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| longest unit of geologic time |
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| eons are divided into smaller units of time called eras |
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| eras are subdivided into periods |
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| period are subdivided in epochs |
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| the extinction of many species on earth within a short period of time |
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| connects two continents that were previously separated |
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| the separation of a population of organisms from the rest of its species due to some physical barrier |
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| the time required for half of the amount of a radioactive parent element to decay into a stable daughter element |
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| a horizontal layer of material, especially one of several parallel layers arranged one on top of another |
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| sediments settling out from bodies of water are deposited horizontally or nearly horizontally in layers that lie parallel or nearly parallel to the earths surface |
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| the principle that in undistrubed rock layers the oldest rocks are on the bottom |
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| cross cutting realtionships |
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| states that the geologic feature which cuts another is the younger of the two features |
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| the rigid outermost layer of earth that includes the uppermost mantle and crust |
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