Cave Natural Arch

(Genetic type)

Examples: Wild Horse Arch, Sam Bass Arch, Egg Shell Arch, Hole in the Bridge Arch, West Rim Arch, unnamed arch

This type of natural arch results when roof collapse occurs over a cave, leaving a portion (or portions) of the cave roof suspended by the walls of the cave. When roof collapse happens in a cave, it is common for multiple sections to collapse, creating multiple entrances and lintels. Very complex morphologies can result. In the simplest of cases, however, there is only one entrance through the roof and hence one lintel and one opening. This is an upright L-shaped opening with the upper, horizontal entrance in the cave roof and the lower, vertical entrance at the mouth of the cave. The lintel is usually much wider than it is thick, but this is not always true. The lintel also tends to be flat, but again this is not always true.

Determining the maturity of a cave natural arch is somewhat complex and depends upon how many roof entrances there are. A cave natural arch with a single opening is considered young if the roof entrance is small compared to the total area of the roof. It is considered an adult if most of the roof has collapsed.

When there are multiple roof entrances, a young cave natural arch will have more intact roof than roof entrance, i.e., the combined area of the entrances is small compared to the area of remnant roof. If the natural arch has less roof than roof entrance, it is an adult.

A cave natural arch should only be considered old if it has a single lintel. If, by coincidence, the lintel is arched and compression strengthening has occurred, the natural arch can survive for long periods of time, even longer than the cave itself. An old cave natural arch will have an arched and well weathered lintel. In extreme cases, where the cave has mostly disappeared, the opening may have become a semicircular aperture. It is usually difficult to distinguish between an old cave natural arch and an old pothole natural arch. The determination is made solely upon context, i.e., it is clear that the adjacent depression was a cave whose roof collapsed rather than a pothole.