LAYERS OF IMAGINATION – A Geologic Mystery Unfolds

I’ve walked this route, just a few blocks from my home here in Minneapolis, countless times. I always find something new, beautiful, amazing. Today, it was this elegant stacking of rock layers, encountered as I scaled the path down some
70 or 80 feet to the base of the Mississippi River gorge.

Here in the upper reaches of the great river, the water has, over the last 12-14,000 years, incised the earth’s crust, laying bare strata of ancient sedimentary stone. How did these layers form? What was happening around here back then?

I’ve always believed such geologic records, like a tree’s rings or layers of glacier ice, bear chronological evidence of changing seasons and climate trends, floods, fires and volcanic events. So is that the case here with what I think I’ve identified as Platteville limestone?

Could that possibly explain the wide range of thickness in these layers—from five or six inches, to a phyllo-like eighth-inch, and back again? Was there really that much difference between the amount of sediment or water flow from one year (or whatever the cycle was) to the next?

          During this period the land that is 
          now Minnesota lay along the equator.

ROCKY NOTIONS OF TIME AND SPACE
In reading up on the subject, I’ve learned that, instead of looking at this in terms of yearly cycles, I should be thinking much, much longer-term. This Platteville limestone formed between 488 and 436 million years ago—a period of some fifty million years.

Hard as it is to comprehend, the land that is now Minnesota then lay along the equator. Tropical seas ebbed and flowed over the area in cycles spanning not years, decades or even centuries, but millions of years, depositing silt and the shells of marine organisms which, with time, pressure and chemical reactions, eventually turned to stone.*

So what is the answer to the wide variance in these layers’ thickness? It turns out that this is actual not all the same rock. According to geologists, the thick, topmost layers are indeed Platteville limestone, a chemically-affected, calcium-carbonate, sedimentary rock comprising mostly-pulverized marine fossil remains.

The much thinner middle-layer strata, seen about three-fourths of the way down my photo, are Glenwood shale, another sedimentary rock made up of much finer particles of mostly mineral-based mud.

Finally, the thickest layers—seen at the bottom of my photo—are St. Peter sandstone, a mix of mostly granular quartz and feldspar with a binder of silica- or calcite-based cement. **

ILLUSTRATION: United States Geological Survey

A BRIEF STROLL…IN PRIMORDIAL TIME
Some folks might ask me why I’d be interested in how rocks formed half a billion years ago. If you believe in your soul, as I do, that everything—animals, plants, microorganisms and, indeed, the earth—are connected, you might understand.

If you’re susceptible to awe—like the kind you might experience observing details of the moon’s surface through a powerful telescope, or getting close enough to touch the plate some passenger was eating from when the Titanic struck that iceberg—then you most certainly get it.

I thought I was going down to the river for a walk. I ended up witnessing—no, actually touching—something that happened hundreds of millions of years before human beings’ earliest ancestors existed. The awesomeness of that is still sinking in. To me, that was a good day.

* Paul Nelson, “Minnesota and Wisconsin’s Own Platteville Limestone Totally Rocks” - MinnPost, 8/30/16 
** geocaching.com