Principles of relative dating of geologic events

Cross cutting relations: The principle of cross-cutting relations states that if one geologic feature cuts across another, the feature that has been cut is older.For example, if an igneous dike cuts across a sequence of sedimentary beds, the beds must be older than the dike.As we will see, painstaking work over many years eventually allowed geologists to assign numerical age ranges to fossil species.Of note, some fossil species are widespread, but survived only for a relatively short interval of geologic time.Thus, once a fossil species disappears at a horizon in a sequence of strata, it never reappears higher in the sequence or, put another way, extinction is forever.Smith’s observation has been repeated at millions of locations around the world, and has been codified as the principle of fossil succession.

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Geologists apply geologic principles to determine the relative ages of rocks, structures, and other geologic features at a given location.Building from the work of Steno, Hutton, and others, the British geologist Charles Lyell (1797–1875) laid out a set of formal, usable geologic principles.These principles continue to provide the basic framework within which geologists read the record of Earth history and determine relative ages.Such species are called index fossils (or guide fossils), because they can be used by geologists to associate the strata with the specific time interval.Different ways to correlate: -Physical continuity- can physically trace a rock unit from one area to another.

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