Caprock Natural 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.