geologic timescale definition of geologic timescale in the Free Online Encyclopedia.

geologic timescale, a chronological scale of earth's history used to measure the relative or absolute age of any part of geologic time. Of the numerous timescales, the most common is based on geologic time units, which divide time into eras, periods, and epochs. Each division is based on a specific set of geological or paleontological conditions that make it different from the other divisions, such as varying rock type or fossils within the strata. The largest unit is the eon; eons are subdivided into eras; eras into periods; and some, usually more recent periods, into epochs. In some timescales, epochs are further divided into ages. Each geologic time chart varies, depending on the latest findings dating rocks and fossils of that particular age, or on the country where the chart originated. For each unit and its subdivisions, which are listed in the table entitled Geologic Timescale Geologic Timescale
Era Period Epoch Approximate duration
(millions of years)
Approximate number of years ago
(millions of years)

Cenozoic Quaternary Holocene 10,000 years ago to the present
Pleistocene 2 .
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No references found	Geochemists have long maintained that the proportion of oxygen in the atmosphere has varied on a geologic timescale, but few biologists have considered how such changes would have affected life. Ancient animals got a rise out of oxygen by Monastersky, Richard / Science News	geohydrology Geoid geoidal horizon GEOINT geoisotherm Geok-Tepe Geokchai Geokhimiia Geoksiur geolith Geologic Age geologic climate Geologic Column geologic dating geologic erosion geologic log geologic map geologic noise geologic province Geologic Section geologic structure geologic thermometer geologic thermometry geologic time geologic time scale geologic timescale Geological Committee Geolkom Geological Contact Geological Documentation Geological Education Geological Epoch Geological Era Geological Institute GIN Geological Journals Geological Maps geological oceanography Geological Pick-Hammer Geological Prospecting Geological Sciences, Institute of Geological Series Geological Stage geological steering Geological Survey Geological Survey, United States Geological Surveying Geological Thermometry geological transportation Geologiia i Geofizika Geologiia Nefti i Gaza Geologiia Rudnykh Mestorozhdenii geologist	Geologic formation Geologic Hazard Abatement District Geologic Hazards Overlay Zone Geologic Hazards Working Group Geologic history geologic log Geologic map Geologic Map Data Model geologic noise geologic oceanography geologic process Geologic province Geologic Remote Sensing Field Experiment Geologic Repository Operations Area Geologic Resource Assessment Program Geologic rift Geologic rift Geologic rift Geologic Sciences Geologic Sciences geologic section geologic structure Geologic Testing Consultants geologic thermometer geologic thermometry geologic time geologic time Geologic time scale Geologic time scale geologic timescale geological geological geological Geological Advisory Committee Geological and Geophysical Geological and Land Management Division Geological and Mineralogical Association of Canada Geological and Mining Development Agency Geological Applications of Remote Sensing Geological Association of Canada Geological Association of New Jersey Geological Borehole Information System Geological Characterization Report Geological Committee Geolkom Geological Contact Geological Curators' Group geological cycle geological dating Geological Disaster Geological Documentation Geological Education Geological Electronic Information eXchange System Geological engineering Geological engineering Geological epoch geological era geological era geological fault

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Geochemists have long maintained that the proportion of oxygen in the atmosphere has varied on a geologic timescale, but few biologists have considered how such changes would have affected life.