Index fossils and relative dating

Geologic units from the same time but different parts of the world often look different and contain different fossils, so the same time-span was historically given different names in different locales.

A key aspect of the work of the International Commission on Stratigraphy is to reconcile this conflicting terminology and define universal horizons that can be used around the world.For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life.Older time spans, which predate the reliable fossil record (before the Proterozoic eon), are defined by their absolute age.Over the course of the 18th century geologists realized that: The Neptunist theories popular at this time (expounded by Abraham Werner (1749–1817) in the late 18th century) proposed that all rocks had precipitated out of a single enormous flood.

A major shift in thinking came when James Hutton presented his Theory of the Earth; or, an Investigation of the Laws Observable in the Composition, Dissolution, and Restoration of Land Upon the Globe Hutton proposed that the interior of Earth was hot, and that this heat was the engine which drove the creation of new rock: land was eroded by air and water and deposited as layers in the sea; heat then consolidated the sediment into stone, and uplifted it into new lands.

For example, the lower Jurassic Series in chronostratigraphy corresponds to the early Jurassic Epoch in geochronology.