Sierra Grande is an extinct volcano in northeastern New Mexico that rises about 2,200 feet above the surrounding plain and dominates the nearby landscape as the largest volcano in the inactive Raton-Clayton volcanic field. Formed between about 3.8 and 2.6 million years ago, it is composed largely of two-pyroxene andesite, a rock type uncommon elsewhere in the field. Sierra Grande also has notable geographic distinctions: its eastern slopes mark the easternmost point in the United States above 8,000 feet, and its summit is the highest point in the country east of 104 degrees west. Highways, rail lines, and local streams trace its influence on the region.