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method of age determination that makes use of layers of ash (tephra). Tephra layers are excellent time-stratigraphic markers, but, to establish a chronology, it is necessary to identify and correlate as many tephra units as possible over the widest possible area. Because of the large number of violent volcanic explosions in Iceland, Sigurdur Thorarinsson, an Icelander who was the founder of the science of tephrochronology, was able to establish a detailed chronology of preoccupational and postoccupational geologic and archaeological events there. Tephrochronology enabled Thorarinsson to make a thorough study of the changes in climate in Iceland and the effect of agriculture on Icelandic ecology.