![]() Two laws, or principles of geology explain why rock layers are formed in this way. We’re not talking about an abstract diagram: this is the actual record of the earth’s crust, recorded in rocks around the world.īut how do we know this evolutionary sequence of layers, one on top of the other, is accurate? Why is there any order at all to rock layers? As you move towards the surface, you find fish, then amphibians, then reptiles, mammals, birds, and finally modern mammals including humans. At the bottom of the timeline there are no fossils of modern animals. It’s as if rock layers are a vertical timeline. Basically, scientists have learned that rocks are stacked in layers containing fossils with the oldest fossils at the deepest layers, and the youngest, or most recent fossils, near the top. One evidence is rock layers specifically, what is called the geologic column. While everybody understands that black bears are related to grizzly bears and we can even figure they are related to extinct bears, lots of people wonder how scientists can be so sure that bears are related to salmon as well.
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