Relative age dating and absolute age dating
The basis for stratigraphy seems quite intuitive today, but its applications were no less than earth-shattering to archaeological theory.
For example, JJA Worsaae used this law to prove the Three Age System.
For more information on stratigraphy and how it is used in archaeology, see the Stratigraphy glossary entry.
Seriation, on the other hand, was a stroke of genius.
First used, and likely invented by archaeologist Sir William Flinders-Petrie in 1899, seriation (or sequence dating) is based on the idea that artifacts change over time.
Like tail fins on a Cadillac, artifact styles and characteristics change over time, coming into fashion, then fading in popularity. The standard graphical result of seriation is a series of "battleship curves," which are horizontal bars representing percentages plotted on a vertical axis.
Cross-dating of sites, comparing geologic strata at one site with another location and extrapolating the relative ages in that manner, is still an important dating strategy used today, primarily when sites are far too old for absolute dates to have much meaning.
Plotting several curves can allow the archaeologist to develop a relative chronology for an entire site or group of sites.
For detailed information about how seriation works, see Seriation: A Step by Step Description.