Saturday, November 28, 2015

Thanksgiving Leftovers

You're probably pretty busy this weekend. How about something light, like some photos? These all come from a few site visits over October and November, taking advantage of the very pleasant autumn weather conditions in the Twin Cities metro.

This and the next photo come from the U.S. Route 10 roadcuts, in the Shakopee Formation (Prairie du Chien Group). All of the little stone rainbows are small domed stromatolites. At very close range, you can distinguish between layers that are "crystalline", so to speak, representing minerals deposited by the microbes, and layers of sand (a grain or two thick). There is a band populated by these small stromatolites about as thick as the area photographed here that extends for at least a few tens of meters. (I do not recommend casual visits along this busy road; the couple of times I've stopped have been Sunday mornings.)

A cross-section through a high-spired snail, U.S. Route 10 roadcuts. Body fossils are pretty rare in the Prairie du Chien; the only forms I saw on this visit were a few snails, mostly high-spired like this one.

This natural cast of a nautiloid was found by an eagle-eyed Junior Paleontologist on a fossil walk at Coldwater Spring. It comes from the Platteville Formation.

I'm getting better at finding horn corals. Here's the pointed end of one in the upper Platteville near Coldwater.

A shell bed in the upper Platteville in the area across from and south of the University of St. Thomas. This platform on the bluffs is dotted with brachiopods.

In the bluffs east of the river a few blocks north of Fort Bridge, I have observed a fair number of chips of bifoliate bryozoans in the lowermost shaly part of the Decorah Shale (see below the penny for an example). To the north, in the area near the site of the shell bed photo above, I have observed ramose forms at about the same interval. Different local environments? (If, of course, the responsible beds are of the same age.)

The week before Thanksgiving, it rained quite a lot from Monday to Wednesday, and then the temperature went below freezing, so all of that water seeping through the ground did the natural thing. Here are beds of the Mifflin Member festooned in ice on the north side of the Shadow Falls ravine.

This is too darn cool. This is a conulariid in the upper Platteville (probably Magnolia Member), in situ, in the Shadow Falls area. I can't tell you how excited I was to find this!

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