Final answer:
Geologic time is divided into units based on evidence from fossils & key beds. Scientists use clues within rocks, such as rock types and fossils, to determine the age of the rocks in a comparative sense.
Step-by-step explanation:
The units in geologic time are divided based on evidence from fossils & key beds. Scientists have pieced together a progression of rocks through time to construct the geologic time scale. They use clues within the rocks, such as rock types and fossils, to determine the age of the rocks in a comparative sense. This process is called relative dating.