Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 313. Long Valley caldera (Fig. 1) is located at the western edge of the Basin and Range Province straddling the eastern frontal fault escarpment of the Sierra Nevada, in which it forms a reentrant or offset commonly referred to as the "Mammoth embayment." The floor of the caldera ranges in elevation from 2000m in its eastern half, where it is dominated by Lake Crowley and sage- and grass-covered Long Valley, to 2600 m in its western half, which is hillier and heavily forested. The caldera walls rise steeply to elevations of 3000-3500 m on all sides except the east and southeast, where the floor rises only 150 m before merging with the Volcanic Tableland at 2300 m elevation. The Mono-Inyo Craters volcanic chain extends from the western part of Long Valley caldera northward from Mammoth Mtn. to Mono Lake, a distance of 50 km. Although commonly described as subparallel to the Sierran front, the chain trends nearly due north at a noticeable angle to the northwest-trending Sierran faults (Figs. 1, 2). The prevolcanic basement in the area is mainly Mesozoic granitic rocks of the Sierra Nevada batholith and Paleozoic metasedimentary and Mesozoic metavolcanic rocks of the Mt. Morrison and Ritter Range roof pendants. The late Tertiary terrain upon which Long Valley volcanism was initiated was a maturely eroded upland drained by westward-flowing streams.