Dunlap's Creek Bridge. An integral part of the National Road, this was the first metal arch bridge in the United States, built 1836-39. Replacing several earlier bridges on this site, including an 1809 Finley suspension bridge, this 80-foot span was built of cast iron by the U.S. Army Corps of Engineers. Capt. Richard Delafield and Lieutenant George W. Cass oversaw construction.