Alcove Natural Arch

(Genetic type)

Examples: La Ventana Arch, Apex Arch, unnamed arch, unnamed arch, unnamed arch

This type of natural arch occurs on or next to the vertical face of a cliff. It can only form where a vertical joint has previously formed behind and parallel to the cliff face. It is very common for stress relief exfoliation at a cliff face to result in the formation of this type of joint. Consequently, the alcove natural arch is also quite common. Its observable attributes vary somewhat depending on where it is in its lifecycle.

For a young alcove natural arch, the lintel is at the edge of the cliff and is supported above what appears to be a shallow recess into the cliff face. The opening is a prone, slotted aperture connecting the air above the cliff edge with this recess. The recess is actually the result of wall collapse. Because of the joint, the cliff face has become a wall of rock that is structurally isolated from the mass of rock behind it. Even though the wall is in contact with the rock behind it, it still supports its own weight. Hence, wall collapse can occur. When erosion widens the joint enough to separate the lintel from the cliff face, usually at the apex of the recess, the natural arch is formed. This is why the initial opening is slotted (aligned with the joint) and prone (connecting the apex of the recess with the air above the cliff). Usually, one can only see light through the opening when standing at the base of the natural arch looking up.

As erosion widens the vertical joint, the natural arch becomes better defined and separated from the rock mass behind the wall. What had been a recess into a cliff face is now seen as a semicircular aperture that is occluded by an adjacent cliff face. This appearance defines the adult stage of the alcove natural arch.

Compression strengthening may make the natural arch more resistant to subsequent weathering and erosion than the cliff face. This, as well as several other factors, can lead to the cliff face retreating from the wall in which the natural arch occurs. An old alcove natural arch will still have a semicircular aperture, but the cliff face will only partially occlude the opening.

If the cliff has retreated to an extent that the opening is no longer occluded, the natural arch can no longer be classified as an alcove type, or any other genetic type. The evidence of formation has disappeared. Very old alcove natural arches may evolve into abandoned or arc type natural arches, or, of course, collapse.