Page 329 - ABCTE Study Guide_Neat

By the 19th century, geologists had begun to classify strata according to the types of fossils found within
them. This made it possible to identify similar strata on different continents if they contained the same
kinds of fossils. These observations led to the development of the relative geologic time scale. However,
the actual age of these rock strata and the fossils they contained remained a topic for heated debate for
many decades. It wasn't until after the discovery of radioactivity, in the late 1890s, that a better method
would be developed to determine the age of rocks, fossils, and other material found in any rock strata.
This tool would give geologists what was needed to date rock strata and to develop a chronology of the
Earth's history.

The Geologic Time Scale

Geologists have known for some time that the entire history of the Earth is preserved in the strata. There
you can easily see the clues to the geological and biological processes of ancient times. The geologic
time scale displayed below was developed over a long period of time and was most recently updated in
2004 by the International Commission on Stratigraphy.

Eon Era Period Ended MYA*
Phanerozoic Cenozoic Neogene Now

Paleogene 23
Mesozoic Cretaceous 65
Jurassic 140

Triassic 205
Paleozoic Permian 245
Pennsylvanian 290

Mississippian 325
Devonian 355
Silurian 415

Ordovician 440
Cambrian 495
Proterozoic Neoproterozoic Ediacaran 542

Cryogenian 630
Tonian 850
Mesoproterozoic Stenian 1000

Ectasian 1200
Calymmian 1400
Paleoproterozoic Statherian 1600

Orosirian 1800
Rhyacian 2050
Siderian 2300

Archean Neoarchean 2500
Mesoarchean 2800
Paleoarchean 3200