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Relative dating of rock layers is to determine which layer is older than the other. This research is part of the discipline of stratigraphy. This is opposed to absolute dating in which a certain age of a rock can be researched.
If layers are not disturbed by folding and mountain building, the younger layer is always on top of the older layer (relative age). These layers can be read like the pages of a book. Each layer has its own characteristics and fossils, and by examining many outcrops on Earth, geologists could make the classification of geological periods. At this point therefore, we know which pages of the geological book come after the other. Charles Leyll already came with some principles (in 1830) that even today are mostly valid:
- Principle of superposition: layers above other layers, are younger.
- Principle of original horizontality: all layers were originally deposited more or less horizontally.
- Principle of lateral extension: a geological layer continues horizontally, until another structure or modification disrupts the layer.
- Principle of transection: a structure that intersects other layers, is younger.
- Principle of Inclusion: a structure that is enclosed by another structure, is older.
- Principle of uniformity: the physical processes that once formed these layers, are still working today.
See also the information page: How old are fossils?
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