A. Relative Age Determination (relative timing):
1. Law of Superposition: In a series of sedimentary layers, the bottom layer is the oldest.
*Assumes the layers have not been overturned by plate tectonic activity.
2. Cross Cutting Relationships: younger events (faults, igneous intrusions, erosion, metamorphism) "cut" or are superimposed on something. That something had to be there to be "cross cut" or modified.
B. Interpreting Geologic Histories:
1, Use relative timing relationships (use basic information in rocks, superposition, and cross-cutting relationships) to work out general events.
2. We also use examples of processes and relationships observed in the modern world to help interpret the past.
This principle is called: uniformitarianism (sometimes also referred to actualism). This principle is summarized by: Understanding the present is key to understanding the past.
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