Stromatolites in situ

A few weeks ago, during the elongated-dry-autumn-with-long-nights that has been substituted for winter in these parts this year, I visited a stromatolite patch in the Prairie du Chien Group and took the usual digital heap of photographs. The photos aren't quite as sharp as I would have liked, but they show a variety of aspects of stromatolites both up close and in situ, unlike the usual circumstances of getting only one of those two properties.

First off, here's a general idea of what we're dealing with:

Click to embiggen, as usual; the thing about stromatolites is sometimes they're more apparent at a distance, and sometime they're more apparent when your nose is practically on them. In this photo, there are columns a couple of centimeters across in the lower part, leading to a small shelf with a knobbly surface representing the tops of said columns, followed by an interval of more obscure growth.

The prolific interval was up to about three quarters of a meter thick, with some significant variation, microbial mounds not being big on standardization. Within this interval it was possible to see where a mound's growth had been cut off, or changes between narrow columns and broader stacks.

Here we have a broad mound of numerous coalesced small centers that is cut off starkly about two-thirds of the way up the photo.

Another example where there appears to be discontinuity between the growth lower in the photo and that in the upper part.

This tall mound is fairly broad at the base, then goes into narrower columns, then appears to show column consolidation near the top.

Part of why the photos may have lacked some clarity is the weathering of the surfaces (and the bright light). The stromatolites had a sort of artistic appearance in places, a bit like fingerprints in rock. I could see paintings of these done with heavy strokes to emphasize the tactile appearance of weathering layers.

There's a certain melted quality to this exposure.

I'm not sure what happened here, but this one looks like it's breaking up (and there seems to be a big rounded pebble in the upper left).

Another interesting feature was the occasional exposure of the top surface of a stromatolitic interval. Given the preponderance of smallish columns, it should not come as a surprise that such surfaces are knobbly.

A close look will reveal the more or less concentric layers of individual columns that have been truncated by weathering.

Finally, here's a surface showing columns that apparently grew out laterally. We usually think of the original microbial colonies growing vertically, to reach the light, but again microbial mounds aren't big on standardization, and will grow as conditions influence them. (Or maybe this mound was simply knocked over at some point; unfortunately, we're missing most of it.)

Preservation is different in this one as well, with layers still being evident but expressed less colorfully.

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...)

Your Friends The Titanosaurs: Gandititan cavocaudatus?

Here we are, already through January of 2024, and clearly we are not starting out like 2023, where the entire month had nary a new dinosaur. Today's entrant was the third of five announced in January 2024 (a couple of weeks ago, actually). You may notice the question mark in the post title; although described as a basal titanosaurian, Gandititan cavocaudatus fits a certain "type" of sauropod (mid-Cretaceous East Asian titanosauriform) that delights in phylogenetic instability. This to me is a subtle signal that something is going on we don't understand yet, so I'd best hedge my bets. So, maybe it's a titanosaur, maybe it isn't, but at any rate it's worth an introduction.