Like the mountains in Glacier National Park, Mt. Helen is composed of sedimentary rock laid down during the Precambrian to Jurassic periods. Formed in shallow seas, this sedimentary rock was initially uplifted beginning 170 million years ago when the Lewis Overthrust fault pushed an enormous slab of precambrian rocks 3 mi (4.8 km) thick, 50 miles (80 km) wide and 160 miles (260 km) long over younger rock of the cretaceous period.
Climate
Based on the Köppen climate classification, Mt. Helen is located in an alpine subarctic climate zone characterized by long, usually very cold winters, and short, cool to mild summers. Temperatures can drop below −10 °F with wind chill factors below −30 °F.