Sandstone arches

The are more than 100 sandstone arches in the Cumberland Mountains of Kentucky and Tennessee. These spectacular structures were formed over some 300 million years, starting with deposition of sand, consolidation into rock, uplift, and erosion. Softer rock erodes faster than harder rock, so when harder rock overlay softer rock the stage was set for differential erosion. As fissures widened into cracks and cracks became valleys, narrow rock ridges were formed under the harder layers. Eventually the lower, softer rock layers began to erode from the sides. This process first produces what are locally called rock shelters, rock houses, or sand caves. These shallow shelters were used historically by native Americans as temporary shelters, such as on hunting trips, and later by European long hunters and even modern hunters. When rock shelters formed on both sides of a narrow rock ridge, it was possible the two eventually joined, in which case a natural arch formed.

This particular arch is in the Daniel Boone National Forest in southern Kentucky and is accessible from a day use area. The cliff line it eroded through extends for some 10 miles.

To get a sense of scale, note the young couple and their toddler just above the steps leading up to the opening. And did you notice the Sarvis blooming near the right side of the arch?

Perhaps America's best-known arch is Natural Bridge in Virginia. It has been a tourist attraction since the days of Thomas Jefferson. The Big South Fork National River and Recreation Area has at least nine arches that are readily accessible. You can check them out here.