The tallest sand dune in America is in Great Sand Dunes National Monument outside of Alamosa, Colorado. This bizarre 46,000-acre landscape of 700-foot sand peaks was the creation of ocean waters and wind more than one million years ago.
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Colfax Avenue in Denver, Colorado is the longest continuous street in America.
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Mesa Verde, Colorado features an elaborate four-story city carved in the cliffs by the Ancestral Pueblo people between 600 and 1300 A.D.