Twilight at Mormon Lake To the north, the San Francisco Peaks continue to catch the sun. Nestled below the horizon but in front of these mountains is the city of Flagstaff. The tallest of these Tertiary Period volcanos is Humphries Peak. Its Navajo name, D'ook'ooslih, means "Earth Mother." It is one of the four mountains, held to be sacred, that mark the corners of the Navajos' ancestral territory.