Caprock Natural Arch

(Genetic type)

Examples: Musselman Arch, Skylight Arch, Metate Arch, unnamed arch, unnamed arch

This type of natural arch occurs where a relatively thin, horizontal layer of rock overlays, and is supported by, a thicker layer of rock that is softer than the overlaying layer. Thus, the lintel is always flat. Differential erosion is always evident with the softer, supporting layer eroding faster than the harder layer above it. As the harder layer looses support from this process, it fractures and collapses. Partial collapse can lead to the formation of a natural arch. When this occurs along a cliff, the resulting lintel is suspended from the edge of the cliff. It can also happen that the harder layer protects an island of the softer rock from erosion. When this occurs, an opening can form between pedestals of the supporting layer of rock.

Usually the harder layer, or caprock, is a different member from the softer layer. Thus, the differential erosion that led to natural arch formation is most frequently between adjacent members. However, differential erosion in the same member can occur if flat layers of that member were deposited with different strengths of cohesion.

Since the lintel of this type of natural arch is always flat, not arched, and relatively thin, caprock natural arches are relatively short-lived. Furthermore, since little development can occur between formation and collapse, they do not exhibit any maturity attributes. However, because they are formed easily and frequently, caprock natural arches, especially small ones, are fairly common.