Lecture 8 Geology 110

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7.2 How are geologic events placed in relative order?

• The principle of superposition states that many rocks are formed in layers at Earth's surface, one above the other, so that the lowest rocks formed first and each higher layer is younger than the one below.
• The principle of original horizontality states that sedimentary layers are horizontal, or nearly so, when they were deposited. Nonhorizontal layering indicates disruption following deposition.
• The principle of cross-cutting relationships states that geologic features, such as dikes and faults, that cut across otherwise continuous rocks must form after the rocks they cut across.
• The principle of inclusions requires that objects enclosed by rock must have formed prior to inclusion in the rock.