Some of the volcanic activity in Simcoe Mountains happened at the same time as eruptions of Goat Rocks to the northwest. For the last 350,000 years Simcoe was active, there was also activity in the Mount Adams region, but Mount Adams itself had not yet started forming. Indian Heaven, a volcanic field farther to the west, is also generally much younger than Simcoe, becoming active only in the most recent 200,000 years of activity at Simcoe Mountains Volcanic Field. Mount St. Helens, like Mount Adams, entirely postdates activity at Simcoe Mountains.