A natural bridge or natural arch is a formation (or landform) where a rock arch forms, with a natural passageway through underneath. Most natural bridges form as a narrow ridge, walled by cliffs, become narrower from erosion, with a softer rock stratum under the cliff-forming stratum gradually eroding out until the rock shelters thus formed meet underneath the ridge, thus forming the natural bridge. Natural bridges commonly form where cliffs are subject to erosion from the sea, rivers or weathering.
Natural bridges form when erosion or weathering (sub-aerial processes) find weaknesses in rocks and work on them, making them bigger until they break through the rock. On coasts this can form two different types of arch depending on the geology. On discordant coastlines, where rock types run at 90° to the coast headlands form. Wave refraction concentrates the wave energy on the headland, and the arch forms when caves break through the headland, e.g., London Bridge in Victoria, Australia. When these eventually collapse they form stacks and stumps. On concordant coastlines, where rock types run parallel to the coastline, with weak rock (such as clay) protected by stronger rock (such as limestone) the wave action breaks through the strong rock and then erodes the weak rock very quickly. Good examples of this is at Durdle Door and Stair Hole near Lullworth cove on the Dorset Jurassic Coast in south England. When Stair hole eventually collapses it will form a cove.
Natural bridges can form from natural limestone caves, where paired sinkholes collapse and a ridge of stone is left standing in between, with the cave passageway connecting from sinkhole to sinkhole.
Like all rock formations, natural bridges are subject to continued erosion, and will eventually collapse and disappear. One example of this was the double-arched Victoriann coastal rock formation, London Bridge which lost an arch after storms increased erosion.
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