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Let's say in the first outcrop, he found an upper rock layer containing ammonite fossils and a lower layer containing scallops.
In the second outcrop, miles and miles away, he also found two layers; but these layers were different.
Well, let's go back to our surveyor, William Smith.
He was often presented with the problem of finding two different rock outcrops from two different periods.
We'd want to use a more short-lived fossil, like the dodo bird.
We also want our index fossils to be common, widely-distributed species that are easy for scientists to identify.
Index fossils help us to distinguish between rock strata from different time periods, so it's important that they don't cover too much historical ground.
We wouldn't want to use a horseshoe crab fossil, because horseshoe crabs have existed for over 400 million years and are still alive today!
He began to identify rock layers by the fossils they contained, and he even noticed that the general order of strata was identical over many different parts of the country.
Smith was the first person to understand the principle of fossil succession.
Now, when we use fossils to date rocks, we have to be careful. Remember that some species of animals and plants lived for a very long time, while others existed only for a short period of time.Tags: Adult Dating, affair dating, sex dating