"The Antiquities Act of 1906 authorizes the President to proclaim national monuments on federal lands that contain 'historic landmarks, historic and prehistoric structures, and other objects of historic or scientific interest.' The President is to reserve 'the smallest area compatible with the proper care and management of the objects to be protected.' From 1906 to date, Presidents have established 157 monuments and have enlarged, diminished, or otherwise modified previously proclaimed monuments. On April 26, 2017, President Trump issued an executive order requiring the Secretary of the Department of the Interior (DOI) to review national monuments established or expanded by Presidents since 1996. (See Table 1.) The order requires review of national monuments where the size at establishment or after expansion exceeded 100,000 acres or where the Secretary determines that the action was taken 'without adequate public outreach and coordination.'"
CRS Insight, IN10696
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html