Arc Natural Arch
Mountain Arch, Ribbon
This type of natural arch, due to erosion after its initial formation,
no longer shows any conclusive evidence for a specific process of formation.
The erosion processes that govern subsequent development are weathering
and compression strengthening.
The lintel is always a very long, very slender arc of rock that is arched,
although only slightly so. A cross section of the lintel varies little
over much of its length. To qualify as an arc natural arch, the span
must be at least ten times the lintel breadth. The lintel is typically
supported by massive abutments, but this is not absolutely necessary.
An arc natural arch is presumed to be very old and near the end of
its lifecycle. Although quite rare, there are enough examples to warrant
a taxonomy type. That there are any examples at all is because they
are structurally strong. Only the very strongest natural arches can
survive for any length of time in such a state. That strength is certainly
due to the lintel being shaped like a full or partial catenary, the
result of compression strengthening. Of course, the catenary shape that
the lintel has developed must also be only slightly arched for it to
have a span ten times its lintel breadth. Massive abutments are typical
because they allow the natural arch to have distributed a greater weight
when it first formed, and hence to have experienced greater compression