Natural arch

Did You Know?

• Natural bridges can form from limestone caves where paired sinkholes collapse, leaving a ridge of stone with a connecting passageway.
• Moon Hill in Yangshuo, Guizhou Province, China is an arch formed from a 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, forming arches when caves break through. Examples include London Bridge in Victoria, Australia, and Neill Island in the Andaman Islands. Collapse of such arches produces stacks and stumps. On concordant coastlines, rock types run parallel to the shore; wave action breaks through stronger rock and rapidly erodes weaker rock behind it. Durdle Door and Stair Hole on Dorset's Jurassic Coast are examples; Stair Hole's collapse will create a cove.

Weather-eroded arches

Weather-eroded arches begin as deep cracks in sandstone. Erosion widens these cracks, isolating narrow walls called fins. Alternating frost and thaw crumbles the porous sandstone, cutting through some fins. The resulting holes enlarge through rockfalls and weathering into arches, which eventually collapse into buttresses that erode further. Many such arches are found in Arches National Park, Canyonlands National Park, and Grand Staircase–Escalante National Monument in southern Utah.

Water-eroded arches

Some natural bridges form in the path of streams that wear through rock. Pothole arches arise from chemical weathering as water collects in depressions and cuts through to lower layers. 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 natural bridge in the Western Hemisphere at 234 ft span (laser measurement, 2007). Xianren Bridge (Fairy Bridge) in Guangxi, China, is the world's largest known natural bridge, with a span of 400 ft recorded in 2010.

Arches as highway or railway bridges

In a few places, natural arches carry highways or railroads. In Virginia, US Route 11 crosses Natural Bridge. Kentucky has two: a limestone cave erosion arch in Carter Caves State Resort Park with a paved road, and a weather-eroded sandstone arch called White's Branch Arch (the Narrows) in Natural Bridge State Park, topped by a dirt road. In Romania, the village of Ponoarele has God's Bridge, a 30 m long, 13 m wide road segment over a stone arch 22 m high and 9 m thick. A railroad from Lima, Peru crosses the Rio Yauli on a natural bridge near kilometer 214.2.