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.