The eon is the broadest category of geological time. This is most easily done by first breaking the time scale into its component parts: eons, eras, periods, and epochs. Learning the geological time scaleīecause of its usefulness for communicating about events in Earth's history, it is important that all students of geology, paleontology, and evolutionary biology commit the geological time scale to memory.
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It is based almost entirely upon careful observations of the distributions of fossils in time and space. The geological time scale provides a global summary of countless small-scale temporal correlations of rock layers made at local and regional scales. This is critically important because no single place on Earth preserves a complete geological history, or even a small fraction of it. More generally, it has allowed us to reconstruct the geological history of Earth by comparing rocks and fossils from place to place. Temporal correlation has made many people very, very rich by allowing them to predict the locations of valuable geological resources such as fossil fuels. Temporal correlation allowed Smith to construct the first geological map (see Section 2.4) of an entire country. By documenting these sequences of fossils, Smith was able to temporally correlate rock layers (or, strata) from place to place (in other words, to establish that rock layers in two different places are equivalent in age based upon the fact that they include the same types of fossils). As he studied layers of rocks to determine where to build canals, he noticed that he found the same ordering of fossil species from place to place Fossil A was always found below Fossil B, which in turn was always found below Fossil C, and so on. The principle of faunal succession was developed by an English surveyor named William "Strata" Smith (1769-1839). This is because evolution and extinction are facts of nature. Faunal succession is the principle that different kinds of fossils characterize different intervals of time. This relates to a third important principle of relative age dating (see Section 2.1 for the other two): the principle of faunal succession. For example, if you find a rock with a trilobite fossil upon it, you will immediately know that the rock is Paleozoic in age (541 Ma to 252 Ma) and not older or younger knowing the species of trilobite allows even greater precision. Knowing when major groups of fossils first appeared or went extinct is therefore incredibly useful for determining the ages of rocks in the field. Most of the boundaries on the geological time scale correspond to the origination or extinction of particular kinds of fossils. For example, a paleontologist can call her colleague and say, "I just found an awesome new trilobite from the Devonian of New York" and her colleague will immediately understand when in geological time that trilobite lived.
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It is a reference and communication system for comparing rocks and fossils from throughout the world and is geology's equivalent of the periodic table of the elements.
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The geological time scale-shown above in a simplified form-is one of the crowning achievements of science in general and geology in particular.