The mountain grew in three phases. Garibaldi's first phase resulted in the creation of a broad composite cone made of dacite and breccia that has been potassium-argon dated to 250,000 years old. 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.