After I'd come across the new exposures at Uŋčí Makhá Park last fall, I was very curious about how a Minnesota winter and spring would treat them. After all, these were fresh, with no previous direct exposure to snow, ice, and freeze-thaw cycles. Would they rapidly degrade, or were they made of sterner material? Last week I had the opportunity to spend some quality time at the park, in preparation for and leading a training session for Mississippi National River & Recreation Area seasonals (and if any of the participants happen on this post, hello! I hope you had a good time!).
What were the results of this natural experiment? A few observations:
The Carimona Member of the Decorah (blue-gray upper interval), particularly
the blocks used as landscaping, suffered appreciably more than the Magnolia
Member of the Platteville (tan lower interval). I attribute this to the
greater shale content of the Carimona.
More Carimona landscaping showing exfoliation.
This indicates that the Carimona blocks will weather faster than the Magnolia blocks; eventually, both lithologies will reach equilibrium with their new surroundings, but the "fucoidal" surfaces on the landscaping are going to go away faster than the shell beds.
|Note the burrows popping off the surface in some places.|
It wasn't all smooth sailing for the Magnolia, though. Although many blocks and beds seemed fine, others had definite signs of damage.
|Here a thin bed is breaking up.|
|This isolated block appears to be shattered. (Colors are weird because when I took this photo, I'd forgotten to reset the lighting from tungsten bulbs.)|
Unlike last fall, which was a time of drought, this spring we can also definitely see where the seeps are.
|And they're concentrated at the bentonite layers in the Carimona.|
Many fossils and features came through without particular damage, though. I included a photo of a bivalve in the fossil guide post. Here it is last week:
|Dare I say that it's "happy as a clam"? (Ignore the color balance differences.)|
With that out of the way, did we find other fossils I hadn't seen in the fall? Well, of course! Tune in next week for some less-typical fossils!