Mount Whittier is a prominent summit in southwestern Washington within the Mount St. Helens area, notable as the region’s second-highest peak after Mount St. Helens. Though moderate in elevation for the Cascades, it rises sharply above nearby valleys and drains into tributaries of the North Fork Toutle River. Its landscape reflects the wet climate of the western Cascades as well as a long geological history shaped by volcanism and Pleistocene glaciation. The mountain is formed of resistant granodiorite from the Spirit Lake pluton, emplaced about 20 million years ago, and its slopes were stripped of vegetation by the lateral blast of Mount St. Helens’ 1980 eruption.