Cave Natural Arch
Horse Arch, Sam
Bass Arch, Egg
Shell Arch, Hole
in the Bridge Arch, West
Rim Arch, unnamed
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.