Here is the complete 'British Bridges' set from 1968. The upper left stamp illustrates the Tarr Steps, a 55 meter long bridge in Exmoor National Park, Somerset. It is made of stone slabs each weighing as much as 5 tons. While the stamp describes it as prehistoric, this is debated, as many clapper style bridges were erected in the Medieval period.
Moody photo of the steps from the 'Bridges' 10v. Issued: March 5 2015. *Wanted, not in my collection.