Sediment layer chronology?

Question

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.

Answer 1

You are correct that the sediment layers became exposed top down as the river "carved" the canyons. One thing to keep in mind is that this carving was also aided by the uplift of the Colorado Plateau region which created a steeper gradient to base level (the lowest level to which a river flows, often sea level). The steeper gradient aided in the down-cutting by the Colorado River creating the majestic exposed walls of the Grand Canyon.

With regards to the fossilized life embedded in these layers or strata, each sedimentary layer represents a different depositional environment in which sediment was laid down from the bottom of the canyon upwards. Sandy beaches, shallow marine environments and fluvial (river) deposits all trap plant and animal matter/evidence that has the potential to become fossilized over time. Thus the living matter was incorporated at the time of deposition of sand, mud, and calcium carbonate providing evidence not only of the life forms associated with each time period of rock but also the environment at the time the organism was living. Now, as the river down-cuts the banks these fossilized remains become exposed allowing geologists a glimpse into the past environment and biota (life).