Did you know that—Minnesota, and much of N. America, was undersea? I did not. But strange as this seems, it was soon made plausible by our teacher last week, Gary Wagenbach, shown here addressing his class during our field trip to Rice Co. Wilderness Park, near Faribault.
Guided by some of this course’s materials, my spouse and I drove over to a spot near Goodhue, MN, where one easily finds a ledge from the Ordovician period, about 500 million years old, the so-called Decorah formation. Here is a photo from that greenish-grey layer, showing a snail shell, captured and on display for all to see.
that— At that time, Minnesota lay near the equator? I did not. But again, references in our public library came to the rescue. In a fine video by Prof. Ed Buchwald of Carleton, and a book by Ojakangas and Matsch, the curious history of our earth is revealed as an ongoing process. Our land masses began as a supercontinent, Pangea, about 225 million years ago, when the N. and S. America were still connected to Eurasia and Africa. The westward march of our continent began, and continues today, at the present rate of about 1cm/year. This motion accounted for a warm climate and shallow seas over our continent at one time. A fascinating example of how this could have affected our neighborhood can be seen in the Barn Bluff at Red Wing. Shown here is the well-known Red Wing Fault, a clear separation of rock types, with the buff-colored St. Lawrence siltstone on the left, and the greenish Franconia sandstone on the right; these two layers shifted about 150 feet, perhaps from seismic activity. And we learned at the Goodhue Co. Museum, above downtown Red Wing, that this greenish color (hold on to your hats!) —–is thought to be caused by a mineral called glauconite, which is thought to have been formed and excreted by ancient sea worms!
and that— a key reason that we see almost no lakes east of Interstate 35 in SE Minnesota was that this corner of the state was a “driftless zone”, never exposed to glaciers, and hence very different from the area near Faribault, populated by many glacier-carved lakes. And as you’ve guessed, I didn’t know that either. So this is my little confession about how little I know about the geology of where I grew up and went to college. The good news is that it’s not too late for me, and perhaps others, to learn at long last what lies underfoot; and in learning this, the possibility also exists that the wider world will also make better sense. For this, my heartfelt thanks go out to Gary -Ed