Buttress Natural Arch
Rock Arch, White
Mesa Natural Arch, Corona
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