The fundamental principles of tephrochronology were de- veloped in Iceland through the classic research of Professor Sigurdur Thorarinsson in the 1940's and 1950's. By his studies on the volcanic ash layers (tephra) produced by the historic explosive eruptions of the volcano Hekla, he established that individual tephra layers could be correlated over large areas of Iceland. As the deposition of such layers was essentially instantaneous on a geological time-scale, the tephra layers provided a distinctive and widespread isochronous stratigraphic marker. Since Thorarinsson's pioneering work, the...