Dartmoor National Park is famous for its ancient clapper bridges. A clapper bridge is basically a large flat stone laid across a stream. It's about as basic as a bridge can get! This one is at Teignhead.
Beautiful Wild Dartmoor: "It's aptly called England's last wilderness, and the feeling of space and silence, broken only by the trilling of skylarks and the crunching of boots on stone, is intoxicating.