Shelter Natural Arch

(Genetic type)

Examples: Star Gap Arch, Natural Arch of Kentucky, Carlotta Arch, Leland Arch, unnamed arch

This type of natural arch always occurs on a ridge-top and is usually the result of cavity merger. In this case, the two cavities are shallow, arched recesses, often called rock shelters, that have formed on opposite sides of a narrow extent of a ridge-top. Differential erosion (usually, but not always, in one member) deepens the cavities. These eventually join to create a cylindrical opening under an arched lintel. Frequently, the floor of the opening is flat, but this is not always the case. Compression strengthening then acts to resist subsequent erosion. Consequently, this type of natural arch is long-lived and very common.

Where conditions are right, e.g., a tall, thin wall of rock on a ridge-top, a shelter natural arch may also be the result of wall collapse. In this case, a semicircular aperture forms. Since wall collapse is a quite different process than cavity merger, one might ask why natural arches formed in this way are not considered a different type. The reason is the difficulty of unambiguously discriminating between them in a large number of cases. The cylindrical opening formed by cavity merger is always arched on the top and usually flat on the bottom, i.e., the lintel is arched and the opening floor is flat. Such an opening looks much like a semicircular aperture that has been stretched in width so that the distance between its parallel entrances is comparable to its span and/or height. Since ridge-top natural arches exhibit a continuum of opening widths, there is no obvious point at which one could unambiguously discriminate between a semicircular aperture and a cylindrical opening with an arched top and flat bottom. Thus, it is not possible to clearly separate the two formation processes based on observable attributes. As a result, these natural arches are combined in a single type with the label 'shelter natural arch'. Where it is clear what formation process dominated, one can modify the type label with the appropriate genetic attribute, e.g., a wall collapse shelter natural arch, or cavity merger shelter natural arch.

Although all varieties of shelter natural arch last a long time, it is difficult to unambiguously define maturity attributes. As a shelter natural arch ages, its opening enlarges and the lintel becomes more delicate and arched. However, a large opening and delicate lintel might also be the result of the shape of the ridge-top prior to natural arch formation. At the extremes, some conclusions are safe. A small, squat opening below a massive lintel is almost certainly a young natural arch. Similarly, a smooth, slender arc of rock at the apex of a large semicircular aperture has clearly experienced a significant amount of subsequent development and may be called old. However, it is probably best to label anything but these obvious extremes as an adult natural arch. It is possible for a shelter natural arch to evolve into either an abandoned or arc natural arch before it collapses.