The Gateway Arch - a 630-foot monument in St. Louis in the U.S. state of Missouri. The Gateway Arch is the world's tallest arch and the tallest man-made monument in the Western Hemisphere. The arch reflects St. Louis' role in the Westward Expansion of the United States during the nineteenth century. Designed by Finnish-American architect Eero Saarinen in 1947 and was completed on October 28, 1965.