Nature’s architecture is full of fascination: the castle crags in the high Sierra, sea stacks and steep fjords cut by glaciers long ago. But nature is perhaps at its most impressive in the formation of arches. Carved by time itself and the diminutive act of frozen water over millions and millions of years, the process is as amazing as the product, thousand pounds of rock suspended effortlessly and perpetually (it seems) hundreds of feet in the air.
In the United States, the desert Southwest is renowned as the epicenter of arch formation. Utah in particular contains one of the largest concentrations of naturally forming arches in the world. Six of the world’s 15 largest arches are found here, including at least one on this list. Arches National Park alone has over 2,000 documented arches. The sandstone bedrock of the Colorado Plateau provides a particularly favorable environment for the development of these arches. The porous rock, complemented by a climate that is adequately but not overly wet, erodes under runoff streams without fully collapsing. Additionally, as a sedimentary rock, the difference between soft and hard layers of rock allows arches to formation—like Bryce Canyon’s hallmark hoodoos.
Wherever there’s sandstone, it’s liable to erode in patterns that create arches. And while the conditions of the desert Southwest are optimal for arches, it isn’t the only place where they occur. The bedrock of Kentucky’s Red River Gorge is also sandstone, and Natural Bridge is likely the most visited natural bridge in Kentucky. There’s a difference between a natural arch and a natural bridge: they appear artificially created by humans, or they were created by a flowing body of water that eroded the opening. Likewise, a natural bridge is a type of natural arch, but not necessarily vice versa.
On the Big Island of Hawaii, the City of Arches is a complex of arches that is relatively hidden from the more popular snorkeling spot, Pae’a, 2 miles to the south. These, like most of the Hawaiian Islands, are formed of volcanic rock gradually eroded by the power of the ocean and salt.
Some of the most striking arches in the country are indeed created by humans, or otherwise born of a more deliberate process. The Bixby Bridge in California’s Big Sur is iconic, a photography hotspot, and one of the world’s tallest single-span concrete bridges. Its graceful arch architecture was designed and completed by the Ward Engineering Company and designer F. W. Panhorst in 1932, and it remains to this day a scenic vestige of its time. It spans 714 feet over Bixby Creek, which flows into the Pacific Ocean 260 feet below the bridge.