One outcrop shows layers from one geologic time period, while the other outcrop represents a different time. Can he put the pieces together to make the story more complete? Let's find out how scientists deal with this common problem by using the fossils inside the rocks.
Back in 1793, there lived a land surveyor named William Smith.
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.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.You may already know how to date a fossil with a rock.But did you know that we can also date a rock with a fossil?
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.We talked about relative dating of rocks and how scientists use stratigraphic succession to compare the ages of different rock layers.You should already understand that the lower rock strata are generally older than the strata found higher up in the rock.When a scientist finds a section of rock that has lots of different strata, he assumes that the bottom-most layer is the oldest, and the top-most layer is the youngest.But sometimes, a scientist finds a couple of rock outcrops that are separated by a wide distance.