The way things happen now is the same way things happened in the past. Earth processes have not changed over time. Mountains grow and mountains slowly wear away, just as they did billions of years ago. As the environment changes, living creatures adapt. They change over time. Some organisms may not be able to adapt. They become extinct, meaning that they die out completely.

Historical geologists study the Earth’s past. They use clues from rocks and fossils to figure out the order of events. They think about how long it took for those events to happen.


SCI-MS.ESS1.04 Construct a scientific explanation based on evidence from rock strata for how the geologic time scale is used to organize Earth’s 4.6-billion-year-old history.


Lesson Objectives

  • Explain how stratigraphy can be used to determine the relative ages of rocks.
  • State how unconformities occur.
  • Identify ways to match rock layers in different areas.
  • Describe how Earth’s history can be represented by the geologic time scale.