LARGEST ARCHES & BRIDGES

Delicate Arch in Arches National Park, Utah

Coastline. On coasts two different types of arches can form depending on the geology. On discordant coastlines rock types run at 90° to the coast. Wave refraction concentrates the wave energy on the headland, and an arch forms when caves break through the headland. Two examples of this type of arch are London Arch—previously known as dLondon Bridge—in Victoria, Australia, and Neill Island in the Andaman Islands, India. When these arches eventually collapse, they form stacks and stumps. On concordant coastlines rock types run parallel to the coastline, with weak rock such as shale protected by stronger rock such as limestone. The wave action along concordant coastlines breaks through the strong rock and then erodes the weak rock very quickly. Good examples of this type of arch are the Durdle Door and Stair Hole near Lulworth Cove on Dorset’s Jurassic Coast in south England. When Stair Hole eventually collapses it will form a cove.

Weather-Eroded Arches

Metate Arch, Devils Garden, a very thin arch near the end of its life
Weather-eroded arches begin their formation as deep cracks which penetrate into a sandstone layer. Erosion occurring within the cracks wears away exposed rock layers and enlarges the surface cracks isolating narrow sandstone walls which are called fins. Alternating frosts and thawing cause crumbling and flaking of the porous sandstone and eventually cut through some of the fins. The resulting holes become enlarged to arch proportions by rockfalls and weathering. The arches eventually collapse leaving only buttresses that in time will erode.
Many weather-eroded arches are found in Arches National Park, Canyonlands National Park, and Grand Staircase-Escalante National Monument (GSENM), all located in southern Utah, United States.
Some natural bridges may look like arches, but they form in the path of streams that wear away and penetrate the rock. Pothole arches form by chemical weathering as water collects in natural depressions and eventually cuts through to the layer below.
Natural Bridges National Monument in Utah protects the area surrounding three large natural bridges, all of which were formed by streams running through canyons, the largest of which is named Sipapu Bridge with a span of 225 feet (69 m).
Rainbow Bridge was also formed by flowing water which created the largest known natural bridge in the Western Hemisphere with a span of 234 feet (71 m), based on a laser measurement made in 2007.
Xianren Bridge, also known as Fairy Bridge, in Guangxi, China is currently the world’s largest known natural bridge with a span recorded at 400 feet (120 m) by the Natural Arch and Bridge Society in October 2010, with a precision of ±15 feet (4.6 m).

Cave Erosion
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 Victorian coastal rock formation, London Bridge, which lost an arch after storms increased erosion.
Moon Hill in Yangshuo, Guizhou Province, China, is an example of an arch formed by the remnant of a karst limestone cave.

Arches as Highway or Railway Bridges

Natural Bridge, Virginia
In a few places in the world, natural arches are utilized by humans as transportation bridges with highways or railroads running across them.
In Virginia, US Route 11 traverses Natural Bridge. Two additional natural arch roadways are found in Kentucky. The first, a cave erosion arch made of limestone, is in Carter Caves State Resort Park and has a paved road on top. The second, a weather-eroded sandstone arch with a dirt road on top, is on the edge of Natural Bridge State Park in Kentucky. The latter arch is called White’s Branch Arch (also known as the Narrows) and the road going over it is usually referred to as the Narrows Road.
In Europe, the Romanian village of Ponoarele has a road segment called God’s Bridge that is 30 m (98 ft) long and 13 m (43 ft) wide, passing over a stone arch 22 m (72 ft) high and 9 m (30 ft) thick.
The railroad from Lima, Peru crosses the Rio Yauli on a natural bridge near kilometre 214.2 as it approaches the city of La Oroya.

LONGEST NATURAL ARCHES in the WORLD 
All known natural arches with span lengths of 180 ft (54.9 m) or greater are included in the list. * indicates ones I have seen.

Note: An arch in the Purcell mountains of British Columbia has been estimated to be about 80m long making it one of the largest true arches in the world. It is well known locally even though it does not have a name, but has not received much attention until mentioned in a FB post and I notified the Natural Arch & Bridge Society.
It is also in an impossible position on a knife edge ridge at over 8300′ elevation. Access is via a long drive on a 4WD road, a backpack and then a 7-pitch rock climb. Most evidence of the length comes from a climber who belayed someone over the top of  the arch – at about the centre of the arch, 40 metres of rope had been let out. It potentially could be the 2nd longest in the world. See my post Hole in the Wall Arch.