Devils Tower, a prominent landmark in northeastern Wyoming, is a mammoth rock obelisk formed from molten rock forced upward from deep within the earth, though geologists continue to debate whether it is solidified lava from the neck of an ancient volcano whose walls eroded away long ago or a sheet of molten rock injected between rock layers. Its characteristic furrowed columns apparently resulted from uniformly arranged cracks that appeared as the volcanic magma cooled. Geologic estimates place Devils Tower at greater than fifty million years old, although erosion likely uncovered the formation only one or two million years ago. Its unique geological attributes stimulated early preservation efforts: in 1892 Wyoming Senator Francis E. Warren persuaded the General Land Office to create a timber reserve surrounding the Tower and also launched an unsuccessful effort to declare the entire area a national park. In 1906 Congress passed the Antiquities Act, empowering the President to grant national monument status to federally owned lands containing historic landmarks, historic or prehistoric structures, and other significant historic or scientific objects, and President Theodore Roosevelt quickly used the act to designate Devils Tower the nation's first national monument. The National Park Service was created in 1916 and eventually assumed administrative control of all national monuments.