Natural bridge developed in Upper Paleozoic sandstones in Wyoming, USA.

Rock arches are rare erosional features. Rock arches that are not formed by river or stream erosion are called "natural arches". Those that are formed by river or stream erosion are called "natural bridges". Natural bridges are rarer than natural arches. The highest concentration of natural arches on Earth is Arches National Park in eastern Utah, USA. Very small erosional openings in rocks are called windows. Larger erosional openings are arches. Examples next to inland bodies of water are called lake arches. Examples along ocean shorelines are called sea arches.

The natural bridge seen here is developed in reddish and light-colored sandstones of the Casper Formation in Wyoming. The feature formed by erosional undercutting of a bedrock meander neck.

From park signage:

Natural Bridge and the Oregon Trail

The Oregon Trail crosses LaPrele Creek about one mile downstream from Natural Bridge. Before the modern road was built into the gorge, Natural Bridge was difficult to access, and it was only rarely visited by emigrants of the covered wagon era. From time to time, however, a few ambitious travelers made their way through the heavy brush and down the steep walls of the canyon to see this remarkable work of nature.

While Native Americans were probably well aware of Natural Bridge, the earliest to record their visits were New Orleans newspaperman Matthew Field and Steadman Tilghman, a young doctor from Baltimore. Both were traveling companions of Scottish nobleman William Drummond Stewart. An early day tourist, Stewart had organized several hunting and exploring expeditions into the Rocky Mountains and traveled strictly for pleasure. In 1843, he was making his final trip west.

On July 12, Field wrote: "Rode off in advance of the camp with Sir William, to visit a remarkable mountain gorge - a "natural bridge" of solid rock, over a rapid torrent, the arch being regular as though shaped by art - 30 feet from base to ceiling, and 50 to the top of the bridge - wild cliffs, 300 feet perpendicular beetled us, and the noisy current swept along among huge fragments of rock at our feet. We had a dangerous descent, and forced our way through an almost impervious thicket, being compelled to take the bed of the stream in gaining a position below. We called the water "Bridge Creek" !

Doctor Tilghman: "The Natural Bridge" is perhaps one of the greatest curiosities we saw in the while of our interesting expedition. It is at the extremity of a valley formed of an immense chasm, with rocky sides - and a perpendicular height of 300 feet - through which flows a beautiful chrystal stream."

In 1846, James Frazier Reed of the ill-fated Donner Party was aware of the bridge. In his diary he wrote, "We made this day 18 miles and camped on Beaver Creek. Here is a natural bridge 1.5 miles above camp."

During the California Gold Rush, a few "Forty-niners" found time to visit Natural Bridge. In a letter dated July 4, 1849, while camped at Deer Creek, Cephas Arms of the Fayette Rovers wrote: "Where we camped last night, and we meant to spend the 4th, instead of coming eighteen miles through the dust thick enough to choke us, if we could find grass, was quite a natural curiosity in the shape of a natural bridge. It is thrown over the river where we camped. "Fourche Boise River", and is a perfect arch one hundred feet long and eighteen feet high of solid stone. On either side the perpendicular rocks rise to the height of one hundred and fifty feet. The bridge is just at teh foot of the mountain through which the stream passes. The mountain is three or four hundred feet above the plain below, and the river rushes through a gorge in the mountain with perpendicular walls to the top of the hill, the whole forming one of the wildest scenes I ever beheld. The bridge has never been named until today. We have christened it Welch's Bridge in honor of one of our company from Michigan, who pronounces it only second to the far famed Virginia bridge. But I have not time to describe the half I have seen. Scenery the most beautiful and grand I ever saw."

The bridge was named after Adonijah S. Welch of Jonesville, Michigan. A graduate of the University of Michigan, Welch was later the first president of iowa State University.

On June 26, 1850, Isaac R. Starr wrote: "Up near the high cliffs there is an arch of solid stone over this river, 40 or 50 feet wide and 15 feet high. I passed up the river, rose through beneath the arch, and viewed with delight the grand works of nature."

From park signage:

Formation of Ayres Natural Bridge

Meandering LaPrele Creek drainage cuts into alternating layers of sandstone and sandy limestone of the Permian-Pennsylvanian Casper Formation.

LaPrele Creek erodes both the upstream and downstream sides of the outcrop.

Undercutting by the creek collapses the lower level of stone, forming the bridge. The creek then follows the shortcut, flowing under the bridge.

Stratigraphy: Casper Formation (also known as the Tensleep Formation), Middle Pennsylvanian to Lower Permian

Locality: Ayres Natural Bridge over LaPrele Creek, southern end Natural Bridge Road, south of Interstate 25, west of the town of Douglas, Converse County, eastern Wyoming, USA

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