Buttress Natural Arch

(Genetic type)

Examples: Feather Rock Arch, White Mesa Natural Arch, Corona Arch, unnamed arch, unnamed arch

This type of natural arch always occurs at the end of a wall or vertical slab of rock. The wall can be an isolated fin or dike, or it can be a projection of rock from the face of a cliff. Such projections frequently form between the heads of adjacent erosion valleys. Differential erosion and other mechanisms can also cause such projections to form. Regardless of how the wall was formed, this type of arch can always be described with the contextual attribute 'projecting.'

The weight of the wall or projection is distributed at its end along half of an inverted catenary shape, where the division into halves is along the axis of symmetry. This can lead to compression strengthening and partial wall collapse. The resulting opening is always either a half-semicircular aperture (roughly the shape of a pie quarter) or an upright oval aperture. The opening shape depends on the specific details of weight distribution, wall collapse, and maturity. [NOTE: A projecting natural arch with an upright slotted aperture and a vertical lintel is not a buttress type, but rather a propped natural arch.]

As a buttress natural arch ages, the opening can enlarge somewhat, but there are usually severe constraints on how far this can happen before the arch collapses. A more reliable indication of maturity is how far the lintel has eroded to conform to the half-catenary shape. In the case of an adult buttress natural arch, only rock that participates in weight distribution remains in the lintel. The lintels of younger examples can have unusual shapes since excess rock is still present. Older buttress natural arches have very elongated and slender lintels, since compression strengthening has only been able to preserve the core of the catenary shape. In very unusual and extreme cases, this type of natural arch can evolve into an arc natural arch.