To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events.

What layer of rock is the oldest?

The law of superposition states that rock strata (layers) farthest from the ground surface are the oldest (formed first) and rock strata (layers) closest to the ground surface are the youngest (formed most recently).