Geologists establish the age of rocks in two ways: numerical dating and relative dating.
Numerical dating determines the actual ages of rocks through the study of radioactive decay.
Another visually powerful way of representing Earth's history is via the use of the geological `clock' (Figure This website has an `infographic' showing geological time on a clock.
Each hour on the clock corresponds to a period of the Earth's history, and shows the characteristics of each period and the key events within them.
Your goal is to study the smooth, parallel layers of rock to learn how the land built up over geologic time.