Waterfall Natural Bridge
Natural Bridge, Three
Turkey Natural Bridge, Black
Rock Natural Bridge, Death
Valley Natural Bridge, Murl
Emery Natural Bridge
This type of natural arch occurs when subterranean
stream piracy undercuts, and eventually isolates, a portion of rock
streambed, leaving a lintel supported by what was, and may still be,
the two banks of the stream. This can happen at the lip of a waterfall
when cracks and joints permit the water to shorten its course through
the rock under the lip. It can also happen for long stretches of rock
streambed where no waterfall is apparent. The shapes of the lintel and
opening vary greatly depending on the type of rock, depositional mode
of the rock (e.g., the extent and nature of cross-bedding), and stream
characteristics. Therefore, the specific morphology of this type of
natural arch is not usually helpful in its classification. Subterranean
stream piracy must be deduced from contextual attributes. These in turn
may be dependent on the relative age of the natural arch.
The contextual attribute that must be present is a stream. The natural
arch must span either an active stream or an abandoned streambed. If
the natural arch occurs through the neck of an incised
meander, then it is a meander natural
bridge rather than a waterfall natural bridge. The presence of a
waterfall either at, or upstream of, the natural arch is also a clear
indicator of a waterfall natural bridge. However, as stated above, a
waterfall need not be apparent for the natural arch to be of this type.
When a waterfall is apparent, its relative closeness to the natural
arch is an indication of maturity, i.e., a tracer of subsequent development
due to erosion. The waterfall retreats upstream from the natural arch
as time passes. Thus, a waterfall natural bridge that is collocated
with, i.e., still part of, a waterfall, is young. A waterfall natural
bridge that is clearly separated from, but still within linear sight
of, a waterfall should be considered an adult example. Once the waterfall
has retreated from the natural arch beyond linear sight, e.g., around
a bend in the stream, it should be considered old.
It is often the case that the waterfall is not apparent. This is usually
due to the stream undercutting a long stretch of rock streambed. In
this case, the precursor to the waterfall natural bridge is a sink and
rise. The stream seems to disappear into a sinkhole and then rise from
the rock down stream. The distance between sink and rise may be several
miles. Vreeland labeled such a feature as a "siphon natural bridge."
However, unless air is permanently present above the water as it flows
through this underground stretch, this type of feature does not meet
the requirements of our definition of a natural arch. Nevertheless,
such a feature frequently evolves into a waterfall natural bridge. Erosion
lowers the stream and isolates the undercut stretch of former streambed,
creating a roof over an underground passageway. Almost all of the natural
arches that have been labeled as "natural tunnels" in other taxonomies
fall into this category.
It frequently happens that roof
collapse occurs in one or more section of such an underground passageway.
Roof collapse can cause multiple waterfall natural bridges to form in
sequence along a single streambed. Also, if roof collapse occurs near
one end of the underground passageway, a natural arch can form there
even when the air gap between the streambed and roof has only formed
over part of the distance between sink and rise. Such a natural arch
would still be considered a waterfall natural bridge. Finally, the natural
arch may be all that remains of a once extensive roof. In such cases
there is no waterfall at all. This is not because it has retreated to
a great distance, but because it was never apparent in the first place.
When a waterfall is not apparent, the best indication of maturity is
the ratio of opening height to span. When the height is small compared
to the span, this type of natural arch is young. When they are comparable
it is an adult. When the height is large compared to the span, it is
A very special subclass of waterfall natural bridge occurs when a petrified
log is found over a streambed. Here again, the flow of water has undercut
the log, now rock, leaving it suspended above the streambed, with its
two ends embedded in the banks of the streambed. A petrified log differs
from other waterfall natural bridge lintels only in how the rock itself
was originally formed, i.e., in the geologic attributes of the natural
arch. There is no difference in the contextual or genetic attributes.
Hence, these unusual features are considered waterfall natural bridges.
They are very fragile and very rare.
The waterfall natural bridge is the only type of natural arch where
flowing water is the primary cause of formation.