Learning About Rocks

Bob reminded me the other day, via a piece he is writing, how my preparations for the field camp in Colorado this summer got me learning about rocks. For the first time, I was going to be leading the physical geography section of the camp, and I knew next-to-nothing about the geology of the region (this still is the case, though I can recite a rough stratigraphic sequence). So I got some books, and set to learning.

Way down deep is the Precambrian. At the field site in Colorado, which is a little mountain valley the size of the island of Manhattan, these rocks are at the head of the valley, exhumed from beneath the earth when the modern Rockies were made. They are old, 1.7 billion years, and they show their age in their warped and wrinkled expression, like an old man who just wants to go to back to sleep. A Great Unconformity (always capitalized, a major event of some billion years in which anything and everything that was deposited got eroded away, at least in the story told by geologists) separates those rocks from the next ones in the sequence, the Fremont and Harding and Manitou dolomites and limestones. At about 400 million years old, they've just entered retirement, perched majestically high above the valley, resting formidably on the back of the Precambrian curmudgeon.

But the reason for my post is actually in the next sequence, the Williams Canyon limestone of the Mississippian and Pennsylvanian Age - 320 million years old. In this part of Colorado, these limestones are not well expressed, mostly because the Ancestral Rockies punched through them a short 20 million years later and forced them off into rivers and seas and oceans. They have been tilted, crunched, eroded, eaten away by water into caves -- just not much left. Almost imperceptible, and just a footnote in the sequence as you go up the geologic column (which mercifully for the reader, I won't do here).

Suffice it to say, I spent those three weeks in constant geologic-learning mode, anxious that I just didn't know enough about the rocks to actually be teaching these students something about them. And so, going to sleep each night, I would recite the sequence of strata over in my head: Precambrian to Fremont to Williams Canyon to Fountain to Morrison to Dakota and on and on and on until you get to the present.

Fieldcamp's over. I get back to Columbia, happy to be home. My house is so big (compared to the cabin), my mattress so restful, my routines so comfortable, my family so welcoming and loving. I stopped learning about rocks. The feeling was something like this: I'm home now; I don't need to learn about this place.

Three or four days back, driving around town, it strikes me: I have absolutely no idea what formation these rock outcrops are here, whereas in Colorado, I would have been compulsively looking them up in a guidebook. The outcrops here in Columbia, I realized, mean something different to me, something of a lesser order: they are sidenotes on the trip to the grocery store, something that had to be blown away to put Stadium Drive in, something to pass by without notice. They are flat in orientation, drab, uninteresting - no mountain came through. They haven't been othered, made exotic.

They are Mississippian. Different formation than the Williams Canyon, but Mississippian nonetheless: mostly Keokuk and Burlington limestones. (The latter is the formation in which the Rock Bridge caves occur). As you head north in town, where I live, it transitions into Pennsylvanian - coal, shales, some clays - these formations are less resistant and so do not form the nice outcrops you see on the road cuts in the middle and southern parts of town. They tell us that we were near the equator then. These stones are depositional relics of tropical seas, the limestones made from the calcium rich shells of marine life, the coal from the decomposed organic matter of dead things living in swamps near the shore.

They are the rocks here, at home.