Sunday, March 18, 2012

Strangers in a Strange Land: The Value of a Road Cut

As teachers, we hope to go in the field with our students in hopes that they can related cartoons in a textbook with exposures in the real world.  Then we get there, and we find together that sometimes the "field" does not readily give up secrets. There is this phenomena called "weathering" that renders rocks indistinguishable from other rocks. Take the above scene, from an area near Shoshone, a little village east of Death Valley National Park. If we were back east somewhere, there would be a complete covering of green organic material that biologists call trees and shrubs covering most of the rocks, rendering identification and interpretation practically impossible (and believe me, I appreciate and admire those who figured out the story of places like the Appalachians and the Canadian Shield).

But even here, where the climate is so dry that plants can scarcely survive, the rocks are still obscured. The geologists who have training in field methods can make sense of this sort of landscape, but students who are in the field for the first time can feel intimidated. A few gray rocks in the foreground, some tan colored rocks across the small valley, and a difficult time understanding how they relate to each other. It would be a challenge. But once in a while, the department of transportation does us a real favor. They want to make their roads straighter, and when nature hasn't cooperated by eroding into straight lines, they make roadcuts for us.

For instance, if the scene shown above doesn't look all that remarkable, look what happens when you turn around...
That's an outcrop! This is one of those sweet times when the real world looks a lot like the textbook diagrams, so the students get to do some basic interpretation of rock relationships without having to visualize obscured contact lines. Many field trips visit this outcrop, so much so that the department of transportation has left behind a parking area across the street that is big enough to accept even buses.

So, what would you do with your students here?

Oh, and by the way, all you mineralogists out there, what do you think this nice botryoidal mineral might be? I have my strong suspicions about a relatively simple answer, but I've just assumed it for a long time. The host rock is a rhyolite tuff, and hot vapors and groundwater have probably both influenced the outcrop.
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