Apr
16
2010

Geology's at work in Minnesota, too

Debris from recent collapse of river bluff in St. Paul park: A massive shelf of Platteville limestone still hangs ready to come down.
Debris from recent collapse of river bluff in St. Paul park: A massive shelf of Platteville limestone still hangs ready to come down.Courtesy Mark Ryan
As Thor posted below, there’s been a whole lot of geological activity going on around the world lately what with all the large earthquakes and the volcano in Iceland spewing hot ash into the friendly skies. Luckily, Minnesota is situated on the very stable and essentially cold and dead North American craton. Devastating earthquakes and volcanoes are in our distant past, but that doesn't mean we don't get our share of geological action. Just last weekend I stumbled across evidence of a recent catastrophic event in one of St. Paul's parks.
I was out fossil hunting, scouring the sedimentary layers on the east banks of the Mississippi River. The strata there are the well-known record of the deposits of the Ordovician seas that once covered this part of Minnesota some 450 million years ago.

River bluff rock layers: The softer massive St. Peter sandstone and thin Glenwood shale (not indicated) are capped by harder, more erosion-resistant Platteville limestone.
River bluff rock layers: The softer massive St. Peter sandstone and thin Glenwood shale (not indicated) are capped by harder, more erosion-resistant Platteville limestone.Courtesy Mark Ryan
In the Twin Cities area, the base of the rock sequence is comprised of St. Peter sandstone, a soft and easily eroded rock that grades into a very thin layer of equally soft Glenwood shale. Immediately above those soft layers is the much more resistant Platteville limestone. The sandstone base has a habit of eroding away in the elements, often leaving a limestone overhang. Sooner or later, gravity wins out and the Platteville shelf collapses. This is what apparently happened in Hidden Falls Park in St. Paul recently. A whole lot of the limestone came crashing down, taking out some good-sized trees and blocking entry into the park. This gradual (and sometimes catastrophic) natural process has been going on for tens of thousands of years, ever since the glaciers exposed the layers. Usually, on our puny human timescale, it goes unnoticed. But it’s all part of the on-going rock cycle. Our famous Minnehaha Falls and its present location are the result of this relentless erosion process. As is St. Anthony Falls near downtown Minneapolis, which used to be located much farther down river from where it is today. In the process, the sandstone gets broken down into sand again, and the limestone blocks are eventually reduced to rubble. All of it will someday end up in the Gulf of Mexico unless some other forces intervene.

Hikers add scale to the rock slide
Hikers add scale to the rock slideCourtesy Mark Ryan
I said earlier the scope of the rock slide impressed me, but maybe "unnerved" is a better word. I often scale the river bluffs looking for fossils, and I sure wouldn't want to be underneath something like this when the forces of nature decide to have it collapse. Across the river at Lilydale Regional Park there was an unfortunate incident several years ago when an overhanging shelf of limestone (in this case the Cummingsville member) fell and killed a fossil hunter. Luckily, no one was injured in the Hidden Falls event (at least as far as I know) but as you can see in the photos, there’s still quite a lot of the Platteville still up there, jutting out, and ready to come down at anytime. I wonder if the city of St. Paul plans to help it along? Whatever the case be careful when exploring along the river bluffs.

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