<P> In 1907, scientists Darton and O'Harra decided that Devils Tower must be an eroded remnant of a laccolith . A laccolith is a large mass of igneous rock which is intruded through sedimentary rock beds without reaching the surface, but makes a rounded bulge in the sedimentary layers above . This theory was quite popular in the early 20th century since numerous studies had earlier been done on laccoliths in the Southwest . </P> <P> Other theories have suggested that Devils Tower is a volcanic plug or that it is the neck of an extinct volcano . Presumably, if Devils Tower was a volcanic plug, any volcanics created by it--volcanic ash, lava flows, volcanic debris--would have been eroded away long ago . Some pyroclastic material of the same age as Devils Tower has been identified elsewhere in Wyoming . </P> <P> The igneous material that forms the Tower is a phonolite porphyry intruded about 40.5 million years ago, a light to dark - gray or greenish - gray igneous rock with conspicuous crystals of white feldspar . As the magma cooled, hexagonal (and sometimes 4 -, 5 -, and 7 - sided) columns formed . As the rock continued to cool, the vertical columns shrank in cross-section (horizontally) and cracks began to occur at 120 degree angles, generally forming compact 6 - sided columns . The nearby Missouri Buttes, 3.5 miles (5.6 km) to the northwest of Devils Tower, are also composed of columnar phonolite of the same age . (Devils Postpile National Monument in California and Giant's Causeway in Northern Ireland, are also columnar basalt, which are superficially similar, but with columns typically 2 feet (0.61 m) diameter .) </P> <P> Devils Tower did not visibly protrude out of the landscape until the overlying sedimentary rocks eroded away . As the elements wore down the softer sandstones and shales, the more resistant igneous rock making up the tower survived the erosional forces . As a result, the gray columns of Devils Tower began to appear as an isolated mass above the landscape . </P>

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