Pothole Natural Arch
Bar Arch, Rainbow
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.