Clapper bridge.

The Clapper bridge in the small village of Wycoller in East Lancashire, England.
Camera Nikon D90. Processed with Nikon Capture NX2.

A clapper bridge is an ancient form of bridge found on the moors of Devon (Dartmoor and Exmoor) and in other upland areas of the United Kingdom including Snowdonia and Anglesey, Cumbria, Yorkshire and Lancashire. It is formed by large flat slabs of stone, often granite or schist, supported on stone piers (across rivers), or resting on the banks of streams. Although often credited with prehistoric origin, most were erected in medieval times and some in later centuries. They are often situated close to a ford where carts could cross.
(Wikipedia).