During the last century geologists constructed a relative time-scale based on correlation of palaeontological and stratigraphic data. Depositional rates of sediments have also been employed as a dating method, but only recently has absolute dating been made possible through the use of radioactive isotopes. Of the various methods the last is obviously the most precise, but fossils, lithologies, and cross-cutting relationships do enable the geologist to give an approximate relative age in field studies. See also absolute age; radioactive decay; radiometric dating; isotopic dating; radio-carbon dating; dendrochronology; geochronology; geochronometry; and varve analysis.
Subjects: Environmental Science.