The history of the formation of the Cascade Mountains dates back millions of years ago to the late Eocene Epoch. Geological events occurring many years ago created the diverse topography and drastic elevation changes over the Cascade Range leading to various climate differences. During the Pleistocene period dating back over two million years ago, glaciation advancing and retreating repeatedly scoured and shaped the landscape. With the North American Plate overriding the Pacific Plate, episodes of volcanic igneous activity occurred. The lateral blast from the 1980 eruption of Mount St. Helens was pointed toward Mount Whittier and stripped the vegetation from its slopes. Due to Mount St. Helens' proximity to Mount Whittier, volcanic ash is common in the area. Mount Whittier and nearby Minnie Peak are composed of resistant granodiorite of the Spirit Lake pluton, which is a complex of once-molten rock that intruded the surrounding rocks approximately 20 million years ago.