A road had existed from pioneer days through spectacular Glenwood Canyon, some of whose cliffs tower 2,000 feet above the Colorado River near present-day Glenwood Springs, Colorado. Photos of teams and wagons negotiating a rough trail through the canyon date from the 1880s. This curved portion of a vast system of bridges, tunnels, and viaducts through the twisting canyon, begun in 1980 and completed in 1992, was the final piece of Interstate Highway 70 that begins in Maryland and ends in Utah. As many as 500 highway workers were employed on what has often been called an engineering marvel. The Glenwood Canyon project required 30 million points of structural steel, 30 million pounds of reinforcing steel, and 400,000 cubic yards of concrete weighing 1.62 billion pounds. Photo by Carol Highsmith, 2015.