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There is more than one method, but one of the most relied upon methods is to use stratigraphy. Stratigraphy is the study of sedimentary rock formations. The study of stratigraphy contains some basic principles that were refined and furthered by Nicholas Steno in the 17th century and can be widely applied to stratigraphy (with some exceptions):
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1. Law of superposition- sedimentary layers are deposited in a time sequence, with the oldest on the bottom and the youngest on the top
2. Principle of original horizontality- when sedimentary layers are first formed, gravity is the key force that controls and limits a layer to a horizontal plane 3. Principle of lateral continuity- when sedimentary layers are first formed, a layer extends laterally in all directions |
Stratigraphy is especially useful for relative dating of fossils; that is, which fossil types came first, and which fossil types followed later.
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Relying on stratigraphy alone for absolute dating of fossils is more difficult, though when supplemented by alternative methods, stratigraphy can even be used for absolute dating of fossils.
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Keeping it all in perspective: if you are adventurous and would like to view a very long drawn to scale timeline for the history of the Earth, please click on this link:
geological timeline for planet Earth. |