A part of the
Basin and Range Province, the volcano's most recent eruptive activity dates to approximately 15 million years ago (the
Miocene), forming during a period of active volcanism including eruptions of
ignimbrite and
ash. It formed around the same time as
Three Fingers,
Castle Peak, and three other volcanoes. Today the volcano appears gnarled due to
erosion and is topped by
pine forests. The caldera is narrow and shaped like a ridge, with precipitous slopes and an
escarpment on the northwest flank. It consists of
rhyolite tuff and
sedimentary layers.
Leslie Gulch lies within the depression of the volcano. Layers of ash and tuff are evident in the formation, and leftover volcanic rocks sit in it as well. The gulch features an array of rock formations and ash erupted from the volcano approximately 15.5 million years ago. == History ==