Arc Natural Arch
(Morphologic type)
Examples: Landscape Arch,
Bridge Mountain Arch,
Ribbon Arch, Castle Arch,
Long Arch
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 strengthening.