Volcanic activity of the third eruptive period occurred between 1,100,000 and 200,000 years ago. A thick sequence of andesite lava flows were erupted from the volcanic plug of Devastator Peak, creating the Pylon Assemblage. With a maximum thickness of more than , the Pylon Assemblage is the largest rock unit comprising the Mount Meager massif. The lava flows are layered, separated by a thin layer of lapilli tuff and reddened breccia. A concentration of subvolcanic intrusions and coarse volcanic breccia clasts more than in length suggest that Devastator Peak is a major vent.