How do rock layers and fossils reveal Earth’s geologic history?

Rock layers and fossils serve as the primary record of Earth’s geologic history, acting as a "calendar" that allows scientists to reconstruct the evolution of the planet’s continents, oceans, and life-forms.12

The Role of Rock Layers

The law of superposition is a fundamental principle of stratigraphy used to interpret this history. It states that in an undisturbed sequence of sedimentary rock, the oldest layers are at the base, with progressively younger layers deposited above them. By studying these strata, geologists can trace environmental changes over time.32

The Role of Fossils

Fossils provide the essential means for correlating these rock layers across different regions. Through the principle of faunal succession, geologists recognize that specific intervals of geologic time contain unique assemblages of animal and plant life.14
* Index Fossils: These are particularly useful because they are distinctive, abundant, and existed for a short, well-defined period, allowing scientists to date the strata in which they are found.5
* Environmental Indicators: Fossils also reveal past climates and environments; for example, finding fossils of organisms that require warm, shallow water in polar regions suggests that the Earth's crust has shifted over time.6

By integrating these observations, geologists build a continuous record of Earth's history, which is further refined and quantified using radiometric dating.41