Pillar Natural Arch

(Genetic type)

Examples: The Grotto, The Colonnade, unnamed arch, unnamed arch, unnamed arch

This type of natural arch is the result of cavity merger. In this case, the cavities involved are two adjacent caves or recesses whose backs have joined, leaving a remnant partition separating the mouths of the caves. This partition is the lintel. Thus, the lintel of a pillar natural arch is always vertical. There is a prone C-shaped opening whose entrances are the mouths of the caves.

Miniature and minor pillar natural arches are very common. They are frequently found in clusters. This striking configuration happens when several adjacent recesses reach a vertical joint that is close to and parallel with a cliff face. Preferential erosion along the joint joins the backs of the recesses creating a series of pillar natural arches that comes close to resembling a latticework. Significant pillar natural arches are much rarer. Most are solitary, but a few examples of clusters do exist.

The lintel of a pillar natural arch is seldom of structural importance to the cavity behind it. If the lintel collapsed, it is unlikely that the cave roof would also collapse as a result. The lintel is more fa�ade than load bearing. Consequently, compression strengthening seldom occurs. Thus, as erosion expands the cave, the lintel just disappears. This is probably why few large examples exist. It also implies that pillar natural arches have a relatively short lifecycle.

A maturity attribute can be assigned based on the size of the lintel in relationship to the opening. A young pillar natural arch has a lintel breadth comparable to or larger than the horizontal extent of the entrances adjacent to it. If the lintel breadth is much smaller than the entrances' horizontal extents, the natural arch is an adult. If, in addition, the lintel is highly scalloped or fractured, it can be considered old.