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Locally, physical characteristics of rocks can be compared and correlated.On a larger scale, even between continents, fossil evidence can help in correlating rock layers.

The first card in the sequence has "Card 1, Set A" in the lower left-hand corner and represents the bottom of the sequence.

If certain fossils are typically found only in a particular rock unit and are found in many places worldwide, they may be useful as index or guide fossils in determining the age of undated strata.

By using this information from rock formations in various parts of the world and correlating the studies, scientists have been able to establish the geologic time scale.

Scientists also use direct evidence from observations of the rock layers themselves to help determine the relative age of rock layers.

Specific rock formations are indicative of a particular type of environment existing when the rock was being formed.

This relative time scale divides the vast amount of earth history into various sections based on geological events (sea encroachments, mountain-building, and depositional events), and notable biological events (appearance, relative abundance, or extinction of certain life forms).