Engineers chose cast iron to construct the Dunlap's Creek Bridge after its wooden predecessor collapsed under the weight of heavy snow. Completed in 1839, the bridge is still in use today.

Credit: From The Old Pike published 1894; Courtesy Pennsylvania Historical and Museum Commission

Dunlap's Creek Bridge was completed by the U.S. Army Corps of Engineers in 1839. It was the first metal-arch bridge built in the United States. This ca. 1894 photo shows the bridge before sidewalks were added to its flanks.