Original: One of stratigraphy's basic concepts is codified in the Law of Superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.
Simplified: One of stratigraphy's basic concepts is codified in the law of superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.