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Radiocarbon dates of yr B.P. and 10,360 ± 50 overlie mastodon remains from the M & M Mastodon Site, Prescott National Forest, Arizona, indicating that mastodon extinction in Arizona may have been contemporaneous with that of the other Pleistocene megafauna in the New World. The cranial fragments are embedded in a filled stream channel north of the Hassayampa River. Thirty species of diatoms are identified in the channel sediment; the most abundant taxa are Epithemia spp. (10%) Alacoseria spp. (22.5%), and Pinnularia (45%). The presence of Nitzschia amphibia, indicates fluctuating water levels, and abundant Pinnularia and Eunotia suggest circumneutral pH with moderately low mineral content. Pollen analysis of 6 channel samples shows upward-decreasing percentages of sedge and cattail pollen, and of algal spores. The upland pollen is dominated by pine (90%) at the top of the …