Shelter Natural Arch
(Genetic type)
Examples: Star Gap Arch,
Natural Arch of Kentucky,
Carlotta Arch,
Leland Arch,
unnamed arch
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.