Shelter Natural Arch
Gap Arch, Natural
Arch of Kentucky, Carlotta
This type of natural arch always occurs on a ridge-top and is usually
the result of cavity merger. In this
case, the two cavities are shallow, arched recesses, often called rock
shelters, that have formed on opposite sides of a narrow extent of a
ridge-top. Differential erosion
(usually, but not always, in one member) deepens the cavities. These
eventually join to create a cylindrical opening under an arched lintel.
Frequently, the floor of the opening is flat, but this is not always
the case. Compression strengthening
then acts to resist subsequent erosion. Consequently, this type of natural
arch is long-lived and very common.
Where conditions are right, e.g., a tall, thin wall of rock on a ridge-top,
a shelter natural arch may also be the result of wall
collapse. In this case, a semicircular aperture forms. Since wall
collapse is a quite different process than cavity merger, one might
ask why natural arches formed in this way are not considered a different
type. The reason is the difficulty of unambiguously discriminating between
them in a large number of cases. The cylindrical opening formed by cavity
merger is always arched on the top and usually flat on the bottom, i.e.,
the lintel is arched and the opening floor is flat. Such an opening
looks much like a semicircular aperture that has been stretched in width
so that the distance between its parallel entrances is comparable to
its span and/or height. Since ridge-top natural arches exhibit a continuum
of opening widths, there is no obvious point at which one could unambiguously
discriminate between a semicircular aperture and a cylindrical opening
with an arched top and flat bottom. Thus, it is not possible to clearly
separate the two formation processes based on observable attributes.
As a result, these natural arches are combined in a single type with
the label 'shelter natural arch'. Where it is clear what formation process
dominated, one can modify the type label with the appropriate genetic
attribute, e.g., a wall collapse shelter natural arch, or cavity merger
shelter natural arch.
Although all varieties of shelter natural arch last a long time, it
is difficult to unambiguously define maturity attributes. As a shelter
natural arch ages, its opening enlarges and the lintel becomes more
delicate and arched. However, a large opening and delicate lintel might
also be the result of the shape of the ridge-top prior to natural arch
formation. At the extremes, some conclusions are safe. A small, squat
opening below a massive lintel is almost certainly a young natural arch.
Similarly, a smooth, slender arc of rock at the apex of a large semicircular
aperture has clearly experienced a significant amount of subsequent
development and may be called old. However, it is probably best to label
anything but these obvious extremes as an adult natural arch. It is
possible for a shelter natural arch to evolve into either an abandoned
or arc natural arch before it collapses.