The Geologic Time Scale
The geologic time scale is a record of events and life forms on earth. Scientists created the geologic time scale by finding index fossils (Widely spread organism fossils that were alive during a short period of time) and rock layers around the world, and studying them. Radioactive dating helped find exact age of rocks. They decide when an era ends by looking at major changes in life forms. The geologic time scale is a useful tool for scientists.
This is an example of a geologic time scale model