Within the reserve is the scheduled monument Tarr Steps , a clapper bridge over the River Barle. The name "clapper bridge" comes from the Medieval Latin ""claperius"" which means "pile of stones". It is an ancient form of bridge constructed with large unmortared slabs of stone resting on one another; this is the largest example of its type. There are 17 spans across , the top slabs weigh 1-2 tons and are about above normal water level. The largest slab is over long and is about wide. This is one of the best known monuments on Exmoor. Its age is unknown, as several theories claim that Tarr Steps dates from the Bronze Age but others date them from around 1400 AD. It has been restored several times in recent years, following flood damage. Over the years the damage provides a good indicator of the strength of each flood. Some of the top slabs have been washed away in extreme flood conditions and they have now all been numbered to facilitate replacement. The Exmoor National Park web site says