The Grand Canyon's walls have millions of years of geological history in them. The "stripes" are actually rock layers that happen over time, stacking on top of each other, younger over older. For example, if you take a picture a day and just lay it down and slowly stack it up, the older ones will be toward the bottom of the stack, not toward the top. This is called superposition.
     The Grand Canyon also has places in the walls where layers of rock seemed to have grown upward instead of to the side. This shows a cross-cutting relationship. The rock cutting through the other layers has to be younger than the other layers because you can't cut something that's not there. It's like trying to cut a pizza that's not there, although some people could argue that imagination is key.

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