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Published online by Cambridge University Press: 27 June 2018
Three separate dates were determined by Arnold and Libby from materials submitted by L. S. Cressman from Oregon (samples 247, 428, 430). Because of the significance of the date of the Mt. Mazama (Crater Lake) eruption to archaeological dating due to Mazama pumice having covered certain archaeological cave deposits, the eruption date of 6453 ± 250 (based upon an average of four runs of sample 247 ranging from 5938 ± 400 to 7318 ± 350) is important. Williams (Cressman and Williams, 1940, p. 76. See also Hansen, 1942, p. 105; Cressman, 1942, pp. 135, 24) estimated this eruption as occurring not less than 4000 nor more than 10000 years ago. Williams (1942, pp. 212-4) later revised this estimate to approximately 5000 years with a minimum of 4000 and maximum of 7000 years.
* Very recently Libby has determined that the bat guano lying directly upon the Pleistocene gravels (probably deposited during the third cycle of Lake Lahontan and possibly contemporaneous with Mankato) is 11199 + 570 years (9249 B.C.).
* B.P., as used here and elsewhere, means Before Present.