saturated with spring snowmelt.

The park is also great for these fossil walks because of the fossiliferous building stone and the presence of several areas with lots of small eroded blocks of the Platteville. I can bring families to the building stone used in the parking area and near the Spring House to give them an idea of what the fossils look like, and then the kids can rummage around in the loose stone. It's a great time: if your family is here, you're probably already the kind of kid who likes to rummage around in rocks; the Platteville is a reliable producer of shelly fossils, so everybody should get to see something; there's that paleontologist guy who can tell you what you've found, and if it's really interesting he'll call everybody over to see what it is; and there are also lots of interesting bugs and spiders and so forth if you're striking out on the fossil front, without anything too dangerous (one of the perks of exploring in Minnesota, although we do scare the bejeezus out of many innocent pillbugs). I get lots of questions, and nobody seems to mind that it's "catch and release" here (NPS property). I've been around the rock pile a few times, so I'm jaded. It takes more than a couple of brachiopod molds to get my interest. But if this is your first time to visit fossils in the field, to turn over a rock and maybe be the first person to ever see the brachs on that particular slab, you can't do much better.

First shells!

Historic Twin Cities geologic maps and photos

I've been a bit busy lately, so no full post, but here are some historical geologic images and maps of the Twin Cities, 1877–1941. Enjoy!

A map of the surficial geology of Minneapolis and western St. Paul, from Winchell (1877). Note "Finn's Glen" at the end of Summit Avenue, today's Shadow Falls. Also note Bassett's Creek in Minneapolis, once a major waterway but now built over. Some now-defunct quarries and waterfalls are also marked. As far as I know, this and the other maps included here are the oldest published geologic maps for their respective areas.

Small quarries of Platteville stone once dotted the Twin Cities. This is a fairly late example, pictured in Sardeson (1916). Kansan till refers to a now-obsolete system of glacial episodes

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 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.

Where to see metro geology, part 3: Coldwater Spring

Picking up from where we left off last time...

Going south from Minnehaha Park on the bike trail network, specifically the Fort Snelling State Trail, we encounter Coldwater Spring. Coldwater Spring, also known as Camp Coldwater, is a small parcel situated between property of Minnehaha Park, Fort Snelling State Park, the Veterans Hospital, and other landowners (there's actually a number of small land parcels in this area, but this is a paleontology and geology blog, not a land ownership blog. If you're curious, there's a map here, with Coldwater as #8, Department of the Interior). The site is currently owned by the National Park Service as the only part of Mississippi National River and Recreation Area that is not an island. Before that, it hosted a Bureau of Mines complex. The complex was vacated during the 1990s after the bureau was closed by Congress, but the buildings still stood until 2011. In fact, if you are using the older version of Google Maps, the buildings reappear when you've zoomed in far enough to transition from flat to 45 degrees. All that's left of the complex today is a few chunks of foundation and the concrete ore storage bins. Before the Bureau of Mines, the titular spring was used by Fort Snelling, taking over what had been a squatter's camp in the 1820s and 1830s. Going back before the Fort, we get into the tribal period of use. The spring is in the Platteville Formation, which is not surprising given that the different members of the formation can promote spring outflows at the contacts (water flows through one member, then runs into another member which isn't as permeable).

Watching the river flow

Nothing in particular, just a few observations:

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.

Where to see metro geology, part 2: Minnehaha Regional Park

When we left off in Part 1, we were hanging around the observation area of Lock & Dam 1. Directly west of the observation area is Minnehaha Regional Park, famous as the home of Minnehaha Falls. Minnehaha Falls is one of a series of cataracts that have formed along the Mississippi in the Twin Cities. The basic processes went like this: each time the glaciers rolled through, they filled up the preexisting river system with glacial debris, and each time they retreated, the river system reestablished drainage. The last time the system was reset, the Minnesota–Mississippi network was grinding its way through the Platteville until it scoured into an old channel in the vicinity of Dayton's Bluff. It's a lot easier to wash out loose debris than it is to erode the Platteville, and a waterfall (proto-St. Anthony Falls) formed where the two met. Over the years, the waterfall eroded upriver, and every time the falls passed a tributary, a smaller waterfall ended up forming on that creek or river, too. This is how we got Hidden Falls, Shadow Falls, and a host of falls that are now mostly to well and truly extinct (human intervention in the drainage systems), as well as Minnehaha Falls. There is something of a cottage industry among Minnesota's geologists of reckoning the elapsed time since deglaciation by calculating the rate of retreat of St. Anthony Falls, using historical observations of its locations before the 1870s, the properties of the underlying formations, and the starting point, but that is for another time.

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.

Divisions of the Platteville Formation

There comes a time when we must speak of the Platteville Formation in detail. If there wasn't a Platteville Formation, there would be no narrow gorge in the Twin Cities, no Minnehaha Falls or Hidden Falls, and most importantly no Saint Anthony Falls: if there's no Saint Anthony Falls, Minneapolis develops in a radically different way. No falls, no easy water power, no mills. The Twin Cities owe their existence to a 455 million year old carbonate bank and the vagaries of glacial and river erosion.

And today the Falls of St. Anthony are mostly artificial. Turns out that drilling holes through the Platteville into the St. Peter Sandstone near the edge of a cataract can be unsafe.

Fossils from Loring Park

For the first post on something specific, I'm going a bit outside of "equatorial" and heading, instead, for approximately the end of the Pleistocene. The geologic time scale is worth a post on it own, but for now, if you're not familiar with the geologic time scale, a brief version is available through the University of California Museum of Paleontology, or you can get all the nitty-gritty from the International Committee on Stratigraphy; the ICS version is slightly more recent, so some of the dates are slightly different because they're a bit more refined. For our purposes, the Pleistocene was a time when glaciers advanced and retreated through Minnesota.

In 1923, the Northwestern National Life Insurance Company was building a new home office across from Loring Park. This building still exists, by the way (see this for its qualifications as a historic site), although the insurance company has long since moved to larger digs. During excavation, workmen cut through old moss beds and other deposits with fossils, eventually described by William S. Cooper and Helen Foot in 1932. It isn't stated, but because both authors were affiliated with the University of Minnesota, I can only guess that this is where the fossils ended up (with the exception of the snails and clams, which went to the Museum of Natural History of the University of Illinois). The following information comes from their publication, except where otherwise indicated.