Pothole Natural Arch
(Genetic type)
Examples: Pritchett Arch,
Window Rock,
Gold Bar Arch,
Rainbow Arch,
Honeymoon Arch,
Flying Eagle Arch
This type of natural arch occurs when part of the wall or floor of a pothole ruptures, leaving a
section of its rim suspended above the new opening. The process that created the pothole in the
first place, chemical exfoliation, is also the cause of the
opening.
A natural arch of this type can only form if the pothole is reasonably near a cliff wall.
Frequently, as chemical exfoliation deepens the pothole, a harder layer of rock is encountered.
This tends to divert the progress of erosion laterally. If a cliff wall is reached, a natural arch
can be formed. As this lateral entrance expands, the harder layer beneath remains less susceptible
to erosion. Since this harder layer is usually flat, many pothole arches, especially older ones,
have a semicircular aperture for the opening.
The other contributing factor to forming a semicircular aperture is
compression strengthening. Once the initial opening is
formed, compression strengthening and weathering take over
from chemical exfoliation as the primary processes governing subsequent development. Lintels that
become arched through these processes can last a long time, allowing the opening to grow to a span
commensurate with the diameter of the pothole. Lintels that do not become arched remain weak,
leading to a short life for the natural arch.
In many cases the pothole forms over a recess in the cliff wall. When this happens, chemical
exfoliation can create an opening without any lateral diversion. The result is an upright L-shaped
opening. Usually, the lower, vertical entrance is oval. Since the pothole stops growing once the
opening forms, little subsequent development occurs beyond erosion of the lintel. Again, weathering
and, in some cases, compression strengthening are the key processes involved.
Note that the opening of a pothole arch occurs either in a single member of rock or at a boundary
where a softer member overlays a harder member. Potholes can also form through a hard, flat layer
overlying a softer, supporting layer. In some cases such a pothole, combined with
differential erosion of the softer layer, can form a
natural arch. However, this would be considered a
caprock natural arch in this taxonomy.
A young pothole natural arch has a span that is smaller than the diameter of the pothole that
formed it. An adult pothole natural arch's span is commensurate with the diameter of the pothole.
Since a pothole need not be round, comparison should be made with the diameter that is parallel
with the lintel. A pothole natural arch might be considered old if, in addition to having a span
commensurate with its pothole's diameter, it has a delicate, arched lintel above a semicircular
aperture.