Limited test excavations at two stoneworking camps in Long Valley, Mono County, resulted in a large collection of Casa Diablo obsidian debitage. The investigation focused on obsidian hydration analysis, since obsidian production and exchange networks constitute a major research issue on both sides of the Sierran crest. Obsidian hydration data from several stoneworking sites in the Mammoth area have been used to identify changes through time in the production of Casa Diablo obsidian; various environmental and social factors have been offered as possible explanations for the observed fluctuations. Our research corroborates previous findings for the eastern Sierra region as a whole, but documents potentially meaningful variability in production within the region. Inter-site differences may result from localized effects of volcanism on prehistoric lifeways or may reflect use by different producer groups.