A method for the relative dating of horizons in volcanic regions by identification of different layers of ash (tephra). Tephra layers (beds) are ideal stratigraphic markers because they are deposited instantaneously. Also, the chemical content of tephra (volcanic ash) is unique for each eruption. If artifacts lie below tephra known to have come from a certain eruption, the artifacts predate the eruption. Tephra layers may be dated by potassium-argon dating and fission track dating and they can sometimes be tied in to absolute chronology where radiocarbon dates can be obtained from material contemporary with the deposit. To establish a chronology it is necessary to identify and correlate as many tephra units as possible over the widest possible area. In the Mediterranean, deep-sea coring produced evidence for the ash fall from the eruption of Thera, and its stratigraphic position provided important information in the construction of a relative chronology. The identification of multiple tephra beds may give bracketing ages for intervening strata. Tephrochronology has also been used to date glacial advances, sea level changes, and alluvial fans.