[VadeRetro]

How can each layer be so distinct if it blends zillions of years?

Each layer doesn't blend zillions of years; that's the point. It's one particular period of sediment deposition which occurred in one set of conditions at one spot. Thus, the Grand Canyon has marine limestones and desert sandstones--at different times. In between the periods of deposition can be--and typically are--long periods of no deposition, even erosion.

[metacognative]

So we have sharp changes of climate and ecology. Hmm, we see that happening all the time in the real world. No wait! That's catastrophy theory! Let's make this up as we go, OK?