Upon a determination of the Secretary of the Interior that sufficient land and other property have been acquired by the United States for national-monument purposes, as provided in section 450kk of this title, such property shall be established as the "Fort Union National Monument" and thereafter shall be administered by the Secretary of the Interior in accordance with the laws and regulations applicable to national monuments. An order of the Secretary, constituting notice of such establishment, shall be published in the Federal Register.
Following establishment of the national monument, additional properties may be acquired as provided in section 450kk of this title, which properties, upon acquisition of title thereto by the United States, shall become a part of the national monument: Provided, That the total area of the national monument established pursuant to this section and section 450kk of this title shall not exceed one thousand acres, exclusive of such adjoining lands as may be covered by scenic easements.
16 U.S.C. § 450kk-1 (2018)