Fin Natural Arch
(Genetic type)
Examples: Full
Moon Arch, Big
Eye Arch, Top
Story Window, Double-O
Arch, Teakettle
Rock
This type of natural arch is the result of erosion acting on a thin
fin or wall of rock. The fin of rock can be created through any number
of different processes. How the fin formed is not relevant to classifying
a fin natural arch. The important distinction is that wall
collapse did not cause the opening. Rather the opening was
caused by any combination of weathering,
differential erosion, exfoliation,
or bedding plane expansion. As a
result, the opening is either an upright oval or upright slotted aperture.
It is not a semicircular aperture. Indeed, the opening is almost always
elevated. The lintel is usually, but not always, massive. Many of the
natural arches that have been labeled as "natural windows" fall into
this category. Unfortunately, the term "natural window" has been applied
so broadly and inconsistently that there is not really much of a correlation.
Several possibilities exist for why an opening might be present in
a rock fin in the absence of wall collapse. The opening may be the coincidental
result of an area of relatively weak cementing between the grains of
the rock, e.g., in sandstone. This usually leads to an oval aperture.
Alternatively, cross-bedding may have led to a localized weakness in
the rock that weathering and gravity have evacuated. A slotted aperture
is the likely result. In either case, if the opening is small enough,
it has little structural impact on the fin, i.e., the weight above the
opening is supported by the tensile strength of the rock and not through
redistribution. Another possibility is that the opening may be so young
that wall collapse has not yet started, i.e., the opening is a small
precursor to what will become a semicircular aperture. In this case,
the fin natural arch will evolve into a shelter
natural arch or, in certain cases, a meander
natural bridge. Finally, if an oval aperture expands upward such
that a catenary shape is approximated under a relatively slender lintel,
the opening can become quite large and last a long time. It is possible
for a fin natural arch to evolve into either an arc
natural arch or abandoned natural
arch before it collapses.
Trying to assign a maturity attribute to a fin natural arch is problematic
at best. It might be possible where the specific process of hole formation
is evident, but this is usually not the case. Therefore, the taxonomy
does not attempt it.