Geologic Age Determination. http://www.nps.gov/archive/grca/photos/images/T501.jpg. Uniformitarianism: (uniform = the same) Geological processes occurring today, like plate movement, mountain formations, etc., have been occurring since the Earth was formed.
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Correlation: The matching of equivalent rocks in different geographic areas.
The Law of Superposition is one of the most basic principles of geology . Younger rock layers are deposited on top of older layers, during normal conditions of deposition. The basic principle of stratigraphy (the study of sedimentary rock layers), is still the single best method that geologists have for determining the relative ages of rock sequences.
Sedimentary rock layers (and lava flows) are always formed in a horizontal orientation. Any folding or tilting must have happened sometime after the layers were originally formed.
**If there is any folding, tilting, or faulting, you know that it occurred after the rock was layered because of the Principle of Original Horizontality
The Principle of Cross-Cutting Relationshipsstates that an igneous intrusion (or a fault) is always younger than the rock it cuts across.
The Principle of Inclusionsstates that the rock unit that contains inclusions is younger than the rock that the inclusions came from.
In the diagram at the left, the igneous intrusion (E) must be younger than rock layers A, B, C, and D because it contains inclusions from those layers.