 In the 20th century, African American migration from the Southern United States into the Midwestern states changed Chicago, St. Louis, Cleveland, Milwaukee, Kansas City, Cincinnati, Detroit, Omaha, Minneapolis, and many other cities in the Midwest, as factories and schools enticed families by the thousands to new opportunities.Chicago alone gained hundreds of thousands of black citizens from the Great Migration and the Second Great Migration.The Gateway Arch monument in St. Louis, clad in stainless steel and built in the form of a flattened catenary arch, is the tallest man-made monument in the United States, and the world's tallest arch.