Clapper Bridge at Postbridge It seems hard to believe that the narrow span of the clapper bridge carried all Dartmoor traffic over the River Dart for 500 years. The clapper bridge owes its long life to the skills of the bridge builders who used four granite slabs, 15ft long in its construction and sheathed its piers in granite cladding as protection against the rushing waters of the Dart. Local legend suggest that the structure is prehistoric, but it is medieval and thought to date from the 13th century.