During the first eruptive period between 2,200,000 and 1,900,000 years ago, eruption of intermediate to felsic pyroclastic rocks occurred at the southern end of the massif. Basal breccia, perhaps from an exhumed vent, underlies andesite and tuffs, flows, lava domes and breccia of Devastator Peak. It has a maximum thickness of and overlies a high ridge of bedrock that formed between 251,000,000 and 65,500,000 years ago during the Mesozoic era.