- Geology of the Snoqualmie Batholith (washingtonminerals.com)
Snoqualmie Batholith - late Oligocene to early Miocene (28 Ma[million years ago] to 22 Ma). Uplift and erosion of the Old Cascade Volcanic Arc exposed plutonic rock of the Snoqualmie Batholith near Snoqualmie Pass between Interstate 90 and State Highway 2. The batholith consists of at least eight intrusive phases, emplaced at a depth of about 4,000 and 8,000 feet. Intrusive rock types found within the Snoqualmie Batholith range in composition from granite to gabbro. Granodiorite is the most prevalent rock type, making up about 80 percent the batholith (Erikson, 1969). Other lithologies within the batholith include porphyritic dacite, andesite, aplite, and intrusive breccia (Livingston, 1971).