Meander Natural Bridge

(Genetic type)

Examples: Arch Rock, Pont d'Arc, Kachina Natural Bridge, Owachomo Natural Bridge, Juanita Arch, High Falls Arch

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.