Marker horizons are stratigraphic units of the same age, with distinctive composition and appearance. So, though they are in different geographic sites, there is certainty about their age-equivalence. Fossil faunal and floral assemblages, marine and terrestrial, make for distinctive marker horizons. Volcanic ash (tephra) has a geochemical fingerprint. Tephra is often used as a dating tool in archaeology, because the dates of some volcanic eruptions are well-established.