They were formed about 1.6 million years ago in the early Pleistocene Epoch by volcanic activity. They are the remnants of a volcano that has been dormant for about 1.4 million years. Some geological references suggest that the volcano represents the southernmost boundary of the Cascade Volcanoes, but there are significant differences in age and form compared to the other volcanoes in that range. Others suggest that its age places it with the volcanic elements of the California Coast Ranges; its composition is closer to that range but does have significant differences.