A TL/ESR Study of the Hearth Feature at the Calico Archaeological Site, California

Abstract

TL/ESR measurements were made to test whether a semicircular arrangement of cobbles at the Calico site was a man-made hearth. Results of an earlier magnetic study of one of the cobbles suggested the inner portion had been heated to 360ºC. Such a temperature would destroy radiation-induced defects on the inner portions of the cobbles, which can then be measured by TL/ESR. TL/ESR signals of the inner and outer portions of four cobbles were measured and showed no significant differences. Moreover, all samples appeared to be at full TL and ESR saturation. Progressive heating of a control cobble taken from outside the hearth feature indicated that destruction of the TL/ESR signal occurs at 200ºC. Estimates of the dose rate at the site indicates that at least 400,000 years are required for an annealed rock to attain full TL/ESR saturation, a much longer time than the estimated 200,000-year age of the deposit.