Sherwin deposits also are found between the basalts and the Bishop Tuff in the Owens River Gorge. The Bishop Tuff was deposited more than 700,000 years ago, starting with a Plinian eruption followed by a series of pyroclastic flows from multiple source vents within Long Valley. The caldera, a 17 by 32 km elliptical structure, formed as the roof foundered above and collapsed into a partially depleted magma chamber. Following caldera collapse volcanism continued in Long Valley with smaller volume rhyolite eruptions. Holocene eruptions of the Inyo and Mono chain of craters and domes occur in an arc that intersects the northwest margin of Long Valley caldera, possibly the initiation of a new caldera cycle.
Pyroclastic flows originate from explosive fragmentation of volatile-rich magma.
These gravity driven flows owe their mobility to rapidly expanding gases, and are capable of traveling hundreds of kilometers. Once emplaced, cooling and compaction of the hot deposits can alter an initial homogeneous mass of glass, ash, and crystals into a layered unit of rock indistinguishable from one deposited as a lava flow. Tuff is an excellent insulator, retaining heat in thick deposits that compact, or weld, under its own weight or overburden. The high temperatures also promote secondary crystallization within the sheet-like deposits. The ultimate product is a layered formation with varying degrees of compaction and crystallization.