Mount Bailey is part of the High Cascades in the western United States. The High Cascades have long been glaciated, by both Pacific-bred storms and natural, elevation-caused, glaciation. In fact, glaciation probably formed on them as early as the late Miocene. Over time, as the range built up, newer activity diminished older Tertiary age rock. Creating lava plateaus, Pliocene activity, mostly basaltic and andesitic, was probably responsible for the original cones at Bailey, Thielsen, and Union Peak.