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Although Devils Tower has long been a prominent landmark in northwestern Wyoming, the origin of the mammoth rock obelisk remains somewhat obscure. Geologists agree that Devils Tower consists of molten rock forced upwards from deep within the earth. Debate continues, however, as to whether Devils Tower is solidified lava from the neck of an ancient volcano, the walls of which eroded long ago, or whether it is a sheet of molten rock which was injected between rock layers. The characteristic furrowed columns are apparently the result of uniformly-arranged cracks which appeared during the cooling of the volcanic magma. Geologic estimates have placed the age of Devils Tower at greater than fifty million years, although it is likely that erosion uncovered the rock formation only one or two million years ago.
The unique geological attributes of Devils Tower stimulated several early preservation efforts, In 1892 Wyoming Senator Francis E. Warren persuaded the General Land Office to create a timber reserve which surrounded the Tower. Senator Warren also launched an unsuccessful effort to declare the entire area a national park. In 1906 Congress passed the Antiquities Act which empowered the President to bestow national monument status upon federally owned lands that contain historic landmarks, historic or prehistoric structures, and other significant historic or scientific objects. President Theodore Roosevelt quickly invoked the Antiquities Act, designating Devils Tower the nation's first national monument in 1906. The National Park Service was created in 1916 and eventually assumed administrative control of all national monuments.