The Casselman River Bridge, completed in 1813 (!) still stands, though it is closed to motorized traffic these days.
Still, IMO, it qualifies as a bridge engineering masterpiece. And it is still a beautiful structure.
Washington Post: Maryland bridge tells U.S. transportation story (http://www.washingtonpost.com/blogs/dr-gridlock/wp/2014/05/27/maryland-bridge-tells-u-s-transportation-story/)
QuoteIn far western Maryland stands a magnificent old stone bridge arching high over the Casselman River. The land around it is Maryland's smallest state park, yet it's easy to see the draw for many tourists.
QuoteIt's beautiful, and impressive. When built in 1813, the 80-foot stone arch span was the longest of its kind in the nation. The stone bridge was so cutting-edge that skeptics doubted the structure would stand once its supporting timbers were removed at the end of construction. What a tribute to American engineering.
There are many, many 19th-century stone bridges left along the National Road, from MD through PA and well into OH. US 40 even still crosses a few of them!