Rockfall

One of the characteristic aspects of the Mifflin Member of the Platteville Formation is its habit of planar jointing. The faces of outcrops often look like someone took a rock saw to them. Nor are they necessarily single flat planes; sometimes joints intersect to form sharp angles. The heavy thunderstorms the previous week inspired a large chunk of Mifflin outcrop to collapse along intersecting joints.

Tumbled down

The joint planes did not form overnight, which can be seen by the amount of roots and soil in the new outcrop faces. There were some pretty big roots in there as well, but whatever tree(s) had once produced them is long gone.

A view into the wedge more or less along one of the two joints.

This particular rockfall was about as polite as possible, occurring not at the top of a stereotypical 30-foot bluff but from a much lower bluff, adjacent to a bike path. The orientation of the wedged stack shows that it toppled out of its former position. The top of the stack is therefore farthest from the bluff. Perhaps it failed at the base first, due to poor support from the Pecatonica, then flopped over.

History going from left to right

When to stay away from outcrops and exposures

Some recent events have provided photographic fodder for a brief unscheduled revisit of safety concerns in the rocks of the Twin Cities, but first I am going to plug the reborn Park Paleontology newsletter. I'll hit it some more this weekend in more detail, but I particularly want to call out Chapter 3 by Emily Thorpe, who was a Geoscientists in the Parks intern over the winter at Salinas Pueblo Missions National Monument. Among other discoveries, she's got the first vertebrate from the Yeso Group (Arroyo de Alamillo Formation), the part and counterpart articulated back half of a skeleton. We've had a lot of great projects from GIPs the past few years, and I'd strongly encourage college students in the geosciences to have a look when the next batch of positions comes out.

Meanwhile, back in the Ordovician...

You may remember the following photo from this post, or this post. Coincidentally, in both posts the photo is being used as an example of a hazard. It was taken back in June 2013 along the road into Crosby Farm Regional Park, before you get to Watergate Marina.

I wonder where this is going...

If you take that road today (May 2017), this is what you'll see:

...the answer appears to be "down".

This is why you should not stand too close to the walls of the bluffs, or to the edge on top.

Meanwhile, over at Shadow Falls, the Decorah Shale presents a different issue, one that can be expressed as a recipe titled "Reconstituted Ordovician Seafloor": take a hillslope of weathered marine shale, and over the course of six days add nearly five inches of rain. It's amazing!

Hiking boots? More like cleats.

The running water is Nature's subtle way of telling you to stay off.

It's not the mud itself that's the big problem (unless you hate mud, in which case you're really in the wrong place), but the slippery sloppy footing. If you go over into the ravine, it's not going to be easy to get you out!

Practical considerations and hazards

If you do choose to explore Twin Cities geology beyond the most convenient couple of places, it is worth knowing something about the terrain. Most practicalities are the same as you would use for any hiking trip: sturdy boots, sunscreen, insect repellent in the buggy times of year, a hat, and so forth. I recommend long pants and sleeves, to protect your arms and legs from banging against things and to give you less to worry about in terms of bloodsucking multi-legged friends, but I realize that this can get extremely uncomfortable in the summer, when the atmosphere transitions from hot and humid to hot and gelatinous. At any rate, choose clothes that you wouldn't mind getting ripped, roughed-up, or coated in mud. Have water on hand, too.