Geologic record
Segments of rock ( strata ) in chronostratigraphy Time spans in geochronology Notes to geochronological units
Eonothem Eon 4 total , half a billion years or more
Erathem Era 10 defined , several hundred million years
System Period 22 defined , tens to ~one hundred million years
Series Epoch 34 defined , tens of millions of years
Stage Age 99 defined , millions of years
Chronozone Chron subdivision of an age , not used by the ICS timescale
Discordant strata example
USGS correlation diagram Correcting for discordancies can be done in a number of ways and utilizing a number of technologies or field research results from studies in other disciplines . In this example , the study of layered rocks and the fossils they contain is called biostratigraphy and utilizes amassed geobiology and paleobiological knowledge . Fossils can be used to recognize rock layers of the same or different geologic ages , thereby coordinating locally occurring geologic stages to the overall geologic timeline . The pictures of the fossils of monocellular algae in this USGS figure were taken with a scanning electron microscope and have been magnified 250 times . In the U.S. state of South Carolina three marker species of fossil algae are found in a core of rock whereas in Virginia only two of the three species are found in the Eocene Series of rock layers spanning three stages and the geologic ages from 37.2–55.8 MA . Comparing the record about the discordance in the record to the full rock column shows the non-occurrence of the missing species and that portion of the local rock record , from the early part of the middle Eocene is missing there . This is one form of discordancy and the means geologists use to compensate for local variations in the rock record . With the two remaining marker species it is possible to correlate rock layers of the same age ( early Eocene and latter part of the middle Eocene ) in both South Carolina and Virginia , and thereby `` calibrate '' the local rock column into its proper place in the overall geologic record .
The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sands etc.). This includes all its fossil content and the information it yields about the history of the Earth: its past climate, geography, geology and the evolution of life on its surface. According to the law of superposition, sedimentary and volcanic rock layers are deposited on top of each other. They harden over time to become a solidified (competent) rock column, that may be intruded by igneous rocks and disrupted by tectonic events.