The "law of superposition" is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields dealing with geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the oldest strata will be at the bottom of the sequence.
This is of great importance to stratigraphic dating, which assumes that the law of superposition holds true and that an object cannot be older than the materials of which it is composed. The law was first proposed in the 17th century by the Danish scientist Nicolas Steno.
When combined with the related law of faunal succession, the law of superposition provides a very powerful tool for the dating of rocks and strata.