Natural Bridge

Definition: A natural arch, bridge or tunnel is a void beneath still standing bedrock, usually of short extent, and allowing human passage from one end to the other, at least part of the time.

A natural arch is an arch of rock formed by erosion (weathering).
A natural bridge a bridge of rock spanning a ravine or valley and formed by erosion.
A tunnel is a nearly horizontal cave open at both ends, fairly straight and uniform in cross-section.

A natural bridge is somewhat shorter than a tunnel.

Image: Veliki naravni most, a natural bridge in the Rakov National Park near Postojna in Slovenia.

There are two different types of natural bridges/arches depending on their formation. The following explanation focusses on the second type of natural bridges!

Natural bridges and arches - like in Arches NP in USA - are formed by multiple erosive processes, like fluvial erosion (flowing water, rivers), frost erosion and wind erosion. They may be formed in not soluble rocks like basalt or sandstone, but also in limestone.
Natural bridges and tunnels in karst areas are the ruins of caves.

In large cave systems collapses are very common. Very often they result in domes that are optimal to stand the pressure of the overlaying rocks.

Sometimes the caves are large and very near to the surface, so the ceilling gets too thin and collapses. As erosion goes on, the collapsed rooms are widened to small valley with a river, the former subterranean river. The valleys grow larger, the connecting cave remains get shorter.

Sometimes the remaining caves are short enough, to see the other end and the next valley. This may be the time to call them tunnel.

Natural Bridges on showcaves.com

Veliki naravni most in Rakov Škocjan Nature Reserve near Postojna, Slovenia.

Natural Bridge

Natural Bridges on showcaves.com

See also