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.