Meander Natural Bridge
Natural Bridge, Owachomo
Natural Bridge, Juanita
This type of natural arch is always associated with an active stream
or streambed. The opening is a semicircular aperture and the lintel
is arched. Most examples have a flat lintel, but there are some notable
exceptions to this. Nevertheless, this characteristic shape of the lintel,
flat on top and arched underneath, is so reminiscent of a man-made bridge
that these features have consistently been labeled natural bridges in
all previous taxonomies. Retaining this concession to anthropomorphic
description in this taxonomy was deemed both necessary (to avoid the
confusion that would certainly arise from any new type label) and tolerable
(because of the consistent previous usage).
Although flowing water in the streambed plays a crucial role in the
formation of this type of natural arch, it is not the only erosion process
involved. Wall collapse must
also occur. Indeed, this is the reason the opening is always a semicircular
aperture. Flowing water is the cause of the wall in the first place
and lateral stream piracy completes
the picture after the opening has formed. Nevertheless, because there
must also be wall collapse, it can not be said that flowing water is
the primary cause of this type of natural arch.
Formation begins when either a stream meander
or two parallel tributaries are incised into rock as a result of rapid
uplift. The flow may be either
permanent or occasional. This results in a thin neck or wall of rock
that separates two streams of flowing water. In the case of a meander,
the flow is due to one stream that has doubled back on itself. In the
case of parallel tributaries, two separate but adjacent streams cooperate
to create the wall.
Once the wall has reached a sufficient height, i.e., once the stream
action has sufficiently incised the rock, wall collapse may occur. The
initial opening may be oval, but soon expands downward to the level
of the stream and takes on the characteristic shape of a semicircular
aperture. At that point, occasional floods trigger lateral stream piracy
so that the stream eventually flows through the opening. Subsequent
development is due to further wall collapse, weathering,
and compression strengthening.
In rare cases, lateral stream piracy can occur upstream of the opening,
leaving the floor of the opening dry.
However, there must be evidence that a stream once flowed through the
opening. It is possible for wall collapse to only proceed down to a
harder layer of rock that is still above the level of the stream. Continued
uplift and deepening of the incised streambed may leave the opening
isolated above the flow. In this case, the feature is a shelter
natural arch, not a meander natural bridge. It is also possible
that an opening might form from some other process than wall collapse
and again not enlarge to the point where stream piracy occurs. In this
case, the feature is a fin natural arch.
A significant amount of subsequent development due to wall collapse
can occur after the initial formation of a meander natural bridge, whether
or not a stream continues to flow through the opening. In some cases,
the stream flows at the foot of one of the abutments, preferentially
enlarging that end of the opening. But it is usual for the opening to
expand in all directions away from the stream flowing through it. This
process continues until either the lintel can no longer assume a catenary
shape (becomes unable to support its own weight) or one of the abutments
becomes too thin to support the weight of the lintel.
These are the observable indicators of maturity. A meander natural
bridge is young if the stream only flows through the opening when in
flood, or if the stream fills the floor of the opening. If at least
one of the abutments is significantly removed from the streambed, it
can be considered adult. If the lintel has become delicate or has lost
its catenary-shaped underside, or if one of the abutments has narrowed
to the point that much further expansion of the opening will result
in collapse, the natural arch is considered old.