Natural arch

Did You Know?

• Natural bridges can form from limestone caves where paired sinkholes collapse, leaving a ridge of stone with a cave passage connecting them.
• London Bridge in Victoria, Australia, a double-arched coastal formation, lost an arch after storms increased erosion.
• Moon Hill in Yangshuo, Guizhou Province, China, is an arch formed from the remnant of a karst limestone cave.

Coastline

On discordant coastlines, where rock types run at 90° to the coast, wave refraction concentrates energy on headlands, and arches form when caves break through. Examples include London Bridge in Victoria, Australia, and Neill Island in the Andaman Islands, India. Collapse of such arches produces stacks and stumps. On concordant coastlines, rock types run parallel to the coast; wave action breaks through strong rock and rapidly erodes weaker rock behind it. Durdle Door and Stair Hole on Dorset's Jurassic Coast are examples; Stair Hole's eventual collapse will form a cove.

Weather-eroded arches

Deep cracks in sandstone layers are where weather-eroded arches start. These cracks grow wider as erosion wears away exposed rock, leaving behind narrow walls called fins. Porous sandstone in the fins crumbles and flakes from repeated freezing and thawing, which eventually cuts through some of them. Rockfalls and further weathering turn those holes into arches. Over time, arches collapse into buttresses that slowly erode away. Southern Utah’s Arches National Park, Canyonlands National Park, and Grand Staircase–Escalante National Monument contain many examples.

Water-eroded arches

Some natural bridges form in the path of streams that wear away and penetrate rock. Pothole arches develop through chemical weathering as water collects in depressions and cuts through to the layer below. Natural Bridges National Monument in Utah protects three large bridges formed by streams, the largest being Sipapu Bridge with a 225 ft span. Rainbow Bridge National Monument's namesake, formed by flowing water, is the largest known natural bridge in the Western Hemisphere at 234 ft. Xianren Bridge in Guangxi, China, the world's largest known natural bridge, has a span of 400 ft recorded in 2010.

Arches as highway or railway bridges

In a handful of locations, natural arches double as transportation routes. Virginia’s Natural Bridge carries US Route 11. Kentucky features two: a limestone arch in Carter Caves State Resort Park topped with pavement, and White’s Branch Arch (the Narrows) in Natural Bridge State Park, a sandstone arch eroded by weather that supports a dirt road. Romania’s God’s Bridge in Ponoarele spans 30 meters long and 13 meters wide, with a stone arch 22 meters high and 9 meters thick beneath a road. Near Lima, Peru, a railroad crosses the Rio Yauli on a natural bridge at about kilometer 214.2, heading toward La Oroya.