Geologic Time Scale - Natural History Notebooks
The geologic time scale is a schema that relates geologic strata (rock layers) to time in Earth's history. Layers are laid down in succession; newer—younger—ones are deposited on top of older ones.
Earth scientists have been able to identify a sequential, chronological order to the layers based on relative age. They are also able to determine absolute age of the rock layers, usually through radiometric dating methods.
Spans of time/strata are classified into categories, which have been further named and subdivided. For example, the Phanerozoic Eon includes the Cenozoic Era. This era contains the Paleogene Period and Neogene Period. The Paleogene Period includes the Paleocene Era, Eocene Era and Oligocene Era. And so on.
Usually, the dated boundaries are based on major events in Earth's history. For example, the boundary between the Cretaceous Period and the Paleogene Period is marked by the great extinction event of approximately 65 million years ago, in which 50% of species alive at the time (including dinosaurs) died out. The event is represented in the geologic record: the fossils of many species that are found below the boundary do not appear above the boundary, which tells us that those species disappeared around that time. Also, in many places, the boundary is marked by a thin layer of iridium-enriched dark clay.
The adjectives Upper, Middle and Lower are used when referring to the rocks themselves, such as Upper Jurassic sandstone. The adjectives Late, Middle and Early are applied to time, as in Late Cretaceous reptiles.
Geologic time scale (International Chronostratigraphic Chart) by the International Commission on Stratigraphy.