Bridges That Build Themselves

The two oldest types of bridge are the arch bridge and the corbel bridge.  It takes a lot of time and effort to build them, but they can last for thousands of years.

Rockfalls can make natural bridges over streams, but the 'design' is very inefficient.  The water channels are easily blocked, with resultant flooding of the adjacent banks.

If I told you that a tornado could pick up rocks and drop them across a river so as to form a perfect arch bridge, you wouldn't believe me.  I hope.  The only way to build a stone arch bridge is to set up a type of scaffolding, shuttering, or former - more properly known as centering.