West Crater forms part of the Quaternary Marble Mountain-Trout Creek Hill volcanic field in southern Washington state, formed by small basaltic and basaltic andesite shield volcanoes and cinder cones. This field encompasses an area of , running from Marble Mountain to the area to the south of Trout Creek Hill. Located south of Mount St. Helens, it extends for and has a width of . It consists of mostly monogenetic scoria cones, each of which produced three or four lava flows. There are at least 22 cones, and all volcanic landforms older than 12,000 years have been glaciated, the oldest the Soda Peaks basalt lava flows, dated to 0.36 million years old.