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.