The history of the Earth can be displayed in the form of a calendar which is based on the observation of rocks formed through time. Depending on the location, the formation of rocks has recorded different (regional) histories describing the magmatic, tectonic, hydrospheric, or biospheric evolutions, and thus different calendars have been generated. Geologists (stratigraphers) are attempting to unify these different calendars. Stratigraphers define rock units bracketed between two boundaries that can be correlated worldwide. The succession of these modern units is the global geological time scale.