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 straight toward The Dome and stripped the vegetation from the slopes. Due to Mount Saint Helens' proximity to The Dome, volcanic ash is common in the area.