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The geologic time scale is a scale that represents Earth’s long history and important event. It includes life forms, geologic events, and landforms. The whole geologic time scale is split up into eras, and within those eras are periods. Scientists made the geologic time scale using rock layers and index fossils. They placed the rocks in order of relative age. Relative age is the general age of something, or about how old something is. Also, radioactive dating helped determine the absolute, or exact, age of rocks. Scientists got the names of eras from ancient rocks that they studied. Major changes in life forms, and geologic events are among the things that would make scientists decide to end one era and start another.