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Crosscutting and the Principle of Superposition

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Geologist in ACTION
Sewer Crosscut
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Location: U. S. Geological Survey, Menlo Park
About: : The stripe of light-colored pavement in this picture hasn't always been there. Originally, the pavement was all one color, but then there was a problem with the sewer line. To replace an old sewer line, workers dug a trench that cut across the old pavement, and then filled it back in with a lighter colored asphalt. The lighter stripe is younger than the darker material around it. When in the sequence do you think they painted the words "STOP" on the ground? How can you tell?
Black Layer cuts across Grand Canyon Layers
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Copyright Ramón Arrowsmith, Arizona State University

Location: Grand Canyon National Park, Arizona
About: Blocks of rock that look like stripes cutting across existing layers are also common in nature. Here, the reddish-brown layers accumulated over time. After they were laid down, hot magma pushed its way through the layers towards the surface. The dark "stripe" is where some of that magma solidified before reaching the surface. The fact that the dark layer seems to cut so cleanly through the layers is evidence that it came along after they were deposited. (It is not possible to cut layers before they exist!)

Key Concepts:
  • Newer rocks are deposited on top of older rocks. The newer rocks cover up the older rocks.
  • For sedimentary or volcanic rocks, the oldest layers are therefore on the bottom and the youngest layers are on the top. We call this "The Principle of Superposition" (super = top).
  • When one type of rock cuts through other rocks, it had to form after the rocks that it cuts. (You can't cut a cake until you have baked a cake).
Classroom Activities: GeoSleuth Murder Mystery

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