国際標準模式層断面及び地点
The [[geologic time scale]] covers the extent of the existence of Earth, from about 4600 million years ago to the present day. It is marked by [[Global Boundary Stratotype Section and Point|Global Boundary Stratotype Sections and Points]]. Geologic time units are (in order of descending specificity) eons, eras, periods, epochs, and ages; and the corresponding [[Chronostratigraphy|chronostratigraphic]] units, which measure "rock-time", are eonothems, erathems, systems, series, and stages.