It shall be the duty of the commissioners, under the direction of the Secretary of the Interior, to survey, locate, and preserve the lines of the opposing armies in said battles, to open, construct, and repair such roads, highways, paths, and other approaches as may be necessary to make the historical points accessible to the public and to students of said battles and for the purposes of the park, to ascertain and mark with historical monuments, markers, tablets, or otherwise, as the Secretary of the Interior may determine, all breastworks, earthworks, gun emplacements, walls, or other defenses or shelters, lines of battle, location of troops, buildings, and other historical points of interest within the park or in its vicinity, and to establish and construct such observation towers as the Secretary of the Interior may deem necessary for said park, and the said commission in establishing the park shall have authority, under the direction of the Secretary of the Interior to employ such labor and services at rates to be fixed by the Secretary of the Interior, and to obtain such supplies and materials as may be necessary to carry out the provisions of sections 425 to 425j of this title.