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.