Wednesday, April 17, 2013

Accretionary Wedge #56: Because Every Picture of the Earth Tells a Great Story

Andrew Alden at About Geology is hosting this month's Accretionary Wedge, and the topic is landscape topography through the eyes of a geologist:  

Once upon a time, you took a picture of something that lots of people photograph. However, because you are a geologist, it didn’t turn out the way it does for most people. Show us that picture, tell us what you see in it, and tell us about the way you take pictures.

No matter where you go, no matter where you are, there is a story to be told by the rocks. And ultimately, no matter how dull the scene might appear, the story is fascinating. These are the first words that I utter in every class I teach, and I cannot look at a landscape, any landscape, without wondering what lies beneath. And sometimes the scenes are far from dull.

To many, the story to be told by a picture may be "I was standing in front of some scenery". Few geologists can ever see a landscape that way. The picture above is one of my favorites, because such a rich history is encapsulated in the small frame of the photograph.

We are standing in a small arch located in Arches National Park in Utah, which actually happens to be called Frame Arch. The view extends across the fault graben of Salt Creek Valley to the distant La Sal Mountains. The reddish rock is the Entrada Formation, a sandstone deposit that developed in the tidal zone of a coastline in Jurassic time. Dinosaurs once climbed coastal dunes in this spot where I stood snapping a picture.

The rocks are tilted because there is a huge body of salt beneath the surface. Water trickling down into the fissures and joints dissolved away the salt near the surface, and the Entrada layers collapsed into the void.

The La Sal Mountains in the distance are volcanic, or to be more correct, laccolithic. The high peaks are made of the more resistant rock that squeezed between and inflated the space between sedimentary layers. Later on, the rocks were lifted up and the overlying layers were eroded away. The eroded mountains are the hearts of ancient volcanoes.

How did this photo "turn out different"? It's because I was more or less ignoring the reason this opening in the rock was called Frame Arch...just to the left in the distance, you can see the slightest opening to what may be the most famous arch in the world, Delicate Arch. It's the symbol of the park, and almost everyone climbs up to Frame Arch uses it to frame Delicate (below). But the geologist's eye takes in more than icons.
Thanks, Andrew, for hosting the Accretionary Wedge this month!
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