IRON MOUNTAIN. Iron Mountain is seven miles northwest of Marathon in northern Brewster County (at 30°18' N, 103°16' W). Its summit, at an elevation of 5,413 feet above sea level, rises 900 feet above the Iron Mountain Ranch at its northeastern base. Iron Mountain is a dark-colored peak of intrusive rock from the Tertiary period, between 2 and 66 million years old, which stands out among the surrounding light-gray sedimentary rocks. The local soils are shallow and stony and support scrub brush and grasses.