Over the next 100 million years, more than 7,000 feet (2,100 m) of younger sedimentary rocks were deposited above the Sundance Formation. Although erosion removed all these younger rocks at the monument, these strata have been preserved in the Powder River Basin to the west. The amount of material removed during this episode of erosion is equivalent to five times the height of the Sears Tower in Chicago (1,450 feet [442 m])—the tallest building in the United States.
From within these eroding sedimentary layers, a stunning igneous tower was revealed. Proceed to “Igneous Rocks” to learn about the formation of Devils Tower.