Using relative dating principles and the position of layers within rock, it is possible to reconstruct the sequence of geologic events that have occurred at a site. In the image below, cliffs along the Snake River show signs of volcanic activity and deposition. The thick, dark, gray layer at the bottom is made of basalt. Red-colored layers are sandstone.
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 important 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.