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Geology Marches Forward

Reading sand beds for clues about the past. Credit: Ale Borunda
Reading sand beds for clues about the past. Credit: Ale Borunda

For most of our drive, I stare out the window and ask Mike questions: “Is that a glacial moraine?” and “How tall were the Andes originally?” and “Why are those sediments white?” He can respond to a stunning number of these questions. I love being around field geologists; the way they make sense out of visual clues that most of us overlook is both mysterious and fascinating.

We drive through a wide, deep canyon, with clearly exposed sedimentary sequences. “What are those dark layers?” I ask, and Mike explains that we’re driving through a Cretaceous sedimentary sequence. Some layers may be siltstone or shale-like deposits laid down when this area was submerged under a shallow sea.

Suddenly, Jay, who is driving ahead of us, stops his truck by the side of the canyon. “I just had to check this out,” he says, pulling his rock hammer out of the truck bed. “Look at this place!” Those dark layers are paleosols, or old layers of organic-rich soils preserved under other sediment, he tells us. And there’s siltstone, he points out. Barbara, his wife, tells us that they’ve found tons of fossils in similar siltstone beds on former trips.
A little further on we see thick, wind-deposited sand beds marked by a layer of volcanic tuff. The sand beds look like the beds forming at the surface today. I don’t know how strong the winds were when those beds were formed or how much sand and dust was in the air but the process that created them is the same that makes sand dunes today. There may have been dinosaurs stomping around and wildly different plants waving in the stiff breezes. But the winds picked up dust particles in the same way they do today. Biology tries out new body plans, reproductive systems and behaviors, and through it all, geology marches forward.

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Exposed Cretaceous beds near Lago Argentino. Credit: Ale Borunda.
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