But not all the magma that welled upward during this restless period reached the Earth’s surface. Extensive masses were trapped far below the surface, where they gradually cooled and congealed. The Missouri Buttes and Devils Tower, however, are believed to be necks of extinct volcanoes. Geologic evidence indicates the Missouri Buttes formed first in two separate eruptions. The magma hardened, plugging the plumbing underneath. A third eruption to the southeast resulted in Devils Tower. During the ensuing tens of millions of years, the gradual erosion of the overlying rock strata revealed these intruded plugs of volcanic rock. Since this dense, hard igneous rock resists erosion much better than the surrounding sedimentary rock, these formations will continue to stand out as features. That ancient land of sedimentary rock through which the molten mass of Devils Tower penetrated may at one time have been as high as the golden eagle I now see drifting high above the Tower. Circling slowly in its morning hunt, the eagle spirals upward on the currents of warm air rising off the sun-heated rock. Perhaps it now soars at the elevation of the land long ago when the heavy, ringing-hard rock of Devils Tower oozed like paste far below the surface. Today the top of the Tower is 386 meters (1,267 feet) above the Belle Fourche River. If that warm, Cretaceous landscape rested 600 meters (2,000 feet) above the present summit of the Tower, then more than 900 vertical meters (3,000 feet) of sedimentary rock has been pared away in the last 60 million years. The relentless physical agents of erosion—running water, wind, and frost action—together with chemical breakdown of rock particles, continue to alter the landform. Given enough time, even the very hard rock of the Tower itself will waste away. Sixty million years ago, when dinosaurs _Triceratops_ and _Tyrannosaurus Rex_ duelled beside the lush river banks of the predecessor of the Belle Fourche, the ancestor of the golden eagle was flying overhead. Millions of years hence, a descendant of the eagle might soar above this same Wyoming landscape. Missing will be the unique shaft of fluted rock we call Devils Tower. And what of the men, who for a mere eyeblink of time, hunted in its shadow or came to wonder at its somber countenance in the morning sun? The Tower’s Geological Story [Illustration: About 60 million years ago, in early Tertiary times, a mass of molten magma forced its way upward through the relatively level layers of red, yellow, green, and gray Jurassic sedimentary rocks that make up northeastern Wyoming. The mass cooled into a hard, igneous rock called phonolite porphyry. An earlier flow of magma occurred about 6.5 kilometers (4 miles) to the northwest.]