The process of showing that rocks or geologic events occurring at different locations are of the same age is called correlation. Geologists have developed a system for correlating rocks by looking for similarities in composition and rock layer sequences at different locations.

What to look for when correlating rocks?

To correlate rock units, something distinctive must be present in each. This can include an index fossil, a unique rock type, a key bed, or a unique sequence of rocks. A key bed can be global. An example is the iridium layer that was deposited at the time of the Cretaceous-Tertiary extinctions.

Why is correlating rock layers important?

Correlation trying to fit together sedimentary strata found in different places. Geologists try to determine the relative age of widely separated strata. They are especially interested in determining layers in separate strata that are exactly the same age. ... This sequence allows for confident correlation.