An ordered, internally consistent, internationally recognized sequence of time intervals, each distinct in its own history and record of life on Earth, including the assignment of absolute time in years to each geologic period. The geologic time scale (see table) has a relative scale, consisting of named intervals of geologic history arranged in historical sequence; and a numerical (or absolute) time scale, providing absolute ages for the boundaries of these intervals. In order to establish a geologic time scale, an independent means of dating rocks is required. Before the discovery of radioactivity, crude estimates of the length of a geologic history were made based on the total thicknesses of sedimentary rock and assumed rates of erosion and sedimentation. These estimates varied by as much as a factor of 10. Geologic time scale Eon | Age at beginning of interval, 106 years | Interval length, 106 years | Era | | | Period [system] | | | Epoch [series] | | | Phanerozoic | Cenozoic | | 65 | Quaternary (Q)* | | 1.8 | Recent | 0.01 | 0.01 | Pleistocene | 1.8 | 1.79 | Tertiary (T) | 65 | 63.2 | Pliocene (Tpl) | 5.3 | 3.5 | Miocene | 23.8 | 18.5 | Oligocene (To) | 33.7 | 9.9 | Eocene (Te) | 54.8 | 21.1 | Paleocene (Tp) | 65 | 10.2 | Mesozoic | 250 | 185 | Cretaceous (K) | 144 | 79 | Jurassic (J) | 206 | 62 | Triassic (Tr) | 250 | 44 | Paleozoic | 543 | 297 | Permian (P) | 290 | 40 | Carboniferous (M, P) | 354 | 64 | Devonian | 417 | 63 | Silurian | 443 | 26 | Ordovician | 490 | 47 | Cambrian | 543 | 53 | Precambrian | Proterozoic | 2500 | 1957 | Late (Z)† (Neoproterozoic) | 900 | 357 | Middle (Y) (Mesoproterozoic) | 1600 | 700 | Early (X) (Paleoproterozoic) | ?2500 | 900 | Archean | 3800 | | Late (W) | 3000 | 500 | Middle (U) | 3400 | 400 | Early (V) | >3800 | >400 | *In parentheses are the symbols for the periods and epochs used on geologic maps and figures in North America, as well as other parts of the world. †Letter designations of Precambrian age intervals are used by the U.S. Geological Survey. The modern geologic time scale is based on many measurements of various rock types by quantitative isotopic chronometers such as uranium-lead (U-Pb) and potassium-argon (K-Ar). | 
