The authority for him to do so was the newly passed Antiques Act of 1906, a Congressional response to widespread looting of Indian burial grounds beginning at the end of the 19th century. According to the National Park Service, "[the act] it authorized permits for legitimate archaeological investigations and penalties for persons taking or destroying antiquities without permission. And it authorized presidents to proclaim 'historic landmarks, historic and prehistoric structures, and other objects of historic or scientific interest' as national monuments -- 'the limits of which in all cases shall be confined to the smallest area compatible with the proper care and management of the objects to be protected.'
"So it was originally expected that national monuments would be proclaimed to protect prehistoric cultural features, or antiquities, and that they would be small."
TR did not think small. Devils Tower National Monument is about 1,347 acres, and during his presidency, TR created 18 national monuments, including Muir Woods in California, Jewel Cave in South Dakota, and Petrified Forest and the Grand Canyon -- the latter some 800,000 acres -- both in the Arizona Territory.