Geologic Time and the History of Cartography

The geologic time scale is the standardized framework geologists use to organize Earth's 4.6-billion-year history into a hierarchy of nested intervals: eons, the broadest divisions, are subdivided into eras, then periods, then epochs, and finally ages. This framework rests on the science of stratigraphy, which studies layered rock and is guided by a few foundational principles: the principle of superposition, which holds that in an undisturbed sequence of sedimentary layers, the oldest layer lies at the bottom and successively younger layers lie above it; the principle of original horizontality, which holds that sediments are deposited in roughly horizontal layers, so tilted or folded strata indicate later deformation; and the principle of faunal succession, which recognizes that fossil assemblages change in a consistent, recognizable order through the rock record, allowing geologists to correlate rock layers of the same age across different regions even when they differ in composition. Formal boundaries between named intervals are today defined by internationally agreed reference points in specific rock outcrops, known as Global Boundary Stratotype Sections and Points, or "golden spikes", which anchor the time scale to physical, verifiable rock.