Relative dating rock layers worksheet

Watch this video to find out how we use index fossils to establish the relative ages of rocks.In previous lessons, we talked about the Geologic Time Scale and how scientists use it to piece together the history of the earth.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!Well, let's go back to our surveyor, William Smith.

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So, how exactly is an index fossil used for relative dating of rocks?The upper layer had scallop fossils, and the lower layer had trilobites.Smith would have brought these two arrangements together, overlapping the common scallop layer, to produce a larger succession of three rock strata!An index fossil is a fossil representing a plant or animal that existed for a relatively short duration of time.These are the fossils that we want to use for relative dating.Fossil succession is based on the observation that certain assemblages, or groups, of animals and plants have lived during certain time periods over geologic history.

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