Final answer:
Geological time periods are divided based on fossil evidence and rock formations. Fossils provide clues about past organisms and rock formations help determine the age and environment.
Step-by-step explanation:
Geological time periods are divided into segments based on two pieces of information: fossil evidence and rock formations. Fossil evidence refers to the remains or traces of organisms that lived in the past, such as bones, shells, or imprints. These fossils provide important clues about the types of organisms that existed during specific time periods. Rock formations, on the other hand, can be used to determine the age and environment in which the rocks were formed. By studying the layers of rocks and the types of rocks present, geologists can establish a relative timeline of Earth's history.