 Basaltic lava flows from the Quaternary period form the black ridges to the east and west of the historic area of St. George city.The volcanic eruptions producing these flows are thought to be 1.2 million years old.Other points of geologic interest include the Virgin River anticline; the rock has eroded away in the center, leaving sheer walls surrounding the "Purgatory Flats" area to the east of St. George.