Three principal volcanoes comprise the southern segment along with several smaller edifies. The largest and youngest principal volcano, Mount Garibaldi, is a dissected stratovolcano that began its formation 250,000 years ago. This eruptive period built a broad composite cone made of dacite and breccia. Parts of this proto-Garibaldi or ancestral volcano are exposed on Garibaldi's lower northern and eastern flanks and on the upper of Brohm Ridge. Around where Columnar Peak and possibly Glacier Pikes are now located, a series of coalescing dacite lava domes were constructed. During the ensuing long period of dormancy, the Cheekye River cut a deep valley into the cone's western flank that was later filled with a glacier. After reaching its maximum extent the Cheekye Glacier and Cordilleran Ice Sheet were covered with volcanic ash and fragmental debris from Garibaldi. This period of growth began with the eruption of the Atwell Peak plug dome about 13,000 years ago from a ridge surrounded by the ice sheet. As the plug dome grew, massive sheets of broken lava crumbled as talus down its sides. Numerous Peléan pyroclastic flows accompanied these cooler avalanches, forming a fragmental cone and an overall slope of 12 to 15 degrees. Some of the glacial ice was melted by the eruptions, forming a small lake against Brohm Ridge's southern arm. The volcanic sandstones seen today atop Brohm Ridge were created by ash settling in this lake. Glacial overlap was most significant on the west and somewhat to the south. Subsequent melting of the Cordilleran Ice Sheet and its component glaciers initiated a series of avalanches and mudflows on Garibaldi's western flank that moved nearly half of the original cone's volume into the Squamish Valley where it covers to a thickness of about . Gaps left by melting ice caused minor to moderate cone distortion where the Cordilleran Ice Sheet was thin and major distortion where it was thick. The ice was thickest in and thus cone distortion was greatest over the buried Cheekye valley. Later volcanism occurred from Dalton Dome, which forms Garibaldi's western summit. Lava flows mantled the landslide headwall on Garibaldi's western flank. Around the same time, a voluminous dacite lava flow from Opal Cone travelled down Ring Creek on Garibaldi's southeastern flank without encountering any residual glacial ice. These latest eruptions of Mount Garibaldi occurred in the early Holocene shortly after remains of the Cordilleran Ice Sheet retreated in regional valleys between 10,700 and 9,300 years ago.