The next eruptive period, the Vulcan's Thumb stage, built an edifice that grew upon the southwestern slope of the ancestral Mount Cayley stratovolcano. This began with the eruption of massive dacite flows and blocky agglutinated breccias onto basement and older volcanic rocks of the Mount Cayley stage. These rocks partially form a ridge south of Wizard Peak and comprise the prominent summit ridge pinnacles of Pyroclastic Peak, including the Vulcan's Thumb.