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In the new Cosmos series, Neil DeGrass Tyson in explaining different ways the age of the world has been determined, shows how sedimentary layers at the Grand Canyon are a fossil record of a lot of the Earth's history. He points out how each layer is (obviously) laid on top of the last, showing the chronology of various stages in the biosphere.

I remember learning how the Colorado river "carved" the canyons, on a trip there as a kid. You can visualize this -- the huge open canyons being eroded over millions of years, the riverbed turning grooves into valleys until enormous mesas are separated by the river. But, if the river "carved" the canyons, doesn't that imply the sediment layers were exposed from the top layers down, chronologically? How was all this fossilized life embedded in rock that at one point was a mile under the surface, that was later exposed by the river's erosion?

I'm not doubting the science at all, just trying to understand how the canyons formed.

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    $\begingroup$ Could you be a bit more specific to what you don't understand? Erosion is a 3-D process, it can create flow pipes that eventually become canyons. $\endgroup$ – Neo May 16 '14 at 0:58
  • $\begingroup$ +1 @Neo: Guessing the issues is that the OP didn't understand the river has only been around 17 million years, but nearly two billion years of Earth's geological history have been exposed by the Colorado River. $\endgroup$ – blunders May 16 '14 at 2:17
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In simple big picture terms, Sediments were deposited first from the bottom upwards (as you understand). Then the area was uplifted and the river eroded from the top down.

Most erosion is from the top down (there are scenarios such as dissolution and caves which are relatively rare exceptions). So here in Texas we find a large amount of the center of the state was covered with Cretaceous sediments which have since eroded away. We see signs of this from "outliers" - basically remnant caps of Cretaceous on the tops of hills.

As I say this is big picture viewpoint. The very bottom of the Grand Canyon has metamorphic rocks. And sedimentation will not have been continuous. There are gaps from erosion, uplift, etc. But yes the bottom is about 2 billion years old, and erosion of the canyon itself is "relatively" recent.

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    $\begingroup$ Pedantic comment: erosion is not always from the top down. Sea and subterranean caves are counter examples. $\endgroup$ – Paul A. Clayton May 17 '14 at 14:15
  • $\begingroup$ Ah that is simple, and I feel stupid! Thanks for the answer. $\endgroup$ – Jason Boyd May 18 '14 at 2:48
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    $\begingroup$ Paul, true; and there are a lot of other issues like there will have been multiple episodes of uplift and erosion: It wasn't a simple deposition, uplift, erosion cycle. It was a first level "big picture question" so I gave a first level "big picture answer". $\endgroup$ – winwaed May 18 '14 at 16:16

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